In our experience in injection mould tooling, it’s important to fully validate part designs prior to manufacture. This prevents any defects or issues with a mould tool’s build that can negatively affect it’s performance. Which have knock on effects including surplus costs in revisions as well as project timeline delays.
Applying a thorough Design for Manufacture (DFM) process, where tool designs are evaluated in detail prior to manufacture ensures that oversights are avoided and issues resolved early.
One particular aspect of a tool’s design we look for is draft angles.
What are draft angles?
Draft (taper) is often applied to vertical faces in plastic component designs to help a part eject cleanly from a mould tool.
Because thermoplastics shrink as they cool, a plastic component tends to grip to the inside of a mould, making the introduction of draft a necessity.
Typical draft is usually as little as 0.5° to 2°.
What happens when draft angles are not accounted for
When a part does not have appropriate draft angles designed into the vertical faces of a part design, a few common mishaps happen:
- The parts come out scratched and/or with other surface defects because they do not eject cleanly from the mould.
- Parts can stick to the fixed half of the mould and pull over when the tool opens.
- Unintended wear and tear on the mould, reducing it’s lifespan and performance.
It can be surprising to people who are not familiar with the injection mould tooling process at how seemingly minute details such as draft can contribute to large issues.
How would I know how much draft to apply?
The necessary draft angle to apply to custom injection mould components depends on various factors such as part geometry, surface finish and material type.
Part geometry- Deeper walls create more surface contact with the mould tool where a larger angle is needed to break the vacuum quickly during ejection.
A good rule of thumb is to add 1° of draft for per 25mm (1 inch) of depth.
Surface finish- Surface finish affects the moulding process because textured surfaces can grip the mould, requiring more draft than a smooth surface which would release easier from a mould.
Alternatively, textured surface depth can be reduced if further draft angles can’t be achieved.
Material type- The polymer of the plastic component also greatly impacts how much draft angle is necessary. Some core rules of material considerations are:
- High shrinkage materials require a larger draft because they grip mould cores more tightly.
- Sticky or flexible materials require larger draft angles to prevent material damage such as tearing or stretching during ejection.
- Rigid materials still require draft angle, as scratches on part surface and mould can occur if draft angle is too low.
DFM process
Draft angles are just one part of our extensive Design for Manufacture (DFM) process, where we rigorously check part designs prior to manufacture. This includes 3D CAD analysis, and Mould Flow Analysis- we wrote more about what we use to validate injection mould components here.
With such an array of elements to take account for, it can feel overwhelming for a smaller team or perhaps a less experienced team to feel fully confident before undergoing the expensive process of cutting steel and beginning manufacture.
It’s always worth receiving a second opinion and oversight on a project like this, even just for peace of mind.
And if you’re considering going direct to a supplier/manufacturer, it’s even more important to ensure everything is correct beforehand.
If you’re currently working on a component design, you can fill in this quick form to receive a free evaluation and review of your design.
