Map Cause to Fix: Sink Mark Prevention Checklist for Engineers
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Map Cause to Fix: Sink Mark Prevention Checklist for Engineers

2026-09-22 01:33:44

Engineer inspecting a molded plastic sink mark

The most reliable fixes are uniform wall thickness, correctly tuned pack and hold pressure, targeted cooling at thick sections, and, when needed, a resin or tooling change. No single lever solves every case. Sink marks almost always come from a combination of geometry and process, so the fastest path to a clean surface is pairing a CAD review with process trials rather than chasing one variable in isolation. The sections below break out design rules, machine settings, material and tooling options, and a troubleshooting sequence you can run on the press.

TL;DR:

  • Most sink marks result from non-uniform wall thickness, early gate freezing, or cooling channel deficiencies, which are best addressed through design adjustments.
  • CAD modeling should accurately reflect actual rib and boss thicknesses, and simulation tools like Moldflow help identify sink-prone geometries early.
  • Process improvements, such as optimizing pack and hold pressure, gate timing, and temperature settings, can reduce sink marks without tooling modifications.
  • Material selections, including using less shrink-prone resins or filled grades, and advanced cooling techniques like conformal channels, significantly impact sink formation.
  • Validating fixes through short-run injection molding trials on prototype tools is crucial before investing in permanent production molds.
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Table of Contents

What Causes Sink Marks in Plastic Parts

Sink marks are shallow depressions on the surface of a molded part, and they form because thick sections cool slower than the thin walls around them. As the outer skin freezes first, the still-molten core keeps shrinking, and that shrinkage pulls the surface inward before it has a chance to solidify flat. Industry testing has traced this directly to differential cooling between thick and thin regions.

You’ll see sinks show up in predictable places:

  • Opposite ribs, bosses, and screw towers, where the added mass behind a thin wall traps heat
  • Near gates, where the last material to pack often cools unevenly
  • At sharp internal corners and thick fillets, where two walls intersect and create a local hot spot

A part can pass every functional test and still ship with a visible sink, because the defect is cosmetic and dimensional rather than structural. That’s exactly why it gets flagged in QA even when the part works fine.

Sink Mark Troubleshooting: Primary Causes to Check First

Before touching the tool, run through the usual suspects in order of likelihood.

  1. Non-uniform wall thickness. Ribs, bosses, and mounting features that carry more mass than the surrounding wall are the number one cause of sink marks in molded parts.
  2. Insufficient packing or early gate freeze. If the gate seals before the thick section finishes shrinking, no amount of hold pressure downstream can reach it.
  3. Cooling-channel shortcomings. Drilled channels that miss a thick boss or deep rib leave a thermal hot spot that never cools at the same rate as the rest of the part.
  4. Material shrinkage behavior and trapped moisture. Some resins simply shrink more than others, and moisture-generated gas can create voids that read as surface sink.

Most sink problems trace back to one or two of these, not all four at once. Isolating which one applies before adjusting settings saves a lot of trial shots.

Optimizing Mold Design to Prevent Sink Marks

Design fixes are the cheapest and most permanent way to prevent sink marks, because they remove the root cause instead of compensating for it downstream.

Keep wall thickness as close to uniform as the part’s function allows, and core out solid sections wherever geometry permits. For ribs and bosses, industry guidance calls for keeping rib thickness at roughly 40% to 60% of the adjoining wall, sometimes called the 60/40 rule for rib design. Adding a moat or relief channel around a boss also helps by giving the thick section somewhere to shrink without dragging the visible face down with it.

A few more design moves worth building into the CAD model early:

  • Replace thick structural walls with gussets and ribs instead of adding bulk
  • Minimize mass buildup at fillets and wall junctions, where two features intersect and double up on thickness
  • Route gates and runners so they feed thick sections directly, and consider multiple or sequential gates on large or asymmetric parts
  • Leave room in the tool design for retrofit fixes like boss pins or beryllium copper inserts, in case process tuning alone doesn’t solve it

Running the design through simulation before cutting steel catches most of this. Autodesk Moldflow is standard practice for flagging sink-prone geometry ahead of tooling, and it’s especially valuable when changing part geometry later would be expensive.

Pro Tip: *Model your ribs and bosses at their actual nominal thickness in CAD, not a rounded-up “safe” number.

Process Parameters That Reduce Sink Marks

If the design is sound and sink marks are still showing up, the fix usually lives in the process window.

  1. Set pack and hold pressure first. Industry guidance points to a starting band of roughly 50% to 80% of injection pressure for many resins, adjusted for gate size and part geometry.
  2. Run a gate-seal test. Increase hold time in steps and weigh the part at each interval; once weight stops increasing, the gate has frozen and you’ve found your true hold time. This is the correct method for determining hold time, according to Plastics Technology’s testing coverage.
  3. Balance melt and mold temperature. Too cold and the skin freezes before packing reaches the core; too hot and cycle time balloons without fixing the sink.
  4. Tune injection speed and switch-over point. A late switch-over to pack often leaves thick sections underfed.
  5. Use multi-stage packing profiles or sequential gating on parts with more than one hot spot, so each thick region gets its own packing window instead of sharing one flat profile.
  6. Document every trial against a visual and dimensional acceptance standard, so you’re not relying on memory to judge whether a change actually helped.

None of these adjustments require new tooling, which makes them the logical second step after a design review. Get the pack profile and gate timing right and a surprising number of “material problem” sink marks disappear entirely.

