Raw Material Blending and Mixing

 Raw Material Blending and Mixing for Extrusion

Manufacturers often blend or mixing multiple ingredients before extrusion to meet the final product’s specifications. When you use more than one polymer or additive, you must ensure uniform distribution before materials reach the extruder feed throat.

Key Challenges in Blending Multiple Raw Materials

• Separating powders from pellets

• Distributing low-concentration additives uniformly

• Avoiding ingredient segregation in flood-fed hoppers

• Controlling particle size and regrind levels

• Adding liquid additives to single-screw extruders

• Mixing powder/powder blends consistently

Best Method: Gravimetric Feeding Above the Feed Throat

You ensure precise formulation by installing gravimetric (loss-in-weight) feeders for each component directly above the feed throat. This setup provides:

• Accurate metering of resin, fillers, and additives

• Minimization of segregation during feed

• Stable and uniform material supply

However, this method requires sufficient factory space and budget for feeders—especially when blending three or more ingredients or handling varied material flows (e.g., <1% additives vs. >15% resin).

Pre-Blending When Feeders Are Limited

If you cannot install a feeder for each component, use pre-blending in a low-intensity mixer:

1. Weigh pellets A and B to target ratios.

2. Use a tumble blender, V-cone, ribbon blender, drum roller, or paint shaker.

3. Blend pellets and powders together.

4. Add a light coat of mineral oil to pellets before adding powder—this helps reduce segregation during transport and hopper loading.

5. Perform trials to ensure oil won’t harm material properties.

Mixing Powder/Powder Blends

Use low- or high-intensity mixers for powder/powder blends. High-intensity mixers, similar to kitchen blenders, spin fast enough to create a vortex and thoroughly mix your batch. Many models include jackets for heating or cooling. These systems help—especially with PVC blends—by making additives like stabilizers and plasticizers adhere to pellet surfaces. Properly mixed blends resist separation during feeding.

Handling Low-Concentration Additives—Masterbatch Approach

To add trace additives uniformly:

1. Pre-mix resin with high concentrations of additives (e.g., C at 10%, D at 1.6%) in a masterbatch.

2. Feed masterbatch + base resin via two feeders:

   • Feeder #1: masterbatch (e.g., 10 lb/hr)

   • Feeder #2: resin (e.g., 190 lb/hr)

3. This blend achieves the required final concentration (e.g., 0.5% C, 0.08% D) while keeping feed rates manageable and precise.

Adding Liquid Additives to Single-Screw Extruders

Liquid additives present unique challenges:

• Pre-blend small quantities with pellets (in tumble or ribbon blenders)

• Alternatively, pump liquid additives directly into the feed throat, but watch for pellet slippage

• Use gravimetric liquid pumps for consistent dosing

• If using volumetric pumps, calibrate pump rpm vs. feed rate at different temperatures

• Monitor ambient temperature near the pump to avoid feed-rate drift

Alternative: Downstream Liquid Injection

To avoid slugging in the feed zone:

1. Modify a two-stage extruder’s vent port to include a liquid injection nozzle

2. Use a small side injection screw to integrate the liquid into the melt

3. Downstream injection improves feed reliability but demands proper screw design or static mixing to ensure full integration