Injection Screw & Plastic Materials

The basic types of injection screw and main parameters:


Injection screw is generally divided into three sections compression section homogenization section.


feed section——bottom diameter is smaller, the main role is  to transport raw materials to the rear section. So the main problem is the transmission capacity. Parameters (L1, h1), h1= (0.12-0.14) D.


compression section—— variable bottom diameter, the main role is  to compact and melt plastic materials, create pressure. Parameters the compression ratio ε=h1／h3 and L2. Gradation degree A = (h1 – h3)/ L2.


homogenization section(metering section) —— push the materials melted in the compression section to the front of the injection screw quantitative in a constant temperature. Parameters (L3, h3), h3 = (0.05-0.07) D.


For whole screw, the parameter L/D ——length-diameter ratio.


L/D pros and cons: L/D and revolution speed n, are the important factors of injection screw capacity and effect. If L/D is large, the materials will has a long residence time in the barrel. It is propitious for plasticizing, and lessen pressure flow and leakage current in the meantime. It’s improves the plasticizing capacity, while avail the materials which has high requirements on the temperature distribution. But it’s also has negative impact on the manufacture, assembly and use. L/D generally equal with 18-20, but the trend has increased.

Pitch S, lead angle φ =πDtgφ, generally, if D = S, then φ =17°40'.


φ impacts on the plasticizing capacity, in general, the larger φ the faster transmission speed. So different materials shapes with different φ values. Powder desire φ around 25°, cylindrical materials desire φ around 17°, square materials desire φ around 15°. But the different φ values, in terms of processing, will also be more difficult, so it is usually taken φ17°40′.


Rib width e, in terms of low viscosity materials, e try to get bigger, too small to leakage, but too big will increase power consumption and easy to overheat. e =（0.08～0.12）D.

Materials features are listed below, you can choose an appropriate injection screw base different materials.

• PC materials(polycarbonate)

Features: ① non-crystalline plastic, no obvious melting point, glass transition temperature of 140 ° ~ 150 ℃, melting temperature of 215 ℃ ~ 225 ℃, mold temperature 250 ℃ ~ 320 ℃.

② large viscosity, temperature sensitive, good heat stability in the normal processing temperature range, does not break down and long-stay in 300 ℃, begins to decompose when more than 340 ℃, the viscosity has less impact under shear rate.


③strong absorbent

• PMMA(plexiglass)

Features: ① glass transition temperature of 105 ℃, melting temperature higher than 160 ℃, decomposition temperature 270 ℃, forming temperature in a wide range.


② good viscosity, poor mobility, good thermal stability.


③ strong water absorption.

• PA (Nylon)

Features: ①crystalline plastics, more types of different species, and different melting point and narrow melting point range, generally used in PA66 melting point of 260 ℃ ~ 265 ℃.


② low viscosity, good liquidity, a relatively sharp melting point, poor thermal stability.


③general water absorption.

• PET (polyester)

Features:① melting point of 250 ℃ ~ 260 ℃, blow molding grade PET has a large range of the molding temperature, about 255 ℃ ~ 290 ℃.


② PET blow molding grade has high viscosity and temperature has large impact on the viscosity, poor thermal stability.

• PVC (polyvinyl chloride)

Heat-sensitive materials, usually divided into hard and soft, the difference is the amount of plasticizer added to raw materials, less than 10% for the hard, more than 30% of soft.


Features: ① no obvious melting point, 60 ℃ soft, 100 ℃ ~ 150 ℃ viscoelastic state, melt in 140 ℃, decompose in the meantime, decompose rapidly at 170 ℃, softening point close to the break point, break down and release of the HC1 gas.


② poor thermal stability, temperature, time all will lead to decomposition, poor liquidity.


(Edit by market02@sinogroupe.com SINO MOULD)