November 2006 Design Tip

Shut offs, what are they good for?

This month we are moulding a box. The box looks like an “A-frame” house with 4 vertical walls and a roof. On one of the vertical walls there is a cut out that would resemble a door and on the roof there is a dormer with a window. See Figure 1a.

The A-side is the exterior of our box and the B-side is the interior core. Figure 1b is the b-side of the mould. In order to create the door and dormer window, the faces of the mould must slide past each other until the mould closes where they create a seal which creates a hole in your part. These are "shut-offs", shown in red in Figure 1b.

Shut offs in straight pull moulds will allow you to design many different features that fall in to 3 basic categories; Holes, Hooks, Long through holes without the use of side actions (cams).

In the following images you will see samples of the 3 designs. The Blue faces show the B-side of the mould, the Green faces show the Aside of the mould and Yellow lines represent the plane where the mould faces touch to form a shut off. The plane should have a minimum of 3 degrees draft with the A-side and B-side faces connected by the yellow lines planar to the shut off face.

Figure 1a

Figure 1b

Figure 1a shows the door in our box, or a hole in the wall. The door is formed by a pad that lives on the B-side core (Figure 1b). The pad is the same thickness as the wall section of the part and seals against the vertical wall of the A-side cavity. If this seal is damaged or wears too aggressively, the plastic will sneak past the seal and create flash. The dormer window on the roof of the part is formed the same way, but shows this form of shut off surrounded by plastic.

Shut offs require greater draft to reduce the amount of wear on the mould and form a better seal under clamping force during moulding. The minimum draft of 3 degrees allows greater clearance for the sealing faces on opposite sides of the mould to pass by one another without the risk of gouging and wear.

Figure 2

Figure 2 shows a through hole across a part 100mm wide. With side action pull limitations, you need to be creative at times. Creating a segmented trough on the A-side and a corresponding segmented trough on the B-side creates a series of shut offs that form a through hole across the entire part! Cool huh? This is perfect for hinges, bolt holes, pivot pins, etc.

Figure 3

Figure 3 is the biggy. Undercuts, most notably snap features, tend to be no no’s in the world of simple straight pull rapid injection moulds. A pass through core (square in this instance) is a standing block of aluminium on one side of the mould that mates to a pocket in the opposite side of the mould. One face of the block forms the inside leg of the hook while the other 3 faces of the block are drafted at 3 degrees and create shut offs against the mating sides in the pocket. Once the mould is opened and the part is ejected, you will have a hook that snaps to your mating part.

Shut offs are powerful tools that allow creative designers and engineers to solve difficult tooling and design issues using drafted walls sealing against drafted walls, eliminating the need for post moulding machining or the need to remove functional geometry.