Plastic Injection Mould Maintenance Programmes

In the maintenance of plastic injection moulds there are various levels depending on how restrictive the maintenance to be carried out must be. In this article they have been divided into 5 categories, ordered from least to most exhaustive.

The five levels into which maintenance is classified are as follows:

• The five levels into which maintenance is classified are as follows:
• Level 2. Simple preventive maintenance.
• Level 3. Inspection maintenance.
• Level 4. General maintenance.
• Level 5. Complete maintenance.

It should be emphasised that even if maintenance is level 1, the least restrictive, it cannot be omitted under any circumstances, as not carrying it out or delaying it could cause irreparable or very costly damage to the mould.

Level 1. Injection mould maintenance routine.

This is the maintenance to be carried out every time a mould is to be lowered from a machine..

For injection moulds that cool with cold water (below room temperature), before removing the mould from the machine, it must be heated to evaporate the droplets that remain on the surface of the mould due to condensation of water vapour. It must be ensured that there is no water left on the surface and that this could be the beginning of oxidation of the cavity.

All water circuits must be drained and blown free of all residual water to prevent rust build-up due to stagnant water, which will create flow reductions or even plugging of the lines. This will cause the mould to overheat and therefore may fail.

While the mould is still hot, the internal surfaces of the mould should be gently wiped with shop-safe solvent to remove residual dirt. The ejector pack should be completely removed and sprayed with a rust inhibitor spray, ensuring beforehand that the ejector pack moves correctly and the mould can be opened and closed.

Injection moulds with a gloss or mirror finish must be protected according to specific instructions.

Check that all bolts, plates, clamps, etc. are in place and tightened correctly.

The last injection moulded part from that production run should be placed in a bag, with the intention of remembering the quality level of the previously moulded parts. These parts should be kept together with the mould.

Level 2. Simple preventive maintenance.

This maintenance is foreseen every time a mould is taken from the warehouse.

Abra el molde de Inyección. Usando un disolvente de seguridad del taller, retire la grasa o sustancia de prevención de óxidos con una almohadilla libre de pelusa con especial cuidado de no rayar nada.

Existe una mala práctica, muy extendida, que es inyectar las piezas antes de limpiar el molde, para arrastrar la capa protectora con las primeras inyecciones. Esto puede derivar en rayado del molde y taponar las salidas de gas, tan importantes durante el proceso de inyección de plástico.

Level 3. Inspection maintenance.

This maintenance is carried out at the end of a production run or every 40,000 cycles by the shift manager or a mould shop operator.

Using the mould check list and visual inspection techniques, the mould is checked and any minor repairs that are necessary are recorded in the mould maintenance book. It is important for each mould to have one and to know the mould’s repair history so that they can be evaluated together in the future. If the mould needs any repairs, a visible red sticker will be placed on the side of the mould so that anyone in the plant is aware of it and does not take the mould back into production without having solved the problems.

Any missing components or voided cavities should be noted and addressed.

The mould must be washed with safety solvent to remove grease and impurities accumulated during the moulding process.

Las salidas de gas deben revisarse y limpiarse para un correcto funcionamiento del molde. En caso de ver suciedad excesiva por el quemado de gases habría que marcar una incidencia en el Libro de Mantenimiento del molde de Inyección y pasarlo a taller para su análisis y solución.

Level 4. General maintenance.

Only trained workshop technicians should perform this maintenance.

This maintenance is foreseen every 100,000 cycles or every 15 production runs.

The first step is to separate all the plates from the mould and clean them.

Mirror polished surfaces should NOT clean with brushes or rags as they will scratch and lose the finish. Only with uncontaminated facial tissues or cotton. Remove dirt by spraying cleaning liquid into the cavity and gently wipe with cotton.

Do not touch with your fingers, dust or shavings on your fingertips can scratch a polished surface.

Be careful when blowing a cavity, there may be dust or rust in the air circuits which can damage the polished cavity.

