Trega's Permanent Molds Page

Trega's Permanent Mold Process
(A.K.A. Gravity Die Casting)

	The permanent mold process has been used in Europe for decades, and is rapidly gaining popularity in the U.S. Everything from lawn mower decks to pistons are being produced with the permanent mold process. (referred to as PM from here on). This casting technique consists of pouring molten metal into metal molds under gravity pressure only. Unlike sand casting molds, the PM molds are permanent and are used over and over again. From one mold, tens of thousands of parts may be produced until the mold eventually wears out. Mold life depends on the complexity of the part and desired tolerances. Once the cavity is machined in the mold a system of runners and gates are machined in the mold to direct molten metal to the cavity. There are two basic methods of PM that Trega uses to produce castings: Vertical Pour Tilt Pour
	Vertical Pour is similar to a vertical gated sand mold. Molten metal is poured directly into a runner and is directed into the mold cavity, usually on the side or bottom of the part.
	Tilt Pour In the tilt pour process, the metal is ladled from the furnace into a cast iron cup attached to the front of the mold and connected to the runners. When the operator is done filling the cup the machine begins to tilt back. This starts a gradual filling of the mold cavity. This slow and controlled filling of the mold reduces turbulence, thereby reducing dross inclusions and trapped air in the casting. This results in a quality casting with mechanical properties higher than sand casting and comparable to pressure die Surface finish for PM parts are better than sand casting, but tend to be a little rougher than die cast parts. This is due to the mold release agent used on the mold. Trega has had excellent result casting O-ring grooves which do not have to be machined, and very fine lettering down to .125" high.
	The PM process yields very good tolerances, as compared to sand casting. Casting tolerances as low as plus or minus .005" can be maintained with the only variable being the thickness of the mold coating. This combination of surface finish and tolerances allows many of our customers to use parts "as cast", with no additional machining required. Cast-in features can also be achieved with the PM process. Steel sleeves, cylinder linings, and threaded inserts are easily cast into parts with the PM process. Most permanent molds are made of gray cast iron. This material has good resistance to erosion from the molten aluminum, transfers heat well and can be shaped into molds economically. Molds generally cost 1/5 to 1/10 the cost of a die cast mold. Additionally, repair or modification to a cast iron mold is less expensive than to a hardened steel mold.
	Designing for PM draft on all walls perpendicular to parting line should be 2° ( 1° is possible ) minimum wall thickness of .125" keep consistent wall thickness across entire part if possible By getting involved early in the casting design process, both Trega and the customer benefits. This partnership in design is something Trega strives for with all their customers.