The Golden Rule: Why Uniform Wall Thickness is Key to Quality Parts
By Ichiplas Design Engineering Team · Oct 2025 · Updated Nov 2026 · Reviewed by the Ichiplas engineering team
You've designed a part with a beautiful, flat, Class-A exterior surface. You've applied the correct draft angles. Yet, when the first parts come out of the machine, you see ugly indentations and the whole thing is slightly twisted.
What went wrong? The likely culprit is violating the golden rule of part design: maintaining a uniform wall thickness.
Plastic is not a static material. As it cools in the mold from a molten state, it shrinks. The core principle to understand is that thicker sections of plastic cool slower and shrink more than thinner sections. This differential shrinkage is the root cause of two of the most common cosmetic defects: sink marks and warping.
The Enemy: Sink Marks and Warping
Sink Marks:
A sink mark is a small depression or dimple on the surface of a part. It occurs when a thick section (like the base of a mounting boss or a support rib) is designed opposite a flat, cosmetic wall. As the thick internal section cools and shrinks, it pulls the still-soft material on the outer surface inward, creating the sink mark.
Warping:
Warping happens when different areas of a part cool and shrink at drastically different rates. This creates internal stresses that physically pull and twist the part out of its intended shape as it's ejected from the mold. A part that was perfectly flat in your CAD model can come out looking like a potato chip.
The Solution: Consistent Wall Thickness
The best way to ensure your part cools and shrinks uniformly is to design it with a consistent wall thickness throughout. In CAD software, this is often achieved with the “Shell” command, which hollows out a solid model to a specified thickness.
When you need to add internal strengthening features, you don't make the wall thicker. Instead, you design thin ribs and bosses that provide the necessary structure without creating a large mass of plastic that will cause sink. We have a dedicated article on designing those features correctly.
Recommended Wall Thickness by Material
These are typical nominal wall ranges for unfilled grades. Stay near the middle of the range for most parts; go thinner only with short flow lengths, and thicker only where structure genuinely demands it.
| Material | Typical wall (mm) | Typical wall (in) | Notes |
|---|---|---|---|
| ABS | 1.2 – 3.5 | 0.045 – 0.140 | Forgiving; good general-purpose choice |
| PC | 1.0 – 3.8 | 0.040 – 0.150 | Needs higher pressure; sensitive to sharp corners |
| PC/ABS | 1.2 – 3.5 | 0.045 – 0.140 | Behaves between ABS and PC |
| PP | 0.8 – 3.8 | 0.030 – 0.150 | High shrink; very prone to sink on thick sections |
| PE (HDPE/LDPE) | 0.8 – 3.0 | 0.030 – 0.120 | High shrink; warps if walls vary |
| Nylon (PA6/PA66) | 0.8 – 3.0 | 0.030 – 0.120 | Flows well into thin walls |
| POM (acetal) | 0.8 – 3.0 | 0.030 – 0.120 | High shrink; keep walls very consistent |
| PBT | 0.8 – 3.0 | 0.030 – 0.120 | Good for thin-walled connectors |
| PMMA (acrylic) | 0.6 – 3.8 | 0.025 – 0.150 | Thick sections risk voids in clear parts |
| TPE / TPU | 0.5 – 3.0 | 0.020 – 0.120 | Often overmolded over a rigid substrate |
Guideline values for unfilled resins. Glass-filled grades, very long flow paths and thin-wall packaging follow different rules — confirm with the resin datasheet or a mold flow analysis.
Frequently Asked Questions
What is the ideal wall thickness for an injection molded part?
Most injection molded parts work best with a nominal wall between about 1.5 mm and 3 mm (0.06–0.12 in). The right value depends on the material, how far the plastic must flow from the gate, and how much stiffness the part needs. Choose the thinnest wall that meets the structural requirement, then keep it consistent across the part.
How much can wall thickness vary within one part?
As a rule of thumb, keep wall thickness within about 10–15% of the nominal value. Where a change is unavoidable, transition gradually with a taper roughly three times as long as the thickness difference, rather than with an abrupt step. Abrupt steps cause sink marks, warpage and flow hesitation.
What happens if a plastic part wall is too thick?
Thick walls cool slowly and shrink more than the surrounding material, causing sink marks, internal voids and warpage. They also lengthen the cycle: cooling time rises roughly with the square of wall thickness, so doubling a wall can roughly quadruple cooling time and significantly increase unit cost.
What happens if a wall is too thin?
Walls that are too thin for the flow length can freeze before the cavity is full, causing short shots, hesitation marks and weak weld lines. Filling them needs higher injection pressure, which can require a larger press and increases molded-in stress.
How do I fix a thick section without changing the outside of the part?
Core it out. Remove material from the non-cosmetic side so the section matches the nominal wall, and use thin ribs or gussets to restore any stiffness that was lost. This keeps the external geometry identical while eliminating the mass of plastic that causes sink and voids.
What's Next?
You know your part needs to be hollowed out and have consistent walls. But what about adding features like clips or holes that seem to break the rules? In our next article, we explore how to do it correctly: Designing for Complexity: An Engineer's Guide to Undercuts.
Getting your design ready for production can feel complex. Our team in Johor Bahru lives and breathes this process every day. If you want to ensure your design is optimized for manufacturing from the start, reach out to us. We're here to help.
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