Many injection molded parts fail design review for the same reason:
Wall thickness.
The CAD model may look correct. The geometry may even be mathematically valid.
But once a manufacturing engineer reviews the part, the first question is almost always:
Are the wall thicknesses consistent?
Wall thickness directly affects:
- mold filling
- cooling behavior
- shrinkage
- warpage
- cycle time
Inconsistent walls are one of the most common reasons parts require redesign before tooling.
Why Wall Thickness Matters in Injection Molding
Injection molding works by forcing molten polymer into a closed mold cavity.
The plastic must:
- Flow through the cavity
- Fill the geometry completely
- Cool uniformly
- Shrink predictably during solidification
When wall thickness varies too much, these conditions break down.
Common consequences include:
- sink marks
- warping
- voids
- short shots
- long cycle times
Manufacturing engineers typically look for uniform wall thickness because it allows the plastic to cool evenly.
Uneven cooling is one of the primary drivers of part distortion.
Sources:
- Protolabs Design Guide for Injection Molding — protolabs.com/resources/design-tips/injection-molding
- Xometry Injection Molding Design Guide — xometry.com/resources/injection-molding
Recommended Wall Thickness Ranges
Different plastics have different flow characteristics and structural requirements.
Typical ranges recommended by manufacturers include:
| Material | Typical Wall Thickness |
|---|---|
| ABS | 1.2 – 3.5 mm |
| Polycarbonate | 1.0 – 3.8 mm |
| Nylon | 0.8 – 3.0 mm |
| Polypropylene | 0.8 – 3.8 mm |
| Polyethylene | 0.8 – 3.0 mm |
These values appear consistently across multiple injection molding design guides.
Sources:
- BASF Plastics Design Guide — plastics-rubber.basf.com
- Protolabs Material Design Guidelines — protolabs.com/resources/design-tips
The exact thickness depends on:
- material viscosity
- part size
- structural loads
- molding process parameters
But consistency is more important than the exact number.
What Happens When Walls Are Too Thick
Designers often increase thickness to improve strength.
But thick sections introduce manufacturing issues.
Sink Marks
When thick areas cool slower than surrounding material, the outer surface solidifies first.
The inner material continues to shrink, pulling the surface inward.
Result: Visible depressions called sink marks.
This is especially common near:
- ribs
- bosses
- reinforced corners
Source: Autodesk Moldflow explanation of sink marks — knowledge.autodesk.com/support/moldflow
Internal Voids
In thick sections, the center of the part may cool too slowly.
This can create voids inside the plastic. These are not always visible externally but can weaken the part.
Longer Cycle Times
Thicker plastic takes longer to cool.
Cooling often represents 60–80% of the injection molding cycle time.
Even small thickness increases can significantly increase manufacturing cost.
Source: Rosato & Rosato – Injection Molding Handbook.
What Happens When Walls Are Too Thin
Very thin walls create a different set of problems.
Molten plastic may:
- cool before the cavity fills
- require excessive injection pressure
- create incomplete parts
This often leads to short shots, where parts are only partially formed.
Thin sections are especially problematic in: large parts, long flow paths, and high-viscosity materials.
The Real Problem: Thickness Transitions
The biggest issue is usually abrupt changes in wall thickness.
Example: A 2 mm wall connected directly to a 5 mm section.
The thick region cools slower and shrinks differently. This creates:
- warpage
- internal stress
- cosmetic defects
Instead, designers should transition gradually between thicknesses.
Typical rule of thumb: use gradual transitions (e.g. 3:1 ratio) between different thicknesses to avoid stress and sink.
Source: Xometry Injection Molding Design Guidelines — xometry.com/resources/injection-molding
Ribs Instead of Thick Walls
A common strategy is to replace thick sections with ribs.
Instead of making a wall thicker: add reinforcing ribs on the interior.
Typical rib guidelines:
- rib thickness ≈ 50–60% of the wall thickness
- proper draft angles
- smooth transitions
This maintains strength without creating sink marks.
Source: Protolabs Design for Injection Molding Guide — protolabs.com/resources/design-tips/injection-molding
Why These Problems Often Appear Late
Most CAD systems validate geometry, not manufacturability.
So a model with inconsistent walls, thick bosses, or abrupt transitions may pass design review.
The problem only appears during:
- supplier DFM review
- mold design
- moldflow simulation
At that point, redesigns can delay tooling by weeks.
Checking Wall Thickness Before Manufacturing
Before sending parts to tooling or suppliers, teams typically verify:
- minimum wall thickness
- thickness variation
- rib proportions
- boss thickness relative to surrounding walls
These checks are part of DFM pre-flight. Instead of discovering problems during mold design, engineers validate them earlier in the workflow.
The Bigger Picture: Manufacturing Readiness
Wall thickness is only one example of a manufacturability check.
Before production, engineers also validate: draft angles, undercuts, geometry integrity, and export errors in STEP files.
If you're interested in geometry issues that break manufacturing workflows, see: