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Technical Information for Experts     01/99e

Screw designs in                                Processing     Testing


injection molding

   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.
Optimized injection molding plasti-
cating units
                                                  N            D    hM       P                             hF
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                         LM           LC                             LF
not independent of one another but
rather mutually interact with one                                                                      L
another.                                        D     External screw diameter
                                                L     Effective length of screw                                           20– 23     D
The aim in general is to achieve a              LF    Length of feed section                                            0.5-0.55     L
quantitative and qualitative improve-           LC    Length of compression section                                     0.25-0.3     L
ment. However, in doing so it cannot            LM    Length of metering section                                             0.2     L
be excluded that a conflict will arise          hM    Flight depth in the metering section
between target parameters. A typical            hF    Flight depth in the feed section
case which is relevant in practice is the       P     Pitch of screw                                                    0.8-1.0      D
requirement for a higher mass flow rate         N     Nonreturn valve
(shorter plasticizing time) while at the
same time melt quality is improved and       Figure 1: Plasticating unit with a three-section screw
the melt temperature is reduced. In
injection molding in contrast with ex-
trusion there is frequently the additional   three different sections (feed, compres-                           metering zones is between 2 and 3.
difficulty that a wide processing range      sion and metering sections) which                                  The flight depths recommended by
for the most varied shot weights is          have different functions.                                          BASF are shown in Figure 2 as a func-
required. Accordingly, it is necessary in                                                                       tion of the screw diameter.
many cases to define priorities or to        At the tip of the screw there is usually
accept compromises.                          also a nonreturn valve to prevent the                              In the flight depths illustrated in Figure
                                             melt in the space in front of the screw                            2 a distinction is made between stan-
Furthermore, when drawing up the             from flowing back during the injection                             dard screws and shallow-flighted
specification of requirements                and pressure hold-on phase.                                        screws. Shallow-flighted screws pick
constraints arising from financial or                                                                           up less material and hence the resi-
technological considerations also have                                                                          dence time in the plasticating unit is
to be taken into account. For example,       Three-section screw                                                shortened. This can be advantageous
when optimizing the plasticating unit of                                                                        in the case of thermally sensitive mate-
an existing injection molding machine        Three-section screws which are also                                rials.
constraints due the machine size             commonly known as "universal
(screw length), the drive power, etc.        screws” are designed in such a way
must be borne in mind. In this way           that they can process as much ther-
solutions which are too costly in pro-       moplastic as possible at
                                                                            12 h = flight depth in the feed section
                                                                                 Flight depth h (mm)




duction terms often drop out even            an adequate level of                 F
though they would offer processing           quality. Due to the sim-           h M = flight depth in the
advantages.                                  ple geometry the struc-                  metering section
                                                                            10
                                             ture of the screw allows                   Standard screw
Accordingly, in most cases the techni-       low-cost production.                       Shallow-flighted
cally feasible solution is not the opti-                                     8          screw
mum design but rather the compro-            Modern standard screws
mise solution which emerges from             have an overall length of
weighing up all the relevant factors         20 – 23 D (length as a          6
affecting the qualitative and quantita-      multiple of the diameter)
tive requirements.                           the length of the feed             h
                                                                             4 F
                                             section accounting for
In injection molding technology today        approximately half the
conventional reciprocating screw injec-      length of the screw. The        2 h
                                                                                  M
tion units are employed almost exclu-        compression and meter-
sively. For the processing of thermo-        ing sections have
plastics these are frequently equipped       approximately the same               30 40 50 60 70 80 90 100 110 120 130
with a single-flighted three-section         length, the pitch is usu-                                    Screw diameter D (mm)
screw (Figure 1). As the name already        ally 1 D (0.8 – 1 D) and
implies this type of screw is character-     the flight depth ratio        Figure 2: Flight depths for three-section screws
ized by the division of the screw into       between the feed and

