Page 182 - Injection Molding Advanced Troubleshooting Guide
P. 182

172   18 Bubbles




               18.3.4.3„ Material: Contamination
               If the molding material is contaminated with a foreign material, adverse reactions
               can occur. In many cases, if a lower melting temperature material is contaminating
               a higher melting temperature material the lower melting temperature material will
               likely degrade during the melting process and in turn create volatiles that outgas.
               These outgassed volatiles will be carried along in the melt stream and in turn may
               lead to a bubble.
               Keys to avoid contamination of material include the following:
                  ƒ Ensure that containers are covered to reduce opportunities for contamination.
                  ƒ Clearly identify all materials, additives, and regrind to avoid mixing materials.
                  ƒ Establish clear guidelines for introducing a new bag, box, or silo feed of material.
               Contaminated material may lead to a variety of visual defects, but will also compro-
               mise the physical properties of the material potentially leading to parts that will
               not perform in end-use application.

               18.3.4.4„ Material: Unmelts
               Unmelts are unmelted pellets that make their way through the barrel without
               melting. They can sometimes be misidentified as bubbles because they often leave
               a pronounced mound on the part. If unmelts are suspected, cut the part in the area
               of the bubble to see if there is a pellet under the mound.
   177   178   179   180   181   182   183   184   185   186   187