PETpla.net Insider 12 / 2011

PREFORM PRODUCTION 23 PET planet insider Vol. 12 No. 12/11 PET planet print Vol.6 The rapid guide to perfect preforms - part 26 Splay Troubleshooting guides have suffered in the past from being locked away in offices and not being available to the people that actually need them. Considering the high cost of machine and tools high scrap rates can be a costly proposition eating away at already tight margins. PETplanet’s “Rapid Guide to Perfect Preforms” takes a different approach. It has been written for the people on the floor of injection moulding companies to support their daily work in making the best preforms possible. The product of two industry experts, Rick Unterlander and Ottmar Brandau, it gives concrete advice on 35 of the most common problems in preform manufacture, most illustrated by pictures. From high AA levels to wavy bodies processors can find help when they need it. Read chapter 26 of the guide. Symptom: Numerous translucent streaks on the surface of the preform. Usually local- ized in one area of the preform. They are shaped as U’s and/or V’s with the closed ends towards the preform gate. Cause: Air / gas has merged with the melt and was drawn to the surface of the preform during injection. Solutions: 1. Check dryer for alarms and cor- rect as necessary. A high dewpoint (above –40ºC (-40ºF)) or low inlet resin temperature (should be minimum of 160ºC (320ºF)) may cause extru- sion problems and must be rectified. 2. Check for adequate resin drying time in hopper (should be around 4h at 175ºC (347ºF) and up to 6h at 160ºC (320ºF)) 3. Check all temperature readings and correct if necessary. PET resin should be melted at about 280ºC (536ºF) +/- 10ºC (18ºF). Decrease as neces- sary. 4. Check nozzle tip heat settings. This area has a major impact on splay in the gate area. Settings too high may cause the melt in the nozzle tip to overheat resulting in gases to be mixed and conveyed into the cavity. Settings too low may cause plastic to build up in the nozzle tip and gate area, interfering with the melt flow, resulting in air being trapped in the melt. The settings should be adjusted to optimize melt quality and minimize the risk of air / gas entrapment. 5. Check injection time isn’t too low (injection rate too fast) and adjust if necessary. Injection rate should be about 10 – 12g/sec/cavity. 6. Check that the valve stems are open- ing properly. A partially open valve stem may restrict melt flow. 7. Check to ensure that the valve gate compressed air pressure is sufficient to open the valve stems properly. Typical pressures are in the range of 8 – 12bar (120 – 145psi). 8. Check throughput of plastic in system doesn’t exceed output of extruder screw. The processed kg/h of plastic should be below the manufacturers rating of the screw. Contact manu- facturer for spec, and if exceeded, raise extruder temperatures and/or increase cycle time to compensate. 9. Check extruder throat cooling isn’t cooling resin pellets too much (should be minimum 160ºC (320ºF) inlet temperature). Restrict cooling as nec- essary. NOTE: Do not restrict when using colorants that are susceptible to jelling or coagulation. 10. Make sure no bridging (clogging) has occurred in the extruder barrel and if necessary clean out the plastic bridged material. WARNING: When preparing to release bridged mate- rial, you must completely cover your body with protective gear including facemask, safety helmet and thermal gloves. Gases may build up inside the barrel and may release when the bridged material is removed, causing molten plastic to be sprayed out. 11. Check for adequate extruder back pressure. This must be high enough to squeeze out gases as the plastic is being melted and compressed. Typical range is 15 – 20 bar (200 – 300 psi). 12. Make sure the extruder screw cushion is sufficient to prevent the screw from bottoming out. This should be a minimum of 25mm to compensate for any dynamic recovery fluctuations. (Two-stage machines only) 13. Check for proper resin flow at extruder throat. If interrupted, air pockets may be drawn into extruder. 14. Check that the decompression (pull- back) time and position is not exces- sive. This should be set as low as pos- sible to minimize the risk of drawing in air, yet enough to eliminate drooling into the cavity from excess melt pres- sure. Typical settings for valve gated moulds are: Time = 0 – 1sec; Dis- tance = 0 – 10mm. Non-valve gated moulds may require higher values to minimize gate stringing. 15. If using regrind, check that the chip size is fairly uniform and not too large. 16. Check for any colorant build up in the extruder throat and remove if nec- essary. 17. Check hardware for worn, damaged or failed components, specifically the mould nozzle tips, tip insulators or PET filled space at tips, the resin in-feed line, and misaligned shut-off nozzle. www.petpla.net