Wall Thickness Guidelines for Injection Molded Parts

Wall thickness is the single most influential geometric parameter in injection mold design. It determines how plastic flows through the mold, how the part cools, where shrinkage occurs, and whether the finished part will be dimensionally stable. Most of the defects that appear on injection molded parts : sink marks, warpage, short shots, and residual stress, trace back directly to wall thickness decisions made during design.

This guide covers the recommended wall thickness ranges by material, the rules for wall thickness transitions, how to handle ribs and bosses, and the specific defects caused by walls that are too thick, too thin, or non-uniform.

 

Why Uniform Wall Thickness Matters

Injection molded plastic cools from the outside in. Thicker sections retain heat longer than thinner ones. When a part has non-uniform wall thickness, different sections solidify at different rates. The result is differential shrinkage: areas that cool faster shrink to their final dimensions while adjacent thicker areas are still contracting.

This differential produces several problems. Residual stress builds at the boundary between thin and thick sections. The thicker section continues shrinking after the thinner section is rigid, pulling the surface inward and creating sink marks. If the stress is high enough, the part warps. In severe cases, the flow front freezes in thin sections before the thick sections are fully filled, producing short shots.

Uniform wall thickness does not mean every wall must be the same. It means transitions between different wall thicknesses must be gradual, and that thick local features like ribs and bosses must be designed to minimize their thermal mass relative to the nominal wall.

 

Recommended Wall Thickness by Material

Every thermoplastic has a practical wall thickness range based on its flow properties, shrinkage rate, and thermal behavior. The table below covers the six most commonly specified materials in engineering injection molding.

Material	Min Wall (mm)	Recommended Range	Max Practical	Common Defect at Excess
ABS	0.8	1.5-3.0 mm	4.0 mm	Sink marks, extended cycle time
Polycarbonate	0.9	1.0-3.5 mm	4.5 mm	Stress cracking, warpage
Nylon (PA6/66)	0.8	1.5-3.0 mm	3.5 mm	Excessive shrinkage, sink marks
Polypropylene	0.8	1.2-3.5 mm	4.0 mm	Warpage, poor surface finish
TPU / TPE	0.6	1.0-3.0 mm	4.0 mm	Tear on ejection, surface drag
Glass-filled nylon	1.0	2.0-4.0 mm	5.0 mm	Warpage, fiber orientation issues

 

Glass-filled materials need extra attention: glass fibers align with flow, creating anisotropic shrinkage. Glass-filled parts are more prone to warpage than unfilled equivalents, and wall thickness recommendations are wider to allow more packing and reduce differential shrinkage.

 

Wall Thickness Transitions

When wall thickness must change, the transition geometry determines whether the change causes a problem.

Gradual Tapers

The standard approach for thickness transitions is a gradual taper over a length at least three times the magnitude of the thickness change. For a wall transitioning from 2.0 mm to 3.5 mm, the taper should run over at least 4.5 mm (3 x 1.5 mm change). This allows flow to fill the transition smoothly and cooling to proceed without abrupt thermal gradients.

Core-Outs

When external geometry requires a thick section, a core-out removes material from the inside of the thick area without changing the external surface. This reduces the thick section to a shell closer to the nominal wall thickness, eliminating the sink mark risk while maintaining the external geometry. Core-outs are a standard approach for thick bosses, ribs that are too tall, and any area where functional geometry requires mass that would otherwise cause sink marks.

Radiused Transitions

At the junction between a nominal wall and a rib or boss, inside corners must be radiused. A minimum inside radius of 0.25 times the wall thickness prevents stress concentration and flow turbulence at the transition. Sharp inside corners are a common cause of cracking in areas that experience cyclic loading or thermal cycling.

 

Rib and Boss Design Rules

Ribs and bosses are the two features most commonly responsible for sink marks in injection molded parts. Both add local material that cools more slowly than the nominal wall, creating a shrinkage differential that pulls the opposite surface inward.

Rib thickness should be 50 to 60 percent of the nominal wall thickness. A part with a 2.5 mm nominal wall should have ribs no thicker than 1.3 to 1.5 mm. Rib height should not exceed three times the rib thickness. Taller ribs have flow and packing problems. Both dimensions require their own draft angle, typically 0.5 to 1 degree per side.

Boss wall thickness follows the same logic. The outer wall of a boss should be 60 percent or less of the nominal wall. Tall bosses require gusset support to the nearest wall rather than increased wall thickness. These rules, along with gate placement and draft angle requirements, form the core of DFM review for any injection molded part. RPM Fast reviews all incoming designs against these parameters before tooling begins. More details are in the DFM checklist for injection molded parts.

 

Defects Caused by Incorrect Wall Thickness

Each wall thickness violation produces a characteristic defect. Understanding the defect tells you what the thickness problem was.

Sink marks appear opposite thick features: ribs, bosses, or abrupt thick sections. The outer surface has solidified, but the core is still contracting. The surface is pulled inward by the shrinking interior. Fix: reduce rib thickness, add core-outs, or reduce packing time issues at the gate.

Warpage occurs when differential shrinkage across the part creates internal stress that deforms the part after ejection. Non-uniform wall thickness is the primary cause. Fix: redesign for uniform wall, reduce thick sections, adjust gate location and cooling channel placement.

Short shots occur when thin walls freeze before the mold fills. The flow front solidifies before reaching the end of the cavity. Fix: increase wall thickness in flow path, relocate gate, increase melt temperature or injection pressure within material limits.

 

Frequently Asked Questions

What is the recommended wall thickness for injection molded parts?

The recommended wall thickness range for most injection molded engineering thermoplastics is 1.5 to 3.0 mm. The specific minimum and maximum depends on the resin: ABS and nylon typically run 1.5 to 3.0 mm, polycarbonate 1.0 to 3.5 mm, and glass-filled materials 2.0 to 4.0 mm. Uniform wall thickness throughout the part is more important than the specific value.

What causes sink marks in injection molded parts?

Sink marks are caused by localized volumetric shrinkage during cooling. They appear opposite thick wall sections, ribs, and bosses where the core is still contracting after the outer surface has solidified. Prevention requires uniform wall thickness, rib thickness at 50 to 60 percent of the nominal wall, and adequate packing pressure during the injection cycle.

Can wall thickness vary in an injection molded part?

Wall thickness can vary, but transitions must be gradual. Abrupt thickness changes cause differential cooling rates that produce warpage, residual stress, and sink marks. A gradual taper over a length of at least three times the thickness change is the standard approach. Where a significant thickness change is unavoidable, a core-out reduces the thick section without changing the external geometry.

What happens if injection molded walls are too thin?

Walls that are too thin cause short shots: the mold does not fill completely before the plastic solidifies and the flow front freezes. Thin walls also reduce part strength, make ejection more difficult, and can tear or deform during demolding. The minimum reliable wall thickness for most thermoplastics is 0.8 to 1.0 mm on well-optimized tooling, though 1.5 mm is the practical minimum for most production parts.

 

Applying Wall Thickness Rules

Wall thickness decisions made at the concept stage cost nothing to change. The same decisions made after tooling is cut cost thousands and weeks. Reviewing a part against the guidelines in this post before DFM review catches the issues that most commonly require mold modifications.

RPM Fast is ISO 9001:2015 certified and reviews every submitted design for wall thickness, rib geometry, and boss design before any tooling is cut. If your design is ready for review, request a quote from RPM Fast and we will provide DFM feedback and a quote within 1 to 2 business days.