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What should a buyer agree before accepting sink marks on an injection molded part?
Identify the affected surface and approved appearance requirement, then agree inspection conditions, reference samples and any measurement method needed. Link the observations to the actual part, cavity, resin and process revision. Review the proposed correction and its retest evidence separately from dimensional and functional approval. Do not use an unexplained photograph, a universal sink-depth limit or a supplier's verbal assurance as the complete release basis.
Decision checklist
- Mark the affected visible zones on the current drawing or a controlled appearance map.
- Agree sample presentation, lighting, viewing method and the approving person.
- Record cavity, resin, tool and process identity for the rejected and replacement samples.
- Evaluate the impact of each proposed design, tooling, material or process change.
- Keep appearance approval, drawing compliance and functional acceptance as separate decisions.
Define the customer problem before choosing a remedy
Imagine an equipment housing with a shallow depression opposite an internal mounting boss. The supplier calls it normal molding appearance, while the buyer expects a smooth visible face. Before arguing about a machine setting, establish which surface is controlled and what the purchase agreement actually requires. This is an illustrative purchasing example, not a GEERPOWER customer result.
Autodesk describes sink marks as surface depressions associated with localized shrinkage at thick sections. Protolabs illustrates how a substantial boss behind a cosmetic surface can contribute to sink and why design, material and processing tradeoffs may need review. These references support a structured investigation; they do not establish the cause of an unidentified sample.
This guide is a buyer acceptance framework for molded components and export-mold trials. It is not a machine operating procedure and supplies no universal wall ratio, process setting or allowable sink depth. A qualified engineering team must assess the actual part and define any changes.
Write an appearance agreement that both teams can use
Use this original worksheet before approving trial samples. It organizes the purchasing decision rather than prescribing a testing standard. A concealed mounting surface and a front-facing display surround need not share the same appearance requirement, but the distinction must be documented instead of decided after shipment.
Where appearance is critical, retain identified boundary or reference samples if the parties agree they are suitable. State how samples are stored, replaced and linked to revisions. A reference that has no identity or approval record can create another dispute rather than resolve the original one.
| Agreement field | Record before approval | Hold point |
|---|---|---|
| Controlled surface | Drawing revision, visible-zone map and feature location | Do not approve an unspecified surface. |
| Viewing conditions | Agreed lighting, orientation, distance and review method | Reconcile inspection conditions before comparing judgments. |
| Acceptance reference | Approved sample ID or documented appearance requirement | Do not substitute a compressed photograph for the agreement. |
| Measurement, if required | Method, support, reference surface and product-specific limit | A depth value without its method is incomplete evidence. |
| Sample identity | Cavity, material lot, tool and process revision | Do not mix before-and-after samples. |
| Release authority | Approving person, concessions and remaining checks | Keep unresolved items visible in the handover. |
Keep the baseline sample and the proposed correction linked
Ask for a marked defect map, photographs under the agreed conditions and the relevant trial record. Identify whether the observation is repeated across cavities, concentrated at a particular feature or associated with a recorded material or process change. Record what is known and what is still a hypothesis.
Require the supplier's correction proposal to state what it will change, why that change is proposed and what evidence will be submitted afterward. A revised process sheet without identified replacement samples does not demonstrate that the disputed appearance has been resolved. Conversely, a selected good-looking sample without its trial basis may not explain the proposed production route.
Preserve the baseline even after an improvement. Keeping both submissions makes it possible to review what changed, identify unintended effects and explain the release decision later. Do not describe a proposed correction as successful before the retest record exists.
Review tradeoffs rather than approving a cosmetic fix in isolation
A design change, gate revision, alternative material or changed process basis should be reviewed against the component's other requirements. Ask the responsible engineer which dimensions, interfaces, appearance zones and functional checks need to be repeated. This article does not rank those remedies for an unspecified part.
Protolabs' cosmetic examples show that a solution to one appearance problem can introduce another design or processing tradeoff. The buyer's practical response is to request an impact list and a defined retest scope, rather than assume that the cleanest photograph is the best complete solution.
Keep approval for appearance separate from fit, assembly and required performance. An acceptable visible surface does not prove an internal condition or service requirement. If the application calls for additional inspection, the product owner should identify it explicitly; do not infer an internal defect or its absence from surface appearance alone.
Turn the review into a usable quotation and handover scope
When comparing quotations, list the appearance review, sample preparation, agreed measurement, correction work and repeat trial responsibilities separately. Identify which party supplies the approved reference and who can authorize a concession. Do not leave the cost of an unagreed design change hidden inside a general promise of good quality.
For mold shipment or production release, keep the signed requirement, sample IDs, correction record, retest results and open-item list together. State whether the approval covers development samples, a defined trial or an agreed production submission. The purchase order should not imply a broader release than the evidence supports.
Send Geerpower Holdings Limited the drawing, marked appearance zones, resin, baseline samples or photographs, current trial information and your unresolved acceptance question. Actual cause, correction results, testing availability, approved limits, tooling scope, price and delivery remain NEEDS_EVIDENCE until confirmed for the project. No measured customer outcome or owned laboratory is claimed here.
Common buyer questions
Is there one acceptable sink depth for all molded parts?
No universal limit is set here. The product requirement, controlled surface and agreed inspection or measurement method must define acceptance.
Can a photo prove a sink mark is acceptable?
A photograph can identify location and support comparison, but it should not replace agreed viewing conditions, reference samples or required measurements.
Does removing visible sink prove the part is fully approved?
No. Review the drawing, assembly and functional requirements independently and repeat any checks affected by the correction.
What should we send for a review?
Send the drawing revision, marked surface, resin, cavity identity where known, trial record and the exact appearance requirement or unresolved decision.
Sources and references used for this guide
This guide supports early project planning. Final machine selection and commercial configuration must be confirmed from the actual part, mold, material, factory utilities and production target.