 2
In addition, high-performance screws         during injection, as a result of which              An increase in throughput with suitable
possessing mixing sections are con-          the screw channels are not completely               homogeneity can only be achieved
structed with a screw length of up to        filled with material (Figure 3).                    when the melting efficiency is simulta-
approximately 26 D for high-speed                                                                neously improved. For this reason a lot
machines (e.g. for packaging materi-         The more air that is drawn in by length-            more thought has been devoted
als). Long screws are not advisable for      ened metering or injection strokes the              recently to structural modifications of
processing thermally sensitive material      more difficult it is to ensure that this air        the screws. At present this includes
grades since overlong residence times        escapes via the hopper and does not                 research into the use in particular of
(throughput rates are usually lower          get into the space in front of the screw.           double- or multiple-flighted screws,
than in extrusion) can result in thermal     Aspirated air which is then occluded in             barrier screws and polygonal screws
damage to the material.                      the melt and gets into the mold pro-                combined with shear and/or mixing
                                             duces streaks, so this must be                      sections in injection molding machines.
The three-section screw comes up             avoided. The minimum condition for
against its limits as a result of height-    flawless parts is that there should be a            All shear and mixing sections have in
ened demands for molding quality and         sensible ratio between the maximum                  common the basic principles of screw
throughput rates. In particular, the         possible metering stroke and the effec-             clearance and the division and recom-
three-section screw without shear and        tive length of the screw (i.e. that the             bination of the stream of melt. The
mixing sections encounters its perfor-       maximum metering stroke for a screw                 Maddock and spiral shear section and
mance limits in direct coloring.             20 D long should be limited, for exam-              the toothed disk or faceted mixing
Although the depth of experience is          ple, to 3 D in order to achieve ade-                section are predominantly used (see
currently greatest with this type of         quate quality).                                     Figure 4). These sections should be
screw the constantly growing demands                                                             designed as far as possible to be neu-
cannot always be adequately fulfilled.       In three-section screws it is very diffi-           tral in terms of pressure so that the
                                             cult to further increase the plasticizing           throughput rate is not reduced, to
Growing competitive pressures require        rates and shot weights attained so far              minimize wear and in order not to have
the selection of injection molding           while obtaining the same or an im-                  a detrimental effect on the melt tem-
machines having an injection and             proved level of homogeneity. Due,                   perature.
clamping unit which is as small as           however, to the continuous shortening
possible in order to minimize article        of the cooling and movement times the
costs through low investment and             throughput rates must be increased so
operating costs. The trend in injection      that the plasticizing time does not
molding machines is accordingly going        become the variable determining the
in the direction of larger shot volumes      cycle time.
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-                                             Partly filled screw channels
ever, means a conflict in target para-
meters when improvements in product                                    V
quality are simultaneously being aimed                                     Injection
for. For that reason priorities have to be
defined or compromises have to be
accepted.                                    Figure 3:
                                             Trickle feed of
                                                                                                        Injection stroke
Larger shot volumes are achieved at          material during
low cost by the simple expedient of          injection
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                                             Maddock shear section           Spiral shear section
these problems the screws must be
lengthened but for structural reasons
this cannot be done to an unlimited
extent in injection molding plasticating
units.
                                             Figure 4:                        Toothed disk mixing section        Faceted mixing section
There is a further risk in longer meter-     Three-section
ing strokes that ever more air is drawn      screw with
in, especially during injection. This is     commonly used
because the screw moves only axially         shear and mixing
(no rotation) under the hopper opening       sections

