Hot Dip Coating of Plastisol is a Common Method of Applying a Protective

Hot dip coating of plastisol is a common method of applying a protective, decorative, or functional plastic vinyl coating on a variety of metal parts.  Typical applications provide a protective layer against sharp or rough materials, a protective plastic layer to prevent corrosion,  or just a beautiful and soft coating to enhance the product's appearance.

It is much more cost effective to do a one step dip to coat a product, for instance a tool grip,  than it is to injection mold a sleeve and then apply it to the handle. The coated grip is softer, has no seam lines, and adheres better to the product to be coated.  Applications for dip coating are extensive and include:    • Hand tools    • Toys    • Medical Instruments    • Electrical Equipment    • Plumbing Fittings

Timeline to ProductionCompleting your plastic dip coating project is a straight-forward and comprehensive process. We work with you throughout the production cycle to ensure the final product meets your design and material specifications.

1. Provide the coating specifications and material properties; i.e., color, durometer, and thickness.2. We submit a quote for your acceptance3. You provide samples of the product to be coated.4. We test dip coat the product and provide finished samples to spec5. Upon approval, you ship the product to be coated to us.6. Proceed to full production 

Dip Coating process step by step.

 1. Part CleaningParts to be coated should be chemically cleaned to remove contaminants .  This results in superior primer adhesion as well as improved corrosion resistance.2. Part PrimingIf adhesion to the part is desired, application of a primer  is required on all sections were polymer coating will be applied.  The primed part is flash dried and baked to set the primer and ready the part for the hot dipping operation.3. Part DippingThe primed part, hot from the primer conversion bake, is immersed in the liquid plastic polymer. All hot parts of the product immersed in the liquid polymer will coat upon withdrawal from the bath with a layer of semi-fused plastic. The amount deposited will depend on the length of time the part was immersed, the metal temperature that the part was immersed at, and the general formulation of the plastic compound used. The higher the metal temperature, the longer the immersion time, and the greater the film thickness.4. Fusion (Curing) plastic polymerThe part covered with the semi-fused polymer is baked to a temperature between 300°F and 350°F, depending upon the specific plastic compound formula. This completes the fusion of the coating, and adhesion to the product.5. Cooling the Coated PartWhen the hot part coated with fused polymer comes out of the oven, it is very sensitive to surface marring. The part is cooled down in a tank of circulating cool water to a maximum of 120°F before handling.6. Complete end productThe finished coated product is then removed from the conveyors

and readied for packing and shipment.

Advantages of Dip CoatingSouthwest Latex dip coatings provide safe ergonomics and beautiful finishes. This process deposits a coat of PVC plastisol adding a protective layer of cushioning which enhances the handling and appearance of your product.1. Dip coating provides a protective shield that resists corrosion.2. Insulates against heat, cold, stress and electrical currents.3. Adaptable to high volume orders requiring fast delivery.4. Durable and UV resistant.5. Alternative colors and finishes (glossy and matt) can be created easily and economically.6. A wide range of thicknesses, textures, and durometers (hardness) are available.7. Dip Coating is sound damping and eliminates the need to debur base materials

[from http://swlatex.com/dip-coating.htm#coatsamples]

Dip coating is a production approach that involves the application of some type of protective coating onto the finished product. The main function of dip coating is to increase the useful life of the product, although dip coating may also be used for decorative purposes as well. Products that are routinely subjected to dip coating are found in both the home and in the workplace.

The most common form of dip coating involves covering metal with plastic. A good example of this type of dip coat application is the simple wire hanger. In order to protect clothing from the possible development of rust on the bare metal of the hanger, the hangers are coated with a thin layer of plastic. The plastic protects the metal from exposure to the open air and limits the possibility of rust. At the same time, the protective coating prevents direct contact between the fabric of the clothing and the metal, and thus minimizes the chances of some sort of stain developing on the material.

In actual practice, the process of dip coating is not difficult. In fact, dip coating can be broken down into a simple three-step process. First, the object is cleaned thoroughly and the immersed in a vat or container of the hot plastic. By immersing the object, there is a better chance of achieving an even coating all along the surface of the product.Once the object is immersed, the next phase of dip coating involves determining how long to leave the object in the hot plastic. This is known as the dwell time. Depending on the construction of the object and the type of melted plastic in use for the coating, the dwell time may vary anywhere from a few seconds to several minutes.

The final phase of dip coating is known as the withdrawal cycle. This involves methodically removed the object from the hot plastic. Care is taken to remove the object as a constant rate of motion. This helps to minimize the chances for the development of judders along the surface of the object. The actual speed helps to determine how thick the final coat dip coat actually is. Generally, faster withdrawal from the plastic will result in a thicker coating along the surface of the object.

Dip coating is often added as a protecting coat to electrical wiring and other items that need to be insulated for safety purposes. At other times, the dip coating may be used to add color to the finished product, making it more visually appealing. In both situations, the presence of the coating usually extends the life of the object, allowing consumers to enjoy more of a return for their investment in the product.

Understanding The Process Of Dip CoatingBy John M StrongDip coating is a process wherein a finished product is coated with a protective coating. The products are dip coated to prolong their useful life. Aside from making the products more durable, dip coating is also used to make the products look more decorative. Products that undergo this process are commonly found in the home and in the workplace.

Dip coating is commonly used in metal products that are coated with plastic. The perfect example of a product that has undergone this process is the wire clothes hanger that you commonly see in clothing stores and inside the home. The metal of the hanger if left bare can easily get rusty and the rust can transfer to the clothes and ruin the cloth. To prevent this from happening, the metal frame of the hanger is coated with plastic. A thin or thick coating of plastic will protect the metal part of the hanger from being exposed to air and moisture which are the main causes of corrosion or rusting. The plastic coating of the hanger will also protect the clothes from discoloration or staining from being in contact with the metal.

Dip coating is a very simple process that can be done in three simple steps. The first step is cleaning the object very thoroughly. When the object is thoroughly clean it is pre heated and submerged in a container that is filled with plastic. Dip coating is more effective than any other process of coating the object because it ensures an even coat all over the surface of the object. It makes sure that the entire object is evenly coated with the plastic.

The next step in dip coating is the dwell time or the length of time that the object should be immersed in the plastic. The length of the dwell time will depend on how the object is constructed as well as the kind of plastic that is used in the medical process. The dwell time can take a few seconds to a few minutes depending on the factors mentioned.

The last step in the process of medical is the withdrawal cycle which is the removal of the object that was immersed in the plastic. It is important to carefully remove the object from the container of hot plastic. There should be constant motion while the object is being removed because it prevents the formation of judders on the surface of the object. This ensures that the plastic coating is smooth. The speed at which the object is removed from the plastic determines the thickness of the plastic coating.

