Tuesday, 28 June 2016

Polymer injection moulding & reciprocating screws: successful since ever, but what will happen in the future?

Figure 1: cross-sectional design of Willert's plasticizing unit.
Recently, PTonline posted an article about the invention birthday of the reciprocating screw and the injection moulding apparatus patented by William H. Willert (Clinton). This was what triggered me to have a look at the current plasticizing systems and related future concepts:

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!
[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

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