Tips for Injection Molding

» When using a manual purge on heat-sensitive materials, such as PVC, it is important to keep the following two things in mind:

  1. After processing is complete, the barrel should not be emptied or else oxidation of the material can occur in as fast as 10 minutes.
  2. Generally, temperatures should not be raised above the processing temperature of the heat-sensitive material or it will degrade quickly.  

» Knowing or estimating your barrel capacity prior to purging will allow you to carefully control the amount of purge being used – typically 1-3 barrels full of product are sufficient. This will allow you to control inventory, and minimize costs.

» When using mechanical or chemical purges, follow these 3 rules to promote agitation:

  1. Use maximum safe screw speed while purging
  2. Use maximum safe backpressure while purging
  3. Keep the screw completely forward while purging

» When using chemical purges, follow the manufacturer’s instructions on soak times – it is critical that the material have time for the chemical reaction to occur, as well as any resulting expansion. Typical soak times are in the 15-20 minute range.

» Chemical purges usually perform best when front barrel zones are raised to maximum allowable temperature, not to exceed the manufacturer’s recommended temperature.

» All commercial purging compounds are different. Some are better at cleaning the screw flights, for instance, while others are better at cleaning hot runners. If your purging compound is not meeting your needs, try another brand or grade.

» Varying screw speed, or stopping and starting (“Disco Purge”) can help dislodge stubborn deposits.

» Once the screw and barrel are clean, use another half barrel of purging compound to perform a series of high velocity air shots to clean the check ring and nozzle.

» Generally, manual purging compound producers recommend never running the screw empty. Whether purging or not, an empty screw can allow oxygen to react with the plastic, and decomposition will begin. If a machine needs to be shutdown, it should be done with a full barrel of the appropriate purging agent or thermally stable resin.

» Some chemical purging compound suppliers suggest that leaving the barrel empty at shutdown is preferable. So, it is imperative that the operator know whether the purge is mechanical or chemical, and that product-specific procedures be followed.

» Some purging compounds can be molded – allowing the processor to clean sprue bushings, runners, and cavities.

» Glass filled purging compounds should never be run into a hot runner or mold, unless the supplier is consulted first.

» Regrind is not free.