Article Source: http://EzineArticles.com/7225452

Dip CoatersKSV NIMA Dip Coaters are robust computer controlled instruments for precise thin film deposition. W… MORE INFO

Dip Coating

Dip coating is the precision controlled immersion and withdrawal of any substrate into a reservoir of liquid for the purpose of depositing a layer of material. Many chemical and nanomaterials engineering research projects in academia and industry make use of the dip coating technique.

Many factors contribute to determining the final state of a dip coated thin film. By controlling the functionalisation of the initial substrate surface, submersion time, withdrawal speed, number of dipping cycles, solution composition, concentration and temperature, number of solutions in each dipping sequence and environment humidity, a large variety of repeatable dip coated film structures and thicknesses can be fabricated. The technique of dip coating can give uniform, high quality films even on bulky, complex shapes.The dip coating technique is used for the fabrication of thin films by self-assembly and the sol-gel technique. Self-assembly can give film thicknesses of exactly one monolayer. The sol-gel technique creates films of increased, precisely controlled thickness, determined mainly by the deposition speed and solution viscosity.  Self assembly Self assembly is the process where components spontaneously organize or assemble into more complex objects, typically by bouncing around in a solution or gas phase until a stable structure of minimum energy is reached. Self-assembly is crucial to the assembly of biomolecular nanotechnology, and is thus a promising method for assembling atomically precise devices. Components in self-assembled structures find their appropriate location based solely on their structural properties (or chemical properties in the case of atomic or molecular self-assembly). Self-assembly is by no means limited to molecules or the nanoscale and can be carried out on just about any scale, making it a powerful bottom-up assembly method.Surfactant molecules can assemble into larger aggregates in solutions varying from round balls to circular rods and lamellar structures, whereas if a solid substrate is

immersed in a liquid containing (functionalized) surfactant molecules a monolayer of these components can spontaneously form on the solid substrate either by physisorption, covalent binding or electrostatic interactions. A self-assembled monolayer (SAM) is a two-dimensional film, one molecule thick, covalently organized or assembled at an interface. The classical example of a SAM is the reaction of alkanethiols with a gold surface. Another example is the reaction of silanes with glass, quartz or SiO2 surfaces.Sol-gel techniqueThe sol-gel technique is deposition method widely used in material science to create protective coatings, optical coatings or ceramics to name a few. This technique starts with the hydrolysis of a liquid precursor (sol) which undergoes polycondensation to gradually obtain a gel. This gel is a bi-phasic system containing both a liquid phase (solvent) and a solid phase (integrated network, typically polymer network). Step by step, the proportion of liquid is reduced. The rest of liquid can be removed by drying and can be coupled with a thermal treatment to tailor the material properties of the solid. Layer-by-Layer assemblyThe layer-by-layer assembly (LBL) is a simple and relatively cheap method to deposit alternate layers of materials. Thin films are created by depositing alternatively layers of opposite charges, providing a high degree of control of the film thickness: negatively and positively charged layers are deposited successively until the desired thickness is reached.

[http://www.ksvnima.com/dip-coating]

http://books.google.it/books?id=RSNoaMvY5EoC&pg=PA293&lpg=PA293&dq=dip+coating+cost&source=bl&ots=9SgaFYBwEa&sig=5hEFjsgXrE7_wqfYip006j4DmMY&hl=it&sa=X&ei=CwlGUeTnO_P07AbQy4CQDg&ved=0CH8Q6AEwCDgK#v=onepage&q=dip%20coating%20cost&f=falseIndustrial Plastics: Theory and Application Di Terry L. Richardson,Erik Lokensgard

from www.schott.com

http://www.polyone.com/en-us/docs/Documents/Applications%20of%20Primers%20and%20Vinyl%20Plastisols_TAB%20SC005.pdf

http://www.polyone.com/en-us/docs/Documents/Hot%20Dip%20Coating%20of%20Vinyl%20Plastisols_TAB%20SC006.pdf

Plastics Engineering Handbook Of The Society Of The Plastics Industry a cura di M. Berins

Paint Technology Handbook Di Rodger Talbert

Machinehttp://www.glenro.com/conformaldip.html

http://www.glenro.com/

conformal-auto.html

http://www.glenro.com/conformal-custom.html

http://www.alibaba.com/product-gs/540789718/

Dip_Coating_Machine.html?s=p

http://www.alibaba.com/product-gs/629393108/Dip_Coating_Machine.html

http://www.alibaba.com/

product-gs/243891436/Automatic_Ultrasonic_Cleaning_and_Dip_Coating.html

PLASTIC SOL

http://www.alibaba.com/product-gs/562205353/Dip_coating.html

http://www.alibaba.com/product-gs/739986730/vinyl_dip_coating.html

There are two basic dip coating process concepts. In the web coating process, the substrate is partially submerged into a coating pan containing the solution to be coated and a wet film is withdrawn onto the moving web, Figure 1. In the discrete method, the irregularly shaped discrete objects to be coated, nails, circuit boards, architectural steel members to optical components are inserted into a tank containing the coating solution. As it is withdrawn a film is attached to the part, which is then dried, so that the part is useable.The continuous web coating process is still widely used because of the following reasons:            • It can apply a range of coating thickness with reasonable coating quality.             • The coating  equipment is inexpensive and relatively simple to operate.            • High-speed capability of the process should lead to low costs and high productivity.            • Scale-up from laboratory coaters is much easier than for precision coaters such as slot-die and curtain coating.            • Excess coating material can be removed by doctoring devices such as Mayer Rod, Air Knife, blades, and squeegee rolls to allow wide range of coating weights.            • Understanding the limits of dip coating is important to understanding the behavior of many other coating techniquesThe wet thickness for the unassisted dip coater is determined by:            • Fluid properties            • Coating speed            • Withdrawal angle.The  range of operating parameters for dip coating are as follows:Viscosity 20–2000 cP or mPa-sWet thickness 10–200 μ (0.4-8 mil)Line speed 0.5–7.5 m/s (100–1500 fpm)Coverage uniformity ± 10 % The wet coating thickness increases with viscosity and coating speed. Therefore, when thin coatings are required at high speeds and viscosity a doctoring device is needed to obtain the desired coverage.

http://www.convertingquarterly.com/blogs/web-coating/id/4748/dip-coating-a-simple-effective-coating-technique.aspx

http://www.google.it/patents/EP1712296A1?hl=it&dq=dip+coating&ei=cMdJUbzFD9Cw7Ab--4GwDQ&cl=en

Conformal Coating Dip Systems FAQ's

How do you control the coating thickness in the dipping process and what is the tolerance of the

coating thickness?

