Though dry plunger lubricants for plunger mechanisms of High-Pressure Die Cast Machines have been an available technology for more than 30 years, its acceptance level has not been as high as with conventional mineral oils.
The main reasons for the low acceptance levels could be attributed to the following,
1. The need for the die-caster to invest in a dedicated and specific dry plunger lubricant dispensing unit.
2. The resistance of die-cast machine operators in implementing new procedures to use and maintain a dry plunger lubricant dispensing system, and,
3. Doubts remaining with the die caster as to the effectiveness of the friction reduction properties of the solid lubricants for the plunger system.
Those who have converted into the path of using dry lubrication seldom revert to liquid technologies, as several benefits prove to be consequential for the die casting operations.
How does it work?
The lubricant is in the form of pellets or beads, measuring not greater than 2-3 mm in diameter, composed primarily of waxes that may or may not be additivated with graphite. These beads, as a solid in its original form, exhibit a low melting point (< 80oC), and thus liquefies to provide lubrication upon its entry into the sleeve hole. The sleeve maintains a relatively high temperature relative to the bead’s melting point owing to its constant exposure to molten aluminum. Its conversion into a liquid inside the sleeve chamber provides the friction and wear reduction properties at the tip and sleeve friction interface.
To dispense the beads, a special applicator illustrated below is mounted above the plane of the sleeve, and is connected to an air line, as well as to the plunger-return limit switch. As soon as the plunger tip retracts, the die-cast machine signals the applicator to dispense a pre-determined quantity of beads into the sleeve hole. The cycle repeats upon each retraction of the plunger tip.
There are 4 key benefits in utilizing dry plunger lubricant in a die-cast machine,
- Lower quantities required per casting shot, as compared to liquid lubricants – While dry lubricants are in no way ‘superior’ to mineral oils lubrication-wise, a smaller quantity suffices with beads since a significant percentage of any mineral oil applied to the sleeves and tips are actually wasted via drippings, considering the open-type lubrication system in plunger mechanisms. With pellets, usage of lubricant is reduced by as high as 70%. This eventually reflects well in the die casters bottom-line.
- A clean and dry working environment – The significantly reduced use of plunger lubricants also allows for a relatively clean and dry work environment. This minimizes the risk of manpower accidents (eg slipping on oil), as well as improving the environment-friendliness of the die casting process — important considering the demands for environmental certifications in this modern world.
- Improved recyclability of aluminum dross – Again, due to the reduced oil usage, any aluminum dross surrounding the shot-end of the high-pressure die-cast machine is relatively dry and oil and contamination-free, allowing the caster more freedom to re-use and re-melt the dross.
- Improved casting quality and appearance – The incidences of plunger oils stains, and porosity, are significantly reduced, as a result of lesser lubricant used per casting cycle.
In closing, many die-casters should weigh the hindrances or challenges in using the dry plunger lubrication system, against its numerous benefits, and should look at this technology as a viable option in the future.