The Kunststoffe editorial team is entering 2021 in a new constellation. As the new editor-in-chief, Susanne Schröder will be greeting you here more often from now on.

The Kunststoffe editorial team is entering 2021 in a new constellation. As the new editor-in-chief, I will be greeting you here more often from now on. For the past six years, I was responsible for FORM+Werkzeug – also at Carl Hanser Verlag. During this period, I formed close ties with the mold and tooling sectors, where both metal processing and plastics play a major role. When I visited plastics trade shows, I was always fascinated by the sheer versatility of plastic: high tech, a functional element, lightweight engineering or as a design object. Now, I‘m looking forward to diving deeper into this material. Previously, my reporting usually ended at the point where the finished article drops out of the mold. But in future, the repertoire will be significantly wider.

So what is changing? You as a reader can be sure that we will continue to represent the amazing world of plastics. With many exclusive articles and in-depth information. Personally, I now face a steep learning curve. I want to quickly make contact with my “new world of plastics” and hope that, after phone calls and video conferences at first, I will soon be able to travel and make personal contacts again. My induction will be greatly eased by a competent team, which supports me in every respect. Since, as Michael Jordan so neatly put it:

Susanne Schröder [<a href="mailto:susanne.schroeder@hanser.de">susanne.schroeder@hanser.de</a>]

16:9

Editorial Susanne

“Talent wins games, but teamwork and intelligence win championships.”

Schröder, Susanne

“Unfortunately, the Only Globally Accepted Ideology Is Money”

Alfred Stern, CEO of Borealis, Talks about the Circular Economy

In this interview with Kunststoffe, Alfred Stern, CEO of Borealis, explains why he considers fixed recycling quotas to be necessary and why voluntary commitments do not work.

Recycled plastics had a difficult time last year, with demand for them falling sharply. As a result, environmental associations and recycling companies are calling on politicians to take measures to promote the circular economy as a result. They are not alone – plastics producers such as Borealis have also joined in. In this interview with Kunststoffe, Alfred Stern, CEO of Borealis, explains why he considers fixed recycling quotas to be necessary and why voluntary commitments do not work.

As a result of the coronavirus crisis and the low price of oil, demand for recycled plastics slumped in 2020. Recycling companies and associated trade associations therefore urgently called on politicians to enact legislative measures to stimulate and stabilize demand. Alfred Stern, CEO of Borealis, also sees these developments as a threat to the circular economy. Kunststoffe spoke with him about the regulations that are necessary and the problems that still exist in plastics recycling.

Kunststoffe: Over the past year, there has been a substantial drop in demand for recycled materials. This is not only due to the coronavirus. Do you see this as a once-off phenomenon or more as a long-term trend?

Alfred Stern: Owing to the coronavirus crisis, many fresh food counters, for example, have been, and still are, closed. Sausages and cheeses were and still are only available in packaged form. There is therefore a growing perception that plastic packaging is not just waste, but that it also has a function, namely to keep food fresh and to aid compliance with hygiene rules. For this reason, demand for plastics has developed very well at our company. For me, this is a sign that we have to take the circular economy much more seriously. The pandemic has shown that the functional role played by plastics is not one that can be easily replaced. Hence the considerable increase in pressure to implement the circular economy and recycle larger quantities of plastics.

Kunststoffe: Nevertheless, demand for recycled materials has decreased.

Stern: Borealis is one of the largest plastics recyclers in Europe. We carry out recycling at three plants, the profitability of which has indeed suffered greatly over the past year. This is because raw material costs for virgin material fell sharply during the crisis. On one hand, virgin material became cheaper to procure but, on the other, the price of collecting, sorting and cleaning remained the same. That is why recycled materials have become less competitive.

Kunststoffe: Do you expect this to continue over the long term?

Stern: That risk certainly exists. It depends on how raw material prices develop. In order to shoulder the massive investments that are necessary to get the circular economy rolling, we need to put a supportive legal and fiscal framework in place whose task is to create conditions that make it attractive for companies to invest in this area.

Kunststoffe: Which measures are necessary?

