Most injection moulding defects are process problems wearing a material costume. The machine is blamed, then the mould, then the resin, and somewhere near the end of that list somebody blames the colour. Occasionally the colour is genuinely at fault. Far more often it is the last variable that changed, which is not the same thing.
This guide covers the fourteen defects that account for most of the scrap on a typical Indian moulding floor. For each one: what it looks like, what causes it, and what to change first.
Work in this order
Before the table, one habit that saves more money than any single fix.
Change one variable at a time, and change the cheapest one first. Process settings are free. Mould modifications are not. Material changes are the most expensive of all, because they invalidate every approval you have already paid for.
The order that works: process, then mould, then material. Most shops do it backwards because material is the easiest thing to argue about with someone else.
The 14 defects at a glance
| Defect | What you see | First thing to check |
| Short shot | Part not fully filled | Injection pressure, melt temperature |
| Flash | Thin fin at the parting line | Clamp tonnage, mould wear |
| Sink marks | Depression over thick sections | Holding pressure and time |
| Voids | Internal bubble in thick sections | Holding pressure, gate size |
| Warpage | Part twists out of shape | Cooling uniformity, ejection temperature |
| Weld line | Visible seam where flows meet | Melt and mould temperature |
| Flow marks | Wavy or ripple pattern near gate | Injection speed profile |
| Jetting | Snake-like mark from the gate | Injection speed, gate location |
| Burn marks | Brown or black scorching at the far end | Venting, injection speed |
| Silver streaks | Silvery splash marks | Moisture in the resin |
| Black specks | Dark contamination points | Barrel residue, regrind, purging |
| Colour streaks | Uneven or swirled colour | Dispersion, screw mixing, dosing |
| Delamination | Surface peels in layers | Contamination, incompatible material |
| Brittleness | Part cracks under normal load | Degradation, over-regrind, drying |
The rest of this article works through the ones where the answer is not obvious.
Short shot
The cavity does not fill. It is the most common defect and usually the simplest.
Check in this order: is there enough material in the shot (cushion), is the melt hot enough, is injection pressure hitting a limit, is the mould venting so trapped air can escape, and is the gate frozen too early.
A short shot that appears only on one cavity of a multi-cavity tool is a balance problem, not a machine problem. Runner sizes or gate dimensions differ, and no amount of pressure fixes it evenly – you will overpack the good cavities to fill the bad one.
Where material comes in: a very low melt flow index resin fills thin sections poorly. If you have moved from an 18 MFI grade to a 6 MFI grade to improve impact strength, short shots are a predictable consequence, not a surprise.
Sink marks
A depression on the surface, almost always over a thick section – a rib, a boss, a wall junction.
Plastic shrinks as it cools. Where the section is thick, the outside skin solidifies first and the still-molten core pulls away from it. If holding pressure cannot push more material in before the gate freezes, the skin gets dragged inward and you see a sink.
The fixes, in order of cost:
1. Increase holding pressure and holding time. Free. Try this first, always. 2. Lower melt and mould temperature. Reduces total shrinkage. 3. Increase gate size. Lets you keep packing for longer before the gate freezes. 4. Redesign the rib. The standard rule is rib thickness at 50-60% of the wall it sits on. A rib at 100% of wall thickness will sink and no process setting will hide it.
Sink marks are much more visible on a glossy dark surface than on a textured or light one. This is why the same tool run in black shows sinks that nobody noticed in natural. That is not the black masterbatch causing sink – it is the black revealing it. A textured surface or a lighter colour hides the same geometry.
Warpage
The part comes out of the tool the right shape and is the wrong shape an hour later, or it comes out already twisted.
Warpage is differential shrinkage. If every part of the moulding shrank by the same amount in every direction, you would get a smaller part, not a bent one. You get a bent one when one region shrinks more than another, or shrinks more in one direction than another.
Three causes, in descending order of how often they are the real one:
Uneven cooling. One side of the tool runs hotter than the other. The hot side shrinks more. Measure both mould halves with a surface probe – do not assume the water is doing what the drawing says. Blocked or scaled cooling channels are extremely common in Indian plants running hard water.
