Common Types Of Surface Treatments For Plastic Injection Mold Tools – Which Is Best

1. Plating

One of the earliest forms of mould treatment is chrome plating. This increases tool surface hardness so it’s much more resistant to abrasion when using glass-filled resins. Chrome helps to prevent rust but it’s not resistant to chemical outgassing when using corrosive resins such as PVC. Also, chrome plating requires the use of a separate conformal anode that goes down inside the mould cavity, this is because if you only have the external anodes, the chrome will not be attracted down inside the cavity and is more likely to build up on the convex corners at the mouth of the cavity. Electroless nickel plating doesn’t need an anode at all, so it can be used to coat all features inside the mold uniformly. It helps the mould resist chemical attack but it’s not as good at handling abrasive fibers.

Nickel-Boron Nitride, another common form of plating, also improves abrasion and corrosion resistance without adversely affecting heat transfer properties.  But its main advantage is that it greatly reduces friction,  so it’s used for moveable sliders, cores and other high-wear areas in the mold.

2. Physical Vapor Deposition (PVD)

In this process, the “source” is the object that will be vaporized. It’s usually a rod or ingot of ceramic or metal that is struck with a high-energy ion beam that causes the material to “sputter” into a plasma. This plasma bonds to the tool wall in very thin layers with great adhesive strength. How thin are these layers? Some are nanometers, certainly less than a micron, so not enough to affect the final part dimensions in most cases. One of the most common applications on tools is Titanium Nitride or  TiN. It’s exceptionally durable and increases hardness greatly, while offering better corrosion resistance than chrome. It can withstand the chemical attack of PVC resins while enhancing lubricity. One potential drawback is that it’s applied at temperatures of 800C  so this can affect the tool’s heat treatment. Another limitation is that this is a line-of-sight application, so it can’t be applied to hidden areas.

3. Chemical Vapor Deposition (CVD)

Unlike PVD, chemical vapor deposition uses a chemical reaction to produce a gaseous thin film.  CVD coatings are thicker than PVD and applied at even higher temperatures. The advantage is that diamond coatings can be applied, offering the highest possible wear resistance and increased tool hardness for an extremely long tool life. CVD coatings also don’t need to be applied in line of sight, so the whole tool can be treated. One drawback to CVD is that, because of the high application temperatures, mould tools must be heat treated a second time after coating.

4. Spray-On Coatings

Another way to alter the surface tribology of a mould is with a spray-on mould release agent. These do not improve hardness or durability but are designed to increase lubricity, by lowering the coefficient of friction, so that parts can be removed more easily. Lower friction also aids in faster mould packing time. The type of resin being molded will determine the best type of coating to use,  although most are based on formulations of silicone, molybdenum, lithium or PTFE,  Polytetrafluoroethylene, also known by its brand name Teflon,  which has a surprisingly high melting temperature of 327C or 600F. Newer spray-on coatings use nanotechnology to create semi-permanent thin films that are more durable and greatly reduce friction. This helps to improve production efficiency by decreasing cycle times and prolonging tool life. Plus, it’s pretty cool saying “nano” about anything.