Manufacturing Processess of Plastics

VACUUM FORMING

Vacuum forming is a technique that is used to shape a variety of plastics. In school it is used to form/shape thin plastic, usually plastics such as; polythene and perspex. Vacuum forming is used when an unusual shape like a ‘dish’ or a box-like shape is needed.

A TYPICAL VACUUM FORMER

THE STAGES INVOLVED IN VACUUM FORMING

1. First, a former is made from a material such as a soft wood. The edges or sides are shaped at an angle so that when the plastic is formed over it, the former can be removed easily.

2. The former is placed in a vacuum former

3. A sheet of plastic (for example, compressed polystyrene) is clamped in position above the mould.

4. The heater is then turned on and the plastic slowly becomes soft and pliable as it heats up. The plastic can be seen to 'warp' and 'distort' as the surface expands.

5. After a few minutes the plastic is ready for ‘forming’ as it becomes very flexible.

6. The heater is turned off and the mould is moved upwards by lifting the lever until it locks in position.

7. The 'vacuum' is turned on and this pumps out all the air beneath the plastic sheet. Atmospheric pressure above the plastic sheet pushes it down on the mould. At this stage the shape of the mould can be clearly seen through the plastic sheet. When the plastic has cooled sufficiently the vacuum pump is switched off.

8. The plastic sheet is removed from the vacuum former. The sheet has the shape of the former pressed into its surface.

9. The excess plastic is trimmed so that only the plastic bowl remains - the completed item. An enlarged view of the final dish is seen opposite.

When plastic is used to make a large number of identical products, industrial processes are used. Specialized machines are used to mass produce products.

To the right is an example of a vacuum formed toy. The simple 'lorry' mould has been placed in a vacuum former and a compressed polystyrene sheet has been placed above it. The polystyrene has been heated and then vacuum formed to the shape of the mould.

The mobile phone seen below has plain packaging. The back is made from card with a lacquered, gloss finish. The front is manufactured from vacuum formed high density polystyrene. The polystyrene is transparent which means that the mobile phone can be seen through it. This type of packaging is called 'blister packaging'.

Materials such as polystyrene, nylon, polypropylene and polythene can be used in a process called injection moulding. These are thermoplastics - this means when they are heated and then pressured in a mould they can be formed into different shapes.

The DVD Storage unit seen opposite has been made in one piece using this process

Injection Moulding

What similarities and differences can you identify between the vacuum forming and injection moulding processes?

Injection moulding

Injection Moulding (Gas Assisted)

Developments over the past years in Gas Assisted Moulding or (Gas Assisted Molding to use the US spelling or Gas Injection Moulding being a common reference term) have resulted in advances in the way in which injection moulded components are manufactured. Enhanced quality, reduced cycle times and component weight reductions can be achieved by the process.

Typical Materials Used:Acrylonitrile-Butadiene-Styrene

Typical Products Produced:Power-tool housing, Telephone

Blow MouldingBlow moulding follows the same stages of manufacture as injection moulding. HDPE (high density polyethylene) is ideal for this type of manufacturing technology .The HDPE granules / powder is placed into the hopper. It drops down on to a rotating thread and travels along its distance. During this process the granules / powder is heated and becomes fluid.

However, when it reaches the mould compressed air is also blown into the mould. This pushes the HDPE around the edges of the mould, forming a hollow shape, in this case the recycling bin.

What similarities and differences can you identify between the blow moulding and vacuum forming processes?

Blow moulding

Blow Moulding is infact Injection Blow Moulding

When the HDPE is cool it is removed from the mould and excess HDPE is trimmed away - leaving the bin.

Injection Stretch Blow Moulding

Injection stretch blow moulding is used for the production of high quality containers.

