Injection moulding is the largest segment of plastics processing. Today the moulders have to use moulds with better coating that lasts longer and further understand preventive maintenance, as they are increasingly processing polymers or composites that are more aggressive to the surface of the moulds. Aggressive conditions of out gassing and moisture acidity often accompany abrasive wear as potential damage to expensive tooling. Further, the use of glass filler and wood fiber, etc creates more challenges. The growing tool complexity involves tinier, more intricate flow passages and more frequent use of moving cores and slides. All these factors have prompted development of a wider variety of mould coatings that can keep moulds operating longer between repair.

Typically hard chrome plating with Rockwell hardness of about 72 is found to be quite good. Newer coatings with better performance are now available. A major drawback is chrome's environmental impact, since chromium is a carcinogen. Electro less nickel has been used successfully for years, particularly to protect moulds where corrosive off gassing is created by materials such as PVC or halogenated fire retardants.

Several new coatings and treatments have been developed to prolong tool life or add performance.
Nickel-cobalt can be an economical alternative to hard chrome. Hard chrome requires construction of a conforming anode to coat the mould. The more the detailing in the mould, the more time it takes to build the anode and the more expensive the process becomes. NiHard requires no anode, and because of its electroless properties, it plates much more uniformly. Cobalt gives it good abrasion resistance, but its hardness is 62 RC, 10 points lower than hard chrome.

Hard chrome and NiHard offer two very good solutions for abrasion resistance, but for very high-wear conditions, an even newer product called diamond-chrome offers exceptional protection. Developed by Bales, it has an RC rating greater than 85. It is a chromium-matrix composite with a dispersion of nanometer-size, spherical diamond particles. Since diamonds are unmatched for hardness, this coating offers protection beyond the norm. It outperforms titanium nitride coating, though the two have similar Rockwell ratings, because it won't compromise the dimensional integrity of the plated tool. NiHard is applied at only about 50 degree C. Titanium nitride (TiN) requires application temperatures of 400 degree C or higher.

Diamond-chrome can plate pre-hardened, heat-treated, or nitrided steel and other base materials such as aluminum, beryllium-copper, brass and stainless steel. Recommended uses include cores, cavities, slides, ejector sleeves and rotating and unscrewing cores.

Diamond-chrome is also very strippable and has no adverse effect on the base material, saving time and money when maintenance is needed. TiN is strippable as well, but it can take several days to remove with a polishing or peroxide-based solution. Diamond-chrome can be stripped in minutes using reverse electrolysis in a caustic solution. Diamond-chrome can coat complex details, while TiN has very limited coverage of complex details. While TiN is very lubricious, with a coefficient of friction of 0.4 (against steel), diamond-chrome has a COF of 0.15, nearly three times more lubricating.

To provide a combination of excellent release properties and high resistance to wear, heat, and corrosion, Bales recently introduced a specialty coating called Nibore. It is an electroless nickel-phosphorus matrix containing boron nitride particles. It has a very low coefficient of friction (0.05 against steel) and an RC hardness of 54, which can be increased to 67 RC after heat treating-a unique characteristic. Nickel-boron nitride can be applied to any substrate at only 80 degree C and can be easily stripped without compromising the base material. Because applying Nibore is an utocatalytic process, it requires no anode, therefore saving time and money. In addition, Nibore will not compromise thermal conductivity of the mould. Applications include unscrewing cores for closures, where reduced cycle times are essential.

Where very high lubricity is needed for deep ribs, zero-draft cores, textured surfaces and "sticky" polymers, a coating of Bales' Nicklon nickel-PTFE composite will greatly improve part release and enhance resin flow by as much as 4% to 8% for shorter cycle times. Its COF is 0.10 against steel. It should be noted that applying pure PTFE to the mould adds high lubricity, but only as a very short-term benefit. PTFE by itself has no hardness, so it won't last. But a dispersion of 25% PTFE by volume in a co-deposit with nickel results in 45 RC hardness for added wear protection.

When it comes to lubricity and release properties, the mould finish as well as the possibility of using a coating must be taken into account. Certain mould finishes may increase the need for a mould coating. After an initial investment in mould coating to improve tool performance, a preventive maintenance program is always a good idea to ensure maximum benefit. Coating wears out after a certain time, and producing substandard parts from a mould with a worn coating leads to a waste of time and money. It is important to monitor moulds during production, and, to minimize damages, spot the deterioration in coating at the initial onset, especially in high-wear areas such as gates and runners. Missing important 'wear signals' will mean more costly repairs and additional polishing expense. One of the most important aspects of a preventive program is pulling the mould for maintenance before the coating wears through.