Key Principles of Injection Mold Design

To ensure high-quality, cost-effective, and durable plastic parts, follow these core design guidelines:

1. Parting Line & Ejection Direction

• Define the ejection direction early to minimize side actions (sliders/cores).

• Align ribs, snaps, and bosses with the ejection direction to avoid undercuts.

• Optimize parting line placement to hide seams on non-cosmetic surfaces.

2. Draft Angles

• Essential for smooth demolding and defect prevention.

• Smooth surface: ≥ 0.5°

• Fine texture (e.g., sandblasted): > 1°

• Coarse texture: > 1.5

• Uneven draft can cause sticking, drag marks, or toppling.

• For deep cavities: outer wall draft > inner wall draft ensures even wall thickness and prevents core shift.

3. Wall Thickness

• Maintain uniform wall thickness (typically 0.5–4 mm).

• Walls > 4 mm lead to sink marks, warpage, and longer cycle times.

• Non-uniform walls cause:

• Sink marks

• Voids

• Weld lines

4. Ribs (for Stiffness)

• Increase rigidity without increasing wall thickness.

• Rib base thickness: ≤ 0.5–0.7T (to prevent sinking).

• Draft angle: > 1.5° per side.

• Height: ≤ 3× wall thickness recommended.

5. Fillet Radii

• Avoid sharp corners to reduce stress concentration and cracking.

• Improve mold durability — sharp internal corners increase risk of steel fracture.

• Use fillets to enable high-speed milling (R-tools), reducing EDM use.

• Choose fillets carefully — they may affect parting line location.

6. Holes

• Prefer round holes aligned with ejection direction (avoid cross-core).

• For length-to-diameter ratio > 2, add draft (size based on minimum diameter).

• Blind hole depth ≤ 4× diameter (to avoid broken core pins).

• Distance from hole to edge > hole diameter.

7. Side Actions (Sliders/Cores)

• Use only when necessary — sliders increase cost, reduce mold life, and create parting lines.

• Redesign features (holes, ribs) to align with ejection direction to eliminate side actions.

• Consider cavity/core shut-off instead.

8. Living Hinges (PP Only)

• Design hinge as integral part of the product (material: PP).

• Hinge thickness: < 0.5 mm, uniform.

• Gate must be placed on one side only to ensure proper flow and strength.

9. Inserts

• Used to add strength, threads, or precision features.

• Common materials: metal (brass), plastic.

• Design for retention: knurling, holes, bends, shoulders.

• Surround insert with sufficient plastic thickness to prevent cracking.

• Ensure precise positioning in mold (pins, slots, magnets).

10. Embossed Logos / Markings

• Place on flat, non-cosmetic internal surfaces.

• Use embossed (raised) text/icons with adequate draft (>1°).

• Align normal direction with ejection path to avoid drag marks.

11. Dimensional Tolerances

• Plastic shrinkage is variable; tolerances are looser than metal parts.

• Follow standards: GB/T 14486-93 (China) or ISO 20457.

• Select tolerance based on material, function, and process capability.

12. Warpage Control

• Avoid large flat areas — prone to bending.

• Use ribs, gussets, or flanges for stiffness.

• Balance wall thickness and cooling.

13. Snap-Fit Joints

• Use multiple snaps for reliability (one failure ≠ total failure).

• Add fillets to improve strength.

• Tight control of undercut dimensions:

• Too much → difficult assembly or breakage

• Too little → loose fit

• Design for easy mold modification (e.g., easy add-metal for adjustment).

14. Welding Methods

• Suitable methods: hot plate, ultrasonic, vibration welding.

• Benefits:

• Stronger joints than adhesives

• Simplified assembly and design

15. Design vs. Process Trade-offs

• Balance appearance, performance, and manufacturability.

• Accept minor defects (sink, weld lines) if hidden in non-visible areas.

• Prioritize function over perfection where needed.

  16. Boss & Screw Design

Boss Design Guidelines:

• Connect bosses to sidewalls or ribs for strength and better melt flow.

• Max height ≤ 2.5× diameter — higher risks air traps, burn, short shots.

• If taller, reinforce with ribs.

• Round shape preferred — easier to machine.

• Keep away from edges/corners; maintain spacing.

• Add "volcano" recess around boss base to prevent sink marks.

• Draft: 0.5° external, 0.5°–1° internal.

Summary: Core Goals of Mold Design

• Minimize undercuts → Reduce sliders

• Uniform walls & proper draft → Prevent defects

• Smart reinforcement → Ribs, bosses, gussets

• Design for manufacturability (DFM) → Lower cost, higher quality, faster cycles

Follow these principles early in design to avoid costly mold changes and production delays.