Plungers
Higher process security with better plungers for High pressure die casting.
Introduction
The complicated process of die casting is influenced by different parameters. In spite of all
Development in automation and observation of the installations many parameters are not controlled or even mastered. There are factors which a long time was unnoticed but which have a great influence in the process of production concerning quality and economy.
Such an aspect is the system of plunger and sleeve and their correct function is the requirement for a reproductive filling of the die.
Previous technology
The plunger transports in the sleeve the melt into the die. Until now an unsolved problem is the insufficient density between plunger and sleeve. The main reason is the difference of temperature within the sleeve and between plunger and sleeve. Because of different thermal expansion plunger and sleeve which fit in a cold condition will not do so after warming up.
The result is that the gap between plunger and sleeve very soon passes the critical size which makes it possible for melted aluminium to get in. This means that aluminium rests between plunger and sleeve impede the movement of the plunger and a jerky movement will result.
It even can be that the plunger gets stuck. This doesn´t only result in sign of wear of plunger and sleeve but also in less productivity because of many machine stops for changing plunger or sleeve.
It is even more difficult to get an opinion of the influence on the product quality. Out of considerations for requirements of more casting technical complicated geometries on the one hand and process security on the other an insufficiently controlled plunger movement can cause inexplicable quality problems.
The plunger technology
A sleeve is only after producing straight and rather round inside. As soon as it is in operation it can become like a banana and inside oval. In addition the length has a various medium-sized diameter. Traditional plungers cannot adapt to changing conditions. After intensive working with this problem the company Allper – producer of plungers and copper alloys – decided to give up a monolithic plunger. The new plunger construction consists of three parts. Deciding is using an elastic plunger ring which can conform different diameters. At the same time the design of the plunger was conformed to optimize working.
With this system you don´t fit the plunger directly on the rod. You use a special plunger holder with drillings and slits for an optimal distribution of the cooling water. With a bayonet fitting it is easy to put on the plunger. The plunger is made of a copper alloy with a high durability and a special thermal conductibility. The plunger also has an elastic ring made of steel and with a high durability. The ring is split in order to fit different diameters in the sleeve.
When required you can very quick change the ring with special pliers. The ring diameter is
a little bigger than the inside diameter of the sleeve which means that the ring always fits tightly to the wall and has optimal density. When starting with this system you can normally use the sleeve you have with some small modifications.
Advantages
This construction brings clear advantages concerning wear and costs. Clearest is the life span of a plunger. A bigger foundry in Germany made more than 30.000 shots with a plunger Ø160 and a cast weight with more than 30 kg. They needed 3 rings. The machine stops could be reduced with 15 hours.
Xxxxxxxxxxxxxxxxxxxx The American die casting foundry Muskegon Castings Corp., Muskegon, Michigan, found out that the part of the cost factor of the plunger in the calculation could be halved from 4 to 2 cents. The traditional plunger was worn after
2.000-3.000 shots and had to be changed. Now the first change is necessary after
25.000 - 27.000 shots. In addition only the ring must be changed and the plunger stands 3-4 rings, that is 75.000-100.000 shots before it must be changed because of cracks from heating.
For a medium-sized die casting machine running in 3 shifts with approximately 7.000 machine hours/year and a time of on average 1 minute you can reach a saving of about 60.000 SEK. Further advantages like less stops and less wear of the sleeve are to be added. After changing to the new plunger system the sleeve has about a three times so long lifetime. The optimal tightness between ring and sleeve is a special advantage when using vacuum.
Sleeve concept
As plunger and sleeve are a common system it is not always enough to optimize only the plunger. You also have to gain 
control of the sleeve. Allper has long worked with this matter. After many measures, calculations and computer simulation they have concluded that sometimes the inside diameter of the sleeve can be so deformed that not even a ring can compensate the differences. For this case the temperated sleeve should be added. To temper the sleeve it is made with borings in the sleeve and a connected to a temperature unit so that you get a temperature difference in the sleeve with maximum 150-200°C.
Conclusion
The use of the Allper system began about 20 years ago. In Scandinavia we started from the beginning and therefore quicker than in other parts of Europe. Today about 250 machines in 80% of the Scandinavian foundries are equipped with plungers from Allper and we have a permanentnt increase.
