Raw Material Blending and Mixing
Depending on the product requirements, some pre-blending or ingredient mixing may be required prior to extrusion. (Blending and mixing are covered in more detail in Part 5, “Auxiliary Equipment.”) Unless a single polymeric material is being added to an extruder, the best way to combine different raw materials and keep them uniformly distributed prior to entering the extruder feed throat depends on different factors. Some factors to consider are:
• Separation of powder and pellets
• Uniform distribution of additives introduced at low concentrations
• Separation of ingredients in flood fed hoppers
• Proper mixing
• Introduction of different levels of regrind and/or the effect of regrind particle size
• Addition of liquid additives to a single screw extruder
• Uniform distribution of powder/powder blends
The best way to meter materials and guarantee uniform component distribution is to gravimetrically feed each component with different feeders directly above the extruder feed throat. Assuming there are enough space and feeders to accommodate the various components in the formulation, gravimetric or loss-in-weight feeding ensures each component is added in the correct proportion, while addition directly above the feed throat minimizes any ingredient segregation. The downside of this approach is the cost of gravimetric feeders, the space required if there are more than four or five components, and if different size feeders are required. Assuming some components are added in very low concentrations (<1%) while other components are added in high concentration (>15%), the feeder size, feeder accuracy, and material (powder, pellets, flake, free-flowing versus compressive powder, fiber, etc.) being fed are critical to the feeder performance. If all feeders are properly sized, designed for the materials being fed (single screw feeder, twin screw feeder, vibratory, weigh belt, etc.), and there is enough room to use a gravimetric feeder for each component, multiple feeders is the best method to ensure a repeatable, uniform formulation is being introduced to the extruder.
In many applications, a feeder is not available for each ingredient, requiring pre-blending. Blending depends on the ingredients being mixed and the way material is handled after blending and prior to extrusion. Assume pellets A and B are approximately the same size and are required to be premixed; proper concentrations of A and B are individually weighed and added to low intensity
blending systems. Typical low intensity blending systems include tumble blenders (wide range of sizes), Vcone blender, ribbon blender, cement mixer, drum roller, or paint shaker for small lots. The same equipment can be used to mix pellets and powder. However, pellets and powder are more likely to separate when transporting the blend or loading it to a feed or extruder hopper after the blending is complete. The powder can flow between the pellets; consequently, at the beginning of an extrusion run the product may be rich in the powder component, while at the end of the extrusion run the product may be rich in the pellet component. One method to minimize this separation is to coat the pellets with a small amount of liquid such as mineral oil to provide a surface to which the powder can adhere. Of course, experimentation is required to verify the mineral oil does not affect the final product properties or performance.
Powder/powder blends can be mixed either in low intensity mixers described above or in high intensity mixers. High intensity mixers operate on the same principle as kitchen blenders. A mixing blade rotates at high speed, forming a vortex in the blender as it mixes the components. Due to the intense mixing, heat is generated and care must be taken not to melt the blend components.
High intensity blenders may be jacketed to heat components or remove heat during the blend cycle. With PVC, heat softens the particle surface, allowing the heat stabilizers and plasticizers to adhere to the surface. Powder/powder blends, once properly mixed, tend not to separate during transfer, assuming the particle sizes of the different components are similar.
Uniform additive addition at low concentrations creates a mixing and blending challenge. Obviously, the best method is to feed each component directly into the feed stream with a small gravimetric feeder. However, this is not always practical or feasible. An alternative approach is to mix the additive (assume it’s a powder) with some resin powder being used in the formulation and produce a masterbatch on a high intensity mixer. As an example, assume two additives, C and D, must be added at 0.5% and 0.08%, respectively, to resin B to produce a profile of material Z. A blend or masterbatch is produced by combining resin powder B with high concentrations of C and D and letting that blend down in an individual feeder. The masterbatch is added using feeder #1, and pellets of B are added via feeder #2 to produce the correct ingredients ratio in the final product. A 100-pound masterbatch is produced containing 10 pounds (10%) of component C,
1.6 pounds (1.6%) of component D, and 88.4 pounds (88.4%) of resin B. This masterbatch is let down in a 19:1 ratio, with resin B feeding at a 190 lbs/hr rate and the masterbatch feeding at a 10 lbs/hr rate to produce the correct additive ratio in the final product Z. If components C and D were fed directly to the extruder at 0.5% and 0.08%, the feed rate for each component would be 1.0
lbs/hr for component C, 0.16 lbs/hr of component D, and 198.84 lbs/hr of resin B to produce a 200-lbs/hr rate of product Z. Using a masterbatch makes feeding small concentrations of ingredients uniformly more practical.
The addition of a liquid colorant or other additive to a single screw extruder can be difficult. If the liquid is added in low concentration, it can be preblended with pellets in a tumble blender, ribbon blender, etc. Liquid can be introduced into the feed throat with a liquid feed pump, assuming the liquid does not create extruder feed problems due to pellet slippage on the barrel wall. It is critical to monitor the liquid feed rate very carefully. When using a gravimetric or loss-in-weight liquid feed pump, the pump rpm changes to keep the gravimetric feed rate constant. However, assume a volumetric liquid feed pump is being used (runs at constant rpm); the feed rate is dependent on the liquid temperature, which affects its viscosity and consequently the feed rate. Initially, a volumetric liquid feed pump must be calibrated and a graph generated showing motor rpm versus output rate in lbs/hr. On the same graph, throughput rate curves versus motor rpm
curves need to be generated at different temperatures. If the liquid temperature changes during the run, the feed rate will vary and the liquid concentration will change over time. Assuming the liquid additive and feed pump are close to the extruder, it is possible the liquid temperature may increase during the run as the ambient temperature in the room increases due to the heat generated by the extruder. This results in an increase in the liquid feed rate and the wrong product formulation, unless the liquid feed pump rpm is changed to compensate for the temperature
change or a gravimetric liquid feed pump is being used.
An alternative to feeding liquid into the feed throat is to pump the liquid into a two-stage extruder vent port. Modifying the vent port to accept a liquid injection nozzle connected to the liquid feed pump and a two-stage screw to accept the extra volume are the changes required. This approach eliminates the potential for feed problems associated with pellet slippage on the barrel wall in the feed zone due to the liquid. Feeding downstream does minimize mixing in the extruder and requires an appropriate screw design or static mixer to accomplish the mixing objectives.