For previous reading, please refer to Injection Molded Part Wall Thickness Design 2: Material Properties and Recommended Wall Thickness Va.
Previous two chapters shared general design principles and materials for wall thickness. After meeting general design principles and recommended material benchmarks, final determination and distribution optimization of wall thickness must be driven by target performance.
As engineers, we need to use key performance characteristics (such as appearance, strength, and stiffness) as a guide, working backward to find optimal wall thickness solution, comprehensively utilize structural optimization, molding processes to compensate for performance shortcomings.

 

For surfaces with high appearance requirements, controlling wall thickness to prevent shrinkage marks and flow marks is the highest design priority. Core strategy is to implement "wall thickness uniformity" principle to extreme and strictly constrain all local structures that may cause thick wall accumulation.
Appearance Quality Orientation: Eliminate shrinkage marks and flow marks
Core Strategy: Execute "Wall Thickness Uniformity" Principle to Extreme
Wall Thickness Range Control: Thermoplastics: 0.5~4mm (Economic Upper Limit 4mm); High-Gloss Shell: Main Wall Thickness 1.0~2.5mm; Consumer Electronics: 1.0~1.2mm (Mobile Phones/Headphones)
Reinforcing Ribs/Bosses Thinning: High-Gloss Area: Rib Thickness 40~50% of Main Wall Thickness; Boss Root: 50~75% of Main Wall Thickness; Golden Rule: (Total Thickness - T)/T x 100% < 8% > Less prone to shrinkage
Strictly control rate of change: Transition gradient must be gentle; abrupt changes are strictly prohibited; Transition zone length = 3 x wall thickness; Crystalline plastic change ≤ 10%; Amorphous plastic ≤ 25%
High-gloss finish compatible; Draft angle: ABS/PC > 2°; PMMA > 3°; Gate located in thick-walled zone, melt flows from thick to thin; Distance between boss and sidewall > 2T (recommended ≥ 3mm)
Strict Wall Thickness Uniformity and Range Control
Ideal State: Ideal wall thickness distribution is that thickness remains uniform across any cross-section. This is primary principle for preventing defects (shrinkage marks, flow marks, warpage) caused by differences in cooling shrinkage.
General Scope: Generally, wall thickness range for thermoplastics is controlled between 0.5mm and 4mm, with 4mm as the economic upper limit. Excessive thickness inevitably leads to slower cooling, increased shrinkage, and prolonged production cycles.
Typical Values for High-Gloss Surfaces: For high-gloss casings such as PC and ABS, main wall thickness is typically 1.0mm to 2.5mm. For example, main wall thickness of consumer electronics casings (mobile phones, earphones) is mostly between 1.0mm and 1.2mm.
Strict Control of Wall Thickness Variation Rate
High-shrinkage crystalline plastics (PP, PA, POM): Wall thickness variation should be limited to within 10%.
Low-shrinkage non-crystalline plastics (ABS, PC): Permissible wall thickness variation can reach 25%. Thickness differences in product design should be controlled within 25% of basic wall thickness as much as possible.
If wall thickness must be changed, gradient must be gradual. Abrupt step changes are strictly prohibited; slopes or rounded transitions should be used.
Transition zone length should be three times wall thickness (3:1 ratio).
Wall thickness variation limit:
High-shrinkage crystalline plastics (PP, PA, POM): Wall thickness variation should be limited to within 10%.
Low-shrinkage non-crystalline plastics (ABS, PC): Permissible wall thickness variation can reach 25%. Thickness differences in product design should be controlled within 25% of basic wall thickness as much as possible.
Extreme thinning design for reinforcing ribs and bosses:
Ribs, pillars, and other protruding structures are main source of surface shrinkage marks and their root thickness must be significantly reduced.
Rib thickness: In high-gloss areas, rib thickness should not exceed 40%-50% of main wall thickness. A general principle is that rib thickness ≤ 50%-75% of main wall thickness.
Boss thickness: Thickness at intersection with main wall should be 50%-75% of main wall thickness (50% for crystalline materials, 75% for non-crystalline materials).
Golden Rule of Thumb: When (Total thickness at ribs - Main body wall thickness) / Main body wall thickness × 100% < 8%, shrinkage is less likely to occur.
Layout Optimization: Distance between boss and sidewall should be greater than twice nominal wall thickness (at least 3mm is recommended) to prevent material buildup and shrinkage marks.
Specific Requirements for High-Gloss Surfaces
Increased Draft Angle: To ensure smooth demolding and prevent surface damage, a draft angle > 2° is recommended for high-gloss parts such as ABS/PC; > 3° is recommended for transparent parts (such as PMMA). For textured surfaces, draft angle should be further increased according to texture depth.
Gate Location Strategy: Gates should be preferentially placed in thick-walled areas, allowing melt to flow from thick to thin areas, which facilitates pressure holding and shrinkage compensation, reducing defects caused by insufficient pressure holding or premature cooling at flow front.

