Troubleshooting · Defect guide
Splay and silver streaks
Where the silver lines come from, and what to fix first
Splay is gas carried in the melt that gets smeared open on the surface as the melt flows into the mould. That gas has three sources: moisture, volatiles from degradation, and air. This guide shows how to find the source, starting with the cheapest checks, how to decide what really needs drying, and why correct drying matters far more than any additive.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Confirm it is splay
Splay shows as silver or milky streaks on the surface that point along the flow direction. They are usually densest near the gate and fade further away. Small surface bubbles sometimes appear with them, and the surface can feel slightly rough.
- Moisture splay: spread across the surface in a U or V shape around the gate, usually on every shot.
- Volatile / degradation splay: silver streaks with a yellow or brown tint, sometimes with black specks, worse after the machine has been idle and restarts.
- Air splay: streaks or bubbles that come and go between shots, often linked to the decompression setting.
- Colour streaks from masterbatch are not splay: they are darker or lighter bands of colour with a smooth surface. See the troubleshooting table.
Mechanism: how gas becomes silver streaks
Inside the barrel, moisture turns into steam and volatiles turn into gas, held in the melt under pressure. As the melt shoots through the gate into the cavity, pressure drops, the gas bubbles expand and surface at the flow front. The flow front rolls out against the mould wall, so burst bubbles are flattened and freeze onto the skin as long streaks in the flow direction. That is why splay always points the way the melt flowed.
The practical takeaway: splay is a gas problem, so you have to know where the gas comes from before fixing it. Guess wrong, for example drying pellets when the real source is air, and you spend time and energy for nothing.
Four groups of causes
1. Material: moisture
The most common cause. Hygroscopic resins such as ABS, PC, PA and PET must always be dried to the temperature, time and dew point the resin maker gives in the TDS. PP and PE absorb almost no water, but what you blend in can bring it: highly filled masterbatch and washed recycled pellets. For recycled material, see Recycled PCR / rPP compounds.
2. Process: heat and residence time
Melt temperature that is too high, or melt sitting in the barrel too long, for example a small shot on a large machine, degrades the polymer or some additives into gas. A screw speed that is too high adds shear heat as well.
3. Process: air
Pulling the screw back after plasticising (decompression / suck-back) too far draws air in through the nozzle. Back pressure that is too low fails to push out the air trapped between pellets.
4. Mould: gate and vents
A small or sharp-edged gate creates high local shear and heat, so splay starts at the gate even when the material is dry. Blocked vents keep gas from leaving the cavity. For machine-setting basics, see Injection molding.
Step-by-step checks, cheapest first
- Purge into the air. Watch the melt coming out of the nozzle. Bubbles or a crackling sound mean there is gas in the melt before it reaches the mould.
- Mould virgin resin as a reference. Run the base resin alone without masterbatch or regrind. If the splay disappears, the source is in the blend.
- Dry only the suspect component. Dry the masterbatch or recycled pellets you suspect, separately, and mould again. If the splay disappears, it is moisture from that component.
- Audit drying of hygroscopic resins. Compare temperature and time with the resin maker's TDS, confirm the dryer air is actually dry, and make sure dried pellets do not sit in the open before reaching the machine.
- Check residence time and melt temperature. If the shot uses only a small part of barrel capacity, or splay gets worse after idle periods, lower temperatures within the resin maker's range or consider a smaller machine.
- Reduce decompression and raise back pressure step by step. If intermittent splay disappears, the source was air.
- Inspect the gate and vents. If nothing above helps and the streaks always start at the gate, the fix is in the mould, the most expensive step.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Hygroscopic resin under-dried | Compare drying conditions with the TDS · measure moisture if you have an analyser | Dry as the resin maker specifies, using a dry-air (desiccant) dryer |
| Dried pellets regain moisture | Check how long pellets wait between dryer and hopper | Convey straight from the dryer to the machine; keep the hopper closed |
| Moisture from highly filled MB | Dry only the MB and mould again | Dry the MB before use; store it sealed |
| Moisture from washed recycled pellets | Dry only the recycled pellets and mould again | Dry them, or ask the supplier to control moisture per lot |
| Melt temperature too high | Measure the temperature of purged melt | Bring it into the resin maker's recommended range |
| Long residence time in the barrel | Compare shot weight with barrel capacity · splay worse after idling | Lower temperatures during stops, purge before restarting, match machine size to shot |
| Too much decompression | Reduce step by step and watch the streaks | Set just enough to stop drooling, no more |
| Back pressure too low | Increase step by step and watch the streaks | Set enough back pressure to expel air without overheating the melt |
| Small or sharp gate · blocked vents | Streaks start at the gate even with dry material · inspect vents | Enlarge or radius the gate; clean vents on a schedule |
How much can additives help?
The honest answer is only in specific cases. The main fix for moisture splay is drying correctly. These are the material options that actually exist:
- Desiccant masterbatch, usually calcium oxide in a carrier, can capture moisture carried in by fillers or recycled pellets in PP and PE. Use it once that source is proven and drying the whole blend is not worthwhile. It does not replace drying hygroscopic resins.
- Antioxidants reduce polymer degradation during melting and help when the gas comes from degradation, especially in recycled resin that has seen several heat histories. If temperature or residence time is far too high, fix the process first.
- Masterbatch whose carrier and additives suit your process temperature. Some formulation ingredients can volatilise or decompose if the melt runs hotter than they were designed for. Ask your MB supplier what temperature the formulation is rated for.
What additives cannot fix: under-dried hygroscopic resin, air drawn in by decompression and back pressure settings, and a gate that creates excessive shear.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Splay questions we hear often
PP and PE don't absorb water, so how can they show moisture splay?
PP and PE pellets themselves absorb almost no moisture, but what you blend in can bring water with it: highly filled masterbatch with CaCO₃ or talc, washed recycled pellets that were not fully dried, and pellets stored somewhere humid with condensation on the surface. To prove it, dry only the suspect component and mould again. If the splay disappears, that component is the source.
We dried the resin and still see splay. What does that mean?
There are three possibilities. First, drying did not reach what the resin maker specifies: temperature or time too low, the dryer air itself is wet, or dried pellets sat in the open and picked moisture back up. Second, the gas is not moisture but volatiles from degradation caused by high melt temperature or long residence time in the barrel. Third, air is being drawn in by too much decompression or too little back pressure. Check them in that order.
How do I tell splay from colour streaks caused by masterbatch?
Splay is silvery or milky streaks on the surface that look like bubbles dragged open along the flow direction. It usually starts near the gate and can feel slightly rough. Colour streaks from poorly dispersed masterbatch are darker or lighter bands of colour, the surface is smooth, and you can see them through the wall when you cut the part. If you mould virgin resin without masterbatch and the splay is still there, colour is not the cause.
Can a desiccant masterbatch replace drying?
It should not. A desiccant masterbatch suits cases where moisture is proven to come from fillers or recycled pellets in PP or PE and drying the whole blend is not worthwhile. Hygroscopic resins such as ABS, PC, PA and PET must always be dried as the resin maker specifies. A desiccant can only absorb a limited amount of water and does nothing for volatiles or air.
Dried, retuned, and the splay is still there?
Send photos of the part and of purged melt, plus the resin and blend you use, on LINE. Our technical team will help you work out whether the gas is moisture, volatiles or air.