Influence of melt pump on extrusion processing of recycled materials

Plastic recycling not only helps to reduce carbon emissions, but also reduces the amount of plastic that ends up in landfills, rivers and oceans. The vacuum system helps to remove the entrained gas and can assist the reaction in the process of solid recycling of PET, HDPE, LDPE, PC, ABS and even liquid recycling of PET.

The use of a large amount of recycled materials in the processing process often affects the stability of the output. In addition, the widespread use of recycled materials purchased from various sources in order to reduce product costs and improve the impact on the environment makes this problem more complicated. This is because, compared with general internal recycled materials, recycled materials from many different products have more changes in their bulk density and feeding characteristics after the pelletizing equipment is resized. In the product molding process, pure resin mixed with recycled materials will make the production process more and more complicated.

When the overall bulk density of recycled materials (pure resin plus recycled materials) changes more than 20% to 25%, the extruder screw is no longer competent to control the changes in product performance. The screw is a volumetric device. For recycled materials with very different bulk densities, the screw's ability to adjust the bulk density of the recycled materials is limited. For a specified screw, the specific volume of the screw groove of the feeding section is constant. If the bulk density of the recycled material changes, the amount (weight) of the polymer entering the feeding section will increase or decrease with the change of the bulk density.

Most screws are designed to be suitable for the processing of pellets, but they are built-in and have a certain flexibility to receive a certain percentage of low bulk density recycled materials. Of course, this is achieved by using a compression ratio greater than that required for full pellet feed. In addition, to achieve this goal, you need to understand the bulk density of the material and the particle flow characteristics of various recycled materials.

If you need to use a melt pump to control the product size during the process, and you want to use different amounts of recycled materials, you must face this situation. There are several things you can do to minimize this instability. First of all, it is necessary to "reduce" the response of the screw speed to pressure changes at the discharge port. This is easy to achieve, only by significantly increasing the proportional band or using a PID controller (which acts as an interface between the suction pressure and the screw speed). This causes the controller to ignore rapidly changing bulk density.

Second, increase the pressure of inhalation. For a given screw design, the exhaust pressure level determines the degree to which the screw is filled with material. When the feeding rate is changed, the material filling will change continuously, and the filling length of a given feeding moment will affect the output output. By increasing the suction pressure, the filling volume increases and the filling length increases, so that the percentage change of the exhaust pressure becomes smaller.

Third, it is not incremental changes but significant changes that make these adjustments easier to achieve. For example, the minimum increase in suction pressure should be 300 pounds per square inch, but the best significant change may be 500 pounds per square inch. Similarly, after regaining stability, only fine-tuning is required.