Injection Screws

Screw Designs in Injection Molding

Page 1
The classical three-section screw can be
employed with adequate quality for the pro-
cessing of many thermoplastics. When
demands on throughput performance and
molding quality rise, however, the three-sec-
tion screw comes up against its limits. If
requirements with regard to throughput and
melt homogeneity are high double- and multi-
ple-flighted screws and also barrier screws
afford significant advantages. Homogene-
ity may also be improved by means of addi-
tional shear and mixing sections. Efficient
optimization of screw geometry taking the
many mutually interacting parameters into
consideration is possible by means of com-
puter simulations. BASF possesses on the
one hand a suitable simulation program and
on the other hand has many years of experi-
ence in the optimization and use of screws.
Technical Information for Experts 01/99e
Design
Materials
Information
System
Processing Testing
Screw designs in
injection molding
------------------------------------------------------------------------
Page 2
2
Optimized injection molding plasti-
cating units
On account of the numerous tasks and
demands to be fulfilled the design and
optimization of injection molding plasti-
cating units is an optimization problem
with a number of objectives. At the
same time the individual variables are
not independent of one another but
rather mutually interact with one
another.
The aim in general is to achieve a
quantitative and qualitative improve-
ment. However, in doing so it cannot
be excluded that a conflict will arise
between target parameters. A typical
case which is relevant in practice is the
requirement for a higher mass flow rate
(shorter plasticizing time) while at the
same time melt quality is improved and
the melt temperature is reduced. In
injection molding in contrast with ex-
trusion there is frequently the additional
difficulty that a wide processing range
for the most varied shot weights is
required. Accordingly, it is necessary in
many cases to define priorities or to
accept compromises.
Furthermore, when drawing up the
specification of requirements
constraints arising from financial or
technological considerations also have
to be taken into account. For example,
when optimizing the plasticating unit of
an existing injection molding machine
constraints due the machine size
(screw length), the drive power, etc.
must be borne in mind. In this way
solutions which are too costly in pro-
duction terms often drop out even
though they would offer processing
advantages.
Accordingly, in most cases the techni-
cally feasible solution is not the opti-
mum design but rather the compro-
mise solution which emerges from
weighing up all the relevant factors
affecting the qualitative and quantita-
tive requirements.
In injection molding technology today
conventional reciprocating screw injec-
tion units are employed almost exclu-
sively. For the processing of thermo-
plastics these are frequently equipped
with a single-flighted three-section
screw (Figure 1). As the name already
implies this type of screw is character-
ized by the division of the screw into
three different sections (feed, compres-
sion and metering sections) which
have different functions.
At the tip of the screw there is usually
also a nonreturn valve to prevent the
melt in the space in front of the screw
from flowing back during the injection
and pressure hold-on phase.
Three-section screw
Three-section screws which are also
commonly known as "universal
screws” are designed in such a way
that they can process as much ther-
moplastic as possible at
an adequate level of
quality. Due to the sim-
ple geometry the struc-
ture of the screw allows
low-cost production.
Modern standard screws
have an overall length of
20 – 23 D (length as a
multiple of the diameter)
the length of the feed
section accounting for
approximately half the
length of the screw. The
compression and meter-
ing sections have
approximately the same
length, the pitch is usu-
ally 1 D (0.8 – 1 D) and
the flight depth ratio
between the feed and
metering zones is between 2 and 3.
The flight depths recommended by
BASF are shown in Figure 2 as a func-
tion of the screw diameter.
In the flight depths illustrated in Figure
2 a distinction is made between stan-
dard screws and shallow-flighted
screws. Shallow-flighted screws pick
up less material and hence the resi-
dence time in the plasticating unit is
shortened. This can be advantageous
in the case of thermally sensitive mate-
rials.