How do I monitor the viscosity?

What is the loading mechanism for the DS100 dip system?

Can the DS100 utilise in line curing?Can I horizontally dip into the conformal coating using the DS101?

Is the argon blanket an important option for the DS101?

How often do you recommend a viscosity check on the conformal coating material in the DS101 tank?

SCH have an option of automatic viscosity control and top up system for the DS101 dip coating system. Can you advise how this works?

How is the height of the conformal coating material in the tank controlled on the DS101 dip coating system?

Can the capacity of the DS101 tank be reduced?

How do you control the coating thickness in the dipping process and what is the tolerance of the coating thickness?The dip speed is controlled by air over oil hydraulics which can control the speed down to 2" per minute which is very slow and more than adequate for conformal coating. Repeatability is about getting accurate viscosity control since the withdrawal rates and viscosity control the coating thickness. Typical speeds for conformal coatings at viscosity of 200 cps are 6" withdrawal speed giving a 25-50um coating for an acrylic coating. This does vary from coating material to material so control measures should be put in place if accurate thickness is required. The typical coating tolerance is depending on what you measuring on? If it is a flat coupon, then with a known viscosity of product you should be able to achieve +/- 5um.

How do I monitor the viscosityThe viscosity is monitored by utilizing a zahn type measuring cup. This is essentially an egg cup with a hole in the base on a wire. You dip the cup into the liquid and lift it out of the coating. Since the volume of the cup is known and fixed, if you time the flow of the coating out of the cup you get a relatively accurate and simple method of measuring viscosity which is certainly suitable in 99% of cases.

What is the loading mechanism for the DS100 dip system?The PCBs are hung on the cross rods by hooks (we can supply or you make / buy your own?) or any other form of jigging ( we can custom build). The immersion and withdrawal of the cross rods is carried out by a custom piston which moves up and down at a controlled rate down to 1-2”/min since we use an air over oil system.

Can the DS100 utilise in line curing?This is a batch dip system without inline curing. If you wish inline curing you will be adding a zero to the price. Sort of range is £30K-70K depending on size of system, rate of cure, type of material to use.

Can I horizontally dip into the conformal coating using the DS101?

It is perfectly possible to horizontally dip PCBs using the automated batch systems such as the DS101 conformal coating dip system. There are factors such as the size of the board, the depth of dipping allowed, the type of conformal coating to be used and the masking & components on the board that must be considered but SCH regularly use this technique to minimise costs for customers.

Is the argon blanket an important option for the DS101?

The argon blanket is a useful accessory for overlaying conformal coatings that are sensitive to moisture such as moisture cure silicones like Dow Corning 2577. The principle is the argon gas is heavier than air and using a series of valves a blanket of argon gas is bled over the conformal coating tank, effectively trapping the solvents under the argon. This is also effective in regions of high humidity and can help prevent moisture ingress.

How often do you recommend a viscosity check on the conformal coating material in the DS101 tank?

Checking viscosity is down to the time the tank lid is off. If it is off all day I would suggest twice a day check to ensure control. If it is a few hours then each time you start. The main issue is building a pattern of use. As you use the dip coating system, record the hours, volume of boards and the conformal coating thinners used, and chart this. Then, you will be able to anticipate when it needs checking and when it doesn't. Also, contributes to the lean manufacture processing.

SCH have an option of automatic viscosity control and top up system for the DS101 dip coating system.  Can you advise how this works?

Automated viscosity control is a sophisticated process reserved for companies who are going to process a lot of PCBs and change the viscosity of the tank a lot of times in a shift or for companies who want a totally closed loop process without manual adjustment.

The viscosity system is an inline viscometer with feedback monitoring the tank constantly. There is a dosing system that would adjust the coating as required, feeding back into the conformal coating tank either conformal coating material or conformal coating thinners.

How is the height of the conformal coating material in the tank controlled on the DS101 dip coating system? 

On the DS101 dip coating system there are two parts to the tank. These are the dipping area and the sump, which are separated by the weir edge. The material is pumped into the tank, over the weir into the sump. Therefore, the weir edge holds the height constant and the weir sump drops with material use.

Monitoring the weir sump depth is crucial again to learn how fast it is drained. We recommend a 25mm (1”) edge difference between the top of the weir and the sump material and keeping the material close to that avoids a wide evaporation area over the weir which means more solvent evaporation. This can also be critical with materials that do not re-dissolve into the material when dry like water based coatings. If they do

dry /cure then these bits float around in the sump and then eventually could clog the pump. We would recommend using a stainless steel basket which we have designed to catch these bits like a sieve and any other bits floating (or even PCBs dropped!).

Can the capacity of the DS101 tank be reduced? 

SCH can make the tank any size you want. However, there is a ratio of surface area of the tank to depth that is important. If you have too little volume of coating in relation to the surface area, the evaporation of the solvent will change the viscosity of the tank too quickly and you will constantly be monitoring it. We have no formula for this but making it too shallow could be a problem. onformal coating Application Process FAQ's

Which conformal coating application method should I choose? Do I dip, spray or brush?What is Semi Automatic Conformal Coating Dipping?How do I set up a conformal coating spray facility?

Which conformal coating application method should I choose? Do I dip, spray or brush?Application selection depends on several criteria including areas such as material selection, volume of PCBs to be coated, budget, throughput speed, type of coverage required and ease of masking. Unfortunately, it can be a combination of factors that effect choice rather than individual factors and it is important to look at all the information collectively. For example, do not choose a material first and then select the equipment after. This could be extremely costly!

So how do SCH do this when quotations are prepared in our coating service?

Well, the first question that is asked is what material is to be used?

This can be critical since some coatings lend themselves to spray processing compared to dip and vice versa. For example, a moisture cure coating is not ideal for dipping since eventually the whole dip tank will cure without inerting precautions which can be an expensive exercise!  

Other factors like volume and throughput are also important. It would be easy to choose selective robotic coating for all boards, assuming an automation process would be the most cost effective. However, cost of set up and program time against volume need to be considered. Also, some PCBs just dont coat well on a robot or the application finish requirements are too stringent to complete with a selective spray system.

A good example is where a batch size may only be a monthly drop of 10 boards at a time and it costs more to set up the robot than to coat the board and the speed advantage has been lost. Another point with robotic systems is whether the board design is suitable for selective spray? If the board is very 3D in nature or has areas which are critical as no go for coating then it may be a difficult

option and at least some of the process may have to be manual in nature. 

Finally, factors such as budget are very important. Amortisation of costs other a number of boards gives’ the final cost of the project. It just may be the case that cost of capital equipment cannot be returned easily and lower cost solutions may need to be found such as aerosols or subcontracting the work out. 

