New Spiral Mandrel Dies1

Peter Fischer, Johannes Wortberg

1 Extended version of a paper presented at the SKZ conference on "Innovations in Extrusion" on 13-14.05.98 in Würzburg

2

Although spiral mandrel dies have been state-of-the-art in

extrusion technology for 25 years, new die concepts are

more topical than ever before. Different melt distribution

systems are combined with spiral mandrels in a cylindrical,

conical or radial configuration. In the case of coextrusion dies, in

particular, this gives rise to good thickness uniformity with a

broader range of processing parameters (raw materials,

throughput, temperatures), short residence times (material or

colour changes), low pressure losses, good thermal control, a

long service life, low space requirements, easy assembly,

dismantling and cleaning and, not least, favourable production

possibilities. Sample designs from sheathing, pipe and tube

(co)extrusion demonstrate the process, production and

operational engineering advantages of innovative spiral

mandrel dies.

1. Optimization criteria

Outstanding results have been achieved with spiral mandrel

dies for the extrusion of rotationally symmetrical

products (blown film, tubes, pipes, hollow articles) for many

years, in the form of a uniform volume-flow and product wall

thickness distribution with no weld lines to impair quality

[1,4,6,9,10,11,12]. Alongside these key aspects, present-day

designs are also required to fulfil

additional criteria, particularly in the case of coextrusion:

Rheological criteria

• low pressure loss and a

short residence time (flow channel cross-sections and

lengths)

• compliance with minimum

and maximum wall shear stresses (risk of deposits

and melt fracture) for the specified range of

processing settings

• avoidance of interfacial

instability in coextrusion, particularly with extreme

layer thickness ratios (individual layers account

for only 1% of the overall layer thickness at times)

• avoidance of non-uniform elastic deformation (and

orientation)

• avoidance of local stagnant

zones and dead spots (material or colour

changes).

Thermodynamic criteria

• attention to dissipation and

heat exchange for the raw materials in question

(heating, insulation) including for the individual

layers in coextrusion dies, where appropriate

• attention to thermal symmetry in the die and

melt flow.

Operational engineering criteria

• rigidity and freedom from warpage

• simple assembly, dismantling and cleaning

(with no melt, or only a small quantity of melt

sticking to the flow channel surfaces)

• tight parting planes

• easy centering

• shortest possible material and colour change times

(for the system in its entirety, including the

extruder, melt filter, adapter and die), and hence a

reduction in the number of rejects.

3

2. Dimensioning

When dimensioning spiral mandrel dies, it is necessary to

consider the relevant areas of the flow channel between the

extruder barrel or screen-change adapter and the annular

slot at the end of the die. The following areas of the flow

channel need to be dimensioned in respect of their

cross-sections and lengths:

1. adapter to the die mount

2. melt flow branches inside the die

3. melt pre-distribution systems (e.g. spider legs, branched

holes, coat-hangers)

4. spiral mandrel melt

distributor

5. restricted flow zones or

relaxation zones

6. the point where the individual

layers are brought together in coextrusion

7. joint flow channel from the point where the melt meets

up to where it reaches the die orifice in coextrusion

8. die gap geometry in the diameter-dependent die unit

(gap width, parallel gap length).

2.1 Simulation

Computed simulations can

sensibly be used to achieve the correct dimensioning of the flow

channels for specific

applications. This involves

calculating the flow processes in circular and annular channels

for single-layer flows, using uni-dimensional formulations with

the assumption of isothermal conditions.

Two-dimensional models (networks) for isothermal flows

have now proved successful for the computation of spiral

mandrel geometries. whether

these be cylindrical, conical or

radial in design (Fig. 1) [10]. Although computation models

for three-dimensional, non-isothermal flow are available

(FEM, BEM), it has so far proved difficult to interpret the additional

information obtained (spatial velocity and temperature

profiles) for purposes of improving on the flow channel

design. Despite this, valuable

Fig. 1: Network model for the computation of a spiral mandrel manifold

Fig. 2: Pressure loss optimization at the die gap

4

qualitative assistance is

obtained for optimizing geometric details, such as feed

throat geometries, deflections and stream divisions.

