CHAPTER FIVE Polymer Forming

Transcription

1 CHAPTER FIVE Polymer Forming Plastic raw materials are available in a variety of forms including Powder, viscous fluids, pellets and granules. Product manufacturers use a wide range of processing machinery to convert these materials into components and products. The most important methods are; extrusion, blow molding, injection molding, compression molding and vacuum molding. البثق 5.1 Extrusion This is the process used to make 'long' products like drainpipes and Curtain rails. Fig. 5.1 shows the main element of the process. Fig. 5.1: Polymer extrusion Plastic granules are fed from a hopper on to a rotating Screw. The screw forces the plastic through a heated tube MDP024 - Prepared By Dr. Mohamed Ahmed Awad

2 where it becomes molten before being forced under pressure through a die. The die contains a hole whose shape corresponds to thatt of the required Article. As it leaves the die, the 'extrusion' is cooled in a water bath or In jets of air. The hardened extrusion is then cut into lengths or coiled, depending on the product. Thermoplastics such as polythene, PVC and Nylon are commonly used in extrusion. 5.2 Extrusion blow-molding This process as shown in Fig. 5.2 is used for making large hollow sections products like bottles and hollow toys. Fig. 5.2: Blow molding The process may be divided into three steps as shown in Fig. 5.3 MDP024 - Prepared By Dr. Mohamed Ahmed Awad

3 Step 1 Extrusion Step 2 Blowing Step 3 Extraction Fig. 5.3: Blow molding steps Air is blown into a section of extruded plastic tube, causing it to expand and take up the shape of the mould. The mould is then opened and the product removed. PVC, polythene are common blow molding materials. MDP024 - Prepared By Dr. Mohamed Ahmed Awad

4 5.3 Injection molding This is the process of forming articles by injecting molten plastic into a mould. An injection molding machine, shown in Fig. 5.4, is similar to that used for extrusion, the difference being that the feed screw (in addition to feeding plastic from the hopper) is used as a ram. The screw is moved backwards, as it rotates, until a measured quantity of plastic is at position A in the diagram. The screw is then driven forwards by a hydraulic ram, forcing the molten plastic into the mould. Fig. 5.4: Injection molding The mould consists of two or more parts which fit together forming a cavity of the required shape. Cold water is circulated through the body of the mould to reduce the cooling time of the molding. After a short time the mould can be opened and the molding removed. The complete cycle can then be repeated. MDP024 - Prepared By Dr. Mohamed Ahmed Awad

5 Injection molding produces components and products which have consistently accurate dimensions and a high quality finish. A wide range of complex forms can be produced which could otherwise only be manufactured by expensive machining processes. Production is fast and the process produces very little waste. An enormous range of products are manufactured in this way. These include: kitchenware - pedal bins, bowls, buckets, jugs, cutlery and containers; cases for electrical appliances - hairdryers, vacuum cleaners, food mixers etc; toys and games; products for the car industry; component parts for many other products. Polythene, polystyrene, polypropylene and nylon are typical injection molding materials. MDP024 - Prepared By Dr. Mohamed Ahmed Awad

6 التشكیل بالضغط 5.4 Compression molding In compression molding, huge forces are used to squeeze a measured quantity of polymer into shape between عصر heated moulds as shown in Fig The polymer can be in the form of a powder or 'slug'. A slug is simply powder which has been compressed into a cube shape. Slugs can be handled more easily than powder, and can be preheated in a high frequency oven. This reduces the 'cycle time' in the molding machine. Fig. 5.5: Compression molding MDP024 - Prepared By Dr. Mohamed Ahmed Awad

7 Compression molding is used for thermosetting plastics. The heat from the mould triggers the chemical reaction known as 'cross-linking'. After a short period of time (known as the curing time), the cross-linking is complete and the mould can be opened and the molding removed. The moldings have a high quality finish requiring only the removal of 'flash'. Electrical fittings (plugs and sockets for example), saucepan and cutlery handles, bottle tops and toilet seats are just a few of the many products which are manufactured in this way. Phenol, urea, and melamine formaldehyde are typical compression molding materials. Fig. 5.6: Compression molding products examples MDP024 - Prepared By Dr. Mohamed Ahmed Awad

8 التشكیل بالتفریغ 5.5 Vacuum forming This is one of the processes used to make articles from thermoplastic sheet. The sheet is first cut to size and clamped above a mould. A heater then raises the temperature of the sheet until it becomes soft and rubbery as shown in Fig Fig. 5.7: Vacuum forming Finally, air is evacuated from beneath the sheet as shown in Fig This allows the normal 'outside' air pressure to push down on the softened sheet, forcing it to take up the shape of the mould. Fig. 5.8: Air evacuated MDP024 - Prepared By Dr. Mohamed Ahmed Awad

9 After a suitable cooling period, the hardened molding can be removed from the mould as shown in Fig Fig. 5.9: Product extraction The process described above is used mainly for shallow products made from thin sheet. When large or complex moldings are produced using thicker sheet, a pressure chamber may also be used above the sheet. Vacuum forming enables large irregular shaped moldings to be produced which could not be manufactured by any other plastic forming process. Further, the equipment is relatively cheap and requires the use of only one mould. Products manufactured in this way include egg boxes, chocolate box liners and numerous other food and confectionery packaging, seed trays, shop signs and fittings, some motor car dashboards, wash basins and baths. MDP024 - Prepared By Dr. Mohamed Ahmed Awad