Detailed introduction

30

2018

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07

Screw cleaning method


 
  Cleaning screwThere are ordinary replacement method, special screw cleaning agent cleaning method, the former method will use a lot of resin.
Extruder screw
1. Water cleaning method
Water cleaning method, using a small amount of water to clean the screw can reduce the cleaning time and avoid crystal points during production. The specific steps are as follows (the following amounts are set based on a typical extruder with a screw diameter of 90mm and L/D = 28).
① Set the screw to rotate at low speed, generally 10r/min.
② Add the cleaned material head with coating grade LDPE.
The screw runs at high speed for about 10min, so that LDPE completely replaces the original material.
Insert a plastic pipe into the hopper of raw materials, close to the screw and slowly continuously inject 100~200ml of water.
⑤ After a few minutes, first start foaming from the middle of the melt curtain, and then develop to all foaming,
⑥ When the foaming process is over, the melt becomes clear and clean again. After 15~20min, reduce the screw speed and add new materials.
⑦ Complete the operation and start production.
2. Variable speed flushing method
Another commonly used screw cleaning method is variable speed flushing method, commonly known as disco flushing method, the steps are as follows:
① The screw runs at medium speed to eliminate the stock in the machine;
② Change the flow sequence, as shown in the following table.
Fractional ratio of maximum screw speed/%
Screw rotation time/min
Table 1 Operation sequence of variable flow
③ If they are not washed clean, they have crystal spots and sundries, continue the above sequence until they are clean.
Selection of cleaning materials:
1) Replace light color resin with high viscosity resin;
2) Timely maintenance of equipment;
3) Keep the material handling equipment clean;
4) Use an intermediate resin to fill the difference in processing temperature (for example, when replacing PA with POM, PS can be used as cleaning material), etc.
Materials to be repurged
Recommended applicable cleaning materials
Polyolefin
  HDPE
  PS
Cast Acrylic, PS
  PVC
General Purpose PS,ABS, Cast Acrylic Resin
  ABS
Cast Polypropylene Resin, PS
  PA
PS, low melt flow rate HDPE, cast polypropylene resin
  PET
Resin to be processed
  PBT
PS, low melt flow rate HDPE, cast polypropylene resin
  PC
Cast acrylic or PC return, no PA or ABS
  POM
PS, avoid any resin containing PVC
engineering resin
PS, low melt flow rate HDPE, cast acrylic resin
fluoropolymers
Cast Acrylic, PE
polyphenylene sulfide
Cast Acrylic, PE
Polysulfone
PC return, extrusion grade PP
Polysulfone/ABS
PC return, extrusion grade PP
polyphenylene ether
General Purpose PS, Cast Acrylic Resin
thermosetting polyester
Similar mixture materials without catalyst
Add filler or reinforcing material
cast acrylic resin
flame retardant mixture
Direct use of natural resin without flame retardant, mixed with 1% sodium stearate
Table 2 Selection of cleaning materials
In general, powdered or pulverized cast acrylic resins and polyvinyl materials are the main clear materials. In fact, cast acrylic is suitable for cleaning of any resin.
For the cleaning of the extruder, special cleaning materials are usually required, and sometimes the die head, filter screen, etc. need to be disassembled. Polyvinyl compounds usually contain release agents and are often used to clean "soft" thermoplastics (polyolefin, styrene and polyvinyl chloride, etc.). For extruders, it is sometimes not applicable due to many requirements and restrictions.
EVOH Cleaning
1. Cleaning
Switching from LDPE to EVOH can be used directly without other changes. Switching from HDPE,PP and LLDPE to EVOH is first replaced with LDPE and then extruded with EVOH. Especially after the use of PA, PVC, be sure to completely replace the resin in the cylinder and die with LDPE, and then EVOH extrusion.
2. Resin temperature
Please pay attention to the temperature setting of the barrel and die not to exceed 240°C, and control the speed of the screw.
3. Retention time
Based on 30 minutes, if the operation is interrupted for a long time, please select the low-speed rotation of the screw to keep running.
4. Cleaning after operation
If it is replaced with LDPE, it can be cleaned effectively in a short time.
5. Attention Points for Cleaning Resin
When pre-cleaning or using cleaning, please do not use PP or HDPE. When these resins come into contact with EVOH, problems may occur, such that cleaning is difficult, and caking may occur in some cases.