                                                                                                                                          3
Double- and multiple-flighted                       It must furthermore be taken into              Barrier screws
screws                                              account that in a multiple-flighted
                                                    design the improved melting efficiency         There has recently been an upsurge
A possibility for raising melting effi-             is not always desirable in the case of         of interest in barrier screws for the
ciency and hence throughput consists                amorphous and/or temperature-sensi-            injection molding process on account
in constructing the screw with multiple             tive plastics. If melting occurs too           of the results achieved in extrusion. In
flights. By comparison with the sin-                quickly unwelcome temperature rises            injection molding, however, the bar-
gled-flighted design the increase in the            and possible damage to the material            rier screw must be adapted to the
number of flights in multiple-flighted              may be produced over the remaining             changed boundary conditions (batch
screws having otherwise unchanged                   length of the screw. Accordingly, a            mode of operation, shortening of
screw geometry yields smaller melt film             double-flighted screw design should be         screw as a function of metering
thicknesses at the cylinder wall as                 chosen when high mass flow rates               stroke, etc.) by comparison with
can be seen in Figure 5.                            (high-speed machines) combined with            extrusion.
                                                    high melting efficiencies (e.g. in the
As a result of the lower film thickness             case of polyolefins) are required.
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                                                          Hopper opening
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-                                    Double-flighted                    Single-flighted
put rate). As the screw diameter rises                                                            Second flight
the effect of the additional screw flight                                                         (reduction of
becomes steadily less unfavourable                                                                cross section)
(starting from about 80 mm).

                                                                         Figure 6: Charging cross sections in single-
                                                                         flighted and double-flighted screws




     Single-flighted screw zone         Double-flighted screw zone
                                                                                                       Melt channel
                Melt film          Barrel wall           Melt film
 Barrel wall




                                                                                                                              Solids channel
         Solids bed                              Solids bed                      Barrier flight
                                  Screw flight
 Screw flight
                  Melt vortex                            Melt vortex


Figure 5: Section through the melting section of a single-               Figure 7: Principle of a barrier screw
flighted and a double-flighted screw



 4
In principle all barrier screws have the     The pitch is frequently increased right      Optimization and simulation
same mode of operation. The charac-          at the beginning of the barrier zone in
teristic feature is the division of the      order to provide the cross section           In the injection molding process prob-
screw channel into a solids channel          needed for a second channel. At the          lems such as
and a melt channel (see Figure 7). The       same time it proves to be advanta-               difficulties in feeding
solids channel is separated from the         geous for the width of the solids chan-          excessively high melt temperatures
melt channel by the barrier flight. The      nel at the inlet to the barrier zone to be       unmelted material
barrier flight has a greater gap width       the same as the channel width ahead              streaks and voids
than the main flight so that only fused      of the barrier zone. This avoids abrupt          material and thermal
material or particles which are smaller      deformations of the solids bed during            inhomogeneities
than the gap width in at least one           passage into the barrier zone. In order          fluctuations in shot weight and
direction can pass into the melt chan-       to ensure controlled melting the outlet          plasticizing time
nel. On flowing over the barrier flight      of the solids channel should be closed           wear of the screw and cylinder
these particles are exposed to an addi-      so that the material gets into the melt
tional defined shear stress which            channel and hence into the space in          can occur.
results in further melting of the residual   front of the screw only via the barrier
solid particles. The barrier flight, more-   gap.                                         When they occur the injection molding
over, contributes to homogenization of                                                    plasticating unit is frequently cited as
the melt.                                    Computer-supported simulation is             the general cause of these problems
                                             useful especially in the design of barrier   but without more precise specification.
In the barrier zone the cross section of     screws because the barrier zone pos-         The reason for this is that the plasticat-
the solids channel reduces in the direc-     sesses a relatively high number of           ing unit is regarded as a black box
tion of the tip of the screw while at the    degrees of freedom in its design.            which is not susceptible to direct
same time the cross section of the melt      Although the screw geometry can be           observation. Only a few variables such
channel increases. In the various types      better adapted to a specific application     as the melt temperature, heating zone
of barrier screws this change in cross       the system also responds more sensi-         temperatures, torque and the pressure
section is achieved by varying the flight    tively under certain circumstances and       in the space in front of the screw can
depths and/or channel widths.                contains more possible sources of            be called upon to evaluate the operat-
                                             errors.                                      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-
              BA




