Injection moulding is essentially injecting molten plastic into a cavity between tightly clamped pieces of steel. The plastic cools and solidifies, the mould opens and the solid part is ejected; this is referred to as a ‘cycle’ which is repeated to mass produce parts.

When it comes to good part design, the main factors that should be considered are the amount of material that is being used per impression, the time it takes to produce the part and of course the cost.

In short, good part design will serve its purpose and satisfy its requirements using as little material as possible and being processed in as short a time as possible. Material costs money therefore the less you use the less it will cost.

The time on a ‘press’ (the machine which puts the mould to work) also costs money so the quicker the cycle time the less money will be spent on labour and use of a moulding facility.

Understanding the moulding process and optimising your design for production can go a long way to improve the efficiency and subsequently the overall competitiveness of the end product by keeping costs down.

If assembly is required it’s important to note that minimising separate components by moulding sections as one piece where possible is often the best route, especially if cost is a main driver of the product. The more separate components you have as part of your design, the more moulds or cavities will be required and moulds can be expensive.

Material

Careful consideration of the materials used will help to reduce costs, many materials have similar properties and would work just as well as more expensive alternatives if the options are laid out clearly and criteria matched appropriately.

It’s almost always good practice when designing for injection moulded parts to observe common methods of maintaining structural integrity, function and aesthetics of other parts as there is usually good reason behind the tried and tested features and forms you will find on these designs.

General rules of thumb include minimising thick wall sections and maintaining good relative proportions to avoid part distortion (which is again caused by the retracting material which can pull other important features and forms out of their respective profile).

Consider the effect a feature might have on the opposite surface of a part, for example having a boss feature on one side of a flat surface may cause subtle ‘sink’ on the opposite side. Again this can be minimised by incorporating tried and tested features used ‘as standard’ in most injection moulded designs; there really is no need to reinvent the wheel in most cases.

Material shrink also influences part integrity. Thermoplastic materials are heated at high temperatures in the barrel of the molding press and injected into the mold cavity. As the part cools in the mold, it shrinks.

Thick cross-section areas cool at a substantially lower rate than thin cross-sections, and press cycle time is based on the cooling rate of the thickest cross-section. Therefore, only one relatively thick cross-section area of the part will increase the press cycle time thereby reducing the number of parts per hour and increasing the cost per part. .

Mechanical characteristics needed for part integrity mean an understanding of the function/design requirements of the part are an important consideration. If this part is under constant load it may require a thicker wall section than a part that is not under any load for example.

Also, the uneven rate of cooling of these thick and thin cross-sections is likely to result in distortion of the part, even after it has been ejected from the mould.

This distortion is often severe enough to prevent the part from meeting specifications. A thick cross-section is also likely to result in a depression on the surface called a sink mark, particularly if the cross-section is of varying widths.

Amount of taper or “draft” refers to the amount of taper of molded parts perpendicular to the parting line. The draft taper should be determined early in the plastic part design process. The need for part surface tapering facilitates ejection of the part from the mold, especially in high speed, high volume production applications.

For any plastic part featuring corners, strong part design is heavily dependent upon radius. Sharp corners create high stress areas; when a plastic part breaks, the break often occurs at or near a sharp corner. The radius of a corner generally should be designed as large as possible without inadvertently creating a thick wall section.