Signs of mould wear: when is intervention needed?

An injection mould is one of the most valuable assets in your production process. The investment can quickly amount to tens of thousands of euros, and for complex multi-cavity or hot-runner moulds even hundreds of thousands, yet in practice maintenance is often postponed, skipped or reduced to a simple purge between two production runs. Until something goes wrong.

In this article, we explain why preventive mould maintenance is not a cost item but an investment, which warning signs you should never ignore, and what a good maintenance programme looks like in practice.

A poorly maintained mould rarely breaks from one day to the next. What usually happens is more gradual and more insidious: the quality of the parts produced slowly drifts, cycle times increase, and at some point the products fall outside tolerance without there being a clearly identifiable tipping point.

A well-maintained steel mould can easily last 500,000 to more than 1 million cycles. Moulds made from hardened steel such as H13 or S136, correctly designed and regularly maintained, can readily exceed one million shots (source: Alpinemold, Injection Mold Life Expectancy). Top-quality moulds (SPI Class 101) with excellent maintenance can even achieve several million shots in practice. But the same mould, with identical material and an identical design, can start showing problems after only a fraction of that number if neglected. The difference lies almost entirely in the maintenance.

Most mould problems are not inevitable. They are predictable, and therefore preventable. The key is a maintenance system based on fixed cycle counters rather than “repair it when it breaks”.

A practical system works on three levels (the cycle intervals below are guideline values; see the factors listed further on):

The exact intervals depend on the type of plastic (abrasive materials such as glass fibre-filled compounds significantly accelerate wear), the production speed and the steel grade of the mould.

At every production start and stop, a visual inspection should be carried out for signs of flash, sticking marks or surface damage. Moving parts such as slides and ejectors are lubricated regularly. After use, the mould is cleaned to remove material residues and outgassed gases; this helps prevent burn marks caused by trapped gases, the so-called diesel effect (source: Fleur Plastics, Defecten bij spuitgieten voorkomen). These basic actions may seem self-evident, but under time pressure they are the first to be skipped, even though they contribute most to a long mould service life.

At regular cycle intervals, all ejector pins and mechanical components are checked for wear and damage. The cooling channels are checked for flow. The presence of limescale deposits in the cooling channels is a common and underestimated problem: the buildup of limescale greatly reduces cooling efficiency, leading to production delays and thermal imbalance in the mould (source: Karel Clean, Het reinigen van koelkanalen bij Final Plastics).

At specific milestones, full disassembly is recommended. The mould is completely dismantled, all components are measured (3D measurement) and compared with the original drawings, cooling channels are flushed or chemically cleaned, and worn parts are replaced or repaired. An overhaul at the right time costs only a fraction of what unplanned downtime or a damaged mould would cost.

The cycle-controlled system described in the previous section is already a major step forward compared with “repairing it when it breaks”. But there is another step higher on the ladder. While preventive maintenance works with fixed intervals — inspecting every 10,000 cycles regardless of the actual condition — condition-based maintenance (CBM) starts from the actual condition of the mould. You do not intervene because the counter reaches a certain number, but because measurement data shows that intervention is becoming necessary (source: TecDoc Maintenance, Best practices for condition-based maintenance in industry).

The difference is more than semantics. A fixed interval is, by definition, a compromise: set broadly enough to protect the mould, and therefore too early some of the time; you disassemble and inspect while nothing was actually wrong yet. If, on the other hand, the mould wears faster than expected, for example due to an abrasive batch or a cooling channel that clogs more quickly, the fixed interval comes just too late. CBM solves both sides: you perform maintenance when the condition calls for it, not earlier and not later.

The modern injection mould is well suited to condition monitoring, because many of the wear signals discussed in this article translate into measurable parameters:

• Cavity pressure and temperature. In-mould sensors record the pressure and temperature profile with every shot. Deviations in the pressure curve reveal early flash, incomplete filling (short shots) or process drift before they become visible on the product, thereby ensuring dimensional stability shot after shot (source: Kistler, Monitoring and control of injection molding processes).
• Ejection force. An increasing force on the ejector pins indicates contamination or deposits in the cavity, as well as ejectors that are running roughly or sticking — a direct signal that cleaning or lubrication is required (source: Kistler, Predictive maintenance and mould monitoring).
• Strain on the mould plates. Strain and distance sensors detect gradual deformation and changes in the interaction between the mould halves, revealing wear on the sensitive parting line at an early stage (source: Kistler, Predictive maintenance and mould monitoring).
• Cooling performance. Monitoring the flow rate and the temperature difference between supply and return makes limescale build-up or blockage visible long before the cycle time noticeably increases — exactly the creeping problem described in signal 4.