Stern: Three very simple things come to mind. First, we should not be exporting our plastics waste – the raw materials for the circular economy – out of Europe. Shipping waste to Asia is not helpful. We must put a stop to that. Second, recycling quotas are needed. And third, EPR systems (extended producer responsibility – editor’s note) should be designed in an eco-modulated way, such that recycled products cost less than those made entirely from virgin material.

Kunststoffe: Various models of fixed recycling quotas are in discussion. These could apply, say, to products or to companies. In the latter case, every manufacturer would be given a specific quota to fill. Which model makes more sense?

Stern: The key lies in generating demand for recycled plastics. This demand has to emanate from the market and not from the manufacturers. Consequently, quotas should apply to products. If demand rises and there is insufficient production capacity to cover it, prices will rise. And companies will then automatically invest in greater capacity. If, on the other hand, the producers have to meet quotas, there is a risk that they will offer more of the corresponding goods but will be unable to sell them because there is no demand. That scenario does not constitute good use of market forces.

Kunststoffe: So the main problem in plastics recycling is not enough demand and thus insufficient profitability?

Stern: At present, the prices of recycled materials are to a certain extent linked to the prices of virgin materials. If virgin materials become cheaper, the prices of recycled materials must also decrease so that they can be sold. That is why it is so important to generate demand for recycled material – it will enable this link to be broken. Of course a number of progressive companies are already forcing the issue independently and are willing to pay higher prices for such materials. Some companies, for instance, have set themselves targets to have a certain percentage of recycled materials in their products. But the massive changes needed to transform the industry cannot be achieved by just a few pioneers on their own; broad-based support is called for. We need massive investment. And that will only happen if profitability is assured.

Kunststoffe: So voluntary commitments on their own are not enough?

Stern: In my opinion, there is no evidence anywhere in the world to show that environmental compatibility is increased by voluntary commitments. Neither as regards CO2 emissions nor other issues. The decisive criterion is profitability. Unfortunately, the only globally accepted ideology we have is money.

Kunststoffe: With its New Green Deal, the EU Commission has formulated measures and goals for a more sustainable economic policy in Europe. Are these measures and goals conducive to strengthening the circular economy?

Stern: The Green Deal contains the right visions. It represents an opportunity not just for Europe, but for the whole world. But the specific steps taken next will be crucial. A good example is the planned plastic tax. It is important that consumers, industry and politics work together to apply it in such a way that it genuinely has a steering effect. It could have a strong impact, for example, if implemented via an eco-modulated EPR system. If this is done sensibly, the entire industry will jump on board to optimize the cost structure. However, it needs to be implemented carefully and must take all the consequences into account. We need to avoid creating a “tropical island” within global competition only to find that we are no longer competitive.

Kunststoffe: Plastics converters also frequently criticize the quality of the recycled materials. Is that not a major problem, aside from profitability?

Stern: It’s true that there is huge variation in the quality of recycled materials on offer. However, it is already possible at the present moment to provide guarantees of quality. Standards of quality can be raised through innovations and technologies. But we mustn’t lose sight of the bigger picture. At Borealis we are concerned with “designing for circularity”, which starts off with “designing for reuse”, i. e. applications that are used several times before being disposed of. In addition, the products must be designed for easy recycling. Mono-material solutions can be processed very easily, e. g. by mechanical recycling. This means that it is possible to achieve high quality with low energy consumption. Where waste cannot be recycled mechanically, the next step is to send it to chemical recycling. In this connection, we are collaborating, for example, with our principal owner, OMV. Chemical recycling yields synthetic oil which can be processed in full into new goods of the same quality. In addition, plastics from renewable raw materials such as food waste are also being used. We are already operating all these technologies commercially, albeit only in small quantities.

Kunststoffe: So the desired standard of quality of recycled materials could be achieved if all processes were fully utilized?

Stern: Yes. However, the problem at present lies in obtaining sufficiently large quantities in consistent quality. So, I am appealing once again for framework conditions to be created that offer incentives for investment. This is the only way to achieve the necessary quantities.

