Machine Design #94: Plastic Wall Thickness — Uniformity Matters More Than Being Thick
Plastic wall thickness must keep function, accuracy, manufacturability, inspectability, and serviceability when the boundary changes.
Function before geometry
Write the input condition, expected result, acceptance limit, failure symptom, and measurement method before selecting a pair, joint, draft, or wall. Assign an owner to every value and change.
Core checks
- Functional stiffness and minimum wall: record value, source, method, owner, and pass/fail evidence.
- Uniform thickness and transition radius: record value, source, method, owner, and pass/fail evidence.
- Flow length, gate, weld line, and air trap: record value, source, method, owner, and pass/fail evidence.
- Sink, warpage, cooling, and fiber direction: record value, source, method, owner, and pass/fail evidence.
- Measurement and molding capability: record value, source, method, owner, and pass/fail evidence.
- Design change and tool trial evidence: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Pair or joint undefined | Wrong fit or replacement | Audit identity and assembled function |
| Process omitted | Warp, gap, sink, or mismatch | Walk the process and measure |
| Documents out of sync | Correct name, wrong revision | Baseline every reference |
Why uneven thickness creates defects
Molten plastic is injected into the mould, cools and shrinks. A thick region cools more slowly than a thin one, so it shrinks later — by which time the outer skin has already frozen. The result is one of two things:
| Phenomenon | Where it appears | How it shows |
|---|
| Sink mark | On the outer surface opposite the thick region | A depression visible under raking light, spoiling the appearance |
| Void | Inside the thick region | An internal cavity that weakens the part while looking perfect from outside |
On a load-carrying part a void is far more dangerous than a sink mark: it does not show up in a visual inspection but it reduces the section that actually carries load.
This is the reason behind "uniform matters more than thick": adding thickness to be safe usually backfires — the added thickness is exactly where voids and sink marks originate.
If it needs to be stiffer, add ribs, not thickness
| Way to add stiffness | Consequence |
|---|
| Increase wall thickness | Creates sink marks and voids, lengthens cooling time, increases material and cost |
| Add ribs | Far more effective stiffness per unit of material, keeps the wall uniform |
| Add flanges, turned edges, curved faces | Stiffens without creating a local thick region |
| Change to a stiffer plastic | More expensive, and does not solve the uneven thickness problem |
Ribs give much better stiffness than the same amount of material spread into a thicker wall — but the rib has to be proportioned to the wall, otherwise the rib root itself becomes the thick region that produces a sink mark.
Thickness transitions have to be gradual
When thickness has to change, do not change it abruptly. A square step both obstructs the flow and creates a stress concentration. The remedy is to taper or blend over a length long enough for the plastic flow and the cooling rate to change gradually.
The general rule: go from thick to thin along the flow direction, and keep the thickness steady along the path the plastic travels. Plastic flowing from thin into thick produces defects more readily than the other way round.
Weld lines: an invisible weak spot
Where two flow fronts meet again — after a hole, around a pin, between two gates — there is a weld line. That region bonds less strongly than the rest and can be the first place to crack.
Three things the design can do: place holes and pins so the weld line does not fall in the main load-carrying area; keep the thickness uniform so the flow does not split early; and discuss gate positions with the mould maker early, because the gate decides the flow and therefore where the weld line ends up.
Handling thick sections and transitions: core it out, do not leave it solid
Knowing that uneven thickness causes defects is one thing; handling a spot that has to be thick is another. The basic move is coring: hollow out the thick mass so the wall returns to near the nominal thickness, instead of leaving a solid lump that cools slowly and sinks.
- Solid mass → hollow it out: a thick boss or rib should be cored into a shell of even thickness,
keeping a stiffening rib where rigidity is needed, rather than left solid.
- Gradual transitions: where thickness changes, blend it over a ratio of about 3:1 (one part of
thickness change over three of length), not an abrupt step — a step makes a sink mark and traps flow.
- Radius the inside corners: a sharp inside corner concentrates stress and disturbs flow; blend a
radius of about 0.5 times the wall thickness.
Flow-length-to-thickness ratio and gate location
The melt only fills the mould over a certain distance before it cools and freezes. The flow-length to wall-thickness ratio (L/T) tells you how far one gate can fill; a thinner wall allows a smaller L/T, so you place the gate closer or add gates.
Design consequences:
- A wall too thin for the fill distance short-shots; fix it by thickening the flow path slightly, or by
adding a gate.
- Put the gate in a thick zone so flow runs thick-to-thin, not thin-to-thick (the thick zone cools last,
draws melt away, and forms voids).
- The gate location leaves a mark and a weld line where two flows meet; choose it for both appearance and
strength, because a weld line is weaker than the base material.
MINATA release checklist
- [ ] Function, boundary, pair or feature, and failure symptom are written.
- [ ] Datums, ownership, process, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, inspection, and maintenance were tried.
- [ ] Revision, supplier, material, and configuration records agree.
Good engineering is an explicit chain that survives manufacture, operation, maintenance, and change. For plastic wall thickness, evidence that accuracy and function remain reliable is the MINATA standard.
Frequently asked questions
Why does a plastic part show a depression on its outer face?
Because behind that spot the material is thicker than elsewhere — usually a rib root, a boss root or a solid block. The thick region cools slowly and shrinks after the outer skin has frozen, pulling the surface inward. The remedy is to reduce the thick region, not to raise the holding pressure.
Can wall thickness be increased to make a part stiffer?
Usually it backfires: the added thickness produces sink marks and voids, lengthens the cooling time and raises cost. Ribs or flanges give far better stiffness for the same material while keeping the wall uniform.
Are internal voids dangerous?
On a load-carrying part they are more dangerous than sink marks, because they do not show in a visual inspection but reduce the real load-carrying section. A part that looks perfect can still hold a cavity inside the thick region.
How should a change in wall thickness be made?
Tapered or blended over a sufficient length, never as an abrupt step. A square step both obstructs the plastic flow and creates a stress concentration — two problems at once in exactly one place.
Where do weld lines sit and can they be avoided?
Where two flow fronts meet again, usually after a hole or around a pin. They cannot be avoided entirely, but they can be moved: place holes and pins so the weld line does not fall in a load-carrying area, and discuss gate positions with the mould maker early.
Frequently asked questions, continued
How do I handle a thick, solid plastic mass?
Core it out so the wall returns to near the nominal thickness, keeping a stiffening rib if rigidity is needed. Left solid, the mass cools slowly, sinks on the opposite face, and tends to form internal voids.
Should a thickness change be a step or a gradual blend?
A gradual blend, at a ratio of about 3:1. An abrupt step makes a sink mark and traps flow; a gradual blend lets the melt fill evenly and reduces stress at the section change.
Why put the gate in a thick zone?
So flow runs thick-to-thin. If you gate thin-to-thick, the thick zone cools last, draws melt away, and forms voids. Remember the gate also leaves a mark and a weld line, so choose the location for both appearance and strength.
A quick table for the shop floor
Wall thickness is easier to set when the situation, the way it is given and the purpose are lined up:
| Situation | How to give or control it | Purpose |
|---|
| Main walls | Keep nearly uniform along the flow | Reduce sink and warp |
| A change in thickness | Transition it with a radius | Avoid hot spots |
| A stiff area | Add a rib instead of more mass | Shorten the moulding cycle |
A worked case
Doubling the wall around a screw hole usually makes a sink mark appear on the outer surface. The better fix is a cored boss tied in with ribs, keeping the thickness ratio the material recommends.
Conclusion
Plastic Wall Thickness is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
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