PVC resin is a powder, and how that powder is graded decides how your line actually runs. Two numbers on the published indicator table carry most of that information: the fraction retained above 0.25 mm and the fraction passing 0.063 mm.
The published figures
| Grade | Retained on 0.25 mm | Passing 0.063 mm |
|---|---|---|
| SG1 to SG6 | 2.0% (premium / first), 8.0% (qualified) | 90% (premium / first), 80% (qualified) |
| SG7, SG8 | 2.0% (premium / first), 8.0% (qualified) | 90% (premium / first), 80% (qualified) |
So roughly speaking: a premium or first grade batch is about 90% fine material passing a 63-micron screen, with a small coarse fraction and a tightly controlled upper limit.
The coarse fraction: 0.25 mm
The material held above a 0.25 mm sieve is the coarse end of the distribution. A wider distribution — more of it, or more variability — has two consequences:
- Slower fusion. The coarse particles take longer to absorb plasticiser. Fusion becomes the rate-limiting step instead of mixing, and the residence time needed to get a homogeneous melt goes up.
- Duller surface. Particles that have not fully fused at the die stay visible as rough surface in the finished profile.
The premium and first grades hold the coarse fraction at 2.0%, which is tight. The qualified level allows 8.0% — four times as much. That is a real difference in how the material behaves, and it is the reason a qualified grade is not simply "premium but cheaper".
The fines: 0.063 mm
The fine fraction is the mirror image, and it causes its own problems:
- Dust and static. Very fine material is difficult to handle in a hopper and generates electrostatic charge that makes it stick to equipment.
- Regrind. Dust that gets into the process comes back as regrind, and regrind in a clear or rigid product is visible.
- Bulk density. A higher fines content lowers the apparent density, which changes how much resin is in the hopper at any given level.
What this means for throughput
Because you are buying a powder, throughput is not simply "kilograms per hour". It is kilograms per hour at the density you ordered. The apparent density figure is the number that tells you how much resin is in your hopper, and it changes with the grade level: 0.45, 0.42 and 0.40 g/ml for premium, first and qualified.
If you are comparing two suppliers on price per tonne, the density difference is worth a calculation before you conclude that one is cheaper — the same hopper volume holds a different weight.
Practical advice
- Running clear work? Particle size and the fish eye figure matter more than the viscosity band. Specify tightly.
- Chasing output? Look at the coarse fraction before anything else. A wide distribution is the usual reason a line that used to run fast now takes longer to reach steady state.
- Handling problems at the hopper? Check the fines content and your bulk density. Also check whether the silo is being over-aerated — static and bridging are often an equipment problem rather than a resin problem.
- Switching supplier? Compare the sieve figures on the certificates of analysis for both batches before you change anything else.
Send us your line speed, your filler loading and the throughput you are trying to hit, and we can tell you which grade level is worth specifying for it.