PETpla.net Insider 09 / 2012

Vol.6 PET planet print PREFORM PRODUCTION 20 PET planet insider Vol. 13 No. 09/12 www.petpla.net The rapid guide to perfect preforms - part 32 32. Unmelts 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 proposi- tion 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 Unter- lander and Ottmar Brandau, it gives concrete advice on 35 of the most common problems in preform manufacture, most illustrated by pic- tures. From high AA levels to wavy bodies pro- cessors can find help when they need it. Read chapter 3” of the guide. Symptom Partially melted or totally unmelted resin pellets in the preform. Cause Unmelted pellets are present in the pre- form from insufficient melting of the PET resin. Solutions 1. Check dryer for alarms and correct as necessary. A high dewpoint (above –40ºC (–40ºF)) or low inlet resin temperature (should be mini- mum of 160ºC (320ºF)) may cause extrusion problems and must be recti- fied. 2. Check for adequate resin drying time in hopper (should be around 4h Table of Contents i........................ About the Authors ii.................................Introduction Chapter 1.............................. AA Level High 2........................ Accordion Preform 3....................... Air hooks / Streaks 4............................... Black Specks 5...................................... Bubbles 6..................................Burn Marks 7...........................Colour Streaking ................................... /Peeling 8..................................Crystallinity 9.................................Drool Marks 10.......................................... Flash 11......................... Gate Depressions 12............................... Gate Peeling 13..................Gate Voids / Pin Holes 14......................... Internal Gate Sink 15................................Internal Sinks 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). Increase as neces- sary. 4. Check throughput of plastic in system doesn’t exceed output of extruder screw. The processed kg/ hour of plastic should be below the manufacturers rating of the screw. Contact manufacturer for spec, and if exceeded, raise extruder tempera- tures and/or increase cycle time to compensate. 5. 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. 6. Check extruder back pressure. This must be high enough for adequate mixing and shearing. Typical range is 15 – 20 bar (200 – 300 psi). 7. Check screw rpm. Screw speed may be increased to increase shear and improve melting. 8. Ensure PET resin pellets have not been degraded. This may raise the pellet crystallinity level and raise its melting point. Check with resin manufacturer for testing resin characteristics. 9. Check hardware for worn, damaged or failed components, specifically the resin in-feed line, and the extruder screw. www.petpla.net 16........................................ IV Loss 17............................ Long Gate Nub 18...............................Milky Preform 19............................ Moisture Marks 20................Neck Finish out of Round 21....................... Parting Line Crease 22.................Parting Line Deformation 23......................... Preform Yellowing 24.................................. Short Shots 25.......................................... Sinks 26......................................... Splay 27........................... Stretched Necks 28..................................... Stringing 29......................... Surface Blemishes 30......................... Thread Flow Lines 31........ TSS (Top Sealing Surface) Sink 32.. ...................................... Unmelts 33.................Wall Thickness Variation 34.................................Wavy Body 35.................................. Weld Lines