A sink mark is what you see when the inside of a section is still shrinking after the outside has already frozen. The skin solidifies against the cold mould wall while the core is molten. As the core cools and contracts, it pulls the skin inward, and you get a depression – almost always over a rib, a boss, or a wall junction.
That is the entire mechanism. Every fix works by changing one of three things: how much the material shrinks, how much extra material you can pack in before the gate freezes, or how thick the section is in the first place.
Where sinks appear, and why there
Sinks form over local thick sections, because thickness controls how long the core stays molten.
The classic locations:
– Ribs – a rib is a thick feature attached to the back of a surface, and the surface opposite is where the sink appears – Bosses – screw bosses are effectively ribs in the round, with the same result – Wall intersections – where two walls meet, the material at the junction is thicker than either wall – Gate areas, if the gate freezes before packing is complete – Thick pads for logos, labels or fixings
If you can see a sink and you cannot see anything on that face causing it, look at the other side of the part.
Fixes, in the order to try them
### Free: process settings
1. Increase holding pressure. This is the first move, every time. Holding pressure forces more material into the cavity to compensate for volumetric shrinkage. More material in means less inward pull.
2. Increase holding time. Pressure is only useful while the gate is still open. If the gate freezes at 3 seconds and you are holding for 5, the last 2 seconds are doing nothing. Find the actual gate freeze time – increase holding time in steps and weigh the part; when part weight stops rising, the gate has frozen and holding longer is wasted cycle.
3. Lower melt temperature. A cooler melt has less thermal contraction to undergo. It also raises viscosity, so watch for short shots.
4. Lower mould temperature. Freezes the skin faster and reduces total shrinkage – but a colder mould also gives a duller surface and can worsen weld lines, so this is a trade.
5. Extend cooling time. Lets the section solidify more completely before the constraint of the mould is removed.
6. Check the cushion. If the screw bottoms out, holding pressure is not actually being applied to the melt no matter what the setting says. A worn non-return valve does the same thing intermittently, which produces sinks that come and go for no visible reason.
That last one is worth emphasising. Intermittent sink marks on a process that has not changed usually mean a leaking non-return valve.
### Cheap: the gate
Increase gate size, or move the gate to the thickest section.
A gate is a thin restriction that freezes long before the part does. Once it freezes, no further packing is possible regardless of pressure – the part is on its own. A larger gate stays open longer and extends the useful packing window. Gating into the thick section means the packing pressure reaches the part of the moulding that needs it most.
This is the highest-value change on many tools, and it is a small modification compared with reworking wall sections.
### Expensive: the geometry
If process and gate changes have been exhausted, the part is asking to be redesigned.
| Feature | Rule |
| Rib thickness | 50-60% of the wall it attaches to |
| Boss outer wall | 60% of the nominal wall |
| Wall thickness change | Gradual transition, not a step |
| Corner radius | Generous, to avoid a thick material pocket |
A rib at 100% of wall thickness will sink, and no process setting will hide it. This is the single most common design cause. It gets specified because a thicker rib feels stronger, and the stiffness gained is not worth the surface defect and the extended cycle time it buys.
Other geometry answers:
– Core out thick sections. Replace one thick boss with a cored one of equivalent stiffness. – Use more, thinner ribs rather than fewer thick ones. Stiffness rises with rib height far more than with rib thickness. – Add a texture or a design feature where the sink appears. If it cannot be eliminated, it can be hidden – a textured surface hides a shallow sink almost completely.
### Material
Semi-crystalline polymers – PP, PE, PA, POM – shrink roughly 1 to 2.5%. Amorphous polymers – ABS, PS, PC – shrink roughly 0.4 to 0.8%. A part with marginal geometry in PP may be entirely acceptable in ABS, purely because there is less shrinkage to accommodate.
Filled grades shrink less overall, which helps with sink, though they bring their own warpage behaviour – see warpage in injection moulding.
Sink marks and voids are the same problem
A void is an internal bubble in a thick section. The mechanism is identical to a sink: the core shrinks away from the frozen skin. The difference is only which one gives way.
If the skin is weak or still soft, it is pulled inward and you get a sink. If the skin is rigid enough to resist, the core tears internally instead and you get a void.
