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Alternatives:
If your part is so complex that you need variations on your wall thickness, look for an alternative. You may want to use design features such as coring or using ribs. At the very least, try not to make the transitions between thicker and thinner sections too abrupt. Try using a gradual transition or chamfered corners to minimize the dramatic change in pressures inside the mold.
Draft
Most injection molded plastic parts include features such as outside walls and internal ribs that are formed by opposing surfaces of tool metal inside a closed mold. To properly release the part when the mold opens, the side walls of the mold are tapered in the direction that the mold opens. This tapering is referred to as “draft in the line of draw”. This draft allows the part to break free of the mold as soon as the mold opens. The amount of draft required can depend on the surface finish of the mold. A smooth, polished tool surface will allow the part to eject with less draft than a standard tool surface.
Consider the fabrication of the hollow plastic box seen to the right. Once the plastic has hardened around the mold, the mold must be removed. As the plastic hardens, it will contract slightly. By tapering the sides of the mold by an appropriate "draft angle", the mold will be easier to remove.
The amount of draft required (in degrees) will vary with geometry and surface texture requirements of the part. Below are several rules for using draft properly:
• Be sure to add draft to your 3D CAD model before creating radii
• Use at least 1 degree of draft on all "vertical" faces
• 1 ½ degrees of draft is required for light texture
• 2 degrees of draft works very well in most situations
• 3 degrees of draft is a minimum for a shutoff (metal sliding on metal)
• 3 degrees of draft is required for medium texture
Sink Marks
When the hot melt flows into the injection mold, the thick sections don’t cool as fast as the rest of the part because the thicker material becomes insulated by the outside surface of faster cooling plastic. As the inner core cools, it shrinks at a different rate than the already cooled outer skin. This difference on cooling rates causes the thick section to draw inward and create a sink mark on the outside surface of the part, or worse, completely warp the part. In addition to being unattractive, the mark also represents added stress that is built into the part. Other less conspicuous areas where sink occurs include ribs, bosses and corners. These are often overlooked because neither the feature nor the part itself is too thick; however, the intersection of the two can be a problem.
One way to avoid sink marks is to core out the solid sections of the part to reduce thick areas. If the strength of a solid part is required, try using cross hatched rib patterns inside the cored out area to increase strength and avoid sink. As a rule-of-thumb, make sure that all bosses and locating/support ribs are no more than 60% of the thickness of the nominal wall. Also, textures can be used to hide minor sink marks.
Textures
Texturing is a process used to apply patterns to a mold surface. This process allows flexibility in creating the final appearance of your parts. Texturing is an integral piece in overall product development and should be considered during the design process to achieve the desired results. Texture can be a functional component of design as well. Imperfect parts can be camouflaged by the right texture. Is the part designed for frequent handling? Texture can be used to hide finger prints and improve the grip for the end user. Texture can also be used to reduce part wear from friction.
A wide variety of textures are available for injection molded parts such as:
• Natural/Exotic
• Matte Finishes
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