EUEvery screwed-together plastic enclosure depends on screw boss design: the cylindrical protrusion around or inside a plastic part that receives a screw, creates a fixed fastening point, and carries the stress from installation and service loads. Get the wall ratio, diameter, height, spacing, or base radius wrong, and the defect doesn’t appear in the model — it shows up weeks later, as a stress-whitened crack around the screw, a sink mark on the opposite face, or a warped part after molding and assembly.
For engineers, product designers, OEMs, and procurement teams working on molded plastic parts, this guide breaks down the boss dimensions that control strength and moldability, including wall thickness, outer diameter, bore size, height limits, draft, and base radius. It also covers spacing, ribs and gussets for reinforcement, material and shrinkage effects, and the fixes for the three failure modes behind most boss-related rejects.
What Is a Screw Boss
A screw boss is a cylindrical protrusion on the interior wall of an injection-molded part, bored through the center to receive a self-tapping screw, a threaded insert, or another fastener. It serves two functions: providing a fixed connection point between assembled components, and carrying the stress generated by screw insertion torque, vibration, and external load after assembly. A poorly proportioned boss shows up directly as a sink mark, a crack, or a loose joint after assembly. Most failures traced back through a plastic part end at the boss.
Core Design Parameters
Boss geometry is not an arbitrary choice — each parameter maps to a specific failure mode. All ratios below are calculated against the nominal wall thickness of the part body.
Wall Thickness
Boss wall thickness is generally held to 50%–70% of the part’s nominal wall thickness, though published guidance varies, with many sources citing 40%–60% or roughly 60%. The shared point across sources is that it should not exceed the nominal wall. Go above the ratio and, in injection molding, thick sections and thick plastic sections increase cooling time, restrict molten plastic packing, and create differential cooling, producing a sink mark on the opposite surface and, in worse cases, internal voids. Maintaining sufficient wall thickness while avoiding excess mass helps prevent sink marks, especially where the outer wall intersects the boss. Go below it and the boss lacks the hoop strength to resist expansion when the screw is driven in, leading to cracking. A common sizing rule is to keep the boss outer diameter around 2.0T to 2.5T relative to the adjacent wall thickness to prevent sink marks.
Outer Diameter and Bore Size
Outer diameter is typically set at 2 to 2.5 times the bore diameter; in practice, the selected screw boss size and screw boss’s outer diameter should be based on load and wall thickness, while thread diameter represents the screw’s outer thread size, so some references calculate it instead as 2 to 3 times the screw’s major diameter. Bore size is set from the screw itself — for self-tapping screws, the pilot hole is usually 75%–90% of the screw’s minor diameter, with the ratio adjusted by material toughness: brittle materials use the higher end, tougher materials can go tighter, and boss hole diameters should match the root or pitch diameter of the chosen self-tapping screw to ensure proper engagement. Bosses may also be sized to accommodate threaded inserts where repeated assembly is required.
Height Limit
Boss height should not exceed three times its outer diameter. A boss taller than this accumulates material mass at the base, and heat cannot escape efficiently through the core pin during cooling, which makes the cored hole harder to control and throws off the final bore dimension. Tall bosses can also extend mold clamping to ejection time because thicker features cool more slowly. A tall boss also deflects laterally under assembly load. When a taller boss is unavoidable, ribs distributing the lateral force are the standard fix to provide adequate support and maintain structural integrity — thickening the wall further is not.
Draft Angle
Use draft angles on the outer surface of the boss, with a minimum draft angle of at least 0.5° on the exterior; inner bore draft should be at least 0.25°. This minimum draft helps facilitate easy removal, and good geometry means the boss helps easy ejection. If draft is insufficient, the injection molding process is more likely to run into release problems when the part must come out of the mold, which at minimum scores the boss surface and at worst deforms the surrounding wall.
