Stone paper is 60–80% calcium carbonate held together by a small amount of polyethylene. That ratio is the whole business. Push the filler up, and the cost falls. Push it too far and the film tears at the fold.
Most formulation problems on a stone paper line look like machine problems. Screw wear, die drool, gauge variation, pinholes. In our experience, the cause usually sits in the recipe, not the hardware.
Three levers do most of the work: filler loading, filler quality, and binder choice. Change one at a time. kkjko
Start with the filler
◆Loading. Film grades usually run 65–75% by weight. Below 60%, you lose the cost advantage. Above 80%, elongation collapses, and the screw works harder for less output.
◆Particle size. A d50 of 2–8 µm suits thin film. Coarser filler, 10–20 µm, disperses easily and costs less, but the surface gets rougher and tear strength drops. Very fine filler under 2 µm gives opacity and strength, and fights you on torque.
◆Coating. Stearic acid at 0.8–1.5% on filler weight is the norm. Too little and the particles agglomerate. Too much and the melt slips, the coupling fails, and plate-out appears on the die.
◆Moisture. Dry to below 0.3%, ideally 0.1–0.2%. Moisture shows up as pinholes and bubbles, and it looks exactly like a venting problem.
Then the binder
HDPE is the backbone. An MFI of 0.3–2 g/10 min is the working range. Higher MFI flows better and lowers torque, at the cost of tear strength.
Blend in LLDPE, typically 20–35% of the polymer fraction, when the film has to survive folding and bag-making. It buys elongation cheaply.
Add a coupling agent, usually PE-g-MAH at 1–3%, once loading passes 70%. It improves the filler–binder bond, lifts tensile strength, and cuts dust on the line. Skipping it is the most common false economy we see.
Watch the lubricant balance
Internal lubricants, PE wax and similar, lower torque and help dispersion. External ones, zinc stearate, reduce die pressure and improve release.
Too much external lubricant is the classic mistake. The film runs beautifully, then fails at the seal and refuses printing ink.
Keep melt temperature between 180 and 210 °C. Stearic acid starts to volatilise above roughly 220 °C. That is where die drool and the burnt smell come from.
A starting point
Component Share by weight
Calcium carbonate, coated, d50 3–5 µm 70%
HDPE, MFI 0.5–1 22%
LLDPE 6%
PE-g-MAH 1.5%
Lubricant package 0.5%
Run it at 180–200 °C on a 40–48 L/D twin screw with a coarse screen pack, 40/60 mesh. Bimetallic barrels are worth the money. Abrasive filler will eat a standard one.
Then move one variable at a time, and keep the previous sample.
Seven variables, one table
Variable Typical Raise it and you get The cost
Filler loading 65–75% Lower cost per kg Brittleness, screw wear
Particle size 2–8 µm d50 Easier dispersion Rougher surface, weaker tear
Stearic coating 0.8–1.5% Fewer agglomerates Plate-out, poor coupling
Moisture under 0.3% - Pinholes, bubbles
Binder MFI 0.3–2 Lower torque Lower tear strength
LLDPE share 20–35% Better elongation Slightly softer film
Coupling agent 1–3% Stronger bond, less dust Added cost per tonne
Measure before you change anything
Four numbers tell you whether a change worked.
Ash content by burn-off gives the real filler percentage. Suppliers drift. So does your own dosing.
Density should land between 1.2 and 1.5 g/cm³. A sudden drop usually means voids or moisture.
Tensile and tear in both directions, machine and cross. Stone paper is anisotropic, and cross-direction tear is where bags fail.
Check dispersion on a thin film under a microscope or with a pressure-filter test. Agglomerates invisible on the line become pinholes on the customer's shelf.
Three changes that backfire
Adding lubricant to stop die drool. Drool is usually volatiles or moisture. More lubricant hides it for a week, then makes the seal fail.
Raising temperature to improve dispersion. It lowers viscosity and raises volatilisation at the same time. Fix dispersion with screw geometry and coating, not heat.
Raising filler to win a price. It works until the first complaint about tearing, and then the whole batch is suspect.
About the author
Written by the technical and export team at Baoding Yashen Technology Co., Ltd., Baoding, Hebei. Founded in 2013.
We make PVC curtain strips, PVC sheets, food-grade PVC wrap, phenolic rods, and the industrial machinery that processes them. Our materials carry CE, FDA, RoHS, and REACH certifications.
We ship to 100+ countries, from Southeast Asia to the Americas and the Middle East.
We supply the whole chain: raw materials, machines, formulas. Formulation work is the part customers ask about last and argue about the longest.
Send us your current recipe, the screw diameter, and the fault. We will tell you which variable to move first.