Together, these measurements create a continuous, objective picture of the mould’s condition. The same data also make it possible to stop estimating maintenance intervals and instead substantiate them with real measurement values (source: Kistler, Monitoring and control of injection molding processes).

Condition monitoring is not an end in itself. Sensors, data processing and their integration cost money, and not every mould will earn back that investment. It makes the biggest difference above all for high-volume moulds, fast-running moulds with short cycle times, expensive multi-cavity tools and applications with tight tolerances, such as healthcare and high-tech — precisely where unplanned downtime or scrap has the greatest impact. For moulds with lower volumes or wider tolerances, a disciplined cycle-based maintenance schedule remains the right and most cost-efficient choice.

Two final comments. First, CBM stands or falls with the same documentation discipline that every maintenance programme requires: measurement data without history and without someone to monitor the trends delivers little value. Second, condition-based maintenance is not the same as predictive maintenance. CBM tells you what the condition is now; predictive maintenance uses that same data plus analysis to estimate when a component will fail. The former is a realistic, achievable step for most injection moulding companies today; the latter is the horizon that lies beyond it.

The service life of an injection mould is not only a matter of maintenance. Several factors play a decisive role:

• Steel grade and hardness: hardened steel (typically 48–52 HRC for the hardest mould steels such as H13 and S136) offers the best resistance to wear and significantly extends service life compared with softer pre-hardened steel (source: VEM Tooling, Injection Mold Life Expectancy).
• Mould design: complex geometries with thin walls, sharp corners and deep cavities create additional wear and stress points.
• Processed material: abrasive or corrosive plastics (glass fibres, flame retardants) greatly accelerate wear and require adapted steel grades or surface treatments.
• Surface treatment: coatings and treatments such as nitriding or PVD increase the wear and corrosion resistance of the cavity surface and often provide a cost-effective alternative to a more expensive steel grade for abrasive materials.
• Production speed: fast-running moulds with cycle times under 10 seconds require more frequent maintenance than moulds with longer cycles (source: Erkotech, Spuitgietmatrijzen).
• Quality of maintenance: the most decisive factor in the long term

Maintenance without documentation is not a programme; it is ad hoc intervention. A proper mould maintenance log contains at least:

• The number of cycles at the time of intervention
• The actions performed and the materials used
• Any deviations identified
• The name of the technician who carried out the work

This history is valuable. It makes it possible to identify trends, such as a cooling channel that repeatedly becomes blocked at the same point or an ejector pin that wears faster than expected, and to adjust maintenance intervals based on factual data. Digital registration also makes it possible to link automatic notifications to cycle counters (shot counters), so that maintenance is scheduled proactively rather than reactively (sources: Erkotech, Spuitgietmatrijzen; TecDoc Maintenance, Best practices for a preventive maintenance plan in industry).

Postponed maintenance may seem like a short-term saving. However, the hidden costs are considerable:

Product rejects due to quality deviations lead to additional material consumption, sorting time and, in the worst case, customer complaints or recalls.

Unplanned downtime is structurally more expensive than planned maintenance. An emergency stop requires urgent intervention, higher labour costs, priority shifts in the workshop and a longer recovery process because damage has accumulated.

Premature replacement of the mould is the worst-case scenario. A mould that should have lasted two million cycles but fails to reach even half that represents a loss of tens of thousands of euros in tooling investment, caused by avoidable neglect.

Preventive mould maintenance is not only an internal task for the manufacturer. A tooling partner that thinks along with you from the engineering phase lays the foundation for a mould that continues to perform reliably in the long term. At Tumag, that is exactly what we do: we advise on the steel grade that offers the longest service life for your specific application, design cooling channels optimally for minimum cycle time, and where possible provide the critical spare parts that limit downtime during a future overhaul.

But Tumag is also ready to support existing moulds. Our mould maintenance programme includes preventive maintenance, overhauls and spare part management. For urgent cases, we offer a 24-hour laser welding service that allows damaged moulds — such as the impacted shut-off surfaces or damaged cavities mentioned in the signals above — to be repaired quickly and accurately, keeping your production downtime to an absolute minimum.

The mould that leaves our workshop today will need maintenance in a few years’ time. The better that future is prepared for today, the less that day will catch you by surprise. Contact the Tumag team to discuss what we can do for your tooling.