These processes are very important. They always come up in exams. You must know the different processes, the plastics that are used and common components that are produced

Manufacturing in Plastics There are different manufacturing processes for the two different types of plastic. This is due to the chemical reactions that take place when the different materials are heated. The processes we will cover are: Thermoplastics Strip Bending Vacuum Forming Injection Moulding Extrusion Blow Moulding Rotational Casting Thermosetting Plastics Compression Moulding Lamination Glass Reinforced Plastic (GRP) All the manufacturing processes left involve the production and use of a mould. The quality of the mould is very important and dramatically affects the quality of the component produced.

Moulds Moulds have common features for whichever of the processes they are used for. The mould may be a simple wooden form used to bend acrylic accurately or a very complex metal mould in several parts used for injection moulding   The surface must be hard and very well finished as the softened plastic will take its finish from the mould surface. Moulds can be textured so that the plastic is formed with the texture pressed into its surface. If used for mass production the moulds must be very hard wearing so are often made from special steels. The sides of the mould must slope to allow the plastic product to be lifted off or pulled out. This slope is called the DRAFT angle. It should be between 5-10 degrees.

Corners should be radiused to allow better plastic flow Corners should be radiused to allow better plastic flow. Sharp corners lead to an area of stress and weakness.   Any recesses in vacuum forming moulds must have small vent holes drilled in them to prevent trapped air stopping the plastic sheet forming.

Thermoplastic Processes

Strip Bending This is a very simple process. The plastic in sheet form is heated along a line by a heating element. As the plastic heats it softens allowing it to be bent. As it cools it will retain its shape. It is often done in schools using acrylic sheet because it is a low cost process.   It is often a good idea to make a wooden former to ensure accurate bending as shown below.

Vacuum Forming This process uses plastic in sheet form. It is used to make many different products from packaging, helmets, masks, toys & baths.

The following processes all use plastic in granular or powder form The following processes all use plastic in granular or powder form. The plastic is heated until it melts to produce a liquid. Injection Moulding This is a continuous process that is ideal for mass production. Numbers produced are often in thousands. This is important because the equipment and moulds are very expensive. The process is very versatile producing items such as bowls, buckets, model construction kits, chairs, packaging, toys eg Lego. It is easy to identify an injection moulded product because you can see the sprue point where the plastic was injected.

Plastic granules pass from the Feed Hopper into the Barrel Plastic granules pass from the Feed Hopper into the Barrel. The rotating Screw Thread pushes the plastic down the Barrel where it is melted by the Heaters. At the end of the Barrel the cone compresses the plastic where it is injected into the Mould. When full the mould is cooled then split allowing the component to come out.

Injection Moulding Moulds can be very complex allowing products to be moulded in one piece. Consider the production of a bottle top.

Extrusion This is very similar to injection moulding but instead of injecting into a mould the plastic is pushed through a DIE. This gives a continuous length of a product with the same cross section. It is ideal for producing products like pipes, guttering & uPVC window frame sections.

This process can be easily adapted to provide a coating on other products such as electrical wire or paper to produce washable vinyl wallpaper.

Blow Moulding This process starts off as the extrusion of a tube which then passes into a split mould which closes sealing both ends. Air is then blown in pushing the plastic out to the sides of the mould. It produces hollow containers like bottles or barrels. It is easy to identify the method because you can see the seam where the plastic tube was trapped in the mould.

Rotational Casting This process is like the process used to make Easter Eggs. It is lower cost than the other methods so can be used to produce batches of products. It is very flexible making products like dolls, traffic cones or large containers.

Thermosetting Plastic Processes Consider the processes we have looked at. Why can you not use them for Thermosetting Plastics?

Compression Moulding This uses a split mould in a similar way to injection moulding but there is no entry hole. The amount of plastic needed is calculated and placed inside the mould before it is closed like rotational casting. The mould is heated and closed under pressure. The plastic flows around the mould and the strong bonds form which prevent the plastic being reshaped again. These plastics are heat resistant so are used for electrical fittings, kitchen ware & handles.

Laminates These are formed by soaking sheets of fabric or paper in a thermosetting resin, then compressing them under pressure until the resin sets. The sheets are laid at right angles to each other which gives an effect like plywood.   The paper may have a design printed on it which will show through or a texture may be pressed into the surface to give an effect. Look at your kitchen worktop at home. Laminates are very strong and are often combined with manufactured boards to give a durable, waterproof surface like Formica or Melamine

Glass Reinforced Plastic (GRP) This is a combination of a thermosetting resin which is hard and brittle and glass fibres that are very flexible. When combined they produce a material that weight for weight is tougher than steel.   GRP is very flexible and suitable for one off or large scale production. It is used for boat hulls, car bodies, train bodies, furniture, baths, childrens playground equipment and many other products. It is easy to identify because it has one very smooth side and one rough side. It is usually formed by hand. The mould is first covered in a gell coat resin which gives the good finish then layers of glass fibre are layered over the top. More resin is added which is stippled into the glass fibre so that it soaks it up. More layers of glass fibre are added until the desired thickness is reached.

When using GRP the mould has to be made the correct way to give the smooth side facing the direction you want.

Plastic Manufacturing Questions 1 Explain the process used to bend acrylic sheet. Why is this a popular process in schools?   2 Why are injection moulding moulds made from steel? 3 Explain what is meant by Draft? 4 Explain what is wrong with the Vacuum forming mould below?

5 What are the function of vent holes in a vacuum forming mould?   6 What happens to acrylic if it is left on the strip heater too long? 7 Why can’t Thermosetting Plastics be put into an injection moulding machine? 8 Why is injection moulding ideal for mass production?

9 How do you recognise an injection moulded product?   10 What is the difference between a Die and a Mould? 11 Describe how 3 core electrical cable is produced? 12 What advantages do extruded guttering and drainpipes have over the old cast iron ones?

13 Explain why rotational casting can be used for batch production?   14 When compression moulding a little more plastic is added to the mould than actually needed. Why? 15 Why were the intercity 125 train cabs made from GRP instead of metal? 16 Why has the introduction of Formica made a big impact on kitchen hygiene?

17 Which Sports Car manufacturer uses GRP for its car body shells?