In the end it is the balance of factors which matters and this is the reason SCH use many different conformal coatings and a variety of application methods available since there is no perfect way to coat all boards.

Contact SCH cirectly or click here if you want to discuss further which conformal coating method and material is right for you.

What is Semi Automatic Conformal Coating Dipping?

Semi Automatic Dip Coating is one of the most efficient methods for application of conformal coatings and is excellent for all volume production whether large or small.

The process of dipping a circuit board into a conformal coating material contained in a tank at a controlled speed of immersion and withdrawal ensures complete coverage, including underneath components and around difficult large 3D boards and there is no over spray or material wastage.

The control of the speed in and out combined with the viscosity of the material defines the coating thickness of the Printed Circuit Board and the tolerance can be controlled extremely accurately.How do I set up a conformal coating spray facility?

With over 50 years experience in conformal coating SCH Technologies can offer advice on setting up a turnkey spray room for companies planning on running their own in house conformal coating operation.

General RequirementsThe coating room area should be clean & dust free since PCBs are susceptible to particles sticking to the drying coating. The temperature range should be reasonably controlled within sensible limits since the viscosity of the coating will vary with temperature. However, a more critical point than temperature is the humidity which needs to be controlled to be above 35% (for ESD reasons) and <55% due to moisture issues which can effect the coating integrity and the application method.

Extraction will be required if atomised spraying. The grade of the extraction will depend on whether solvents are being used. If so, then bifurcated fans should be used to ensure that unnecessary explosions are avoided!

With regards spraying solvents and electrical power in coating rooms there are HSE rules and guidelines with spraying where the zone 1 area on the front of

the booth must not have live power or switching within 2m.

Spray Coating EquipmentA basic coating facility could simply be an extracted area, a UV source and the coating. However, for setting up a Spray Coating Facility that can produce high quality results regularly there are several options that will aid the operator and ensure that the best results are achieved. A basic set up could include: Spray booth For even, repeat application of the coating(s) using a spray gun.

Drying CabinetFor storage of drying PCBs after application to ensure minimal contamination from the atmosphere.

UV Inspection BoothFor inspection of the fluorescing coating under long wave UV to ensure coating coverage to IPC A 610 Class III standards or alternatives.

Solvent Exposure AlarmSEA 500

For monitoring the exposure of the operators during the process to ensure that they are alerted of any issues and a permanent record of the process is created.

 Spray boothThe spray booth should be primarily designed to extract the fumes efficiently whilst allowing coating to be applied easily and reliably.

Using a high quality spray gun is a must to give the best coating finishes repeatably over long time periods. Although, low cost guns work reasonably well, over time they wear and they are inferior to the finish achieved with a high quality gun.

During spraying, a UV light above the PCB aids the operator in visually ensuring good coating coverage. It also ensures that shadowing effects are avoided. A manual or automatic turntable also aids spraying again to help with 3D shadowing effects.

The system shown is the CB100 Conformal Coating Spray booth.Drying CabinetOnce coated the PCBs should be stored in a cabinet whilst the coating dries.

The curing cabinet has four main roles. These are to extract away the air around the PCBs to aid drying and remove dangerous fumes, ensure that the PCB is held still and the coating does not flow, the PCBs are not touched to damage the coating and that any particles in the atmosphere do not settle onto the PCBs and stick.

The system shown is the CC100 Conformal Coating Drying cabinet.

UV Inspection Booth

To inspect to IPC-A -610 standards the operator needs to use UV lighting.

Also, if there are any finishing requirements to take place such as hand touch up with coating and a brush, extraction will be required. The operator has a dark, enclosed area to study the PCB with the UV lighting intensity maximised using a UV-transmitting plastic window in the ceiling. Also, ESD

points need to be attached to ensure correct handling of the PCBs.

The system shown is the IB101 Conformal Coating Double inspection booth.

 Solvent Exposure Alarms (SEA)

The SEA range of solvent monitoring detectors allows operators to be aware of any breaches in exposure limits. Also, the SEA 500 will data log the results and ensure that the operators have a clear record of their exposure.

These easy to use systems give real time results without requiring timely analysis or delays.The system shown is the SEA 500 measurement, alarm and data logging system.

The System shown is the SEA Aeroqual 500 Solvent Monitoring SystemFor further advice on setting up a conformal coating spray room please contact us.

SCH have written a technical bulletin on this topic. Select this from the Technical Bulletin area of the website, click here to link directly or request a copy through [email protected] .

How long do I have to cure my coating and why do I need to bother?Conformal coating curing is dependent on the material type used. For instance, solvent based acrylic conformal coatings generally have a three stage curing mechanism. This is tack free, cured enough to handle and fully cured. Typically, the timescale's are approximately 10-15 min, 30-90 min and 3-4 weeks, respectively but can vary depending on factors such as coating, extraction rates, temperature of the coating room and thickness of application. Now, a coating could be applied to a PCB and after a period of two hours look fully cured. However, there will still be trace solvents in the film which have not evaporated at that point and which may be affecting the coating electrical properties enough that the PCB may fail in test.  Therefore, the question of when is a coating fully cured is when it stops affecting the performance of the PCB. Therefore, curing is only critical if the PCB needs to be operated, tested or calibrated and the coating cure level affects these tasks. If this is found to be the case then the key issue is to accelerate the cure process enough by baking the coating for an extended period at an elevated temperature according to the material TDS instructions. Typically, 12 hours at 70C works effectively to fully cure a coating but it will depend on the coating thickness, the material itself and other factors.

Further, if polyurethane conformal coatings are used there is another cure mechanism of polymerisation where the conformal coating achieves its chemical resistance. This is

typically after 24-72 hours depending on the coating type, the temperature, air flow and coating thickness.

Is it advisable to clean my boards before coating? What are the advantages disadvantages of this and how clean do my PCB’s have to be before coating?This is a difficult question to answer simply. However, the best advice is if the product is safety critical and you have not tested the product for long term reliability it may be worth cleaning!

The problem lies in not knowing what contaminants are on the surface of the board before coating. These contaminants could be from a variety of sources including the bare board manufacture, the solder resist used and whether it is compatible with the coating, the assembly processes including fluxing and the handling process.

Determining if the board contaminants are relatively benign is possible using techniques like Surface Insulation Resistance (SIR) testing? However, it can be complex and could be quite costly depending on the level of investigation. This cost however needs to weighed against the potential costs of returns, reputation and consequential losses if it suddenly goes wrong.

