Sink Marks in Injection Molding: Causes and How to Prevent Them

A sink mark is a frequent cause for cosmetic rejection, appearing as a shallow depression on an otherwise flat surface. The defect manifests on the exterior, but its root cause is almost always internal geometry—specifically, a thick section, a rib, or a boss. Understanding the mechanical relationship between internal mass and external shrinkage is critical for preventing sink during the design phase. This guide is part of the injection molding defects section.

What a Sink Mark Is

A sink mark is a shallow depression on the surface of a molded part, typically located opposite a thick section or behind a rib or boss. It forms during the cooling phase. The outer skin of the part solidifies first against the mold wall. As the thicker internal material continues to cool and shrink, it pulls the still-pliable surface inward. The defect is not a lack of material, but a localized collapse driven by uneven cooling rates.

Consequently, sink marks frequently appear on highly visible A-surfaces while the offending geometry resides on the hidden B-surface. A glossy finish highlights sink marks, whereas a textured finish can mask minor depressions, making mold surface finish an integral part of sink mitigation.

Where Sink Comes From

Like most defects, sink can originate in more than one place. For a buyer, the useful split is between causes you influence through design and causes the supplier manages through process:

Source	What’s going on	Who primarily addresses it
Thick sections	Heavy walls cool slowly and shrink inward	Design (uniform walls)
Ribs and bosses too thick	A rib thicker than 40–60% of the wall it joins sinks opposite itself	Design (proportioning)
Insufficient packing	Not enough hold pressure/time to compensate for shrink	Process (supplier)
Low mold or high melt temperature	Cooling imbalance worsens shrink	Process / temperature (supplier)
Gate location and size	Far or undersized gates can’t pack thick areas well	Tool / design (gate planning)

The pattern worth remembering: local geometry is an important sink-risk input, and process changes may have limited room when a section is heavy or difficult to pack. Gate seal, packing response, material shrinkage, cooling, and tool conditions should still be reviewed before geometry is declared the root cause.

The one ratio that explains most sink marks

Rib proportioning is where the majority of avoidable sink originates, and there is a published number for it rather than a feel. Protolabs’ wall thickness guidance puts ribs and bosses at 40 to 60 percent of the wall they attach to.

Draw a rib at full wall thickness and you have created a section roughly twice as thick as its surroundings at the junction. It cools last, shrinks most, and pulls the show face in behind it. This is not a marginal effect that careful processing manages away — it is geometry, and it happens in every resin.

The corollary is worth stating because it catches people the other way: a rib thinned to well under 40 percent may stop sinking and start failing to fill, or become too weak to do the job it was added for. The range exists because both ends have a failure mode.

Why the same design sinks differently in a different resin

Sink is shrinkage made visible, so the resin’s shrinkage behaviour sets how much geometry you can get away with.

The clearest published case is filled versus unfilled. SABIC puts glass-fibre-reinforced compounds at one-third to one-half the shrinkage of the unreinforced resin. So a heavy section that sinks visibly in unfilled nylon may not in 30 percent glass — which sounds like a fix and is a trap, because the same guidance notes that reinforced crystalline resins shrink anisotropically, differently along the flow direction than across it. You can trade a sink mark for a warp.

Wall thickness ranges move too. Protolabs’ published figures run from 0.030–0.115 in for nylon to 0.080–0.750 in for polyurethane. A section that is routine in one resin is a heavy section in another, which is why a material substitution on an existing tool deserves a sink review rather than a purchase order.

How Design Prevents Sink

Most sink problems are cheaper to prevent on the model than to chase on the press:

• Keep walls uniform. Even wall thickness cools evenly and is the single biggest defense against sink. Abrupt thick areas are the usual culprits, the same principle covered in the DFM guide.
• Proportion ribs and bosses to the wall. A rib or boss that’s too thick relative to the wall it joins will sink opposite itself. Keeping these features appropriately thinner than the main wall is the standard fix; the exact ratio depends on the resin, so confirm it with your supplier.
• Core out thick sections. Where a heavy section exists for strength, hollowing it (coring) and adding ribs often gives the same stiffness without the thick mass that sinks.
• Consider gate placement. Packing out a thick area is easier when the gate can feed it well, which ties sink to gate design.

What the Supplier Controls

A capable molder can reduce sink through process, raising and holding pack pressure longer, tuning mold and melt temperatures, and adjusting cooling time. If sink appears at first samples, this is the first conversation. But there’s a limit: if the geometry is the problem, no amount of process tuning fully removes the mark, and the honest answer is a design change. A supplier who explains that trade-off clearly is giving you good information, not making excuses.

What a Buyer Should Do

• Flag your cosmetic faces so sink on a show surface is treated as a reject, not shipped as acceptable.
• Review thick features before tooling, ribs, bosses, and heavy walls are the candidates.
• Ask at sampling whether any sink is process-related (tunable) or geometry-related (needs a design change), so you know which lever applies.
• Decide finish with cosmetics in mind, since texture can mask minor sink that gloss would reveal.

Buyer FAQs

What causes sink marks in injection molding?

Sink marks form when local shrinkage is not adequately compensated before the surface becomes rigid. Heavy walls and thick rib or boss junctions are common risk patterns, while gate location and seal, packing response, material shrinkage, and cooling also affect the result. Ask the supplier to verify which combination limits the part before approving a geometry or process correction.

How do you prevent sink marks?

The most reliable prevention is design: keep walls uniform, proportion ribs and bosses thinner than the main wall, and core out heavy sections rather than leaving thick mass. A capable supplier can also reduce sink by increasing and holding packing pressure and tuning temperatures. Prevention in design is more dependable than correction on the press, because process can’t fully remove sink caused by geometry.

Can sink marks be fixed without changing the part?

Sometimes. If the sink is driven by process, too little packing or a cooling imbalance, a molder can often reduce or eliminate it by adjusting settings. If it’s driven by a thick section or an oversized rib or boss, process tuning has limits and a design change is usually needed. Asking your supplier which case you’re in tells you whether it’s a settings fix or a tooling/part revision.

Are sink marks always a defect?

Not necessarily. It depends on where they are and what you’ve specified. A faint sink on a hidden internal boss usually doesn’t matter; the same mark on a visible A-surface is typically a reject. This is why defining cosmetic surfaces in your RFQ matters: it sets the standard against which sink is judged, so you and the supplier aren’t negotiating it part by part.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.