D
L
L
L
D
0.5-0.55
0.25-0.3
0.2
0.8-1.0
D
L
L
F
L
C
L
M
h
M
h
F
P
N
20–23
h
F
h
M
N
P
L
L
M
L
C
L
F
D
External screw diameter
Effective length of screw
Length of feed section
Length of compression section
Length of metering section
Flight depth in the metering section
Flight depth in the feed section
Pitch of screw
Nonreturn valve
Screw diameter D (mm)
Flight depth h (mm)
30 40
60
80 90
130
2
4
6
12
50
70
100
10
8
110 120
h
F
h
M
h
F
= flight depth in the feed section
h
M
= flight depth in the
metering section
Standard screw
Shallow-flighted
screw
Figure 2: Flight depths for three-section screws
Figure 1: Plasticating unit with a three-section screw
------------------------------------------------------------------------
Page 3
3
In addition, high-performance screws
possessing mixing sections are con-
structed with a screw length of up to
approximately 26 D for high-speed
machines (e.g. for packaging materi-
als). Long screws are not advisable for
processing thermally sensitive material
grades since overlong residence times
(throughput rates are usually lower
than in extrusion) can result in thermal
damage to the material.
The three-section screw comes up
against its limits as a result of height-
ened demands for molding quality and
throughput rates. In particular, the
three-section screw without shear and
mixing sections encounters its perfor-
mance limits in direct coloring.
Although the depth of experience is
currently greatest with this type of
screw the constantly growing demands
cannot always be adequately fulfilled.
Growing competitive pressures require
the selection of injection molding
machines having an injection and
clamping unit which is as small as
possible in order to minimize article
costs through low investment and
operating costs. The trend in injection
molding machines is accordingly going
in the direction of larger shot volumes
and higher mass flow rates for the
same size of plasticating unit in order in
this way to increase the economic
efficiency of the process. This, how-
ever, means a conflict in target para-
meters when improvements in product
quality are simultaneously being aimed
for. For that reason priorities have to be
defined or compromises have to be
accepted.
Larger shot volumes are achieved at
low cost by the simple expedient of
lengthening the metering stroke (> 3
D). Lengthening of the metering stroke
has the consequence that the effective
screw length is shortened. This can
result in unmelted material and inho-
mogeneous temperatures. To eliminate
these problems the screws must be
lengthened but for structural reasons
this cannot be done to an unlimited
extent in injection molding plasticating
units.
There is a further risk in longer meter-
ing strokes that ever more air is drawn
in, especially during injection. This is
because the screw moves only axially
(no rotation) under the hopper opening
during injection, as a result of which
the screw channels are not completely
filled with material (Figure 3).
The more air that is drawn in by length-
ened metering or injection strokes the
more difficult it is to ensure that this air
escapes via the hopper and does not
get into the space in front of the screw.
Aspirated air which is then occluded in
the melt and gets into the mold pro-
duces streaks, so this must be
avoided. The minimum condition for
flawless parts is that there should be a
sensible ratio between the maximum
possible metering stroke and the effec-
tive length of the screw (i.e. that the
maximum metering stroke for a screw
20 D long should be limited, for exam-
ple, to 3 D in order to achieve ade-
quate quality).
In three-section screws it is very diffi-
cult to further increase the plasticizing
rates and shot weights attained so far
while obtaining the same or an im-
proved level of homogeneity. Due,
however, to the continuous shortening
of the cooling and movement times the
throughput rates must be increased so
that the plasticizing time does not
become the variable determining the
cycle time.
An increase in throughput with suitable
homogeneity can only be achieved
when the melting efficiency is simulta-
neously improved. For this reason a lot
more thought has been devoted
recently to structural modifications of
the screws. At present this includes
research into the use in particular of
double- or multiple-flighted screws,
barrier screws and polygonal screws
combined with shear and/or mixing
sections in injection molding machines.
All shear and mixing sections have in
common the basic principles of screw
clearance and the division and recom-
bination of the stream of melt. The
Maddock and spiral shear section and
the toothed disk or faceted mixing
section are predominantly used (see
Figure 4). These sections should be
designed as far as possible to be neu-
tral in terms of pressure so that the
throughput rate is not reduced, to
minimize wear and in order not to have
a detrimental effect on the melt tem-
perature.
Figure 3:
Trickle feed of
material during
injection
Partly filled screw channels
Injection stroke
Injection
V
Figure 4:
Three-section
screw with
commonly used
shear and mixing
sections
Maddock shear section
Spiral shear section
Faceted mixing section
Toothed disk mixing section
------------------------------------------------------------------------
Page 4
4
Double- and multiple-flighted
screws
A possibility for raising melting effi-
ciency and hence throughput consists
in constructing the screw with multiple
flights. By comparison with the sin-
gled-flighted design the increase in the
number of flights in multiple-flighted
screws having otherwise unchanged
screw geometry yields smaller melt film
thicknesses at the cylinder wall as
can be seen in Figure 5.
