Literature review on extruders which are used in 3D printers such as Mendel and Darwin
Introduction
An extruder is a machine that is used to melt plastic polymer into molten form and then
converts it to a given shape. In the modern world, plastic is commonly used and different shapes
are made from the plastic extrusions. There are different types of extrusion processes including
the injection molding, cast film extrusion and blown film extrusion. The heating of the polymers
is either done externally or internally. Though the internal heating is produced through the
friction produced by the plastic polymers, there is some heating that is induced to the extruder
externally. When the plastic granules stick to the screw, it helps in the transport of the molten
plastic forward.
The typical screws that are used in the film extrusion have feed transition and metering
zones. When the polymer is melted, it is passed through the die so as it will achieve the final
shape. When the polymer achieves the desired shape, it then cooled to have the final product.
The History of Extruders
The first thermoplastic extrusion machine was first built in 1935 by Paul Troester in
Germany. Initially, the extruder machines were being used for rubber extrusions. The initial
machines were ram steam heated and screw extruders and had short length to diameter. With the
advancement in technology, electrical extruders were made and had increased length and twin
cams. Roberto Colombo of LMP discovered the twin extruders which had co-rotating twin
screw. Roberto obtained a patent of this product in many countries though he allowed some
companies to use the patent right. The first details about the extrusion were concerned with the
melting and pushing process.
By early 1950s, there was an increased scientific study concerning the extrusion
processes that were being increased. The study about the extrusions was first being done by the
investigators and workers in the polymers industry. The study however attracted the
academicians which brought about the gap between the extrusion theoreticians and those
practicing extrusion technologies.
Uses of extruders
The extrusion process has many types. In the plastic extrusion, the raw plastic is melted
and transformed to melted form which is used to mould different items including the rods, seals,
plastic tubes, adhesive tapes and sheets among other items using extruders. The commonly used
plastic materials include the nylon, polycarbonate, polyethene, acrylonite butadience and acrylic.
Blown film extrusions- The blown method of extrusion is used to manufacture the shopping bags
and objects that are similar. This has a close similarity with the regular extrusion except for the
die. The die in this method is used like a small cylinder that has a circular opening that can be
few centimeters to several meters. The greatest advantage of this method is the quality variations
it provides.
Sheet/film extrusion- In this method of extrusion, the materials are cooled through pulling them
through a series of cooling rolls. In this method, the materials are able to achieve the desired
texture and thickness. This process is then followed by thermo forming whereby the plastic is
heated until it’s soft enough to form a given shape. When vacuum is used during this process, it’s
referred to as vacuum forming.
Tubing extrusions-This method is used to make tubes and straws. There is no much difference
with other processes except for the die that is being used. Medical tubes and drinking straws are
made using this process. In this process, the mandrel or a pin is used to extrude the hollow part
of the tube being made. Depending on the number of holes that are required, different or several
lumens can be used.
Compound extrusion- This process that combines several polymers in creating a plastic
compound. Powder, liquid or pellets can be used in the injection for molding and extrusion. In
this type of the process, machines are very helpful; they can be the small laboratory machines or
the big industrial machineries for molding large items.
The above extrusion processes are the commonly used today. They each have their own
advantage over the others. Each process requires a different machineries and equipments.
Types of extruders
Twin Screw Extruder
A twin screw extruder is an extruder machine that has two Archimedean screws. There
are different twin extruders divided into different classes. These different twin extruders are
operated differently and have varied designs and applications. This makes its hard to have a
general comment regarding the twin screw extruders. The difference in twin screw extruders are
larger compared to the single screw extruders. This is because the twin screw extruders have
various functions and rotations and the degree of intermeshing compared to the single screw
extruders. There are different classifications of twin screw extruders. The classification can be
based on the geographical configuration.
