Designing for Manufacture, Part 2 – Material Selection

June 15, 2017

designing-part2-detail (1) So you need to select the correct material for your project? – in order to determine the best material for your project, there are 3 key questions to be asked;

What is the intended end use of the part?- Does the part need to be rigid or flexible, withstand pressure or weight, temperature variations and will it be exposed to any other elements?
Are there any special requirements regarding appearance? – Does it require a specific finish, colour match or any embossing/printing?
What about any regulatory requirements, and do these apply? – Does it need to be food safe, child safe or is it likely to be exported?

All of the above should be taken into consideration when choosing the best material for your project and polymer choice is down to a number of different attributes – price, heat resistance, chemical resistance, shrinkage, flame-resistance, tensile strength, flexural strength. Remember to take into consideration plastics that are amorphous as these are less free-flowing but tend to shrink less, compared to crystalline or semi-crystalline plastics which flow better but have higher shrinkage rates. Plastic suppliers will give guidance on shrinkage, temperature and melt flow rates.

Amorphous

These polymers possess widely different mechanical and physical properties owing to their structure and temperature. Transparent with a low shrinkage rate, are soft, but have poor chemical resistance, do not have a sharp melting point, but have a high gas permeability.

Semi-Crytalline

As crystalline polymers contain considerable amounts of amorphous material they are generally called semi-crystalline polymers. Opaque or translucent, with a high shrinkage rate, are hard, have good chemical resistance, but a low gas permeability.

Basic characteristics of polymers and their applications

All plastics fall into 2 basic categories:

Thermosetting Polymers (known as Thermosets), which set during processing via a chemical reaction and cannot be re-melted or re-shaped, are heat resistant, have dimensional stability and can resist chemicals. Commonly used in the manufacture of handles and lid knobs on cooking pots and in electrical insulation. These can also be categorised into different types, such as; epoxy resin, melamine formaldehyde, polyester resin which have varying degrees of stiffness, textures and level of resistance to chemicals.
Thermoplastic Polymers (known as Thermoplastics), which are heated then cooled in a mould to form a part, can be re-used and more importantly recyled. These can also be further categorised into; nylon, acrylic, polypropylene, polystyrene, low-density polythene and high-density polythene, which all have different properties of colour, degree and malleability and weight.

Commodity resins are often used for everyday applications and mass produced as they are easy to process and inexpensive ie containers and bottles. Engineering resins are more expensive but offer better strength and resistance to both the environment and chemicals, ie electrical components, auto parts. Speciality high-performance resins are expensive and used in aerospace.

To keep costs low, manufacturers can use additives or fillers to obtain the qualities they need at a much lower cost.

Common additives used and their applications

Antimicrobial – used in food related or high contact consumer products

Antistatics – used to decrease the static electricity conduction

Plasticizers and Fibres – used to make the resin more pliable and fibres add strength and stiffness

Flame Retardants – used to make products resistant to combustion

Optical Brighteners – used to improve whiteness

Colourants – used to add colour or special effects such as pearlescence or fluorescence

Choosing the right material is key to creating perfect plastic parts. It is important to work with an injection moulder who has extensive knowledge and experience in this area and good relationships with resin suppliers. Do you need any further information? Let us know by contacting our sales and technical team, who are always happy to help.

PART 3 – PART DESIGN