Case Study: Vitropurge and Vitrolite

This study was performed at a College Laboratory. This Laboratory is continuously retained by Industry to solve Color Matching and Quality Control issues of numerous thermoplastic polymers.

The study started with a color match in a typical injection molding process using Polypropylene and a deep blue color pigment. The color pigment, Phthalocyanine Blue, is known to be very difficult to clean/purge from a screw injection molding machine. The machine was a 22 ton screw injection molding machine producing two color chips per shot. The machine settings of barrel temperature profiles, injection pressure, hold pressure and screw speed were optimized for the Polypropylene. The machine was operating on an automatic 30 second cycle.

The following procedure was used to clean the blue polypropylene from the machine after the required number of color chips were produced:

  • The same uncolored polypropylene was put in the cleaned machine hopper. No other materials or additives were added to avoid the presence of any uncontrolled variables.
  • The machine remained on the 30 second automatic cycle.
  • Each shot of color chips was visually evaluated for the presence or absence of any remaining blue color including a visual evaluation of any reduction of blue contamination.
  • After 30 consecutive shots a visual evaluation showed the color chips did not show any blue color and the chips appeared to look like the uncolored polypropylene.
  • The color chips were visually consistent in color from chip 30 through chip 35.
  • No further purging of the phthalocyanine was thought to be necessary.
  • Without interrupting the 30 second automatic cycle, Vitropurge at 1.5% by dry weight was dry blended (called “shake and bake” by many operators) with the same polypropylene.
  • This dry blend was added to the cleaned hopper.
  • The FIRST shot of the Vitropurge cleanout material through the molding machine was very blue in color. This clearly showed the blue pigment had not been totally removed from the machine barrel.
  • After 10 additional cycles the polypropylene color chips were totally clear of any color and/or contamination. Visually, the tenth chip appeared better than the thirtieth chip made with just uncolored polypropylene.

This study shows clearly, and without any doubt, that Vitropurge powder at a 1.5% by weight addition accomplishes a superb job of removing residual colorant from the machine. Since Vitropurge is totally inorganic, Vitropurge can be used with any thermoplastic polymer and additionally has no temperature limits for its use.

The huge success of the quick, complete and time saving use of Vitropurge generated a possible follow-on use of a companion product, Vitrolite, to the process described above.

If Vitropurge was so effective in purging, would the use of Vitrolite bring additional improvements? Could the addition of Vitrolite at 0.5% by weight to uncolored polypropylene bring positive improvements above and beyond the optimized machine cycle used in the molding/purge cycle described above. The following study was undertaken:

  • The molding machine setup parameters as described above were unchanged at the start of this experiment.
  • Vitrolite at 0.5% by weight was dry blended with the injection grade polypropylene. No other additives including colorants were added to this batch to preclude any uncontrolled or unknown variables. The injection machine molding machine produced uncolored chips starting with the original machine parameters on the 30 second automatic cycle.
  • At this point, machine settings were altered in sequence allowing for each setting change to reach equilibrium.
  • The changes made were:
    • Reduce barrel temperature profiles
    • Reduce injection pressure
    • Increase injection speed
    • Reduce hold pressure
    • Reduce hold time
    • Reduce the automatic molding cycle time to accommodate the hold time reduction.
  • Chips produced with these changes should result in improved chip physical properties and production cost savings.
  • The final step was to refill the machine hopper with the injection grade polypropylene virgin polymer that did not contain any Vitrolite or any other additive and continue molding at the changed machine settings.
  • The astonishing result observed was the injection molding machine at these new settings from above was unable to mold any complete chips! In fact the machine stalled and was quickly shut down to prevent damage to the machine.

This quick test on the injection molded color chips clearly showed the effectiveness of a 0.5% addition by weight of Vitrolite to the polypropylene will result in better quality products at lower cost. Another advantage would be since the Vitrolite is an inorganic additive, it is not polymer sensitive. This means Vitrolite may well be successfully used with any thermoplastic material.

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