Several commonly used plastic injection screw selection points

The screw of injection molding machine has the functions of conveying, melting, mixing, compressing, measuring and exhaust, etc. The following to introduce a few commonly used plastic injection molding screw selection of the main points.

Polycarbonate PC

PC features are:

Amorphous plastic, no obvious melting point, glass transition temperature of 140~ 150℃, melting temperature of 215~225℃, molding temperature of 250~320℃. High viscosity, more sensitive to temperature, good thermal stability in the normal processing temperature range, 300℃ for a long time to stay basically does not decompose, more than 340℃ began to decompose, viscosity is less affected by the shear rate. Strong water absorption.

Screw injection molding PC should pay attention to the following points:

(1) according to its characteristics of good thermal stability and high viscosity, a large length-diameter ratio is selected as far as possible to improve the plasticizing effect, usually 26. Because its melting temperature range is wider, the compression section can be longer, so the gradient screw is used. L1=30%L, L2= 46%L.

(2) compression ratio: gradient A needs to adapt to the melting rate, but the melting rate cannot be calculated by express. According to the processable properties of PC from 225 ° c to 320 ° c, the gradient A value can be relatively higher than the average value.

(3) because of its high viscosity and strong water absorption, mixing structures are added to the screw before the homogenization section and after the compression section to strengthen the disintegration of the solid bed, and at the same time, the water entrainment can become gas escape.

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

Nylon PA

The characteristics of PA are as follows:

The melting point of crystalline plastic varies from variety to variety, and the melting point range is narrow. Generally, the melting point of PA66 is 260~265°C. Low viscosity, good fluidity, obvious melting point, poor thermal stability. Average water absorption.

The screw of injection molding PA should pay attention to the following points:

(1) mutant screw with a length-diameter ratio of 18~20 was selected.

(2) the compression ratio is generally 3~3.5, in order to prevent overheating decomposition h3=(0.07~0.08)D.

(3) due to its low viscosity, the gap between the stop 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 stop ring, and the nozzle should be self-locked.

(4) other parameters can be designed according to the universal screw.

PMMA

The characteristics of PMMA are as follows:

The vitrification temperature is 105℃, the melting temperature is more than 160℃, the decomposition temperature is 270℃, the forming temperature range is very wide. High viscosity, poor fluidity, good thermal stability. Strong water absorption.

Screw injection PMMA should pay attention to the following points:

(1) select the gradient screw with the aspect ratio of 20~22, and generally L1=40%L and L2=40%L according to the molding accuracy requirements of its products.

(2) the compression ratio is generally 2.3~2.6.

(3) in view of its hydrophilicity, mixing ring structure is adopted at the front end of the screw.

(4) other parameters can be generally designed according to the general screw, the gap with the barrel should not be too small.

Polyester PET

The characteristics of PET are as follows:

The melting point is 250~260℃, and the molding temperature of blown PET is a little wider, 255~290℃.

Screw for injection PET should pay attention to the following points:

(1)L/D is generally 20, L1=(50%~55%) L, L2= 20%L.

(2) screw with low shear and low compression ratio, compression ratio 1. Generally, 1.8~2, and hs= 0.09d to prevent overheated shear from causing discolation or opacity.

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

(4) because this kind of material is more sensitive to the temperature, and generally manufacturers use recycled materials, in order to increase the output, generally used is low shear screw, so can appropriately increase the motor speed, in order to achieve the purpose. At the same time in the use of recycled materials (most of the sheet materials), according to the actual situation, in order to increase the conveying capacity of the feeding section, also adopted the way of increasing the blanking caliber in the barrel, and achieved a better effect.

Polyvinyl chloride PVC

PVC is heat sensitive plastic, generally divided into hard and soft, its difference lies in the amount of plasticizer added in the raw material, less than 10% is hard, more than 30% is soft.

Characteristics of PVC:

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 in HCl gas. The disadvantages are poor thermal stability and poor fluidity.

The screw of PVC injection molding should pay attention to the following points:

(1) screw design should be low shear as far as possible to prevent overheating.

(2) screw, barrel to prevent corrosion.

(3) generally speaking, screw parameters are L/D=16~20, h3= 0.07d, e=1.6~2, L1 = 40%L, L2= 40%L.

(4) in order to prevent material storage, the head taper is 20°~30°, suitable for soft adhesive; If the product requirements are higher, can be used without measuring section, separated screw, this kind of screw is more suitable for rigid PVC; In addition, for the purpose of temperature control, cooling water or oil holes are added to the screw in the feeding section, and cold water tank or oil groove is added to the barrel. The temperature control precision is about 2℃.