How to choose the screw of the injection molding machine

2021-03-23 10:56:29 admin

As we all know, the "screw plasticization component" of the injection molding machine is the main core unit of the injection molding machine, which directly reflects the performance of the injection molding machine. The screw, known as the "heart", is a vital part, and its performance determines the quality and efficiency of the processed products.


With the rapid development of the plastics industry, injection molding equipment has matured. As designers and users of injection molding equipment, it is necessary to clearly understand which screws are suitable for the processing of which materials, especially in today's diverse materials, the types of screws on the market are also There are many changes every day, but in actual use it is rather chaotic. Users often use a certain screw to process plastics with unsuitable physical properties.


There is also a misunderstanding of concept: "Thinking that a screw is omnipotent", but it is not. In this way, a brief introduction of several commonly used screws with main structure is provided to provide some reference when processing products.


(1) Features of PC material (polycarbonate):


1. Amorphous plastic, no obvious melting point, glass transition temperature 140℃-160℃, melting temperature 215℃-225℃, molding temperature 230℃-320℃;


2. High viscosity, high processing temperature, narrow range, easy to decompose, avoid overheating when selecting screw;


3. Sensitive to moisture, the resin is easily hydrolyzed under high temperature, the drying temperature is 120℃-130℃, and the time is about 4 hours.


Parameter selection:


1. In view of its characteristics of good thermal stability and high viscosity, L/D should choose a large aspect ratio as far as possible in order to improve the plasticizing effect;


2. Due to its wide melting temperature range, long feeding section, short homogenization section and moderate compression ratio, barrier type over-rubber heads can be used to increase the plasticizing effect;


3. If adding flame retardant or glass fiber reinforced PC material, you can choose to use corrosion-resistant and wear-resistant high-quality stainless tool steel screw and three small pieces.


(2) PMMA (plexiglass) features:


1. The glass transition temperature is 105°C, the melting temperature is greater than 160°C, the decomposition temperature is 270°C, and the molding temperature range is 180-250°C;


2. The viscosity is large, the fluidity is poor, and the shear rate is more sensitive, so the temperature must be strictly controlled during processing;


3. The temperature of the barrel is 240℃-250℃, and the residence time does not exceed 8 minutes when it exceeds 260℃;


4. Strong water absorption, drying temperature 80℃-90℃, time over 3 hours.


Parameter selection:


1. L/D selects the ratio of length to diameter 20-22, and the gradual type screw generally takes L1=35-45%, L2=35-40%;


2. The compression ratio ε is generally 2.3-2.6;


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