As a result of the lower film thickness
there is on the one hand higher heat
transfer from the cylinder to the solid.
On the other hand the lower film thick-
ness gives rise to higher shear speeds
in the melt film which results in higher
energy dissipation and hence
improved melting efficiency.
However, in a multiple-flighted con-
struction of the screw it must be borne
in mind that a reduction of the cross
section of the screw channel is pro-
duced, especially in the feed section
(see Figure 6). When the channel width
is too small there is not enough room
for the material to trickle unimpeded
from the hopper into the screw chan-
nels. This effect is especially notice-
able when the screw diameters are
relatively small (feed behavior, through-
put rate). As the screw diameter rises
the effect of the additional screw flight
becomes steadily less unfavourable
(starting from about 80 mm).
It must furthermore be taken into
account that in a multiple-flighted
design the improved melting efficiency
is not always desirable in the case of
amorphous and/or temperature-sensi-
tive plastics. If melting occurs too
quickly unwelcome temperature rises
and possible damage to the material
may be produced over the remaining
length of the screw. Accordingly, a
double-flighted screw design should be
chosen when high mass flow rates
(high-speed machines) combined with
high melting efficiencies (e.g. in the
case of polyolefins) are required.
Barrier screws
There has recently been an upsurge
of interest in barrier screws for the
injection molding process on account
of the results achieved in extrusion. In
injection molding, however, the bar-
rier screw must be adapted to the
changed boundary conditions (batch
mode of operation, shortening of
screw as a function of metering
stroke, etc.) by comparison with
extrusion.
Figure 5: Section through the melting section of a single-
flighted and a double-flighted screw
Figure 6: Charging cross sections in single-
flighted and double-flighted screws
Figure 7: Principle of a barrier screw
Barrier flight
Solids channel
Melt channel
Hopper opening
Double-flighted
Single-flighted
Second flight
(reduction of
cross section)
Barrel wall
Melt film
Screw flight
Solids bed
Melt vortex
Screw flight
Solids bed
Melt vortex
Melt film
Barrel wall
Single-flighted screw zone
Double-flighted screw zone
------------------------------------------------------------------------
Page 5
5
In principle all barrier screws have the
same mode of operation. The charac-
teristic feature is the division of the
screw channel into a solids channel
and a melt channel (see Figure 7). The
solids channel is separated from the
melt channel by the barrier flight. The
barrier flight has a greater gap width
than the main flight so that only fused
material or particles which are smaller
than the gap width in at least one
direction can pass into the melt chan-
nel. On flowing over the barrier flight
these particles are exposed to an addi-
tional defined shear stress which
results in further melting of the residual
solid particles. The barrier flight, more-
over, contributes to homogenization of
the melt.
In the barrier zone the cross section of
the solids channel reduces in the direc-
tion of the tip of the screw while at the
same time the cross section of the melt
channel increases. In the various types
of barrier screws this change in cross
section is achieved by varying the flight
depths and/or channel widths.
The pitch is frequently increased right
at the beginning of the barrier zone in
order to provide the cross section
needed for a second channel. At the
same time it proves to be advanta-
geous for the width of the solids chan-
nel at the inlet to the barrier zone to be
the same as the channel width ahead
of the barrier zone. This avoids abrupt
deformations of the solids bed during
passage into the barrier zone. In order
to ensure controlled melting the outlet
of the solids channel should be closed
so that the material gets into the melt
channel and hence into the space in
front of the screw only via the barrier
gap.
Computer-supported simulation is
useful especially in the design of barrier
screws because the barrier zone pos-
sesses a relatively high number of
degrees of freedom in its design.
Although the screw geometry can be
better adapted to a specific application
the system also responds more sensi-
tively under certain circumstances and
contains more possible sources of
errors.
Optimization and simulation
In the injection molding process prob-
lems such as
difficulties in feeding
excessively high melt temperatures
unmelted material
streaks and voids
material and thermal
inhomogeneities
fluctuations in shot weight and
plasticizing time
wear of the screw and cylinder
can occur.
