Chapter 6

CHAPTER 6 : PLASTIC6.0 Plastic Plastic : An organic polymer material (with

carbon and hydrogen base) with the ability to flow into a desired shape when heat and pressure are applied to it and retain the shape when they are withdrawn

Polymer : A material consisting of long molecular chains or networks of low-weight elements to form a single compound

6.1 The History of Plastic1. Regnault (1835) a France chemical scientist introduce a chemical material called vinyl chloride

monomer transformed in white powder (polyvinyl chloride - PVC)

but not popular because of not enough raw materials at that moment

2. John Wesley Hyatt (1866) an American, finally came upon the solution with

celluloid produced billiard balls using celluloid as a

substitute

3. Alexander Parkes (1855) origin from Birmingham, England won a bronze medal at the 1862 Great

International Exhibition in London produced a hard but flexible transparent material,

which he called Parkesine from an organic material derived from cellulose

claimed that this new material could do anything rubber was capable of, but at a lower price

Parkesine soon lost its luster due to the high cost of the raw materials needed in its production

4. Leo Hendrik Baekeland (1907) a Belgian-born American living in New York state developed a liquid resin that he named Bakelite

6.2 Properties of Plastic1. The general properties :

i. lightii. processableiii. durableiv. resist corrosionv. colour stayvi. translucentvii. transparentviii. good electric insulatorix. good thermal insulatorx. wear resistance

6.3 Polimer1. Polymera chemical substance made up of a lot of mer or

repeating units or molecules to form a long flexible chaina material consisting of long molecular chains or

networks of low-weight elements to form a single compound

2. Polymerizationa process of linking the monomers together to form a

polymera chemical reaction in which high-molecular-mass

molecules are formed from monomers3. Monomer : a single molecule or a small molecule that may become chemically bonded to other monomers to form a polymer.4. Homopolymer : polymeric materials which consist of polymer chains made up of single repeating units.

5. Copolymer : consist of polymer chains made up of two or more chemically different repeating units which can be in different sequences.

Linear polymer M – M – M – MAlternating copolymer M – O – M – O – M – ORandom copolymer O – M – O – O – M – O – MBlock copolymer M – M – M – O – O – O – M

6. Amorphous (non-crystalline) a substance having no specific space lattice,

the molecules being distributed at random a structure not having the long range repetitive

pattern of atoms arrangement

6.4 Polymer Structure

It is a long linkage structure. The chains of molecules bonded by a weak

force and flexible causes the carbon bondage to move and rotating in the chains.

Polymer exists in two structures : amorphous (non-crystalline) and semi-crystalline.

6.4.1 Amorphous Structure (non-crystalline) At a high temperature, the polymer became a

viscous liquid where the chains moved and glide into one another in tangled state.

The chains arrangement are randomize and not in certain geometry pattern.

When the temperatures are reduced, the flexibility of molecule properties is blocked and it is similar to the glassy state.

6.4.2 Semi-Crystalline Structure With slow cooling, the molecules will have the

certain structure. Causing the molecule to packed together and

increased the forces between them, with higher strength, rigid and brittle.

The polymer formed with 90% in crystalline chains and the remain in amorphous state.

6.5 Types of Polymer Chains There are 4 types of molecule chains by polymerization process as shown

in the figure below :i. Linear chain polymers

a. in the form of a long molecular chainsii. Branched chain polymers

b. the linear chain with a series of branchediii. Cross-linked chain polymers

c. with short links which connected the closer chains togetheriv. Network chain polymers

d. molecular structure in 3 dimension networks

6.6 Polymerization Processes

1. Polymerization : a chemical reaction in which high-molecular-massmolecules are formed from monomers.

2. Two types of polymerization process :a. Addition Polymerization

chain reaction involves the straight-forward addition of monomers of the same kind or of different kinds

it is occurs between molecules or monomers contains carbon bonding when temperature, pressure and certain catalyst are given

examples :i. Polyester are formed from ethylene polymerizationii. Polyvinyl Chloride (PVC)

b. Condensation Polymerization step reaction involves between two monomers with the elimination of a simple by product, such as water, hydrogen chloride, etc it is occurs when two monomer substances react together to produce polymer and small molecules (usually water) will be expelled