Base Radius and Top Chamfer
The base and top transition areas are typically stressed, so the base radius is set at 25%–50% of the nominal wall thickness as a minimum radius to reduce stress concentrations. Its function is to reduce stress concentrations at the boss base, where the junction between the boss and the part wall is where cracking originates most often. A chamfer or recess at the top guides the screw or insert into the bore, reducing misalignment and localized stress during assembly while supporting proper engagement.
Spacing
Center-to-center spacing between adjacent bosses should provide adequate spacing between bosses and be at least twice the nominal wall thickness. Tighter spacing causes the cooling rates of the two bosses to interfere with each other; when a screw boss close to another boss leaves thin areas or starts creating thin areas in the mold wall that are difficult to cool, it can lead to hot blade creation, a hot spot between them, and reduced mold life, compromising dimensional accuracy.
Screw Boss Considerations
Ribs and Gussets
Ribs placed around a boss raise its load capacity without creating thick sections that increase sink risk. Thickening the wall instead only worsens sink marks — ribs are the more economical fix. Ribs or gussets provide adequate support and help maintain structural integrity, especially when the boss is near an external wall. Rib thickness is typically 50%–60% of the adjacent wall, and two to three ribs are usually sufficient; adding more increases mold complexity without a proportional gain in strength.
Connecting to Sidewalls
A standalone screw boss has limited rigidity, so tying it to the nearest side wall with a coring rib improves material flow. This connection meaningfully increases overall stiffness and helps both the part and the boss share load more effectively instead of leaving the feature to carry the full assembly load on its own. If the screw boss close to the external wall, the rib also improves local rigidity and reduces sink risk.
Counterbore and Displaced Material
Driving a self-tapping screw displaces plastic. Without a counterbore or relief space at the bore opening, that displaced material has nowhere to go and pushes outward against the boss wall, producing visible stress whitening or cracking. A counterbore exists specifically to give that material somewhere to move.
Material Selection
Select a resin with suitable mechanical properties for the expected load and assembly method, since brittle materials such as unfilled PC or PS crack under the expansion stress of screw assembly and are not suited for boss features. ABS, Nylon, and PC/ABS blends tolerate that stress better and are the more reliable choice, and screw bosses should match the material of the molded part for more stable performance. Materials with low and uniform shrinkage also provide superior dimensional accuracy. When a brittle plastic must be used, blending it with a softer material helps bosses accept screws easily.
Cooling and Shrinkage
Material shrinkage — commonly in the 0.5%–0.7% range — directly affects the dimensional accuracy of reliable screw bosses. Cooling time also increases with thicker boss dimensions. Cooling channels need to be routed specifically around the boss region; without that adjustment, differential cooling around thicker features in the injection molding process will cause the actual bore size to drift from the design value and can affect high quality plastic parts.
Common Failure Modes and Fixes
Cracking / splitting Root cause: local stress exceeds the material limit during screw insertion. In injection molding, screw boss design guidelines also allow bosses to be positioned around PCBs and connectors for better design flexibility. Fix: increase the base radius, add ribs, switch to a tougher resin.
Sink marks and voids Root cause: boss wall thickness too high relative to the nominal wall. Fix: hold the wall ratio to the recommended range, core out material at the base.
Warpage Root cause: height exceeds the recommended ratio, or cooling is uneven. Fix: control the height-to-diameter ratio, select a material with low, uniform shrinkage.
These three failure modes are connected. Excess wall thickness can trigger both sink marks and warpage. An insufficient base radius is both a direct cause of cracking and a factor that amplifies shrinkage differences at the stress concentration point. When troubleshooting, check the wall-thickness ratio and the height ratio first — most problems resolve at that step, before ribs or a material change become necessary. The same screw boss design guidelines also help when designing screw bosses as bearing surfaces for boss screws, facilitating gear rotation and supporting gear rotation; for molding bosses, one-time thread-forming can be acceptable, but for repeated assembly cycles, insert molding with threaded inserts is usually the better choice.
