Since every plastic has its own specific characteristics, making the right choice before building a mould is essential. The type of steel and dimensions of the mould are selected depending on the plastic chosen. Switching to another plastic after the mould has been made can result in problems, as this can adversely affect the dimensions of the product or the tool life of the mould.
The list below provides a brief overview of typical plastics used for compact injection moulding and thermoplastic foam casting (TFC). Taking the time to choose the right plastic is important. Using the most appropriate plastic for your application means choosing the best solution in terms of technical properties, design and cost.
What requirements should your product satisfy?
An extensive choice of injection-mouldable plastics (thermoplastics) is both a blessing and a burden. Nonetheless, one can make a choice fairly quickly by filtering through them using the following properties:
- chemical durability
- dimensional stability
- colour or transparency
- temperature resistance (both - and +)
- strength (mechanical load)
Of course, plenty of other points should be considered when making products for specific applications. For instance, one should always consider the environment in which the product will be used. For example: outdoors on the motorway (the effect of weather, noise, exhaust gasses), in the food industry (suitable as packaging material, good for vacuum packaging, cheap), in the medical sector (hospital use, easily cleaned with alcohol, retains its colour), in the aviation industry (fireproof, lightweight), etc.
One should also consider any further processing that the product may undergo, including printing, lacquering, inserts (and screws), glueability, chrome coating, etc. Not every plastic is suitable for every process.
A sound choice can only be made once the environmental factors, possible processing and the properties that the product must have are clear.
Nowadays, there are a huge number of injection-mouldable plastics (so-called thermoplastics) to choose from. Often they are variations of one and the same plastic. In essence, it comes down to the following list.
Acrylonitrile Butadiene Styrene (ABS)
This material is best known for its use in children’s toy bricks, but is very widely used. It is ideal for use in housing components where dimensional stability and colour are important. ABS is a material that also works well when the product has to be lacquered and where inserts or screws have to be inserted directly into the plastic.
Perhaps it needs to be a little prettier? And in particular, must it retain its colour? Then Acrylonitrile Styrene Acrylate (ASA) is a good choice. ASA is much more UV-resistant and it can also be made flame-retardant when combined with polycarbonate (PC).
Polypropylene (or polypropene) is also widely used, but it is generally undervalued. This is because its dimensional stability is not that good and it is known as being relatively cheap. However, its chemical stability is unprecedented. Polypropylene (PP) is easy to colour and easy to injection mould. Lacquering, gluing and printing are unfortunately more difficult.
Polyethylene (or polyethene) (PE) is usually made as high-density polyethylene (HDPE). However, the density is still quite low (just like PP by the way). PE is also relatively cheap, easy to process and can be made to be very transparent.
Polystyrene (PS) or Styrene Butadiene (SB)
The TFC material of choice is high impact polystyrene (or HIPS). It is also called styrene butadiene (SB). This material is sufficiently dimensionally stable, strong, hard, easy to lacquer and mechanically process and relatively cheap. The flame-retardant version also has an acceptable price. It is very extensively used to manufacture medical equipment, large analyser housings and HVAC components.
Polycarbonate is primarily chosen for its excellent transparency. It is a material that is used for cover caps, for example, and because it has very good mechanical properties. It is inherently flame retardant. However, with PC you have to be careful with certain cleaning products, chemicals and oils, as contact with these agents can cause stress cracking that causes the plastic to crack.
PC is often blended (mixed by the manufacturer) with other plastics, such as ABS, ASA and PBT, in order to utilise the good properties of all of these materials.
Do you want something really transparent and/or with a super high-gloss finish? Polymethylmethacrylate (PMMA) (best known under the brand name Plexiglas, but there are several other brands) lets more light through than polycarbonate (PC), but it is flammable. Just like PC, PMMA is also sensitive to stress cracking. It is ideally suited for outdoor applications that require a long lifespan.
Polybutylene Terephthalate (PBT)
Polybutylene terephthalate - an engineering plastic - is used increasingly. It is a material that can be used where dimensions have to be very accurate and we use it for the platforms of analysis equipment, for example. It is rigid, hard, easy to use at higher temperatures and wear-resistant. It also provides good resistance to organic solvents, oils and fats.
Who hasn’t heard of the Nylon brand? But there are numerous other suppliers of more or less the same material, and this is widely used as an injection moulding material. Polyamide has plenty of advantages, such as good chemical resistance, impact resistance, wear resistance and good sliding properties. This material is often used for more technical parts, especially when it concerns PA6, PA6.6 or PA12 with glass filling.
The material of choice for snappers is Polyoxymethylene (POM). This flexible material can be bent and returns to its original shape easily. It is also often used for cogs and bearings, for example, because it is so wear-resistant. This material is dimensionally stable but has a high shrinkage factor.
A material we use almost exclusively for the aviation industry is polyetherimide. It is best known under the brand name Ultem. It combines a low specific weight (important in aviation!) with very good fire resistance. Colouring these materials is possible but difficult. This is due to the material’s high processing temperature.