                                                                                          mized screw geometry (Figure 8).
                                         n
           ba

                  SF




                                    tio
             se




                                    ta
                    kn




                                                   Plasticizing time
              do




                                 pu
                  ow erie
                   ne




                                m
                    led nc




                               Co




                                                   Pressure profile
                      xp

                       ge e




                                                   Temperature profile                    Apart from attaining the targeted mass
                                                                                          flow rate complete and proper melting
                                                   Melting profile
                                                                                          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



                                                                                                                                 5
With the aid of the simulation program                                 In addition to the rise in the curve of        In injection molding unlike extrusion it
the dimensionless solids bed width Y is                                the dimensionless solids bed width the         is more difficult due to the axial dis-
calculated. This is defined as the ratio                               presence of residual solids at the end         placement of the screw to fix the
of the solids bed width x to the channel                               of the screw or at the beginning of a          beginning and the length of the barrier
width b (to the solids channel in the                                  shear or mixing section must be pre-           zone. It is a problem here that shot
case of barrier screws). Figure 9 shows                                vented. In the case of relatively small        weights and hence metering strokes
a favorable and an unfavorable melting                                 amounts of residual solids this would          should vary. That is to say that
process. The interpretation of the                                     result in an inhomogeneous melt con-           depending on the metering stroke the
progress of melting allows statements                                  taining isolated particles of solid. If the    start of the barrier zone is closer to or
about the relationship between the                                     amount of solids is higher, back pres-         further away from the hopper, the site
conveying performance of the screw                                     sure can build up in shear sections and        of initial melt formation remaining
and its melting performance and a                                      throughput can fluctuate. If the gap           unchanged. For problem-free opera-
qualitative estimate of the level of                                   were clogged up high pressure loss             tion it must be ensured that the site of
homogeneity of the melt achievable in                                  (lower output) would additionally occur.       initial melt formation is located ahead
the space in front of the screw. At the                                                                               of the barrier zone in order that sepa-
same time the two effects illustrated in                               Blockage of shear webs can occur not           ration into solids and melt can be
Figure 9 (increases in the dimension-                                  only in shear sections having sealed           effected by the barrier web. Otherwise
less solids bed width and residual                                     channels but also in barrier zones hav-        the melt channel will only be partly
solids content at the beginning of a                                   ing closed inlet and outlet channels.          filled which is synonymous with a
shear or mixing section) are of particu-                               This is the case when the melting effi-        reduction in throughput or an increase
lar importance.                                                        ciency of the barrier zone is too low or       in plasticizing time. Local overheating
                                                                       the site of melt vortex formation is           can, moreover, occur in the solids
An increase in the dimensionless solids                                located in the barrier zone.                   channel (especially in the closed outlet)
bed width can cause the solids bed to                                                                                 if insufficient material is melted which
break apart due to high deformation                                    By comparison with shear sections,             then flows into the melt channel.
and result in the formation of individual                              barrier zones have the advantage here
islands of solids which are no longer                                  in that on account of their normally           With the correct sizing the result illus-
effectively melted by shearing. Espe-                                  greater length blockages due to melt-          trated in Figure 10 is obtained: com-
cially in the case of high-melting poly-                               ing efficiency occur only locally. How-        plete separation of solids and melt and
mers (high enthalpy requirement) in                                    ever, these local blockages can result         a full melt channel. In this way high
association with compression which is                                  in unwanted fluctuations in throughput         throughputs of adequate homogeneity
applied too early or too strongly there                                and should, therefore, be avoided.             can be achieved. It must, however,
is the risk that the dimensionless solids                                                                             also be noted that this result cannot
bed width again rises to the value of 1                                                                               be achieved with a single geometry in
which is synonymous with “clogging”                                                                                   optimum manner for the entire spec-
of the screw. In this case in practice                                                                                trum of materials and all operating
restriction of the mass flow rate and                                                                                 points, as is also the case for other
homogeneity problems can be                                                                                           screw designs.
expected.
                                                                                                                      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-
                                 1.0
 Normalized solids bed width Y