Apart from optimization of the spiral mandrel geometry, special

importance has to be attached to the establishment and

favourable configuration of the pressure loss. The procedure to

be followed for optimizing a die orifice is shown in Fig. 2 in

qualitative terms.

The simulation of multilayer

flows in coextrusion dies is aimed, inter alia, at avoiding

interfacial instability. Only simple basic rules can be derived for

the dimensioning of a specific flow channel, however. Hence, it

can be recommended that the gap widths be adjusted to the

specific layer distribution in the product before and after the

point where a number of different layers are combined

[10]. The frequently-asked question about the bandwidth of

the melt throughput or volumetric flow rate in

coextrusion dies cannot, however, be answered on a

generalized basis. This will depend, among other things, on

the elastic properties of the melt, and taking these into

consideration in flow simulations involves a high outlay,

particularly where complex shear/elongational flows prevail.

Restrictions due to the rheological (in)compatibility of

certain raw materials, on the one hand, and pressure loss and

residence time criteria, on the

other hand, mean it is not possible to predict the limits of

the processing parameter range sufficiently accurately in

theoretical terms. Experimental results are generally only

available for reference products, and hence it is necessary to be

cautious and not always accept the confirmation given with

regard to the feasibility of certain new, multilayered products (e.g.

with throughput ranges of 1:10, or greater).

2.2 Different materials

A spiral mandrel die can and,

indeed, must be optimized for a number of different materials in

most cases, making allowance for their respective processing

temperatures and melt throughputs. Where a conflict

prevails between pressure loss and residence time, the flow

channels must be dimensioned for the melt with the highest

viscosity, specifying a maximum tolerable pressure loss in each

section of the flow. Residence time criteria may be more

important than local pressure losses in areas containing

critical deflections or flow branches, and also for the spiral

geometry, while low pressure losses may take precedence in

simple, circular flow channels (drilled holes), with a still

acceptable residence time.

Shear rates should not be set at

below 5 s-1 or above 50 s-1 for design purposes, so as to

ensure that there is still sufficient wall shear stress to permit an

acceptable material or colour

change. In a few cases, such as with cable and conductor-

sheathing dies, or with low-viscosity melts, shear rates may

be as high as 1000 s-1.

2.3 Geometric degrees of

freedom

In some cases, the simulation

models can provide useful assistance with the specification

of geometric degrees of freedom, such as the type and

extent of melt pre-distribution, the number of turns in the spiral,

spiral overlap (design height), the shape of the spirals and the

spiral inlet and outlet. In all cases, however, it will still be

necessary to make allowance for production engineering

aspects and the cost, such as when determining the number of

spirals and the shape of the spiral cross-section. Computer

simulation has doubtless made less of a contribution to the wide

variety of different design concepts that are available

today than has the creativity of engineers.

A number of design solutions for spiral mandrel dies with different

pre-distribution systems and spiral mandrel configurations for

coextrusion dies are presented and discussed in what follows.

5

3. Melt pre-distributors

For a long time, the only approach adopted was to equip

spiral mandrels with pre-distributor systems comprising

branched drilled holes or circular channels so that each

spiral was fed from a hole of its own. Systems have also

become established today where pre-distribution is

performed in an annular gap. Contrary to the previously-used

smear devices placed behind the spider-leg mandrel [1], the

spiral mandrel manifold is retained as the key element for

definitive melt distribution here.

In other words, almost all the

melt flowing in from the annular

gap on the pre-distributor system is pressed into the spiral

inlets. Following this, the redistribution that results from

the overlap of the axial and spiral flows leads to sickle-

shaped partial flows distributed over the circumference and,

finally, to a uniform melt distribution at the end of the

spiral mandrel manifold (Fig. 3) [14]. The designs that have

proved successful in practice extend from the conventional 1/1

solution (hole/spiral) through to a form of pre-distribution

involving a slot die reproduced on a cylinder, as it were, with a

downstream spiral mandrel manifold (Fig. 4).

