Highly-filled engineering plastics can cause wear on conventional steel molds constructed with insufficient hardness. Of the factors causing wear, fiberglass content has the most influence with a Mohs hardness of 5 to 7. Comparatively, carbon fiber has a Mohs hardness of 2, and common tool steels have a hardness of 4.
Wear can be minimized by proper processing and properly hardened tool steel cavities, cores, runner systems and sprue bushings. Cavities must be vented at the end of fill to minimize trapped gasses, which could cause pitting from high temperatures. Gates can be affected by the fast injection speeds used in processing glass-filled Long Fiber Compounds. These speeds can cause high temperatures and a loss of hardness.
The mold cavity and core finish play an important role in tool longevity, and machining marks have been shown to accelerate wear. A 4 microinch (0.0001 mm) or better finish is recommended for high production cores and cavities. Gates should be hardened and replaceable to obtain mold longevity.
Many tool steels are able to resist the erosion caused by glass filled materials. The choice of tool steel is dictated by economics, location within the mold and life expectancy required.
The following are tool steels with good abrasion resistance:
- A-2 Steel resists serious abrasion when hardened to 58-60 Rockwell C(Rc)
- D-2 Steel contains more chromium, is more resistant to abrasion, and is somewhat harder to machine than A-2. D-2 is limited to smaller components due to its brittleness.
Mold plating is an excellent way to improve the service life of a mold. Effective abrasion-resistant coatings include electroless nickel plating, slow deposition dense chrome, and nye-carb plating.
For long production molds, A-2 or D-2 tool steel hardened to Rockwell C~60+ is recommended. Of these, A-2 steel is a little more flexible and forgiving. For low volume runs, S-7 and H-13 are acceptable softer steels.
