Avoiding part failure: 5 design flaws in injection moulding – and how proper prototyping can identify them

1. Inconsistent wall thicknesses → stresses & warping
Typical fault:
Significant variations in wall thickness lead to non-uniform shrinkage behaviour. The result: warping, sink marks or stress cracks.
Troubleshooting tip:
During quality inspection, specifically analyse shrinkage and warping (e.g. by measuring and comparing with CAD data).
The following may help:
• Adjusting the filling parameters
• Targeted temperature control
• Or design modifications (e.g. making wall transitions smoother)

2. Demoulding problems caused by undercuts and geometries
Typical error:
Components are designed too complexly, without taking demoulding into account. Consequence: scratches, stresses or damaged moulds.
Troubleshooting tip:
During the initial trial run, pay close attention to the demoulding process – does the part come out smoothly? Are there any visible marks?
Corrections can be made by:
• Polishing / demoulding chamfers
• Demoulding using ejectors
• Modifying the component design

3. Incorrectly dimensioned ribs → Sink marks & air bubbles
Typical fault:
Ribs too thick or incorrectly positioned → Air entrapments, sink marks on the surface, inconsistent stability.
Troubleshooting tip:
Sink marks can quickly become visible during sample inspection.
Action:
• Adjust process parameters (holding pressure, injection speed)
• Or: Revise the design – e.g. reduce rib thickness (approx. 60–70 % of the wall thickness) or reposition them.

4. Unsuitable gate locations → Filling problems & flash
Typical fault:
A poorly positioned gate leads to:
• uneven filling
• flash at critical points
• visible defects
Troubleshooting tip:
Check the filling analysis during sample testing – an experienced eye can immediately tell whether the flow path is correct or whether problems are to be expected.
Often, minor adjustments are sufficient:
• a different gate position
• mould modification
• control of the injection speed

5. Design without regard to manufacturing → unnecessary costs & rework
Typical mistake:
The component is designed to look ‘perfect on screen’ – but is not suitable for injection moulding. Consequence: an expensive mould, a poor process or component failure.
Troubleshooting tip:
It becomes apparent as early as the prototype stage whether the component is suitable for series production – for example, based on:
• Filling behaviour
• Mould wear
• Cycle time
Standard tools such as Moldflow are helpful – but nothing replaces experience in the real-world process. I regularly support customers with practical optimisation in this area.

The initial sampling (sample inspection) is not just a mandatory step – it is the best opportunity to identify risks and highlight design flaws.
Unfortunately, this step is often ‘rushed through’ without structured troubleshooting. This is precisely where I support companies – with the following services:
My support during sample testing:
• On-site or remote support
• Analysis of component issues using actual sample parts
• Constructive suggestions for improvement (even at short notice)
• Support with communication with toolmakers and moulded part manufacturers

✅ Conclusion: Checking early on saves a lot later on
Many problems in injection moulding can be avoided – if the design, tool and process are considered together from the outset.
Proper sample testing with an external specialist helps to identify risks early on – and ensures component quality in the long term.