VERY USE FULL FOR ALL INJECTION MOLDING STUDENT. Screw is the heart for most of the ideal process parameters in injection molding.

1. BY
NAIK DEVANG
18/06/2011

2.  Converting solid granules in to homogeneous
melt is known as plasticizing.
 The screw has three sections. FEED zone,
COMPRESSION zone and METERING zone.
 Plastic material under shear changes its
viscosity. This is Shear Thinning
characteristic of plastics.
 At the tip of the screw non return valve is
fitted.

3.  Standard three zone screw.
 Mixing screw.
 Barrier screw.
 Screw for Thermo set.

4.  Higher plasticating power.
 Lower torque requirement.
 Much better homogenization.
 Longer residence time.

5.  Feed zone promotes out put rate. Higher
channel depth at feed zone would
increase the o/p.
 Channel volume at feed zone is not
fully occupied by the material because
of inevitable space between granules.
 Higher the diameter of the screw,
higher channel volume per pitch, hence
higher the o/p.

6.  FEED ZONE
 COMPRESSION ZONE
 METERING ZONE

7.  Feed zone.(Feed Zone 60 to 50 %)
 Compression zone.(20 to 25%)
 Metering zone.(20 to 25 %)

8.  Nitralloy : A tool steel machined & nitride for 72 hrs.An
excellent low cost alternative when excessive wear is not a
primary consideration.
 4140: A tool steel with good strength when surface threted or
hard faced.
 D-2 A high carbon/high chrome tool steel. Good wear resistance
but low on torque strength.
 CPM-9V : Excellent for abrasive application with very good
wear resistance.
 Surface Treatment.
 NitriDiNg: Increase the hardness & life of tool steel screw such
as 135M & 4140.Very cost effective.
 ChroMe PlatiNg: Offers hardness & low friction with
corrosion protection.
 CarbiDe CoatiNg: Carbide Coating-Guaranteed to double
the life of a 9V screw.


9.  Longer feed zone creates a greater potential
throughput.
 Longer compression zone results in less shear heat
and more time to compress and melt the plastic.
 Longer meter zone allows the screw to develop more
pumping pressure and greater opportunity to
homogenize.
 Shorter meter zone allows less time to assure uniform
melt temperature and viscosity.

10.  Feed and compression zone channel depths are
depended on the selected compression ratio and
screw profile.
 Meter zone of the screw is decided by the plastic to
be processed.
 Higher channel depth in feed zone conveys more
material and hence more torque is required to rotate.
 Shear rate is lower on the polymer in screw with
deeper channel depth at lower rpm.It is preferred for
shear sensitive polymers.
 Shallow channel depth screw requires higher rpm
 And lower torque for a given o/p rate.

11.  L/D ratio
 Signification of L/D ratio.
 Influence of changing the L/D ratio.?

12.  This is the ratio of the length of the screw The
flighted length) to its diameter.
 For thermoplastics most suitable ratio is 20:1.
 Advantage for small L/D.1Less residence time in the
bareel.2 Occupies less space.3 Requires less torque,
making strength of the screw and amount of power
less important. 4 Less investment cost.
 Advantage of large L/D.1 Allows a screw design for
greater o/p or recovery rate,sufficent torque is
available.2 Uniform o/p and greater mixing.3 Screw
can be design to pump at higher pressure.

13.  Normally it is between 1.5 & 4.5 for universal screw,
which is capable of processing most of the polymers.
 The CR should be high enough to compress the low
bulk density un melted plastic into solid plastic
without air pockets
 Engineering polymers recommended CR is 2 to 2.5.
 Higher the compression ratio, greater the resulting.
 Shear heat imparted to the polymer.
 Heat uniformity of melt.
 Energy consumption.

14.  Some additives are corrosive in the nature.
 Some has limited residence time.
 Degrade and generate corrosive gases.
 Flame retardant grades produce corrosive acids at
certain temperature.
 % utilization should be such that melt remains stable
avoiding critical residence time.

15.  Plasticizing rate is the amount of homogeneous
melt produced by the screw per unit time. It is
specified as gms per sec/kg per hr.
 Cycle time = Tot shot per
remolded(gms)/Maximum plasticizing
rate(GR/Sec)

17.  Hot gloves, Brass Gauze round
wire brush,Stearic acid
flake,Several cotton rags.
 Don't use Steel Screw
drivers,Scrapers,Or Bars.

18.  NAIK DEVANG