Fig. 3: Operating principle of a spiral mandrel manifold

Pre-distributor + Spiral mandrel

n feed holes + n spirals

(„conventional design“)

m feed holes with + n spirals coat - hangers

Important: more than 90% melt from pre-distributor into spirals

Fig. 4: Allocation of pre-distributor and spiral mandrel manifold

6

3.1 Conventional solution

Figure 5 shows a conventional solution by way of example, with

radially oblique pre-branching channels running directly into

the spiral inlets (n = number of holes or circular channels). By

depicting the configuration laid out flat, it is clear that the

hatched areas cannot be made round on a lathe but have to be

machined with a milling cutter. Otherwise, dead spots between

the spiral inlets would impair the functionality of the spiral

mandrel.

3.2 Coat-hanger

pre-distribution system

One way of feeding an annular

gap at the end of the pre-distribution system is by using a

coat-hanger pre-distributor (frequently used in slot dies and

sometimes on deflector torpedo heads). The variants that have

already been constructed start with just one coat-hanger on the

circumference of the pre-distribution system, which is fed

in axially or radially from a drilled hole or a channel, and go right

through to combined pre-branching systems

incorporating a number of coat-hanger distributors around the

circumference. Decoupling the feed channel and the spirals

means that the number of spirals can be determined in

isolation and can vary from n to a multiple of n, depending on

the task on hand and the design constraints.

On the spiral mandrel in Fig. 6,

the melt is fed laterally into the coat-hanger channel "wound

around" the cylindrical mandrel. The coat-hanger channel then

transfers the melt to the spiral mandrel proper via an annular

gap. A basic die is obtained with just two components (in this

case having a spiral mandrel diameter of ≤100 mm) which

offers all the advantages of the spiral mandrel principle by

comparison with mandrel supports, circular channel and

heart-shaped manifolds.

Fig. 5: Spiral mandrel distributor with central feed and

drilled pre-branching n holes ! n spirals

Fig. 6: Side-fed spiral mandrel

manifold with coat- hanger pre-distribution

1 drilled hole/coat- hanger ! 6 spirals

7

Figures 7, 8 and 9 show different

designs of spiral mandrel with multiple pre-distribution systems

in the form of coat-hangers.

Decoupling the number of

spirals from the pre-distribution system permits a more

favourable rheological and technological design, with an

increased number of turns, particularly in coextrusion dies

where the number of individual pre-branching holes is limited by

design constraints. As a rule, it is best to have a larger number

of spirals with smaller cross-sections. This is more important

for high-molecular raw materials, some of which display a high

visco-elasticity (e.g. PE-HMW-

HD), than for low-viscosity products (e.g. PA).

4. Concentric spiral mandrel dies and stack

dies (pancake dies)

Coextrusion dies with non-

concentric spiral mandrels have been available on the market

since the start of the Nineties, intended particularly for blown

film dies. Their distinguishing design characteristics are the

radial or conical arrangement of the spiral mandrels, with an

upstream pre-branching system, and a melt feed from the side

[2,5,7,15,16]. The relatively flat

design means that coextrusion

dies can be made up by stacking individual discs

(pancakes) on top of each other. The fact that these pancakes are

interchangeable and can be added to (such as for the

extrusion of three to five, or seven layers) means that these

dies offer the user the greatest possible number of

combinations.

While most of the familiar stack

die concepts incorporate a radial arrangement of spiral

mandrel manifolds, side-fed conical designs with pre-

branching potentially constitute the better alternative, since

these permit greater spiral overlap with smaller outside

diameters and lower heights for the individual modules. A

corresponding concept has recently been introduced as the

"Multicone" modular film blowing head [15].

