February 2006 Design Tip

When two parts are better than one

Imagine a typical, hand-held electronic device – a phone, PDA, or MP3 player for example. The plastic shell almost certainly consists of two (or more) pieces that screw or snap together. The obvious reason for a two-part shell is ease of access to the parts inside. But this approach also simplifies moulding, as the inside of a one-piece shell would be extremely difficult, or impossible, to mould.

In fact, any feature of a part, large or small, that would "hang up" on a straight-pull mould increases the complexity of manufacturing. One solution, wherever such features exist, is to use side-pull cams to create the undercut. Another option, of course, is to redesign the part, eliminating the undercut feature; this can sometimes be accomplished without impacting functionality at all. A third, and somewhat less obvious alternative is to separate the part into two or more parts, turning "hidden" surfaces into accessible outside surfaces. Of course the separated parts then have to be assembled, but this can usually be easily accomplished in any of a number of ways.

For Protomold customers, there are several possible benefits to breaking a large part into smaller ones.

  • It allows the production of designs that would be too large, as a single part, for Protomold's process.
  • It allows the duplication of tall, narrow features that might be difficult to cut into a mould.
  • It eliminates the problem of ejecting parts that would be too "deep" for easy removal from the mould.

Some of these designs could possibly be produced as a single part using traditional injection moulding techniques rather than Protomold's rapid injection moulding. The tradeoff, however, would be much higher cost and a significantly longer wait for parts.

Time/Cost Comparison:
Traditional vs. Rapid Injection Moulding
  Protomold rapid injection moulding Traditional injection moulding (China) Traditional injection moulding (EU)
Price range £2000 to 10,000 £10,000 to 60,000 £20,000 to 120,000
Production time 3 to 15 days 4 to 8 weeks* 8 to 16 weeks
Technology Direct  3D CAD to aluminium mould Direct or indirect CAD to steel mould Direct or indirect CAD to steel mould

* May require additional time for rework

Converting complex parts to simpler components can often be a very straightforward process. Take, for example the "elbow" shown in Figure 1. Neither a straight-pull mould nor a side-pull cam could form this part's inside curve.

Figure 1

Dividing the part into two halves, however, allows each piece to be easily formed in a straight-pull mould. The addition of bosses permits the two halves to be bolted together into a single part closely resembling the original design. Other assembly options include snaps or the use of adhesives.

Figure 2

The finished part could be made air- or fluid-tight by adding a seal groove along the mating surfaces. If, like the sample shown above, the part is rotationally symmetrical ( See June 2005 Design Tip – Cut Costs with Rotational Symmetry ), both halves would be identical and could be made in a single mould, further reducing the cost.

Protomold's ProtoQuote online quoting engine analyzes all submitted designs for undercuts and other " entrapments." If these features are essential to the design, separating complex parts into simple components may be the quickest, most cost-effective way to achieve the design you want and functionality you need.