A “Toolbox” for Forensic Engineers

Accidents in the Workplace 261
is an emergency, there may not be time to do anything about it — that is
the nature of an accident. It is one of the responsibilities of management to
recognize the possibility of such situations and insist on the observation of
all safety procedures at all times.
8.2 Accident with a Pressure Die-Casting Machine
Pressure die-casting is a process widely used for the mass production of
accurate small to medium sized castings in aluminum or zinc alloys, having
good surface finish, sometimes incorporating inserts of other metals such as
steel pins and screw threads. It is a process similar to injection molding of
plastics. Essentially, the machines comprise a set of water-cooled metal dies
and cores that are clamped firmly together while a measured quantity of the
molten alloy is injected under high pressure. The clamping force has to be
sufficient to prevent any liquid from escaping. A short time is allowed for
solidification, during which period the dies have to remain clamped. When
the casting is solid and able to support itself, the dies are opened and the
casting ejected. After cleaning the opened mold, the faces of the cavity are
recoated with a release agent and then clamped together again ready for the
next casting. Different sizes of casting require different time cycles that have
to be set according to the mass of metal injected and the time necessary for
the heaviest section to solidify. It is a mass production process, so in order
to achieve maximum output the cooling part of the time cycle is absolutely
critical; too long a time slows down the production rate, and if the time is
too short for complete solidification, castings either break up when the dies
are opened or become distorted by the ejector pins.
In some pressure die-casting processes the liquid metal is drawn from
near the bottom of the reservoir and injected with minimum turbulence, in
order to eliminate dross and avoid air entrapment. In other processes the
metal is ladled out of a large melting pot and a “shot” sufficient to make the
casting poured into a receiver and then injected. Castings made in this way
are very likely to carry air into the mold and, although the outer skin of the
casting may be free of porosity and well formed, the air and any other gases
evolved during solidification end up within the thicker sections. The internal
gas pressure thus set up may sometimes be sufficient to cause parts of the
casting to swell when first taken from the mold while the outer skin is still
soft and relatively weak.
8.2.1 The Accident
This particular accident occurred when two men were operating a machine of
the latter type making castings in an aluminum alloy. Both were experienced