Processing problems and what usually causes them

Usually the lubricant did not move with the filler. External lubricant controls friction between the melt and the metal, so if loading goes up and the wax stays where it was, the motor carries the difference.

• Check external wax dosage against the new loading rather than the old one.
• Look at the top end of the particle size distribution, not only the D50 — coarse material takes more energy to disperse.
• At the same loading, uncoated carbonate raises melt viscosity more than coated. If you moved from coated to uncoated to save on the tonne rate, this is where it comes back.

This is dispersion, not contamination, in almost every case. The filler is going in as clumps and coming out as clumps.

• Uncoated carbonate agglomerates at high loading. A stearic acid coating is what stops it, which is the main reason activated grades exist.
• Check there is enough external lubricant to wet the filler before the melt forms. Too little and nothing carries the particle apart.
• Check mixing time and the order material goes into the blender. Filler added before the lubricant tends to stay as it went in.

Nearly always over-lubrication, or a lubricant system pulling in two directions.

• Reduce external lubricant first and change one thing at a time. Plate-out responds quickly, so a single step tells you whether you are on the right track.
• An over-coated filler carries free stearic acid that was never bound to the calcite, and it migrates. Ask for the coating level — 1.0 to 1.2% is the working window on our activated grade.
• Check the filler has not picked up moisture in storage, which pushes the same fault.

Three common causes, and they are easy to tell apart by where the fault appears.

• Rough throughout the section points at the coarse end of the filler. The D50 can be on specification while the top end is not.
• Rough only at the surface, with drag marks, points at too little external lubricant at the die land.
• A dull, slightly clouded finish points at moisture rather than particle size.

Either the pigment is not dispersing, or it is being run past its heat limit.

• Check the let-down ratio and whether the carrier is compatible. A pigment that is strong in tint needs less, and under-dosing shows as streaks rather than a lighter shade.
• Wax does dispersion duty for pigment as well as filler. If you cut the wax to fix something else, the colour goes first.
• Watch the temperature against the pigment. Pigment Yellow 12 and 14 are stable to 200–220 °C; alpha blue 15:1 holds to 260 °C. A yellow run at profile temperatures can shift, and no amount of dosing fixes that.

Moisture, in the great majority of cases, and the filler is the usual way it gets in.

• Uncoated carbonate picks up moisture in storage. Coated is hydrophobic, which is why it is the safer choice in a humid godown or a monsoon month.
• Check the bags are intact and stored off the floor, and whether a part-used bag sat open.
• If the fault appeared with no change to the formulation, look at the weather and the storage before you look at the recipe.

Usually the opposite problem to high torque — there is too much lubricant, and the feed zone has lost the friction it needs to convey.

• External lubricant is the first thing to bring back down.
• If torque and feeding are both erratic in the same run, the lubricant balance is off rather than simply high — internal and external do different jobs.

Some of this is arithmetic: filler displaces polymer, and the polymer is what carries impact. The question is whether the drop is proportionate.

• A gradual fall as loading rises is expected. Decide what the floor is for the application and work back to a loading.
• A sudden fall is a dispersion problem, not a loading problem. Agglomerates act as flaws, and each one starts a crack.
• Coating improves the interface between filler and polymer, so a coated grade usually holds impact further up the loading curve than an uncoated one.