Shot Sleeve and Plunger Tip Materials for Die Casting

The two parts that meet the melt first

Molten metal is ladled into the shot sleeve and injected into the die cavity by a water cooled plunger, which also transmits the intensification pressure to the solidifying casting. Both parts therefore see thermal and mechanical stress and direct attack from the molten metal at the same time. The materials for the sleeve and the tip are selected together, to give maximum life and minimum maintenance, and neither can be chosen on its own. A worn plunger tip is cheaper to replace than a shot sleeve, so the general rule is to keep the sleeve harder and let the tip wear.

The shot sleeve is a nitrided steel part

The shot sleeve is generally manufactured from a steel that is nitrided to a depth of about 0.25 mm, giving a surface hardness of up to 70 HRC. The nitrided case is what carries the sliding of the tip and resists the attack of the aluminium. It is worth noting that a nitrided sleeve and a nitrided tip of the same steel at the same hardness are a poor pair. When both parts are of the same material at 70 HRC, additional lubrication is required to prevent seizure and galling, and the tip also has a tendency to embrittle and to develop surface cracks under the combination of heat and abrasive wear. Copper alloy plunger tips have largely taken that problem away, and where they are used the sleeve no longer has to be nitrided at all.

Why the sleeve and the tip must not match in hardness

The seizure and galling trouble is reduced if the hardness of the two parts is significantly different, and that is the reason copper alloys took over the plunger tip. A copper alloy tip runs at up to about 40 HRC against a sleeve at 70 HRC, and the difference is what keeps the pair sliding. The choice is not free, though, because a copper alloy expands more than the steel around it and needs a correspondingly larger clearance. Aluminium bronzes and nickel aluminium bronzes, whose thermal conductivities are very similar to the nitriding steels, were used for a time but were not completely satisfactory as bearing materials. The beryllium bronzes that followed have a higher thermal conductivity than aluminium bronze and can be heat treated for more strength and hardness.

Plunger tip materials and their properties

Meehanite is relatively cheap and more resistant than nitriding steels to attack from molten aluminium, but it suffers from lower toughness and fatigue resistance, and an improvement has been obtained by spraying the tip with bronze weld and grinding it to fit. Among the copper alloys, the beryllium copper materials known by their trade names of Ampcoloy 83-20, with 2 percent beryllium, and 91-20, with 0.5 percent beryllium, 1 percent cobalt and 1 percent nickel, have been used successfully in many parts of the world. Both are precipitation hardened by a low temperature heat treatment. Beryllium is an expensive element and all beryllium compounds are toxic, which is what drove the later beryllium free and low beryllium materials. Ampcoloy 940, a copper-silicon-nickel-chromium alloy, and the modified low beryllium alloy BB1-MOD, with 0.6 percent beryllium, 1.5 percent nickel and 0.8 percent cobalt, both appear in the table below.

Materials used for cold chamber plunger tips with the relevant properties, as published in the source. Thermal conductivity is given in CGS units, so the column is comparable within the table rather than against a table in SI units.

Clearance and the thermal expansion of copper

The higher thermal expansion of a copper alloy has to be paid for in clearance. Tolerances of about 0.01 mm per cm are usually adequate between the outside diameter of the tip and the bore of the hardened and ground sleeve. Under the operating pressures used, the high ductility of the copper alloys gives a self-peening action on the surfaces of the tip, which helps it hold a close fit in the sleeve. The higher thermal conductivity of the copper alloys compared with nitriding steels also improves tip cooling and heat dissipation, which reduces distortion under repeated heating and cooling. When a copper alloy tip wears, the relatively easy machining allows it to be turned down to the next lower size and put back into service.

How the rest of the machine is built

The parts around the sleeve and the die are far less demanding. The source compares a popular medium sized hot chamber machine as built in 1938 with a similar model built in 1981, and the two lists show that most components of a die casting machine are amply strong in carbon steel, low alloy steel or alloy cast iron. The tie bars take some stress but low alloy steels are usually adequate, and only where the operating conditions indicate that the tie bars will be highly stressed is an EN 24 nickel chromium molybdenum steel called for. Platens and toggle links are commonly a 0.3 percent carbon steel.

Machine part materials for a medium sized hot chamber machine as built in 1938 and in 1981, as published in the source.

The pair in one view

Set side by side, the two parts that meet the melt are chosen on opposite principles. The sleeve is made as hard as the process allows, and the tip is made softer on purpose, in a material that also takes the heat away faster than the steel it runs in.

The shot sleeve and the plunger tip, their usual material and condition, and the reason each is chosen that way.

Where to go next

The die the melt is pushed into is covered on the die casting die component materials page, which lists the same components by material and hardness, and the broader selection by casting metal on the die casting die steel page. The H13 that both the sleeve and the die inserts are commonly made from is on the H13 tool steel page, and its heat treatment on the hot work heat treatment page. The nitriding that hardens a sleeve is set out on the nitriding guide, and the other surface treatments used on die components on the case hardening comparison page. How the inserts and cores themselves are chosen is on the core and insert selection page, the cooling of the die on the die cooling line design page, and the broad failure picture on the die wear and failure page. The choice of grade by die casting process is on the high pressure die casting selection page, and the way a grade is matched to a part on the materials by tool type page.

Source, Die Casting Metallurgy, A. Kaye and A. Street, Butterworths Monographs in Materials, 1982, Chapter 21, The metallurgy of die casting machines.