Non-uniform wall thickness. Thick sections shrink more than thin ones. This is a design problem and the mould has to compensate.
Molecular and fibre orientation. Material flowing in one direction shrinks more across the flow than along it. In glass-filled grades this effect is severe and predictable – a glass-filled PP part will always want to bow.
Ejecting too hot turns a part that would have been fine into a warped one. If cycle time has been cut recently and warpage started, that is your answer.
Weld lines
Where two flow fronts meet – around a hole, a core pin, or from two gates – they form a visible line. It is also a mechanical weak point, typically retaining 40-80% of the base material’s strength depending on how well the fronts fused.
The fronts fuse properly only if they are still hot and mobile when they meet. So: raise melt temperature, raise mould temperature, raise injection speed. Move the gate so the fronts meet somewhere that matters less. Add a vent or an overflow well at the weld line so the cold front material pushes past the visible area.
The colour connection is real here. Metallic and pearlescent effects make weld lines dramatically worse, because the flake pigments align with the flow and the meeting line shows as a sharp visual seam. This is a known limitation of special effect masterbatches, not a defect in them – it should be discussed at the design stage, and gate position chosen with the effect in mind.
Silver streaks
Silvery, splash-shaped marks radiating from the gate. In nearly every case this is moisture.
Water in the resin flashes to steam at melt temperature and is dragged along the flow front. Hygroscopic polymers – PET, PA, PC, ABS, PMMA – absorb enough moisture from Indian ambient humidity in a few hours to cause this.
Dry the resin properly. And dry the masterbatch if it needs it: a masterbatch carried on a hygroscopic carrier needs the same treatment as the resin. On PET especially, an undried colour concentrate will cause both silver streaking and a measurable drop in intrinsic viscosity.
If drying is correct and you still see streaks, look at excessive back pressure, screw wear, or trapped air from a starved feed throat.
Burn marks
Brown or black scorching, always at the last place to fill. This is not the material burning in the barrel – it is trapped air being compressed so fast that it ignites the polymer. It is a diesel effect, and the tell is that it appears at the end of fill, never near the gate.
Fix the venting. Reduce injection speed at the end of the fill profile. Clean the vents – they clog with plate-out over thousands of shots and nobody checks them.
Black specks and dark contamination
Dark points in an otherwise clean part. This one gets blamed on masterbatch more than any other defect, and it deserves its own treatment – we cover the full diagnostic in black specks in plastic parts.
The short version: specks that are hard, angular and consistent in size are usually degraded polymer from a dead spot in the barrel or a worn non-return valve. Specks that appear immediately after a colour change are purging residue. Specks that track exactly with masterbatch dosage are the only ones where the masterbatch is genuinely implicated – and the test is to run the same lot at half dosage and see whether speck count halves.
Colour streaking and poor dispersion
Swirls, streaks or patchy colour. Distinguish two different problems that look similar:
Poor distribution means the masterbatch has not been spread evenly through the melt. You see broad, soft variation. Fix it with better mixing – a mixing screw, higher back pressure, a longer residence time, or a dosing unit that meters consistently instead of dumping.
Poor dispersion means the pigment agglomerates have not been broken down. You see fine, sharp streaks and specks of concentrated colour. This is a masterbatch quality problem – the pigment was not properly wetted and milled during compounding.
The practical test: run a film press-out or a thin flat plaque. Agglomerates that are invisible in a 3 mm part are obvious in a 100 micron film.
Carrier compatibility matters more than people expect. A masterbatch on a PE carrier dosed into PP will disperse acceptably in many cases and badly in some. A carrier whose MFI is far below the host resin will not melt and spread in time. When you buy, ask for the carrier polymer and its MFI – not just the dosage. We explain why in what is masterbatch.
Delamination
The surface peels away in thin layers, like mica. This is almost always incompatible material contamination – PP in PE, PA in PP, a pocket of something that never fused with the matrix.
Sources: contaminated regrind, an incompletely purged barrel, or a masterbatch on a carrier that is genuinely incompatible with the host polymer. The last one is rare with a competent supplier but it is the reason carrier polymer belongs on your specification, not left to the supplier’s discretion.
Excessive moisture and severe overheating can also cause it.
