Designing for Smart Injection Molding

Injection molding is the most common method of producing plastic components, so when designing plastic parts, it is important to understand what you can do to ensure the part can be manufactured as efficiently as possible.

A badly designed plastic part is difficult to produce, which means there may be quality issues with the part, which will mean it is expensive, both in the production of the part and the on-going life of the part. The expense and difficulty can be avoided though with a few simple thoughts when designing the part.

Thickness – Plastic parts can have a much thinner wall thickness than most designers realize. A small cap approximately 1" (25mm) diameter can have a wall thickness of just 0.040" (1mm) and function well as a protection cap or plug. This thin wall means the parts can be produced quickly, will maintain their shape and size during the production process and will have a short cycle time in the Injection molding machine. All of those elements will ensure you get a part that is consistently made to drawing, with production costs kept to a minimum.

Uniform Thickness – It is crucial to ensure that you design a part with a uniform thickness throughout the part. Parts with areas where the material is much thicker than the rest of the part are prone to becoming misshapen. If a plastic part cannot cool evenly at the same rate it is liable to become twisted or have areas that become dish shaped as they cool. All of this means that the dimensions of the part will not be consistent.

Radii – Plastic has to flow in and around the tool in order to produce the part, so it is crucial to include a radius on every sharp corner. Where you have a corner in the design, make the internal radius approximately half the wall thickness and ensure the outside radius is one and a half times the wall thickness, in order to maintain a consistent wall thickness around the corner.

Draft – The part has to come out of the tool efficiently and cleanly, so it is important to add a draft angle to faces to allow the tool to eject the part efficiently. A draft on the part will ensure that you do not see deep ejector pin marks on the part or marks on the part where it has been forced out of the tool. As a general guideline, aim for 1 to 2 Degrees of draft when designing the part. Don’t forget to maintain the wall thickness when adding the draft angle.

Text – Having a name or logo on the part is a great way to advertise or to provide the part number for the part and it is easy to do on plastic parts. Generally, the text will need to be raised on the part and a final height of 0.020” (0.5mm) is a good height. Choose a font such as Century Gothic, Verdana or Arial.

A good plastic injection molding company who designs their own tools will be able to help you with the design of your part; but thinking about the elements mentioned above will help both you and they get off to a smart start.

Written by John the article was published in Medical Plastics News Magazine.

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