Screw & Barrel Design

Screw & Barrel Design
Screw design varies widely depending upon the manufacturer. Almost all machines are
being made with single flighted standard type screws with one manufacturer using a
doublewave type of barrier screw.

1; flight land width : 0.1 (screw diameter)
2 flight/cylinder clearance : 0.001 (screw diameter)
3 screw length/diameter ratio : commonly 20-25 : 1
4 feed/transition/metering sections : typically the screw length will be divided 50%
feed, 25% transition and 25% metering.
5 materials : screws are frequently made from AISI 4140 heat treated steel. Flight
tips are usually hard surfaced with Stellite. Chrome plating of the screw is
common, for ease of cleaning. Barrels are normally bimetallic, with an Hastalloy
inner lining for wear resistance.
6;flight pitch : almost always square (0 = 17.7o)
7 mixers : some type of mixing device is frequently added to the screw to improve
melt homogeneity and pigment dispersion. Since pigments are commonly added
in the form of a masterbatch, a high-shear mixer is required to improve dispersion
(Maddock type).
8 compression ratio : C.R. is in the range of to 4 : 1; most commonly 2-3 : 1

9 check valve : screws are equipped with a one way valve at the tip to prevent melt
from flowing back through the screw during the ram injection stage. Two most
common types in use are the ring type and ball type.
10 injection nozzle : the function of the nozzle is to convey the melt from the
injection cylinder to the mould, with minimum pressure drop and heat loss.
Nozzles are generally kept short, with open streamlined flow passages to
minimize pressure drop and heat loss and to avoid material hang-up