After working on injection molded parts for so many years, do you really understand wall thickness design?
Many engineers, upon hearing words "wall thickness," think it's simple—just a little thicker or thinner, right? But true experts know: Wall thickness is foundation of injection molded part structural design.
It directly determines product's manufacturing feasibility, final quality, cost, and performance. Today, let's talk about the most basic and important—general design principles.

 

Core Design Principles
Three principles form theoretical foundation of injection molding wall thickness design:
Uniformity Principle: Primary principle for avoiding most injection molding defects. The more uniform wall thickness distribution, the fewer defects.
Appearance defects: shrinkage marks, porosity, weld lines; Performance issues: warpage, stress concentration; Process issues: uneven cooling, prolonged cycle time.
Rationality Principle: Wall thickness must comprehensively consider external forces, supporting functions, structural characteristics, and material properties.
Determine wall thickness requirements based on external forces and functions; strive for thinnest possible wall thickness while meeting performance requirements; column positions and overhanging structures require separate verification.
Economy Principle: Achieving a balance between "too thin" and "too thick," considering both cost and manufacturability.
Too thin: difficulty in filling, insufficient strength; Too thick: material waste, prolonged cooling; Optimal: the thinnest wall thickness that meets functional requirements.
Principle 1: Uniformity—Uneven Wall Thickness is Root of All Evil
This is the first ironclad rule for avoiding most injection molding defects.
Ideal wall thickness distribution is that thickness of any cross-section of product is as uniform as possible. Once wall thickness is uneven, it will cause three types of problems:
① Appearance Defects: Shrinkage marks (dents), porosity, weld lines, uneven color... Nine times out of ten, these surface problems are caused by abrupt changes in wall thickness.
② Performance and Dimensional Issues: Warping, dimensional instability, internal stress concentration, and weakened strength and rigidity—these problems are often more serious than appearance defects because they affect the product's functionality and lifespan.
③ Process Issues: Inconsistent cooling times, extended molding cycles, and filling difficulties directly impact production efficiency and cost.
For example: A product's main body wall thickness is 2mm, but a section is suddenly thickened to 5mm—shrinkage marks will almost certainly appear in this area. This is because thicker wall cools slower and shrinks more, creating a significant height difference with thinner section.
Therefore, uniformity is primary principle in wall thickness design. Without exception.
Principle Two: Rationality—Thicker Isn't Always Better. Wall thickness needs to be determined comprehensively based on following factors: external forces product will bear, product's supporting function, structural characteristics (such as column positions, cantilever structures), and selected plastic material.
Under premise of meeting mechanical performance and functional requirements, the smaller wall thickness, the better.
Principle 3: Economy and Moldability
Too thin wall: High injection resistance, difficult melt filling; insufficient product strength.
Too thick wall: Increased material costs; significantly prolonged cooling time; highly prone to defects such as shrinkage marks, bubbles, and uneven shrinkage.
⚠️ Many beginners mistakenly believe that "thicker is stronger"—this is a misconception.
Correct approach: Use structural reinforcement (stiffening ribs, bosses) to increase strength, not by increasing wall thickness.
Wall thickness variation rate: Different materials have different requirements.
Crystallic plastics (POM, PA, PBT, etc.): Wall thickness variation ≤ 10% (large shrinkage rate, large variation = severe deformation)
Amorphous plastics (ABS, PC, PS, etc.): Wall thickness variation ≤ 25% (relatively uniform shrinkage)
General specification: Thickness difference should be controlled within 25% of basic wall thickness.
Relationship between Draft Angle and Wall Thickness
When wall thickness of a plastic part is thicker, molding shrinkage increases, and a larger draft angle should be used. This is a fundamental factor that must be considered when determining draft angle; values cannot be obtained solely from tables.
Using Structure to Enhance Thickness: Correct Reinforcement Method
Structural Strength Orientation: Using Ribs to Replace Thickness as Core Path
Maintaining a relatively thin main body wall thickness, meeting load-bearing requirements through rib design
Rib "Golden Rule"

Design Points and Layout: Prefer more short and narrow ribs to fewer deep and wide ribs; Gradually reduce height at rib ends to reduce air trapping; Follow direction of maximum stress and offset, consistent with melt filling direction; Pay attention to reducing glue at rib intersections to prevent local over-thickness; Arrange ribs symmetrically on non-appearance surfaces to avoid local stress concentration;
① Ribs (Struts) – The most effective way to enhance rigidity and strength. Root thickness should not exceed 60% of main wall thickness.
② Optimize Cross-Section Shape – Use channel, arch, or box-shaped structures to achieve higher structural efficiency.
③ Boss (Screw Column) Treatment – Root of boss should avoid direct connection to sidewall, which would cause wall thickness accumulation; use ribs (corner plates) for connection.
④ Smooth Corners – All internal and external corners should be designed with rounded corners. R/T ratio is recommended to be between 0.2 and 0.6, with an ideal value of R/T = 0.5.
�� Golden Rule: "Uniformity and Gradual Transition" Based on this, reasonable feature design (reinforcing ribs, bosses, rounded corners, etc.) can achieve "trading thickness for structure" - achieving the best economy and manufacturability while ensuring product performance and quality.