Last updated: 2026-09-05 · 5 min read

A sink mark is a shallow depression on a molded surface, and it is not a cosmetic accident — it is the surface reporting a volume problem underneath: plastic shrank somewhere thick while the skin had already frozen, and nothing was left to push material into the gap. The three levers that decide whether sink appears are geometry (local thickness and rib proportion), packing (how much extra material is pushed in after the cavity fills), and cooling (how fast the skin locks in place). Sink is largely predictable before steel is cut, which is why it belongs in the DFM conversation — not in the argument after T1 samples arrive.

Key facts

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  • Sink = local volumetric shrinkage pulling the frozen surface inward; it appears where material is locally thick.
  • Top trigger zones: opposite ribs, bosses and gussets; thick-to-thin transitions; corners and label recesses.
  • Rule of thumb: ribs and bosses should run about 50–60% of the nominal wall they attach to.
  • Process-side relief — packing pressure, hold time, gate freeze — can mask sink but cannot out-pump bad geometry.
  • Semi-crystalline resins (PP, PA, POM) sink more visibly than amorphous ones (ABS, PC, PS) at equal geometry.

    • Sink marks — Sink marks — local depressions on the part surface caused by uneven cooling. Occur at ribs, bosses and thick-to-thin transitions. Root cause: the core of a thick section shrinks more than the skin. Fix: reduce rib thickness to 50–60% of nominal wall, increase packing pressure, optimize cooling. The related guide covers adjacent topics.

    What is a sink mark, physically?

    Every thermoplastic shrinks as it cools — the cavity is deliberately oversized to compensate. Shrinkage is not uniform, though: a thick section stays molten longer than the surrounding wall, and while it finally cools and contracts, the surface skin around it has already solidified. If no additional melt is packed in to replace the shrinking volume, the still-soft surface gets pulled inward. The result is a smooth, shallow dent that follows the outline of the thick feature beneath it.

    Sink mark — a localized depression on a molded surface caused by volumetric shrinkage of a thicker underlying section after the surface skin has frozen. Its twin is the void: when the skin is too rigid to be pulled inward, the shrinkage tears open an internal bubble instead. Same cause, two failure modes.

    That void distinction matters for diagnosis: if you see surface dents, the skin was flexible enough to move, which means packing and geometry changes can still influence it; if X-ray or sectioning reveals internal voids in thick bosses, the material problem is the same but the surface will not warn you about it.

    Where do sink marks show up, and why there?

  • Opposite ribs. The classic case. A rib adds stiffness, but its intersection with the wall is locally thick; if the rib is nearly as thick as the wall, the outside face sinks directly above it.
  • Opposite bosses and screw towers. Mounting bosses concentrate material. Undersized coring of the boss leaves a fat column that sinks on the visible face.
  • Thick-to-thin transitions. Sudden steps in wall thickness freeze unevenly; the step edge telegraphs through as a read-out line or dent.
  • Corners and gussets. Inside corners accumulate material by geometry alone; gussets sized to the full wall thickness repeat the rib problem.
  • Engraved or recessed zones. Text, logos and label recesses locally thicken or thin the wall; shallow engraving over a thick zone often turns into a visible halo of sink.
  • Far from the gate. Packing pressure decays along the flow path, so distant thick zones receive less compensation and sink first.

    • How to predict sink before the mold is cut

    Sink is one of the most forecastable defects in molding, because its inputs are geometric and measurable before any steel moves. The prediction sequence:

    1. Map local thickness. A wall-thickness analysis on the CAD flags every zone where material stacks up — rib intersections, bosses, corners. Every flagged zone is a sink candidate and should be named in the DFM review.
    2. Check feature proportions. Ribs, bosses and gussets get measured against the nominal wall. Anything above roughly 60 percent of the wall thickness is a redesign candidate; below 50 percent, sink risk drops off sharply.
    3. Factor in the resin family. Semi-crystalline materials shrink more and crystallize unevenly, so the same geometry that looks acceptable in ABS may sink visibly in PP or POM. The resin choice and the sink review belong to the same conversation.
    4. Run the mold-fill simulation. Volumetric shrinkage simulation turns the geometric guess into a map: predicted sink locations, with magnitude. It will not replace a T1 sample, but it moves the argument from opinion to a documented prediction.
    5. Verify at T1 against the prediction. The trial should confirm or refute the simulation zone by zone. A factory that predicted sink locations and shows samples matching its own forecast is demonstrating process control, not luck.

    This is also why sink belongs in the tolerance discussion: a sunk surface is a dimensional deviation with a cosmetic face on it, and the acceptable depth depends on where the zone sits — see the tolerance reality check for what can be held where.

    Fixing sink at the machine: what process tuning can and cannot do

  • Raise packing pressure and extend hold time. More material pushed in after fill compensates more shrinkage — the standard first move. It works until the gate freezes; hold time beyond gate freeze adds nothing.
  • Confirm gate freeze with a study. Incrementally lengthen hold time and weigh the parts; when part weight stops rising, the gate has frozen. That number defines the real process window instead of a guess.
  • Lower melt and mold temperatures within range. Faster skin freeze-in reduces the time the surface can be pulled inward, at the cost of flow length and residual stress — a trade, not a free fix.
  • Balance cooling around the thick zone. A hot spot at the boss keeps material shrinking long after the rest of the part is stable; conformal or added cooling near the problem zone shortens that tail.
  • Know the ceiling. Process tuning typically buys you a fraction of a millimeter of relief. When geometry is fundamentally thick, no setting will pack enough material through a freezing gate — the fix must move into design.