6.7 Differences between Thermoplastic and Thermosetting The plastic are divided into 2 large groups :

a. Thermoplastics- group of plastics that can be softened every time they are heated- with no curing (chemical change) takes place

during the moulding operations- they then can be reshaped- e.g. Polyethylene, Nylon, PVC (Polyvinyl Chloride)

b. Thermosetting plastics- group of plastics that only can be heated and

formed only once- undergo chemical change (curing) during moulding- can never again be softened by heating- e.g. Epoxy, Polyesters

6.8 Thermoplastic Thermoplastic materials belong to the linear and

branched chain polymers that are obtained by addition or condensation polymerization of monomers.

They can be softened, hardened or resoftened repeatedly by application of heat.

Thermoplastics are solids at room temperature , melted or softened by heating, placed into a mold and then cooled to give the desired shape.

The monomer linear chains structure of thermoplastics :M – M – M – M – M – MM – M – M – M – M – MM – M – M – M – M – M M is molecule or monomer When heated, the chains will move separately and took

place to a new position and maintain to that position after the heat and pressure removed

The additives of thermoplastics :1. Filleri. wood flour, calcium carbonate, glass fiber, asbestos, aluminium powder, mica granulesii. to improve the impact strength and reduce shrinkage during moulding2. Plasticizersi. to make plastics softer and more flexibleii. to improve flow propertiesiii. to reduce rigidity and brittleness3. Lubricantsi. substance that reduces friction when applied as a surface coating to moving parts to fix the processing and flowability4. Colourantsi. subdivided into dyestuffs, organic and inorganic pigmentsii. pigments more able to resist the temperature and lights5. Antioxidantsi. prevent oxidation, the polymer reacting with oxygenii. oxidation can cause loss of impact strength, elongation, surface cracks and discolourationiii. antioxidants help prevent thermal oxidation reactions when plastics are processed at high temperatures and light-assisted oxidation when plastics are exposed to UV lightiv. stabilizers

6.8.1 Properties of Thermoplastics can be softened and hardened repeatedly by

applying the appropriate thermal and pressure can be attached/ jointed using heat and pressure not liquidify but flow at appropriate pressure and

heat for injection moulding when blowed, acts like glasses, can be shape as

bottles and round by using pressure or vacuum technique

are linear chain polymers form by a long carbon chains through covalent bonding

6.9 Thermosetting Plastic Thermosetting materials belong to the three-dimensional cross-linked or

network polymers. Once they have been set and hardened, they cannot be remelted and

returned to their original state (cannot be recycled or reused) The three-dimensional cross-linked structure :

Stronger material and insoluble to the solvents because impossible for the solvents to enter and breaks the chains.

6.9.1 General Properties of Thermosets melted material when the first time heated and then settled or preserved

into hard and rigid shape at that temperature after the first formation, no changes will happen eventhough heat and

pressure applied usually harder, stronger and more brittle than thermoplastic

6.10 The Advantages Of Plastics1. Excellent electrical insulation properties.2. Good insulation properties.3. Certain plastics has good chemical resistance.4. Good resistance to shock and vibration.5. Transparent and translucent.6. Easily coloured.7. Easily assembled or jointed.8. Repairing the self lubricating.9. Good wear and scratch resistance.

6.11 The Disadvantages Of Plastics1. Measurement dimension changed by humidity and high thermal extension.2. Low temperature duration for operation (150 - 600˚F).3. Brittle at low temperature.4. Softer and less elastic than metal.5. Certain plastics are easily affected by ultra-violet.6. Creeps occurs at any temperature.7. Easily burnt.

6.12 Factors For Plastics Usages1. Processable and assembleability compared to metals e.g. plastic moulding production- do not need any finishing or tidying process2. Plastics materials are cheaper than metals e.g. nylon replacing zinc3. Plastic properties e.g. insulation, self lubrication, etc4. Plastic-metal composites can be produced to achieve better properties e.g. plastic-metal reinforcement are good electrical

conductors, lightness and easily produced5. Multiple functions in the same time e.g. housing or covers, insulators and self

lubricating bearing

6.13 Plastic Manufacturing Process In plastic products manufacturing process,

varies of machines used depends upon the shape products and types of resins used.

