PETpla.net Insider 07+08 / 2010

Vol.6 PET planet print PREFORM PRODUCTION 18 PET planet insider Vol. 11 No. 07+08/10 www.petpla.net The Rapid Guide to Perfect Preforms by Rick Unterlander and Ottmar Brandau 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. Bound in laminated paper that is easy to clean off, the small format of the book of 108mm by 151mm (4.25” by 6”) allows easy storage in back pockets or tool boxes. 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. PET planet print vol.6 The Rapid Guide to Perfect Preforms machine no./name the shut-off nozzle must be opened before the valve stems are opened for the next cycle. This will pressurize the hot runner at the beginning of the next cycle and help fill in the empty space in front of the valve stem as it opens. Do this by increasing the valve gate open delay timer and/or by increasing the cooling time to provide more transfer before the mould opens. The shut-off nozzle delay time should be set to 0sec. 7. Check all temperature readings and correct if necessary. PET resin should be melted at about 280ºC (536ºF) +/- 10ºC (18ºF). Too low a melt temperature may raise the viscosity enough to cause poor melt flow around the valve stem / nozzle tip area. 8. Check nozzle tip heat settings. This area has a major impact on gate voids / pin holes. Settings too high may slow the solidification process. The settings should be adjusted low enough to minimize gate deformation yet not too low to cause crystalline formations. 9. Check that the decompression (pullback) time and position is not excessive. This should be set as low as possible to minimize the risk of drawing in air, yet enough to eliminate drooling into the cavity from excess melt pressure. Typical settings for valve gated moulds are: Time = 0 – 1sec; Distance = 0 – 10mm. 10. Check to ensure that the valve gate compressed air pres- sure is sufficient to close the valve stems properly. Typical pressures are in the range of 8 – 10bar (120 –145psi). 11. Check hardware for worn, damaged or failed components, specifically the mould gate inserts, valve stems, valve stem seals, nozzle tips (check manufacturer for best suited tip design), tip insulators, and clogged core / cavity cooling channels. 13. Gate Voids / Pin Holes Symptom: A small hole or empty space has formed directly on or under the gate nub; this may or may not be a complete hole. Cause: Plastic in the gate nub was not properly formed or solidified. Solutions: 1. Check water cooling temperature. This should be 7 – 10ºC (45 - 50ºF) for optimum cooling. 2. Check water cooling flow to mould. This should be at the mould manufacturers rating (for a given preform size and cavitation). 3. Check that the cooling time is sufficient for the preform being moulded. This typically ranges from about 1 to 3sec depending upon the preform gate thickness. Add time if necessary to improve gate solidification. 4. Check that the hold (packing) time isn’t too little for the preform being moulded. Increase as necessary to improve gate formation. 5. Check the valve gate close delay timer and decrease as necessary to maximize nub cooling. Typical settings are 0 – 0.3sec. 6. Ensure that the empty space in front of the valve stem (when initially opened) is properly re-filled with melt at the start of injection. For this to occur, the melt in the barrel must be transferred to the shooting pot (2-stage machines only) and