Plastics Guide

Why use plastic?
Plastic Process selector
Plastics Materials Selector
Plastics Finishing / Decorating / Printing

WHY PLASTIC?

There is only one catch:-
To obtain the cost benefits of using plastic products you need the tooling to do it with. This can take the form of an extrusion die, a mould tool, or a former. All of these things are expensive, and take time to make. On www.plaslink.com you may well be able to find someone making the plastic bit you want, or who has the tools for it, who will be able to help and advise you, but if not this guide is to help you select the plastics processes and materials to achieve the part you want.

Plastic Process Selection

3 Is high mechanical strength and stiffness required? Yes = use pultrusion / No = use extrusion

4 Is it hollow like a bottle? You have 3 suitable processes:- blow moulding and rotational moulding. If these are not suitable you may need to make the item in two or more parts by other methods and joint them together. See also dip moulding, used for soft components like grips and bellows

5 Can the part be made from a sheet of material stretched to form the desired shape, like bowl, or a sandwich tray? Yes = go to 6 / No = go to 10

6 Is the part thin walled - e.g 1 mm or less? Yes = use thermoforming / No = go to 7

7 Do you require high strength e.g train seats, boat hulls golf clubs? Yes = use fibre reinforced plastics / No = go to 5

8 How many parts do you want?- less than 1,000-20,000? Yes = use vacuum forming / No = go to 10

9 You want a full three dimensional moulded shape with all faces formed in the moulding process. How many parts do you want? 1-1000 = use reaction injection moulding or vacuum casting / 50 upwards = use injection moulding / rotational moulding and fibre reinforced plastics may also be used, and will generally give lower tooling costs with higher unit costs.
For thermosetting materials such as melamine, urea, and phenolic resin compression moulding is normally used. Compression moulded parts are harder and have a more durable surface than thermoplastic materials- often used for handwheels, light switches etc. For development and very low volume use rapid prototyping systems, or vacuum casting or reaction injection moulding.

10 You want an elastic part. You need to decide if the part you need has to have good compression set e.g. how well will it recover after it has been deformed. If you do not need very good deformation resistance then you can use a thermoplastic material. If good deformation resistance is required you will need a fully cross linked (vulcanized) rubber such as natural rubber, silicone, nitrile, etc. These materials are formed by compression moulding , or transfer moulding, or extrusion. They generally require a specialist rubber processor.
If very good deformation resistance is not required a thermoplastic material such as SBS or Hytrel can be used. These materials are formed by heating processes, normally injection moulding or extrusion.

Before you go you should see our comparison table to confirm your choice. If you are not sure how a plastics process works click on that process and we will show you how it works, and give further examples.

Extrusion -
Extrusion is used to make continuous sections only. Parts can however be slit, cut, drilled as secondary operation. eg tubes, gutters, window sections etc. Plastic is squeezed through a two dimensional former or "die", to form a continuous section of material.

Pultrusion -
Similar to extrusion but with much higher Strengths- even used to make road bridges. Glass or other fibres are incorporated into the extrusion and so loadings of up to 60% glass can be achieved with very good fibre alignment. Materials are generally thermosetting type materials such as epoxy.

Rim/Vacuum Casting -
Low volume detailed parts, also used as prototyping system for injection moulding. Chemically curing resins are injected at very low pressures,or drawn into a mould under a vacuum. Long cure times mean that this is an expensive production process.

Injection Moulding -
Volume production system - estimated 80% -90 % of all plastic parts made by this process. Limitation of high tooling cost for low volume applications. Parts are formed by injecting soft or liquid material into a complete mould under very high pressures.

Blow Moulding -
Used to make hollow components, bottles, oil drums, children's furniture etc. Limited choice of materials. Parts are formed by inflating a "bubble" of plastic inside a hollow mould, which gives the finished shape to the product. Variations on process include injection blow moulding, stretch blow moulding, blow moulding using performs.

Rotational Moulding -
Used to make large items, e.g. domestic oil tanks, large bins etc. Items can be hollow, but also used for other products- but generally for parts that form part or all of a shell. Parts are formed by rotating a hollow tool with a charge of powdered plastic inside. The tool is heated, and then cooled, so the plastic fuses to the inside of the tool, to form a shell.

Fibre Reinforced Plastics -
Used where higher strengths and stiffnesses are required than available from other processes, eg golf clubs, vehicle seats. Generally parts are made by various lay up processes and resin is then added and cured by chemical or heat action. Note some Fibre reinforced materials can be injection moulded.

Vacuum Forming -
Used to make large area parts such as trays covers etc. Tooling much cheaper than injection moulding, but three dimensional ribs, bosses etc cannot be formed. Limited choice of materials. Parts are formed by heating a sheet of plastic over a one sided mould or former, until it is soft but not liquid, and then using a vacuum to suck the sheet onto the mould, then allowing it to cool.

Thermoforming -
Similar process to Vacuum forming, but thermoforming refers to thin walled components, such as are used in packaging and display items.

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