What’s PET?

PET is a polyester,which is an abreviation of PolyethyleneTerephthalate. PET material is usuallly used for injection molding for plastic bottle preform,such as mineral water bottle,coca-cola bottle,juice bottle,etc.

Characteristics of PET raw materials

  •  Excellent transparency of over 90%, excellent surface gloss and a glassy appearance.
  • good aroma and air tightness.
  •  Excellent chemical resistance.it is almost resistant to all organic drugs.
  •  Good hygiene, PDA – approved (the largest chemical laboratory in the United States).
  •  No toxic gases will occur when burning.
  •  Excellent strength properties, and can be further improved by biaxial stretching.

why need to dry the PET resin before PET preform injection molding?

1.Remove the moisture from the PET material before injection molding

Because PET material has a certain water absorption, it absorbs a lot of water during transportation, storage and use. Too much content of water of PET material will affect the production as below:

  • AA (Acetaldehyde)  acetaldehyde increase. The odor of the beverage bottle causes the beverage to taste (but has little effect on the human body)
  • IV (Intrinsic Viscosity) viscosity drop. It has an influence on the pressure resistance of the beverage bottle and is easily broken. (essentially caused by hydrolytical degradation of PET)

2. Prepare for high temperature preparation for PET into the injection molding machine for shear plasticization

Water absorption characteristics of PET

The moisture content of the raw material granules usually ranges from 0.1 to 0.3% (1000 to 3000 ppm).

PET moisture absorption

The main points of PET drying

  • Dry set temperature 165-175℃
  • drying time 4-6 hours
  • Feeding inlet temperature is 160℃ or more
  •  Dew point below -30℃
  •  Dry air flow 3.7m³/ kg / h

Degree of drying

The ideal moisture content after drying is approximately: 10 to 40 ppm (+/- 10 ppm), ie 0.001-0.004%

Excessive drying will cause AA increasing and IV decreasing.

how to reduce the cycle time?

  • increase the speed of mold open/close of injection machine
  • increase the speed of ejecting speed of injection machine
  • adjust of the distance between extract plate and eject plate
  • turn-on time of the valve gate
  • adjust of injection speed and pressure of  PET injection molding machine
  • match of the pressure holding and cooling
  • position adjust which allow the robot to enter in

PET preform and bottle

Common quality defects and solution of PET injection molding

AA value too high

  1. The quality of raw materials has quality problems. → The quality of raw materials should be checked before entering the warehouse.
  2.  poor drying effect → check the operation of the dryer (such as: dew point, drying temperature / time)
  3. the barrel heating temperature is too high → try to reduce the temperature under the premise of ensuring the quality,
  4.  the set back-pressure is too high → reduce the back-pressure under the premise of quality (400 ~ 600Psi)
  5. the cycle time is too long → optimize the cycle time (to avoid the raw material staying in the barrel for too long time)

preform trun yellow

  1.  The drying time is too long/the temperature is too high. → The over-dried material should be discharged, and then use new material to be dried again.
  2.  the temperature of the barrel is too high → reuce the temperature(generally,  temperature is 2 ~ 5 °  the temperature that the preform does not occur a white fog is the normal temperature
  3.  the mold temperature is too high → reduce the temperature  to the appropriate temperature (275 ~ 280 ° C)
  4. the raw material stays in the barrel for too long time → inject empty the raw material inside the barrel, then re-charge

Wall thickness change too much

  1. heating temperature of barrel is too high→ reduce the temperature
  2. back-pressure too much → reduce it
  3. injection speed too high → reduce it little by little
  4. temperature of cooling water is too high →  check the temperature, suggest 9-11 ℃±2℃
  5. cooling time too short →  increase it by little, usually 3-4s

Scratch on the surface of body/neck

  1.  the mold has adhesive → clean the mold
  2. mold damage → replace damaged parts or repair
  3.  remove the plate suction tube damaged → replace the damaged suction tube or repair
  4.  the lip ejection position / pressure is too large → adjust the ejection position / pressure to the appropriate state
  5. The position of the take-up plate is not correct. → Check the position of the suction plate and the lip of the plate (generally set to: 22mm)

IV value reduce too high

  1. raw materials storage time is too long
  2. poor drying effect → check the dryer
  3. the barrel heating temperature is too high → try to reduce the temperature
  4. the set back-pressure is too large → try to reduce the back-pressure
  5. the mold heating temperature is too high → reduce the heating temperature of the warmer, generally set: 280 ~ 285 ° C
  6. the preform storage time is too long or the warehouse environment temperature, humidity is too high → check the wet / temperature of the warehouse (generally: 70% / 28 or less), do a first-in-first-out inventory method
  7.  the quality of the raw material itself has quality problems

Shrinkage and deformation

  1. the cooling time is too short → lengthen the cooling time (set: 3 ~ 4S)
  2. The pressure holding time is too short / the pressure is insufficient → increase the pressure holding time,totally 6-8s. and increase the holding-pressure
  3. the mold cooling water temperature is high → reduce the water temperature (set: 9 ~ 11 ° C ± 2 ° C)
  4. the mold circulating water pressure is low → increase the water pressure (set: 6 ~ 7 Kg)