The advantages of these designs have to be set against a

number of drawbacks, however. Table 1 marks an attempt at a

comparative appraisal. What are listed here are fundamental

arguments; the more appropriate concept for an

individual case (application, product, (range of) dimensions)

can only be established through practical experience. Where

alternative concepts may be available, a best-possible

benefits/risk estimate should be conducted on the basis of the

arguments listed before a decision is made [2,5]. Fig. 7: Spiral mandrel with pre-branching (1/2 – 2/4) and coat-hanger

pre-distribution (4/n)

8

Fig. 8: Spiral mandrel from a 6-layer head

Fig. 9: Spiral mandrel with drilled pre-branching holes, coat-hanger pre-distribution and a large number of spirals

9

Table 1: Concentric spiral mandrel dies versus stack dies (pancake dies)

Concentric Spiral Manifold Stack Die (Pancake Die)

Pressure loss Flow resistances identical subsequent to merging. Separate

optimum channel design for individual layers.

Additional flow resistance after each merging point. Unfavourable long flow

paths in large dies.

Residence time, material/colour

changes

Can be determined individually for all the separate layers up to

the point where they merge.

Can no longer be determined individually after each merging point. Unfavourable in

big dies on account of long flow paths.

Distribution,

dependence on operating point

Number of overlaps almost

unlimited. Broad range of operating points.

Number of overlaps limited. Narrow

range of operating points.

Melt merging Can be performed sequentially or at a single point (depending on

layer structure).

Can only be performed sequentially. Unfavourable with some structures (raw

materials).

Layer/flow

stability

Can be optimized through the

merging configuration.

All multilayer flows (2 to n layers) must be

stable.

Variability of layer

structure

Can only be varied to a very

limited degree (such as by an adapter).

Can be varied by changing discs with

height adjustment of the extruder/adapter. Gap widths and

merging only adjustable to a limited extent.

Temperature control of

individual layers

Not generally possible: can only be influenced via joint die

temperature and melt temperature control upon exit

from the extruder.

Individual temperature control for the individual discs (if necessary, by heat

exchange between the discs on the basis of metal contact).

Number of

components

Relatively few individual parts

and screws; few sealing surfaces.

A large number of separate parts and

screws in some cases; a large number of sealing surfaces.

Suitability for internal ducts and

internal cooling

Depends on feed and pre-branching; good with side feed.

Fundamentally suitable by virtue of the side feed.

10

The extent to which the

advantages of variability in the stack die concept outweigh its

potential drawbacks through the limited adaptability of the melt

merging process and the limited influence that can be exerted on

potential interfacial instabilities, will depend to a large extent on

the application in question. In the case of products with

smaller dimensions, the advantages will potentially be

greater than for larger die diameters. The stack die

principle can offer advantages in extrusion coating or cable

sheathing on account of its side feed (inherent to the concept)

and the relatively low height which will also permit the

system to be retrofitted to

existing dies, between the basic

die and the die orifice. It can be assumed that both design

principles will find their specific applications and that neither

concept will come to dominate. The spiral mandrel die concept,

which offers a convincing performance on many points,

has, after all, still not displaced alternative manifold and die

concepts in all applications, including spider-type dies for

PVC [13].

5. Die units for variable

product dimensions

Where identical or uniform

products are to be manufactured in high volumes,

use is frequently made of special-purpose dies. This holds

particularly true for a large

number of die designs from the USA, on account of the specific

conditions prevailing on the market there. Here in Europe,

the situation is different: what is required is flexibility, including

with regard to the product variants that can be produced

with a single basic system. A typical example are pipe dies,

which are required to cover the largest possible range of pipe

dimensions (diameters and wall thicknesses).

Die units are connected up to the basic spiral mandrel dies

(either directly or via interchangeable intermediate

components) with flow channels that shape the melt to the outlet

cross-section. When designing these areas of the flow channel,

allowance must be made for different criteria as a function of

the product and the material, as has already been mentioned in

Section 2.

