Plastic part design for injection molding is to ensure uniform wall thickness throughout the part. While this does not mean every wall has to be identical, maintaining relatively equal wall thickness ensures proper melt flow.
Thick walls cool slower and greater shrinking will occur. Thin walls cool faster as thus, less shrinkage. Create your part with uniform walls and cross section will simplify manufacturing and costing. Uniform wall thickness will help ensure product quality by avoiding sink marks associated with uneven or thick walls.
Parts with uniform walls will not warp, will fill properly and will fit together because variable shrinkage is minimized. Wall thickness variations should not exceed 20% in high mold shrinkage plastics. In fact, even this slight disparity can introduce processing and quality problems. Select the optimal wall thickness :
thinner parts require less raw plastic material and they cool faster, but must consider product requirements : force, pressure, environment etc.
Use the ribs. Ribs increase the bending stiffness of a part. Without ribs, the thickness has to be increased to increase the bending stiffness. Adding ribs increases the moment of inertia, which increases the bending stiffness. The rib thickness should be less than the wall thickness-to keep sinking to a minimum. The thickness ranges from 40 to 60 % of the material thickness. In addition, the rib should be attached to the base with generous radius and corners.
Design part must be without sharp edges. The molding process will naturally create small rounds at all sharps anyways. Rounded corners will ease plastic flow, so engineers should generously radius the corners of all parts. Sharp inside corners result in molded-in stress—particularly during the cooling processes when the top of the part tries to shrink and the material pulls against the corners. Fillets should be min ¼ X Wall thicknesses. Rounding all edges helps to ensure uniform wall thickness and avoid unsightly shrink marks.
Gate location will determine how the plastic melt flows through the part and where weld lines may occur. Weld lines are lines where to flows of plastic meet and form a relatively weak bond. The most effective gate location is when the melt enters at the thickest part of the cavity and then flows to the narrower areas. Gate will be visible and may have a vestige sticking out. This may be unsightly and also can impede function potentially. Be sure to call out gate location on the part drawing as well. Plastic part alsois going to use an ejector pin method of ejection, consider where the knock-out pins will be. Be careful not to place these on weak areas of the part and remember that there will be visible indentations on the part on these areas. Also be sure to call out these locations on the drawing.
Draft angles are needed so that a plastic part can be released from the mold without distortion or damage. The high pressures of injection molding force the plastic to touch all the surfaces of a mold's cores and cavities. The cavity becomes so tightly packed that it is often difficult to remove the part. Sometimes, shrinkage will actually make it easier to take the part out of the mold, but in other cases, shrinkage will cause the part to stick to the mold's cores. These natural occurrences call for draft angles. Draft min of 0.5 deg. For all injection molded parts, draft angles are necessary to ensure proper part ejection with no deformation. Longer surfaces require greater draft angles due to increased surface area. Similarly, textured surfaces require greater draft angles due to a higher propensity for the parts to stick.
Plastic part design guide and tips for best design
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