Powder Blending in Plastics Compounding

A compounding blend is usually a coarse resin with a set of fine additives at a few percent or less: stabilisers, antioxidants, flame retardants, pigments, processing aids. The particle size difference between the resin and the additives is exactly the condition that produces segregation.

What makes it visible is the extruder. Feed variation becomes a property variation in the compound, and a flame retardant or a stabiliser that is unevenly present produces a product that fails a test rather than one that looks different.

Static then complicates handling. Polymer powders charge readily, which makes fine additives stick to surfaces and to the resin unevenly, and a blend that looked right in the mixer can arrive at the throat with the additives on the hopper wall.

And the dust is a hazard. Polymer fines and many additives are explosible, so the mixer, the transfer and the hopper are equipment inside a dust explosion risk even though the pellets everyone is looking at are not.

A blend that presents the additive package to the extruder at the intended level continuously, not on average, with the route from mixer to throat designed to preserve it.

Static managed by grounding and, where necessary, by carrier selection, so additives stay with the resin instead of on the equipment.

Explosion protection and bonding designed for the dust that is actually present, with the whole blending and transfer system treated as one system.

In battery material production the required homogeneity scale is set by the electrode, and the hardened mixing element that survives the duty is also the source of the iron.

The formulation and the handling are looked at together. Where an additive can be supplied on a carrier of similar particle size to the resin, or as a masterbatch, the segregation driving force is removed rather than managed. That is usually cheaper than any equipment answer.

Where powders are blended directly, the mixer is chosen for its ability to distribute fines into a coarse bed: convective rather than tumble, with attention to whether the additive is being distributed or just carried along.

After the mixer the route matters more than the mixer. Short, direct, with mass flow hoppers and minimal drop height, and loss in weight feeding of the additive at the throat where accuracy really matters. Many plants find that feeding separately at the extruder solves what no blending change could.

Grounding and bonding are designed as a system and measured. Hoppers, liners, flexible connections and the mixer itself are bonded, continuity is tested at commissioning and periodically after, and the explosion characteristics of the fines are measured rather than assumed from the polymer grade.

• Additive supplied on a matched carrier or as masterbatch where segregation can be designed out.
• Convective mixing where fines have to enter a coarse bed.
• Short direct route to the extruder with mass flow hoppers and low drop heights.
• Loss in weight feeding at the throat where additive accuracy is critical.
• Full bonding and grounding of mixer, hoppers and flexible connections, measured.
• Explosion characteristics measured on the fines actually present in the plant.

• Particle size ratio between resin and additives, which is the segregation driving force.
• Additive tolerance of the product specification, since some additives fail a test rather than a sight check.
• Static behaviour of the resin and additives and the grounding concept.
• Route to the extruder, counted in drops and metres rather than described.
• Explosibility of the fines, measured on plant material.
• Whether separate feeding at the throat is a better answer than blending at all.