29.0 PROCESS OTHER THAN INJECTION MOULDING
29.1 Introduction:
A variety of special procedures are widely used in the processing of plastics in addition to basic procedures like injection moulding, compression moulding and transfer moulding.
29.2 Different types of process:
Some of the important different processes for thermoplastic materials other than injection moulding are
- Blow moulding
- Thermoforming
- Rotational forming
- Extrusion
- Forming
- Calendaring
29.2.1 Blow moulding:
Blow moulding is a process of forming hollow articles from a soften plastic tube. The process consists of forming a tube of the melt (called parison) and introducing air into the parison to expand, either to a free blown object or against a mould to form it to a definite size and shape.
The equipment used to accomplish this job often consists of an extruder to plasticize the resin; a cross head with die and core to create desired tube or parison, an air source to inflate the tube, and a press to open and close the mould.
Theoretically any thermo plastic resin can be used for a blow moulding operation. However, only those materials which show high hot strength and good stretch properties at the extrusion temperature and suitable for formation of a parison and a subsequent blowing can be used. In all cases the extruder should have an L/D ratio of at least 20: 1 to ensure complete and thorough plasticization of the melt.
29.2.2 Thermo forming:
This is a process where flat sheet material is converted to a three dimensional shape. The process requires that the sheet material be brought to a thermo plastic state by the addition of heat and then forced into a mould, pneumatically or by mechanical means. The process is also called vacuum forming since much of the early work was done using vacuum to provide the forming pressure.
There are number of advantages to the thermo forming process. One of the most significant is that of the tooling. In comparison to the other methods, is relatively inexpensive. Since the technique is particularly suited to the production of many large parts, cost savings can be impressive. Many parts with very thin wall thickness can be made only by this method. The process is also ideal for the production of small number of parts - samples, prototype etc. the process now is used to produce such items as refrigerator door liners, automobile dash board panels, cheese containers, soft drink cup, signs, packaging etc.
29.2.3 Rotational moulding
Rotational moulding is known as roto-forming. Rotational moulding is a process in which a pre determined mass of powder, usually thermoplastics, is charged in a closed mould and caused to simultaneously rotate into two planes while in hot oven. The temperature would be between 260 and 485°C depending on material and product. This causes the powder to melt and uniformly deposit on the mould surface. When the melting face is completed the moulds are moved, while still rotating, from the oven into a cooling chamber where the moulds are cooled by air circulation or water spray, internal cooling to speed the cycle by means of Nitrogen or water fog is currently being investigated as means of lowering the cost.
After the cooling cycle the moulds are opened, and moulded parts are removed. Gasoline tanks, toys, plastics balls etc. are produced by this method.
Advantages of rotational moulding are :
- Tooling is simple and inexpensive.
- Parts produced are strain free.
- Parts are usually made with uniform wall thickness.
- No weld lines, sprue marks, gate marks / ejection marks on the parts
- No scrap, or very little, is produced.
- Hollow parts are easily produced with simple tooling.
- Usually no secondary operations are required.
- Wall thickness change can be easily made without new tooling or tool modification.
Disadvantages are:
- Not suited generally for large production runs of small parts.
- Not suitable for production of parts with wall thickness less than 0.5.
29.2.4 Extrusion:
Extrusion moulding is used to make continuous shapes such as tubes, rods, sheets, pipes, wires, and films out of thermo plastic materials. The extrusion machine is similar to an injection moulding machine. The material is fed into the machine through the hopper. The screw compresses and plasticizes the material until a liquid melt is obtained. The melt is then pumped through the die, which shapes the extrusion. On leaving the die, the extrusion is cooled, sized, solidified and drawn of by some type of take off equipment. The final product is then cut to length, reeled up or coiled.
29.2.5 Foaming
Expanded or foamed plastics are low density materials made by foaming gas bubbles in the plastics while in liquid or molten state. A variety of processes and type of products are available depending upon the type of polymer and the structure. Foe polyurethane foams, bubbles are produced in the low molecular weight mixture of reactants undergoing polymerization, partly arising from gases produced in the condensation process but usually supplemented by blowing agent like volatile fluorocarbon liquid. Flexible polyurethane foams are open cell type, like sponges. Rigid or flexible thermoplastic materials like PVC, polyurethane, polystyrene etc. may be formed in the following methods
1) By dispersing a blowing agent in the material and heating it above its softening temperature. The blowing agent foams a large number of small bubbles in the material, and may be a volatile liquid such as pentane used in expanded polystyrene, or an unstable chemical such as azodicarbonamide, releasing much gas when heated.
2) By dissolving a gas such as nitrogen or carbon dioxide in the heated plastic at high pressure. When the pressure is released, a large number of small bubbles are formed and grow in the soft plastic. This process may be carried out in autoclaves or continuously in extrusion machine, the plastic expanding as it leaves the extrusion tube.
29.2.6 Calendaring:
Soft flexible PVC sheets, plain or embossed, are making in large quantities by feeding hot premix between rollers individually driven and heated to 150 – 180oC with an output up to 4500kgs per hour. When desired, sheet can be embossed or laminated or laminated to another plastic sheet to make items like floor tiles and this may be done immediately as the sheet emerges from the rollers of calendaring machine.Go to next chapter
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