Lecture 14 Processing of Plastics

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LECTURE-14PROCESSING OF PLASTICS- Injection MoldingNIKHIL R. DHAR, Ph. DDepartment of Industrial & Production EngineeringBUET

Department of Industrial & Production Engineering26/2Injection Mold Layout: The plastic melt flows from the injection nozzles and enters the mold at the sprue. From the sprue the plastic flows into the runners and ultimately through the gates into the part. Gate and runner design is an important part of the mold design. To help ensure that the mold fills completely, one should balance the mold so that all cavities fill at the same time. When the cavities are the same, a symmetric layout is used. If the cavities are all markedly different, often the gates and runners must be sized/shaped differently in order to allow all cavities to fill in the same amount of time. GateRunnerSprueMoldCavityDepartment of Industrial & Production Engineering26/3Plunger Type Injection Molding Press: In this molding press, the plastic is fed into the mold when a cylinder plunger extends and forces the plastic into the mold. After the plunger retracts more material can be fed from the hopper to the shooting pot. (Thus the stroke of the plunger determines the additional material fed in each time.) Of course the shooting pot is long enough to hold several shots, so the plastics stays in the pot for a while, giving the band heaters time to heat and melt the plastic. Notice the torpedo, which is basically an obstruction to the plastic flow in the shooting pot. As the plastic moves around the torpedo, it is better mixed.

PlungerHopperShooting PotTorpedoNozzleBand HeatersDepartment of Industrial & Production Engineering26/4Screw Type Injection Presses: The original plunger type has had one important modification. A reciprocating screw now forces material into the mold. This screw action ensures that the same amount of material is always metered in, and it is equally dense along the length of the screw. Additionally the material will be much better mixed by the screw action which helps to maintain better consistency from shot to shot. Since the screw action generally helps to pack the material in better, a given plunger travel will push more material into the cavity. Finally the action of the screw, as it rotates and mixes, adds energy to the melt. However, band heaters are still needed to fully heat the melt. All of this results in a much better and more consistent part. This is why the screw press is essentially the only press found in industry. Small plunger presses are still made for prototype/lab purposes.Screw meters plastic, plunger provides pressure

Band HeatersShooting PotNozzleHopperReciprocating ScrewDepartment of Industrial & Production Engineering26/5Injection Molding ScrewsThe injection molding screw plunges forward to provide holding and packing pressure. The screw rotates as it retracts to meter and plasticize the melt. The screw is broken up into 3 regions. The Feed Section draws material from the hopper and starts movement into the shooting pot. In this section, channels between the flights are deep and the depth is constant. The next section, called the Transition Section, compresses and melts the plastic pellets. Most plasticization occurs in this section. The root diameter tapers, causing the channel depth to decrease. In the last section, the Metering Section, the correct fill is precisely measured out. This section has a constant channel depth. Feed SectionTransition SectionMetering SectionDepartment of Industrial & Production Engineering26/6Screw ConfigurationThere are a number of design variations that can be made to the injection molding screw configuration. The extruder screws are composed of various screw segments connected together to create a complete configuration design.

Dulmage Mixing SectionMixing PinsVented BarrelsVented Barrels: Vented barrels with two stage screws are another options in some design. The first stage meters and compresses the material. Then, the material is vented, compressed and metered forward in the second stage.Dulmage Mixing Section: Some the segments used in plastic extrusions are the dulmage mixing segment and mixing pins segment. The Dulmage Mixing design is used at the end of the screw to enhance mixing. Usually, several sections are put together. Mixing Pins: Another configuration option are the mixing pins. These are usually inserted between the final flights to enhance the mixing. VentMaterial from HopperDepartment of Industrial & Production Engineering26/7Barrier Flight ScrewsIn Barrier flight screws, the channel is split into two sections: one for solids and one for the melted plastic. The barrier flight is not quite as large as the primary flight. The barrier flight has small passage for the melt to flow from the solid channels into the melt channels. The solid channel becomes smaller and the melt larger along the length of the screw. The functioning of the barrier flight is shown here. Solids ChannelMelt ChannelPrimaryFlightBarrierFlightDepartment of Industrial & Production Engineering26/8Press ParametersGenerally, there are three common parameters used to describe the injection molding press capacity: clamping force, shot size, and injection pressure.

