What Is a Wall Thickness?

Wall thickness is the distance between the opposite surfaces of a part's wall. In a molded part it is the nominal thickness the whole design is built around, and the rule is to keep it as thin and uniform as the function allows, because thick or uneven sections cool unevenly and cause sink marks, voids and warpage.

In short: wall thickness decides how a part fills, cools and shrinks, so it drives both part quality and cycle time.

What Is Wall Thickness in Injection Molding?

Designers start with a nominal wall, the base thickness of the shell, and then add secondary features such as ribs, bosses and holes to it. Most molding problems trace back to places where that nominal wall changes suddenly.

The reason is cooling. Plastic shrinks as it cools, and a thick section keeps cooling and shrinking after the thinner walls around it have set. That uneven shrinkage pulls the surface inward (a sink mark), leaves a hollow inside the part (a void), or bends the part out of shape (warpage).

Thickness also sets the cycle. Cooling time generally dominates the injection molding cycle, and a thicker wall takes longer to cool before the part can be ejected. MIT's injection molding course notes give three reasons to keep walls thin and uniform: shorter cycle time, better dimensional stability and fewer surface defects. They suggest holding the nominal wall within ±10%.

Why Not Just Make the Wall Thicker?

A thicker wall is stiffer, but it is rarely the best way to add stiffness. The same course notes list three options: increase the thickness, switch to a stiffer grade (glass-filled polypropylene, for example), or add ribs. Ribs usually win, because they add stiffness without creating a thick section.

Ribs need care too. Where a rib meets the wall, the material is locally thicker. One US patent on molded thermoplastic articles notes that a sink mark forms opposite a rib once it exceeds about 25% of the wall thickness in thermoplastic olefins, and about 40% in many other plastics. That is why ribs are designed noticeably thinner than the wall they support.

Minimum Wall Thickness by Process

The right wall depends on the process as much as the material. These are the minimum wall figures published on NICE Rapid's service pages:

Process	Minimum wall thickness
LSR (liquid silicone rubber) injection molding	0.4 mm (0.016 in)
CNC machining	0.5 mm (0.020 in), material dependent
Pressure die casting	0.8 mm (0.031 in) aluminum; 0.5 mm (0.020 in) zinc
Compression molding	1 mm (0.039 in) recommended
Blow molding	0.5–4 mm (0.020–0.157 in) wall range

How to Design Wall Thickness

• Pick one nominal wall early and design everything else relative to it.
• Where thickness must change, taper it gradually instead of stepping it.
• Core out thick areas such as solid bosses and heavy corners instead of leaving them solid.
• Add stiffness with ribs and gussets, kept thinner than the wall they support.
• Draft both faces of a wall in the same direction so the thickness stays constant.

Wall thickness is one of the first things checked in the DFM review that opens every plastic injection molding project.

Frequently Asked Questions

What is wall thickness in injection molding?

It is the thickness of the part's shell between its inner and outer surfaces. Designers set one nominal wall and keep it as thin and uniform as possible, because thick or uneven sections cool unevenly and cause sink marks, voids and warpage.

What does nominal wall thickness mean?

Nominal wall thickness is the base thickness a molded part is designed around. Ribs, bosses and other features are sized relative to it, and a common guideline is to hold the nominal wall within about ±10% across the part.

Why is uniform wall thickness important?

Because plastic shrinks as it cools, and thick sections cool more slowly than thin ones. Uneven walls shrink unevenly, which causes sink marks, internal voids and warpage, and the thickest section also sets how long the cycle has to be.

Related Services at NICE Rapid

• Plastic Injection Molding — DFM review first; wall thickness, along with part complexity and material, determines cycle time and tooling requirements.
• LSR Molding — wall thickness, parting line, gate location and draft are checked for silicone-specific flow; minimum wall 0.4 mm.
• Request a Quote — send a STEP file and our engineers will flag thick sections and abrupt wall changes.

Sources

1. MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — nominal wall "as thin and uniform as possible" to shorten cycle time, improve dimensional stability and eliminate surface defects; nominal wall within ±10%; reinforcement by thickness increase, stiffer grade or ribs — ocw.mit.edu
2. MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 02: Injection Molding I (Spring 2025) — "Cooling time generally dominates cycle time" — ocw.mit.edu
3. US Patent 7,846,533, Background — sink marks form when a projection exceeds about 25% of the wall thickness (thermoplastic olefin) or 40% (other thermoplastics) — patents.google.com
4. Mitigation of Sink Voids in Thick-Walled Thermoplastic Components via Integrated Taguchi DOE and CAE Simulations, Polymers (PMC) — sink marks and sink voids as common flaws in thick-walled molded parts — pmc.ncbi.nlm.nih.gov