protomold August Design Tip - Rapid Injection Molding

Protomold: Rapid Injection Molding
August Design Tip

 Don't be Square!

We've talked in the past about corners and reasons to make them rounded (radiused) instead of sharp. Let's talk about this in a little more detail and help distinguish between inside and outside corners, both of which should be radiused but for slightly different reasons.

Outside corners

The first thing to keep in mind is that an outside corner of your part is created by an inside corner of a mold, and vice versa. One reason we don't make parts with sharp outside corners is because our molds are made by a vertical milling process that cannot cut a sharp inside corner. The radius of our inside corner (your outside corner) cannot be smaller than the radius of the cutter, which will vary somewhat with the depth of the cut.

Inside corners

Our milling process can produce sharp outside corners when making a mold, so you can have sharp inside corners. The problem is that sharp inside corners can create serious stresses in a part as it cools. The reason is simple. The rate of resin cooling is proportional to surface area. Any corner will have more surface area on the outside of the curve than on the inside. (Think about the advantage of the proverbial "inside track.") On a radiused corner, there is always a difference between the two surface areas, but if the inside of the corner is square, it essentially has a surface area of zero, which maximizes the difference between the inside and outside surface areas.

Example of Sharp Corners vs. Radiused Corner

Example of Sharp Corners vs. Radiused Corner

If the part consisted of two walls meeting to form an "L" shape, the part may tend to warp as it cools, reducing the angle between the two walls. If, however, the corner is in, say, a box whose shape keeps the walls from moving in relation to one another, instead of warping, they'd merely become stressed. The result could be cosmetic problems; a fracture or buckled floor.

In addition, because they are sharp, the outside corners of a mold half can "grab" the part within which they are forming a core, either making ejection difficult or risking damage to the part or mold. And finally, sharp corners can contribute to sink and weakened knit lines. So radius those corners!

Now that we've hopefully convinced you to radius corners, let us describe one situation where you should not radius a corner: at the parting line.

Fig. 1

Fig. 1

If you look at Figure 1, you see the parting line where A- and B-Side mold halves meet to form the sharp edge of the part. In machining molds, there is always some tolerance, but slight movement of the parting line to the left or right will not change the geometry of the edge.

Fig. 2

Fig. 2

In Figure 2, on the other hand, both the mold halves form the parting line edge, so any mismatch in the mold will leave a ledge, changing the shape of the part at the parting line. That's one reason we recommend leaving the parting line sharp.


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Plastics Trivia Question

The plastic-bodied Zest, shown here:

Plastic-Bodied  Zest

  A.  will be introduced at the 2009 Paris Auto Show.

  B.  sells for just under $5000 at FAO Schwartz.

  C.  was designed to sell for about 10,000 euros.

  D.  has a 62 horsepower engine and can reach 150 kph.

  E.  is built on a Brazilian-made VW chassis.

(Honor System: No Googling, Yahooing, or Dogpiling until after you've submitted your guess.)

Last month's question/results:
Intelimer ® polymers:

A.  can change their viscosity, permeability, and volume in response to relatively small changes in ambient temperature

B.  can be used to keep meat fresh without refrigeration

C.  can change their optical properties in response to light in the UV waveband

D.  will bend through an arc of up to 180° in response to electrical stimulation

E.  are selectively permeable to higher-energy molecules but impermeable to lower-energy molecules

The correct answer is
A. can change their viscosity, permeability, and volume in response to relatively small changes in ambient temperature.

The responses are represented in the following chart:

Last  Month's  Results

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