Low filament weight in meltblown fabric usually points to a drop in polymer melt index, a cooler melt point, or a clogged filter bag. Correcting these three variables restores consistent fiber formation and helps keep nonwoven production stable.
- Low filament weight usually means the polymer melt index is too low for the set melt point.
- A cooler melt point can thicken the melt and make the fiber diameter larger than intended.
- Filter bags and melt die gaps should be checked after every resin switch or changeover.
- Record filament weight at the start of the shift to catch drift before it affects output.
- Consistent resin batches and stable cooling water temperatures help prevent repeat issues.
Meltblown fabric relies on a thin polymer stream stretching into fine fibers as it passes through a melt die and into a collection zone. When filament weight drops below the target range, the fabric usually becomes heavier, less open, and harder to process downstream. Engineers often see this symptom after a resin change, a filter bag replacement, or a shift in melt point.
This issue is not just a quality complaint. It changes the fabric structure, affects filtration efficiency, and can trigger rejects in the next stage of nonwoven production. The good news is that low filament weight is usually traceable to a small number of variables.
What Does Low Filament Weight Mean?
Filament weight measures the mass of a single fiber in grams per meter. In meltblown nonwovens, this value sits in a narrow window. A drop in filament weight means the fiber is heavier than designed. The fabric may look normal at first glance, but the structure changes.
The surface becomes less open. The fiber diameter increases. The fabric loses some of the fine, web-like texture expected from meltblown fabric. If the fabric is used for filtration or hygiene, these changes can affect performance.
A useful rule of thumb is to compare the current filament weight against the target and the previous good shift. If the value is outside the accepted range for two or three consecutive readings, treat it as a production issue, not random variation.
Common Symptoms You May See
Before changing settings, check the physical fabric. Low filament weight often shows up in a few predictable ways.
- The fabric feels stiffer or thicker than the last good run.
- The open structure looks less even, with fewer fine fibers visible.
- Filtration tests show a different pressure drop than expected.
- The fabric may feel more like a spunbond or spunbond nonwoven hybrid than a true meltblown sheet.
- Downstream converting may struggle with the changed hand feel.
These symptoms are not always obvious. Sometimes the fabric looks acceptable, but the filament weight is already drifting. That is why routine checks matter.
Likely Causes and Fixes
Most low filament weight issues trace back to three areas: resin properties, melt temperature, and die condition. The table below covers the most common causes and the actions to take.
| Symptom | Likely cause | What to do |
|---|---|---|
| Filament weight rises after a resin change | New batch has a lower melt index than the previous one | Check the resin certificate, verify melt index, and compare against the accepted range for this meltblown fabric recipe |
| Filament weight rises when melt point is lowered | Cooler melt thickens the polymer, increasing fiber diameter | Raise the melt point to the target range or adjust the resin if the batch is too low |
| Filament weight rises after filter bag change | New filter bag is too fine or the die gap is out of spec | Inspect the filter bag rating, clean the die, and verify the gap setting |
| Filament weight rises at the start of a shift | Melt die is not fully at temperature or the filter bag is not properly seated | Allow extra warm-up time, check die alignment, and confirm the filter bag is dry and free of dust |
| Filament weight rises during a long run | Melt index drifts or the die wears unevenly | Monitor melt index over time, inspect the die surface, and clean or replace worn parts |
| Filament weight rises when cooling water is cooler | Cooler melt reduces stretch and increases fiber thickness | Adjust the cooling water temperature to match the process window and recheck filament weight |
The resin is the most common starting point. Polypropylene nonwoven producers use meltblown fabric for its fine fiber structure, and that structure depends on the melt index. A resin with a lower melt index will not stretch as easily. The fiber becomes thicker, and the filament weight rises.
If the resin is the issue, the fix is usually a recipe change. Do not force a low melt index resin into a process designed for a higher one. The melt point may need to be raised, but there are limits. If the melt point is already at the upper end of the accepted range, the resin is not suitable for this line.
Melt Point and Melt Index: The Pair to Watch
Melt point and melt index work together. Melt point is the temperature at which the polymer melts. Melt index is a measure of how easily the resin flows. A higher melt index means the resin flows more easily. A lower melt index means it is stiffer and needs more heat to reach the same flow.
When you change the melt index, you change the behavior of the melt. If the melt index drops, the same melt point produces a thicker fiber. The filament weight goes up. This is why resin changes are a high-risk moment for meltblown production.