An alternative is to ionic test the PCB. This is where the PCB is washed and measured by an ionic contamination system such as an Ionograph which SCH use. The system washes a test board completely, measuring the residue removed off the board. This residue is then expressed as an absolute value which can be checked against an industry standard on cleanliness. The test is low cost, takes approx 30 min and can be run after cleaning to check the process has removed all of the residues needed.

That said, many companies conformal coat over a no clean process and approx 50% of SCH coating service work is no clean. Problems with coating that can occur with “dirty” no clean PCBs include excessive de-wetting and a lack of adhesion of the coating to the board. This in turn leads to excessive re-work / finishing after coating which can cost more than the original cleaning process!

What problems could I see if I do not use cleaning for the conformal coating process?

Cleaning is always a good idea from a reliability point of view, especially if you are using liquid fluxes.  It is our experience that with solder pastes, solvent-containing products generally tend to interact with the paste residues more than solvent-free coatings which can lead to unexpected failure modes.

From the benefit of our experience, as long as the bare boards are of good quality (ionically clean, free of surfactants) and the process is well under control (no handling of bare boards without gloves etc), then the vast majority of no-clean pastes are compatible with coatings.  If you have any specific combinations of flux and coating, we can check our database for any compatibility issues.

The main failure mechanisms are:

Rosin from paste melting at T>80°C and phase change (solid/liquid) resulting in increased volume, which stresses coating, causes

coating to crack. This is a point of ingression for water and a possible failure site.Solvent in coating leaches activators from the rosin, free acids in coating or surface of board which can cause failures.Residues cause coating to de-wet, resulting in no coating present in certain areas.

Thermal impedance of conformal coatings

We are getting asked more and more questions on thermal impedance (resistance) so here is a calculation based on the thermal conductivity of acrylic resin being 0.2 w/mk.

Epoxy is about 0.35 w/mk.

Therefore  0.2 watts/mK = 0.002 watts/cmK =y

Coating thickness 50 µm = 50 x 10E - 4 cms = x

x / y = 2.5 C°/watt for a 1 cm² area.

Put in a simple comparison context, a highly filled thermally conductive polymer would be about 4 watts /mK giving 0.125 C°/watt cm² for the same 50um layer.

Remember the lower the number the better, i.e. less resistance to heat passing through the coating!

But it's not that simple because then you have to factor in the thermal impedance of the area of a board dissipating to free air which depends on its topography, (actual surface area not just the flat square), and thermal emissivity etc etc !

The upshot is, don’t coat heat sinks or surfaces of components that may be used in thermal interfaces.

Introduction to Dip Molding and Coating

Candle making is probably one of the earliest dip molding/coating processes used in the industry. It is still in use today. Dip coating has also been an important branch of food processing. For example, chocolate and/or caramel are often coated on many nuts, cookies, and dry or fresh fruits. Dip molding and coating were widely used even before plastic materials were invented. In this context, however, the scope of

discussion is limited to plastic manufacturing processes.Dip molding is a plastic manufacturing process where a heated metal mandrel(mold) is immersed in a tank of molten polymer resin such as plastisol. The plastic is coated/formed around the mold. The hotter the mold and the longer the dip, the thicker the coating. The mold is then extracted from the bath and goes through a curing or cooling process. The part is stripped from the mold after is cured or solidified. Even parts on complicated molds can be stripped relatively easily because the material is quite elastic. The part may need to be dipped more than once to achieve the desired thickness. Examples of dip molded products are surgical gloves, plastic bags, handlebar grips, and so on.

Dip coating is very similar to dip molding except that instead using a mandrel, an actual part is coated by the desired plastic material. The plastic coating provides a protecting layer as well as a better-looking finish. Multi-dipping can be used when two or more different material characteristics or properties are needed for a single product. The coating does not have to be smooth and shiny. It could be an open or closed cell foam, it could be matte or textured, i could even simulate the look and feel of suede or leather.

Plastisol is the most common dip molding material. It is a mixture of suspended plastic particles (usually PVC) dispersed in a plasticizer. As a liquid, it can be stored at room temperature for years. Once heated, the two components fuse into a vinyl, never to liquify again. Plastisol can be formulated to produce vinyls with almost any durometer, clarity, color, and a variety of chemical and electrical properties.

Other common candidates for dip molding are latex, neoprene, urethane, epoxy, etc. Recently, polyurethane and Silicone are often used to replace latex to avoid allergy related issues.

To control the thickness and the quality of the finish, the following parameters need to be carefully managed in the dip molding process:

Temperature of the moldTemperature of the resinSpeed of dip-inBath timeWithdraw speed

Pros and Cons of Dip Molding

Pros• Short lead time• Suitable for fast prototyping• Low initial setup (tooling) costs• Low production costs• Complex parts can be easily stripped from the mold because they

are highly elasticCons• Relatively slow in production• Precision control of the wall thickness is a challenge

IWTStiftung Institut für Werkstofftechnik

Sol-Gel-Verfahren

Sol-Gel Techniques

Ceramic or organo-ceramics coatings realized by sol-gel process are based on solutions chemical. These techniques are studied on fundamental level as well as for direct industrial applications, for the fabrication of ceramics, nanopowders, fibers or the creation of homogeneous nano-crystallite layers.

The specificity of the sol-gel techniques is that the starting state of the material is a liquid, solution state, which will during the transformation mutate in a gel. Typically, the solution is composed of particles with a size between 1 up to 100 nm, dispersed homogeneously in a aqueous or in a solvent middle. Usually, sol-gel techniques are based on systems including metalo-organic polymer. The transition between the fluid state to the ceramic material is going through a gel state. During the sol-gel transformation, the nanoparticles will create a three-dimensional network in the solution, which is the precursor of the gel. The ceramic state from the gel is caused by a heat-treatment which is dependant of the material state.

Deposition and realization of ceramic

layers by sol-gel

Deposition process is performed by dip- or spin coating. The initial liquid state of the solution is leads to the gel state after a short drying. A heat-treatment on free air is following. For temperature bellow 400°C, organic components will be decomposed in CO2 and in water. The formed film is then amorphous and nanoporous. The crystallographic arrangement will take place at about 500°C. From an amorphous film, it is possible to obtain a crystallographically arranged film, composed of nanocrystals. The chemical composition of the solution, the deposition parameters, those from the heat-treatment (temperature, time, cooling down conditions) will influence the properties of the film.

Deposition techniques

Synthesis of the solution for ceramic films

Solutions used for the coating of sol-gel oxydoceramic are mainly based on metal-alcoolate components on a form of M(OR)n, where M is the metal with 3 covalent liaisons (Aluminum, Yttrium, Bore) or 4 (Silicate, Titanium, Zirconium). The metallic atom is linked, through an oxygen atom, to an acrylic group, as ethylic (-C2H5), propylic (-n(i)C3H7) or butylic (-n(s,t)C4H9). By hydrolysis, the alcoolate groups (-OR) will react with water, and will be replaced by OH- groups. This leads to the condensation and to the formation of monomers chains which will ramify and will create alcoolates polymers.

