Designing for Injection Moulding: What You Need to Know

“It looked perfect in CAD. The 3D print worked like a charm. But the first moulded part… didn’t fit.”

This scenario is all too common. A design performs flawlessly during prototyping, but once mass production begins with injection moulding, unexpected issues arise. Parts may not fit, function as intended, or meet aesthetic expectations.

Injection moulding isn't merely about scaling up a prototype; it introduces new variables and challenges that can significantly impact the final product. Understanding these factors is crucial to achieving parts that meet design specifications.

Why Injection Moulding Isn't Just Scaling Up

Transitioning from prototyping to injection moulding involves more than just increasing production volume. Several factors unique to injection moulding can alter the outcome:

• Material Shrinkage: Plastics contract as they cool, and different materials have varying shrinkage rates.

  
  

• Draft Angles: Necessary for easy part ejection from the mould, affecting the part's geometry.

  
  

• Gate Placement: Influences how the molten plastic fills the mould, impacting strength and appearance.

  
  

• Wall Thickness: Variations can lead to defects like warping or sink marks.

  
  

• Undercuts: Features that complicate mould design and part ejection.

  
  

• Ejector Pins: Their placement can leave marks or affect part integrity.

Neglecting these aspects can result in parts that deviate from the prototype, leading to increased costs and delays.

6 Essential Design Tips for Successful Injection Moulding

Incorporate Appropriate Draft Angles

Why it matters: Draft angles facilitate the removal of the part from the mould without damage.

Best practices:

• Apply a minimum draft angle of 1 to 2 degrees on vertical walls.

• Increase the angle for deeper parts or textured surfaces.

• Ensure uniform draft to maintain consistent wall thickness.

Account for Material-Specific Shrinkage

Why it matters: Different plastics shrink at different rates, affecting final part dimensions.

Best practices:

• Consult the material datasheets for shrinkage values.

• Adjust mould dimensions to compensate for expected shrinkage.

• Consider the directionality of shrinkage, especially in complex parts.

Utilise Mould Flow Analysis

Why it matters: Predicts how the molten plastic will fill the mould, identifying potential issues.

Best practices:

• Use simulation software during the design phase.

• Identify areas prone to air traps, weld lines, or incomplete filling.

• Optimise gate placement and runner design based on analysis results.

Maintain Uniform Wall Thickness

Why it matters: Inconsistent wall thickness can cause defects and affect structural integrity.

Best practices:

• Aim for uniform wall thickness throughout the part.

• Gradually transition between different thicknesses to avoid stress concentrations.

• Use ribs or gussets to reinforce areas without increasing wall thickness.

Minimise or Eliminate Undercuts

Why it matters: Undercuts complicate mould design and can increase production costs.

Best practices:

• Redesign parts to avoid undercuts when possible.

• If necessary, use side actions or collapsible cores to accommodate undercuts.

• Evaluate the cost-benefit ratio of including undercuts in the design.

Strategically Place Gates and Ejector Pins

Why it matters: Their placement affects part aesthetics and structural performance.

Best practices:

• Position gates to ensure even flow and minimise weld lines.

• Place ejector pins in areas that won't affect the part's appearance or function.

• Consider the impact of gate and ejector pin placement on cycle time and mould complexity.

Case Study: Transforming a Problematic Bracket into a Success

A start-up designed a plastic bracket that performed well during 3D printing. However, the first injection-moulded samples exhibited warping, misaligned holes, and surface imperfections.

Identified issues:

• Lack of draft angles, causing ejection problems.

• Failure to account for material shrinkage leads to dimensional inaccuracies.

• Poor gate and ejector pin placement, resulting in cosmetic defects.

Solutions implemented:

• Introduced appropriate draft angles to facilitate ejection.

• Adjusted mould dimensions to compensate for material shrinkage.

• Repositioned gates and ejector pins to less visible areas.

These changes led to parts that met design specifications, demonstrating the importance of considering injection moulding nuances during the design phase.

Ready to Optimise Your Injection-Moulded Parts?

If you're preparing a design for injection moulding, it's crucial to address these considerations early. Doing so can save time, reduce costs, and ensure your parts meet expectations.

Feel free to share your CAD files or questions. We're here to help you navigate the complexities of injection moulding and achieve successful outcomes.