Sink-mark prevention should be planned before cosmetic samples or production lots make a local depression a commercial dispute. For overseas OEM, procurement, quality and engineering teams, the useful question is not whether a generic setting can eliminate every mark. It is whether the supplier has identified the local mass, pressure-transfer, cooling and measurement conditions that can create a visible depression on this part, in this resin and tool. A buyer-ready plan converts that question into design inputs, trial evidence and a part-specific acceptance record.
A sink mark is an observable surface depression; it is not, by itself, proof of one cause or proof of an internal void. Confirm the condition and its location before selecting a process or tool action.
01
Start with a controlled definition of the symptom
Describe the symptom as an observation, not as a diagnosis. Record the affected face, drawing coordinate, cavity identifier where applicable, resin grade and colour, sample age, image orientation and viewing conditions. A shallow depression may be obvious on a glossy cover and difficult to see on a textured or dark surface. It may appear opposite a rib, boss, insert, transition or locally thick section. A surface depression is different from a suspected internal cavity, and the inspection route and functional consequence may therefore differ.
Before tooling release, define acceptance by zone and function. Identify show surfaces, concealed faces, sealing or load-bearing regions, permitted gate or ejector areas, the approved appearance comparator, lighting and viewing distance, and the decision owner. If a numerical depth limit is needed, agree the instrument, measurement direction, sampling plan and repeatability study first. A universal depth limit or a photograph taken under uncontrolled lighting is not a reliable acceptance criterion for every resin, texture or geometry.
02
Separate likely causes from the evidence that can confirm them
Local shrinkage is a common working hypothesis when a mark is opposite added mass. A rib root, boss base, screw feature, corner intersection or abrupt wall transition can retain a hotter melt core while adjacent material and the surface cool. Resin type, filler content, pigment system, local wall ratio, flow direction and post-mold conditioning can alter the response. The same visible mark can also be influenced by insufficient packing, an early gate freeze, a restrictive runner or gate, non-uniform cooling, a cavity-specific condition, or a machine-control issue. These are hypotheses to test, not interchangeable conclusions.
Ask the supplier for a local section map rather than a nominal-wall statement. The map should show the basic wall, local maximum thickness, ribs, bosses, radii, cored regions, gate position, flow length and cooling circuits relevant to the area. Then compare cavity-coded parts from a stable baseline. Useful confirmation checks can include cavity pressure where available, controlled part-weight response through the hold phase, actual melt and mold-temperature records, cooling-circuit performance, gate-freeze evidence, and a review of cushion and non-return-valve function. Evidence should be tied to the exact material grade, mould revision and machine used for the trial.
03
Prevent the geometry-driven risk during DFM
Use the DFM review to remove unnecessary local mass before steel is committed. Ask whether a boss can be cored, a rib root can be thinned or radiused, a solid pad can be relieved, or a stiffness requirement can be met with a gusset or a smoother transition. The design choice must still preserve assembly loads, fastening, sealing, appearance, tool strength, draft and ejection. Resin-maker rib recommendations are useful starting points, but they are not universal pass/fail values; the appropriate ratio depends on the selected resin, base wall, visible-face requirement and local feature geometry.
The confirmation check is a section-based comparison of options, not an assumption that every thick feature must be removed. Request annotated CAD sections and, where the program warrants it, a documented flow-and-cooling assessment that states its material data, boundary conditions and limitations. Compare any proposed geometry change against the critical dimensions, fit, structural need and cosmetic face. The corrective action is the option that reduces the relevant mass or improves pressure and heat transfer without creating an unreviewed risk elsewhere.
04
Validate packing, gate and cooling controls without prescribing a universal setting
Packing is effective only while the gate allows material to compensate for contraction. A hold-pressure or hold-time change that improves one region may also create flash, moulded-in stress, ejection difficulty, dimensional movement or an appearance change elsewhere. Cooling behavior also depends on tool-circuit layout, heat transfer, local steel conditions, resin crystallinity and ejection temperature. Machine size, shot-to-barrel relationship, actual melt temperature and control response can change the result. For these reasons, a buyer should request a controlled study rather than instruct a supplier to use a fixed pressure, time or temperature.