Such as :1. Injection Moulding Process2. Extrusion Moulding Process3. Blow Moulding Process4. Transfer Moulding Process5. Compression Moulding Process6. Rotational Moulding Process7. Thermoforming Process9. Calendaring Process10.Casting Process

Injection Moulding Process1. The basic concept of injection moulding machine is a heat-softened thermoplastic material is forced by a screw-drive cylinder into a relatively cool mould cavity that gives the plastic the desired shape.2. The mould usually made from tool steels or beryllium cuprum and can be multiple cavities so that more than one component can be injected in every cycle of process.3. It was the fastest process and widely used.4. Figure 5 shows the typical components of injection moulding machine.

5. Work principles :i. the raw material (resins or pellets) are inserted into the hopperii. under gravity manipulating, the resins enter the injection channel which surrounded by heateriii. certain temperature applied depends upon types of material used, usually around 75˚C - 95˚C for smaller products and 120˚C - 260 ˚C for larger productsiv. when the resins becomes softer, it will be injected using weither the reciprocating-screw or torpedo plungerv. the softened plastics will be enter directly to the nozzle and enters the mould cavityvi. to ensure the mould was fully injected, pressure between 35MPa to 40 MPa are used

vii. the product will be solidify in the mould cavity under water cooling cycle through the mouldviii. after that, the injector will be pulled back, the mould then opened and the products will be retrieved from the mouldix. the connecting opener will retrieved the product where the ejector pins will ejected the product from the mould

6. The advantages :i. the injector mould usage are faster than pressure mould (torpedo plunger)ii. mould cost are lower because of the simplicityiii. various shape, intricate or thinner products can be producediv. high quality parts can be produced at a high production ratev. the process has relatively low labour costsvi. good surface finishes can be produced on the moulded partvii. the process can be highly automated.

7. The disadvantages:i. sink = caused by moulding temperature or the pressure are higherii. flashing = caused by unfitted mouldiii. bubble = caused by cold mould or the pressure are too lowiv. uncompleted moulding = the material did not enter the mould fully, example : small gatesv. weld line = caused by cold mould, low moulding temperature and time for injection are too fastvi. not economical for small productionvii. certain mould only for certain productviii. high cost of machine means that a large volume of parts must be made to pay for the machineix. the process must be closely controlled to produce a quality products.

Extrusion Moulding Process1. A process that supplies a continuous stream of thermoplastic material/ products with equally cross-section where it is directly produced through a shaping tool or to some other subsequent shaping process (dies) placed directly on the end of the extrusion machine.

2. Work principles :i. the pellets or resins are inserted into the hopperii. the material then fed into the heated cylinder by rotating screwiii. when the material become softer, it will be forced continuously by the rotating the screw ramiv. the products or outcomes are formed into continuous shapev. after the product exiting from the die, it will be cooled by air (air-blast system), water (water-bath system) or become cold by interact will cold roll surface where then it will be solidifies while rollingvi. extruded products such as pipe, rod, etc, extrude this way because it can be curved or bended after extruded with hot water by sinking it in it

3. The advantages :i. continuous extrusionii. accurate measurementiii. good finishingiv. economical for mass production

4. The disadvantages :i. the dies are easily wear outii. need to be closely controllediii. thermosetting materials are not suitable because it solidifies quickly

Blow Moulding Process1. It is suited for the manufacture of bottles and other simple hollow shaped parts, from a preformed plastic tube.2. It is a process producing thermoplastic products using injection or extrusion method where the material are softened will be injected or extruded into the mould and then blowed with air.3. It is a cylinder or tube of heated plastic called a parison is placed between the moulds. The mould is closed to pinch off the ends of the cylinder and compressed air is blown in, forcing the plastic against the walls of the mould.

