Plastic product design mistakes to avoid

Designing plastic products is deceptively tricky. Small decisions in the CAD stage — wall thickness, draft angles, ribs, boss placement and shrinkage allowances — can make the difference between a part that succeeds in production and one that causes cost overruns, poor fit, or early failure. Below are the most common design mistakes we see (and how to avoid them), with practical guidance that applies to rotomoulding, injection and dip-moulding processes. Contact Plastics supports product development from concept to production and can review designs for manufacturability.
 

1. Ignoring draft angles (leading to difficult demoulding)

Problem: Vertical walls with no draft make parts stick in the mould or require excessive force to demould, increasing cycle times and damaging parts.
Fix: Always include a minimum draft angle. For injection moulding small parts, 1°–3° is common; for larger rotomoulded parts, consider larger drafts where possible. When in doubt, ask your manufacturer for the recommended draft for the chosen process.

2. Wrong wall thickness (warpage, sink marks, and longer cycles)

Problem: Overly thin sections can be weak and warp; overly thick sections cool unevenly and create sink marks or internal stresses. Rotomoulding allows variable thickness, but extreme variations still create problems.
Fix: Aim for uniform wall thickness where possible. Use ribs and gussets to add stiffness instead of simply increasing thickness. If variable thickness is unavoidable, step changes should be gradual and supported by analysis or prototyping.

3. Sharp corners and inadequate radii (stress concentrators)

Problem: Sharp internal corners cause stress concentrations and poor polymer flow in mould filling — leading to cracks or premature failure.
Fix: Add generous fillets. For rotomoulded parts, radii help material distribution during heating/rotation and reduce weak points.

4. Poor rib and boss design (weak bosses, sink marks)

Problem: Tall thin ribs and unsupported bosses can lead to sink marks, cracking when fastened, or poor thread performance.
Fix: Use ribs to support large flat walls but avoid ribs that are more than ~60% of the wall thickness in height. Design bosses with support gussets and adequate root radii, and consider metal inserts or captive nuts for repeated assembly.

5. Not accounting for material shrinkage & tolerances

Problem: Expecting perfect dimensional repeatability without accounting for polymer shrinkage will lead to ill-fitting assemblies. Different plastics and processing methods have different shrink rates.
Fix: Specify tolerances and include shrinkage allowances in tooling/cad files. Prototype early and measure to validate final dimensions before committing to production tooling.

6. Designing for one process without considering alternatives

Problem: A design optimised purely for injection moulding might be expensive or impossible with rotomoulding, and vice versa.
Fix: Discuss manufacturing intent early. Contact Plastics offers multiple processes (rotational, injection, dip and compression moulding) and can recommend the best method for part geometry, volume and cost.

7. Overlooking assembly, fastening and secondary operations

Problem: Designers sometimes forget clips, snap-fits, mounting bosses, drainage, or how parts will be painted/printed — causing rework.
Fix: Define how the part will be assembled and finished. If the part needs inserts, incorporate features or tolerances to accommodate them. Add drainage for parts exposed to liquids and plan for post-processing access.

8. Skipping prototype and DFM review

Problem: Jumping from CAD to production tooling without prototyping often hides real-world issues and drives expensive revisions.
Fix: Build functional prototypes (3D print or prototype tooling) and run a Design for Manufacturability (DfM) review with your manufacturer. Contact Plastics provides product development support and prototyping to reduce risk before mass production.

Quick DfM checklist (before you send files to the factory)

  • Add appropriate draft angles.
  • Target consistent wall thickness; use ribs for stiffness.
  • Add fillets to internal corners.
  • Design bosses with gussets and consider inserts for screw retention.
  • Specify tolerances and shrinkage allowances.
  • Confirm process (rotomoulding vs injection) early.
  • Prototype and test early.
 

When to involve your manufacturer

If your product will be rotomoulded, injection or dip-moulded, involve your manufacturer during concept stage. Early collaboration reduces tooling changes and cost, speeds up time to market, and improves product reliability. Contact Plastics offers product development and manufacturing services across these processes and can advise on material selection, tooling trade-offs and prototyping.