Design Guide: Injection Moulding

Plastic parts are needed everywhere, and Injection Moulding is the most popularly manufacturing method for producing these parts. It is neither subtractive or additive but a formative process that shapes molten plastic within a mold cavity, producing complex parts in a single step with high repeatability and precision, making it ideal for mass production.

Plastic injection molding is primary a mid-to-high volume part production process, but it can also be adapted for low-volume runs using a technique called Rapid Injection Moulding. When done right, it produces high-quality parts with excellent surface finish and reliable mechanical properties.

To take full take advantage of this technology, it is important that your 3D model follows a set of established design guidelines. This article covers key injection moulding process considerations, design guidelines, cost reduction tips and summary of the best design practices.

About the Injection Moulding Process

The process uses three essential components—the mould (or tool), resin material, injection mould machine. Using a CAD model (.STEP format preferred), a mould is created and machined with precision to form the part’s features. Next, it is installed into an injection mould press and molten plastic is injected into the mould tool under high pressure. The part is then cooled, ejected and the process repeats.

Injection Moulding Process Considerations

• Material Flow: Proper flow is essential for completely filling the mould cavity. If the material flow is too quick and thin, it can cause flashing or air entrapment. If it is too slow and thick, the flow may lead to incomplete filling or weld lines.
• Shrinkage and Warping: When certain sections cool and shrink at different rates in different directions, it can result in permanent bents in parts due to internal stresses.
• Part Ejection: During ejection, the walls of the part will slide and scrape against the mould, which can result in drag marks.

Injection Moulding Design Guidelines

Wall Thickness

Recommended wall thickness for common resins available at HLH Rapid:

Ribs & Gussets

Ribs are thin, raised features incorporated in injection moulded parts to increase stiffness and strength. However, if not properly designed, ribs can make parts prone to sink marks. The maximum rib thickness should not exceed 0.5 times the nominal wall thickness, and the rib height should be limited to less than three times its thickness.

Gussets are rib-like features that do not extend all the way across a surface. They are used to increase strength and stiffness at stress concentrations without adding too much extra material.

Draft Angles

Threads and Bosses

For best results, avoid adding threads directly on your injection moulded part. Threaded inserts can be overmoulded into the part.

Corners

To avoid flaws on injection moulded parts, avoid sharp corners and round all edges. When designing internal edges, experts recommend using a radius of at least 0.5 times the wall thickness.

Text and Logos

Because a moulded part is essentially the inverse of its machined metal tooling, the rules on engraving/embossing are reversed. For injection moulding, choose embossed over engraved text, as it’s easier to CNC machine such detail into the mould tool. We recommend a minimum height of 0.5mm to ensure readability. Use a font with uniform thickness and a minimum font size of 20 points for better results.

Summary of Best Design Practices

Cost Reduction Tips

When using injection moulding, you have to be prepared to bear the high initial investment associated with making the hard tools. While it is a given that tooling will be costly, there are several ways you can optimize your injection mould design to save on costs:

• Eliminate Undercuts: Avoid undercuts where possible, as they’ll add to the complexity and cost of the tool.
• Choose Embossed Over Engraved: It’s easier to CNC machine text into the mould tool, as it involves the removal of less material which overall translates to less cost.
• Simplify As Much As Possible: Unless it adds value to the function or quality of your design, avoid overly complex features, as they often drive costs up unnecessarily.