4. Work principles for extrusion blow moulding method :i. the resins or pellets are inserted into the hopperii. with gravity manipulating, the materials enter the injection or extrusion chamber or cylinder which surrounded with heateriii. when the materials become softened (plasticizers), it will be injected or extruded using screw rotator or torpedo plungeriv. the hot and softened material called parisonv. the materials will be flow straight to the nozzle and enter the mould cavityvi. top of the parison will be cutted off using a bladevii. the air then blowed into the parison through a cavity at the top of the mould

ix. after that the mould will opened and the product will be retrievedx. the connecting opener will retrieved the products using ejector pin to eject the product out from the mouldviii. the plastic will be cooled, then solidified in the mould under water cooling system cycling in the channels in the mould

5. Work principles for stretch blow moulding method :i. the plastic is first moulded into a "preform" using the injection molded processii. these preforms are produced with the necks of the bottles, including threads (the "finish") on one endiii. these preforms are packaged, and fed later (after cooling) into an blow molding machineiv. in this process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then blown using high pressure air into bottles using metal blow mouldsv. usually the perform is stretched with a core rod as part of the process

vi. the stretching of some polymers, such as PET (polyethylene terephthalate) results in strain hardening of the resin, allowing the bottles to resist deforming under the pressures formed by carbonated beverages

6. The advantages :i. it does not need the finishing or tidyingii. various of products, intricate or thinner can be produce7. The disadvantages :i. not economical for small productionii. certain mould only for certain product

Transfer Moulding Process1. It is a process where the moulding compound is first softened by heat in a transfer chamber and then is forced under high pressure into one or more mould cavities for final curing.2. In this process, the resin is not fed directly into the mould cavity but into a chamber outside the mould cavities. When the mould is closed, a plunger forces the plastic resin from the outside chamber through a system of runners and gates into the mould cavities.3. It is also used for moulding thermosetting plastics such as the phenolics, ureas, melamines and alkyds.4. The plastic material flowing process into the mould causing the heat transferred equally and quickens the chemical reaction for cross linking or curing.

5. Work principles :i. the resins were placed in a chamber (preclosed mould) outside the mould cavity forming a preformed plastic shapeii. then heated at certain temperature where it will be plasticizeiii. when it softened, it then forced into the mould cavity (closed mould) as hot melt plastic material by a plunger through a system of runners and gatesiv. then let it cooled to solidifiedv. after the plastic has cured, the plunger is removed and the mould cavity then openedvi. the parts is then ejected by ejector pin and removed the plastic parts from the mould

6. The advantages :i. the solidifies timing for transfer moulding are faster than compression mouldingii. the inserting timer are shorter because of the usage of bigger preformed material which can be heatened quicklyiii. this process is suitable to form parts that need small metal insertened because the plastic material entering the mould cavity in stages without using higher pressureiv. intricate shape and with variation of bigger cutting can be producedv. no flash is formed during moulding and the moulded part requires less finishingvi. many parts can be made at the same time by using a runner system7. The disadvantages :i. losing material in flowing channelii. mould cost are higher than compression moulding mould

Compression Moulding Process1. It is a process where two parts mould combined together under one compression and then formed a cavity accordingly to the component shape.2. In the process, before the combined, the resin (thermosets) may be preheated, is loaded into a hot lower part of the mould containing one or more cavities. The upper part of the mould is forced down on the resin and then applied pressure and heat melts the resin and forces the liquefied plastic into filling the cavities forming the needed component.3. Many thermosetting resins such as the phenol formaldehyde, urea formaldehyde and melamine formaldehyde resins are formed into solid parts by the compression moulding process.4. The formation is prepared under certain pressure by using upper part of the mould. This is similar to forging process for metal and facing flashing problem where in need for trimming or cutting process.

5. Work principles :i. the materials inserted into the mould as preformed powdered shape because of heat and pressureii. the preheated material is loaded into a hot mould containing the cavityiii. the upper part of the mould is forced down on the plastic resin, pressure and heat are then applied, forces the liquified plastic to fill the cavityiv. continued heating is required to complete the cross-linking of the thermosetting resinv. then the part is ejected from the mould

6. The advantages :i. mould costs are lowii. short flow of material reduces wear and abrasion on mouldsiii. production of large parts is more feasibleiv. more compact moulds are possiblev. expelled gases from the curing reaction (cross-linking process) are able to escape during the moulding process7. The disadvantages :i. complicated part configurations are difficult to make with this processii. inserts may be difficult to hold to close tolerancesiii. flash must be trimmed from molded parts

