Introduction to plastic screw and principle of plasticization

Function of injection screw:

Feeding, conveying, compression, melting, exhaust, homogenization
Important geometric dimensions of the screw:
Screw diameter, feed section, compression section, metering section, feed tooth depth,
Measuring tooth depth

Screw diameter (D)
‧Related to the required injection volume
Injection volume = 1/4π‧D2‧ (injection stroke)‧0.85
‧Generally speaking, D2 is inversely proportional to the highest injection pressure
‧The larger the D, the larger the extrusion rate; Q ≒ 1.29D2HmNr‧60/1000 (kg/ Hr)
Feeding section
‧Responsible for plastic transportation, pushing and preheating
‧Should ensure that the melting starts at the end of the feeding section and preheat to the melting point.
‧Solid specific heat ↑, melting point ↑, latent heat ↑, heating to the melting point needs
There is a lot of heat, and the feeding section should be long. The solid thermal conductivity is ↓, and the heat transfer is slow
、The temperature rise of the plastic center is slow, the feeding section should be long to preheat ↑, feeding
The segment can be short.
‧The longest crystalline material (such as POM, PA); amorphous material
Second (such as: PS, PU); the shortest heat sensitivity (such as: PVC).

Compressed segment
‧Responsible for the mixing, compression and pressurized exhaust of plastics, through this
The raw materials of the section should have almost completely melted, but not necessarily
Will mix evenly.
‧In this area, the plastic gradually melts, and the volume of the screw groove must be corresponding
Decrease, otherwise the material will not be compacted, heat transfer will be slow, and exhaust will be poor.
‧For amorphous plastics, the compression section should be longer, otherwise the screw groove
The volume drops quickly, and the material volume is not reduced, which will cause blockage.
‧Crystalline plastics are actually not completely crystallized (e.g. PE: 40～
90% crystallinity, LDPE: 65% crystallinity), so the mesh
The front compression section has a tendency to lengthen.
‧Generally, it accounts for 25% of the working length of the screw.
‧Nylon (crystalline material) has 2 to 3 turns, which accounts for about 15% of the working length of the screw.
‧High viscosity, fire resistance, low conductivity, high additives, accounting for 40%
~50% of the working length of the screw.
‧PVC can use 100% of the working length of the screw to avoid shock
Strong shear heat.
Metering section
‧Theoretically, at the beginning of the metering section, the material should be completely melted, but until
At least the metering section = 4D to ensure uniform temperature and uniform mixing.
‧Long metering section will result in better mixing effect; too long metering section will cause melting
If the body stays for too long, thermal decomposition will occur; if it is too short, the temperature will not change.
Evenly.
‧Generally, it accounts for 20~25% of the working length of the screw.
‧PVC heat sensitivity, should not stay too long to avoid thermal decomposition (not necessary
Metering section).
Feeding tooth depth, measuring tooth depth
‧The deeper the feed tooth, the greater the throughput in the feed area, but it needs to be considered
Screw strength.
‧The shallower the measured tooth depth, the higher the heating and mixing performance index of plasticization.
But it is necessary to prevent the plastic from scorching, (the measuring tooth depth is too shallow, then the shear heat
Self-generated heat ↑, the temperature rise is too high, especially not conducive to heat-sensitive plastics. )
‧Measuring tooth depth = KD = (0.03～0.07)D
-D ↑, K is small; D↓, slenderness ratio ↑, plastic with poor thermal stability
Material, K chooses the big