Independent buyer resource Evidence before approval No supplier network claims

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

Buyer guide Independent resource 5 min read Reviewed by the PlasticsTechnologyAlliance.com editorial team Updated June 2026

A crack is the one defect a buyer should never argue down to “cosmetic.” Whether it appears at T1, after assembly torque, or weeks later in a box, a crack around a hole, boss, or clip is a structural finding—and the review question is not “can we live with the look” but “do we understand why it cracked, and is the fix proven.” This page is the buyer-side review aid for that decision; the cracking around loaded features explainer walks the same review as an animated workspace.

What Buyers Usually See

Hairline cracks radiating from screw bosses after assembly; splits along a visible line crossing a hole; clips that crack at the root on first flex; cracks appearing at ejection around deep features; or parts that arrive intact and crack days later without obvious load.

Why This Matters Before Approval

Cracks concentrate where parts carry load, and several of the likely patterns—weld lines through holes, voids inside bosses, molded-in stress (including the hoop stress around a molded-in metal insert)—are systematic: every shot carries the same weakness. A waved-through crack at sampling is a recall pattern at volume. This is also a defect where supplier explanations deserve scrutiny, because the convenient answers (“operator over-torqued it”) and the systematic ones (weak weld line) look identical in a photo.

Possible Technical Patterns to Verify

  • Weld lines through loaded features. Flow fronts meeting on the far side of a hole or boss can create a seam markedly weaker than the bulk material—cracks tracking a faint line often point here. See weld lines.
  • Voids inside bosses. A boss with an internal void can snap under screw torque while looking perfect outside.
  • Molded-in stress. Aggressive filling, cold tooling, or sharp corners may leave residual stress that cracks later—including delayed cracking with no apparent load.
  • Boss/rib geometry. Thick roots, missing radii, or sharp transitions concentrate stress where torque arrives; see ribs & bosses.
  • Material condition. Degraded, wet-processed, or over-regrind material can crack at loads the virgin resin would survive—overlaps with brittleness.
  • Chemical exposure. Some resins stress-crack in contact with certain cleaners, oils, or adhesives used in assembly.

Questions to Ask the Injection Molder

  • Does the crack path follow a weld line? Where do your flow simulations or trial observations put the weld lines on this part?
  • Can you section a boss so we can check for internal voids?
  • What drying, regrind ratio, and melt temperature were these parts run with?
  • Has anything in assembly (torque, chemicals, insert heat) been replicated in your testing?
  • What specifically will change—gate location, geometry request, process—and how will we verify the change worked?

Documents or Evidence to Request

  • Annotated photos of crack initiation points and paths.
  • Sectioned samples of suspect bosses (or part-weight comparison data) for void checking.
  • The process record for the affected lot, including drying and regrind.
  • A torque-to-failure or flex test comparison: cracked-lot samples vs corrected samples.
  • Written cause assessment and corrective plan.

When to Delay Mold or Production Approval

  • Any crack on a structural or safety-relevant feature without a written cause assessment.
  • Crack path follows a suspected weld line and no gate/geometry remedy has been evaluated.
  • “Fixed” parts exist but no comparison evidence (test, section, weight) accompanies them.
  • Cracking is intermittent and unexplained—intermittent now usually means frequent later.

What to Include in the Next RFQ

  • Identify the loaded features (bosses with torque values, clips with flex cycles) so bidders see the duty, not just the geometry.
  • Ask where weld lines will land and how the supplier proposes to keep them off loaded features—a gate design question worth asking before steel.
  • State assembly chemicals and processes so chemical-compatibility issues surface at quoting.

Buyer-Side Checklist

  • Crack locations photographed and mapped
  • Weld-line involvement assessed
  • Boss sections or weight data reviewed for voids
  • Material handling (drying/regrind) records checked
  • Fix verified by test, not by appearance
  • Acceptance standard updated (cracks = reject, stated explicitly)

Run the full sample review with the T1 Sample Review Checklist; track recurrence in the Defect Log Template.

Evidence Box

This buyer review note was developed from recurring shop-floor troubleshooting patterns, injection molding process principles, and buyer-side mold trial review logic. Where specific technical claims affect supplier evaluation, material handling, tooling decisions, or production approval, they should be verified against the actual mold design, material grade, supplier processing guide, process sheet, and sample inspection data.

This page is a buyer-side review aid, not a final engineering diagnosis, supplier certification, or guaranteed fix.

Optional Technical Deep Dive

The tooling and design mechanics behind crack patterns are covered in gate design, weld lines, and ribs & bosses. Material-side embrittlement is covered in the brittleness review page.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, perform failure analysis, or certify suppliers. Crack causes are part- and process-specific—confirm with your supplier, and where load or safety is involved, with physical testing.

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