ZVAC® EXPLANATORY NOTES
Gas Porosity and Diecasting in a Vacuum Zone.
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GAS POROSITY IN DIECASTINGS
Gas porosity in diecastings has three main origins:
- Entrapped air
- Entrapped lubricant fumes and vapours
- dissolved hydrogen, mainly in the case of aluminum alloys
The biggest problems are caused by the first two factors.
Hydrogen is relatively easy to minimise or eliminate by well established melting and degassing procedures.
Flow simulation softwares can help the experienced diecaster optimise the fill patterns to expel gases, but there are limitations to what is possible.
Often, the position of feed gates and vents are defined by the part’s shape or the customer’s specifications.
Thin walled diecastings require rapid injection. In most cases this leads to the metal flows becoming fragmented into high velocity divergent flow streams, which at their turbulent confluences, entrap air and fumes.
The flow fragmentations occur where the metal collides with cores or other features in the die cavity; they can also stem from the use of multiple separate gates.
In thin sections, trapped air can also prevent metal flow streams from fusing together, leading to cold lap faults.
Overflow pockets help a little, at expense of extra remelt costs and additional pressurised wet area, which detracts from the machine capacity available for actual cast parts.
To make accurate predictions of where porosity will remain in the pressurised part, by using simulation software, appropriately small finite element mesh sizes have to be used, else the porosity simply disappears from view, only to reappear in the actual part, when it is secondary processed.
The flow velocities of metal into a die are usually between 30 and 55 metres per second; equivalent to 67 mph and 123 mph.
At such speeds, the liquid metal splatters and spreads at every turn, and on curved surfaces, takes the outer convex curved surface until sliding and viscous friction and increasing viscosity slows it, or it hits a face or restriction, which then causes back filling simialar to the hydraulic jump phenomena.
But when using vacuum correctly,with pressures in the reciever preferably below 10 mbar and large valves, vents and good sealing, it has such a dramatic effect on eliminating trapped gases, that imperfections of gating and flow separations caused by the die features, can become much less of a problem or even insignificant.
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Generally, the term 'HIGH VACUUM' (HV) correctly refers to absolute pressures in the range below 1/1000 millibar absolute or 1/10 pascal absolute (pa abs).
Diecasting vacuum equiment suppliers have misapprpriated the term 'High vacuum' to puff their wares, but even with extensive die sealing, their products will not enable vacuum in the die to be anywhere near the lowest-pressure end of the 'rough vacuum' range, which is officially classed as 1 millibar absolute (100 Pascals abs).
When corrrectly applied, the ZVAC method can attain vacuum in die pressures below 50 millibar abs providing, that pumps and receivers are correctly sized according to volume of air and water vapour sucked in per casting machine cycle.
When using vacuum for diecasting, as soon as any air is sucked from the die (mold / mould), the vacuum system receiver pressure momentarilly rises a lot, and gradually recovers according to the pump capacity.
On a system with a very large receiver volume, high flow valves and filters and very well sealed dies, the momentary pressure rise might typically be around 1,000 Pascals absolute (10 mbar abs).
Some of the common vacuum assisted diecasting processes run with receiver pressures in the 'low' or 'rough' vacuum range, 10,000 to 25,000 pa_absolute.
The reasons often being unsealed or inadequately sealed dies, and pumps too small to cope with the volumes of gases and vapours drawn from the dies.
Comparing some vacuum units:
1 Standard Atmosphere = 101,325 pa = 0.101325 mega pascals
= 1.01325 bar
So, 10,000 to 25,000 pa abs would only be equivalent to 90% to 75% 'full vacuum'.
= 14.696 lbf/in2
= 1.0332 kg/cm2
Neither values are near the starting level for 'high vacuum'.
For reference, here are two links to very informative webpages on vacuum fundamentals:
Diecasting companies that claim to be using 'high vacuum', should be also be willing to show that the pressure in the mold before and during injection, is lower than one tenth of a pascal, which is highly unlikely in a production foundry environment.
Problems with traditional vacuum diecasting methods:
Unless shown otherwise, it is believed that non of the traditional vacuum techniques can be classed as being a 'high vacuum' technique.
Usually, they can only attain around 15,000 pa abs (150 mbar abs) in the mould cavity of the average simple 'open and close' die in good condition, and about 25,000 pa abs (250 mbar abs) if sliding cores are present and even worse, if the shot sleeve is a loose fit in the die.
The old methods with unsealed dies, can also suffer variability of vacuum from shot to shot, depending upon die face condition and contamination.
Some of the newer vacuum methods with very large valves and large area connections to the die cavity, can counter some of the affects of leaks, but frequently get blocked by molten metal.
Many diecasters, including large technically competent companies, fail to see any worthwhile benefit from using vacuum. On examination, the reasons were usually at least one the following:
- they were given pumps that had feeble volumetric flow capacities
- used pumps which had inadequate level of maximimum vacuum
- used small capacity receivers
- had small diameter pipes, which severely limited flow rates (i.e. had low conductance and high resistance)
- did not seal the dies either peripherally or internally
- used small vent connections to the die cavities
- had vacuum valve and filter systems with high flow resistances.
- used valves and filters which quickly clogged (blocked-up) and which were difficult to maintain and clean.
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OTHER VACUUM METHODS
There are many varieties of vacuum diecasting technique. Some belong to the companies that developed them for their own products, like Gibbs, Ryobi and Suzuki, others are commercial packages which usually feature very expensive components.