Cracking Around Holes, Bosses and Clips: Questions Before Tool Approval

A crack on a molded part is never only cosmetic. Whether it shows at T1, during assembly, or weeks later in the field, a crack around a hole, boss, or clip is a structural failure. Name the mechanism, then verify that the proposed fix actually works.

Identifying the Pattern

Cracks usually show in load-bearing areas: hairline fractures radiating from screw bosses after torque; splits tracing a visible line across a hole; clips breaking at the root on first use; or parts failing days after molding from residual stress.

Because cracking often comes from tooling or process flaws (a poorly placed weld line, internal voids), a part that cracks in sampling is a warning that field failures are likely at production volume.

Mechanisms to Verify

Several mechanisms crack loaded features. The supplier should verify the cause, not guess.

Weld lines at loaded features: Flow fronts that split around a hole or boss and rejoin leave a weld line. That seam is weaker than bulk material. If a crack tracks a faint visible line, a weld line is likely. Relocating the gate moves the weld line; see weld lines.

Internal voids in bosses: A screw boss can look perfect outside and hold a hollow void inside. Assembly torque collapses the wall and cracks it.

Molded-in stress: High injection pressure, cold molds, or sharp internal corners leave residual stress. Delayed cracking is common: parts break later with no obvious external load.

Geometry failures: Thick rib roots, missing radii, or sharp transitions concentrate stress. Common at snap fits and boss bases; see ribs & bosses.

Material degradation: Wet resin, degraded material, or excess regrind weakens the polymer. The part cracks under loads virgin resin would survive. Overlaps with brittleness.

Chemical exposure: Some resins stress-crack when exposed to cleaners, lubricants, or adhesives used in assembly or end use.

Questions for the Molder

Ask specific technical questions:

• Does the crack align with a weld line? Do flow simulations or short-shot studies confirm a weld line there?
• Have you sectioned the boss to check for internal voids?
• What drying parameters, melt temperature, and regrind ratio were used for this lot?
• Have assembly conditions (torque specs, chemical exposure, insert heating) been replicated in testing?
• What exact variable (gate location, geometry, process) will change, and how will improvement be verified?

Evidence to Request

Do not accept a generic “process adjusted” reply. Require:

• Annotated photos showing crack initiation points and paths.
• Sectioned samples of the cracked features, or comparative part-weight data to rule out voids.
• Full process logs for the affected lot, with drying times and regrind ratios highlighted.
• Comparative torque-to-failure or flex testing of the cracked lot versus the corrected lot.
• A written root-cause analysis and corrective action plan.

When to Withhold Approval

Hold tool or production approval if:

• A structural or safety-critical part cracks and the supplier cannot show a proven root cause.
• The crack follows a weld line and no gate or geometry change is proposed to move it.
• The supplier claims a fix but cannot provide comparative test data (torque or flex) proving stronger parts.
• Cracking is intermittent. Intermittent cracking in qualification usually becomes a high defect rate in production.

Securing Performance in the RFQ

Name loaded features in the RFQ: boss torque specs, clip flex cycles, and similar duty. Bidders need the mechanical job, not only the shape. Ask where they expect weld lines and how they will keep them off loaded features. That is a gate design discussion. List assembly chemicals and processes before material selection locks.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, perform failure analysis, or certify suppliers. Crack causes are specific to the part, tooling, and process. Confirm causes with your supplier and validate fixes with physical testing, especially for load-bearing or safety-critical components.