Hydrolysis and condensation are influenced by partial substitutions of alcoolates groups with organic groups. Typically, organic acids as acetic acid or acetylaceton are used. After hydrolysis and condensation, organometallic polymers are present, this is the so called sol which will make possible the deposition of thin

oxyceramic films as SiO2, ZrO2, TiO2, Al2O3

Properties of ZrO2-films obtained by sol-gel

Interest and development given to ZrO2-films deposited through sol-gel process can be explained by them very interesting properties as well as mechanic, thermal and chemical which are relevant for lot of technical applications. Zirconium oxide films have an hardness of 1100 to 1400 HV, an extreme high resistance to strong acids and bases, and are except few examples, the oxyceramics which resist the better to alkaline attacks. Never the less, Zirconium oxide has a very high thermal resistance and is very well adapted for high temperature applications. Zirconium oxide stabilized by calcium oxide can be used up to 2200°C and this during a long of time in oxidant area and up to 1800°C in reducing medium. Pure Zirconium oxide presents a very high stability against melting metals as chromium (Tm = 1875°C), manganese (Tm = 1245°C), iron (Tm = 1536°C), nickel (Tm = 1453°C) as well as against melting glass. Dilatation coefficient of zirconium oxide is relatively high for a ceramic material (11 - 13 x 10-6 K-1) and can be associated to chromium and chromium-nickel based steels.

Principles properties of zirconium oxide

Limitations in the using of sol-gel coatings

Oxydoceramics films processed by sol-gel which are cracks- and defects free are never thicker than some hundreds of nm. The figure below shows the variation of the weight per area, which allows, through the theoretical density, to calculate the theoretical film thickness. Experimental results showed that the maximal thickness for a crack free film is about 500 nm.

The main reason for the apparition of cracks in the film is the presence of high traction residual stresses. These stresses appear during the expulsion of the solvent, which corresponds to a contraction of the film on a form of a gel. The networking of the polymers chains lead to a reduction of the ductility within the gel which leads to a reduction of the relaxation of traction stresses. If the stresses go higher than a critical value, cracks will appear. Under this border, no cracks will be visible, which proves that the accumulated elastic energy is high enough to stop the formation of cracks.

Thicker oxydoceramic film can be obtained by the accumulation of several layers. For each layer, it's necessary to drive a full process, which leads to a very repetitive and inconvenient process for films thicker than 2 μm. Anyway, the accumulation of defects for each layer leads to a maximal available system including more than 5 layers impossible.

The limitation of the formation of cracks is an important theme of research these last years. The addition of organic components in the solution is one of the way followed for this. With them polymeric structures, these additives lead to a drastic augmentation of ductility during the gel phase and reduce the residual stresses,

and by the way, the apparition of cracks.

An other way to increase the critical thickness before cracks appear is to replace a part of the alcoolates groups by ORganically MOdified SILanes groups, ORMOSILs. These groups allow also to increase the ductility of the film and the relaxation of the residual stresses.

By the way, near the oxydoceramic materials, it exists hybrid materials, organic/inorganic with low concentration of organic components which present very interesting characteristics as high available thickness or ductility.

If you are interested about this subject: Hybrids coatings, organic / inorganic

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What are the sol-gel techniques?Here you can find different information concerning sol-gel techniques, synthesis of solutions, methods of realization and properties of coatings.Ceramic sol-gel coatings and applications

You can find here information about ceramic coatings obtained by sol-gel: synthesis of the solution, coating process, heat treatment, properties and the associated applications:

protection against melting metal corrosionprotection against the corrosion of Beryllium by chlorinecoating for mercury’s vapour lampprotective coatings against the propagation of germs and bacteriasensitive coatings to gas, sensorphotocatalytic active coatingscarburizing protective coatingHybrids coatings, organic / inorganic

Information about sol-gel hybrid coatings , synthesis, process, heat treatment and properties

From http://diogenes.iwt.uni-bremen.de/wt/wb/solgel/verfahren_en.php

Technical applications of sol-gel coatings

Corrosion's protection against aggressive Sn/Pb melt

In the frame of industrial brazing, the melt metal (tin/plumb alloy) is at 400°C and is driven by special pumps. These pumps are equipped with agitating wheels made in stainless steels (X4CrNi 18-10). Exams showed that these wheels are corroded by the melting metal, after six weeks of using, the wheels are extremely damaged and the pump efficiency decreases. With a deposited coating on the wheel, the corrosive attack is drastically reduced, as well as after twelve weeks using, no corrosive signs appears. An analysis showed in fact, that the deposited film stops the reaction between the steel and the melting metal.

Sol-gel layers for the protection of beryllium windows for X-ray analysis

Beryllium is permeable to X-ray and is used in the industry for the construction of X-ray chambers. The components in Beryllium are included in a PVC body. But, the chlorine of the PVC reacts with beryllium and leads to an oxidation of this one. With a sol-gel layer deposited, the beryllium is totally protected from any chlorine attack. Photos below show beryllium pieces coated with a protective layer.

Augmentation of the life time of mercury vapour lamp

UV-radiation has, for a wavelength of 254 nm, a bactericide effect. Micro organisms as viruses, bacteria, yeasts and other fungus are destructed by UV-rays. Chemical agents are then not any more necessary. However, bactericides effect of UV lamp decrease with the time and during prolonged using. The tube of the lamp takes then a dark coloration. The quartz which is the material of the tube is attacked by the gaseous mercury contained within the lamp. This coloration reduces the properties of the lamp.

In collaboration with a reputed fabricant of mercury vapour lamp, a special sol-gel coating has been developed in the way to stop the attack of the mercury on the quartz surfaces. Through this process, the life time of and the using time of the lamp has been drastically increased.

Protective coating against carbon during a low pressure carburizing

The main objective of this application is to be able, locally, to stop the low pressure carburizing of a steel. For this, a new type of process had to be developed. A few micrometers coating is locally deposited on a polished surface. The coating has to be able to stay stable up to 960°C (low pressure carburizing temperature) without to present significant defects.

As it is visible, on the below micrograph, there is no diffusion of carbon in the steel and this deposited layer does not need to be cleaned or suppressed, because few micrometers thickness are not significant during the use of the material.

The GDOEs analysis shows the variation of the carbon diffusion with and without protective coating.

