Durable, corrosion-resistant, anti-sticking coating for injection moulds
For decades, injection moulding has been one of the most common manufacturing methods for plastic commodities. From oversized polycarbonate furniture to polymeric microstructures, injection moulding ensures short production cycles, flexibility of materials and low labour cost.
Researchers at the City University of Hong Kong (CityU) developed an all-in-one composite coating that provides protection and release property to injection moulds while being durable, corrosion and abrasion-resistant.
The coating boosts productivity in major ways and promises much commercial potential. CityU is now looking for partners and investors from all over the world to commercialise their technology.
Despite its advantages, injection moulding entails high initial tooling cost especially when it involves elaborate multi-part mould design. Today’s plastic materials also take their toll on tool life. Additives range from glass-filler and mineral-filler to rice hulls, wood fibre and ground metal – all of which increase mould wear. Worse still, certain plastic formulae such as PVC and halogenated fire retardants produce corrosive gases when heated.
When corrosion couples with abrasion, the damage done to the mould could be immense. Conventionally, hard chrome plating by electroplating is used to combat corrosion and minimise mould wear. However, plating non-uniformity and the presence of micro-cracks may cause coating to wear down easily. Also, the electroplating process is not environmental friendly.
On the other hand, the interaction between the mould and the moulded part could slow down the production cycles, not to mention the residue sticking to the mould after release that needs to be cleaned up manually.
To lubricate the mould, release agents such as polytetrafluoroethylene (PTFE) are applied. However, PTFE has no hardness and doesn’t last for a long time. It needs to be re-applied and cleaned after a number of runs. That again slows down the production cycle.
All-in-one mould coating
In light of this, researchers at CityU have been working on an all-in-one composite mould coating that provides good release property alongside corrosion and abrasion resistance, yet without the need to re-apply or clean up frequently.
Basically, metal nickel is added to brittle chromium nitride coating by the physical vapour deposition (PVD) method. The research team experimented with varying compositions of chromium nitride in different phases (namely face-centred cubic fcc-CrN and hexagonal close-packed hcp-Cr2N) in relation to metal nickel to reach the optimal properties.
There is no chemical reaction between nickel and nitrogen and the metal is added just to improve the toughness while maintaining the hardness. The resulting anti-sticking coating demonstrates enhanced properties, such as high hardness, corrosion resistance, oxidation resistance, wear resistance and toughness.
It is 3.5 times harder than uncoated steel with wear rate improved by 40 times. Its mould release force is very small, which is similar to that of PTFE so that it is ‘slippery’ enough to ensure easy and quick release of moulded parts. Its corrosion resistance property has also improved by 20,000 times over uncoated steel. Resistance to high temperature in air can be up to 500 °C.
CityU is seeking partners and investors from all over the world to commercialise their all-in-one coating for plastic injection mould. The technology, including the coating formula and prescribed manufacturing conditions, is ready for licensing. The team is also available to offer consultancy services to clients who need help in replicating the formulae and process.
Contact information: wenming.ji [at] isis.ox.ac.uk (Dr. Wenming Ji)
• This article is part of the “ITF Projects Commercialisation Programme” hosted by Hong Kong Science and Technology Parks Corporation (HKSTP) in conjunction with Isis Innovation, the technology transfer company of the University of Oxford. The programme aims to expose Hong Kong innovations to international audiences and hence, facilitate commercialisation via the network of Isis. For information on all projects in the programme, please visit
This project is organised by Hong Kong Science & Technology Parks Corporation. Any opinions, findings, conclusions or recommendations expressed in this material/event (or by members of the project team) do not reflect the views of the Government of Hong Kong Special Administrative Region, the Innovation and Technology Commission or the Vetting Committee of the General Support Programme of the Innovation and Technology Fund.