Choosing appropriate materials for injection moulding
So, you have decided that injection moulding is the way you intend to manufacture your components; but you now face a choice as to which polymer or combination of polymers is best suited to your needs. Most importantly, that will meet the design criteria for the part and provide the physical properties demanded by the application.
Plastic injection moulding relies on thermoplastics, a very wide range of polymers, that can tolerate repeated melting and solidification, without significant change to the chemical structure and properties of the material.
Where to start?
There are many grades of engineering thermoplastic resins to choose from and deciding which one is right for your product can be daunting. Fortunately, these resins fall into distinct polymer categories, each of which is likely to offer the end-use properties that you are seeking.
It is important to determine the operating conditions under which the part will be expected to function without risk of failure, as well as the environmental extremes that it is likely to encounter.
Questions to ask might include:
- Does the part need to be tough and flexible, and able to take significant impacts and shock?
- Must it have a high degree of stiffness and rigidity, have a glossy or specially textured surface?
- Is resistance to chemicals and fuels important, and is it necessary to ensure that the surfaces of the part do not degrade in the presence of UV light?
- Will the part be subjected to temperature extremes?
- Does the part need to be resistant to abrasion or wear, and offer low sliding friction?
- Can the injection moulded part accept post production processes such as machining?
- Does the material have to meet regulatory requirements for food or clinical uses?
And then, of course, there are variations on a theme. You can add significant value to your parts using two-, three-, even four-shot injection moulding techniques that use specially designed moulds into which different polymer resins with specifically desired properties are injected in a series of separate ‘shots’ to create overlays, inserts and other interesting features in various colours. One very common method is a two-shot process that over-moulds a specific area of a rigid plastic component with a flexible elastomer to enable the component to be more easily gripped.
Material choices should be discussed at the earliest stages of the design process, and a dialogue established between the designer and the injection moulding company’s materials experts before final decisions on material choices are made.
Popular resins and their properties
The scope of this article doesn’t allow anywhere near full treatment of this subject, but the following six classes of resins give some idea of the breadth of choice that is available:
- ABS (Acrylonitrile Butadiene Styrene) – a tough, impact resistant plastic used in the manufacture of power tool housings, automotive components and similar parts subject to rough, day-to-day handling.
- PC (Polycarbonate) – a highly impact resistant transparent plastic that tolerates high temperatures and which produces glossy, crystalline finishes when injection moulded. Often blended with other polymers, PC is frequently used in the manufacture of medical devices, lenses, electronic housings and automotive components.
- PA (Polyamides) – more commonly referred to as nylons, which come in many grades to provide strength, chemical resistance and good high temperature performance. Widely used in the manufacture of machine components (gears, shims and so on) and, when mixed with fillers, some structural parts.
- PE (Polyethylene) – a low cost resin, notable for its high ductility, abrasion resistance, and chemical resistance. Available in Linear Low Density, Low Density and High Density (LLDPE, LDPE and HDPE) versions, the resin is widely used in the manufacture of large packaging and bulk containers.
- POM (Polyoxymethylene) – a low friction, tough resin more commonly referred to as acetal. Its slippery surface and resistance to fuels and lubricants makes it ideal for use in automotive plain bearings.
- PP (Polypropylene) – another widely used, low cost resin offering toughness and high flexibility. It is a favourite of the packaging industry, which uses the material to make lids and bottle tops.
As one of the UK’s leading trade plastic injection moulders, with over 50 years of experience, our comprehensive range of injection moulding machines gives us the flexibility to fulfil the diverse scope of our customers’ requirements. Processes we carry out include conventional injection moulding, over-moulding, insert moulding and clean room moulding.
We select and source high-quality materials from proven suppliers and have experience in processing most thermoplastics. We manufacture a huge variety of injection moulded parts, including housings, aesthetic parts, two-part mouldings, clear mouldings/lenses, internal technical parts and high-tolerance parts.
Why not speak with one of our industry experts today to see how our plastic injection moulding service can help you.