Injection-molded parts are manufactured by melting and injecting solid thermoplastic resins pellets into a mold. They mold, which already has the shape and form of the desired part, houses the molten plastic till it cools back into the solid state of the new form. The part is then ejected, ready for post-processing or immediate use.
There are many ways that one can increase the strength and performance of injection-molded parts. For one, aligning and following with design guidelines is a solution. The material to be used also determines the durability of the part. Other ways to convey additional strength and improve structural stability includes the use of simple structures incorporated by design.
When dealing with injection molding design, simple structures and design techniques such as use of ribs, gussets, bosses, radii and even draft angles can reduce stress concentrations, ensure printing success and easier part ejection. If a part has significant need for strength, it is possible to maintain its weight and deliver more strength by simply adding ribs or gussets. In order to build the perfect injection molded part, a balance between design and strength considerations must be struck. Here are some considerations on just how to do that.
Ribs, Gussets and Other Structures
• Walls One of the ways to increase the overall strength of your part is to increase the thickness of your walls. Depending on the resin material being used, the recommended minimum wall thickness can range from 1mm (small parts) to 3mm+. The thicker the wall, the likelier that the part sink. Generally a minimum wall thickness of 2mm is ideal for most parts. Walls that are thicker than these will have more stress concentrations within, emphasizing the need for ribs and gussets as additional support structures. Thicker walls will also lead to longer cycle times and parts with sub-par mechanical characteristics. Finally, avoid having sudden and abrupt variations in wall thickness. If there is a need to have uneven wall thickness in an area of your part, ensure that this variation is handled across transition zones to lower the stress concentration and reduce the stress, voids, warps and sinks in such areas.
• Ribs Ribs are structural integrations in manufacturing design used to convey rigidity and support to a part. They are thin-wall like structures in the design used to replace or compliment thick wall sections or bosses in the design. By placing ribs in the design, it is possible to add strength and maintain the recommend wall thickness. Ribs are advantageous because they help eliminate the problems that may arise from using thicker wall dimensions.
Rib Guidelines I. Ensure that the base thickness of the rib is around 50-70 percent of nominal wall thickness to avoid sink marks and excess mass II. Base radius of ribs should be no more than 0.25 to 0.5 times the nominal wall thickness. This helps to reduce stress concentrations. III. Use a 0.5 – 1.5o draft for ribs to ease ejection IV. The maximum rib height is 2.5 to 3 times the nominal wall thickness. This is because deep ribs may stick in the mold and are usually harder to fill. V. Ensure ribs are adequately spaced with the distance between ribs at least 2-3 times the nominal wall thickness to avoid hot spots on the mold.
• Gussets Gussets, like ribs, perform support functions. They are often used to reinforce bosses and walls. Gussets essentially use the same design guidelines that apply to ribs with an exception to how their corners are handled. Gussets may also be considered as ribs that do not extend from one wall to another. Rather than stop at a sharp angle, gussets are usually triangular. When they are not triangular, gussets must be made to have their unconnected corners rounded to avoid sharp angles.
Gusset Guidelines Along with the guidelines that apply to ribs, the following may be true for gusset design: I. Ensure that the thickness of the gusset at any intersection with a nominal wall be no more than 0.5 to 0.66 (2/3) the thickness of the nominal wall II. The space between gussets must be a minimum of 2 times the nominal wall thickness III. Keep the length of the gusset within 0.3 to 1 times its height IV. The maximum gusset height is less than 4 times the nominal wall thickness. The preferred range for gusset height is 2 times the nominal wall thickness. V. Gussets can be up to 0.95 times the height of the boss they attach to. VI. The depth of a strengthening gusset is usually twice the nominal wall thickness
• Bosses Bosses are cylindrical projections that serve attachment and assembly purposes in many injection molding designs. Bosses are designed to accommodate self-tapping screws, threaded inserts, mounting and fastening parts. Bosses are also used to deliver more strength and durability to a part when thickening the nominal walls may consume too many materials and increase part cost.
Boss Guidelines I. For bosses, the standard is internal diameter is 0.5 times the external diameter. This allows a corresponding increase in the wall as the size of the bosses increase. II. Keep the wall thickness at the base of the boss no more than 0.6 times the nominal wall thickness III. Keep the height of the boss less than 3 times the outer diameter IV. To lower stress concentrations and reduce the chances of breakage, design bosses with blended radii and do away with sharp edges at their base. V. Replace excessively long bosses with two shorter, mating bosses and ensure that boss holes extend to base-wall levels. VI. The larger the radii, the lower the stress concentration. However, larger radii may increase the likelihood of voids or sinks.
• Fillets/Radii Fillets and radii offer intuitive solutions to dealing with the fragility and stress concentrations that come with sharp corners. As is the case with gussets, using fillets too small will not be effective in stress reduction. Fillets and radii go hand in hand. When a fillet is to be added to an inside corner, it is usually effective to add a radius to the external corner. Fillets and radius are imperative for avoiding crazing, cracking and structural failure that may arise from the use of sharp corners in part design.
Fillet/Radii Guidelines I. Where possible, avoid sharp corners altogether; radius all outside and inside corners. II. Keep the radius as large as your design can accommodate III. The radius of an inside corner should be 0.5 times the nominal wall IV. The radius of an outside corner should be 1.5 times the nominal wall V. Any radius at the base of a boss must have a minimum radius of 0.015” and be 0.25 times the nominal wall VI. The larger the radii, the lower the stress concentration. However, larger radii may increase the likelihood of voids or sinks. VII. Bases of bosses and ribs are locations for high stress concentrations. Radius the edge where the boss and wall come in contact to reduce the sharp corner.
Conclusion Injection molded parts can be improved and strengthened with an array of techniques. Bosses, gussets, ribs, fillets and internal radii solutions all exist to help reduce stress concentrations, conserve material consumption and prevent unnecessary wall thickness and lower cycle times . Ribs and gussets are particularly beneficial for parts that have thin walls that may lose structural integrity and durability as day-to-day use commences. Adhering to the above guidelines for each component will help you deliver on a part that works just as intended.
Using Simple Structures and Hacks To Design More Durable Injection Molded Parts