PET Troubleshooting FOR TRAINING OPERATOR.pdf

PET Troubleshooting

1. Troubleshooting Guideline for
PET PROCESSING

2. Written by:
Jörg Meier PET Process and Service Technician 59 route de Wasserbillig
L - 6686 Mertert/Luxemburg
Phone: 00352 / 26 74 61 – 34
Fax: 00352 / 26 74 55 – 35
E-mail: [email protected]
Internet: www.mht-ag.de

3. Table of Contens 1
1. SCRATCHES ON BODY
2. SINK MARKS
3. PIN HOLE IN THE GATE
4. BLACK SPOTS
5. UNMELTS
6. SPLAY MARKS
7. PELLING
8. HAZY PREFORM
9. PREFORM BUCKLING
10. FLASH
11. GATE CRYSTALLINITY
12. WHITE PREFORM
13. LONG GATE
14. YELLOW PREFORM
PAGE 5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.

4. Table of Contens 2
15. COLOR STREAKS
16. PULLED GATE NUB
17. WATER LEAKAGE
18.
PAGE 19.
20.
21.

5. Scratches on the preform body
Random marks , or scratches on the
preform body.
Possible Faults
- Random scratches, reason by contact with
other objects.
Suggested Check
- Minimize handling and transfer of robot ejection. - Minimize distance preforms drop from robot to the conveyor.
- Increase preform cooling, to reduce preform
surface temperature.
- Check the hight, for th e free fall into the box.

6. Sink Marks
Sink marks, material to hot on the
surface, more found on thick wall
preforms.
Possible Faults
- Not enough material injected.
- Not enough material packed.
- Execessive shrinkage due to high melt
temperature.
- Excessive shrinkage, not enough mold
cooling.
Suggested Check
- Increase shot size and mold cooling. - Reduce transition position. - Increase hold pressure and hold time. - Check mold cooling channels.
- Reduce machine and mold heats.
- Check water cooling system, pressure, flow
and temperature.

7. Pin Hole in Gate
A small opening/hole in the preform
gate aria
Possible Faults
- To much heat in gate area.
- Melt in gate nub stick to the valve stem.
Suggested Check
- Reduce nozzle tip temperature.
- Check valve gate open timer.
- Increase mold cooling time.
- Check for propper valve stem air pressure.
- Check the nozzle tip heater, for the correct
position.

8. Black Spots
Random particles found in the
preform body, reason of degraded
material.
Possible Faults
- Black particles, reason by degraded melt. Suggested Check - Reduce mold nozzle tip temperature.
- Reduce mold manifold temperature.
- Reduce machine heats.
- Keep system heat up time to a minimum
during start - ups.
- Purge barrel and shoo ting pot clean, for fresh
material to the start up.

9. Unmelts
Resin pellets appearence in the
preform body, reason of unmelted.
Possible Faults
- Incoming resin temperature to cold.
- Resin melting point to high.
- Not correct dryer operation, air flow,
temperatur, hopper residence time.
-
Suggested Check
- Check for proper dryer opertation, hoses,
air flow, and dryer time.
- Check for proper resin flow in the machine.
-

10. Splay Marks
Splay Marks created by injecting hot
or degraded melt into cavity.
Possible Faults
- Flow splays on the preform surface.
- Splay marks created by injecting cold or
degraded melt into the cavity.
Suggested Check
- Increase mold nozzle tip temperature.
- Increase mold manifold temperature.
- Increase machine heats.
- Increase back pressure.

11. Peeling
Cold material sticking on the bottem
area outside.
Possible Faults
- Cold material between nozzle tip and valve
stem.
- Cold material between the vespel insulator. Suggested Check - Inspect mold valve stem tip and gate
insert, and exch ange as necessary.
- Increase nozzle tip temperature.
- Reduce the last hold pressure.
- Playing with the va lve stem, open and
close delay timer.
- Use the MHT anti peeling nozzle tip.

12. Hazy Preform
A cloudy white appearance spread
uniformly throughout the preform
body.
Possible Faults
- Incoming resin moisture level to high, during
the plasticizing pro cess resulting in resin IV
loss.
- Incoming resin temperature to cold.
- Not enough extruder compression during
plasticizing process .
Suggested Check
- Check for proper drye r operation,temperatur,
airflow, and hopper residence time.
- Increase screw back pressure.
- Check the dryer time.