Classification of Twin Screw Extruders
Intermeshing
Co-rotating
Low speed Extruders for profile
Extruders
extruders
extrusion
High speed extruders for
compounding
Counter-rotating
extruders
Conical extruders for profile
extrusion
Parallel extruders for profile
extrusion
High speed extruders for
compounding
Non-Intermeshing
extruders
Counter-rotating
extruders
Co-rotating
extruders
Co-axial extruders
Unequal screw length
Not used in practice
Inner melt transport forward
Inner melt transport
rearward
Inner solids transport rearward
Inner plasticating with rearward
transport
Source: http://files.hanser.de/hanser/docs/20040401_244515431-14400_3-446-21774-6.pdf
Multi-screw extruders
There are other extruders that contain more than two screws and are regarded as multi screw
extruders. The multi screw extruder may look similar to the single screw extruder. The feed
section of the extruder looks similar to the single screw extruder but the mixing section is
different from the single screw. In the planetary roller section of the extruder, the six or more
screws revolve around the main screw. The planetary screw is referred to as the sun screw. The
sun screw intermesh with the planetary screw and the barrel. In this regard, the planetary section
must have grooves that will correspond with the planetary flights on the planetary screw. This
must correspond to the feed of the barrel.
In the first part of the machine, the materials moves forward the same way as in the single screw
extruder. When the material reaches the mixing section, it’s intensively mixed through a rolling
action between the planetary screws, the barrel and the sun screws. The helical design of the
planetary screws, the barrel and the sun screws form a large surface area that is equal to the
length of the barrel. The narrow area between the mating planetary screws and the grooves
allows a thin layer to be exposed to a large surface that helps in control of temperatures and also
in effecting the devolution and exchange of heat.
In the first part of the machine, before the planetary screws, the material moves forward as in a
regular single screw extruder. As the material reaches the planetary section, being largely plasticated at this point, it is exposed to intensive mixing by the rolling action between the planetary
screws, the sun screw, and the barrel. The helical design of the barrel, sun screw, and planetary
screws result in a large surface area relative to the barrel length. The small clearance between the
planetary screws and the mating surfaces, about ¼ mm, allows thin layers of compound to be
exposed to large surface areas, resulting in effective devolatilization, heat exchange, and
temperature control. Thus, heat-sensitive compounds can be processed with a minimum of degradation. For this reason, the planetary gear extruder is frequently used for extrusion/compounding
of PVC formulations, both rigid and plasticized [21,22]. Planetary roller sections are also used as
add-ons to regular extruders to improve mixing performance [97,98]. Another multiscrew
extruder is the four-screw extruder, shown in Fig. 2.11.
Differences between the extruder that uses a motor and the extruder that uses a solenoid
valves and air pressure and also what are the advantage of the second one (frostruder)
Extrusion is done using two basic types of extruders, i.e. (i) Motor extruders in which the
power is supplied from torque produced from interaction of a stator with a magnetic field and (ii)
Solenoid extruders in which the power is supplied by pressure from compressed air and regulated
by means of two solenoids.
Different manufacturers of extruders make them in different designs and sizes, but all of
them have the same common features. Basically, there must be a source of power to provide the
needed pressure for extrusion. A common type of motor extruder consists of a ring shape or
cylinder and pairs of permanent magnet poles on the stator. Windings on the stator line up when
voltage is brought into it. The outer part of the ring consists of the electrified stator with copper
windings. The motor is connected to the thrust bearing of the extruder either directly or through
gears. The speed of the motor rotation is synchronized with the alternating speed the circuits in
the stator. A varying number of magnetic poles of permanent magnets is used in different
motors. The motors used in extruders contain much more magnetic poles which are synchronized
in order to produce a high torque. In an AC motor, each circuit winding on the stator has a
positive or negative polarity. When AC voltage is applied to the motor, the windings alternate
their polarity between positive and negative. This change takes place several times in a second at
a speed that is timed such that it is the same as the motor rotation speed. This is what is referred
to as synchronization. In order to increase the torque, the speed of the motor is increased and
more current is drawn. Modern motor extruders produce a torque of between 2000 and
11,000Nm at speeds of 20 to 500 revolutions per minute.
