How to select screw parameters for different raw materials

(1) PC (Polycarbonate)

Features:

① Non crystalline plastic, without obvious melting point, with a glass transition temperature of 140 ° C to 150 ° C, a melting temperature of 215 ° C to 225 ° C, and a molding temperature of 250 ° C to 320 ° C.

② It has high viscosity and is sensitive to temperature. It has good thermal stability within the normal processing temperature range. It stays at 300 ℃ for a long time and does not decompose. It begins to decompose when it exceeds 340 ℃, and the viscosity is less affected by shear rate.

③ Strong water absorption.

Screw parameter selection:

a. Due to its good thermal stability and high viscosity, L/D should choose a larger aspect ratio as much as possible to improve the plasticization effect.

Due to its wide melting temperature range, a gradient screw is used. L1=30% of the total length, L2=46% of the total length.

b. Compression ratio ε The gradient A needs to be adapted to the melting rate, but currently the melting rate cannot be calculated. Based on the machinability of PC melting from 225 ℃ to 320 ℃, its gradient A value can be relatively higher than the middle value. In the case of larger L2, ordinary gradient screws ε= 2-3.

c. Due to its high viscosity and strong water absorption, a mixing structure is added to the screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed. At the same time, the water carried in it can be transformed into gas and escaped.

d. Other parameters such as e, s, φ And the clearance with the barrel can be the same as other ordinary screws.

(2) PMMA (Organic Glass)

Features:

① The glass transition temperature is 105 ℃, the melting temperature is greater than 160 ℃, the decomposition temperature is 270 ℃, and the molding temperature range is very wide.

② High viscosity, poor fluidity, and good thermal stability.

③ Strong water absorption.

a. L/D selects a gradient type screw with a length to diameter ratio of 20-22, and generally requires L1=40% and L2=40% depending on the accuracy requirements of the product forming.

b. Compression ratio ε ， Generally, select 2.3 to 2.6.

c. Due to its hydrophilicity, a mixing ring structure is used at the front end of the screw.

d. Other parameters can generally be designed according to universal screws, and the gap with the machine barrel should not be too small.

(3) PA (Nylon)

Features:

① Crystalline plastics have many types and different melting points, with a narrow range of melting points. Generally, PA66 is used with a melting point ranging from 260 ℃ to 265 ℃.

② Low viscosity, good fluidity, significant melting point, and poor thermal stability.

③ Water absorption is average.

Screw parameter selection:

a. L/D selects mutation type screws with a length to diameter ratio of 18-20.

b. The compression ratio is generally selected from 3 to 3.5, with h3=0.07 to 0.08D to prevent overheating and decomposition.

c. Due to 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 should be about 0.08. If necessary, depending on the material, the front end can be equipped with a check ring, and the nozzle should be self-locking.

d. Other parameters can be designed according to universal screws.

(4) PET (polyethylene terephthalate)

Features:

① The melting point ranges from 250 ℃ to 260 ℃, while blow molded PET has a wider molding temperature, ranging from approximately 255 ℃ to 290 ℃.

② Blow molded PET has a high viscosity, and temperature has a significant impact on viscosity, resulting in poor thermal stability.

Screw parameter selection:

a. L/D is generally taken as 20, with a three segment distribution of L1=50% -55%, and L2=20%.

b. Using screws with low shear and low compression ratio, compression ratio ε ， Generally, it is taken as 1.8-2, and at the same time, shear overheating causes discoloration or opacity, h3=0.09D.

c. There is no mixing ring at the front end of the screw to prevent overheating and material storage.

(5) PVC (polyvinyl chloride)

Features:

① No obvious melting point, softening at 60 ℃, viscoelastic state at 100 ℃~150 ℃, melting at 140 ℃, simultaneous decomposition, rapid decomposition at 170 ℃, softening point close to the decomposition point, decomposition released into HCl gas.

② Poor thermal stability, temperature and time can lead to decomposition, and poor fluidity.

Screw parameter selection:

a. Temperature control is strict, and screw design should be as low as possible to prevent overheating.

b. The screw and barrel should be corrosion-resistant.

c. The injection molding process needs to be strictly controlled.

d. Generally speaking, the screw parameters are L/D=16-20, h3=0.07D, ε= 1.6-2, L1=40%, L2=40%.

e. To prevent material accumulation, there is no check ring, and the head taper is 20 °~30 °. It is suitable for soft adhesive. If the product requirements are high, a separate screw without a measuring section can be used. This type of screw is more suitable for hard PVC, and to cooperate with temperature control, cooling water or oil holes are added inside the feeding section screw, and cold water or oil grooves are added to the machine barrel. The temperature control accuracy is about ± 2 ℃.