Due to the need for an economical working life for a High-Pressure Die Cast Machines’ tips and sleeves, as well as the requirement to produce sound, defect-free castings, good plunger lubrication is a must. Any ‘oil’ will not suffice, especially considering the following:
- Boundary, non-film lubricating scenario
- Elevated pressures of up to 10,000 psi
- High temperatures influenced by molten aluminum
As such, technologies have been developed specifically for plunger lubrication.
Mineral Oils – the most commonly used plunger oils are based on mineral-oil or petroleum, albeit at very high viscosities to combat high pressures and temperatures. Typically, viscosities would start at 1,000 cst at 40oC, and can go all the way up to 5,000 cst at 40oC. These oils are fortified with, at the most basic level, extreme pressure additives similar to those used in gear oils. Premium formulations introduce solid lubricants such as Graphite, Molybdenum Disulfide, and even PTFE, with the theory that solid lubricants provide better friction reduction properties as compared to mineral-oil / EP combinations.
Mineral Oils are relatively affordable compared to the other technologies (Water-Based and Dry Solid types), thus contributing to its popularity, while wear protection (tip and sleeve life) is ‘satisfactory’. These are applied via a drip system, into either the sleeve hole, or on top of a retracted tip, or applied on both, right after casting is ejected.
Downsides in utilizing mineral-oil based technologies include a dirty, and oily work area, as the excess applied plunger oils will indeed ‘drip’ into the surrounding area. There is no easy way to control drippings, as the sleeve is an open, hollow chamber, other than reducing quantities applied. Thus, mineral oils for plunger lubrication may be deemed as a safety and environmental hazard. Over-applying plunger oils can also stain castings if the quantity is such that the oil ‘gets mixed’ into the solidified casting.
In our next paper, we tackle dry / plunger lubrication and a new water-based formulation for plunger lubrication.
