How to select the correct injection moulding material

There is a lot to consider when choosing the right plastic resin for a part; it’s basically a science! Here we look at the basic characteristics of polymers with some examples of popular resins and their applications.

To select the best moulding material for your manufacturing project, thinking about the questions below will help guide you to the right material.

What is the intended end use of the part?

– Does the part need to be rigid or flexible?
– Does the part need to withstand pressure or weight?
– Will the parts need to withstand certain temperature variations?
– Will the parts be exposed to other elements or chemicals?

Are there special appearance considerations?

– Is a specific finish required?
– Does a colour need to be matched such as branding and corporate colours?
– Is embossing a consideration?

Do any regulatory requirements apply?

– Will the product be exported and need to meet REACH standards?
– Does the part need to be food safe?
– Will children use the product?

A plastics primer – thermoset vs. thermoplastic

Plastics fall into two basic categories, thermoset plastics, and thermoplastics. To help you remember the difference, think of thermosets just as the term implies, they are “set” during processing.

When thermoset plastics are cured, it “sets” the part into a permanent form. The chemical reaction is not reversible, so parts made with thermosets cannot be re-melted. These materials can be a recycling challenge unless a bio-based polymer is used.

Thermoplastics are heated then cooled in a mould to form a part. Once the parts are cooled, they revert to their original state and can be re-melted and cooled again. For this reason, thermoplastics are easier to reuse and recycle. They comprise the majority of the manufactured polymer resins on the market today and are used in the injection moulding process.

To recap:

Fine tuning the resin selection

Thermoplastics are categorised by family and type. They fall into three main categories or families: commodity resins, engineering resins, and specialty/high-performance resins.

Speciality/high-performance resins
The high-performance resins come with a higher cost. That is why commodity resins are often used for everyday applications.

Commodity resins
Commodity resins are easy to process and are inexpensive. They are often found in common mass produced items like packaging.

Engineering resins
Engineering resins are more expensive, but offer better strength and resistance to chemicals and environmental exposure.

Morphology
Within each of these families, there are resins that have different morphology. Morphology describes the arrangement of molecules in a resin and fall into two categories, amorphous and semi-crystalline.

Amorphous resins:
• Shrink less when cooled
• Offer better transparency
• Work well for tight-tolerance applications
• Tend to be brittle
• Lack chemical resistance.

Semi-crystalline resins:
• Tend to be opaque
• Offer good abrasion and chemical resistance
• Are less brittle
• Have higher shrinkage rates.

Examples of available resins

Amorphous
An example of an amorphous, commodity resin is polystyrene or PS. Like most amorphous resins, it is transparent and brittle, but can be used in high-precision applications. It is one of the most widely used resins and can be found in plastic cutlery, foam cups, and plates.

Higher up on the amorphous scale are the engineering resins such as polycarbonate or PC. It is temperature and flame resistant and has electrical insulating properties, so is often used in electronic components.

An example of a specialty or high-performance amorphous resin is polyetherimide or (PEI). Like most amorphous resins, it offers strength and heat resistance. However, unlike most other amorphous materials is also chemically resistant, which is why they’re often found in the aerospace industry.

Semi-crystalline
An inexpensive semi-crystalline commodity resin is polypropylene or PP. As with most semi-crystalline polymers, it is flexible and chemically resistant. The low cost makes this resin the choice for many applications such as bottles, packaging, and pipes.

A popular engineering, semi-crystalline resin is polyamide (PA or nylon). PA offers chemical and abrasion resistance as well as low shrinkage and warp. There are bio-based versions available making this material an earth-friendly alternative. The toughness of the material makes it a lightweight alternative to metal in automotive applications.

PEEK or polyaryletherketone is one of the most widely used semi-crystalline high-performance resins. This resin offers strength as well as heat and chemical resistance and is often used in demanding environments such as bearings, pumps, and medical implants.
Plastic additives to improve characteristics

As we have seen, resin families (commodity, engineering, and high-performance/specialty) contain both amorphous and semi-crystalline alternatives. However, the higher the performance, the higher the cost. To help keep costs low, many manufacturers use additives or fillers to obtain the qualities they need at a lower cost. These additives improve performance or convey other characteristics to the final product. (what are additives or fillers?)

Some common additive applications:

Antimicrobial – Used in food-related applications or high-contact consumer products.
Antistatic – Decreases the static electricity conduction and is often used in sensitive electronics.
Plasticisers and fibres – Plasticisers make a resin more flexible, whereas fibres add strength and stiffness.
Flame-retardants – Make products resistant to combustion.
Optical brighteners – Improve whiteness
Colourants – Add colour or special effects such as fluorescence or pearlescence.

The final selection

Choosing the right material for a project is one of the most important factors in creating perfect plastic parts. The advances in polymer science have contributed to the development of a large selection of resins from which to choose.

It is important to work with an injection moulder that has experience with a variety of resins and applications including resins that are compliant with FDA, RoHS, REACH and NSF.

If you are unsure which resin to select, you consult with your injection moulder to determine the best material.

At WSM Plastics, we are always on-hand to offer expert advice so your product is made with the correct materials.

We are here to help. Contact us today to turn your product idea into a reality.

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