                                                                                                                      rial. There are, for example, great dif-
                                                                                                                      ferences in enthalpy (melting energy)
                                         Y= x
                                            b                                                                         and viscosity (flow properties) between
                                                                                                                      amorphous and semicrystalline plastics.
                                                Unfavorable
                                 0.5

                                                                        x
                                                                        b
                                                                                                                           Barrier zone
                                                       Favorable     Site of melt
                                 0.0                                 vortex formation
                                    25     20    15       10           5           0
                                                Screw length (L/D)
                                                                                                        Solids channel              Solids channel
                                                                                                                     Melt channel                    Melt channel


                                                                                                      Figure 10: Barrier zone with closed inlets and
Figure 9: Comparison of different melting profiles                                                    outlets
 6
Figure 11: Enthalpy, viscosity and melt-
ing profiles for two different materials                                                                         1

                                                                                                                     Ultramid ® A3W
                    800                                                                                              Polystyrene 168 N
 Enthalpy (J/g)




                                 Ultramid ® (PA 66)




                                                                               Normalized solids bed width Y
                    600          Polystyrene (PS)

                    400
                                                                                                               0.5
                    200

                       0
                           0         100         200          300
                           Room                PS melt     PA melt
                           temperature     temperature     temperature
                                                         Temperature (°C)


                                                                                                                0
                    10 4                                                                                             20           16                 10                               0
 Viscosity (PA*s)




                    10 3                                                                                                                   Screw length (L/D)

                    10 2

                    10 1         280°C: Ultramid ® A3W
                                 230°C: Polystyrene 168 N
                    10 0
                           10 - 1 10 0   10 1   10 2     10 3 10 4 10 5
                                                         Shear rate (1/s)




In connection with this Figure 11                                             Conclusion                                                            “How much screw” or which design
shows by way of example the enthalpy                                                                                                                should be employed must be decided
and viscosity curves for an amorphous                                         Even today standard or universal                                      in line with the application. At the
and a semicrystalline material. From                                          screws such as the three-section                                      same time requirements, particularly
the large differences in the energy                                           screw cover a large part of the range of                              with regard to homogeneity, can be
needed for melting and in flow proper-                                        requirements and materials. However,                                  highly variable.
ties it may be inferred that it is difficult                                  there has recently been an increase in
using just one screw geometry to                                              the incidence of cases in which higher                                The aim of this technical information
process a great many materials in                                             demands are imposed on the melt                                       paper was to set out possibilities for
optimum manner and to do this at                                              quality (homogeneity) than can be met                                 screw designs. The universal screw
highly variable operating points.                                             by a three-section screw. In order to                                 which solves all problems using a sin-
                                                                              attain the required homogeneity, addi-                                gle geometry continues to evade us.
To make this clear Figure 11 provides                                         tional barrier zones as well as shear                                 There are, however, enough possibili-
a comparison of the course of melting                                         and/or mixing sections are then                                       ties for fulfilling the requirements
for a polystyrene (PS) and an Ultramid®                                       employed. The extent to which homo-                                   imposed.
(PA) using the same screw geometry. It                                        geneity can be increased by such mea-
is evident that there are great differ-                                       sures can be seen in the example
ences in the patterns (end of melting).                                       shown in Figure 12.
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         Figure 12: Homogeneity results
relatively low demands are
placed on output rate, the
possible shot weights and
homogeneity.




                                                                    Three-section screw                                       Three-section screw with          Barrier screw with
                                                                                                                              shear and mixing section          shear and mixing section
                                                                                                                                                                                           7
                                                                        ® = reg. Tradename of BASF Aktiengesellschaft
                          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.




                                                                          KTX 9908 e 12.2000




BASF Aktiengesellschaft
67056 Ludwigshafen
Germany

				
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