How is a moulded part shaped so it can actually be made well, cheaply and without defects?
Designing for manufacture: mould and pattern design, wall thicknesses, split lines, injection and ejector points, draft angles, location pins, fillets and radius corners, undercuts, shrinkage and thinning, integrated assembly features, and the purpose of bosses, ribs and webs.
An SQA Advanced Higher Design and Manufacture answer on designing for manufacture, covering mould and pattern design, wall thickness, split lines, draft angles, fillets and radii, undercuts, shrinkage and thinning, integrated assembly features, and the purpose of bosses, ribs and webs in moulded parts.
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What this key area is asking
The SQA wants you to know how a part is designed so it can be manufactured well, especially by moulding. This covers mould and pattern design, wall thickness, split lines, injection and ejector points, draft angles, location pins, fillets and radii, undercuts, shrinkage and thinning, integrated assembly features, and the purpose of bosses, ribs and webs. These features are common Section 2 explain questions, and you apply them when planning your assignment outcome.
The mould and how the part leaves it
Mould and pattern design is therefore the starting point: the designer decides how the part splits, where melt enters, where the part is ejected, and how it releases. Getting these right avoids defects and reduces tooling cost.
Wall thickness, shrinkage and the features that prevent defects
The biggest rule is uniform wall thickness. Even walls cool and shrink evenly; uneven sections cool at different rates, causing:
- sink marks (dimples over thick sections);
- warping (distortion from uneven shrinkage);
- internal stress that weakens the part.
So the designer avoids abrupt transitions between thick and thin walls, blends them gradually, and accounts for shrinkage (the part shrinks as it cools, so the mould is made slightly oversize) and thinning of sheet materials (in thermoforming, deep draws thin the sheet). Undercuts (features that trap the part in the mould) are designed out or need costly moving mould parts.
Fillets and radius corners round internal and external corners to remove sharp stress concentrations, ease melt flow, and strengthen the part.
Bosses, ribs, webs and integrated assembly features
Integrated assembly features build the joining into the part itself: moulded snap-fit clips, location pins and lugs that align and hold mating parts, cutting the number of separate fasteners and speeding assembly. Designing these in is a core way to simplify manufacture and assembly.
Where this fits in the course
Designing for manufacture connects the process to the part: the features here are what make moulding work. It depends on the material chosen, links to assembly methods through integrated features, and is applied through the CAD models in graphics and modelling.
Try this
Q1. Explain why a moulded part needs draft angles. [3 marks]
- Cue. A slight taper on faces parallel to the opening direction lets the part release cleanly; a straight wall would jam or scuff in the mould.
Q2. Explain two problems caused by uneven wall thickness in a moulded part. [4 marks]
- Cue. Sink marks over thick sections; warping from uneven shrinkage; internal stress that weakens the part.
Q3. Explain the purpose of a moulded boss. [2 marks]
- Cue. A raised cylindrical feature that takes a screw or locates a pin, giving a fixing point without a separate component.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Advanced Higher6 marksExplain the purpose of draft angles, fillets and uniform wall thickness in the design of an injection-moulded part.Show worked answer →
Worth about 6 marks, so the marker wants the purpose of each feature
linked to making the part well.
Draft angles. A slight taper on faces parallel to the direction the part
leaves the mould lets it release cleanly without sticking or scuffing,
because a perfectly straight wall would jam in the mould.
Fillets and radius corners. Rounding internal and external corners removes
sharp stress points, eases the flow of melt into the mould, and avoids
weak spots, giving a stronger part and a better moulding.
Uniform wall thickness. Keeping walls an even thickness lets the part cool
and shrink evenly, avoiding sink marks, warping and internal stress that
uneven sections cause.
Conclude. A strong answer states that these features make the part
easier to mould, stronger and free of defects, lowering cost and waste,
which is the aim of designing for manufacture.
SQA Advanced Higher4 marksExplain the purpose of bosses and ribs in a moulded plastic product.Show worked answer →
Worth about 4 marks. The markers want the purpose of each feature.
Bosses. A boss is a raised cylindrical feature moulded into the part to
take a screw or locate a pin, providing a fixing or assembly point without
a separate component.
Ribs. A rib is a thin wall moulded across a surface to stiffen and
strengthen the part without adding the weight, material and sink marks of
a thicker wall.
Why they matter. A strong answer notes that bosses and ribs add strength
and fixing points while keeping wall thickness uniform, so the part is
strong, light and free of moulding defects.
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