January 2005 Design Tip

Twins, triplets, and other family members

Since Rapid Injection Moulding is all about making identical objects, wouldn't it make sense to machine moulds that produce more than one at a time? Well, yes...and no. It depends on a number of factors. First, a few definitions. A multicavity mould is machined to produce two or more identical parts at a time. A family mould is machined to produce two or more similar parts at a time. A family mould with runner shutoff is designed to produce two or more different parts, but only one at a time.

Example "multi-cavity" mould shot.
Photo courtesy of MagneticRays.com

Multi-cavity moulds are rarely used in prototyping because of the greater cost and time required for machining and polishing. However, once the parts from a single-cavity prototype mould have been approved, a multi-cavity mould may be a cost effective way to reduce the piece part price. In some situations, if no design changes are involved, Protomold may provide a credit for the initial prototype mould towards the cost of a new multi-cavity mould.

Example shot from a family mould without "runner shutoff."

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.

Example family mould with "runner-shutoff."
Photo courtesy of Advanced Bio-Surfaces

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.

Free Sample Part and Key

In our October Design Tip we offered you a free : Rapid Injection Moulding Sample Part and Key. Well, we've already shipped about 1500 of them! If you'd like us to mail one to you just send an email to marketing@protomold.co.uk and we'll get right on it.

As always, you can visit the Protomold Design Guide for helpful Rapid Injection Moulding design information.