Design for manufacturing (or DFM) is a process for designing parts to make them easier or more cost-effective to manufacture while ensuring the customer’s quality and performance requirements are met. Design for manufacturing should happen as early as possible in the design process to avoid redesigning the part later at additional costs. DFM includes all aspects of the design process, including material selection, part optimization, and design characteristics.
Plastic Material Choice
Injection-molded parts can be made from a variety of thermoplastics which fall into the categories of commodity plastics, engineering plastics, and high-performing plastics. The polymer should be chosen based on the product’s needs (e.g., strength, durability, thermal stability, chemical resistance) and equipment limitations. High-performance thermoplastics, for example, may require modified equipment because of their high melting temperatures. In other cases, costs may be a determining factor.
Number of Components
For products that require multiple components to be assembled, there may be an opportunity to redesign the part to combine two components. Engineers will evaluate how the components are used with each other and determine if they can be combined without negatively impacting molding, costs, or adding additional processes.
It may also be possible to design molds to optimize the number of cavities, which can reduce costs.
Designing the Part
The guidelines provided here are very general as the properties of the polymers vary. Some stronger polymers may require less material to meet strength requirements. Shrinkage and viscosity vary among polymers, which can impact moldability. Always consult with a plastic injection molding expert if you are working with a polymer that you are not familiar with.
One critical design consideration is ensuring consistent wall thickness. Thinner walls use less material, provide a lower weight product, and cool faster, reducing production time, but too thin, and the walls may warp, crack, twist, shrink and possibly fail. The nominal wall thickness should be determined by the functional performance of the part and the polymer being used.
When molding plastics, the melted polymer will take the path of least resistance—flowing toward the thicker wall sections first (racetrack effect), causing weld lines and air traps. The thinner region will create flow resistance (hesitation) from increased viscosity as cooling occurs. This can lead to a frozen layer- a skin forming on the melted plastic.
The outer areas that are in contact with the mold cool first. Cooling causes contraction, so with uniform wall thicknesses, the whole part will contract away from the mold simultaneously. When there are two connected walls where one is thin and one is thicker, the thinner will cool first. The thicker wall will begin to cool on the outside, but the inside will continue to contract, pulling material from the surface and creating a sink mark. The stress can also cause warping, twisting, and cracking where the two walls meet. Having a gradual transition can prevent flow hesitation and the racetrack effect. In these cases, the polymer should flow from thick to thin sections to avoid sinks and voids.
Corners should be radiused. Fillet (called radii in 2D and prevents a sharp transition at the wall) are used where walls intersect the floor of the part. A rule of thumb is that a radius equal to at least 0.5 times the adjacent wall thickness should be used. The radii at the top of the wall should be 1.5 times the thickness of the nearest wall.
Structural features such as bosses, ribs, and gussets need to be designed correctly. Bosses are projections on the plastic component, such as a screw receptacle or a locator for a mating pin, that are typically used to assemble the final part. Bosses should be isolated from the corner as sinkage in the nominal wall can occur. Bosses can be strengthened by connecting them to ribs at the wall or gussets to the base.
Ribs and gussets aid in integrating internal structures and prevent part warpage. A filet can be added to reduce stress but take care not to make it too large and create sink.
Typically, the thickness of bosses, ribs, or gusset bases should be 60% or less of the part’s nominal wall thickness. Rib height should be no more than three times the nominal wall thickness. To increase stiffness, increase the number of ribs and not the height. Space them a minimum of two times the nominal wall thickness apart.
Gussets can be up to 95% of the height of the rib or boss it is attached to. Gusset base length is typical twice the nominal wall thickness.
Other Considerations For Moldability
Design for manufacturing ensures the part can be removed from the mold without damaging it. Draft angles make it easier to remove the part from the mold. Without them, the friction created would create scrap on the sides of the part or ejector pins would be needed, which can leave marks.
The deeper the part, the larger the draft angle should be. For deep cavities, a rule of thumb is to add 1° of angle for every inch of the cavity. With Torlon PAI, draft angles as low as one-eighth of a degree have been used, but the part should be evaluated before using such a low angle. Part complexity and texture play a role in effective draft angles. The deeper the texture, the higher the draft angle, ranging from 1° to 12+° draft. The rule of thumb is to add 1° to 1.5° per 0.001 inches of textured depth.
Be alert to elements of the part’s design that can create challenges. Undercuts are indents or protrusions that run parallel to the mold’s parting line and prevent the part’s ejection from a straight-pull mold. Undercuts should be avoided if possible because they add cost to the mold as they require core pulls or cams.
Turning to the Experts for DFM
When it comes to designing your high-performance part, turn to the experts at Ensinger. Our design for manufacturing (DFM) for high-performance plastics requires in-depth knowledge beyond that required for injection molding with commodity polymers. At Ensinger, we have the in-depth specialist knowledge of the processes and materials required to solve your toughest challenges. Contact us today.