Common Design Specifications for Plastic Injection Molded Parts

1. Material Selection and Wall Thickness

1.1 Material Selection Guidelines

• ABS

• Advantages: High fluidity, low cost.

• Applications: Components with low strength requirements (non-impact, non-durability-tested structural parts) such as internal supports (keyboard brackets, LCD brackets), and electroplated parts (buttons, side keys, navigation keys, decorative electroplated parts).

• Common grades: Chimei PA-757, PA-777D.

• PC+ABS

• Advantages: Good fluidity, balanced strength, moderate price.

• Applications: Parts requiring high rigidity and impact toughness (frames, housings).

• Common grades: Bayer T85, T65.

• PC

• Advantages: High strength.

• Disadvantages: High cost, poor fluidity.

• Applications: High-strength requirements (shells, keys, transmission frames, lenses).

• Common grades: Teijin L1250Y, PC2405, PC2605.

• POM

• Advantages: High rigidity/hardness, excellent fatigue resistance and wear resistance, low creep and water absorption, good dimensional/chemical stability, and insulation.

• Applications: Pulleys, transmission gears, worm gears, and transmission components.

• Common grade: M90-44.

• PA (Nylon)

• Advantages: Toughness.

• Disadvantages: Hygroscopic, brittle when fully dry.

• Applications: Gears, pulleys; key impact-resistant gears require fillers.

• PMMA

• Advantages: Excellent light transmittance (92% sunlight transmittance after 240-hour accelerated aging, 89% after 10 years outdoors, 78.5% for UV), good mechanical strength, cold resistance, corrosion resistance, insulation, and dimensional stability.

• Disadvantages: Brittle.

• Applications: Transparent structural parts with strength requirements (lenses, remote control windows, light guides).

1.2 Shell Wall Thickness Requirements

• Uniformity: Thickness variation should be within 25% of the base wall thickness.

• Minimum Thickness: The overall minimum wall thickness of the part shall not be less than 0.4 mm. The back of this area shall not be a Class A surface, and the area shall not exceed 100 mm².

• Typical Thicknesses:

• Shell thickness (thickness direction): 1.2–1.4 mm; side wall thickness: 1.5–1.7 mm.

• Outer lens support surface: 0.8 mm; inner lens support surface: minimum 0.6 mm.

• Battery cover: 0.8–1.0 mm.

1.3 Key Principles for Thickness Design

• Excessive Thickness Issues: Increased material cost, extended molding/cooling time (reduced efficiency), and defects such as bubbles, shrinkage cavities, and sink marks.

• Insufficient Thickness Issues: High melt flow resistance (difficult molding for complex/large parts) and poor part strength.

• Transition Requirements: Avoid sharp angles between thick and thin sections; use gradual transitions. Thickness should decrease along the melt flow direction to prevent uneven shrinkage, bubbles, sink marks, warpage, and internal stress.

2. Draft Angle

2.1 Core Guidelines for Draft Angles

• General Range: 0.2° to several degrees; 0.5°–1° is ideal for most cases.

• Direction Rules:

• For inner holes: Use the small end as the reference (per drawing); draft angle expands outward.

• For outer shapes: Use the large end as the reference (per drawing); draft angle tapers inward.

• Precision and Size Considerations:

• High-precision parts require smaller draft angles.

• Tall/large-sized parts require smaller draft angles.

• Material and Thickness Factors:

• Materials with high shrinkage require larger draft angles.

• Thick-walled parts (increased molding shrinkage) require larger draft angles.

• Tolerance Notes: Draft angles are generally not included in part tolerances.

• Special Cases:

• Reinforcing ribs <3 mm high: 0.5°; 3–5 mm: 1°; >5 mm: 1.5°.

• Cavities <3 mm high: 0.5°; 3–5 mm: 1°; >5 mm: 1.5°.

• Transparent parts: Larger draft angles to avoid scratches (PS >3°, ABS/PC >2°).

• Textured/sanded surfaces: 3°–5° draft angle (varies with texture depth). Recommended formula: 1° + H/0.0254° (H = texture depth). Example: Texture 121 → 3°, Texture 122 → 5°.

• Through-cut surfaces: 1°–3° draft angle.

• Shell outer surfaces: ≥3° draft angle.

• Other shell features: Standard 1° draft angle. Exceptions:

3. Reinforcing Ribs

3.1 Design Purpose

3.2 Key Design Principle

4. Pillars and Holes

4.1 Pillar Design Guidelines

• Prevent shrinkage at the pillar’s wall thickness.

• Add surrounding reinforcing ribs to improve pillar strength.

4.2 Hole Design Guidelines

• Hole-to-Hole Spacing: ≥2 times the hole diameter.

• Hole-to-Edge Spacing: ≥3 times the hole diameter. For space-constrained or mounting holes, use bosses to reinforce the hole edge.

• Side Hole Requirements: Avoid thin-walled cross-sections to prevent sharp edges (safety hazards) and short shots.