Eliminating Side-actions for Fun and Profit

Quick, how would you make the part shown in Figure 1? Well, since by now you're aware of the fact that Protomold supports up to four side actions per mold, the obvious answer is to use a mold with a side-action to create the bottomless box with the window. After all, without a side-action, a mold feature that protrudes inward from the A-Side of the mold (or outward from the B-Side) would be entrapped in the window when the mold opened, right?

Fig. 1

Wrong! This part can be made in a two-part straight pull mold without side actions. It works for one simple reason: the side walls are drafted. That means that the mold surface will begin to pull away from the part surface as the mold begins to open. So if your design can support the draft and you'd like to save a little money and skip the side action altogether, it's worth taking note. Take a look at Figure 2 to see how this can be applied.

Fig. 2

Figure 2 shows a cross-section of the window area in the closed mold in which:

• the gray areas are the mold walls
• the window is being formed partially by an extension of the mold's A-Side and partially by an extension of the B-Side
• the dotted line represents the plane of the outside surface of the wall
• the red line is the shutoff where the metal faces of the two mold halves meet when the mold is closed

Note that the shutoff runs at an angle across the window, dividing the parts of the window that will be produced by the A and B mold halves. Because the wall is drafted, the two mold halves move away from the part (and from one another at the shutoff) as the mold opens and the part is ejected. No part of either mold half is entrapped in the window; hence, no side action is required.

There is one important consideration when designing a part using this technique: the draft angle of the shutoff. To avoid damage to the mold the shutoff must be drafted a minimum of 3°. Because the shutoff is angled slightly relative to the wall itself, the draft of the wall must be greater than 3° to allow a 3° draft of the shutoff. The required amount of wall draft will vary directly with wall thickness and inversely with window height ("shutoff height," as shown in Figure 2). Most CAD programs can help determine the proper degree of wall draft to create a minimum of 3° draft at the shutoff.

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Plastics Trivia Question

Which of the following is true of Neoprene?:

 A. Its chemical name is polyfluoroprene.

 B. It's original brand name was Polyprene.

 C. It was introduced in 1938 when natural rubber became scarce.

 D. It was developed by the Goodyear Rubber Company.

 E. Its air spaces can be filled with nitrogen for increased insulation.

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

Last month's question/results:
Which of the following is not true of Engineered Cementitious Composites (ECC), a form of concrete reinforced with polymer fibers?:

A. It is 500 times more resistant to cracking than ordinary concrete.

B. It is ductile (bendable) under load.

C. It is 40 percent lighter than ordinary concrete.

D. Under strain, the embedded fibers slide within the cement.

E. It was originally developed for shipbuilding applications.

The correct answer is
E. It was originally developed for shipbuilding applications.

The responses are represented in the following chart:

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March 19-20, 2008
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June 3-5, 2008
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