When they occur the injection molding
plasticating unit is frequently cited as
the general cause of these problems
but without more precise specification.
The reason for this is that the plasticat-
ing unit is regarded as a black box
which is not susceptible to direct
observation. Only a few variables such
as the melt temperature, heating zone
temperatures, torque and the pressure
in the space in front of the screw can
be called upon to evaluate the operat-
ing behavior. Matching these quantifi-
able variables to their causes is often
difficult and requires a great deal of
know-how. There is a major informa-
tion gap here with regard to interde-
pendencies capable of supporting
efforts towards process optimization or
redesign of the unit.
The combination of the empirical
knowledge gathered in BASF over a
long period of time with the results of
simulation computations is an ideal
basis for rendering the plasticization
process more transparent and more
predictable. This is essential for discov-
ering weaknesses and if possible arriv-
ing reliably and speedily at an opti-
mized screw geometry (Figure 8).
Apart from attaining the targeted mass
flow rate complete and proper melting
is of decisive importance in the opti-
mization of a plasticating unit. The
reason for this is that the attainment of
the maximum possible throughput only
makes sense when complete melting
and adequate homogeneity can be
ensured.
Figure 8: More predictable plasticating units as a
result of simulation
Computation
BASF knowledge
based on experience
Plasticizing time
Pressure profile
Temperature profile
Melting profile
------------------------------------------------------------------------
Page 6
6
With the aid of the simulation program
the dimensionless solids bed width Y is
calculated. This is defined as the ratio
of the solids bed width x to the channel
width b (to the solids channel in the
case of barrier screws). Figure 9 shows
a favorable and an unfavorable melting
process. The interpretation of the
progress of melting allows statements
about the relationship between the
conveying performance of the screw
and its melting performance and a
qualitative estimate of the level of
homogeneity of the melt achievable in
the space in front of the screw. At the
same time the two effects illustrated in
Figure 9 (increases in the dimension-
less solids bed width and residual
solids content at the beginning of a
shear or mixing section) are of particu-
lar importance.
An increase in the dimensionless solids
bed width can cause the solids bed to
break apart due to high deformation
and result in the formation of individual
islands of solids which are no longer
effectively melted by shearing. Espe-
cially in the case of high-melting poly-
mers (high enthalpy requirement) in
association with compression which is
applied too early or too strongly there
is the risk that the dimensionless solids
bed width again rises to the value of 1
which is synonymous with “clogging”
of the screw. In this case in practice
restriction of the mass flow rate and
homogeneity problems can be
expected.
In addition to the rise in the curve of
the dimensionless solids bed width the
presence of residual solids at the end
of the screw or at the beginning of a
shear or mixing section must be pre-
vented. In the case of relatively small
amounts of residual solids this would
result in an inhomogeneous melt con-
taining isolated particles of solid. If the
amount of solids is higher, back pres-
sure can build up in shear sections and
throughput can fluctuate. If the gap
were clogged up high pressure loss
(lower output) would additionally occur.
Blockage of shear webs can occur not
only in shear sections having sealed
channels but also in barrier zones hav-
ing closed inlet and outlet channels.
This is the case when the melting effi-
ciency of the barrier zone is too low or
the site of melt vortex formation is
located in the barrier zone.
By comparison with shear sections,
barrier zones have the advantage here
in that on account of their normally
greater length blockages due to melt-
ing efficiency occur only locally. How-
ever, these local blockages can result
in unwanted fluctuations in throughput
and should, therefore, be avoided.
In injection molding unlike extrusion it
is more difficult due to the axial dis-
placement of the screw to fix the
beginning and the length of the barrier
zone. It is a problem here that shot
weights and hence metering strokes
should vary. That is to say that
depending on the metering stroke the
start of the barrier zone is closer to or
further away from the hopper, the site
of initial melt formation remaining
unchanged. For problem-free opera-
tion it must be ensured that the site of
initial melt formation is located ahead
of the barrier zone in order that sepa-
ration into solids and melt can be
effected by the barrier web. Otherwise
the melt channel will only be partly
filled which is synonymous with a
reduction in throughput or an increase
in plasticizing time. Local overheating
can, moreover, occur in the solids
channel (especially in the closed outlet)
if insufficient material is melted which
then flows into the melt channel.
