Injection Molding: Wall Thickness Design Principles

Specific thickness requirements exist based on material selection. Deviations from optimal thickness cause significant problems:

• Excessive Thickness: Leads to sink marks, voids, internal bubbles, increased material costs, longer cycle times (due to extended cooling), and reduced production efficiency.

• Insufficient Thickness: Creates high flow resistance in the melt, making complex or large parts difficult to fill. It also compromises part strength.

Design Recommendations:

1. General Rule: Larger parts typically require thicker walls; smaller parts can use thinner walls.

2. Uniformity is Paramount: Ensure consistent wall thickness wherever possible to prevent uneven shrinkage during cooling. Non-uniform walls cause defects like sink marks, warpage, voids, and high internal stresses.

3. Transitioning Thickness: When thickness changes are unavoidable:

◦ Avoid sharp corners or acute angles at transitions.

◦ Use gentle, gradual tapers.

◦ Transition thicknesses along the flow direction (thicker to thinner).

◦ Never allow abrupt changes.

4. Thickness Range: For common thermoplastics, walls between 1.0-2.0 mm are typical. While walls below 0.3 mm are generally difficult to fill, very soft plastics or rubber can sometimes fill down to 0.2-0.3 mm.

The core principle is to design walls that fill completely without causing sink marks or voids. Ultimately, designers must balance achieving necessary part strength with minimizing material usage and manufacturing costs.

Specifications for Design of Plastic Structures

Common Material Options & Applications:

a. ABS: High fluidity, low cost. Suitable for non-structural parts not subject to direct impact or structural durability demands (e.g., internal support brackets like keyboard/LCD brackets). Preferred for electroplated components (buttons, side keys, decorative parts). Common Grades: Chimei PA-757, PA-777D.
b. PC+ABS: Good fluidity and strength, moderate price. Ideal for parts requiring high rigidity and impact toughness (e.g., frames, housings). Common Grade: Bayer Bayblend T85, T65.
c. PC: High strength, expensive, lower fluidity. Used for high-strength applications (e.g., structural housings, buttons, transmission frames, lenses). Common Grades: Teijin Panlite L1250Y, PC2405, PC2605.
d. POM: High stiffness, hardness, fatigue/wear resistance; low creep/water absorption; excellent dimensional stability, chemical resistance, and insulation. Typical uses: pulleys, gears (spur, worm, turbine), transmission parts. Common Grade: M90-44.
e. PA (Nylon): Tough, hygroscopic (absorbs water); becomes brittle when completely dry. Used in gears, pulleys. Impact-critical gears require additives for enhanced strength. Common Grade: CM3003G-30.
f. PMMA (Acrylic): Excellent light transmittance (92% after 240h aging, 89% after 10yrs outdoors, 78.5% UV). High mechanical strength, cold/corrosion resistance, good insulation, stable dimensions. Brittle. Used for transparent structural parts with strength requirements (e.g., lenses, remote control windows, light guides). Common Grade: Mitsubishi VH001.

Key Finished Product Requirements:

When selecting a material, also evaluate these critical attributes:

• Environmental Resistance: Chemical, UV, flame, or extreme temperature exposure?

• Mechanical Performance: Required strength? Need for flexibility under load?

• Safety & Stability: Stable physical properties? Biocompatibility (especially medical)? FDA compliance (food contact)?

• Sustainability: Biodegradability? Compatibility with recycled materials?

• Aesthetics: Need for painting or pre-colored resin? Opacity/transparency requirements?

• Functionality: Use in electromagnetic environments?

Aligning Material Choice with Design:

Always cross-reference these requirements with the material’s recommended wall thickness. A material is only viable if it can be injection molded to your product’s required dimensions and geometry while meeting all performance criteria. When nearing a material decision, consult TONGDA LINK mold engineers for direct advice or a connection to our material supplier experts.

Material Modification & Alternatives:

• Example: Nylon 6/6 offers good flow for thin walls and high impact resistance, but may be rejected for average strength and heat resistance. Adding glass fiber significantly increases strength and heat resistance, reduces sink in thick sections, but can increase warpage risk in thin areas.

• Alternative Materials:

  • For thick optical components, acrylic (PMMA) often outperforms polycarbonate (PC) by reducing sink, voids, bubbles, and improving detail.

  • Optical-grade Liquid Silicone Rubber (LSR) provides superior clarity and light transmission, enabling designs with minimal thickness constraints and fine features.

  • K-Resin (styrenic) can be a suitable alternative to ABS or PC in large structural parts.

  • Liquid Crystal Polymer (LCP), often glass-filled, offers high strength and exceptional capability for very thin walls.

Hundreds of materials and countless modifications (blends, additives, fine-tuning) exist to achieve desired results. Explore further resources on the TONGDA LINK materials page.

Optimizing wall thickness is critical for part performance, manufacturability, and appearance. Adhere to the following principles:

1. Uniformity is Critical: Maintain consistent wall thickness throughout the part. Thickness variations should generally not exceed 25% of the nominal wall thickness. The absolute minimum wall thickness for any section should be ≥ 0.4 mm.

2. Recommended Thickness Ranges:

   • Main Shell Body: 1.2 – 1.4 mm

   • Side Walls: 1.5 – 1.7 mm

   • Outer Lens Support Surface: 0.8 mm

   • Inner Lens Support Surface (Min): 0.6 mm

   • Battery Cover: 0.8 – 1.0 mm

3. Gradual Transitions Between Thick & Thin Sections:

   • Avoid abrupt changes. The thinner wall should be ≥ 40-60% of the thickness of the adjacent thicker wall.

