Injection molding DFM (design for manufacturability) is the check a part goes through against the rules of the process before anyone cuts a mold. Five things decide most of it: draft on every face that slides against the mold, walls held to one uniform thickness, ribs and bosses kept thinner than the walls they join, a gate placed where the thickest section can be packed, and a parting line that lands somewhere you can live with. Each prevents a specific defect, and each is far cheaper to settle in CAD (computer-aided design) than in a finished tool.
The numbers below are starting points taken from published design guidelines for injection molding by resin makers and molders. Resin, part size, and cosmetic demands all move them, so the final word belongs to your molder and the data sheet for the exact grade. For how the same thinking carries over to routing and 3D printing, see our overview of design for manufacturing across processes.
How Much Draft Does a Molded Part Need?
Draft is the taper on any face that slides against the mold as the part is ejected. Plastic shrinks onto the core as it cools, so an undrafted face drags, scuffs, and can stress or even break the part on the way out. Protolabs’ guidance is that 1 to 2 degrees works in most situations, and it strongly advises at least half a degree on every vertical face.
Resin and polish shift that floor. DuPont’s design guide lists drafts of a quarter degree or less on shallow draws for some of its unfilled resins, which makes very low draft something to agree with the molder in advance.
Texture needs more. DuPont’s guide and other published design guides use the same rule: add 1 degree of draft for every 0.025 mm, or 0.001 inch, of texture depth, so a 0.05 mm grain needs about 2 degrees more than a smooth version of the same face. Protolabs sets 3 degrees as its minimum for a light bead-blast finish and 5 for a medium one.
We would put draft into the model from the first prototype, even when that prototype is 3D printed and doesn’t need it. Protolabs makes the same point from the molding side: printing and machining will build a part with no draft at all, so a design proven only in prints can reach the molder with every vertical face still square.
Wall Thickness, Sink Marks, and Corners
Uniform wall thickness is the rule the others serve. Thin sections solidify faster than thick ones, so a thick area keeps shrinking after its thinner neighbors have frozen. The result is a sink mark on the surface, warpage in the part, and stress locked into the material.
Thickness costs cycle time as well. Komaspec’s design guide notes that cooling time scales with the square of wall thickness, so doubling a wall roughly quadruples the cooling portion of every cycle, and DuPont’s guide charts cycle cost climbing as parts get thicker.
Where a part needs stiffness, ribs do the job better than a thicker wall, and where a section has to be heavy, core it out from behind. Where thickness has to change, blend it over a gradual transition and gate the heavier side, so that section can be packed as it cools.
Corners follow the same logic. An inside radius of half the wall thickness removes most of the stress concentration a sharp corner creates, and DuPont’s stress data shows little further gain beyond that. Pair it with an outside radius equal to the inside radius plus one wall, and the thickness stays constant around the bend.
Rib and Boss Proportions
Ribs add stiffness without thickening the wall, but every rib puts extra mass where it meets that wall. Protolabs and other molders keep the rib base at 40 to 60 percent of the thickness of the wall it joins, so a 2.5 mm wall takes a rib between 1 and 1.5 mm at its base. Above that range, the junction cools last and shows as a sink mark on the opposite face.
Bosses, the raised cylinders that take screws and inserts, follow the same limit on their walls: no more than about 60 percent of the surrounding wall, with an outside diameter at least twice the hole. Stand a boss off the sidewall and tie it back with ribs or gussets. A boss fused into a wall creates exactly the thick section the rest of the design works to avoid.
Rib layout matters even when no injection pressure is pushing the material. The arcade controller handles we reverse engineered for Barcade were originally injection molded in thin-walled ABS (acrylonitrile butadiene styrene). The replacements were resin cast in silicone molds, with the resin vacuum degassed but no pressure chamber, and the internal ribs were redesigned to help the resin flow evenly through the part.
We would treat the flow path as a design input on any molded or cast part, because the ribs decide where the material has to travel before it sets. The Barcade Quick & Crash controller case study covers the full project.
Gates, Weld Lines, and the Parting Line
The gate is where plastic enters the cavity, and its position sets the fill pattern. Gate into the thickest section and let material flow from thick to thin. Gated into a thin area, the gate can freeze before packing pressure reaches the heavy section downstream, and that section sinks.
Weld lines form wherever two flow fronts meet, which happens behind every hole and core pin. They are weaker than the material around them, and filler makes the gap wider: DuPont’s guide notes that weld lines in heavily glass- or mineral-filled resins can have as little as 60 percent of the strength of an unreinforced material. Keep them off screw bosses and load paths by moving the gate or the holes, and ask the molder for a mold-flow simulation before the tool design is final.
The parting line is where the mold halves meet, and it leaves a witness line on every part. Choose where it lands before the tool is designed, ideally along an edge or a change in geometry rather than across a cosmetic face. Features that lock the part into one half, called undercuts, add moving parts such as slides or collapsible cores to the tool, and our guide to injection molding undercuts and side actions walks through the options.
Why Run an Injection Molding DFM Review Before Tooling?
Every check above costs an engineer’s time in CAD. After the tool is cut, the same fix means reworking the mold itself, which is why the injection molding DFM review belongs before the tool is quoted.
Prototypes help, within limits. On the Barcade controller, 3D-printed prototypes were test-fit on one of Barcade’s deployed cabinets before the molds were committed, which confirmed the ergonomics and the fit of the internal parts. What a print can’t show is how the molded part will fill, where its weld lines will fall, or how it releases from the tool. For low volumes, printing and molding are a decision of their own, covered in our comparison of 3D printing and injection molding for small runs.
Headed for a mold? Send the CAD through our project inquiry page before the tool is quoted, or phone 718-557-9578. Include the resin, the expected volume, and which faces are cosmetic, since those three answers shape every rule above.
