How to Incorporate Shut-offs in Injection Molding Design
To take a closer look at shut-offs—a design element that allows engineers to solve difficult tooling and design issues—let's consider the injection molding of a box. The box looks like an A-frame house with four 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 mold. In order to create the door and dormer window, the faces of the mold must slide past each other until the mold closes where they create a seal which creates a hole in your part. These are the shut-offs, shown in red in Figure 1b.
Shut-offs Help Create Part Features
Shut-offs in straight-pull molds will allow you to design many different features that fall in to three basic categories: Holes, hooks, and long through holes without the use of side-actions (cams).
In the following images you will see samples of three designs. The blue faces show the B-side of the mold, the green faces show the A-side of the mold and yellow lines represent the plane where the mold 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 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 mold and form a better seal under clamping force during molding. The minimum draft of 3 degrees allows greater clearance for the sealing faces on opposite sides of the mold to pass by one another without the risk of gouging and wear.
Figure 2 shows a through hole across a 4 in. wide part. 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.
Hooks and Snap Features
Figure 3 is the big one. Undercuts, most notably snap features, tend to be no-nos in the world of simple straight-pull rapid injection molds. A pass through core (square in this instance) is a standing block of aluminum on one side of the mold that mates to a pocket in the opposite side of the mold. One face of the block forms the inside leg of the hook while the other three faces of the block are drafted at 3 degrees and create shut offs against the mating sides in the pocket. Once the mold 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-molding machining or the need to remove functional geometry.
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