Method to Decrease Die Swell

Modifying the molecular structure of HDPE in the extruder using free-radical initiators such as peroxides can reduce die swell (bottle weight) and broaden the MWD of the base resin to provide faster bottle fabrication rates [34,35]. The addition of 100 ppm of an organic peroxide to HDPE prepared with a Phillips catalyst lowered bottle weight 4.5%, as shown in the data in Table 6.13. 

The addition of air (40-160 ppm based on resin weight) to the extruder has also been reported [36] to modify the molecular structure of HDPE resin with either a unimodal or bimodal molecular weight distribution. The 

Sample PPM Peroxide MI H FI/MP ADSf Bottle Weight (grams) 

The weight and diameter of the parison is controlled by the geometry of the annular die opening and the resin being extruded. As the molten polymer is extruded through the die, it swells to a thickness and diameter that is greater than the dimensions of the die opening. The amount of swell is dictated by a complex relationship between the internal shape of the die and the viscoelastic properties of the polymer. 

Small-scale tests are generally best for process quality control. When developing a product using a new catalyst or a new process, the small-scale tests can only provide general guidelines that must be periodically field tested to ensure the desired performance. 

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