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 it hits an end face or restriction and back fills in a manner akin that of the hydraulic jump phenomena.

Using high vacuum has such a dramatic effect on eliminating trapped gases, that imperfections of gating and flow separations caused by the die features become insignificant.

Both values are nowhere near the startng level for 'high vacuum'.

For reference, here are two links to webpages fundamentals about vacuum:

www.absolute-vacuum.com     (Pending availability).

The www.engineeringtoolbox.com     (Pending availability).

Companies that claim that they are using 'high vacuum', should be able to demonstrate that the pressure in the mold before and during injection is lower than one tenth of a pascal. That is unlikely to happen in a production environment.

As far as can be determined, non of the traditional vacuum techniques can be classed as 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 the shot sleeve is a loose fit in the die.

Those methods can also suffer huge vacuum variability from shot to shot depending upon die face condition.

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 they can also frequently get blocked by molten metal.

The reason most diecasters have failed to gain from vacuum, is probably because: they were given small pumps and receivers of severely limited flow capacity and attainable vacuum, not advised to seal the dies either peripherally or internally, not told to use really massive vent connection areas to the die cavities and given valve and filter systems with high flow resistances. To this group, after suffering those setbacks, 99% full vacuum in a receiver could indeed be thought of as 'HIGH VACUUM'.