Simply increasing wall thickness is not the best way to improve strength. A more economical and effective strategy is to meet load-bearing requirements through refined reinforcing rib design while maintaining a relatively thin main body wall thickness.
Structural Strength Orientation: Using Ribs to Replace Thickness as Core Path
Maintaining a relatively thin main body wall thickness, load-bearing requirements are met through rib design.
Rib "Golden Rule"

Design Considerations and Layout: Prefer more short and narrow ribs to fewer deep and wide ribs; Gradually reduce height at rib ends to reduce air trapping; Direction should follow maximum stress and offset, consistent with melt filling direction; Pay attention to reducing glue at rib intersections to prevent localized excessive thickness; Symmetrically arrange on non-surface surfaces to avoid localized stress concentration.
Core Dimensional Relationships of Reinforcing Ribs (“Golden Rule”)
For thin-walled parts (wall thickness ≤ 1.0mm), with mold manufacturer's approval, reinforcing rib thickness can be equal to or even slightly greater than wall thickness.
At reinforcing rib intersections, attention should be paid to reducing material thickness to prevent localized excessive thickness.
Rib Thickness (Root Width): This is a critical parameter. It is typically taken as 40% - 60% of main body wall thickness T.
Rib Height: It is recommended not to exceed 3T.
Root Fillet: This must be set to eliminate stress concentration and improve flow. Fillet radius R = (0.25 - 0.5)T, and generally not less than 0.3mm - 0.5mm.
Draft Angle: A draft angle of 0.5° - 1.5° is required.
Rib Spacing: Center distance between multiple reinforcing ribs should be > (2 - 4)T.
Design Considerations and Layout
It's better to have a large number of short, narrow ribs than a few deep, wide ribs; Direction of rib extension should follow direction of maximum stress and maximum offset, ideally be consistent with melt filling direction; Arrange ribs symmetrically on non-visible surfaces to avoid localized stress concentration; If ribs do not extend to boundary, height should gradually decrease at the ends to reduce air trapping.

 

To resist bending and torsional deformation, goal is to increase moment of inertia of section, not simply to increase its thickness.
Stiffness and Deformation Control: Comprehensive Structural Strategy
Objective: Increase moment of inertia of cross-section, rather than simply increasing its thickness.
Structural Reinforcement (Recommended): Rib Stiffener Solution
Material Increase: +7%; Stiffness Increase: 2x
Simply Increase Thickness (Not Recommended): Increase Wall Thickness Solution
Material Increase: +25%; Stiffness Increase: Poor Effect
Other Reinforcement Methods
Flange Design: Increase Edge Stiffness; Box Section: Maximize Moment of Inertia; Arched/Wave Surface: Improve Bending and Torsional Resistance
CAE Analysis and Pre-Deformation Compensation: Complex/high-precision parts must utilize mold flow analysis (CAE) to predict warping trends, and pre-deformation (reverse deformation) compensation can be employed during mold design stage. Uniform and efficient mold cooling is key to controlling deformation caused by uneven cooling.
Structural reinforcement is superior to simple thickness increase; By adding ribs, stiffness can be significantly increased (e.g., by 2 times) with very little material (e.g., only 7% more material), far exceeding effect of simply increasing wall thickness (requiring 25% more material).
Other Geometric Reinforcement; Designing flanges, using box sections, or using arched / wave-shaped surfaces can effectively increase moment of inertia and improve resistance to deformation.
Materials and Process Synergy
Glass Fiber Reinforced Materials: Adding glass fiber (GF) can significantly improve material rigidity and strength, reduce shrinkage, helping to control deformation. However, anisotropic shrinkage caused by this should be noted.
Process Control: Uniform and efficient mold cooling is key to controlling deformation caused by uneven cooling. Water barriers and water spray pipes are often used to enhance cooling in thick-walled areas and near ribs.
CAE Analysis and Pre-deformation: For complex or high-precision parts, mold flow analysis (CAE) must be used to predict warping trends, pre-deformation (reverse deformation) compensation can be adopted during mold design stage.