Material not melt

  1. The barrel heating temperature is too low → increase the temperature appropriately
  2. the screw slips can not cut the raw materials
  3. the back pressure  of charge is too small → increase the back pressure
  4. the drying temperature is low → check if the feed temperature of screw meets the requirements (normal requirement is 160 ° C)

Bottle mouth shortage

  1.  Insufficient injection volume → increase injection volume (the remaining position after injection is generally 5~8mm)
  2.  the injection pressure is too low → increase the injection pressure
  3. the injection speed is too slow → speed up the injection speed (optimal injection time: 3.5 ~ 4s)
  4. the pressure holding position is too big→ reduce the pressure holding position (injection pressure: 900 ~ 1200Psi)
  5. The holding pressure is too small → increase the holding pressure
  6. the mold cooling water temperature is too low → increase the cooling water temperature (set: 8 ~ 10 ° C)

Long sprue

  1. The drying effect of raw materials is poor → stop and dry again
  2.  valve needle cylinder of hot runner blockage → maintenance hot runner
  3. hot runner / nozzle temperature is too high → properly decrease the temperature
  4. the pressure holding time is too short → increase the pressure holding time
  5. The holding pressure is too small → increase the holding pressure
  6. the cooling time is too short → lengthen the cooling time

sprue has the long filar material

  1. The hot runner nozzle cannot be closed or the nozzle mouth is worn out → maintenance hot runner cylinder or replacement of new nozzle
  2. The hot runner nozzle temperature is too high → reduce the nozzle heating temperature
  3. the delay closing time of nozzle vavle is too long → reduce  delay closing time of the nozzle valve
  4. control valve of needle valve gate failure → maintenance control vavle

water mark

  1. The temperature of the mold cooling water is too low → increase the temperature of the circulating water of the mold
  2.  The ambient temperature of the workshop is too high or the humidity is too high, causing the mold to hydrate the water → lowering the air conditioning temperature
  3. the mold leak water → check the mold leakage and eliminate

Bottom crystallization

  1. The hot runner and nozzle temperature are low → increase the nozzle and hot runner temperature
  2. the holding pressure is too big→ reduce the pressure
  3. The mold cooling water temperature is too low or high → check if the cooling water temperature is reasonable and then adjust
  4. mold cooling water channel blockage → maintenance mold water circulation channel

white fog on the preform body

  1. Insufficient drying of raw materials → stop and dry the raw materials
  2. the screw heating temperature is too low → increase the screw heating temperature
  3. screw slip → (general production of color oil embryos appear) increase the temperature of the barrel
  4. back pressure is too low → increase back pressure

black spot / pollution

  1.  The raw materials are polluted or have impurities in them. The raw materials with impurities should be released  out(the feeder should always pay attention to whether the raw materials are contaminated)
  2. The raw materials are contaminated during drying → take out the problem material
  3. barrel temperature / hot runner / nozzle temperature is too high → reduce the temperature to the appropriate temperature
  4. The production environment or equipment is not clean and hygienic → keep the workshop and equipment clean and hygienic
  5. The preforms are stored in the warehouse and get contaminated → keep the warehouse sanitation and the packaging intact

Flash

  1. mold neck,mold core plate or mold cavity may damaged → check and change if need
  2. injection pressreu too big → reduce the injection pressure
  3. holding pressure too big → reduce the pressure or position
  4. barrel heating temperature too high → reduce
  5. clamping force not enough → increase it
  6. material stay inside the heating section of barrel too long time → reduce the cycle time

Peeling on the bottom of preform

  1. The nozzle temperature is too high or too low → adjsut it
  2. the mouth of the copper nozzle is worn → replace the new nozzle and the new nozzle insulation sleeve
  3. hot runner valve pin deformation or wear → replace the new valve pin
  4.  The third stage of the holding pressure is too big→ reduce

Internal shrinkage of the bottom of the bottle

  1. Excessive scale or rust of the core cooling channel→ cleaning the core cooling channel
  2. the core cooling water pipe is loose → check and tightening water pipes
  3. Insufficient water pressure → raise water pressure (generally: 6 to 7 bar)
  4. the water temperature is too high → adjust the temperature of the chiller

burn mark

  1.  the nozzle temperature is too high → reduce the nozzle heating percentage
  2. nozzle insulation bush aging → replace the new nozzle insulation sleeve
  3. The cooling water channel at the bottom of the cavity is blocked → clean the mold cavity and the bottom of the cavity
  4.  Insufficient water pressure → increase water pressure
  5.  the water temperature is too high → lower the water temperature

Fishbone stripe

  1. The mold hot runner temperature is too high → reduce the hot runner temperature
  2.  The mold nozzle heating temperature is too high → reduce the nozzle temperature
  3. mold nozzle insulation sleeve aging → replace nozzle insulation sleeve
  4. The nozzle valve pin does not move smoothly → maintain the hot runner or replace the valve pin

pinhole sprue

  1.  The mold nozzle temperature is too high → reduce the nozzle temperature
  2.  The position of the suck-back is too small → adjust the position of the suck-back
  3.  The mold hot runner temperature is too high → lower the hot runner temperature
  4.  the raw material drying effect is poor → stop and re-dry (about 2 to 3 hours)
  5.  the injection speed is too fast → slow down the injection speed
  6. The pressure and time are too small → increase the pressure and time