Castool Community

The danger of exceeding a maximum allowable gap between the plunger tip and the shot sleeve has been understood by die casters for many years.  The technology of both plunger tips and shot sleeves has improved appreciably in the interim.  The parameters of the gap, however, remain unchanged.  The interaction between the plunger tip and the shot sleeve is certainly the most critical in the die casting process.   It can only be most effective, however, if both components are operating at an equal level of efficiency.

Also, an effective vacuum can only be achieved if the seal between plunger and shot sleeve remains intact at all times.  If the seal is broken, an effective vacuum is obviously impossibly.  The challenge is to maintain a minimal gap without interference, while using the least amount of lubrication.

Four thousandths of an inch is the maximum gap between the plunger tip and the shot sleeve during the casting process.  If at any time during the shot, the gap exceeds four thou, the alloy is likely to penetrate the space, and flash or blow-by will occur. This will inevitably cause excessive wear on both plunger and sleeve.

It is essential, therefore, that a gap of less than four thousandth of an inch be maintained at all times during the casting cycle If the gap becomes much less than four thou, there is a danger of interference and inconsistent shot velocity.  Scrap will result.  . The problem is that when metal is heated, it expands.  The importance of controlling the temperature of both plunger tip and shot sleeve is shown by the fact that if the temperature of a 150 mm copper tip is allowed to rise by 95ºC, the diameter will increase by more than 0.28 mm.

Shot sleeves are getting bigger, but whatever the size of the sleeve, that critical maximum gap of four thousandths of an inch unfortunately remains unchanged.

Plunger tips were originally made of steel.  Steel tips are still used, primarily for their durability and economy.  A steel tip, of course, has the same coefficient of thermal expansion as the shot sleeve in which it slides.  Since the plunger tip is exposed to more heat than the sleeve, the expansion of a steel tip is difficult to control very precisely.  The next step in the development of the conventional plunger tip was to make it of beryllium copper which has a coefficient of thermal expansion more than 50% greater than that of steel.  This made the expansion of the tip much easier to control.  It was then possible to maintain the thermal, and therefore the dimensional, stability of the tip throughout the length of the stroke.

Die casters usually reduce the temperature of their plunger tips with water. Excessive plunger tip expansion and wear is nearly always simply the result of insufficient coolant.   Even experienced die casters sometimes neglect this.

Rate of flow is easily measured, and should be monitored constantly.    Maintaining an adequate flow of water is vital to controlling plunger tip expansion,

There are some proprietary cooling-intensive plunger tips that utilize the cooling water much more effectively than conventional tips.