Typical Materials Used:Polyethylene - Terephthalate (PET)

Typical Products Produced:Carbonated and soft drink bottles, Cooking oil containers, Agrochemical containers Health and oral hygiene products and Bathroom and toiletry products

Insert Moulding Inserts can either be incorporated at the time of the moulding process, or be inserted as a post moulding operation. When incorporated as part of the moulding process, the inserts can either be loaded by robot assistance or by the process operator. If the inserts are applied as a post moulding operation, they can usually be applied any time after the moulding process. In some cases, they are inserted as part of a series of post moulding operations. This helping to minimize the actual cost of insertion. The fundamentals of process speed (and economics), and process repetition must be borne in mind when making the decision when to apply the insert. It is not good practice to allow the cycle time to vary from shot to shot; this can have an adverse effect on the quality

Rotational Moulding

Rotational moulding (often referred to as Rotamoulding or Rotomoulding) is a process used for producing hollow plastic products. By using additional post-moulding operations, complex components can be produced enabling the process to compete effectively with other moulding and extrusion practices.

Rotational moulding differs from other processing methods in that the heating, melting, shaping, and cooling stages all occur after the polymer is placed in the mould, therefore no external pressure is applied during forming.

This provides the following advantages Economically produced large products, Minimum design constraints, Stress-free products, No polymer weld lines and Comparatively low mould costs.

What is the difference between male and female moulds?

Compression moulding

Moulding through the force of compression is another very common industrial process. The materials used are melamine formaldehyde, phenol and urea. These materials can be formed into different shapes through applying both heat and pressure, other plastics do no have these properties

The stages are as follows:

1. The mould is heated.

2. A 'slug' or piece of the selected plastic is placed into the mould and warms up.

3. The hydraulic press begins to move down when the plastic has reached the correct temperature..

4. As the upper and lower mould meet, the plastic is compressed into the shape of the mould.

5. The upper mould moves upwards and the plastic piece (in this case a tray) is removed.

What products can you think of that have been manufactured using a strip heater?

Line bending

The desk organiser seen opposite has a steel wire frame and plastic shelves. The selves have been folder on a strip heater. This heats the plastic along a line so that it becomes soft and flexible. It can then be folded to almost any angle. Thermoplastics such as Acrylic (Perspex) are most suitable for this type of product as they can be shaped when heated.

The diagram below shows the ‘disassembled’ desk organiser. The shelves have been removed to reveal the steel frame behind it. When the shelves clip back on to the frame, the organiser it forms can ‘free stand’ on a desk. The desk organiser can be used to hold folders and paper etc.....

Extrusion

Materials such as polystyrene, nylon, polypropylene and polythene can be used in a process called extrusion. These are thermoplastics - this means when they are heated and then pressured in a mould they can be formed into different shapes and sections

  A machine used to extrude materials is very similar to the injection moulding machine. A motor turns a thread which feeds granules of plastic through a heater. The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape. The extrusion is then cooled and forms a solid shape. The shape of the die determines the shape of the tube.

Can you think of any other products that are manufactured using extrusion?

Extrusion

How an extruder works

Below are examples of the type of shapes (sections) that can be extruded using an extrusion machine.

Plastics have excellent surface qualities. As they are self-finishing, plastics require little or no surface finish. However, when designers want a specific colour or texture, spray paints can be used.

Use wet and dry paper (wet) to smooth the surface down and create a ‘key’ for the primer to grip to.Primer is used to create an undercoat to protect the plastic and provide a good surface finish for the top coat. The primer should be rubbed down using wet and dry paper between coats.Top coat is applied once the primer has completely hardened. There are different surface textures; gloss, matt and metallic being the most common. Top coat is applied in several layers to build up a thick, tough surface layer on the top of the primer.

Finishing techniques

Plastics are very useful materials. They are durable, easy to join, self-finishing, easy to mould and not susceptible to rot or sust. However, sometimes their properties are enhanced by combining them with other materials.

Carbon fibre can be added to thermosetting plastics to improve their strength. Golf clubs, tennis racquets, cycle frames and fishing rods are made using carbon fibre composite.

Plastics combined with other materials

Working with plastics – keywords