Mould quality is poor. Is the mould really the problem?

Rarely. Poor mould quality is usually the visible result of decisions about material, design freeze and manufacturability that were not controlled before the steel was cut.

The moulds were already made. The quality was not acceptable.

A company that makes a compact medical device, with a charging case and several plastic parts, was in exactly this position. It had built its tooling without independent governance. The first question was natural: can the existing moulds be corrected?

That is where most teams start. The visible problem is in the tooling, so the repair is planned in the tooling. But the review pointed somewhere else. The moulds were not the starting point of the failure. They were the physical record of decisions that had not been controlled early enough.

What the mould review found

The company shared its mould analysis report and the 3D design data. We reviewed both and presented the findings as a risk-prevention and learning discussion, not as a search for someone to blame.

The analysis was clear. For most of the moulds, changes were not possible in the existing tool. For one mould, a clear part, a change was technically possible but not advisable. The reasons were practical:

• A change of material had left a shrinkage mismatch against the cavity dimensions.
• Correcting the geometry meant extensive welding inside the cavities.
• There was not enough space for lifters and sliders, and the cooling lines were in the way.
• A clear part with a mirror finish carries a high cosmetic risk on any welded surface.

A repair that is technically possible is not automatically an acceptable production solution. Heavy welding reduces dimensional stability, affects mould life and adds variation in production.

The design problems behind the moulds

The 3D data showed three design interferences, found before any further tooling work:

1. Charging case, hinge and lock. When the case closed, a corner feature near the hinge clashed with the lock area. This can cause binding, wear and difficulty closing.
2. Middle case and sensor key. The two geometries overlapped, which can cause assembly stress and deformation.
3. Main circuit board and bottom case. The board clashed with internal features of the bottom case, which risks board damage and assembly stress.

A perfectly made mould can only reproduce the geometry it is given. It cannot remove a clash that is already in the approved design. This is why tooling review and product design review need to be one connected activity.

Where the project really broke down

The pattern across the moulds mattered more than any single defect. Design revisions arrived after the tool concept was approved. Recovery was attempted through welding, local corrections, cancelled layouts and rework. Tool limitations became visible only after the steel decisions had been made.

Underneath that pattern were five gaps:

• Manufacturability was not validated early.
• There was no hard design freeze.
• Material shrinkage changed after the tool reference was set.
• Welding was assumed to be a way back.
• No single owner controlled decisions across functions.

These are governance gaps, not workmanship gaps. A toolmaker works inside the design, the material assumptions and the tool concept it is given. When those inputs move after the critical decisions, the options narrow fast. Tooling problems were outcomes, not root causes. Trials did not create these problems; they revealed decisions made earlier.

Three gates to close before any steel is cut

These are general good practice for any injection-moulded product.

Gate 1: Material finalisation. Lock the resin grade and align its shrinkage with the cavity sizing. After that, a material change needs a documented impact review, because it is never only a purchasing decision once cavities are sized.

Gate 2: Engineering DFM. "The part can be moulded" is not a review. Check that wall thickness is harmonised, the insert strategy is validated and undercuts fit within the mould envelope. Check plastic parts, electronics and other bought-out parts together for fit and interference.

Gate 3: Design freeze. Work from one frozen CAD baseline, with a change-control protocol that records the effect of every revision on the product and the tool concept. A freeze does not forbid change. It makes sure no change silently cancels an earlier approval.

Four tooling decisions to lock early

Beyond the gates, four decisions set the risk level of the tool. They do not set its quality, and they cannot be fixed later by good workmanship.

• Modify an old tool, or build a new one?
• Will welding be allowed on clear parts, or is the policy zero weld?
• Family mould or single cavity?
• When space is tight, does cooling or the mechanism come first?

For more on closing decisions before you order, see What to decide before you order a mould. If you are still comparing toolmakers, start with how to choose a mould supplier.

What this company did next

After the review, the company chose to restart the programme under governance and build new moulds on the latest design baseline. The programme was run again step by step: design, bought-out parts and the statement of requirements were each controlled in sequence, in the order concept, engineering DFM, design freeze, tool concept approval, tool manufacturing and controlled first trials. Their confidence in the approach became complete. The project is still in development.

Tooling cost is visible the day the moulds are ordered. Tooling risk is decided much earlier, when material, geometry and assumptions are approved. Governance exists to control that earlier, invisible risk.

A self-check before you commit to tooling

1. Is the final material approved, with shrinkage aligned to the cavity dimensions?
2. Has an engineering DFM review confirmed wall thickness, inserts, undercuts and mechanism space?
3. Have all plastic parts been checked for interference with mating parts and bought-out assemblies?
4. Is there one formally approved CAD baseline?
5. Does every design change need a documented tooling-impact decision?
6. Have the mould type, welding policy and cooling-versus-mechanism priority been approved?
7. Does one accountable owner coordinate decisions across design, engineering and tooling?

Common questions

Can a poor-quality mould always be repaired?

No. It depends on the cavity design, the material shrinkage, the space for mechanisms, the cooling layout and the surface requirement. Extensive welding can add dimensional instability and variation. Treat repair as an engineering decision, not as the default.

Why can a correctly moulded part still fail at assembly?

Because the approved design may already contain a clash between mating parts. Plastic housings, moving features and electronics must be checked together in 3D. Moulding a part correctly does not remove an interference built into the design.

Should changes be banned after design freeze?

No. A freeze creates a controlled baseline. Changes can still be made, but their effect on material, tool feasibility, fit and earlier approvals must be reviewed first.

Who should own decisions before tooling begins?

One accountable owner. Different functions give technical input, but open decisions and changes should never pass between them without a named person responsible.

Related articles: What to decide before you order a mould: four decisions to close first