Brittleness
The part is the right shape and breaks under a load it should survive.
Degradation from over-processing – melt temperature too high, residence time too long, screw running at very high shear. Every pass through an extruder or moulding machine costs the polymer some molecular weight.
Too much regrind, too many times. Each regrind cycle shortens chains. There is no universal safe percentage; there is a percentage your part can tolerate, which you find by testing.
Undried hygroscopic resin. In PET and PA, moisture causes hydrolytic chain scission during processing – the polymer is chemically shortened, permanently. The part looks fine and is weak.
Wrong grade. A high-MFI, easy-flowing grade is short-chained by definition, and short chains mean lower impact strength. If someone switched to an easier-flowing resin to fix a short shot, brittleness is the bill for it.
Where masterbatch genuinely belongs on the list
Being honest about this is more useful than defending the material.
Masterbatch is a plausible cause of: poor dispersion streaks, some black specks, delamination from an incompatible carrier, a measurable shift in flow at high dosage, and colour variation between lots.
Masterbatch is almost never the cause of: short shots, flash, warpage, weld lines, burn marks, silver streaks, voids or sink marks. Colour can reveal sink and weld lines by changing surface gloss, which is a different claim entirely.
The test that settles most arguments takes twenty minutes: run the tool in natural, unpigmented resin at the same settings. If the defect is still there, it was never the colour.
The specification that prevents most of this
A large share of colour-related moulding trouble is written into the purchase order, or rather left out of it. A masterbatch specification that a supplier can actually be held to should name:
– The host polymer and grade, with its MFI – The carrier polymer required, and its MFI relative to yours – The process – injection, film, pipe, rotomoulding – because dispersion demands differ enormously – The dosage you intend to run at – Colour tolerance and the standard it is measured against – Any regulatory requirement – food contact, toy safety, RoHS – Whether regrind is in the feed, and at what level
We set that out in detail in how to write a masterbatch specification.
Talk to us about the material half
Life Color Pigments & Masterbatches supplies colour, white, black and additive masterbatches to moulders across India, and we manufacture to ISO 9001:2015.
If you are chasing a defect and want to rule the material in or out properly, tell us your polymer, your process and your dosage and we will tell you what the masterbatch can and cannot be responsible for.
FAQs
What are the most common injection moulding defects? Short shots, flash, sink marks, warpage, weld lines, flow marks, burn marks, silver streaks, black specks, colour streaking, voids, delamination, jetting and brittleness. Short shots and sink marks are the two most frequently reported on Indian moulding floors.
What causes warpage in injection moulding? Differential shrinkage. The usual causes are uneven mould cooling, non-uniform wall thickness, fibre or molecular orientation in filled grades, and ejecting the part before it has cooled enough to hold its shape.
How do you fix sink marks on plastic parts? Increase holding pressure and holding time first, then lower melt and mould temperature, then increase gate size so you can pack for longer. If the rib is thicker than about 60 percent of the wall it sits on, the geometry has to change.
Is masterbatch responsible for black specks? Sometimes, but it is the least likely of several causes. Degraded polymer from a dead spot in the barrel and residue from an incomplete purge are far more common. Test by running the same masterbatch lot at half dosage – if speck count does not halve, the masterbatch is not the source.
What causes silver streaks in moulded parts? Moisture, in almost every case. Water in the resin flashes to steam at melt temperature and streaks along the flow front. Dry hygroscopic polymers such as PET, PA, PC and ABS properly, and dry the colour concentrate too if its carrier is hygroscopic.
Why do burn marks appear at the end of the part and not the gate? Because they are caused by trapped air being compressed and igniting, not by material overheating in the barrel. The air collects at the last area to fill. Improve venting and slow the end of the injection profile.
How do I tell poor dispersion from poor distribution? Poor distribution gives broad soft colour variation and is a mixing problem on your machine. Poor dispersion gives fine sharp streaks and concentrated specks, and is a compounding quality problem in the masterbatch. A thin film press-out makes the difference obvious.
How can I prove a defect is not caused by the colour? Run the same tool at the same settings in natural, unpigmented resin. If the defect persists, the masterbatch is not the cause. This single test resolves most disputes between moulder and supplier.