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

A sink mark is a shallow depression on an otherwise flat face, and it is a frequent cosmetic reject. The mark shows on the outside, but the cause is almost always internal geometry: a thick section, a rib, or a boss. If you understand how internal mass drives external shrink, you can stop most sink in design instead of arguing about it at T1. Related pages sit in the injection molding defects section.

What a Sink Mark Is

A sink mark sits opposite a thick section or behind a rib or boss. During cooling, the outer skin freezes first against the mold wall. Thicker internal material keeps cooling and shrinking, and it pulls the still-pliable surface inward. The defect is not missing plastic. It is localized collapse from uneven cooling.

Sink often lands on highly visible A-surfaces while the heavy geometry sits on the hidden B-surface. Gloss highlights sink; texture can hide minor depressions. That makes mold surface finish part of sink mitigation, not an afterthought.

Where Sink Comes From

Sink can start in more than one place. Split what you control in design from what the supplier manages in 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)

Local geometry is a major sink-risk input. Process changes have limited room when a section is heavy or hard to pack. Still review gate seal, packing response, material shrinkage, cooling, and tool conditions before you declare geometry the root cause.

The one ratio that explains most sink marks

Rib proportioning is where most avoidable sink starts, and there is a published number for it. 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 create 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. That is geometry. It happens in every resin. Process will not erase it.

The other end of the range matters too. A rib thinned well under 40 percent may stop sinking and start failing to fill, or become too weak for the job. Both ends have a failure mode.

Why the same design sinks differently in a different resin

Sink is shrinkage made visible, so resin shrinkage sets how much geometry you can tolerate.

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. A heavy section that sinks in unfilled nylon may not in 30 percent glass. That sounds like a fix and is a trap: the same guidance notes that reinforced crystalline resins shrink anisotropically, differently along flow 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 heavy in another. A material substitution on an existing tool deserves a sink review, not just a purchase order.

How Design Prevents Sink

Most sink is 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. Same principle in the DFM guide.
• Proportion ribs and bosses to the wall. A rib or boss that is too thick relative to the wall it joins will sink opposite itself. Keep these features appropriately thinner than the main wall; confirm the exact ratio with your supplier for the resin.
• 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 a thick area is easier when the gate can feed it well. That 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, start there. There is a limit. If geometry is the problem, process will not fully remove the mark, and the honest answer is a design change. A supplier who states that trade-off clearly is giving you usable information, not excuses.

What a Buyer Should Do

• Flag your cosmetic faces so sink on a show surface is a reject, not shipped as acceptable.
• Review thick features before tooling: ribs, bosses, and heavy walls.
• Ask at sampling whether any sink is process-related (tunable) or geometry-related (needs a design change).
• 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 forms when local shrinkage is not compensated before the surface goes rigid. Heavy walls and thick rib or boss junctions are common risk patterns. Gate location and seal, packing, material shrinkage, and cooling also matter. Require the supplier to name which combination limits the part before you approve a geometry or process correction.

How do you prevent sink marks?

Design first: uniform walls, ribs and bosses thinner than the main wall (typically 40–60% per Protolabs guidance), and cored heavy sections. Process can help with packing and temperatures, but it cannot fully erase geometry-driven sink. Put rib/boss ratios in the DFM notes before steel is cut.

Can sink marks be fixed without changing the part?

Sometimes. If packing or cooling is the driver, settings may reduce or remove it. If a thick section or oversized rib/boss is the driver, expect a design or tooling revision. Ask which case you are in before you authorize more trials without a CAD change.

Are sink marks always a defect?

It depends on location and your spec. A faint sink on a hidden internal boss often does not matter; the same mark on a visible A-surface is typically a reject. Define cosmetic surfaces in the RFQ so you and the supplier inspect to the same standard instead of negotiating 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.