A practical trial starts with a documented stable baseline. Hold the material grade, colour, drying method, regrind policy, tool and inspection conditions constant where possible. Change one defined variable or one planned tool feature at a time, then compare cavity-coded samples, visual results and the agreed functional or dimensional checks. Part weight or cavity-pressure response may help identify when additional hold time no longer changes the part, but the method and conclusion should be recorded. If process evidence resolves the symptom within the approved operating window and without adverse effects, a steel change may not be justified. If it does not, use the evidence to prioritize a gate, runner, cooling or geometry review.
05
Make supplier evidence and acceptance criteria part of release control
Ask the supplier to provide a prevention dossier with the approved CAD and tool revision, exact resin grade, colour or additive direction, material-handling record, cavity map, process baseline, controlled trial plan, sample traceability and inspection results. For visible areas, retain approved sample images under the agreed viewing conditions and link them to physical boundary samples where the program needs them. For a suspected internal condition, define whether sectioning, CT, another validated non-destructive method, or functional testing is appropriate before release. Do not call an internal indication acceptable or unacceptable without its location, size, service condition and measurement basis.
The release record should state the acceptance criterion, sampling level, inspection frequency, reaction plan and change triggers. Useful triggers include a resin-grade or colourant change, regrind-ratio change, gate or cooling modification, machine transfer, cavity repair, a persistent cavity-specific trend or a process setting outside the validated window. If a mark reappears, first contain and map the symptom, then review the evidence against those triggers. Corrective action should address the demonstrated mechanism and include post-action samples under the same acceptance method; it should not rely on a generic promise that sink marks will never occur.
Practical takeaway
Before you move
to the next review.
- Define cosmetic, concealed, sealing and load-bearing zones; agree viewing conditions, comparator samples and ownership of the appearance decision.
- Request an annotated local section map covering wall changes, ribs, bosses, radii, cored features, gate position, flow path and relevant cooling circuits.
- Confirm the proposed resin producer and exact grade, colour or additive direction, drying approach and approved regrind policy before interpreting trial results.
- Require a controlled baseline and one-variable-at-a-time study with cavity-coded samples, process records and the agreed visual, dimensional or functional checks.
- Freeze sink-mark acceptance criteria, sample traceability, reaction plan and re-approval triggers for material, tool, machine and process changes.
FAQ
Common questions
What should a buyer put on the drawing for a sink-mark-sensitive surface?
Mark the visible zone and identify the appearance reference, allowed feature locations, viewing conditions and approval owner. Add a numerical surface-depression requirement only when the measurement method, direction, repeatability and sampling basis are agreed. A broad note such as “no sink marks” can be ambiguous without those controls.
Does a sink mark always mean the supplier used too little hold pressure?
No. Insufficient packing is one possible contributor, but local mass, gate freeze, gate or runner restriction, cooling variation, resin behavior and machine response can also matter. Request a controlled baseline and confirmation checks before assigning a cause or prescribing a setting.
When should an OEM approve a tool modification to prevent sink marks?
Approve it after the supplier has shown why a process-only adjustment within the proposed operating window is inadequate or creates unacceptable trade-offs, and after the geometry, gate or cooling change has been reviewed for fit, strength, appearance, tool maintenance and validation impact. Require a documented post-change sample comparison.
Can one approved sample set cover all cavities and future resin lots?
Not necessarily. Multi-cavity tools can show cavity-to-cavity differences, and material or colour changes may alter shrinkage and appearance. Define cavity identification, the approved material specification, lot traceability, sampling plan and re-approval triggers so the evidence remains meaningful in production.
References
Source material
- [1] BASF, Injection-Molding Problems in Engineering Thermoplastics: Causes and Solutions
- [2] Covestro, The Fundamentals of Shrinkage in Thermoplastics
- [3] Celanese, Celcon Processing and Troubleshooting Guide
- [4] SABIC, NORYL Resin Injection Molding Processing Guide
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.