A hot mould and a hot melt keep the skin soft, favouring sinks. A cold mould and a cold melt make the skin rigid, favouring voids. This is why an attempt to fix a sink by dropping temperatures can convert it into a void – which is worse, because it is invisible until the part fails or is sectioned.
The real fix for both is the same: pack more material in, or make the section thinner.
Why colour changes what you see
This causes more disputes between moulders and masterbatch suppliers than it should, so it is worth being exact.
The pigment does not create the sink. It changes how visible the sink is.
Sink marks are seen as a distortion in the way light reflects off a surface. That distortion is most obvious on a glossy, dark, saturated surface, where reflection is specular and any deviation in the surface normal shows immediately. It is least obvious on a light, matt or textured surface, where reflection is diffuse.
So the same tool at the same settings shows a pronounced sink in gloss black and an almost invisible one in natural or a light textured grey. Nothing about the moulding changed.
Two practical implications:
Qualify appearance in the actual production colour. A tool approved in natural and then run in black will produce complaints that are nobody’s fault and everybody’s problem.
Where a dark gloss finish is required and the geometry is marginal, expect to spend more on the tool. That is a design decision, and it is cheaper to make it before the mould is cut.
There is one genuine material effect worth knowing: some pigments nucleate crystallisation in semi-crystalline polymers, slightly altering shrinkage and therefore sink depth between colours. It is a small effect next to the visibility change, but it is real, which is why dimensionally critical parts should be qualified per colour.
We supply colour, black and white masterbatch with consistent carrier and pigment loading lot to lot, so that the surface you approved is the surface you keep getting.
Quick diagnostic
| Symptom | Most likely cause |
| Sink over every rib, all cavities, always | Rib too thick – design |
| Sink in one cavity only | Gate or cooling imbalance in that cavity |
| Sink appeared after a cycle time reduction | Insufficient cooling or packing time |
| Sink comes and goes randomly | Non-return valve leaking, or inconsistent cushion |
| Sink appeared after a colour change | Visibility, not mechanism – check in natural |
| Sink appeared after a material change | Higher shrinkage grade or polymer |
Talk to us
If a sink argument has landed on the material, the twenty-minute test is to run the tool in natural resin at identical settings. If the sink is still there, it was never the colour.
For the colour side, tell us your polymer, part geometry, surface finish and shade and we will formulate for consistency across lots. Related: injection moulding defects, household, automotive, furniture.
FAQs
What causes sink marks on plastic parts? The core of a thick section continuing to shrink after the surface skin has frozen, pulling the skin inward. They appear over ribs, bosses, wall junctions and thick pads, which are the features that stay molten longest.
How do you get rid of sink marks? Increase holding pressure first, then holding time, then reduce melt and mould temperature, then extend cooling. If those are exhausted, enlarge the gate or move it to the thick section. Geometry changes – thinner ribs, cored bosses – come last because they cost the most.
What is the correct rib thickness to avoid sink marks? Around 50 to 60 percent of the wall it attaches to. A rib at full wall thickness will sink and no process setting will hide it, because the junction is now the thickest region of the moulding.
Why do sink marks appear and disappear at random? Usually a leaking non-return valve or an inconsistent cushion, so holding pressure is not reliably reaching the melt. The setting on the machine looks correct while the pressure actually applied to the part varies shot to shot.
What is the difference between a sink mark and a void? The same shrinkage mechanism with a different outcome. If the skin is still soft it is pulled inward, giving a sink. If the skin is rigid it resists, and the core tears internally instead, giving a void. Cooling the mould to fix a sink can convert it into a void.
Does black masterbatch cause sink marks? No. It makes existing sinks far more visible, because a glossy dark surface reflects specularly and shows any surface distortion. The same tool in a light or textured colour can look acceptable. Prove it by running the tool in natural resin at the same settings.
Can changing material reduce sink marks? Yes. Semi-crystalline polymers such as PP and PE shrink roughly 1 to 2.5 percent while amorphous polymers such as ABS and PC shrink 0.4 to 0.8 percent. A part with marginal geometry can be acceptable in an amorphous polymer and unacceptable in a semi-crystalline one.
How do I find the right holding time? Increase holding time in steps and weigh the part each time. When part weight stops increasing, the gate has frozen and further holding time adds nothing but cycle. That point is the useful maximum.