Family moulds (without runner shut-off) are appropriate for parts that are highly similar
in shape and volume. The similarity is driven by the need to fill both cavities in the same
amount of time. For this reason, mirror image parts can be a good candidate for a family mould.
Since a family mould allows two or more parts to be made at the same time, piece part prices are reduced.
Family moulds (with runner shut-off) are used when parts are dissimilar in shape and volume. A runner
shutoff is a "valve" that directs liquid plastic to one of several cavities in the mould. Set-up time
is less than with individual moulds; however, the parts need to be run one cavity at a time. The piece
part price savings are not as large as they are in a family mould without a runner shut-off.
Of course, there are tradeoffs to consider when considering the use of a family mould. One tradeoff may
be in the time it takes to obtain the first sample parts. Several single cavity moulds can be manufactured
in parallel in less time than it takes to manufacture one family mould to make the same parts. For this reason,
family moulds are generally not good candidates for premium quick-turn deliveries. The other tradeoff has to do
with flexibility in the event of a design change. Any change that requires metal to be added may require a new
mould, and the time and cost associated with remaking a family mould or multi-cavity mould will be greater than
for just a single cavity mould.