My Die Spits at Me |
Everyone who has worked in high pressure die casting has experienced this habit. The plunger starts pushing melt into the cavity - then suddenly a loud bang and a dust of liquid particles rains to the ground. After some seconds the mould opens and a half empty casting appears with foil like metal sheets attached to it.
What has happened was die flashing: at the end of fill the high impact load of mass and velocity was too much for the locking force of the die casting machine. The mould could not be held tight and amounts of liquid metal were released into the free.
Causes for die flashing are numerous and experienced on a daily basis. Some moulds are old and worn out, carrying dents and heavy distortions. Other reasons for flashing include insufficient die casting machine locking force, worn out machine platen, weakened tie bars or insufficient clearance between the mould components. Usually flashing and spitting develop over time as the moulds getting older and worn out, but can be experienced in some moulds from the first casting. Whenever it occurs, the casting quality and mould life are reduced significantly.
Facing these issues, production personnel are limited in their possible actions. In some rare cases the locking force can be increased, undercuts can be worked into the die surface to keep the flash from ejecting and then seal the die for the next shot, or metal sheets can be placed between mould and machine platen to maintain the die seal until the production run is finished.
Elimination of causes should be part of the proper maintenance of mould and machine as preparation prior to the next production run. At this stage in manufacturing, significant changes to the mould design itself are not possible anymore and any design flaw will stay until the castings are not needed anymore. During production only some small changes can be tried, like changing the die temperature profile. Switching the cooling from water to oil heating or eliminate the cooling completely could be worth trying, however.
Designing a mould is always a compromise of placing the casting, ejector pins, metal savers and cooling lines. As bigger the moulds get, the more attention has to be paid to the placement of cooling lines. A thermal expansion coefficient of 0.00002m/m°C for die steel may be ignored for a small die, but in a die in production that is 2000mm (78.5 inch) wide and run at a temperature of 220°C (430°F) this small coefficient calculates to a total expansion of 8.8mm (1/3 of an inch) overall. This theoretical value won’t be found on a production mould due to temperature pattern in the mould itself. While the outside frame or bolster may not exceed 80°C, the cavity surface with contact to the melt can pass 300°C. Internal cooling placed in the mould influences these temperature fields and the expansion of the steel components in its locations. If sufficient attention is not paid to their placement, the mould components could touch, deform and break, the clearance between components can widen or decrease and may not function as planned, or melt simply is pushed out of the die at the end of filling.
Die flashing can be influenced by changing cooling medium / temperature or switch to spot cooling such as bubblers or fountains. Cooling has to be placed close to temperature hot spots and moved away from colder locations. This is, however, easer said than done. In almost every mould, ejector pins and cooling systems compete for space, inserts can’t be crossed and other locations can’t be reached with the cooling volumes necessary. Die components can grow unpredictably based on steel composition, clearances, clamping forces or temperatures introduced. A mould might seal during warming up and at production temperature but will not after a production break. Die components have different temperature profiles; when heated, clearances between them may open and melt will be squeezed into these gaps. When the components heat up again and the clearance is gone due to solidified metal sticking in these openings the die steel will deform. After the next break down or die cleaning the mould flashes even more or, in the worst scenario, leaks and melt splashes out.
To get a forecast what will happen during production die casting process simulation can be utilized to detect originators for flashing as the high impact loads after filling and/or the mould temperature profile. During the mould design process die components used in the simulation can be changed fast, easy and cheap and be optimized to avoid die distortion and flashing before having the mould on the shop floor.
Ralf Kind/ Ke Roth
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