Views: 0 Author: Site Editor Publish Time: 2026-06-10 Origin: Site
In the product design phase, wall thickness is often one of the most overlooked parameters. Many product managers and designers think: "A little thicker means a stronger product" or "A local thick area doesn't matter, it won't affect the look."
This is a dangerous misconception.
Uneven wall thickness—especially locally thick or thin sections—is one of the most common causes of part rejection in injection molding. It doesn't make your product fail immediately. Instead, it triggers a chain reaction: sink marks, warpage, voids, dimensional instability — and eventually, scrap.
Today, let's break down what uneven wall thickness really does to your parts, and how to avoid these pitfalls at the design stage.
What you see: Visible depressions on the surface above thick sections (sink marks), or internal holes when you cut the part (voids).
Why it happens: Plastic shrinks as it cools. Thick sections cool slowly. The surface solidifies while the core is still molten. As the core shrinks, it pulls the surface inward, creating a dent. In severe cases, the core pulls away from itself, creating a void.
Consequences:
Cosmetic parts are rejected (sink marks cannot be hidden)
Structural strength is reduced (voids become stress concentration points)
Typical locations: Rib roots, boss roots, junctions between bosses and walls.
What you see: Flat surfaces become curved, right angles become obtuse, round parts become oval — right out of the mold.
Why it happens: Different wall thicknesses cool at different rates. Thin sections cool and solidify first. Thick sections cool later. This time difference creates residual stress inside the part. When the part is ejected, the stress releases and the part deforms.
Consequences:
Assembly problems (misaligned snap-fits, offset screw holes)
Functional failure (leaking seals, stuck moving parts)
Unacceptable appearance
Typical locations: Flat parts with uneven thickness between edge and center, housings with uneven rib patterns.
What you see: Burn marks, black spots, short shots, or internal bubbles in specific areas.
Why it happens: When the melt flows, a sudden change in wall thickness or irregular flow front can trap air inside the cavity. The compressed air heats up dramatically (over 200°C / 390°F), burning the plastic (black spots) or blocking the melt from filling the area (short shot).
Consequences: Immediate scrap — and this usually shows up on the very first trial molding.
Typical locations: Abrupt transitions from thick to thin, meeting points of multiple melt flows.
What you see: Cracks appear during ejection, assembly, or after days/weeks in storage or use.
Why it happens: Residual stress from uneven wall thickness, combined with environmental factors (chemicals, UV, heat), exceeds the material's strength at a stress concentration point.
Consequences: Safety hazard — especially for load-bearing or enclosure parts.
Typical locations: Around metal inserts, sharp corners, roots of abrupt wall thickness changes.
A customer designed a handheld device housing. The nominal wall thickness was 2.0 mm. But at the base of several screw bosses, they increased the wall to 5.0 mm for extra strength — with no radius transition.
Trial molding results:
Circular sink marks (Ø8 mm, 0.3 mm deep) appeared on the exterior surface directly above the bosses — impossible to hide with painting.
Overall part warpage was 0.8 mm — exceeding the assembly tolerance.
Customer insisted: "Do not change the drawing. This small difference doesn't matter."
Results:
We built the tool as requested. Mass production yield: only 65% .
The customer had to sort every batch. Unit price increased by 40%.
Three months later, the customer came back and approved a tooling modification. Cost: $5,500 USD. Production downtime: 45 days.
If they had optimized the wall thickness from the beginning:
Add a "volcano" structure (material reduction groove) at the boss base
Keep local wall thickness ≤ 2.5 mm
Add R-corners for transition
Extra cost: less than $300
Expected yield: 95%+
Keep wall thickness as uniform as possible. Target: ≤ 25% variation from nominal. For a 2.0 mm nominal wall, the maximum should not exceed 2.5 mm.
Material | Recommended Range | Minimum |
|---|---|---|
ABS | 1.5 – 3.0 mm | 0.8 mm |
PC (Polycarbonate) | 1.8 – 3.0 mm | 0.8 mm |
PP (Polypropylene) | 1.5 – 3.5 mm | 0.6 mm |
PA (Nylon) | 1.0 – 3.0 mm | 0.5 mm |
POM (Delrin/Acetal) | 1.5 – 3.0 mm | 0.6 mm |
Rib thickness = (0.4 – 0.6) × nominal wall thickness
Rib height ≤ 3 × rib thickness
Always add an R-corner at the rib base (R ≥ 0.25 × rib thickness)
Outer diameter = 2 × inner diameter (screw size)
Add a volcano structure (material reduction groove) at the base to keep local wall thickness ≤ 1.5 × nominal
Add gussets / supporting ribs around tall bosses
If changing from T1 to T2, and T2 > 1.5 × T1, use a tapered transition
Transition length L ≥ 3 × (T2 – T1)
Add R-corners at both ends
If your design is locked and cannot be changed due to assembly, appearance, or functional reasons, here are four ways to mitigate the risk:
Method | How It Works | Best For |
|---|---|---|
Optimize gate location | Place gates in thick areas, fill thin areas last | Parts with thick-thin transitions but gate not yet fixed |
Adjust processing parameters | Lower injection speed, higher pack pressure, longer cooling time | Mild sink marks or warpage |
Use foam / micro-foam injection | Gas counter-pressure or chemical foaming reduces stress and sink | Parts where slight surface swirl marks are acceptable |
Accept a tooling modification | Remove material from thick sections ("de-thickening") on the mold | The most reliable fix — moderate cost |
Uneven wall thickness is not a small issue.
It won't show up on your 3D CAD screen. It won't appear in your beautiful renderings. But it will show up at T0 trial molding — in the most direct and brutal way possible:
The 5 minutes you saved in design will become thousands of dollars in tooling modifications, double-digit percentage yield loss, and burned trust with your customer.
So the next time you design an injection molded part, remember:
Uniform wall thickness is the #1 rule
Local thickening is not the same as local strengthening
Ribs are the correct way to increase strength
Volcano structures and R-corners are your best friends
If you're unsure about your design, send your drawing to your mold maker for a Design for Manufacturing (DFM) analysis. That one hour of review will be the highest-ROI time you spend on your entire project.
