How Plastic Material is Made : Extrusion

Plastic injection processors mold material every day .. but do you know how it is produced? There are keys in the extrusion process that can sometimes interpret potential problems on the molding floor. This artticle will explain the process and its relevence to plastic injection.

Material starts as a mixture of powders.. there is a base (such as ABS, Polypropylene,etc.), sometimes additives to improve the blend (in ex: UV to reduce sun damage, or fiberglass to increase rigidity) and pigmented colors to give the material its desired hue.

The extrusion press has similar characteristics of its injection counterpart...it has a screw for blending and moving the material forward. The barrel containing the screw has heater bands to help control primary shear heat. The difference is, the screw does not shoot material...it merely spins. As it rotates, the material is forced out a faceplate with holes drilled into it called a "head". As the material comes out of the holes, it is formed into strands.

These material strands are pulled by the machine operator down a long trough, called a "water bed". The strands run at the bottom of the bed, being held in place by a series of rollers to keep the strands submerged. Towards the end of the bed, the strands are allowed to come above the water through several sets of brushes. The brushes are used to wipe the water off of the strands.

At this point,the strands run through a series of air blowers to remove the remainder of the water. The strands run from the blowers into a large machine similar to a grinder, called a pelletizer. The pelletizer cuts the strands into uniform chunks...the pellet that is familiar to you. Pellet length is controlled by the speed (RPM) that the pelletizer turns. The faster the blades turn, the shorter the pellet size. The pellets also go over a vibrating tray, designed to remove fines.
Pellet diameter is controlled by screw RPM...the faster the screw is ran, the larger the pellet diameter will be.

Now that we have outlined the extrusion process, we will identify elements that can affect the plastic injection side of the equation:

-material changeover: The screw is pulled often when changing material types (such as polypro to ABS) or when going from a dark to light color. This is why it is important to pay attention to lot changes. The beginning and end of an extrusion process is when problems with lots are most likely to occur. When problems point towards a material issue, the lot should be checked for a)whether it was a beginning of lot container and b) material should be checked visually for consistency if it was produced at the end of lot. Pellet diameter and length are important dimensions.

-Base material: The potential for a mistaken base material or improper blend of base, additives and pigment exists. This is generally going to happen at the beginning of a material lot.

-contamination: When running light or clear materials, one of the first things to check when there are scrap issues associated with contamination is a visual inspection of the material being used for contamination within the pellets, and whether the lot being used has changed or is a beginning of run container. The potential exists for an improper screw cleaning, or a contaminated blend receptacle used to blend the powders. The potential for excessive fines (material ran through vibration portion of pelletizer too fast) also exists.

Inspecting for these things can sometimes help you better understand where a problem's roots lie. Most extrusion manufacturers are happy to work with you when problems are found. It takes both extrusion and injection working together to develop product efficiencies.