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TOPIC:   H-13 Tool Steel Surface Enhancement for Polysulfone and Zytel GRZ
 

Submitted:   February, 2002

The intent of this e-mail is to find the optimum surface enhancement for a tool running polysulfone and glass reinforced nylon. The mold is a one cavity with a part geometry forming a 6" radius hemi-spherical dome (approx 1 degree draft on side wall connecting hemi-spherical section near parting line). Part is center-gated. There is a stripper ring around peripheral wall. There are two ribs (forming an X) protruding from the inside of the dome (these are cut into the top of the core). There is a center ejector pin boss connecting the intersection of the X. All the rib walls have double drafted surfaces (1.5 and 3 degrees). One would expect a lot of draft, however this is not the case, as the polysulfone is extremely sensitive to molded in stress as well as ejected,  in stress.

I would like to find an economic long lasting, low coefficient of friction (dynamic and static ), a high temperature resistant (the hot-oil mold runs at near 320F-330F) , high abrasive resistant surface enhancement that doesn't change the core micro-surface finish to give the molded article a slightly frosted affect after the surface enhancement has been added to H-13 tool steel core and lasts up to the rigorous glass fiber abrasiveness when running GRZytel. Could somebody help me to solve this problem?
Envisioning the application described, it is agreed that stress will be a function of this process. Assuming you have already discussed with your material supplier any experiences for lowering the molded in stress, we agree on the route of eliminating the drag during ejection.

Before any treatment on the existing core, common practice would be to evaluate the following: How is the polish in the ribs? Were the ribs cut or EDMed? Were the ribs draw polished? The ribs should be closely inspected for any traces of EDM or cutter marks. They should also have a very clean, sturdy draw polish with 600 stone PRIOR to applying any sort of coating.

Treatments to probably avoid, TiN (to not heatup your one core), Chrome (which may cause a suction issue), and as you mentioned, anything that will alter the finish (Armalloy). Treatments to pursue might be Chrome with Teflon, or Electroless nickle with cobalt.... Something to give hardness and lubricity. Refer to options at www.balesmold.com.

If down the road that core needs replacement, a possible approach will be a conformally cooled core, possibly bimetalic, a high conductivity core with a high hard surface.