Don’t you hate it when you spend hundreds of thousands of dollars on mold design and fabrication, you think through every possible issue and then a part still gets hung up in the tool? We understand. It’s these issues that keep our technical team working late to improve mold coatings.
Release coatings are necessary to support consistent part ejection during the injection molding process. This is especially important for automatic molding systems. If a part is hung up in the tool and not picked by the end-of-arm tooling it can lead to major issues.
Best-case, the ejection error will be sensed by the end-of-arm tooling and stop the production process until the machine can be manually reset.
Worst-case, the part is partially ejected, hangs up, and results in crashing the tool.
To aid with consistent ejection and part release, non-stick surface coatings are often applied. What type of non-stick coating is used? Great question. There are actually a couple of options to consider.
The Baking Approach
A common non-stick coating material used in injection molding applications is PTFE, polytetrafluoroethylene (say that three times fast).
Yes, it’s the same material used on cookware and bakeware. PTFE is applied with a spray coating process and then is baked on in an oven. This results in a non-stick surface.
As great as it is for making your muffins slide out unharmed, there is a weakness to PTFE coatings. PTFE spray coatings result in a mechanical bond to the tool surface. Mechanical bonds can fail faster than chemical bonding.
No big deal when you’re talking muffins, but it’s a little more concern when we’re talking expensive molds. Why does it fail?
Mechanical bonding relies on simple mechanical principles to mechanically attach to the mold surface. Once the mechanical bond starts to break, the entire surface coating will start to wear down. When you see an example of a surface coating chipping off, this is an indicator of a mechanical bond.
A Better Bonding Alternative
Another type of bond is chemical bonding. This results in surface coating molecules covalently bonded with the molecules of the mold substrate. The surface layer is impregnated into the substrate creating a bond that must break atomic layers in order to fail.
More specifically, titanium nitride (TiN) is applied to the tool surface with physical vapor deposition (PVD). The result is a strong chemical bond with the tool substrate that is much stronger than PTFE.
Think of it like the difference between a Hollywood marriage and soulmates tying the knot. One is guaranteed to fail and the other is going to live a long and happy life together.
Let’s take a look at a real-world example.
A custom injection molder was making a thin rubber part with some undercuts using an automatic ejection process. The tool was coated with PTFE. The rubber molding resin was outgassing and left a residue on the mold surface.
This was cleaned off every 10 minutes with a manual process using a lint free cloth. After three months the PTFE coating started to wear off in the area that was frequently cleaned (there goes that Hollywood marriage down the drain).
Titanium Nitride would be a more ideal solution to increase durability of the surface layer and result in a non-stick surface. Since PVD Titanium Nitride coatings are 2 – 6 microns thick (0.0001 – 0.0002”), the coating process does not affect tolerances and keeps the mold within original specs.