Key Design Principles for Injection Molding

To ensure high-quality, cost-effective, and manufacturable plastic parts, the following design principles should be considered during the early stages of product development. Proper design can significantly reduce mold complexity, avoid defects, and extend mold life.

1. Mold Opening Direction and Parting Line

•The mold opening direction must be determined early in the design phase to minimize or eliminate side actions (e.g., sliders, lifters) and to reduce the visual impact of parting lines.

•Design features such as ribs, snaps, and bosses to align with the mold opening direction to avoid undercuts and reduce parting lines.

•Select an appropriate parting line to prevent undercuts and improve both appearance and performance.

2. Draft Angles (Draft Taper)

•A proper draft angle prevents surface scratches (drag marks). Recommended values:

•Smooth surface: ≥ 0.5°

•Fine texture (e.g., matte): > 1°

•Coarse texture: > 1.5°

•Sufficient draft prevents ejection damage such as stress whitening, deformation, or cracking.

•For deep cavity parts, the outer surface draft should be greater than the inner to prevent core misalignment, ensure uniform wall thickness, and maintain strength at the opening.

3. Wall Thickness

•Most plastics have an optimal wall thickness range of 0.5–4 mm. Walls thicker than 4 mm lead to longer cooling times and sink marks.

•Non-uniform wall thickness causes sink marks, voids, and weld lines.

•Design with consistent wall thickness wherever possible.

4. Ribs (Reinforcing Ribs)

•Ribs increase part stiffness and reduce deformation.

•Rib thickness should be 50–70% of the nominal wall thickness to avoid sink marks.

•Include a minimum 1.5° draft on rib sides to prevent ejection damage.

5. Fillets (Corner Radii)

•Small radii can cause stress concentration in the part, leading to cracks.

•Sharp corners in the mold cavity create stress concentration, increasing the risk of mold cracking.

•Use proper fillets to improve mold machinability (e.g., R-mills instead of EDM).

•Be aware that large radii may shift the parting line; choose fillets or sharp corners based on design and aesthetic needs.

6. Holes

•Keep hole shapes simple—preferably circular.

•Align hole axis with the mold opening direction to avoid side cores.

•For length-to-diameter ratio > 2, include a draft angle. Hole diameter should be based on the minimum diameter (maximum material condition).

•For blind holes, length-to-diameter ratio should not exceed 4 to prevent core pin bending.

•Maintain a distance between holes and edges ≥ hole diameter.

7. Side Actions (Cores, Sliders, Lifters) and How to Avoid Them

•Use side actions when the part cannot eject due to undercuts. While they enable complex geometries, they increase mold cost, reduce lifespan, and may cause parting lines or sink marks.

•Avoid side actions whenever possible by reorienting features (e.g., holes, ribs) to align with the mold opening direction, or using core-cavity shut-offs.

8. Living Hinges (Integrated Hinges)

•PP (polypropylene) is ideal for living hinges due to its excellent flexibility and fatigue resistance.

•Hinge thickness should be < 0.5 mm and uniform.

•Gate the part on one side of the hinge only to ensure proper material flow and hinge integrity.

9. Inserts

•Inserts (usually metal) are embedded in plastic to enhance strength, hardness, dimensional accuracy, or to create threaded holes.

•Common insert materials: brass, steel, or even plastic.

•Design features to prevent rotation and pull-out: knurling, holes, bends, flattening, or shoulders.

•Surround inserts with sufficient plastic thickness to prevent stress cracking.

•Consider insert positioning in the mold (via pins, holes, or magnetic fixtures).

10. Part Markings (Logo/Text)

•Place markings on flat, internal surfaces.

•Use raised (embossed) text for better mold release.

•Orient the marking surface as parallel as possible to the mold opening direction to avoid drag marks.

11. Dimensional Tolerances

•Due to inconsistent and unpredictable shrinkage, plastic parts have lower dimensional accuracy than metal parts.

•Do not apply mechanical part tolerances directly. Use standards such as GB/T 14486–1993 (Dimensional Tolerances for Molded Plastic Parts) to select appropriate tolerances based on material and application.

•Consider your factory’s capabilities and industry benchmarks when setting tolerance levels.

12. Warpage (Part Deformation)

•Improve structural rigidity to reduce warpage.

•Avoid large flat areas; use flanges, ribs, or textured surfaces.

•Reinforce with properly designed ribs and avoid isolated thin sections.

13. Snap-Fit Design

•Use multiple snap features in parallel so that failure of one does not disable the entire assembly—this improves durability.

•Add fillets to increase strength at stress points.

•Tight tolerances are critical: too many undercuts cause damage; too few lead to loose fit.

•Design for easy mold modification (e.g., add material later) to fine-tune fit during trials.

14. Joining Methods (Welding Techniques)

•Hot plate, ultrasonic, or vibration welding provides strong, reliable joints.

•These methods simplify product design by eliminating fasteners and reduce assembly cost.

15. Balancing Design, Process, and Performance

•Balance aesthetics, functionality, and manufacturability. Sometimes sacrificing processability improves appearance or performance.

•When defects are unavoidable, design them into non-visible or non-critical areas.

16. Boss Hole Diameter vs. Self-Tapping Screw Size

Screw Size    Recommended Hole Diameter   

M2    1.7 mm   

M2.3    2.0 mm   

M2.6    2.2 mm   

M3    2.5 mm   

Note: Hole size allows for proper thread formation without cracking.

17. Boss Design Guidelines

1. Avoid isolated bosses—connect them to side walls or ribs for better strength and material flow.

2. Boss height ≤ 2.5 × diameter. Taller bosses risk air traps, burn marks, or short shots.

3. For taller bosses (especially away from walls), use ribs for support.

4. Circular bosses are preferred—other shapes are harder to machine.

5. Keep bosses away from corners and outer walls to avoid weak sections.

6. Add "volcano" sinks (local material removal around boss base) to prevent sink marks.

7. Draft angles:

•Outer surface: 0.5°

•Inner surface: 0.5° or 1°