At the low end of the published range, SG7 and SG8 cover the products the rest of the range cannot do well: thin-wall bottles, rigid injection-moulded fittings, and chlorinated PVC resin production. Their specification differs from SG1 to SG6 in a few places, and those differences are the substance of this guide.
The grades and their published uses
| Grade | Viscosity band (ml/g) | Published uses |
|---|---|---|
| SG7 | 95-87 | Bottles, transparent sheet, rigid injection-moulded fittings, chlorinated PVC resins |
| SG8 | 86-83 | Bottles, transparent sheet, rigid injection-moulded fittings, chlorinated PVC resins |
These are the lowest viscosity bands in the range, which means the lowest molecular weight and, in the melt, the easiest flow of the eight grades.
Why a free-flowing melt suits this work
Injection moulding is the demanding process in rigid PVC. The melt has to fill a multi-cavity mould through narrow runners, around cores and into thin sections, all before it stiffens. Lower molecular weight lowers the melt viscosity, and that is what lets a rigid formulation fill the cavity cleanly at a workable injection pressure.
Bottles fall into the same logic: thin walls, a long flow path, and a surface that shows every flow defect. The transparent sheet application is there for the same reason — these grades are specified for clear rigid work, where the grade level on impurities and fish eyes decides the optical result.
Between the two, SG8 is the more fluid melt and the faster fuser; SG7 holds a little more strength in the finished part. Mould complexity, cycle time and the part's mechanical requirements settle the choice, and it is worth making that decision with the technical manager before the specification is fixed.
The CPVC feedstock use
The published application list for both grades includes chlorinated PVC resins — the feedstock for CPVC. This is a distinct industrial use: the resin is supplied to a chlorination process rather than to a plastics processor. Buyers in that line source against the same published specification as everyone else, and the certificate of analysis travels with the shipment in the usual way.
The SG7/SG8 indicator set
The manufacturer publishes a separate indicator set for these two grades. Three rows differ from the SG1-to-SG6 set:
| Indicator (SG7/SG8) | Premium | First grade | Qualified |
|---|---|---|---|
| Impurity particles (per 40 cm²) | 20 | 40 | 100 |
| Apparent density | 0.48 g/ml | 0.45 g/ml | 0.40 g/ml |
| Fish eyes (per 400 cm²) | 30 | 50 | not specified |
The rows that stay the same as SG1 to SG6 are volatile matter (0.3% / 0.4% / 0.5%), the sieve fractions (2.0% retained on 0.25 mm; 90% passing 0.063 mm at premium and first grade) and the residual vinyl chloride figure (8 mg/g premium, 10 mg/g first grade).
Two of the differences are worth reading closely. The apparent density of premium SG7 and SG8 is 0.48 g/ml against 0.45 for the rest of the range — a denser powder, which feeds more resin per hopper volume and is worth a calculation when comparing bulk prices. And the fish eye limits are slightly looser than the SG1-to-SG6 set (30 and 50 against 20 and 40), which is one more reason a transparent application should specify premium rather than first grade.
What to send us
Tell us the product — bottle, sheet, fitting or CPVC feedstock — the quantity and the destination port. For clear work, say so: we will quote premium as the starting grade level and explain what the fish eye figure means for your product.