Clamping force is usually the most common method to refer to the injection molding press capacity. Thus, presses are talked about as being 20 ton, 50 ton, etc. The clamping force is the force available to hold the platens together. The platens contains the mold cavities. Clamping can be achieved using in-line hydraulic cylinders, mechanical toggle clamps, or a combination of the two (called hydro-mechanical).

Shot size is the amount of material that can be transferred into the mold in one shot. Shot sizes are usually specified in cubic centimeters or ounces.

Injection pressure is the pressure at the sprue that forces or injects the plastic melt into the mold. Specification by this parameter refers to the maximum injection pressure. Department of Industrial & Production Engineering26/9Clamping MechanismsThere are a few different ways to provide the clamping force for the mold. The in-line hydraulic cylinder provides good force control, but requires large hydraulics that tend to be slow. The toggle clamps move quickly but provide poor force control. The hydro-mechanical clamping system is a combination of the two clamps. It uses a toggle mechanism for most of the travel and then uses the hydraulic cylinder for the locking force.

In line hydraulic cylinderToggle ClampDepartment of Industrial & Production Engineering26/10Injection Molding DefectsInjection molding can create defects on the finished product. Some of the more common Injection molding defects are

Short ShotFlashingWeld LinesJettingEjector Pin MarksSink MarksWarpage/Residual Stresses

When designing for a part and the associated mold, one should keep these defects in mind as well as the fundamental objectives of filling, holding, packing and removing the part. Department of Industrial & Production Engineering26/11Short Shot: Short shot occurs when there is insufficient material to fill the mold cavity and/or the material solidifies too soon. It has several causes, including insufficient injection pressure, or insufficient time allowed during the injection process. Sometimes the material will freeze in a given section before it can reach the edges of the mold.

Department of Industrial & Production Engineering26/12Flashing: Flashing occurs when there is too much material and it pushes its way out of the die; basically, the material overflows the cavity. This can be caused by too much injection pressure, too much injection time, or insufficient clamping force. It also can be caused by a poorly machined die that does not properly seal off the cavity.

Part Moderate-Heavy FlashDepartment of Industrial & Production Engineering26/13Weld Lines: Weld lines occur when flow fronts meet in the mold. In addition to being aesthetically unappealing, weld lines decrease the strength of the part. This normally occurs around holes or obstructions and causes very weak areas in the molded part. Additionally, weld lines are much more pronounced if flow fronts are moving in completely opposite directions, as opposed to when the flow fronts share some components of velocity. Weld lines are more pronounced if melt is cooler when fronts meet. Unavoidable w/ Solid CoresCan result from poor gate placement

GateWeldLineGateGateWeld LineDepartment of Industrial & Production Engineering26/14Jetting: Jetting is generally caused when one gates a part in such a way that the material flow enters an open section with much space between the gate and the opposite wall. When the flow area is squeezed through the gate, the velocity increases, and the plastic melt shoots into the empty cavity mold. If there is nothing to break its path, it will shoot all the way through to the opposite wall, where it will quickly solidify. Successive incoming material streams then fold over the previously frozen stream, and the stream lines become locked into place. Often air pockets can be trapped in between the successive folds and further folds do not fully join creating a weaker part. To reduce the risk of jetting, one should always gate the part so that incoming material flow is directed into a nearby wall. After the stream has impinged on the wall, the plastic melt will spread in the appropriate fashion. Melt moves rapidly, cools unevenly and traps flow lines. JettingAlternate gating eliminates jetting

Department of Industrial & Production Engineering26/15Ejector Pin Marks: Once the part is sufficiently cooled, the cavity opens and ejector pins push the part out. The pins usually leave marks in the area where the ejector pins pushed the part. There are four different possible causes of the pin marks: 1) the pin is above the flush line, 2) the pin is below the flush line, 3) there is clearance around the pin and 4) material is too soft at ejection and one pushes through the part.1) Pin above flush2) Pin below flush3) Clearance around pin

One tries to eject the part before it solidifies and pins push through the part.

