Every product designer and engineer has experienced a moment of doubt while looking at a CAD model. Will this part mold cleanly? Will it withstand stress? Is it cost-effective to produce at scale?

Identifying and addressing critical manufacturing constraints early in the injection molding design process is vital for optimizing parts. A proactive approach reduces potential production issues by considering material behavior, tooling limitations, and processing conditions, leading to efficient, defect-free manufacturing.

At H&H Molds, we have spent decades assisting engineers in refining their designs for manufacturability. This guide offers essential insights to help designers minimize defects, improve part performance, and optimize production efficiency.

Draft Angles: Ensuring Smooth Ejection & Surface Quality

One of the most critical considerations in injection molding is ensuring that parts are released cleanly from the mold. Insufficient draft angles can cause sticking, drag marks, or excessive friction, especially on textured surfaces. While the specific requirements vary based on material properties, surface textures, and mold geometry, incorporating drafts early in the design process improves moldability and reduces the risk of production problems.

Smoother surfaces usually require a minimal draft, while more textured or complex parts benefit from slightly larger angles to prevent ejection problems. Material shrinkage and flexibility also influence draft requirements. Conducting mold flow simulations early in the design phase helps validate draft angles and avoid costly tooling modifications later.

Wall Thickness: Achieving Structural Integrity & Efficient Cooling

Maintaining uniform wall thickness minimizes shrinkage variations, reduces warpage, and ensures consistent cooling. While optimal thickness depends on material type and application, a well-balanced design prevents defects and supports manufacturing efficiency.

Sudden variations in thickness can create stress concentrations and inconsistencies in flow, resulting in defects like sink marks or incomplete filling. Smooth transitions between different thicknesses help distribute material flow evenly, decrease internal stress, and ensure dimensional stability. Engineers utilize mold flow analysis tools to anticipate potential problems and refine designs accordingly.

Material Selection: Balancing Performance & Processability

Choosing the right material for an injection-molded part requires balancing mechanical properties, surface aesthetics, and manufacturability. Engineers should evaluate options based on performance needs, regulatory requirements, and processing conditions rather than adhering to rigid material recommendations.

Considerations by Application:

Cosmetic Parts: Materials with good surface quality and texture compatibility.

Impact-Resistant Components: Tough, high-strength materials that withstand mechanical stress.

Chemical & Heat-Resistant Parts: Specialized formulations that offer stability in extreme environments.

Reinforced materials enhance strength and rigidity, but careful mold temperature management is required to maintain surface quality. Collaborating with material suppliers and mold designers ensures the best performance and production efficiency fit.

Avoiding Stress Concentrations: Enhancing Structural Durability

Sharp corners and abrupt transitions act as stress concentrators, increasing the risk of cracks, fatigue, and premature failure. Designers should incorporate fillets and radii appropriate to the application to improve structural integrity and moldability.

Larger fillets distribute stress more evenly, minimizing potential weak points. The fillet size depends on the material choice, anticipated load conditions, and part geometry. Stress analysis simulations assist in optimizing these features for improved long-term performance.

Managing Shrinkage & Dimensional Stability

All thermoplastics shrink during cooling after injection molding, at varying rates according to material composition, processing conditions, and mold temperature. Engineers should consider these factors when designing for precision.

Higher molding pressures can minimize shrinkage but may introduce residual stresses that impact part stability. Controlled cooling strategies help mitigate internal stress while maintaining dimensional accuracy. Consulting material datasheets and leveraging mold compensation techniques ensure that final parts meet exact specifications.

Material Selection and Shrinkage Considerations

Different thermoplastics have distinct shrinkage rates that influence the final dimensions of parts. Understanding these rates is essential when designing molds to account for post-mold shrinkage.

Here are some examples:

  • ABS (General Purpose, High Impact): 0.0055 – 0.008 in/in
  • Acetal (POM, Copolymer): 0.016 – 0.025 in/in
  • Acrylic (PMMA): 0.003 – 0.008 in/in
  • ASA: 0.0045 – 0.0075 in/in
  • Nylon 6 (Dry): 0.012 – 0.022 in/in
  • Nylon 6/6 (Dry): 0.012 – 0.022 in/in
  • Polycarbonate (PC): 0.005 – 0.007 in/in
  • PC/ABS: 0.0045 – 0.008 in/in
  • Polybutylene Terephthalate (PBT): 0.013 – 0.018 in/in
  • Polypropylene (PP, Homopolymer): 0.013 – 0.019 in/in
  • Polypropylene (Impact Copolymer): 0.013 – 0.019 in/in
  • PEEK: 0.003 – 0.008 in/in

Given these variations, mold cavity dimensions must be adjusted to compensate for expected shrinkage. Factors such as fiber reinforcement can significantly reduce shrinkage while enhancing mechanical properties.

Gate Placement: Optimizing Flow & Part Quality

Gate location affects flow patterns, weld lines, and packing pressure. Choosing the wrong type or placement can cause defects like short shots, voids, or excessive stress at the gate location.

Gate Types & Considerations

• Edge Gate: Best for simple parts but may leave a visible vestige.

• Submarine Gate: Allows automatic degating but increases shear heating.

• Fan Gate: Ideal for thin-walled parts, improving flow uniformity.

• Hot Runner System: Reduces waste but adds tooling complexity & cost.

Why Design Optimization Matters

A well-designed injection-molded part enhances production efficiency, reduces waste, and improves performance. You can create cost-effective, durable, and manufacturable parts at scale by refining key design elements, including draft angles, wall thickness, material selection, and gate placement.

Design for Manufacturability (DFM) is a crucial strategy for optimizing injection molding processes. It enhances efficiency and minimizes potential defects. Manufacturers can improve mold performance and streamline production by refining the part design and selecting materials early. Thoughtful gating and cooling strategies can enhance cycle times, while effective stress distribution encourages long-term durability.

For experienced injection molders like H&H, DFM offers a practical and widely applicable approach to achieving high-quality results. It ensures that each project is tackled with precision and efficiency.

At H&H Molds, we specialize in guiding customers through these design challenges to ensure successful outcomes. Need expert input on your next project? Contact us today or request a FREE quote to discuss how we can help optimize your injection molding designs for manufacturability. Want to learn more about thermoplastic properties? Check out this resource from M. Holland with detailed specifications on one of the suppliers we use.