Kunststoffe: You have just opened a new recycling plant in Lahnstein, Germany. What is that about?

Stern: We believe the demonstration plant there to be the last word in mechanical recycling plants. We want to use it to show what can be achieved when modern technology and machinery are deployed in a state-of-the-art mechanical recycling plant. This is a joint collaboration between us and the companies Tomra and Zimmermann Recycling, with each partner contributing its know-how. We at Borealis are looking after the commercial side, we know the customers and are familiar with product development and applications. Tomra is known for its collection and sorting technologies, Zimmermann has many years of experience in waste management, including the sorting and recycling of plastics. The plant produces high and low density polyethylene and polypropylene. The materials produced differ from recycled materials currently available on the market in terms of their purity, low odor, high product quality and bright colors. We hope this approach will open up other markets and applications, e. g. in the automotive sector or in caps & closures.

Kunststoffe: What type of waste are you processing there?

Stern: Both rigid and flexible post-consumer. I don’t mean sorted and pre-sorted waste, but rather classic post-consumer household waste.

Kunststoffe: What quantities are being processed?

Stern: It is a demonstration plant. The quantities are large enough for us to conduct qualification tests with customers to show which applications the technology lends itself to.

Kunststoffe: Are you planning to expand the site into a commercial plant with your partners at a future date?

Stern: We did not build the plant without having a vision for the future. But first we are looking at how well it performs and gaging the extent of interest. The point of the demonstration plant is to improve the technology and to gain experience for future large-scale operation.

The interview was conducted by Florian Streifinger and Susanne Schröder, Editors.

Together with Tomra and Zimmermann Recycling, Borealis has built a demonstration plant in Lahnstein. According to its own information, the most modern plant for mechanical recycling is intended to show what is possible with modern technology and equipment in a state-of-the-art plant for mechanical recycling

Borealis - Hauptartikel

Alfred Stern has been CEO of Borealis since July 2018 and a board member at the plastics producer since 2012. Prior to his appointment as CEO, he was responsible for the Polyolefins and Innovation & Technology divisions. Stern has been working for Borealis since 2008. Before that, he held various management positions at DuPont, including Research and Development, Sales and Marketing, and Quality and Business Management. He studied Polymer Engineering and Science at the University of Leoben, Austria, and holds a PhD in Material Science.

“Nowhere in the world is environmental compatibility increased through voluntary commitments.”

Streifinger, Florian

Kunststoff-Aufbereitung

Kunststoffverarbeitung: Verfahren

Recycling

Umweltmanagement

Kunststoff Werkstoffe

Kunststoff

Bauwesen

Baustoffe

Kunststoff-Recycling

Kunststoff-Rezyklat

Feeding Mechanically Recycled Fiber-Reinforced Thermoplastics Back to Production

Organo sheets have become established in several industrial sectors for the series production of highly stressed lightweight components. The utilization of waste material is a promising recycling approach.

Organo sheets have become established in several industrial sectors for the series production of highly stressed lightweight components. Hereby, waste material accumulates during production, which cannot be fed back directly to the manufacturing process. Therefore, the utilization of this scrap material is a promising recycling approach.

Continuous fiber-reinforced thermoplastics exhibit very good technical properties that permit their use in numerous application areas – from sports shoes to housings for electronic equipment through to safety-relevant automotive components such as brake pedals or A-pillar reinforcements. For more than 20 years, Bond-Laminates GmbH, an affiliate of Lanxess, has been producing such fiber composite materials under the brand name “Tepex” in their Brilon/Germany site. Meanwhile, these organo sheets have become established in many mass-produced products.

Tepex organo sheets are manufactured in a continuous process, and are customized for the respective processor. In particular for the automotive industry, where the semi-finished Tepex items are further processed in fully automated manufacturing cells, the inlays are supplied as two-dimensional pre-cuts. In order to minimize the scrap produced when trimming these inlays, the corresponding geometries are interleaved in the semi-finished item – taking the necessary fiber orientation into account – so that an optimum material yield is possible. This procedure is also known as “nesting”.