Check the melt index before every resin change. Do not rely on the supplier name alone. Different batches can vary. A resin that was acceptable last month may not be acceptable this month if the melt index has drifted.
A practical check is to run a small sample and measure filament weight. Compare it to the last good run. If the filament weight is higher than expected, the resin is likely the cause.
Filter Bags and Die Condition
Filter bags are designed to catch contaminants before the melt reaches the die. They also affect the flow. A filter bag that is too fine will slow the melt. A filter bag that is clogged will do the same. Both conditions increase the pressure in the melt zone and can thicken the fiber.
Check the filter bag rating against the process specification. If the bag is finer than the target, the melt will not flow as freely. The filament weight will rise.
The die itself matters too. A die with a worn surface or a gap that is out of spec will change the fiber diameter. A smaller gap can increase the pressure and thicken the fiber. A larger gap can change the stretch. Either way, the filament weight can move outside the target range.
Inspect the die surface for buildup or wear. Clean the die during changeovers. Verify the gap setting with a calibrated tool. Do not guess. The gap is a critical part of the process.
Cooling Water and Stretch
The cooling water temperature affects how the fiber stretches after it leaves the die. Cooler water stiffens the fiber faster. Warmer water allows more stretch. Both directions matter.
If the cooling water is cooler than the process window, the fiber does not stretch as much. The diameter increases. The filament weight rises. This is a subtle issue because it may not show up immediately. It can drift over a shift.
Check the cooling water temperature at the start of the shift and again halfway through. Compare it to the target range. If it is lower than expected, adjust the water temperature or check the cooling system.
A stable cooling water temperature helps keep the filament weight consistent. It also reduces variation between shifts. This is one of the easiest improvements to make in nonwoven production.
Prevention Tips for Stable Filament Weight
Prevention is simpler than troubleshooting. A few habits can keep low filament weight from becoming a recurring issue.
- Record filament weight at the start of every shift. Log the value in a simple sheet or the line computer.
- Check the melt index before every resin change. Compare the new batch to the last accepted batch.
- Verify the melt point and cooling water temperature after every changeover.
- Inspect the filter bag and die after every changeover.
- Keep a small sample of the last good fabric. Compare the new fabric to the sample before accepting a batch.
- Review the filament weight trend over several shifts. A slow drift is easier to catch than a sudden spike.
These habits take little time. They prevent the issue from becoming a quality complaint. They also make it easier to trace the problem when it does occur.
When to Call It a Resin Issue
Sometimes the fix is not on the machine. Sometimes the resin is simply not right for the process. If the filament weight rises after a resin change and does not come back after adjusting the melt point and cooling water, the resin is the likely cause.
The next step is to contact the resin supplier. Provide the melt index, the melt point, the cooling water temperature, and the filament weight. Ask for a replacement batch or a resin with a higher melt index.
Do not accept a resin that requires a melt point above the process window. The fiber will be too thick. The fabric will not meet the target structure.
A good supplier will understand the process. They will provide a resin that fits the meltblown fabric recipe. If they cannot, it is time to look for another resin.
Final Check Before Restarting
After fixing the issue, do not restart the line immediately. Run a short sample. Measure the filament weight. Check the fabric structure. Compare it to the last good sample.
If the filament weight is within the target range and the fabric looks correct, restart the line. If not, check the variables again. The melt point, the melt index, the filter bag, and the die are the four main points.
A quick check before restarting saves time. It prevents a bad batch from going into the next stage of nonwoven production. It also keeps the quality team informed.
Low filament weight is a solvable problem. It usually points to a small number of variables. Check the resin, the melt point, the filter bag, and the die. Keep the cooling water stable. Record the filament weight. The fabric will come back to spec.
Frequently asked questions
Can a lower melt point cause low filament weight?
Yes. A cooler melt thickens the polymer and increases the fiber diameter. This raises the filament weight and changes the fabric structure.
Should I change the melt index or the melt point first?
Check the resin certificate first. If the melt index is outside the accepted range, the resin is the issue. If the melt index is acceptable, adjust the melt point within the process window.
How often should I check filament weight?
At the start of every shift and at the end of every shift. If the fabric is used for filtration or hygiene, check it more often.
Can a new filter bag cause low filament weight?
Yes. A filter bag that is too fine or clogged can slow the melt and thicken the fiber. Check the filter bag rating and condition after every changeover.
Is a small rise in filament weight acceptable?
It depends on the application. If the rise is within the accepted range and the fabric performs correctly, it may be acceptable. If it is outside the range, treat it as a production issue.