Design Requirements for Transparent Components and Brittle Materials
More sensitive to wall thickness uniformity and stress concentration, with more stringent design requirements.
PMMA Acrylic Wall Thickness Requirements

All corners must be rounded to avoid stress concentration and cracking caused by sharp angles.
PC Polycarbonate (Transparent) Wall Thickness Requirements

Draft angle not less than 2 to prevent scratching of glossy or transparent surfaces.
Common Design Requirements
Extreme requirement for wall thickness uniformity: Maintain consistent optical performance, avoid stress whitening and cracking.
Gate and flow strategy: Plastic enters mold cavity from the thickest part to achieve good pressure holding.
Demolding protection: Appropriately increase draft angle to prevent demolding scratches.
Transparent parts (such as PMMA, PC) or brittle materials are more sensitive to wall thickness uniformity and stress concentration, requiring more stringent design.
Extreme Requirements for Wall Thickness Uniformity
Transparent parts must ensure uniform wall thickness to maintain consistent optical performance and avoid stress whitening and cracking; Wall thickness changes must be smooth and gradual, all internal and external corners must be designed as rounded corners to avoid stress concentration and cracking caused by any sharp angles.
Targeted Wall Thickness Ranges
Common wall thickness: 1.2 - 2.3 mm; For transparent lenses, 1.8 - 2.3 mm is recommended to ensure strength; If wall thickness exceeds 3.0 - 3.1 mm, reinforcing ribs or other structural elements should be considered as an alternative to thickening; For small transparent parts, a wall thickness of 1.5 - 2.0 mm is recommended; For large light guide panels or structural components, a wall thickness of 2.0 mm or more is required, and it can even reach 4.0 - 6.5 mm.
Specific Process Requirements
Gate and Flow: Ensure that plastic enters mold cavity from the thickest part to achieve good pressure holding, avoid shrinkage and internal stress.
Demolding Protection: Draft angle needs to be appropriately increased (usually not less than 2° for PC transparent parts) to prevent scratches on glossy or transparent surfaces during demolding.

In practical engineering, if heat resistance or insulation performance requirements exist, a specialized assessment must be conducted based on detailed performance data provided by material supplier, combined with CAE methods such as thermal and electrical simulations, rather than relying solely on wall thickness adjustments.
Comprehensive Considerations for Thermoelectric Deformation Control
This chapter does not include specific design specifications for HDT or relationship between electrical properties and wall thickness; these require specialized evaluation.
Design Recommendations
Heat Resistance Requirements: Based on detailed performance data sheets provided by material suppliers, conduct a specialized evaluation using CAE (Computer-Aided Engineering) simulations, rather than relying solely on wall thickness adjustments.
Insulation Performance Requirements: Based on detailed performance data sheets provided by material suppliers, conduct a specialized evaluation using CAE simulations, rather than relying solely on wall thickness adjustments.
Comprehensive Considerations: Wall thickness design must be combined with specific structure, dimensions, loads, and selected material grade of product, verified and optimized through CAE analysis (mold flow analysis + structural simulation).
Fiberglass/Flame Retardant Materials: Performance data sheets provided by material suppliers for specific model and thickness must be used; general recommended values should not be applied.
Core Principle: All recommended values are initial design references based on historical experience. Final wall thickness must be verified and optimized through CAE analysis, taking into account specific structure, dimensions, loads, and selected material grade of product.