Almost every first enquiry we receive asks the same question in different words: SG5 or SG3?
The short answer is that they are not interchangeable, and the choice is mostly about whether your product is rigid or soft — with one important exception for clear work, where the deciding factor is something else entirely.
The numbers
| SG5 | SG3 | |
|---|---|---|
| Viscosity band | 118-107 ml/g | 136-127 ml/g |
| Impurity particles (premium) | 16 per 40 cm² | 16 per 40 cm² |
| Apparent density (premium) | 0.45 g/ml | 0.45 g/ml |
| Fish eyes (premium) | 20 per 400 cm² | 20 per 400 cm² |
| Main uses | Transparent products, rigid pipe and fittings, monofilament, sleeving, profiles, PVC-U windows | Electrical insulation, agricultural film, irrigation hose, artificial leather, footwear, general soft products |
Viscosity is the polymer's molecular weight expressed in a way you can measure. Higher viscosity means a higher molecular weight, and that cascades into everything else: mechanical strength goes up, fusion slows down, and the processing window narrows.
When you want SG5
SG5 is the rigid-work grade. If you are extruding pipe, profiles, window frames or fittings, this is where you start. The lower viscosity fuses faster, which matters when you are running a rigid extrusion line where the material has to be on the machine for a controlled time before it degrades.
It is also the grade used for transparent work — clear rigid sheet, transparent packaging, clear fittings. If your product has to be see-through, SG5 is the usual starting point.
When you want SG3
SG3 is the soft-goods workhorse. Agricultural film, irrigation hose, artificial leather, footwear, general soft extrusion. The higher viscosity gives a stronger melt, which is what you want when you are drawing down a film or running a soft profile that has to resist tearing.
The exception: clear work
If your product is transparent, the viscosity band is not your deciding factor. Clarity is.
What you actually need to look at is the impurity count and the fish eye figure, because both of those show up directly in the finished product. A fish eye is an inclusion in the resin itself — not contamination, a defect in the polymer — and in a clear sheet it is a visible speck that cannot be screened out.
This is where the grade level earns its keep. Premium holds 16 impurity particles per 40 cm² and 20 fish eyes per 400 cm². First grade holds 30 and 40. Qualified holds 90 impurity particles. If you are running clear film, the difference between premium and first grade is the difference between a sellable product and a rejected batch.
The same logic applies to transparent rigid sheet and to bottles, where the tightest grades are normally specified.
A note on filler loading
Whichever grade you pick, the calcium carbonate loading in your formulation shifts the balance. More filler means a stiffer, cheaper compound that fuses faster and raises the density of the finished product. It also changes how the resin behaves in the melt, so a compound that works at 30% filler may not behave the same way at 50%.
If you tell us your filler loading and your end product, we can tell you which grade and grade level to order — and if your compound needs a different grade than the one you have been buying, it is usually because the filler ratio changed rather than because the resin did.
How to check what you actually received
Every shipment comes with a certificate of analysis covering the batch: grade, grade level and the published indicators for that batch. The table above is the specification; the certificate is the measured result for your lot. Comparing the two is the quickest way to tell a supply problem from a formulation problem.
Still deciding?
Send us the end product, the filler loading and the quantity, and we will tell you which grade and grade level to order. The chat assistant on this site records the whole requirement and it reaches the sales desk directly.