13. Preform Buckling
Preform body collapsing under
ejection pressure,at the thickest and
hottest part of the wall section.
Possible Faults - Buckling faults by not enough mold cooling.
- Not enough packing to compensate for
- shrinkage and proper heat transfer.
- Execessive shrinkage or preform
temperature due to high melt temperature. Suggested Check
- Check water cooling system,pressure,flow, and temperature. - Increase preform cooling time.
- Increase hold time.
- Increase hold pressure.
- Decrease machine and mold heats.
- Check mold channels, or blockages.

14. Flash
Formation of a thin plastic protrusion
between adjacent mold component
surface.
Possible Faults - Plastic flow through parting line and vents
due to an excess amount of material
injected.
- Plastic flows through parting line and
vents due to low melt viscosity.
Suggested Check
- Reduce injection shotsize.
- Increase injection transition position.
- Reduce hold and fill pressure and fill speed.
- Reduce machine and manifold temperature.
- Reduce system heat up time during start up.
- Clean the complete mold and parts.
- Check preload of tapers.
- Check the clamp tonnage from the machine.

15. Gate Crystallinity
White crystalline formation appearing
in the preform gate area.
Possible Faults
- Temperature of melt in mold nozzle tip
too cold during th e time the melt is not
flowing.
-
Suggested Check
- Increase mold nozzle tip temperature.
- Check the nozzle and vespel insulator parts.
- Check the mold gate in sert water channels for
contamination or blockages.
- Check the nozzle tip heater band is not
contacting gate insert.

16. White Preform
Material not enough dryed.
Possible Faults
- Incoming resin temperature to cold.
- Resin melting point to high.
- Not correct dryer operation, air flow,
temperatur, hopper residence time.
- Material not enough dryed.
Suggested Check
- Check for proper dryer operation, hoses,
air flow, and dryer time.
- Check for proper resin flow in the machine.
- Check the hopper hoses inside the machine.
- Increase the back pre ssure from the machine.
- Increase the dryer time and temperature.
- Increase the machine temperature.

17. Long Gate
Elongated gate nub of preform
appearing as a melt protrusion at the
gate vestige.
Possible Faults
- Valve stem not closing properly.
- Nozzle tip temperature to high.
- Air pressure from the system works not
properly.
Suggested Check
- Playing with the valve gate close delay timer.
- Check proper valve stem air pressure.
- Check the nozzle tip and stems or replace as
necessary .
- Check for material in mold nozzle tip and
clean as necessary.
- Check for worn valve stem piston seals.

18. Yellow Preform
Yellow Preform resin degraded by
excess heat.
Possible Faults
- Resin degraded by to much heat .
- Extruder heats to high.
- Mold heats to high.
- Dryer time to long.
- dryer heat to high.
Suggested Check
- Check the dryer for proper operation, temperature,airflo w,and hopper residence
time.
- Reduce mold and machine heats.
- Keep system heat up time to a minimum.
- Reduce shear heat in extruder,reducing
screw speed,back pressure.

19. Color Streaks
Non homogenous mixture of PET melt
and colorant appearing as color
streaks in the preform body.
Possible Faults
- Not enough mixing of color PET pellets.
- Melt not homogenized enough during the
plasticizing process.
Suggested Check
- Increase screw back pressure and speed.
- Increase extruder melt temperature.
- Check for consistent color feeding from
the color dosing unit.
- Check for proper color additive feed tube
location and correct as necessary.
- Use a mold or machine mixer.

20. Pulled Gate Nub
Elongated gate nub preform
appearing as a drawn-out crystalline
vestige.
Possible Faults
- Excess heat in gate area, reason gate nub
to stick in mold gate opening during the
mold open motion.
Suggested Check
- Check chilled water system, pressure, flow and temperature. - Check for contamination or blockages in mold
gate pad cooling channels and clean as
necessary.
- Reduce nozzle tip temperature.
- Increase mold cooli ng time, hold pressure.

21. Water Leakage
Water leakage inside the mold or take
off plate, material mixed with the
water together.
Possible Faults
- Water leakage from the mold side,
gate insert or cavity o-ring broken.
- Water leakage from the take off tube
inside from the piston.
- Water leakage from any hoses, mold or
machine side. Suggested Check
- Check the water flow inside from the mold
for leakage.
- Check all o-rings from the mold side, stacks
cavities, neck-rings and all plates.
- Check for any leakage inside the tubes.