However, with strongly curved geometries that make interleaving highly problematic, systematic trim waste is unavoidable (Title figure). Under mass-production conditions, a waste proportion of just 5 to 10 % results in huge amounts of residual material, which cannot be returned directly to the production process. In order to re-use the material, it must first be recycled.

Suitable Recycling Process  for Organo Sheets

Because Tepex and comparable organo sheets are based on a thermoplastic plastic matrix, recycling is possible with material utilization by means of mechanical rework as well as by means of chemical raw material utilization, i. e. splitting up into individual components by means of hydration, hydrolysis, and pyrolysis. Recycling of residual and waste material offers the greatest economic benefits. Hereby, the Tepex waste must first be ground to a defined particle size.

Conventional rotary cutter granulators are able to shred all types of organo sheet. However, the high cutting speeds normally used for this technique result in a very high wear rate of the cutter blades. Moreover, the regrind has a high dust content and a relatively low medium particle size. In general, the resulting low bulk density of the regrind is unsuitable for conventional processing operations such as regranulation or direct injection molding.

A reduction of cutting speed with uniform shredding performance can be achieved with classical shaft shredders. However, a low speed should be selected for the shear roller. In particular, single-shaft shredders with automatic hydraulic feed are well suited to produce a homogeneous regrind (Fig. 1) [1]. Hereby, large lamellar particles – as can occur with other shredding methods – are avoided. This also applies for thicker materials or particularly brittle carbon fiber-reinforced variants.

Due to the high rigidity, strength, and impact resistance of Tepex materials, a suitable set-up is necessary for efficient shredding. Apart from the smallest possible positive cutting angle, other parameters should be ensured (Table 1) [2]. In addition, a suitable extraction system should always be provided due to the high dust generation.

Reground material produced by shredding can be used directly for typical plastics processing methods such as injection molding or extrusion. In order to ensure smooth production in standard machines without modified feed zones, a medium particle size of 8 to 15 mm is recommended. Such regrinds have a bulk density of about 200 kg/m³ (± 20 kg/m³). If conventional granulate (re-granulates or virgin material) is added to the regrind in an amount ≤ 20 %, the bridging tendency will be reduced so far that normal feed hoppers without forced feeding – i. e. standard versions – can be used. This is confirmed by investigations at Kunststofftechnik Paderborn, a research facility of the Paderborn University, Germany.

Figure 2 shows an automatic process in which a mix of 80 % granulate and 20 % organo sheet regrind was injection molded using a standard 25-mm diameter screw without additional peripherals. Higher amounts of regrind, sometimes even up to 100 %, can also be processed with standard machines, provided that a suitable forced feed for the regrind is used, e. g. a compression screw with high channel depth. For maximizing the mechanical properties in the component, screw geometries with high channel depths in the feed zone that are gentle to the fibers can be used. But usually they are not essential. Tepex regrind with the recommended medium particle size can be processed without problems using standard, unmodified three-zone screws.

For the injection molding process, and depending on the component to be produced, low metering speeds and back pressures are recommended. Normally, the optimum operating point is determined by the machine type used and the component’s design.

The addition of short fiber-reinforced or non-reinforced virgin material permits the component’s fiber content to be adjusted precisely. With Tepex based on polypropylene (PP), a reduction of the regrind to a fiber weight content of 30 % is expedient, because short fiber-reinforced PP-GF compounds with a higher fiber content are hardly available on the market. Due to the non-polarity of PP, an improvement of the mechanical properties by a further increase of the fiber weight content is not a trivial matter, and can usually only be achieved with special long-fiber granulates. Other Tepex types can also be processed without additives, or short fiber-reinforced virgin material with comparable fiber weight content can be admixed.

