How to choose screw for different raw materials?

Time:2021-06-10 16:27:53 / Popularity: / Source:

One. PC

①Amorphous plastic, no obvious melting point, glass transition temperature 140~150℃ melting temperature 215℃~225℃, molding temperature 250℃~320℃.
②Viscosity is large, and it is more sensitive to temperature. It has good thermal stability in normal processing temperature range. It will not decompose after a long stay at 300℃. It will start to decompose when it exceeds 340℃. Viscosity is less affected by shear rate.
③ Strong water absorption.
Screw parameter selection:
a. In view of its characteristics of good thermal stability and high viscosity, L/D should choose a large aspect ratio as much as possible to improve plasticizing effect.
Because of its wide melting temperature range, a gradual screw is used. L1=30% full length, L2=46% full length.
b. Compression ratio ε needs to be compatible with melting rate by degree of gradient A, but melting rate cannot be calculated at present. According to processing characteristics of PC melting from 225℃ to 320℃, value of gradient A can be relatively medium. For upper value, when L2 is large, normal gradual screw ε=2~3.
c. Because of its high viscosity and strong water absorption, a mixing structure is added to screw before homogenization section and after compression section to strengthen disintegration of solid bed, and at the same time, entrained water can be turned into gas and escape.
d. Other parameters such as e, s, φ and gap with barrel can be same as other ordinary screws.
How to choose screw for different raw materials 

Two. PMMA (plexiglass)

①Glass transition temperature is 105℃, melting temperature is greater than 160℃, decomposition temperature is 270℃, molding temperature range is wide.
②Viscosity is large, fluidity is poor, and thermal stability is good.
③ Strong water absorption.
Screw parameter selection:
a. L/D chooses a gradual screw with a length-to-diameter ratio of 20-22, and generally L1=40% and L2=40% depending on accuracy of product molding.
b. Compression ratio ε is generally selected from 2.3 to 2.6.
c. In view of its hydrophilicity, a mixing ring structure is adopted at the front end of screw.
d. Other parameters can generally be designed according to general-purpose screw, gap between barrel and barrel cannot be too small.

Three. PA (nylon)

① Crystalline plastics have many types and different types, their melting points are also different, melting point range is narrow. Generally, melting point of PA66 is 260℃~265℃.
②Low viscosity, good fluidity, relatively obvious melting point, and poor thermal stability.
③General water absorption.
Screw parameter selection:
a. L/D selects a mutant screw with a length-to-diameter ratio of 18-20.
b. Compression ratio is generally selected from 3 to 3.5, in which to prevent overheating decomposition h3 = 0.07 to 0.08D.
c. Because of its low viscosity, gap between check ring and barrel should be as small as possible, about 0.05, gap between screw and barrel is about 0.08. If necessary, depending on material, front end can be equipped with a non-return ring, and nozzle should be self-locking.
d. Other parameters can be designed according to general screw.

Four. PET (Polyethylene Terephthalate)

①Melting point is 250℃~260℃, molding temperature of blow molding grade PET is a little wider, about 255℃~290℃.
②Blow molding grade PET has high viscosity, temperature has a great influence on viscosity and poor thermal stability.
Screw parameter selection:
a. L/D is generally taken as 20, three-stage distribution L1=50%-55%, L2=20%.
b. Use a screw with low shear and low compression ratio. Compression ratio ε is generally 1.8~2. At the same time, shearing overheating causes discoloration or opacity h3=0.09D.
c. There is no mixing ring at the front end of screw to prevent overheating and material storage.

Fives. PVC (Polyvinyl Chloride)

①No obvious melting point, soft at 60℃, viscoelastic state at 100℃~150℃, melting at 140℃, decomposing at the same time, decomposing rapidly at 170℃, softening point close to decomposition point, decomposing and releasing in HCl gas.
②Poor thermal stability, temperature and time will cause decomposition, and poor fluidity.
Screw parameter selection:
a. Temperature is strictly controlled, and screw design should be as low as possible to prevent overheating.
b. Screw and barrel should be corrosion-proof.
c. Injection molding process needs to be strictly controlled.
d. Generally speaking, screw parameters are L/D=16-20, h3=0.07D, ε=1.6-2, L1=40%, L2=40%.
e. In order to prevent material accumulation, there is no check ring, and head taper is 20°~30°, which is more suitable for soft rubber. If product requirements are higher, a non-metering section, separate type screw can be used. This type of screw is more suitable for hard PVC. In order to cooperate with temperature control, cooling water or oil holes are added to screw in feeding section, cold water or oil grooves are added to barrel.Temperature control accuracy is about ±2℃.

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