Plastic Injection Defects: Short Shots, Unfilled part, Shorting

• Die temperature : Effects on Material Viscosity:

  
  

  Material flowfronts are easily affected by mold temperature. It is important to understand that a variance as little as 3 degrees F can affect your process. *Note* Molding cycle time can become longer as a result of mold temp increases... Because of this, care must be taken to also consider increased barrel resonance time. * Refer to the manufacturer's Material Safety Data Sheet* (MSDS)

• Screw failure : Replace the screw check-ring:

  
  

  Note that this is applicable to situations where there is clear variation in the cushion when cut-off has been reached.

• Screw Recovery Variances:

  
  

  - Raise or lower the feed zone temperature. If plastic at the hopper opening is bridging, lower the temperature... Also check the temperature of the feed throat cooling system.

• Correct the back pressure : Insufficient or excess back pressure:

  
  

• Injection speed : Increase the injection speed:

  
  

  If no change in the fill time is noted after setting of a higher injection rate, there is a possibility that the performance of the molding machine is insufficient or that there is excessive pressure loss in the sprues, runners, and gates; You may be pressure-limited. Fill times set at the top of fill window can lead to process variance based on lot, melt and viscosity..

• Plastic temperature : Raise the temperature based on melt temperature:

  
  

  Your material supplier data generally offers information that defines your upper and lower melt temperatures. Standard sampling would start with your material melt temp being set at the middle of this window.. If your process shows signs of high injection or peak pressures, you may need to adjust temp to decrease viscosity and improve flowfront. If viscosity and pressures appear low, the potential for gassing and "gas trap" shorts exist... you may have to lower temperature to slow the end-of fill knit to reduce gas creation and give gas ahead of the flow time to escape through your venting set-up.

• Packing: Increase or Lower Pack/ Hold pressure:

  
  

  After assuring that 98% fill of the cavity has been acheived prior to pack/ hold pressures being added, Consider your approach towards the hold stage. Is it set too high to allow gas to be relieved? Are you packing enough to fill out the part?

• Poor Venting: Short/Long-Term Approaches, Solutions:

  
  

  Short shots can be due to poor venting at the end of fill in a flowfront. One way of determining whether a short is occuring due to "gas trap" is to install a temporary vent in the location of the short. This can be done using tape, a label or "vent tape"... a specialized metal tape that has been designed to withstand the abuse of repeated clamping of injection molding.