   • Always use radial fillets (arc transitions) at wall junctions.

   • Ensure all thicknesses remain within the recommended range for the selected material.

4. Avoid Problematic Geometries: Steer clear of features prone to defects or failure, regardless of wall thickness, including:

   • Long unsupported spans

   • Sharp internal corners

   • Poorly designed bosses (e.g., lacking adequate draft, ribs, or support)

5. Utilize Ribs for Strength & Stiffness:

   • Reinforce tall or unsupported walls with ribs instead of increasing overall wall thickness.

   • Benefits of rib design:

     • Maintains part strength and rigidity without adding mass or increasing sink risk.

     • Reduces part deformation.

     • Can improve material flow during molding.

RIBS DESIGNS

6. Internal Corner Radii
Sharp external corners are acceptable, but internal corners should incorporate radii (where design permits) to enhance structural integrity and reduce warping stress.

7. Boss Design Guidelines
a. Wall Thickness Ratio: Maintain boss walls at 40–60% of the adjacent nominal wall thickness to prevent sink marks.
b. Shrinkage Mitigation: Optimize wall thickness to minimize shrinkage risks during cooling.
c. Reinforcement: Add radial ribs around bosses for increased strength. Design rib width per Figure 3-1.
(See example images: Excessive wall thickness in initial designs causes shrinkage; optimized thickness eliminates this issue.)

8. Wall Thickness

• Minimize wall thickness to reduce material usage and manufacturing costs, while ensuring functional strength and structural integrity.

• Absolute Minimum: 0.6 mm – 0.9 mm (highly material and geometry dependent).

• Typical Range: 2 mm – 5 mm.

9. Draft Angles (Demolding Slopes)

9.1 General Principles

• Draft angles are essential for part ejection and preventing warpage, curl, or surface damage (scratches).

• There is no single fixed standard; angle selection depends on part depth, molding method, wall thickness, plastic material, surface finish, and required precision.

• Ideal Range: 0.5° – 1° per side is generally suitable for many applications.

• Minimum Practical Angle: 0.2° per side (difficult to achieve consistently).

• Direction:

  • Internal Features (Holes/Bosses): Taper should decrease towards the base (smaller end at bottom – conforms to core).

  • External Features (Part Outline): Taper should increase towards the base (larger end at bottom – conforms to cavity).

Generally, the inner hole is based on the small end, which is in line with the drawing. The inclination is obtained from the expanding direction.

9.2 Key Factors Influencing Draft Angle Selection

• a. Precision Requirements: Use smaller draft angles for high-precision areas.

• b. Feature Height/Size: Taller or larger features generally require smaller angles per degree of height.

• c. Material Shrinkage: Higher shrinkage materials require larger draft angles.

• d. Wall Thickness: Thicker walls (increasing shrinkage) require larger draft angles.

• e. Tolerance: Draft angles are typically excluded from part dimensional tolerances.

• f. Surface Finish:

  • Transparent Parts: Require larger angles to prevent ejection scratches (PS > 3°, ABS/PC > 2°).

  • Textured/Surface-Treated Parts (Leather, Sandblast): Require significantly larger angles (typically 3° – 5° or more) based on texture depth (H). Use formula: Recommended Draft = 1° + (H / 0.0254)°, where H is texture depth (e.g., deeper texture requires more draft).

• g. Insert Areas: Draft on surfaces for inserts is typically 1° – 3°.

• h. Outer Shell Surfaces: Minimum draft ≥ 3° (critical for appearance and ejection).

• i. Internal Shell Features & Ribs:

  • Standard Draft: 1° per side.

  • Specific Cases:

    • Ribs < 3mm height: 0.5°

    • Ribs 3mm – 5mm height: 1°

    • Ribs > 5mm height: 1.5°

    • Cavities/Bosses < 3mm depth: 0.5°

    • Cavities/Bosses 3mm – 5mm depth: 1°

    • Cavities/Bosses > 5mm depth: 1.5°

Optimizing Wall Strength Through Design

Even when material limitations exist, strategic design modifications can significantly mitigate internal stresses and weaknesses caused by suboptimal wall thickness.

1. Core Hollow Sections: Parts with thick, continuous profiles (e.g., dumbbells, bobbins) are ideal candidates for coring. This process removes excess internal material while maintaining the structural core – akin to carving out wedges while leaving the apple core intact. Coring effectively prevents sink marks, reduces material consumption and part weight, and can even enhance overall strength.

2. Reinforce Thin Walls: Tall, thin walls (e.g., on box lids) benefit from gussets. Ensure the gusset’s wall thickness adheres to the 40-60% rule relative to the main wall. This reinforcement not only adds strength but also prevents shadowing – uneven cooling caused by section thickness variations.

Upon receiving your part quote from the mold partner, carefully review the accompanying DFM analysis. This report provides crucial feedback to enhance moldability, including:

• Color-coded identification of overly thick or thin areas relative to the nominal wall.

• Recommendations for draft angle adjustments.

• Identification of parting lines, ejector/gate locations, undercuts, side-actions, and hand-loaded inserts.

• Optional flow analysis to simulate pressure points near gates and predict knit line formation.

Contact TONGDA LINK for Expert Guidance

For detailed insights into wall thickness optimization and other critical injection molding design considerations, contact our experts:

• Phone: +86 135 5476 1695

• Email: emma.nestor@tongdalinkmold.com