Injection Molding Design Guidelines

Much has been written regarding design guidelines for injection molding. Yet, the design guidelines can be summed up in just a few design rules.

1	Use uniform wall thicknesses throughout the part. This will minimize sinking, warping, residual stresses, and improve mold fill and cycle times. 1. Wall Section Considerations 2. Voids and Shrinkage 3. Warpage 1. Uniform Walls Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs. Also, thinner parts weight less, which results in smaller amounts of the plastic used per part which also results in lower part costs. The wall thicknesses of an injection-molded part generally range from 2 mm to 4 mm (0.080 inch to 0.160 inch). Thin wall injection molding can produce walls as thin as 0.5 mm (0.020 inch). The need for uniform walls Thick sections cool slower than thin sections. The thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section. This builds stresses near the boundary of the thin section to thick section. Since the thin section does not yield because it is solid, the thick section (which is still liquid) must yield. Often this leads to warping or twisting. If this is severe enough, the part could even crack.  Uniform wall thicknesses reduce/eliminate this problem. Uniform walled parts are easier to fill in the mold cavity, since the molten plastic does not face varying restrictions as it fills. What if you cannot have uniform walls, (due to design limitations) ? When uniform walls are not possible, then the change in section should be as gradual as possible. Coring can help in making the wall sections uniform, and eliminate the problems associated with non-uniform walls.  Warping problems can be reduced by building supporting features such as gussets.  2. Voids and Shrinkage Shrinkage is caused by intersecting walls of non-uniform wall thickness. Examples of these are ribs, bosses, and other projections of the nominal wall. If these projections have greater wall thicknesses, they will solidify slower. The region where they are attached to the nominal wall will shrink along with the projection, resulting in a sink in the nominal wall. Shrink can be minimized by maintaining rib thicknesses to 50 to 60% of the walls they are attached to. Bosses located at corners can result in very thick walls causing sinks. Bosses can be isolated using the techniques illustrated. 3. Warpage  Thick sections cool slower than thin sections. The thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section. This builds stresses near the boundary of the thin section to thick section. Since the thin section does not yield because it is solid, the thick section (which is still liquid) must yield. Often this leads to warping or twisting. If this is severe enough, the part could even crack. Other causes: Warping can also be caused due to non-uniform mold temperatures or cooling rates. Non-uniform packing or pressure in the mold. Alignment of polymer molecules and fiber reinforcing strands during the mold fill results in preferential properties in the part. Molding process conditions--too high a injection pressure or temperature or improper temperature and cooling of the mold cavity. Generally, it is best to follow the resin manufacturer's guidelines on process conditions and only vary conditions within the limits of the guidelines. It is not good practice to go beyond the pressure and temperature recommendations to compensate for other defects in the mold. If runners need to be sized differently to allow for a proper fill, or gate sizes that need to be changed, then those changes need to happen. Otherwise the finished parts will have too much built in stresses, could crack in service or warp-leading to more severe problems such as customer returns or field service issues.
2	Use generous radius at all corners. The inside corner radius should be a minimum of one material thickness. Radius Limitations Radius Sharp corners greatly increase the stress concentration. This high amount of stress concentration can often lead to failure of plastic parts. Sharp corners can come about in non-obvious places. Examples of this are a boss attached to a surface, or a strengthening rib. These corners need to be radiused just like all other corners. The stress concentration factor varies with radius, for a given thickness.  As can be seen from the above chart, the stress concentration factor is quite high for R/T values lesss than 0.5. For values of R/T over 0.5 the stress concentration factor gets lower. The stress concentration factor is a multiplier factor, it increases the stress. Actual Stress = Stress Concentration Factor K x Stress Calculated This is why it is recommended that inside radiuses be a minimum of 1 x thickness. In addition to reducing stresses, fillet radiuses provide streamlined flow paths for the molten plastic resulting in easier fills. Typically, at corners, the inside radius is 0.5 x material thickness and the outside radius is 1.5 x material thickness. A bigger radius should be used if part design will allow it.
3	Use the least thickness compliant with the process, material, or product design requirements. Using the least wall thickness for the process ensures rapid cooling, short cycle times, and minimum shot weight. All these result in the least possible part cost.
4	Design parts to facilitate easy withdrawal from the mold by providing draft (taper) in the direction of mold opening or closing. Draft and Texture The reason for draft Drafts (or taper) in a mold, facilitates part removal from the mold. The amount of draft angle depends on the depth of the part in the mold, and its required end use function. The draft is in the offset angle in a direction parallel to the mold opening and closing. It is best to allow for as much draft as possible for easy release from the mold. As a nominal recommendation, it is best to allow 1 to 2 degrees of draft, with an additional 1.5° min. per 0.025 mm (0.001 inch) depth of texture. See below. The mold parting line can be relocated to split the draft in order to minimize it. If no draft is acceptable due to design considerations, then a side-action mold (cam-actuated) may be required at a greater expense in tooling. The reason for texture Textures and Lettering can be molded on the surfaces, as an aesthetic aid or for incorporating identifying information, either for end users or factory. Texturing also helps hide surface defects such as knit lines, and other surface imperfections. The depth of texture or letters is somewhat limited, and extra draft needs to be provided to allow for mold withdrawal without marring the surface. Draft for texturing is somewhat dependant on the mold design and the specific mold texture. Guidelines are readily available from the mold texture suppliers or mold builders. As a general guideline, 1.5° min. per 0.025mm (0.001 inch) depth of texture needs to be allowed for in addition to the normal draft. Usually for general office equipment such as lap-top computers a texture depth of 0.025 mm (0.001 inch) is used and the min. draft recommended is 1.5 °. More may be needed for heavier textures surfaces such as leather texture (with a depth of 0.125 mm/0.005 inch) that requires a min. draft of 7.5°.
5	Use ribs or gussets to improve part stiffness in bending. This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs. Rib Design
The use of 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. Bending stiffness = E (Young's Modulus) x I (Moment of Inertia) 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 radiusing at the corners.  At rib intersections, the resulting thickness will be more than the thickness of each individual rib. Coring or some other means of removing material should be used to thin down the walls to avoid excessive sinking on the opposite side. The height of the rib should be limited to less than 3 x thickness. It is better to have multiple ribs to increase the bending stiffness than one high rib. The rib orientation is based on providing maximum bending stiffness. Depending on orientation of the bending load, with respect to the part geometry, ribs oriented one way increase stiffness. If oriented the wrong way there is no increase in stiffness. Draft angles for ribs should be minimum of 0.25 to 0.5 degree of draft per side. If the surface is textured, additional 1.0 degree draft per 0.025 mm (0.001 inch) depth of texture should be provided.