With the correct sizing the result illus-
trated in Figure 10 is obtained: com-
plete separation of solids and melt and
a full melt channel. In this way high
throughputs of adequate homogeneity
can be achieved. It must, however,
also be noted that this result cannot
be achieved with a single geometry in
optimum manner for the entire spec-
trum of materials and all operating
points, as is also the case for other
screw designs.
In addition to the effects of the screw
geometry, the process parameters
(speed of rotation) and the residence
time, the melting process is also
affected by the properties of the mate-
rial. There are, for example, great dif-
ferences in enthalpy (melting energy)
and viscosity (flow properties) between
amorphous and semicrystalline plastics.
Figure 10: Barrier zone with closed inlets and
outlets
Figure 9: Comparison of different melting profiles
Barrier zone
Melt channel
Solids channel
Melt channel
Solids channel
25
20
15
10
5
0
Screw length (L/D)
1.0
0.5
0.0
b
Normalized solids bed width
Y
Favorable
Unfavorable
x
b
Y=
x
Site of melt
vortex formation
------------------------------------------------------------------------
Page 7
Ultramid
®
A3W
Polystyrene 168 N
0
20
16
10
0
0.5
1
Normalized solids bed width
Y
Screw length (L/D)
“How much screw” or which design
should be employed must be decided
in line with the application. At the
same time requirements, particularly
with regard to homogeneity, can be
highly variable.
The aim of this technical information
paper was to set out possibilities for
screw designs. The universal screw
which solves all problems using a sin-
gle geometry continues to evade us.
There are, however, enough possibili-
ties for fulfilling the requirements
imposed.
7
In connection with this Figure 11
shows by way of example the enthalpy
and viscosity curves for an amorphous
and a semicrystalline material. From
the large differences in the energy
needed for melting and in flow proper-
ties it may be inferred that it is difficult
using just one screw geometry to
process a great many materials in
optimum manner and to do this at
highly variable operating points.
To make this clear Figure 11 provides
a comparison of the course of melting
for a polystyrene (PS) and an Ultramid
®
(PA) using the same screw geometry. It
is evident that there are great differ-
ences in the patterns (end of melting).
It would be difficult to arrive at one
optimum design for the screw geome-
try for use with both materials. The use
of the same screw for both
materials assumes that only
relatively low demands are
placed on output rate, the
possible shot weights and
homogeneity.
Conclusion
Even today standard or universal
screws such as the three-section
screw cover a large part of the range of
requirements and materials. However,
there has recently been an increase in
the incidence of cases in which higher
demands are imposed on the melt
quality (homogeneity) than can be met
by a three-section screw. In order to
attain the required homogeneity, addi-
tional barrier zones as well as shear
and/or mixing sections are then
employed. The extent to which homo-
geneity can be increased by such mea-
sures can be seen in the example
shown in Figure 12.
Figure 11: Enthalpy, viscosity and melt-
ing profiles for two different materials
Three-section screw
Figure 12: Homogeneity results
Three-section screw with
shear and mixing section
Barrier screw with
shear and mixing section
Temperature (°C)
Enthalpy (J/g)
Room
temperature
PS melt
temperature
PA melt
temperature
0
200
400
600
800
0
100
200
300
Ultramid
®
(PA 66)
Polystyrene (PS)
Shear rate (1/s)
Viscosity (P
A*s)
10
-
1
280°C: Ultramid
®
A3W
10
0
10
1
10
2
10
3
10
4
10
5
10
1
10
0
10
2
10
3
10
4
230°C: Polystyrene 168 N
------------------------------------------------------------------------
Page 8
BASF Aktiengesellschaft
67056 Ludwigshafen
Germany
®=
r
eg. T
radename of BASF Aktiengesellschaft
KTX 9908 e 12.2000
Note
The information submitted in this
publication is based on our current
knowledge and experience. In view of
the many factors that may affect pro-
cessing and application, these data do
not relieve processors of the responsi-
bility of carrying out their own tests
and experiments; neither do they imply
any legally binding assurance of certain
properties or of suitability for a specific
purpose. It is the responsibility of those
to whom we supply our products to
ensure that any proprietary rights and
existing laws and legislation are
observed.

Basf