Boss Design for Self-Tapping Screws in Injection Molded Parts

Boss design for self-tapping screws starts with the screw

A thread-forming screw displaces plastic as it turns. The pilot hole has to leave room for that displaced plastic while keeping enough material for the threads to bite. Screw makers publish hole charts per screw diameter and resin family. Use the chart instead of scaling from an old drawing.

The common starting point is a hole near 80 percent of the screw’s outer diameter. Brittle resins need it opened up; tough ones take it tighter. A hole that is too small drives hoop stress straight into the boss wall. The rim splits, sometimes on the first assembly, sometimes on the tenth when a tired operator leans on the driver.

A hole that is too big gives weak thread engagement, and the screw strips at low torque. Between the two failures, the split boss costs more. A split boss scraps the whole part; a stripped screw only scraps the joint.

The outer diameter follows the screw too. Use about twice the screw diameter for unfilled resins, about 2.5 times for glass-filled grades where fibers cut ductility. That wall is not there for looks. It has to absorb the screw’s radial expansion without cracking and cool fast enough to keep the show face flat.

Keep the boss wall thin on purpose

A boss wall as thick as the part wall always sinks the face behind it. The thick section stays molten after the wall around it has frozen, and the late-shrinking plastic pulls the show face inward. On textured surfaces the sink shows as a polished shadow. The fix is to core the boss. Make it a tube, not a solid post, with the wall at 50 to 60 percent of the nominal wall.

The trade-off runs the other way too. A boss wall below half the part wall lacks the hoop strength to resist the screw. The boss cracks during assembly even when the pilot hole is right. That is the window. Thick enough to survive the screw, thin enough to cool with the part. First trials tell you which side you are on. A sink shadow behind the boss means the wall is too thick. A cracked rim means it is too thin.

At the base, add a fillet of 0.25 to 0.5 times the part wall. It spreads the bending load where the boss meets the floor. Keep the fillet modest, because a generous radius piles material at the root, and that root is exactly where a sink starts. Readers of our wall thickness masterclass will recognize the same uniform-wall logic behind every rib and boss on the part.

Tall bosses need gussets, not thicker walls

Once a boss climbs past about twice its outer diameter in height, torque starts bending it at the base. The instinct is to thicken the wall. That only trades a bending problem for a sink problem. The better answer is gussets. Small triangular ribs tie the boss wall into the floor and carry the torque load down.

Size gussets like any other rib, with about half the part wall thickness at the root and draft on both sides. Four small gussets stiffen a tall boss more than thickening its wall does. Watch the root where the gusset meets the boss. A thick gusset base is just a thick section by another name, and it sinks the same way. Our gusset design for bosses and walls walks through the placement details.

T0 samples are where this shows up. A tall standalone boss that survived the design review will rotate or crack at the base under a real torque driver. Gussets added before the mold is cut are cheap. Gussets added after are not.

Draft and the base fillet decide whether it cracks

A boss has two drafted surfaces, the outside and the core-pin hole inside. Both need 0.5 to 1 degree per side. Zero draft on the core pin builds friction and vacuum. The pin sticks, scuffs the hole, and in bad cases drags the whole boss out of round. The thread then bites unevenly and the screw cants.

The base fillet deserves its own check. Sharp corners at the boss root concentrate stress exactly where the bending load peaks. A fillet of 0.25 to 0.5 times the part wall spreads that load. At the entry, cut a half-millimeter chamfer or short counterbore so the screw starts straight instead of chewing the rim. A screw that enters crooked loads one side of the boss and cracks it from the top down. That failure looks like a material problem, and it almost never is.

Place the boss away from trouble

Position a boss away from outside corners, where stress already runs high. Keep bosses at least two outer diameters apart, centre to centre, or the thick sections merge into one slow-cooling mass. Near a side wall, offset the boss and tie it to the wall with a thin rib. Never merge the boss straight into the wall. The merged section cools last, sinks the face, and can trap a void inside.

These are the same DFM rules for bosses and ribs that catch most molding defects before steel is cut. Gating decides the hidden risk. Melt splits around the core pin and rejoins behind the boss, leaving a weld line. That line is the weak axis, and the screw’s expansion opens it. Gate the part so the fronts meet hot and at a shallow angle, or away from the boss entirely. Weld lines weaken plastic parts exactly where a screw boss needs strength most. A fill simulation shows the weld line position before steel is cut. Check it. Buyers who skip that check meet the same crack at T0, along the same line, on part after part.

Know when a screw boss is the wrong fastener

Self-tapping screws in soft TPE or TPU barely bite. The threads form and then tear on the first service. Metal inserts or snap features hold better there. The other common miss is a joint that gets opened repeatedly. Threads cut into plastic survive a few cycles, then wear, and the screw spins free in its own hole. For anything assembled more than once, a heat-set insert is the better spend. Its boss runs about twice the insert diameter, and the joint survives dozens of cycles.

Glass-filled resins sit in between. They need the wider outer diameter and a slightly larger pilot hole. Their weld lines are weaker, so gate with more care. If the drawing combines a cosmetic face, a self-tapping boss, and glass-filled resin, ask the molder for a torque-tested sample first. I’ve seen a line scrap a week of housings whose bosses passed the drawings but failed under the plant’s worn drivers. A maximum drive torque on the drawing, not just the screw size, is the cheapest insurance here.

Try the interactive boss designer

The interactive boss designer below takes screw diameter and wall thickness as inputs. It returns the boss dimensions from the same ratio rules used in this article. Drag the sliders and watch the pilot hole, outer diameter, and wall limits update.

Boss design reference by Topworks Plastic Mold

Frequently asked questions