Isolation of wires, resisting on high temperature

Today, it is possible to coat thin electric wires with organic coatings. For an use at temperature below 250°C, polyimide or polyamide coatings are used. For higher temperature, ceramic fibres are used, the electric wire is enrolled with the fibres. By this way, temperature up to 1200°C can be achieved without deterioration of the wire. However, this process is expensive and the diameter of the wire is drastically increasing. An other disadvantage of this technique is the lost of flexibility of the wire. A possible solution is to coat the wire, with a hybrid sol-gel coating, to be able to find a place for an utilization for middle range temperatures. The electrical properties of these materials are actually few known, but fact is that mostly all oxiceramics and polymers present an important electrical resistance. These hybrid materials, based on Ormosils show a temperature resistance up to 400°C and possess, opposite to oxiceramic, a high ductility, on the way that hybrid coatings are perfectly adapted for thin wires.

Through this self constructed apparatus, thin wires can be coated

The 2 pictures above show SEM pictures of a wire coated with a Al2O3 film. Left the film is intact and on right, the wire has been elongated of 50%. The the Alelongation under 2%, no damages are visible. This shows the feasibility of the coating on an electric wire. However, oxiceramic as Alto corrosion if water molecules penetrate through micro cracks or porous. That's why a hydrophobic film could be an interesting variant but with a lower using temperature.

http://books.google.it/books?hl=it&lr=&id=ONBNBdPyuzAC&oi=fnd&pg=PR9&dq=Sol-Gel+Technologies+for+Glass+Producers+and+Users+Michel+A.+Aegerter,M.+Mennig&ots=UjVBi-56xX&sig=mFmvJEO0IQntcjDj09jiMvsFsZU#v=onepage&q=Sol-Gel%20Technologies%20for%20Glass%20Producers%20and%20Users%20Michel%20A.%20Aegerter%2CM.%20Mennig&f=false

http://www.chemat.com/chematscientific/Chemicals.aspx

http://www.pcimag.com/articles/sol-gel-hybrid-coatings-for-wood-products-with-improved-surface-durability-and-repellence-properties

LBL techhttp://en.wikipedia.org/wiki/Layer_by_layer

peelable material http://www.alibaba.com/product-gs/765944015/High_Glossy_Water_Resistance_JL_5188D.html

http://www.rippert.de/en/wet-painting/dipping-adp-cdp/http://www.directindustry.com/prod/durr-paint-systems/dip-coating-lines-27501-828355.html

http://www.alibaba.com/product-gs/488461209/Powder_dip_coating_line_for_wire.html

http://www.pfonline.com/articles/waterborne-dip-coating

COLD SPRAY COATING PROCESS

Schematic Diagram of Cold Spray Process

Dr Antolli Papyrin and colleagues at the Russian Academy of Sciences were the first to demonstrate the cold spray process in the mid-1980s.The Cold Spray or cold gas-dynamic spraying process is the next progressive step in the development of high kinetic energy coating processes. Similar in principle to the other thermal spray methods, it follows the trend of increasing particle spray velocity and reducing particle temperature as with the HVOF/HVAF processes, but to a more extreme level that it could be asked whether the process fits under the description of thermal spray.

The Cold Spray process basically uses the energy stored in high pressure

compressed gas to propel fine powder particles at very high velocities (500 - 1500 m/s). Compressed gas (usually helium) is fed via a heating unit to the gun where the gas exits through a specially designed nozzle (laval type convergent-divergent nozzle mostly) at very high velocity. Compressed gas is also fed via a high pressure powder feeder to introduce powder material into the high velocity gas jet. The powder particles are accelerated  and moderately heated to a certain velocity and temperature where on impact with a substrate they deform  and bond to form a coating. As with the other processes a fine balance between particle size, density, temperature and velocity are important criteria to achieve the desired coating. The particles remain in the solid state and are relatively cold, so the bulk reaction on impact is solid state only. The process imparts little to no oxidation to the spray material, so surfaces stay clean which aids bonding. No melting and relatively low temperatures result in very low shrinkage on cooling, plus with the high strain induced on impact, the coatings tend to be stressed in compression and not in tension like liquid/solid state reactions of most of the other thermal spray processes. Low temperatures also aid in retaining the original powder chemistry and phases in the coating, with only changes due deformation and cold working.

Bonding relies on sufficient energy to cause significant plastic deformation of the particle and substrate. Under the high impact stresses and stains, interaction of the particle and substrate surfaces probably cause disruption of oxide films promoting contact of chemically clean surfaces and high friction generating very high localised heating promoting bonding similar to friction or explosive welding.

Coatings at present are limited to ductile materials like aluminium, stainless steel, copper, titanium  and alloys. Hard and brittle materials like ceramics can not be sprayed in the pure form, but may be applied as composites with a ductile matrix phase. Substrate materials are also limited to those that can withstand the aggressive action of the spray particles. Soft or friable substrates will erode rather than be coated.

The cold spray process is still primarily in the research and development stage and only now becoming commercially available.

Cold Spray Process Advantages:

Low temperature process, no bulk particle meltingRetains composition/phases of initial particlesVery little oxidationHigh hardness, cold worked microstructureEliminates solidification stresses, enables thicker coatingsLow defect coatingsLower heat input to work piece reduces cooling requirementPossible elimination of grit blast substrate preparationNo fuel gases or extreme electrical heating requiredReduce need for maskingCold Spray Process Disadvantages:

• Hard brittle materials like ceramics can not be sprayed without using ductile binders

• Not all substrate materials will accept coating• High gas flows, high gas consumption.• Helium very expensive unless recycled• Still mainly in research and development stage, little coating

performance/history dataPossible Uses for Cold Spray Coatings:

• Corrosion protection, where the absence of process-induced oxidation may offer improved performance

• Electrical and thermal, where the absence of process-induced oxidation may offer improved conductivity

• Pre-placement of solders and coatings where purity is important

Brush PlatingArticle From: Products Finishing, Derek Vanek from Sifco Applied Surface ConceptsShare on facebookShare on printShare on twitterShare on oknotizieMore Sharing Services

5

Posted on: 2/22/2011The benefits of plating localized areas with a portable plating system.

Whether for OEM or repair applications, the portability, ease of use, and the minimal solutions required make brush plating an attractive option to consider.

CLICK IMAGE TO ENLARGE (+)

Brush plating is a good option when the part is too large for a traditional plating tank, such as this large shaft being plated with nickel.

Most brush plating deposits have a dense, non-porous and uniform crystalline structure like the sulfamate nickel deposit shown layering a base material here.

