Common Problems of Plastic Extrusion

Common Problems of Plastic Extrusion

To ensure successful extrusion manufacturing, every parameter must be identified, controlled, and monitored. Some of the variables are based on equipment, others on operating conditions. The variables range from the quality of the die and materials to temperatures and pressures.

We recently discovered a challenging situation with the growth of a product line. Product line growth, of course, is a very good situation to be in. However, when this particular product was developed, we expected the order volume to remain low since it was a variation of a high-volume product we were already manufacturing.

To save costs, we had created tooling from a "hodge-podge" of existing dies/calibrators. The tooling worked OK for small volume but when the order volume grew rapidly last year, we experienced high scrap rates that previously had gone somewhat unnoticed.

We then evaluated the tooling from a fresh perspective and discovered our "hodge-podge" wasn't creating the necessary back pressures for proper melting, and that the tooling lead-in and other characteristics were improper. New, properly-designed tooling was developed which led to a significant improvement in product scrap rates.

Some preventative measures can be taken before the manufacturing process even begins. For example:

  • Equipment instruments used to monitor temperature, pressure, RPM and amperage should be calibrated twice a year so the readings don't drift over time.
  • A steady-state process of resin is essential. This begins with how it's stored. It needs to be a in a clean, dry area, without being subject to extreme temperature variation. To ensure that resin is consistent, some companies conduct resin analysis to measure the material and record density, melt index, shear rate vs. viscosity data, and tensile strength. If it's too dry, it may not melt and the resin can't be processed. If a vendor changes from one batch of resin to another mid-manufacturing process, the result can be an altered product. During material lot changes, we monitor machine variables more closely in order to make adjustments as needed to maintain efficient extrusion processing and eliminate quality issues with the product.

Typical extrusion problems fall into a few main categories: aesthetic flaws (e.g., pits, black specs, pinholes, drag marks, die lines, sink marks); size variance (which can be intermittent or contiguous); and dimensional variations.

When a problem occurs, the first step is to examine the components of the manufacturing process starting with the equipment. It's possible the screw design and/or temperature is causing the problem, or the die itself wasn't properly made, or the temperature for a particular zone in the extrusion line is incorrect. Listed below are the main variables that need to be examined when the extruded product doesn't meet finished-product specifications/dimensions:

  • Melt pressure
  • Melt temperature
  • Temperature of the barrel
  • Temperature of the die
  • Heater power
  • Cooling power
  • Speed of the screw
  • The motor load in amps
  • The speed of the line
  • Die wear or improper design (as in the example in the first paragraph)

Extrusion experts say if a normally-stable process isn't working correctly, the extrusion technician usually will start to troubleshoot by comparing the current process conditions (i.e., temperatures, pressures, material lot, etc.) to those when the manufacturing problem didn't exist. Usually tooling, once "tuned-in" and properly maintained, will not be the root of the problem. However, as in our experience with the "hodge-podge" tooling set, even dies/tooling need periodic evaluation to ensure they are adequate for the current volume of orders.