    • Fixing sink in the design and the mold: the durable solutions

    The fixes that actually remove sink rather than hide it are geometric. They are ordered here from least to most expensive, which is also the order a competent DFM will propose them:

  • Core out the thick zone. Remove material from the backside of bosses and thick pads so local thickness approaches the nominal wall. This is the single most effective sink fix and usually costs nothing but a CAD change — if it happens before steel.
  • Resize ribs and bosses to the proportion rule. Rib base at 50–60 percent of wall thickness; bosses cored with an internal hole or reduced outer diameter. Stiffness is recovered with geometry (more ribs, thinner) rather than bulk.
  • Radii and gradual transitions. Replace sharp internal steps with generous radii and taper zones so thickness changes over distance instead of in a step.
  • Move or resize the gate. A gate that faces the thick zone directly gives packing pressure the shortest path to the problem. Late gate changes are steel changes — this is why gate location appears on the DFM agenda with a reason attached.
  • Texture as a mitigation, not a cure. A matte or textured finish hides mild sink by breaking the surface reflection that makes dents readable. It is legitimate camouflage for low-severity zones, and useless against deep sink — and the finish itself has its own draft demands from the surface finish chart.

    • The economics are blunt: every one of these is a design note before the deposit and a change order after steel cutting. If your current quotation came back without a word about rib proportions or thick zones, the sink argument is scheduled for T1 — just later and more expensive than it needed to be. Name the appearance surfaces in the RFQ and ask for a written sink-risk assessment per zone; the question itself filters factories.

    Sink is the most honest defect in molding — it always points at exactly the feature that caused it. When a buyer shows me a sunken boss, I don’t for process parameters first; I for the wall-thickness analysis. The projects that run calm are the ones where rib ratios were argued in the DFM, in writing, while changing them was still free. The stressful ones argued the same numbers at T1, with the samples on the table and the schedule already spent.

    — Arthur, Tooling Engineer at aCatMould

    Sink, void, warp: which defect are you actually looking at?

    Three defects trace back to the same family of shrinkage problems and get confused at the sample table, but each one points at a different fix — so the diagnosis changes the next move:

  • Sink mark. Surface dent over a thick zone; the skin moved inward because packing could not compensate. Fix direction: geometry proportioning and packing path.
  • Void. Internal bubble inside a thick section; the skin was too stiff to move, so the shrinkage tore a gap inside. Fix direction: coring the section, better packing pressure transmission — the surface may look fine while the structure is weakened.
  • Warp. Whole-part bending from uneven shrinkage across the part, not a local dent. Fix direction: cooling balance, gate location, uniform walls — a geometry-and-cooling problem, not a single-feature one.

    • The practical test is simple: run a straightedge along the surface. A local dent under the straightedge is sink; a globally bowed face is warp; and if the surface reads flat but the part fails at the boss, section it and look for voids before blaming the process settings. Naming the defect correctly is what keeps the fix conversation pointed at the right cause instead of cycling through machine parameters at random.

    Frequently asked questions

    Can sink marks be completely eliminated?

    On correctly proportioned geometry with adequate packing, visible sink on appearance surfaces can be effectively eliminated. On unavoidably thick sections, the goal shifts to reducing sink below the visibility threshold of the finish — texture, part color and viewing conditions all set that threshold.

    Why do my samples sink in one spot only?

    Because shrinkage is local. One thick zone — usually a rib intersection or boss — runs out of packing compensation before the rest of the part. A single consistent sink location is actually good news diagnostically: it points at one feature, not a systemic process problem.

    Does every resin sink the same way?

    No. Semi-crystalline resins like PP, PA and POM shrink more and more abruptly, so they sink deeper at equal geometry. Amorphous resins like ABS, PC and PS shrink less and more evenly, and are more forgiving of modest thickness errors.

    Is raising packing pressure always the first fix?

    It is the first trial at the machine, because it costs nothing to try — but it is bounded by gate freeze and by flash limits. If more pack causes flash elsewhere before the sink disappears, geometry is the remaining lever.

    Can texture hide a sink mark?

    Mild sink, yes — matte and textured finishes break the reflection that makes dents readable. Deep sink, no; the surface still deflects and reads as a distortion under raking light. Camouflage is a mitigation strategy, never a substitute for proportioning the geometry.

    When should sink risk be discussed with the mold maker?

    Before the deposit, as a named item in the DFM: which zones are sink candidates, what the simulation predicts, and which fix (coring, rib resize, gate move) applies to each. A written sink-risk assessment turns the risk from a surprise at T1 into an agreed design decision.

    Key takeaways

    1. Sink is local shrinkage pulling a frozen surface inward — a volume problem wearing a cosmetic mask.
    2. It concentrates opposite ribs, bosses and transitions: anywhere material is locally thick.
    3. The proportion rule does most of the prevention: ribs and bosses at roughly 50–60 percent of the nominal wall.
    4. Process tuning (pack, hold, gate freeze, cooling balance) buys limited relief; durable fixes are geometric — coring, resizing, gate placement.
    5. Sink is predictable before steel: thickness mapping, proportion checks, resin factor, simulation, then T1 verification — get it in the DFM, in writing.

    If your part has visible surfaces over ribs or bosses, send us the geometry and name those surfaces. You will get a written DFM that lists each sink-risk zone with the proposed fix and the simulation prediction — from a named engineer, before any money moves.

    Sources
    Autodesk Moldflow — injection molding simulation for sink mark prediction
    MatWeb — material property database for resin and steel datasheets
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · MoldMaking Technology