Chapter 8: Release Materials
Excerpt from the NADCA Publication E-410 by E. A. Herman
When the die has opened and the casting has been ejected, but before the die closes again, the cavity areas of the die receive an application of a release material. The application is usually made by spraying the material as a fine mist. Sometimes the application is not made every time the die is open, but only after some small number of shots. The purpose of the release material is to keep the casting from sticking to the die. The material is called by a variety of names including “lube,” “die spray,” “die lube,” “spray,” “parting” and “release.”
The practical die caster knows that the release material can have a significant influence on the performance of the die casting die. Sometimes, by changing the type or brand of material or by changing the application pattern or quantity, the operating performance of the die is improved. The improvements are not limited to the sticking of the castings to the die. Often surface finish, productivity, porosity or even warpage is affected. The release material and its application is a significant processing variable.
Fig. 8-1. The die designer should supply a water spray diagram like the one shown here for the processing engineer and the operating personnel. The dilution ratio must then be made to get the correct amounts for both water and release material on each area of the die.
Determining Required Performance
The performance factors required from the release material are:
- Lubricity (of the moving mechanical parts of the die)
A thin (and hopefully uniform) film of the material is applied to the cavity surface and to the sliding parts of the die to achieve those performance factors. The film of release material is a complex mixture of organic compounds and sometimes pigments such as graphite and aluminum particles. Except for lubrication for the moving mechanical parts of the die, the basic function of the film is to keep the molten metal from establishing intimate contact with the die steel. Without contact, the casting can not stick to the die.
The film also forms an insulating barrier between the molten metal and the die. That barrier slows the rate at which heat is lost from the metal to the die. The greater the thermal resistance, the better the casting’s surface finish and the longer the filling time can be. These relationships are developed in more detail in the NADCA textbook, Gating. Greater thermal resistance, however, may slow solidification somewhat. Increased thermal resistance of the release film will reduce thermal shock and therefore heat checking of the die steel. The high temperature of the molten metal in combination with the high pressures cause chemical reactions to take place within the film of release material during and just after injection. These reactions can be exothermic (i.e. release heat), isothermic (i.e. have no heat released nor absorbed) or endothermic (i.e. absorb heat). If such reactions add or delete heat, they will affect the way the metal flows into and through the cavity.
The chemical reactions described above result in much of the release material being “burned” away. The burning away is desirable since the material will not then build up onto the die cavity (i.e. “varnish” or “scale”) or “stain” the casting. The pigments, if any, will not burn off. Release materials for the high-temperature alloys such as brass may need to include pigments. The decomposition of the material must happen slowly enough that the cavity has time to fill and the casting’s skin form before the release material disappears.
Some release materials are graded in terms of their “release,” “anti-solder” and “lubricity” properties. These properties are probably not available in engineering units, but the rankings help the process engineer select the best material. The die caster should keep a history of the release materials used, the jobs that each was used on and the performances. Such a history could be a help in selecting the material for a new job.
The release material is applied to the die by spraying. The material is first mixed with a solvent (usually water) and then sprayed with compressed air through spray nozzles. The water boils off the hot die leaving the desired film. The suppliers of the release materials take great pains to formulate the materials to stay in solution with the water (it must not separate in the use tank) and to insure that it will spread-out (i.e. “wet”) on the cavity surface.