Can one cracked molded part show whether the cause was the design or the molding process?
One cracked molded part rarely shows whether the cause was the design or the molding process, because both explanations produce cracked parts that look similar and both are supportable from a single failed example. When molded parts start cracking in the field, two explanations compete from the outset: either the part geometry concentrates stress in a way that makes failure inevitable in every part the tool produces, or the molding process drifted and some parts came out weaker than others.
What settles whether cracking in molded parts is a design defect or a molding process defect is how the failures distribute across the population of parts that was made, and that analysis requires data most investigations do not ask for early enough.
What different failure patterns do a design defect and a molding process defect predict?
A design defect and a molding process defect predict different failure patterns: a geometric design defect is present in every part from the tool, so failures should appear across the entire production history at a rate set by service conditions rather than by manufacturing date, while a molding process defect is variable, so failures should cluster — by date range, by shift, by material lot, or by cavity in a multi-cavity tool.
These two predictions, spread across the whole production history for a design defect and clustered for a molding process defect, are testable, and testing them is considerably more informative than examining one more broken part.
Why is the cavity number the fastest way to tell a design defect from a molding defect?
The cavity number is the fastest discriminator between a design defect and a molding process defect, and it costs nothing but reading the parts: multi-cavity tools usually mark parts with a cavity number, and that single data point often redirects an entire investigation.
If failures concentrate in one or two cavities of a multi-cavity tool while the other cavities are unaffected, the problem is that cavity — its cooling, its venting, its dimensions — and not the design, which is common to all of them. If failures distribute evenly across every cavity of a multi-cavity tool, a cavity-specific process explanation is substantially weakened.
How do date codes show whether a molded-part defect started at a point in time or was always there?
Where molded parts carry date codes, plotting failures against production date, and against the number of parts produced in each period, distinguishes a defect that started at a point in time from one that was always present.
In a plot of molded-part failures against production date, a step change coinciding with a documented material change, a tooling repair or a process adjustment is strong evidence of a defect that started at that point. A flat distribution of failures across years of production points the other way, toward a defect that was always present.
Why do production volumes matter as much as failure counts?
Production volumes matter as much as failure counts because a cluster of failed molded parts means nothing without knowing how many parts were made in the same group. Failures clustering in a date range mean nothing if far more parts were made in that range, and clustering in a cavity means nothing without knowing that cavity’s share of output.
Analyzing how failures distribute across a production population therefore requires production volumes alongside failure counts. Investigations of cracked molded parts that work only from returned parts routinely misread ordinary volume variation as a defect signal, which is one of the more common analytical errors in this kind of investigation.
Can service conditions make molded-part failures look like a manufacturing problem?
Yes: service conditions can make failures of molded parts cluster in a way that reflects exposure rather than manufacturing, because parts do not all experience the same conditions. If the units that failed were disproportionately in hot climates, in heavy duty cycles or in a particular installation configuration, the clustering may reflect exposure rather than manufacturing.
Separating manufacturing variables from service variables requires both to be recorded, and it is why a failure analysis that considers only the molded parts and not their deployment can reach a confident wrong answer.
What should be checked in the part geometry, whatever the failure distribution shows?
Regardless of how the failures distribute, the geometry of a molded part should be assessed on its own terms: internal radii at load-carrying transitions, rib-to-wall thickness ratios that produce sink and voids, bosses sized for the fastener load, and whether a weld line falls anywhere it carries stress.
A part design that concentrates stress at the observed initiation site is relevant even if a molding process factor triggered the failures, because the design establishes how little margin existed.
Which molding process records should be reviewed for the production periods under investigation?
For the production periods in question, the molding process record to review covers melt and mold temperatures, injection and packing pressures, cycle time, regrind percentage and its source, resin lot numbers with certificates of analysis, drying conditions, and any recorded deviations or tooling maintenance.
Regrind and drying deserve particular attention in the molding process record, because both degrade material in ways that do not appear on the incoming resin certificate but do appear in the finished part.
What does testing failed and unused molded parts add to the failure distribution analysis?
Comparative testing on failed parts, unused parts from the same lot and unused parts from unaffected lots turns the analysis of how failures distribute across a production population into a mechanism.
Thermal analysis and molecular weight measurement identify processing-induced degradation in molded parts. Impact and tensile testing quantify how much strength differs between the populations of parts. Where the molded parts that failed measure materially weaker than parts that did not, the molding process explanation has direct support rather than inference.
What evidence should be secured early when molded parts start cracking?
When molded parts start cracking, the evidence to secure early is the failed parts with cavity and date codes legible and recorded; unused parts spanning multiple lots and cavities, including from periods with no failures; production volumes by period; the full process and material record, before retention schedules expire; and the field data — where each failed unit was deployed and how it was used.
The field data matters because without it the analysis of how failures distribute across the production population cannot control for service exposure.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.