Six Problems with the Cold Chamber Die Casting Process
The cold chamber Die Casting process is used to make millions of components around the world every year, including automotive parts, technically demanding parts, and simple consumer products. It has been around for over 100 years and fundamentally it has not changed very much in that time. However, as customer requirements increase, the shortcomings of the traditional cold chamber process become more evident.
1. Premature Solidification
A thin shell of solidified alloy forms in the shot sleeve
As the molten metal is poured into the cold chamber, some of it solidifies at the interface. When the shot is made, these prematurely solidified particles usually end up in the casting, where they can become defects such as inclusions, weak spots, porosity, leak paths and surface imperfections. They can also become stuck in the gate, partially blocking the flow of metal to the cavity, leading to loss of integrity and process inconsistency.
2. Oxide Inclusions
Oxides form as soon as molten Al alloy is exposed to air
These oxides become mixed into the alloy and the castings.
When molten alloy is poured into the shot sleeve there is a great deal of turbulence on the surface where oxides immediately form. These become en-trained in the metal in the casting, reducing the physical properties and becoming a starting site for weakness, cracks and porosity.
3. Gas Porosity
A large amount of air is present in the shot sleeve
The shot sleeve must extend through the fixed platen of the machine to enable the metal to be poured in. Consequently it is usually less than half full. The air that is in the shot sleeve can easily end up in the casting where it becomes gas porosity. The use of shot sleeve lubricants also introduces dissolved gas into the metal which can increase gas porosity in the casting. Vacuum systems can reduce this porosity, but they can never completely remove it.
The speed of the plunger during the first part of its stroke must be very slow to enable some of this air to be vented. However, this also increases the amount of presolidified alloy in the casting, contributing further to these problems.
4. High Impact Shot End
In a cold chamber machine, the diameter of the shot sleeve is small relative to its length. So the hydraulic piston, ram, rod, and plunger must travel at a very high speed in order to fill the cavity in the required time. This causes a high pressure spike at the end of cavity fill, contributing to flash, dimensional variation and the need for a larger clamping force. Hydraulic damping systems can be installed to reduce the spike, but at considerable cost and complexity.
5. Shot-end Friction and Wear
Large temperature differences along the shot sleeve causes differential expansion and therefore distortion. High impact pressures and thermal shocks cause damage to the shot sleeve material.
However, there must also be a tight fit between the plunger and the shot sleeve. Consequently, there is high friction in the shot sleeve, leading to 'stick-slip' action and shot-to-shot variation.
The cold chamber process has continuous shot-end wear and the high associated costs.
Lubricants are applied to the plunger to reduce this shot-end wear. However, the downside is that residues from the lubricants can end up in the alloy and the castings, reducing strength and other properties.
6. Slow Production Rate
The cold chamber process is inherently slow.
The ladling of metal into the shot sleeve and the long slow-shot movement contribute to a slow cycle.
The thick 'slug' or 'biscuit' at the end of the shot sleeve takes a long time to solidify and this often limits the production rate.
All of these shortcomings, and a number of others, are inherent in the cold chamber process. They cannot be eliminated without making major changes to the process. For the last 100 years or so, there has been no alternative.
As a result, parts designed for the cold chamber process are made much thicker than they need to be. A casting that would be strong enough with a 2 mm wall thickness might specified as say 4 mm thick, to allow for the reduction in properties and the process variation. The additional wall thickness adds to the cost of the component, both for the extra alloy and for the slower production cycle required.
Reject Rates
A lot of plants that make high pressure Die Cast components in Al alloys have surprisingly high reject rates. Numbers in the range of 5 to 10 % are not uncommon. This translates to values of 50,000 to 100,000 parts per million (ppm).
However, customers are expecting delivered part defect rates of less than 100 ppm, so it is not difficult to see that plants are expending very large sums to sort and scrap these rejects before they get to the customer.
Although some of these rejects will be due to inadequate Die and Process design, it is clear that many of them are caused, or exacerbated, by the cold chamber process itself.