Quick answer: Ribs and bosses should move load through a controlled section without creating hidden thick masses. Size each feature relative to the adjoining nominal wall, core solid geometry, taper transitions, add radii and verify gate, cooling, ejection and assembly loads together.
Start From the Load and Nominal Wall
Define what the feature carries: bending, screw clamp, insertion, impact, alignment or cosmetic form. Select a nominal wall for the resin and flow path, then size the feature as part of the structural section. Copying a rib ratio without understanding load and material can create either unnecessary mass or inadequate support.
Use section depth, multiple ribs or curvature before making the wall solid. Review fatigue and creep for sustained loads. Reinforced materials may add stiffness but change weld-line strength, orientation and surface appearance.
Design Rib Roots and Intersections Deliberately
Rib thickness at the root should avoid a heavy intersection with the parent wall while remaining fillable and strong. Taper and draft support release; a root radius reduces stress but also adds local mass, so the outside cosmetic surface must be considered. Closely spaced ribs can behave like one thick block thermally.
End ribs gradually and keep them from creating sharp flow stops. Evaluate how the melt reaches and vents the rib. A rib at end of fill can short or burn even when its nominal thickness looks acceptable.
Support Bosses Without Building Solid Columns
A screw boss needs bore control, hoop strength, base support and room for the selected fastening process. Core the boss and connect it to walls or ribs through a load path rather than a large solid pad. Keep boss location and screw axis consistent with assembly datums.
Review weld lines around the boss, insert or screw torque, chemical exposure and long-term preload. A boss can pass initial assembly and crack later from molded stress plus threadlocker or service heat. Validate the actual hardware and installation process.
Engineering Scope
| Feature | Design purpose | Molding risk to control |
|---|---|---|
| Rib | Increase section depth and stiffness | Sink at root, poor fill, drag and fiber orientation |
| Boss | Locate or fasten an assembly | Thick base, cracking, weld line and ejector load |
| Cored pad | Preserve envelope with less mass | Thin shutoff, cooling access and local weakness |
| Wall transition | Connect functional sections | Hesitation, flow mark, stress and differential shrinkage |
Illustrative Boss Redesign
Illustrative engineering example—not a claimed CKMOLD customer result: A screw boss sits on a broad solid pad beneath a gloss surface. Samples show a circular sink and delayed cracking after assembly. The redesign cores the pad, adds two gussets aligned with the load, increases the internal root radius without thickening the outer skin and defines torque plus heat-aging tests for approval.
Use Coring and Transitions to Manage Thermal Mass
Core thick handles, feet, pads and attachment blocks while preserving support. Transition between sections over practical distance without creating a knife-edge or hard flow restriction. If the product envelope prevents ideal coring, consider an insert, separate component or accepted cosmetic boundary.
Coordinate core geometry with mold steel strength, cooling channels, venting and ejection. A theoretically uniform plastic section can leave fragile steel or an inaccessible hot spot. DFM must balance both sides of the cavity.
Predict and Approve the Surface Consequence
Map ribs and bosses behind visible surfaces. Higher-gloss materials and uniform colors often reveal sink more clearly; texture can change visibility but does not remove the dimensional effect. Define viewing light, distance and allowable boundary samples.
Gate and pack access influence sink, but excessive holding pressure can add stress or flash. Use part weight and gate-freeze evidence to understand whether process can reasonably help. Geometry and cooling should carry the main prevention burden.
Validate Features as an Assembly System
At trial, measure wall and feature dimensions, surface sink, boss position, ejection marks and flatness by cavity. Test screws, inserts, snaps or mating parts with controlled installation. Condition parts for relevant temperature, humidity or chemicals before judging delayed failure.
If a correction changes root thickness, rib height or boss support, update CAD and drawings. Keep steel-safe options for critical bores and fits. Production controls should monitor the process variables linked to the verified failure mechanism.
Engineering Record Checklist
- Identify the load, assembly method and nominal wall for each feature.
- Size rib root, height, spacing, draft and radius as a connected section.
- Core bosses and connect them to a deliberate load path.
- Review screw, insert, weld-line, chemical and creep risks.
- Use gradual transitions without creating fragile mold steel.
- Map features behind cosmetic surfaces and define sink acceptance.
- Check gate access, venting, cooling, ejection and tool maintenance.
- Validate dimensions and assembly after representative conditioning.
Related Engineering Resources
Engineering Questions
Why do ribs cause sink marks?
The rib and parent wall form a thicker thermal intersection that cools and shrinks differently from the surrounding skin.
Should a boss be attached directly to a wall?
It should be supported through a stable load path, often using ribs or gussets, without creating an excessive solid mass.
Can texture hide sink?
It may change visibility, but it does not remove dimensional depression or differential shrinkage. Acceptance still needs evidence.
Why can a boss crack days after assembly?
Molded stress, hoop strain, excessive torque, creep, heat and chemicals can combine into delayed environmental stress cracking.
Apply the Wall-Thickness Transitions review to your released design. Share the function, exact resin, annual volume, CTQs, cosmetic limits and launch timing through the CKMOLD project form. If CAD is relevant, send it directly to jerry@ckmold.com; the form does not require an upload.