Tue. Feb 7th, 2023

 

The High pressure Die Casting Process

In theory, the high-pressure die casting process is simple: Molten metal is injected into the mould, solidifies a few seconds later and is then ejected as a casting. The mould gets treated with a lubricant to avoid having the casting stick to the mould then closed for the next “shot”.

In reality, this simple-sounding process is highly unstable. For example, a nozzle intended to spray lubricant on the mould surface may loosen and spray over a wider area than intended – or may clog partially or completely, Die Casting Factories  restricting the amount and area of the spray. Missing a sufficient layer of lubrication, castings may stick in the mould, crack during ejection then have to be found within the production batch and scrapped. Similarly, melt volume in the holding furnace at the die-casting machine can change the melt volume dosing into the machine chamber. This in turn changes the metal fill pattern into the cavity by increasing turbulences and the amounts of air that are entrapped in the casting creating porosities. Or consider a basic variable: the temperature of the mould. In order to eliminate pre-solidification during cavity filling the mould has to be brought to working temperature of approximately 400° F / 200°C.

However, the liquid melt injected into the mould heats it beyond this temperature, which, if left unaddressed, would affect the functional life of the mould. In an attempt to regulate this variable, liquid cooling medium containing flowing water or oil is forced through channels in the die steel. Even so, the mould itself changes over the course of a production run; as it heats from room to production temperature, it increases in size; components that held the mould closed in an unheated state may no longer function correctly or completely. Even the condition of the die-casting machine itself changes during a production week; adjustments made on a cold machine yield different scale readings when the machine reaches production temperatures. Variations are present when unexpected breaks occur; the longer the machine stops, the more difficult it becomes for the operator to equilibrate the temperatures again. Furthermore, mould aging affects casting results: the mould wears at different speeds in different areas, affected by melt flow patterns, location of clamps, scale build-up in cooling lines – all of which require the operator to constantly strive to equilibrate the mould temperature.

In short, the high-pressure die casting process is in continuous flux – and the outcome is in the hands of the shop floor personal.

Process Simulation

Blaming only the shop floor personnel for bad castings would be as easy as it is unfair. In the most cases, poor quality results not from the efforts of the floor personnel, but from far earlier in the engineering process: the production process may have been insufficiently developed; the casting shape may have been poor designed. In either case, a good quality casting will never be achieved. This is where process simulation can be of the greatest help.

In the first stage of process planning, casting simulations can be performed, long before cutting die steel or the release of the final casting design. Using CAD files of the early casting design ideas in combination with theoretical process parameters, simulations directly point to potential problems. At this point of development, the casting design and manufacturing process can be changed easily, quickly and inexpensively. With experience, a well-trained die casting engineer using simulation tools can create a process that yields good quality castings during the first die trials.

In the most cases this success is achieved by designing a mould using one set of possible process parameters only. Given the volume of parameters possible in the casting process, and the range of variation within those parameters, the number of potential interactions that the engineer could consider approach infinity – as would the time needed to research those possibilities. With limited time and resources – and having achieved these good castings through single parameter simulation – the engineer may stop at this point, and turn his attention to another project, leaving shop floor personnel to resolve any further production variations.

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