Surface finishes (textured, chrome, DLC,…) must be redone in case of rust or excessive wear.

All components should be inspected for wear and tear. If any excessive wear is observed, it should be reported and a decision made by competent personnel: repair, replace or continue use.

All areas of damage outside the cavity area should be reworked if they are a problem. Any areas within the cavity detail area with dents, dings or other signs of wear should be considered critical and should be carefully analysed before replacements or repairs are started.

All moving parts should be lubricated if necessary. Use lubricant sparingly on all moving parts that come into contact with plastic parts. In the case of food sector parts, there are greases suitable for this sector.

The vents should be verified to determine the depth, width and surface compared to the specs of the 3D model of the mold.

The seals and O-rings must be checked for integrity and tightness. Water leaks can generate oxides and, if they fall into the hot chamber, short circuits with very high costs.

All water circuits must be pressure tested for leaks and recertified to ensure a good flow rate. In case of clogging and blockage, the circuits must be pressure cleaned with a special cleaning fluid. Specific ultrasonic machines are also available for this purpose.

The ejector pack should be examined for proper alignment. If the ejector holes have become oval, it should be determined whether they should be re-drilled to a larger size and the ejectors replaced with larger diameter ejectors.

The return springs must be replaced to avoid fatigue problems every 50,000 cycles. If they are broken or have lost part of their effect, they shall be replaced earlier.

The material passage areas, both sprues and cooling bushings of the hot runners have to be checked and, in more detail, if they are fibre-loaded parts, because the material may have enlarged the inlets and important areas of the mould.

This data should be recorded in the check list and a decision should be taken to repair, if necessary.

Notes of work to be carried out during General Maintenance should be included on the form for future work. Bent, worn, seized or broken ejectors should be noted.

Level 5. Complete maintenance.

This maintenance must be carried out by qualified mould and die personnel.

It is performed at 250,000 cycles or half of the expected lifetime. It will be activated slightly earlier if excessive wear is observed.

Records in this maintenance are very important, so everything has to be properly recorded.

Before maintenance starts, there must be four complete injections (parts, sprues and runners) delivered with the moulds for the study.

Two injections will be used for initial mould qualification:

• This gives a visual record of what was acceptable when the mould was new and fully functional.
• The second two injections should be the last parts produced before the mould is removed for maintenance.

Comparison of before and after injections will give an excellent indication of tool wear.

All components indicated and authorised for replacement must be removed and new components installed in accordance with the original designs.

All slide guides, bushings and all bearing moving surfaces should be checked for wear and replaced or repaired as necessary.

Plates and mold cavity surfaces should be checked for both parallelism and flatness to ensure they have not changed from the original 3D model.

The mold cavity surface should be cleaned and polished as necessary to meet the original surface and texture requirements.

For high production moulds, cavities must be stripped and stress relieved to eliminate excessive hardening and brittleness of some materials.

Bumps, scratches and dents must be corrected until the surface fully complies with the original surface specifications.

All components that do not meet original part specifications must be repaired, replaced and re-evaluated as necessary (determined by company or customer).

All components that have been surface treated should be disassembled and reworked where wear is visible.

All components which have had special surface treatments for corrosion resistance, lubricity, hardness and the like must be removed to ensure the original intent of the component.

All moving components must be checked for movement, fit and play.

All return springs on the ejector plate must be replaced with new springs to avoid fatigue.

Allwater lines should be flushed with a descaling agent to remove scale build-up.

All O-rings, internal connections and seals must be replaced to ensure watertightness.

The whole set of moulds/cavities must be re-examined and re-qualified as if it were a new mould.

The identification of the mould must be redone indicating that it has passed a thorough check.

With this maintenance carried out at the end of the indicated cycles or when a problem or wear is detected, future errors and problems can be detected before they occur and thus avoid stoppages not contemplated in the work plan, thus saving time and money. In addition, if the moulds are well maintained, their service life can be significantly extended.