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

A crack on a molded part is never just a cosmetic issue. Whether it appears immediately at T1, during assembly, or weeks later in the field, a crack around a hole, boss, or clip indicates a structural failure. The review objective is to understand the physical mechanism behind the failure and verify that the proposed fix actually works.

Identifying the Pattern

Cracks typically manifest in load-bearing areas: hairline fractures radiating from screw bosses after torque is applied; splits tracing a visible line across a hole; clips breaking at the root during first use; or parts failing spontaneously days after molding due to residual stress.

Because cracking often stems from systemic tooling or process flaws—like a poorly placed weld line or internal voids—a part that cracks during sampling suggests a high likelihood of field failures at production volumes.

Mechanisms to Verify

Several distinct mechanisms cause cracking at loaded features. The supplier must verify the specific cause rather than guessing.

Weld Lines at Loaded Features: When flow fronts split around a hole or boss and rejoin, they create a weld line. This seam is significantly weaker than the bulk material. If a crack tracks along a faint visible line, a weld line is the likely culprit. Relocating the gate moves the weld line; see weld lines.

Internal Voids in Bosses: A screw boss might look perfect on the outside but contain a hollow void inside. When torque is applied during assembly, the weakened wall collapses and cracks.

Molded-In Stress: High injection pressures, cold molds, or sharp internal corners leave residual stresses in the part. This often causes delayed cracking—parts break later without any apparent external load.

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

Material Degradation: Processing wet resin, using degraded material, or exceeding the allowed regrind ratio fundamentally weakens the polymer. The part will crack under loads the virgin resin easily survives. This mechanism overlaps with brittleness.

Chemical Exposure: Certain resins stress-crack when exposed to specific cleaners, lubricants, or adhesives used during assembly or end-use.

Questions for the Molder

When investigating a crack, ask the supplier specific technical questions:

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

Evidence to Request

Do not accept a generic “process adjusted” response. Require hard evidence:

• 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, highlighting drying times and regrind ratios.
• Comparative torque-to-failure or flex testing data showing the cracked lot versus the corrected lot.
• A written root-cause analysis and corrective action plan.

When to Withhold Approval

Delay tool or production approval if:

• A structural or safety-critical part cracks and the supplier cannot provide a proven root cause.
• The crack follows a weld line, but the supplier has not proposed a gate or geometry change to move it.
• The supplier claims the issue is fixed but cannot provide comparative test data (e.g., torque or flex tests) proving the new parts are stronger.
• The cracking is intermittent. Intermittent cracking during qualification usually scales into a high defect rate during production.

Securing Performance in the RFQ

Prevent cracking early by providing thorough information in the RFQ. Identify all loaded features—such as boss torque specifications and clip flex cycles—so bidders understand the mechanical duty, not just the shape. Ask bidders where they expect weld lines to form and how they plan to keep them away from loaded features; this is a critical gate design discussion. Finally, list all assembly chemicals and processes to catch compatibility issues before material selection is locked.

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.