The Three Numbers of Screw Boss Design

A boss is a molded post with a hole down its center. It takes a screw, a threaded insert, or a locating pin. Nearly every plastic housing has several, and they cause a large share of field failures.

Three dimensions decide whether a boss survives assembly and service. Get them right on the drawing and most boss problems never appear. Fix them after T1 and you pay for welding, re-cutting, or both.

The outer-to-inner diameter ratio should stay between 2 and 3. Below 2 the wall goes too thin to hold thread or insert loads. Above 3 you are molding plastic that only adds sink risk.

Boss Wall Thickness and the Pilot Hole

A boss wall is a thick section hiding inside a thin part. It freezes after the surrounding wall, shrinks more, and pulls a shallow dimple into the face behind it. Holding the boss wall at or below 60% of nominal keeps that dimple invisible on most resins.

The pilot hole for a self-tapping screw runs slightly under the screw’s outer diameter so the thread can cut its own path. Size it at 75–90% of the screw’s minor diameter. Tougher resins like ABS accept the tighter end. Brittle resins need the looser end or the boss splits on the first drive.

Add a small chamfer, about 0.3–0.5 mm, at the mouth of the hole. It centers the screw and stops the first thread from shaving plastic into the joint. That shaved debris is a common cause of inconsistent seating torque on the line.

Thread engagement should use the full boss depth the design allows. A screw that only bites the top third of a deep boss loads a short thread length and strips under vibration. Our draft and hole design notes cover the release angles that keep deep bosses molding cleanly.

Gussets: When a Boss Can’t Stand Alone

A tall freestanding boss is a cantilever anchored at one end. Any side load bends it at the base, where the bending moment peaks. Tall screw bosses without support flex when the screw bites, and they crack at the root after a few thermal cycles.

Add gussets to any boss taller than about twice its outer diameter. Three or four gussets, spaced evenly around the boss, tie it to the nearest wall or floor. The load then travels into the part structure instead of hanging on the boss base.

Gussets follow the same thickness discipline as the boss itself. Keep gusset thickness at or below 60% of nominal wall. Run the gusset about two-thirds of the boss height, and radius the root. A gusset drawn at full wall thickness trades stiffness for a sink mark on the show face.

Keep gussets on the non-cosmetic side wherever the design allows. A faint sink line opposite a gusset root hurts nothing on a hidden face. On a visible face it becomes a quality complaint.

Threaded Inserts: Molded-In vs Post-Mold

Self-tapping screws cut their own threads and cost nothing beyond the screw. They also wear out. After a few assembly cycles the plastic threads loosen, and the joint never clamps the same way again. Any joint that gets opened for service deserves a metal thread.

Four insert routes cover nearly every case. The right one depends on load, volume, and how many times the screw comes out.

Two rules from insert makers settle most arguments. Torque resistance grows with insert diameter, while pull-out resistance grows with insert length. Size the diameter for the tightening torque and the length for the pull load, not the other way around.

Mold the hole instead of drilling it. The molded skin around the hole is denser than the core material, and that skin carries the insert’s knurls better. This matters most in structural foam parts, where the core is porous under the skin. Plastics Technology’s insert fastener guide walks through the same installation trade-offs with test data behind them.

Match the hole geometry to the insert. Tapered inserts want tapered holes; straight inserts want straight holes. A mismatched hole leaves the knurls half-engaged, and the insert spins the first time someone over-torques the screw. Cut the hole deeper than the insert. Never let the screw bottom out on the hole floor, or it jacks the insert out.

Common Boss Failures and Their Fixes

Most boss failures trace back to the drawing, not the molding machine. Solid screw boss design catches them before steel is cut. These are the five we flag most often, with the fix for each.

A sink mark opposite the boss means the boss wall ran too thick. It freezes last, shrinks most, and pulls the back face in. Thin the boss to 0.5–0.6 × T, core out the base, or move the boss off the cosmetic wall.

A boss that cracks as the screw drives in is failing in hoop stress. The pilot hole is too tight, the screw is oversized, or the base corner is sharp. Open the pilot hole, confirm the outer diameter is at least twice the screw diameter, and add the base fillet.

A crack that follows the weld line at the boss base points at the gate position. Two flow fronts met at the boss, and the weld is the weakest ring on the part. Move the gate so the weld lands in a low-stress area, or move the boss. Our gate location notes show how gate placement steers weld lines.

Threads that strip on reassembly have simply worn out. Self-tapping threads in plastic survive only a few cycles before they loosen. Switch serviceable joints to threaded inserts, whose metal thread handles repeated assembly without wearing the plastic.

An insert that spins or pulls out is usually too short for the load. The hole geometry may also mismatch the insert type, or the install overheated and melted the knurl engagement away. Lengthen the insert, match the hole to the insert, and qualify the install with a pull test on first articles. Engineers Edge’s molding design reference lists the same boss diameter and gusset ratios used above.

Try the Interactive Boss Design Guide

Numbers on paper only go so far. The interactive Boss Design Guide covers boss anatomy, screw-boss sizing rules, and common defects. Its live designer checks your dimensions as you change them. Run your boss sizes through it before you lock the drawing.

Open the interactive Boss Design Guide

Boss design reference by Topworks Plastic Mold

What to Send Us for a Boss DFM Check

Boss review is a fixed line in our DFM reports. Send the 3D file with the resin grade, the screw size and type, and mark which faces are cosmetic. Assembly torque helps if you have it; expected service cycles help more.

We check every boss for wall ratio, pilot hole size, base fillet, and gusset need. We also check where the weld line lands relative to the base. Anything that will sink a cosmetic face or crack under the screw gets flagged before steel is cut. Our DFM review process and molding design guide describe the full check in more detail.