Rotational Moulding Process1. It is a process to forming big hollow component in small or average quantities where a thin metal mould consists of two separated parts and designed to rotating in two axis which 90 to each other.2. Work principles :i. loading a measured quantity of polymer (usually in powder form) into the mouldii. the mould then heated in an oven whilst it rotates, until all the polymer has melted and adhered to the mold walliii. by that homogeneous layer with similarity/ accuracy thickness will be formed accordingly to the mould shapeiv. the material/ product are cooled while the machine rotated before removed it from the mould by bringing the mould to the cooling station where water are sprayed to the mouldv. the mould then brought to the last station where the product will be removed from the mould

3. This process only limited for materials that has low melting temperature in small quantities and protype production such as :a) Polyethyleneb) Polypropylenec) Polyvinyl Chloride

Thermoforming Process1. It is a process when a heated plastic sheet is forced into the contours of a mould by heat and pressure or vacuum.2. Mechanical pressure may be used with mating dies or a vacuum may used to pull the heated sheet into an open die. Air pressure may also be used to force a heated sheet into an open die.3. There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.

4. Work principles for vacuum forming method :i. the plastic sheet/ material was placed in between the opened mould and the heating coils by clamping it to the moveable clamperii. then the sheet was brought closer to the coils to softened it, usually placed on top of the machineiii. when the sheet are softened enough, the clamper with sheet back to its place and in the same time the mould which usually placed below the clamper moving up towards the softened sheetiv. the air between the sheet and the mould then are gasped out or vacuumed causing the sheet to fit tightly to the mould which the air holes can be seen in the opened mouldv. then air are blowed to separated the forming sheet and mould and in the same time cooled the productvi. the mould then moved downward to its original place and the formation that occurs from the sheet are the product that need to be trimmed or cut off

5. The advantage :i. low in mould cost because it can be formed using

plaster, thermosetting plastic, wood or aluminium

6. The disadvantage :ii. hollowed or opened space components cannot be

produced because of the different pressure when the forming process occurs.

7. Materials that can be used :a) Polystyreneb) Acrylonitrile-Butadiene-Styrene (ABS)c) Polyvinyl Chlorided) Polypropylenee) Polyethylene

8. Applications : used to formed plastic sheets for certain shape such as :- food packaging- yogurt package- motorcycle fairing- vehicles bumper

Casting Process1. It is a process by the use of a liquid or powder material that is shaped without the application of significant pressure.2. Casting is a manufacturing process by which a liquid material such as a suspension of minerals as used in plastic is introduced into a mould, allowed to solidify within the mould, and then ejected or broken out to make a fabricated part.3. For thermoplastics, liquid monomer is poured into the mould and, with heat, allowed to polymerize in place to a solid mass.4. For thermosets, they are poured into a heated mold wherein the cross-linking reaction completes the conversion to a solid.5. Casting is used for making parts of complex shape that would be difficult or uneconomical to make by other methods, such as cutting from solid material.6. Encapsulation and potting are terms for casting processes in which a unit or assembly is encased or impregnated, respectively, with a liquid plastic which is subsequently hardened by fusion or chemical reaction.

Calendaring Process1. It is a process to produced sheets or films by passing the soft or half gel thermoplastic materials through a number of pairs of heated rollers which the heat and pressure are applied to the materials and the rolls in combination are called calendars.2. Usually this process applied in stages of rolling before final products with certain thickness achieved.3. Materials used : PVC and copolymers that contains Vinyl Chloride.4. The thickness of products between 0.05 – 0.7mm and with width upto 1m.

5. Work principles :i. polymer powders, adhesives and other additives were mix in normal conditionii. stirred in closed heated mixtureiii. the softened or half gel materials from the mixture then masticated between two heated rolls that squeeze it out into a film or sheetiv. the heated sheet or film then trimmed and then passes around one or more additional rolls (calendars) before being stripped off as a continuous film6. The calendres contains with 3 to 4 rollers made of hardened steel, steel covered with fiber or cast irons.7. The calendering rollers has polished surface and the cylinders have drilled holes to channeled heated liquid, oil or water.