Each SG grade is produced in three grade levels. They are the same polymer chemistry at the same viscosity — what differs is how tightly the batch is controlled on the indicators that affect the finished product.
What actually differs
Looking across the published indicator table for the SG grades, the grade level changes three things and leaves the rest alone.
| Indicator | Premium | First grade | Qualified |
|---|---|---|---|
| Impurity particles (per 40 cm²) | 16 | 30 | 90 |
| Volatile matter incl. water | 0.3% | 0.4% | 0.5% |
| Apparent density | 0.45 g/ml | 0.42 g/ml | 0.40 g/ml |
For SG7 and SG8 the impurity figures are 20, 40 and 100 respectively.
The fish eye figure differs too, but only between premium and first grade: 20 and 40 per 400 cm² for SG1 to SG6, and 30 and 50 for SG7 and SG8. The qualified level does not publish a fish eye figure.
What does not differ is the viscosity band, the particle size distribution and the residual vinyl chloride figure. Those are properties of the grade, not of the grade level.
How to read that
Impurity particles are foreign matter counted over a 40 cm² area. Every speck of it ends up in your product. In a clear film it is visible; in a pipe it is a weak point.
Volatile matter is residual water and light volatiles. Too much of it gives you bubbles, silver streaks and porosity in extrusion — and it moves the weight on the scale, which matters when you are buying by the tonne.
Apparent density tells you how much resin is in a given volume of hopper. A denser grade means more resin per cubic metre, which changes your feeding rate and the economics of a bulk delivery.
Fish eyes are inclusions in the resin that show up as specks in clear sheet. They are the single most visible difference between grade levels.
Which level to buy
Premium if you run clear or transparent work — film, rigid sheet, bottles, fittings. The tighter impurity count is the whole point, and if it is not visible in your product you are paying for a specification you do not need.
First grade if you run opaque rigid work: pipe, profiles, window frames, conduit. The impurity count still matters for pipe wall quality, but you are not paying for optical clarity you will never see.
Qualified if you run general soft extrusion where cost is the dominant consideration and the product has no cosmetic requirement.
The honest caveat
Buying a grade level you do not need is a straightforward way to overpay. Buying one below what your process needs is a more expensive mistake — it shows up as rejects, and it is usually harder to diagnose because the cause is spread across the batch.
If you are not sure which level to take, describe the end product and how tight your customer's own specification is, and we will tell you which one to order. We would rather quote the right grade than the highest one.
Every shipment includes a certificate of analysis for the batch, so you can check what you actually received against what you ordered.