Common Mistakes in Mould Design and How to Avoid Them
Injection moulding remains one of the most cost-effective and scalable manufacturing methods for producing plastic components. But the success of any injection-moulded part begins long before the machine cycles — it starts with the tool design.
Poor mould design can lead to excessive flash, warping, sink marks, long cycle times, and even complete project failure. These mistakes can be costly, both in time and materials, and they often stem from avoidable oversights in the design phase.
In this article, we outline the most common mistakes seen in mould tool design, explain how they affect production, and offer practical advice on how to avoid them — whether you’re a product designer, toolmaker, or project engineer.
Ignoring Draft Angles
One of the most basic — yet frequently overlooked — principles in injection moulding is the need for draft angles on vertical faces. A draft angle allows the part to be ejected cleanly from the mould without dragging or damaging the surface.
What goes wrong:
Designers sometimes create parts with vertical walls (0° draft), which can cause the plastic to grip the steel. This leads to ejection problems, part deformation, or damage to the tool itself.
How to fix it:
Apply a minimum of 1–2 degrees of draft on all vertical walls, especially on textured surfaces which may require up to 5°. You can use CAD tools to check draft automatically before the design is released for tooling.
🛠️ At Newark Tools Ltd, we run all customer designs through a draft analysis before mould design begins.
Poor Gate Placement
The gate is where molten plastic enters the cavity, and its location has a big influence on part quality and mould performance.
What goes wrong:
Placing gates in the wrong area can lead to flow lines, visible blemishes on cosmetic surfaces, air traps, or poor part filling. It can also complicate tooling, making it harder to balance flow or remove the part cleanly.
How to fix it:
Gates should be located to allow uniform flow, avoid weld lines, and leave gate vestiges in non-critical areas. Use flow simulation software like Moldflow to test different gate options before committing to steel.
🌡️ Underrated Cooling System Design
Cooling often represents over 60% of the cycle time in injection moulding. Yet many designs treat it as an afterthought.
What goes wrong:
Inefficient or insufficient cooling leads to uneven shrinkage, long cycle times, warping, and part distortion. Hot spots inside the tool can create inconsistent material flow or surface blemishes.
How to fix it:
Design cooling circuits early. Use baffles, bubblers, or even conformal cooling channels (if using 3D-printed inserts) to ensure even heat dissipation across the tool. Always validate cooling with thermal simulation when possible.
🔥 Our moulds are designed with fully integrated cooling strategies to reduce cycle times and improve part stability.
💨 Lack of Venting
During the filling process, air in the mould cavity must escape. Without proper venting, the air becomes trapped, which interferes with plastic flow and causes defects.
What goes wrong:
Poor venting results in burn marks, incomplete filling (short shots), or weak weld lines. These defects often appear in the same places and are difficult to eliminate post-production.
How to fix it:
Add shallow vents (0.02–0.05 mm deep) at the end of material flow and in areas likely to trap air. Ensure vents are maintained and cleaned regularly — even a well-designed vent can become blocked with material over time.
🧩 Thin Walls or Inconsistent Wall Thickness
Wall thickness is a critical factor in plastic part design, directly affecting how the material fills the cavity, cools, and shrinks.
What goes wrong:
Thin walls or abrupt transitions between thick and thin sections can lead to flow hesitation, sink marks, and warping. Thin areas may not fill properly, while thick areas may cool slower, causing internal stresses.
How to fix it:
Aim for uniform wall thickness throughout the part. If varying thickness is unavoidable, use gradual transitions or fillets to reduce internal stress. Keep walls thick enough to ensure full cavity filling but not so thick that they prolong cycle time unnecessarily.
📐 We review all part geometry to ensure manufacturability before tool design begins.
📏 Not Accounting for Material Shrinkage
All thermoplastics shrink as they cool, and if this isn’t considered during tool design, the resulting parts will be out of spec.
What goes wrong:
Ignoring shrinkage can result in parts that don’t fit, warp, or fail to meet critical tolerances — especially when mating with other components.
How to fix it:
Always use the material supplier’s recommended shrinkage percentage when scaling your cavity dimensions. These values vary widely between plastics — from 0.5% (ABS) to over 2% (HDPE).
🔍 Skipping Design for Manufacture (DFM) Reviews
Skipping a proper design for manufacture (DFM) review is like building a house without checking the foundation — risky and expensive.
What goes wrong:
Parts that look good on-screen may have undercuts, poor flow paths, or tolerance stack-ups that make them difficult or impossible to mould.
How to fix it:
Partner with a mouldmaker who will perform a DFM analysis before cutting steel. A good DFM review will highlight gating options, potential sink marks, draft concerns, and mould complexity. It also opens the door to design improvements that reduce cost.
At Newark Tools Ltd, we don’t just build tools — we build partnerships. From DFM reviews and rapid prototyping to fully hardened production tooling, our in-house team ensures that your mould performs exactly as needed, cycle after cycle.