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Much has been written regarding design guidelines for injection molding. Yet, the design guidelines can be summed up in just a few design rules.

• Use uniform wall thickness throughout the part. This will minimize sinking, warping, residual stresses, and improve mold fill and cycle times.
• Use generous radius at all corners. The inside corner radius should be a minimum of one material thickness.
• Use the least thickness compliant with the process, material, or productdesign requirements. Using the least wall thickness for the process ensures rapid cooling, short cycle times, and minimum shot weight. All these result in the least possible part cost.
• Use ribs or gussets to improve part stiffness in bending. This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs.

Uniform Walls

• Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs.

Also, thinner parts weight less, which results in smaller amounts of the plastic used per part, which also results in lower part costs.

• The wall thickness of an injection – molding part generally range from 2mm to 4mm (0.080 inch to 0.160 inch). The wall injection molding can produce walls as thin as 0.5 mm (0.020 inch)

The need for uniform walls

• Thick sections cool slower than thin sections. Then thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section.
• This builds stresses near he boundary or of the thin section to thick section. Since the thin section does not yield because it is solid, the thick section (which is still liquid) must yield. Often this leads to warping or twisting. If this is severe enough, the part could even crack.

Burn Marks - Problem Solving Guide

Use the following as a checklist for potential solutions:

• Check mold venting
• Remove “fines” from material
• Reduce back pressure
• Reduce injection velocity
• Reduce screw speed (RPM)

Weld Lines - injection Molding – Problem Solving Guide

Use the following as a checklist for potential solutions:

• Increase mold temperature
• Check vents (increase?)
• Runners and/or gates too small
• Sprue or nozzle opening too small
• Increase material temperature
• Increase injection speed
• Check melt for inconsistencies

Flash - Injection Molding - Problem solving Guide

Use the following as a checklist for potential solutions:

• Reduce injection velocity
• Clean parting lines
• Reface parting lines
• Mold faces are not parallel
• Land too large (no shut off)
• Reduce material temperature
• Reduce mold temperature
• Reduce material pressure (pack and/or hold)
• Reduce packing time
• Increase clamp pressure (check for consistency)

Sink mark – Injection Molding – Problem Solving Guide

Use the following as a checklist for potential solutions:

• Increase gate size
• Reduce material temperature
• Texture mold surface (hidden)
• Reduce flow lengths (add gates)
• Increase runner size
• Increase injection pressure
• Check for a “cushion”, increase shot size
• Increase injection/pack time
• Increase injection velocity

Short Shot – Injection Molding – Problem Solving Guide

Use the following as a checklist for potential solutions:

• Increase mold temperature
• Increase injection velocity
• Runner too small
• Inadequate cold slug well
• Gate too small (note enough)
• Inadequate venting
• Hesitation (non – uniform walls, etc.)
• Not enough plastication capacity
• Insufficient feed
• Material temperatures too low
• Increase injection/packing pressure
• Increase packing time
• Increase back pressure

Flow lines – Injection Molding – Problem Solving Guide

Use the following as a checklist for potential solutions:

• Increase gate size
• Increase clamp pressure
• Increase melt temperature
• Increase mold temperature
• Maintain a more consistent cycle