Key Industry Specifications

Brush plating, also known as selective plating, is a portable process used to apply localized electroplated deposits and anodized coatings, as well as for electro-polishing. It is used by OEMs to enhance specific areas on production parts, as well as to correct dimensional errors made in machining. It is also used in repair or job shop applications.

Aplications and BenefitsBrush plating is commonly used in a broad range of industries that include aerospace, oil and gas, power generation, marine, defense and general

manufacturing. There are numerous commercial and industry specifications for brush plating and anodizing.

Brush plating’s portability and versatility make it a great finishing tool. It can be mechanized or automated and used anywhere in the shop or out in the field. It focuses the plating or anodizing onto only those specific areas that require the coating, and can significantly minimize the amount of time spent masking compared with traditional tank plating process. Another key benefit is that it uses very small volumes of solutions to accomplish the job at hand.

Brush Plating BasicsA portable power pack (rectifier) provides the direct current required for plating, anodizing and electropolishing. The power pack has two leads, one of which is connected to the plating tool and the other which is connected to the part being finished. The direct current supplied by the power pack is used in a circuit that’s completed when the plating tool is touching the work surface. A brush plating power pack gives the operator the ability to control and monitor the voltage, amperage and ampere hours for a given job, as well as to change the polarity as required for each of the steps involved in the process.

In a typical operation, the part is first masked, and then a series of base material-specific preparatory steps are conducted to ensure an adherent deposit. The last step is the plating the metal deposit to the desired thickness.Brush plating requires movement between the plating tool and the part. This can be accomplished by moving the plating tool over the part, by moving the part and keeping the plating tool stationery, or by moving both. Dedicated electrodes are used for each operation in the process to electrochemically prepare the part and then to plate the final deposit. The electrodes are covered with an absorbent material saturated with a solution and then applied to the part.

Brush Plated DepositsBrush plated deposits are applied at much faster rates than those achieved in tank plating. With proper surface preparation, the quality of the deposit and adhesion is equivalent or superior to good tank plating practice, typically in excess of 11,000 psi. Most brush plating deposits have a dense, non-porous and uniform crystalline structure.

The manufacturers of brush plating equipment generally offer a number of plating solutions for each of the more important metals. One reason for this is to offer a choice in properties. For example, one application may want a hard, wear-resistant nickel, while another needs an impact-resistant, ductile coating. Since the ductility of metals generally decreases with increasing hardness, it is impossible to meet both requirements with a single solution. New to the brush plating industry are highly specialized metal matrix composite coatings such as cobalt chromium carbide (CoCr3C2) deposit.

When to Brush Plate?Brush plating should be considered:

When you need an accurately controlled thickness on a localized area of a part.When tank electroplating is not an option (e.g. the part is too large for the tank,

masking for tank plating is too comple, or the tank plater’s lead time isn't acceptable)

When the part cannot be moved.When processes such as thermal sprays or welding are not acceptable.Brush plating systems are available for electroplating, anodizing and electropolishing. These systems vary in their degrees of sophistication and coating capabilities.

Small touch-up systems are used to apply cadmium, zinc-nickel, silver, gold or other deposits onto relatively small areas. Larger, more sophisticated systems use power packs with outputs up to 500 A and are capable of producing excellent quality finishes and high thicknesses on large surface areas.

Brush plating is a proven process that offers a high degree of flexibility to apply accurately controlled thicknesses of engineered deposits and coatings onto localized areas of metal components. Whether for OEM or repair applications, the portability, ease of use, and the minimal solutions required make brush plating an attractive option to consider. 

Curtain CoatingFrom Wikipedia, the free encyclopedia

Jump to: navigation, searchCurtain coating is a process that creates an uninterrupted curtain of fluid that falls onto a substrate. The substrate is transported on a conveyor belt at a regulated speed through the curtain to ensure an even coat of the die. The curtain is created by using a slit at the base of the holding tank, allowing the liquid to fall upon the substrate. Most manufactures will also include a catch pan to retrieve and reuse the excess fluid.

Contents  [hide]1 Process2 Edging3 Flow Rate4 Benefits of Curtain Coating5 Deficits of Curtain Coating6 Geometrical Possibilities7 Production Rates8 See also9 References10 External links

[edit]

Process

Curtain Coating is a process in which the object or substrate to be coated is guided through a curtain of fluid located in a gap between two conveyors. The mechanism is formed by a tank of fluid from which a thin screen falls down in between the two conveyors. The thickness of the coating layer that falls upon the object is mainly determined by the speed of the conveyor and the amount of material leaving the tank (Pump Speed). Curtain coating is a premetered method, which means that the amount of liquid required is supplying from the tank to the screen in order to be deposited on the substrate.

Curtain coating is one of the technologies used in the converting industry to modify the properties of substrates.

[edit]

Edging

To be certain the flow of the liquid is disrupted, an edging along both sides of the slit is required. This is to prohibit the surface tension from causing the coating to taper in; sometimes edging alone is not enough and a surfactant must be added to lower the surface tension.

[edit]

Flow Rate

It is also important to note that each fluid or dye has its own minimum flow rate. The minimum flow rate is the smallest amount of dye at a given moment to keep the curtain flowing continuously. The minimum flow rate is directly proportional to the surface tension while viscosity is inversely proportional.

Curtain Coating is a pre-metered coating method, which means that the exact amount of coating needed to coat the substrate can be calculated before the process is actually accomplished. This is can be done by using the ratio of the flow rate (with respect to volume) and width of the substrate to the speed at which the substrate passes under the "curtain" of coating fluid.

Because of the ability to calculate the exact (or nearly so) amount of fluid needed for a given project, film thickness variations on the finished product can be kept within +/- 0.5% of the target thickness .[1]

[edit]

Benefits of Curtain Coating

• Faster coating velocity• Ability to produce a thinner coat• Easily coat abstract surfaces• Lower cost of dyes• Lower waste of coating• Coats a more uniform layer[edit]

Deficits of Curtain Coating

• Minimum flow rate• Air entrainment• Rough surfaces are more likely to have air entrainment• Management of airflow around the apparatus[edit]

Geometrical Possibilities

• Curtain coating is usually only effective on relatively flat substrates. Because of the possibility of air pockets being formed beneath the coating, it is not advisable to use curtain coating on substrates with extremely rough or angular surfaces with grooves and pits etc.

[edit]

Production Rates

Production rates are only limited by the maximum speed that the fluid can be laid down on the substrate without breaking the curtain or getting air bubbles between the substrate and the coating material. This is dependent mainly upon the viscosity of the coating fluid.

http://books.google.it/books?id=fD4jb6nZbCUC&pg=PA78&hl=it&source=gbs_toc_r&cad=4#v=onepage&q&f=false

http://www.tciinc.com/Capabilities.aspx