Working on from the basic

diameter at the end of the spiral mandrel manifold or, in the case

of extrusion dies, the basic diameter at the point where the

individual layers merge, it is possible to conduct the annular

channel either towards the inside, or straight on or towards

the outside. In pipe extrusion, the channel will generally be

reduced to a smaller diameter while, on blown film dies, it will

generally increase in size towards the die orifice. At all

events, care should be taken to ensure that the flow accelerates

steadily.Fig. 10: Die units for colour strip marking on a sheathing die

11

Figure 10 shows the range of

die units for a spiral mandrel die used to sheathe steel pipes with

PE. The space allowed in the basic die for the biggest pipe to

pass through means that all the die orifices are smaller than the

diameter of the spiral mandrel manifold. On this example,

provision has also been made for a unit to apply colour stripes

to the sheathing.

Figure 11 shows the different-

diameter die units for a pipe die.

6. New requirements

placed on extrusion dies

The complexity of new

products, which in many cases is beyond the limits of "standard"

dies on account of the greater range of raw materials used and

the need for multilayer extrudates, means that suitably-

aligned extrusion technology,

and particularly, die technology,

is now more important than ever before. Apart from the geometric

degrees of freedom, surfacing and coating techniques are

gaining increasing importance. In addition to the proven

galvanic coating processes (chrome plating and nickel

plating) for the customary steels used in die making (e.g. C45 or

C60), innovative coatings offering improved friction and

adhesive properties could also be attractive for extrusion die

technology too in the future. Initial experience gained with

coatings of "Diamond-Like Carbon" (DLC) on die

components is highly promising [8]. Of particular interest here is

the influence on the amount of deposits in the flow channel and

at the die orifice. Since PVD or CVD coating processes may

place additional demands on

the base material and the part

geometry, this provides a new challenge to die designers.

Figures 12 and 13 show the results of laboratory tests

conducted with a blown film die whose mandrel received

different coatings. The extent of the deposits was taken as a

reference to assess the surface properties.

Fig. 11: RW-1-25/125 pipe die with dies for pipes – Ø 32 and 110 mm

12

0,509

0,582

0,421 0,4310,45

0,095

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0,55

0,6D

epos

it af

ter 4

hou

rs [g

]

Reference NiP+PTFE 4S-T CrN (Mplas) CrN (MVT) DLC+Si

Type of coating

Weight

Fig. 12: Property comparison for different coatings (Source: 'KKM')

0,043

0,026 0,028

0,010

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

Dep

osit

afte

r 4 h

ours

[g]

NiP Cr NiP blasted Cr blasted

Type of coating

Weight

Fig. 13: Property comparison for chromium and nickel coatings (Source: 'KKM')

13

7. Examples of dies [17]

The dies shown in Figs. 14 to 23 have all proved successful in

practical use and demonstrate the potential range of different

designs for the spiral mandrel principle. An optimum problem

solution can be obtained by establishing a concept that is

suitably tailored to the task and the requirements, and

employing a customized rheological, mechanical and

thermal layout.

Figure 14 shows the cross-

section and a laid-flat view of the pre-distribution system with a

spiral mandrel for an extrusion die that is used to sheathe steel

pipes. The melt is fed in from the side via a branching system

incorporated in the circumference, before being

uniformly pre-distributed over four coat-hangers.

Following this, the final

distribution of the melt takes place in a spiral mandrel

manifold with 16 turns (for this particular size). The basic die is

made up of only two parts and is therefore easy to assemble and

dismantle.

Figure 15 shows the pipe

sheathing die, which also marks the pipe with colour stripes.

Fig. 14: Spiral mandrel die for

sheathing, with a side feed, quadruple pre-branching and coat-hanger pre-distributors

Fig. 15: Sheathing die for steel pipes

14

Figure 16 shows a concept for a

three-layer pipe extrusion die that has been designed for a

layer structure with a relatively thick inner layer and thin central

and outside layers. The pre-distribution system for the inner

layer is a "conventional" system with one feed hole per spiral

turn and an axial central feed. The central and outside layers,

by contrast, have an axially eccentric feed via a single hole

in each case. For each of the layers, a coat-hanger pre-

distribution system performs the pre-distribution of the melt in an

annular gap. Following this, the distribution proper is conducted

in spiral mandrel manifolds, with six spiral turns in this case. In

this "coating task" with a thin

central and outside layer, the

thin layers are first brought together and then the inner layer

added on.

