We see some interesting things with the injection molds that come across our plate. From parts sticking to galling, there are a lot of issues that can kill mold performance. The good news, surface coatings for injection molds can solve many of these issues.
Let’s take a look at common Issues we use surface coatings to remedy.
Part sticking in the mold, not releasing well
Parts sticking to the mold or ejectors and not releasing properly can oftentimes be remedied with the right combination of texture/polish and surface coating. In many cases, adding a light blast texture to a polished surface will reduce the stiction force that sticks the part in the mold. However, it isn’t always as simple as choosing the lowest coefficient of friction surface coating.
PTFE (Tradename: Teflon) is one of lowest coefficient of friction materials at 0.1 +/- 0.05, but the PTFE coating needs to be masked off because the coating thickness will affect mold tolerances. PTFE is soft and can wear off easily as the mold cycles.
Another option to consider is PVD Titanium Nitride (TiN) that has a coefficient of friction around 0.4. While the coefficient of friction is higher than PTFE, TiN is much more durable than PTFE and will last significantly longer before reapplication.
Titanium nitride has a Rockwell hardness of approximately 85Rc, while the Rockwell hardness of PTFE is around 50Rc.
Vents clogging and corroding
One of the most frequent reasons injection molds are taken down for maintenance is to clean the venting. During the injection molding process as plastic fills the mold cavity air quickly escapes through various venting locations in the mold, which can become clogged or corroded over time.
Injection mold vents can’t be too large, or molten plastic will shoot into the vents. If the vents are too small or clogged, the air doesn’t exhaust fast enough and the process cycle time slows down. Polishing vents to an SPI A-3 and applying a thin PVD Titanium Nitride (TiN) surface coating will allow the vents to resist material buildup that will clog the vents and decrease corrosion.
Mold surface/texture wearing away and/or pitting
After 2 – 3 months of production time and 50,000 – 100,000+ cycles, the mold texture, parting line and gate will exhibit signs of abrasion wear and chemical corrosion.
The wear and corrosion results from the molten resin and corrosive injection gases repeatedly beating over the surface of the mold, slowly wearing away the peaks and valleys that make up the surface texture or polish.
Titanium Nitride PVD coating will increase surface hardness to 85 Rockwell C and have better abrasion wear resistance. The surface coating’s inertness also acts to help combat corrosion.
Galling on moving surfaces
Ejector guides and other mold components are other areas where a coating will help. PVD coatings improve lubricity to protect against seizure. With a hard, protective surface, the mold will be hanging in the injection machine making parts versus sitting idle on the tool room bench.
Injection mold performance can be increased with surface coatings, which allows the mold to run longer in between preventative maintenance, ensures faster turnaround when maintenance is needed, and reduces the overall operating cost.