Adding calcium carbonate to PVC is a cost decision, and it is also a processing decision — one that changes which resin grade you need.
What filler does
At a basic level, calcium carbonate makes the compound cheaper per kilogram and stiffer. But the more useful way to think about it is what it does to the fusion step.
Filler particles sit between the polymer chains. The more filler you add, the less polymer there is to transfer plasticiser, and the faster the compound fuses. That is usually a benefit — a faster fusion cycle means more output per hour and less heat history in the material.
The trade-offs come with it:
- Melt strength drops. This is the big one. Filler reduces the strength of the melt, which makes the compound more fragile as it leaves the die. On high-speed film and blown film this shows up as problems with draw-down and with die swell.
- Impact strength falls. Relevant if your product has to survive handling.
- Density rises. A 30% filled compound is noticeably heavier per unit volume than an unfilled one, which changes the shipping weight and the finished product weight.
Why this changes the grade
Because filler changes how the melt behaves, the grade that was right at one loading is not necessarily right at another.
A practical example: a rigid pipe compound at 30% filler runs fast and fuses cleanly on SG5. Take the same formulation to 50% filler and the melt strength has dropped enough that you are closer to the edge of the process window — many lines move to a higher viscosity grade to get the melt strength back.
For soft compounds the direction can reverse. A heavily filled soft compound at low viscosity can become fragile and short-moulding, and the answer is again usually a grade change rather than a process change.
How to work it out
The three variables that interact are:
- The filler loading — by weight, and by volume.
- The processing method — extrusion speed, die and cooling conditions.
- The end product — specifically, whether it needs impact strength or whether it needs to be cheap.
In practice:
| If your compound is… | You are likely to need… |
|---|---|
| Rigid, heavily filled, high output | A mid-viscosity grade that still fuses fast — SG5 |
| Soft, lightly filled, needs strength | A higher viscosity grade — SG3 or SG4 |
| Soft, heavily filled, cheap | The highest viscosity grade you can run without processing problems |
| Clear, any loading | A premium grade level, and usually a low loading |
What to tell us
The most useful thing you can put in an enquiry is the formulation as it actually runs:
- the filler loading in weight percent
- the filler type and grade (coated or untreated, coarse or fine)
- the processing method and the line speed
- the end product and what it has to survive
With that we can tell you which grade and grade level to order, and if your current grade is no longer the right one we will say so rather than sell you the same thing you already have.
One thing to be clear about: we supply the resin. We do not supply filler, and we do not run your formulation. But we have seen enough compounds to be useful about which resin a given formulation needs.