Material and Cooling Choices That Affect Sink Mark Formation

Resin family changes how much a part shrinks and how forgiving it is of process variation. Amorphous resins like ABS and polycarbonate shrink less and more predictably than semi-crystalline resins like polypropylene and nylon, which go through a sharper volumetric change as they crystallize. That difference alone explains why the same wall thickness sinks badly in one material and barely at all in another.

Filled and reinforced grades, glass or mineral filled versions of common resins, reduce shrinkage further and are worth considering when a part geometry can’t be redesigned around the problem. Reviewing shrinkage behavior across available injection molding materials before finalizing a design saves a redesign cycle later.

On the tooling side:

  • Conformal cooling channels follow the part’s actual geometry instead of running straight drilled lines, and they reduce localized thermal gradients far more effectively than conventional cooling near deep ribs or bosses.
  • Beryllium copper inserts pull heat from hot spots that standard steel and standard channels can’t reach.
  • Tool finish and vent condition also affect surface skin quality, and a poorly maintained tool can mask or worsen an otherwise correctable sink.

Advanced Fixes for Persistent Sink Marks

When design and process changes don’t fully clear a sink, a few specialized techniques are worth the added tooling cost.

  • Gas-assist molding injects nitrogen into the thick section after the shot fills, hollowing out the core and eliminating the mass that was causing shrinkage in the first place. It works best on ribs, handles, and other naturally tubular features.
  • Chemical blowing agents, used at very low percentages, create internal gas pressure that pushes against the cooling skin. Surface splay is a common tradeoff, so this stays a specialist approach rather than a default fix.
  • Spring-loaded and protruding boss pins retract slightly as the part shrinks, feeding material into the sink zone. Preload tuning matters enormously here: too much preload and the pin behaves like a rigid pin with no benefit, too little and the melt just pushes it back, making the sink worse.
  • Active thermal pins and Be-Cu inserts for heat removal often outperform simple protruding pins in published testing, though they add real tooling cost and lead time.

Quick Sink Mark Troubleshooting Checklist

Run this sequence before committing to any tooling change.

  1. Map every visible sink against the part’s wall thickness and gate location. Fixes should target that exact zone, not the whole part.
  2. Run the gate-seal test and a pack pressure sensitivity trial, logging visual and dimensional results at each step.
  3. Try a melt and mold temperature sweep in small increments, then adjust injection speed the same way.
  4. If process changes plateau, test one tooling retrofit, a boss pin or a Be-Cu insert, on a prototype tool before committing to full production steel.

Pro Tip: Change one variable per trial shot. Stacking two changes at once (say, hold time and mold temperature together) makes it impossible to tell which one actually fixed the sink.

An Engineer’s Take on Sink Mark Prevention

An Engineer's Take on Sink Mark Prevention — overview diagram

Most sink mark write-ups treat design and process as separate problems, and that’s backwards. The order matters: check geometry, then process, then reach for gas-assist or a spring-loaded pin, because the tooling fixes cost real time and money if the underlying wall thickness was the actual issue all along.

Where this gets genuinely useful is in how fast you can test that sequence. WJ Prototypes runs SLA, SLS, MJF, DMLS, CNC machining, sheet metal fabrication, vacuum casting, injection molding, and die casting for clients across aerospace, automotive, medical, electronics, robotics, oil and gas, and industrial machinery. An ISO certified shop with engineers who see this exact defect across dozens of part geometries a month tends to spot the difference between a geometry sink and a process sink faster than a single in-house trial run ever will.

— Nas

Validate Sink Mark Fixes Before Committing to Production Tooling

Testing a design change, a new pack profile, or a boss-pin retrofit on your own production line means burning press time you probably can’t spare. WJ Prototypes runs short-run injection molding specifically so you can validate a geometry or process fix on a real tool before committing to full production steel, with no minimum order quantity holding up a small trial batch. Submit your CAD file through the instant online quoting system and you get engineering feedback on wall thickness, gating, and cooling layout alongside your quote, not after tooling is already cut. If you’re still narrowing down whether the fix belongs in the design or the process window, that same quote flow works for a quick prototype run too. Upload your part and get a quote to see what a corrected mold looks like before it ever reaches your production floor.

Sources

FAQ

What Are Sink Marks in Injection Molding?

Sink marks are shallow surface depressions that form when a thick section of a part cools and shrinks slower than the surrounding thin walls, pulling the surface inward. They typically appear opposite ribs, bosses, and thick corners, and the fix depends on whether the root cause is geometry, process settings, or the resin itself.

How Do You Remove Sink Marks From a Molded Part?

You generally can’t remove a sink mark from a finished part, but you can eliminate it in future shots by adjusting pack and hold pressure, running a gate-seal test to set correct hold time, or redesigning the offending rib or boss. For parts already in production, a targeted cooling insert or boss-pin retrofit often clears a persistent sink without a full tool rebuild.

What Is the Average Lifespan of a Plastic Injection Mold?

Mold lifespan depends heavily on tool steel, cavity count, and production volume, and it isn’t something this article’s sources quantify. A well-maintained hardened steel tool used for medium production runs typically outlasts several soft-tool prototype builds, which is one reason validating a design on a prototype tool first, through a service like WJ Prototypes’ injection molding, protects the more expensive production mold from avoidable rework.

Can Simulation Software Actually Predict Sink Marks Before Tooling?

Yes. Tools like Autodesk Moldflow simulate cooling and shrinkage across a part’s geometry, flagging thick sections likely to sink before any steel is cut. It won’t catch every process-related sink, since some only show up once real machine variables come into play, but it dramatically reduces expensive tooling iterations tied to geometry problems.