Eliminating Side-Actions with Draft

How would you make the part shown in Figure 1 if you were using Protolabs' injection moulding services? Well, considering that Protolabs supports up to four side-actions per mould, one answer is to use a mould with a side-action to create the bottomless box with the window. After all, without a side-action, a mould feature that protrudes inward from the A-Side of the mould (or outward from the B-Side) would be entrapped in the window when the mould opened, right? Well, that is one correct answer.

However, this part can also be made in a two-part, straight-pull mould without side-actions. It works for one simple reason: the side walls are drafted. That means that the mould surface will begin to pull away from the part surface as the mould 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.

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

• the grey areas are the mould walls
• the window is being formed partially by an extension of the mould'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 shut-off where the metal faces of the two mould halves meet when the mould is closed

Note that the shut-off runs at an angle across the window, dividing the parts of the window that will be produced by the A and B mould halves. Because the wall is drafted, the two mould halves move away from the part (and from one another at the shutoff) as the mould opens and the part is ejected. No part of either mould 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 shut-off. To avoid damage to the mould the shutoff must be drafted a minimum of 3 degrees. Because the shut-off is angled slightly relative to the wall itself, the draft of the wall must be greater than 3 degrees to allow a 3-degree draft of the shutoff. The required amount of wall draft will vary directly with wall thickness and inversely with window height ("shut-off height," as shown in Figure 2). Most CAD programs can help determine the proper degree of wall draft to create a minimum of 3 degree draft at the shutoff.

If you have questions on side-actions or any other design consideration, please contact an applications engineer at +44 (0) 1952 683047 or [email protected].