How to Choose Screw for Different Raw Materials?

I. PC material (polycarbonate) of extruder screw

1. Features

(1) Amorphous plastic, no obvious melting point, glass transition temperature 140°-150°C, melting temperature 215°C-225°C, molding temperature 250°C-320°C.

(2) The viscosity is large, and it is more sensitive to temperature. It has good thermal stability in the normal processing temperature range. It will not decompose after a long stay at 300℃. It will start to decompose when it exceeds 340℃. The viscosity is less affected by the shear rate.

(3) Strong water absorption.

2. Parameter selection of extruder screw

(1) In view of its good thermal stability and high viscosity, L/D should choose a large aspect ratio as much as possible to improve the plasticizing effect. Because of its wide melting temperature range, a gradual screw is used. L1=30% full length, L2=46% full length.

(2) The compression ratio ε needs to be compatible with the melting rate from the degree of gradient A, but the current melting rate cannot be calculated. According to the processability characteristics of PC melting from 225°C to 320°C, the value of the degree of gradient A can be relatively take the upper-middle value, in the case of large L2, the ordinary gradual screw ε equals 2~3.

(3) Because of its high viscosity and strong water absorption, a mixing structure is added to the extruder screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed. At the same time, the entrained water can be turned into gas and escape.

(4) Other parameters such as e, s, φ and the clearance with the barrel can be the same as other ordinary screws.

II. PMMA (plexiglass) of extruder screw

1. Features

(1) The glass transition temperature is 105°C. The melting temperature is greater than 160°C. The decomposition temperature is 270℃. The molding temperature range is very wide.

(2) The viscosity is large, the fluidity is poor, and the thermal stability is good.

(3) Strong water absorption.

2. Parameter selection of extruder screw

(1) L/D selects a gradual screw with a length-to-diameter ratio of 20-22, and generally L1=40%, L2=40%, depending on the accuracy of its product molding.

(2) The compression ratio ε is generally selected from 2.3 to 2.6.

(3) Because it has a certain degree of hydrophilicity, a mixing ring structure is adopted at the front end of the extruder screw.

(4) Other parameters can generally be designed according to the general-purpose screw, and the gap between the barrel and the barrel should not be too small.

III. PA (nylon) of extruder screw

1. Features

(1) There are many kinds of crystalline plastics with different melting points, and the melting point range is narrow. Generally, PA66 has a melting point of 260 ℃ ~ 265 ℃.

(2) Low viscosity, good fluidity, obvious melting point and poor thermal stability.

(3) Average water absorption.

2. Parameter selection of extruder screw

As a professional screw and barrel manufacturer, Lesun will show you some tips for selecting extruder screw.

(1) Select the mutant screw with L/D ratio of 18-20.

(2) The compression ratio is generally selected from 3 to 3.5, among which to prevent overheating decomposition h3=0.07~0.08D.

(3) Because of its low viscosity, the gap between the check ring and the barrel should be as small as possible : about 0.05. And the gap between the screw and the barrel is about 0.08. If necessary, depending on the material, the front end can be equipped with a non-return ring. The nozzle should be self-locking.

(4) Other parameters can be designed according to general extruder screw.

IV. PET (polyethylene terephthalate) of extruder screw

1. Features

(1) Melting point is 250℃~260℃. Blow molding grade PET has a wider molding temperature, about 255℃~290℃.

(2) Blow molding grade PET has higher viscosity. Temperature has a great influence on viscosity. Its thermal stability is poor.

2. Parameter selection of extruder screw

(1) L/D is generally taken as 20, three-stage distribution L1=50%-55%, L2=20%.

(2) Use a screw with low shear and low compression ratio. The compression ratio ε is generally 1.8~2. At the same time, discoloration or opacity caused by shearing overheating h3 is equal to 0.09D.

(3) There is no mixing ring at the front end of the extruder screw to prevent overheating and material storage.

V. PVC (polyvinyl chloride) of extruder screw

1. Features

(1) No obvious melting point. It becomes soft at 60°C and viscoelastic at 100°C to 150°C. It melts at 140°C and decomposes at the same time. Decomposes rapidly at 170°C. The softening point is close to the decomposition point, and the decomposition is released in HCl gas.

(2) Poor thermal stability. Temperature and time will cause decomposition. Its liquidity is poor.

2. Parameter selection of extruder screw

(1) The temperature is strictly controlled, and the screw design should be as low as possible to prevent overheating.

(2) The screw and barrel should be corrosion-proof.

(3) The injection molding process needs to be strictly controlled.

(4) Generally speaking, the extruder screw parameters are L/D=16~20, h3=0.07D, ε=1.6~2, L1=40%, L2=40%.

(5) In order to prevent the material from being trapped, there is no check ring, and the head taper is 20°~30°, which is more suitable for soft rubber. If the product requirements are higher, a non-metering section separate screw can be used. This kind of screw is more suitable for rigid PVC. Moreover, in order to cooperate with temperature control, cooling water or oil holes are added to the screw in the feeding section, and cold water or oil grooves are added to the extruder barrel. The temperature control accuracy is about ±2°C.

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