However, not all motors used in extruders use permanent magnets. The motor may either
be AC or DC – using only electric current to provide the necessary magnetic field for torque
generation. In either case however, the larger the rotor diameter, the greater the number of pole
pairs to generate power, and consequently, the higher the torque produced. An AC motor may
have from eight to forty magnetic pole pairs while a brushless DC motor normally has two, four,
or six pairs. DC and AC motors are magnetized only when electricity is flowing through them.
This absorbs power to create the magnetic field and is less efficient than a permanent-magnet
motor. A brushless DC motor has permanent magnets on the rotor and the windings on the stator,
but with a smaller diameter and many fewer poles.
A different type of extruder is the solenoid extractor. This is the type commonly used for
frosting extrusion. This type is generally called a frostruder and uses a direct drive motor and
two solenoids and uses compressed air for provision of pressure. Each solenoid is attached to a
block and a spring. The purpose of the spring is to provide pressure when the solenoid is
retracted. Basically, the frostruder is a syringe attached to air compressor. The pressure is
controlled by means of two solenoid valves, one to build up pressure (Pressure valve) and the
other to relieve the pressure (Relieve valve). When the Pressure valve is “on”, the syringe is
connected to the main pressure provided by the compressed air and that pressure forces out the
material which is to be extruded from the syringe. To stop the extrusion, the Pressure valve is
closed and the Relief valve is opened. This releases the pressure and the extrusion stops almost
instantly. The materials best extruded by this method are paste-like materials such include food
materials such as frosting, peanut butter, jam etc. and engineering resins such as clay, silicone
and epoxy.
There are many differences between the motor extruders and frostruders, ranging from
their design, size, mode of operation and price. In motor extruders, the torque is generated in the
gap between the magnetic pole pairs on the rotor and pole pairs in the windings on the stator.
Magnets run lengthwise on the rotor, with the number of circuit windings on the stator being
equal to the number of magnets on the rotor. On the other hand, torque in frostruders is provided
by compressed air. Current is required to control the Pressure and Relieve valves.
Advantages of Frostruders over Motor Extruders
i) Solenoid extruders deliver constant torque over their entire speed range, starting from zero,
whereas AC and DC motors lose torque at low speeds.
ii) Frostruders have the advantage of having the capacity to start and stop almost instantaneously.
Once the Pressure valve is turned off and the Relieve valve is on, the pressure in the syringe is
removed and extrusion stops almost abruptly. This however is not the case with motor extruders.
When the motor is turned off in the extruder, it continues to run for some time, still providing
torque which is transmitted to the extruder. This means that the extrusion cannot be stopped
instantaneously as in solenoid extruders.
iii) The torque provided by the extruder motor is limited by the number of the magnet pole pairs
and the speed of rotation. This means that in order to have high torque, the number of magnet
pairs would have to be very high, a practical drawback in the design. On the other hand, the force
supplied to the syringe in frostruders entirely depends on the pressure of the main source, i.e. the
compressed air, which is regulated by the solenoid valves. This makes it possible for the
frostruder to exert pressures of very high magnitudes to the material in the syringe. From an
economical point of view, the manufacture of a motor extruder with a very high torque would
not be viable because it would require having a very long cylinder and great number of
permanent magnet pole pairs.
iv) Frostruders of any size be designed because their operation only depends on their ability to
control pressure from compressed air. The torque produced by motor extruders depends on the
number of magnetic pole pairs which are arranged on the length of the cylinder. The size of a
motor to produce a high torque for extrusion would therefore be very big and also expensive in
terms of procurement and maintenance especially due to room space.
Due to their design, frostruders use very little oil for lubrication because they have few moving
parts and are therefore excellent for food extrusion both at home and industry. Motor extruders
on the other hand use a lot of oil and are not suitable for food extrusion. They are used
commercially for in tubing, blown film, sheet, and continuous-extrusion blow molding. Food
materials such as frosting, peanut butter, jam etc. which cannot be hygienically extruded using
the motor extruders are extruded using frostruders.