At this point, it should be stated

that the way in which the melts are brought together in

coextrusion - either sequentially, at a single point, or through a

combination of both - is of major importance and can determine

the overall serviceability of the system.

The three-layer pipe extrusion die for small corrugated pipes

that is shown in Fig. 17 has been designed for one third of

the melt to pass via each of the spiral mandrel manifolds. This

example is designed to show

that spiral mandrel technology can be used for smaller pipe

dimensions as well. This also holds true for smaller pipes

based on polyamide which are used as single-layer or

multilayer pipes in the automotive sector, such as for

fuel lines.

Figure 18 shows this die with a

protective cover.

Fig. 16: Three-layer spiral mandrel

die for coating the exterior of pipes

Inside:n holes ! n spirals Centre/outside: 1 hole !

1 coat-hanger ! n spirals

Fig. 17: Three-layer spiral mandrel die for corrugated pipes; 1 drilled hole ! m drilled holes / coat-hangers ! n spirals

15

Apart from fixed covers (see

also Fig. 19) contact protection can also be provided through

insulation (Fig. 20).

Fig. 18: Three-layer die for corrugated pipes

Fig. 19: Pipe die with a protective cover

16

Figure 21 shows a five-layer die

for small pipes and hoses. Pre-distribution is conducted in

annular channels here, given the need for a central passage for

coating tasks or for a

temperature-control medium. This is after the melt has been

fed in from the side and deflected in the appropriate

manner. With this type of pre-

branching, 8 or 16 drilled feed holes are required for the spiral

mandrel in question, in line with the 2n power series. The hole

diameters and the flow channel cross-sections in the spiral

mandrel manifolds are aligned to the product specifications (as

explained above) in order to achieve an optimum

compromise between flow resistance and residence time.

Fig. 20: Pipe die with insulation/contact protection

Fig. 21: Five-layer spiral mandrel die with side melt feed and annular channel pre-branching;

1 hole ! 2 channels ! 2n channels ! 2n spirals

17

Figure 22 shows the 5 spiral

mandrels from this die.

The six-layer die shown in Fig.

23 is used for continuous parison production in blow

moulding units for fuel tanks and

filling spouts. Three thin layers (EVOH as a barrier, and two

coupling agents) are embedded in two layers of high-molecular

PE-HD here. The sixth and thickest layer (2nd layer from the

outside) is a coextruded layer with a high percentage of

regrind (parison waste, etc.). One particularly important

feature is the low percentage of the overall composite that is

accounted for by the thin layers. This is only feasible with a

narrow layer thickness distribution [12]. The thick layers

are fed according to the conventional concept (n holes /

n spirals) while the thin layers are fed with the combination of

hole(s) / coat-hangers / n spirals.

Fig. 22: Spiral mandrel manifolds of a five-layer die

Fig. 23: Six-layer spiral mandrel parison die

thick layers: n holes ! n spirals thin layers: 1 hole ! m holes / coat-hanger ! n spirals

18

Figure 24 shows the very good

layer thickness uniformity, including for thin layers that

account for less than 1% of the overall wall thickness in each

case.

The examples given here

demonstrate the flexibility in

design offered by the spiral

mandrel manifold principle in conjunction with different pre-

branching and pre-distribution configurations for specific

applications. Work is now concentrating on the process

and production engineering optimization of die and flow

channel details that are not (yet)

accessible to straightforward mathematical optimization. It

would also seem feasible and worthwhile improving on the

temperature control of extrusion dies.

Fig. 24: Layer thickness distribution in the plastic fuel tank wall

19

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[2] Perdikoulias, J Petric, J.

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Wortberg, J.

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[15] N.N Multicone - modularer Folienblaskopf. Firmenschrift Windmöller & Hölscher, Lengerich1997

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Blasfolientechnik im Spannungsfeld von Preis und Leistung. Kunststoffe 87 (1997) 11, S. 1584 - 1592

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