Page 115 - Module DMV20173

6.0 POLYMER PROCESSING

6.2.3 INJECTION SCREW

1. The comments for the extrusion screw operation apply to the injection screw equally.
2. The compression ratios are usually in the range 2.5:1 to 4:1 and the common L/D ratios

are between 15 and 20 to 1.
3. Most screws are capable of producing injection pressures of 30,000 psi (210 MPa).
4. The one important difference from the extruder screw is the addition of a back-flow
check valve fitted to the end of the screw.

5. The screw of the valve is to stop any back flow of molten polymer across.

6.2.4 NOZZLE

1. A nozzle is screwed into the end of the barrel to provide a means of transferring plastic
melt from the barrel to the mold via the sprue bushing.
2. It acts as a simple "push to seal" connection.
3. The nozzle is a region where the melt can be heated both by friction and conduction

from a heater band before entering the relatively cold channels of the mold.
4. Contact with the mold causes heat transfer from the nozzle and in cases where this is
excessive it is advisable to use nozzle retract during the screw back part of the cycle.
5. There is the possibility of the melt freezing in the nozzle.

6.2.5 CLAMPING SYSTEM

1. The clamping mechanism is the second major portion of the injection molding machine.
2. The function of the clamp mechanism is to make sure the following sequence is carried
out:-

i. Bring the mold together quickly.
ii. Close the mold gently.
iii. Part it slowly.
iv. Move it rapidly to full open.
v. Strip the mold slowly.

3. While carrying out the above actions, the clamp must be capable of holding the mold
closed during the injection and cooling part of the cycle.

BPLK 147 DME 1083