Place the ejector pins on hidden areas of the part.Department of Industrial & Production Engineering26/16Sink Marks: Sink marks are also common injection molding flaws. Sink marks occur at excessively thick wall sections, or where there are abrupt changes in thickness- thick sections solidify too late and shrink away from the wall. Proper design reduces/eliminates sink marks (ribs, core out sections)Bad DesignImprovedDesign

Thick sections cause sink marksDepartment of Industrial & Production Engineering26/17Warpage/Residual Stresses: Warpage is the out of plane distortion of an injection molded part, generated by constraining the part while cooling. Warpage is typically caused by anisotropic shrinkage. Several causes for anisotropic shrinkage are: Variations in thickness, Differing shrink rates due to melt orientation, Uneven cooling, Differences in the mold cavity pressure. If the part is massive enough to resist warpage, residual stresses will result. Since gates are usually highly oriented and have extremely fast cooling rates, residual stresses are always present near the gates.

Suggested design changes to minimize distortionDepartment of Industrial & Production Engineering26/18Less Common MethodsExpansionexpandable polystyrene bead fill mold and bond (steam)

Foaming liquid chemicals combine & cure (isocynate polyal)

Spinningproduces plastic fiberssimilar to extrusion

Solid Phase Formforming plastic below glass temperaturesimilar to stamping or forgingDepartment of Industrial & Production Engineering26/19Common PolymersABS (Acrylanitrile Butadiene Styrene): amorphous, good Impact Strength, excellent appearance, easy to process computer housings, small appliances, automotive interior, & medical componentsAcrylic: amorphous polymers, excellent clarity, excellent weatherability optical & outdoor applicationsCellulosics: among the first thermoplastics developed: smell funny, very flammable Nylon 6: semi-crystalline polymer, good cost to performance ratio, lower numbered nylons, 6 ,6-6, 4-6, absorb moisture and change their properties as a resultPolycarbonate: amorphous material, excellent Impact Strength, clarity, & optical properties currently long lead times for this material.Polyethylene High Density: widely used, inexpensive, thermoplastic, easy to process, good to excellent chemical resistance, soft & not for use above 150 FPolypropylene: semi-crystalline material, low temperature material, excellent chemical resistance difficult to mold to extremely close tolerances Polystyrene High Impact (HIPS): few cents more than crystal styrene, to pay for the rubber modifier, opaque & very widely used, lower modulus, better elongation, & less brittle than crystal styrenePVC Polyvinyl Chloride Rigid: properties similar to ABS (except appearance) at a slightly reduced cost primarily for water pipe and pipe fittings, occasionally for electrical enclosures *in plastic phase PVC is corrosive to molds & machines (non corrosive as a solid) Department of Industrial & Production Engineering26/20Shaping Processes for Thermoplastics

Department of Industrial & Production Engineering26/21Shaping Processes for Thermosets

Department of Industrial & Production Engineering26/22Processing of Polymer-Matrix-Reinforced PlasticsSheet Molding Compound (SMC): Continuous strands of reinforcing fiber are chopped into short fibers and deposited over a layer of resin paste, usually a polyester mixture, carried on a polymer film such as polyethylene. A second layer of resin paste is deposited on top and the sheet is pressed between the rollers. The product is gathered into rolls or placed into containers in layers and stored until it undergoes a maturation period, reaching the desired molding viscosity. The matured SMC, which has a leatherlike feel and is tack free, has a shelf life of about 30 days and must be processed within this period.

Department of Industrial & Production Engineering26/23Prepregs: The continuous fibers are aligned and subjected to surface treatment to enhance their adhesion to the polymer matrix. They are then coated by being dipped in a resin bath and made into a sheet or tape.Boron-epoxy prepreg tape.

Manufacturing process for polymer-matrix compositeDepartment of Industrial & Production Engineering26/24Pultrusion: Long shapes with various constant profiles, such as rods, structural profiles and tubing are made by the pultrusion process. Typical products made by this process include golf clubs; drive shafts and structural members such as ladders, walkways and handrails. The most common material used in pultrusion is polyester with glass reinforcements.

Department of Industrial & Production Engineering26/25Molding Reinforced Plastics

(a) Vacuum-bag forming. (b) Pressure-bag forming. Manual methods of processing reinforced plastics: (a) hand lay-up and (b) spray-up. These methods are also called open-mold processing.Department of Industrial & Production Engineering