Leading producer of specialised PVD coatings for components and tools Oerlikon Balzers says its coatings protect moulds better to enable improved performance, while reducing maintenance and running costs for manufacturers.
Moulds that are used for the manufacture of plastic parts, from water bottles to automobile bumpers, are expensive and essential in the injection moulding process – where a single mould can sometimes cost several hundreds thousand dollars, not including ongoing maintenance and running costs. Despite being made of hardened steel, the process of injecting melted resins at high temperatures and pressure into cavities over many cycles in moulds causes wear erosion to cavity surfaces and moving components. Even thermoplastics can be abrasive or “sticky”, acting like sandpaper or leaving residue that wears down the surface of moulds. Furthermore, the increased use of abrasive material like long glass and composite fibres escalates the amount of abrasion and friction within moulds.
Thus, injection moulders are turning to a variety of coatings to protect their investments as well as reduce maintenance and running costs. Oerlikon Balzers says it offers coatings that can be applied to mould cavities as well as moving, sliding components within a mould. The coatings are available in a variety of styles, from carbon-based coatings, PVD and PACVD deposition to nitriding techniques, sharing an overriding goal – they harden the surface of steel, protecting the surfaces from abuse arising from the processes mentioned above, the company notes.
“These coatings are applied for the simple fact that steel alone is not hard enough to protect these expensive moulds and ensure optimal uptime and productivity,” explains Thomas Vermland, Global Application Manager for Oerlikon Balzers, which has been producing specialised PVD coatings for components and tools for over 30 years.
Reinforced glass fibre products
In the automotive industry, where there is greater emphasis on design and weight reduction, automotive designers increasingly use glass fibres as reinforcements in many parts, including automobile bumpers. As a result, automobile bumpers are constructed using less material than before. Here, increased pressure and higher temperatures are used to inject the plastic in the mould, amounting to a factor of two to four times higher than in the past, Vermland noted. As such, there is inevitably more friction (from sheering effects) that can cause polymers like polyethylene or polypropylene to become unstable during crystallisation. To compensate this, reinforced glass fibres are often utilised as an economical way to create a lighter, stronger part, which retains more flexibility.
Initially, short glass fibres were used – which did not create much of a problem because they did not affect crystallisation significantly. Moreover, the slightly abrasive nature of glass fibres had a positive “cleaning” effect, removing residue on internal cavities caused by out-gassing.
Today, the popular long glass fibres used are more abrasive, resulting in micro cutting within mould cavities and runners by sharp, jagged edges with a hardness of up to1200 HV. This, unfortunately, create significant issues, particularly, along the parting line and sharp contours in large moulds.
For large moulds like those used for automobile bumpers and tailgates, a nitriding process significantly increases the surface hardness of the metal of the mould. Nitriding is a heat-treating process that involves hydrogen, plasma and electricity to create a case-hardened diffusion layer on the surface metals. Because it is not a coating, it does not affect the overall dimension of the component.
Vermland says Oerlikon Balzers’ Balinit Primeform diffusion treatment increases injection mould surface hardness by up to 1400 HV, which makes moulds significantly more robust (including vs. long glass fibres), while reducing residue build-up, spalling and edge embrittlement.
“We can take the entire bumper mould and place it in the machine and treat it in one operation,” says Vermland and added: “With the diffusion treatment process, we can take a base steel and harden the first hundred microns of the surface to Rockwell hardness well above 65. The rest of the steel remains unaffected.” This is the replacement for Hard Chrome in any big mould. Vermland adds that a large steel mould of 65 HRC could not even be manufactured due to its size.