Low-pressure vs. high-pressure die casting
Low-pressure vs. high-pressure die casting
Aluminium components have been used in the automotive industry since the early 20th century. For almost as long, automotive experts have been comparing the different processes for manufacturing these parts in terms of their economic efficiency and suitability. Here, we present a comparison between low-pressure and high-pressure die casting.
An important point to begin with – both low-pressure and high-pressure die casting have situations where they are the most suitable process. This strongly depends on the component complexity, the number of parts and the manufacturing budget. It therefore makes sense at this point to compare how the low-pressure and high-pressure die casting processes work.
High-pressure die casting
One half of the die is attached to a fixed machine plate and the other to a movable one on a die-casting machine that is horizontally aligned. Because of the high pressure used when pouring – up to 1,200 bar – the bolts holding the two halves of the die together must have a high locking force. As these have a relatively high melting point, cold-chamber die casting machines are mainly used for aluminium alloys. Here, the casting assembly is located outside the melt. The molten metal is fed to a shot chamber, from where a piston drives the metal into the die. Once the metal has cooled and solidified, the two halves of the die are opened and the casting is automatically ejected from the die by ejector pins.
High-pressure die casting
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Low-pressure die casting
As with high-pressure die casting, here too the halves of the die are attached to a fixed and moving machine plate, but the machine is aligned vertically. The holding furnace for the molten metal is located beneath the fixed plate. Applying pressure of up to max. 1 bar to the furnace pushes the molten metal (usually aluminium, but also magnesium) up through an intake port into the die chamber – usually a permanent mould, although sand moulds are also possible. The upward movement of the molten metal is against gravity. After filling the die, the pressure is maintained as the metal cools to enable the addition of further molten metal to counter any volume deficits (shrinkage cavities) as the metal passes from its molten to solid state. This naturally ensures solidification that is as even as possible from top to bottom.
Low-pressure die casting
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Requirements for the automotive world
As everyone knows, the sector is being forced to increase efficiency and reduce CO2 emissions. In engine technology, low-pressure die casting is able to use sand cores to implement what is known as a closed deck design for an engine block – this means that the openings on the cylinder head surface of the engine block, previously needed in die casting for demoulding the cooling jacket contours, are not needed in the low-pressure die casting process. This enables a more rigid engine block to be produced; taken together with the improved material properties, this saves weight and improves performance – a substantial contribution to downsizing. The same technology is applied for parts used in the structure and the chassis, where large framework parts can be “hollow cast” using a sand core, thus sub-stantially reducing the component weight.
From the luxury class to the mass production sector
Due to its outstanding material properties, the low-pressure die casting process has been well-known in the automotive industry for decades, but until a few years ago its relatively long casting cycles meant that it was only used in the luxury class, where low quantities and higher unit costs are the norm. Despite the quality achieved in the low-pressure process, the mass production sector was forced to continue with high-pressure die casting because of its extremely fast cycle times. The fact is that a high-pressure die casting machine costs around four times as much as a low-pressure system – until a few years ago, however, the former’s casting cycles were around 4 to 6 times faster. Ways were therefore sought to make the low-pressure process more economical, that means faster. A major step forward came with the increase in size of the whole machine. As low-pressure die casting does not need the massive closing forces required by high casting pressure, the machine size is not dictated by the weight of the cast part – for example, 110 kg can be cast in the die of a low-pressure die casting system.
The Kurtz team has been able to increase the plate dimensions needed for the die, enabling tools with multi-layer charging to be used. Today, for example, engine blocks are cast twice – corresponding to an impressive 50% reduction in the casting cycle! With other additional optimisation of aspects such as the machine’s cooling circuit to accelerate solidification, a further 25% cycle time saving could be made. Low-pressure die casting also scores in terms of the space requirement of the machines, since the vertical arrangement of the system enables two low-pressure die casting machines to be installed in the space required for a single high-pressure machine. As a side-effect, this also leads to reductions in return materials, thus saving money. These developments make the low-pressure process more than competitive in many areas. Talk to us and together we can design the optimum system to meet your specifications!