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| Figure 1: cross-sectional design of Willert's plasticizing unit. |
In Figure 1 the cross-sectional design of the plasticizing system of Willert’s patent can be seen. It is composed of three main elements, i.e. the reciprocating screw, the barrel and the material delivering part. If we compare the plasticizing unit of the figure to those implemented in our nowadays injection moulding machines, we won’t find much of a difference.
Before
we think about implementing new plasticizing systems, a basic question arises: Do we already know (after 60 years)
everything about the melting process during plasticizing, respectively the temperature
of the melt in the antechamber?
There
are analytical models such as the melting model of Tadmor [1], which describe how a solid bed of polymer
pellets are melted in the plasticizing unit by the heat transferred from the
hot barrel and the shear heat, also known as viscous dissipation, generated by the
rotation of the reciprocating screw. In
injection moulding processes, the screw reciprocates until the necessary amount
of polymer is melted. In the best case
scenario, a homogenous temperature distribution of the melt in the antechamber
is attained. In this case, all the melt components, e.g. polymer, colors and
additives are equally distributed throughout and we can say that “the screw has
done a good job”.
However, how can we analyze
the distribution profile of the melt temperature?
Formerly,
the melt would be separately injected into a thermally isolated cup and a thermocouple
would be inserted into it to record the temperature (offline measurement). This
would simply show whether the temperature profile set on the barrel was reached.
Just
very recently and yet at university level, an innovative inline measuring
method has been proposed [2]. This
utilizes an ultrasound sensor-based system which determines the axial profile
of the melt temperature in the screw chamber and channels via reflection. Such
inline measuring capabilities will allow us to further develop and improve the plasticizing
process with the aim to decrease temperature gradients within the injection unit.
The inline accessed data will be crucial to set up and validate more accurate CFD
simulations of the plasticizing process. Therefore, simulating the plasticizing
process utilizing different mixing elements in a reciprocating screw set up will
lead to more assertive conclusions regarding the best design to enhance the
melt homogeneity. Consequently, though the principle of the reciprocating screw
will remain, new screw designs may finally arise in a near future to meet the
requirements of processing ultra and high performance plastics, such as PI,
PEEKs, PAEKs, etc.
Besides reciprocating screws, which other plasticizing
systems can be found in injection moulding?
- Ultrasonic Injection Moulding
In the
field of replication of micro parts (parts containing micro features whose
weight is below 1g) the development of new plasticizing solutions starts to be
noticed. Current micro injection moulding machines use two stage systems, which
allow a separation of the plasticizing phase via extrusion and the injection phase
via a stamper. For precise injection of few grams of melt for moulding micro
parts, plasticizing by means of ultrasonic energy emerges as a new alternative
[3]. Ultrasonic injection moulding machines utilize
ultrasound energy to enable the transition from granule to melt in a homogenous
manner throughout the plasticizing unit. Thus, the residence time of the melt
is reduced to a minimum, which is advantageous when working with materials
prone to degradation by the process temperature and/or shear. Overall, this is
a totally new concept of injection moulding, which requires new strategies for
process control as well. Let’s see how this technology develops, but it looks
certainly promising in businesses, such as healthcare and watch manufacturing
for instances.
- Inverse screw injection moulding
Finally, plasticizing
small melt amounts can also be realized by a new concept based on an inverse
screw system (see Figure 2 below) [4]. The main difference to current injection
moulding reciprocating screws can be found in the internal structure of the
inverse screw: the screw flights for conveying the polymer pellets into melt
are part of the cylinder. Using this internal structure sufficient space for processing
standard pellets in the feeding section can be accomplished. Furthermore, the
diameter of the plunger is reduced. This plunger is placed coaxially within the
cylinder. The main advantage of this design is to have the melting benefits of
a reciprocating screw combined with the precision of a plunger during injection.
This system was already tested for several commodities and engineering
thermoplastics. Benefits include shorter residence times and precise
repeatability, which makes this system also a promising development partner in
the manufacture of high precision parts.
 |
| Figure 2: Schematic representation of the inverse screw unit [4]. |
The above mentioned systems are the pioneers who set foot on the ground of new injection moulding innovations and they will be niche players. In the field of microreplication more and faster movements can be seen in terms of completely new developments that totally disrupt well-known plasticizing systems. Let’s see what will be presented at the K show this year in Düsseldorf.
Greetings and until soon!
Greetings and until soon!
Herwig
Literature:
[1] Z. Tadmor: Fundamentals of plasticating extrusion. I. A theoretical model for melting; Polymer Engineering& Science, Vol. 6, 1966
[2] B. Praher et al.: Non-invasive Ultrasound Based Temperature Measurements at Reciprocating Screw Plastication Units: Methodology and Applications, PPS-30, 2014
[3] Ultrasion S.L. (http://ultrasion.eu/)
[4] Ch. Hopmann et al.: New plasticizing process for increased precision and reduced residence times in injection moulding of micro parts, CIRP Journal of Manufacturing Science and Technology, 2015