Mechanical Properties like Conventional Short Fiber-Reinforced Plastics

Strength, rigidity, and toughness of the recyclate are comparable with those of conventional short fiber-reinforced plastics with corresponding fiber contents.  In order to validate this claim, Tepex regrind (source material: Tepex dynalite 102-RG600(x)/47 %; fiber weight content: 66 %) was mixed with different amounts of a short glass fiber-reinforced Durethan BKV30 (polyamide 6 (PA6), fiber weight content: 30 %), and processed directly in an injection molder. Hereby, the resulting mechanical values for flexural modulus and tensile strength are the same as for Durethan types with comparable fiber weight content (Fig. 3) [3]. The results were confirmed in various investigations – also with other polymers and fiber materials. [1, 4]

For the purpose of verification, a demonstrator component (Fig. 4) was produced in cooperation with Kunststofftechnik Paderborn. The component is used for research work, and originally served for investigating different aspects of the special injection molding process GITBlow [5]. The geometry is similar to components that are already being mass-produced for the automotive industry [6].

The overmolding material consists entirely of a mix of 80 % virgin material and 20 % regrind. The demonstrator shows that thick-walled regions with negligible shrinkage as well as thin-walled regions with long flow paths can be produced completely and without visible shrinkage effects, provided that normal injection conditions are observed.

Without additional investments, recycled trim wastes can be used in planned or even in ongoing series products. Consequently, the process represents an important step towards a comprehensive circular economy for continuous fiber-reinforced thermoplastics.

[1] Schemme, M.; Karlinger, P.; Hummel, S.: “Closed-Loop-Recycling – Strategien für endlosfaserverstärkte thermoplastische Hybridstrukturen“, FOREL Kolloquium TU Dresden, 29.08.2019

[2] Mattus, V.; Hintersteiner, R.; Meier-Kortwig, J.; Buntenbach, S.: „Werkstoffliche Verwertung lang- und endlosfaserverstärkter Thermoplaste zu hochwertigen, langfaserverstärkten Fließpress-Bauteilen“, Abschlussbericht Az: 09220 (Deutsche Bundesstiftung Umwelt), April 1999

[3] „Kombination von Thermoplast-Spritzguss und Thermoformen kontinuierlich faserverstärkter Thermoplaste für Crashelemente (SpriForm)“, Abschlussbericht zum BMBF-Verbundprojekt (02PU2350–02PU2355), 2011

[4] Moritzer, E.; Heiderich, G.: „Mechanical recycling of continuous fiber-reinforced thermoplastic sheets”, PROCEEDINGS OF PPS-31, DOI: 10.1063/1.4942328, March 2016

[5] Moritzer, E.; Landgräber, B.: “Investigations on the production of hybrid structures using the two-stage GITBlow process”, AIP Conference Proceedings 2139, DOI: 10.1063/1.5121664, 2019

[6] “Fast 50 Prozent weniger Gewicht”, Industrieanzeiger, 24.11.2014

Even with a highly optimized production of organo sheets, considerable amounts of scrap occur

20:7

Table 1. Recommended process parameters  for shredding organo sheet trim waste

Fig. 1. Shredded Tepex: during shredding, the type of machine used is decisive. Single-shaft shredders in particular are able to produce a homogeneous regrind

Fig. 2. Processing a mix of 80 % granulate and 20 % organo sheet regrind with a standard  3-zone screw: reproducible metering in every cycle is possible during processing

Fig. 3. Comparison of rigidity and strength between PA6 Durethan BKV30, BKV35, and BKV40, as well as PA6 Durethan BKV30 with additions of 10, 20, and 30 % Tepex 102 regrind: the values are comparable in spite of the addition of regrind

Fig. 4. A demonstrator shows that the regrind is suitable for injection molding plastic components. The overmolding material of the demonstrator is a mix of 80 % virgin material and 20 % Tepex regrind

Lanxess - Hauptartikel

Dr. Stefan Seidel is Head of Research and Development for continuous fiber-reinforced thermoplastic composite materials at Lanxess/Bond-Laminates; <a href="mailto:stefan-seidel@bond-laminates.de">stefan-seidel@bond-laminates.de</a>

Faserverarbeitung

Kunststoffverarbeitung: Maschinen und Komponenten

Mahlgut

Faserverbundwerkstoffe

Andere Werkstoffe

Organobleche (gewebeverstärkte Thermoplast-Halbzeuge)

Halbzeuge und Zwischenprodukte

Kunststoff-Halbzeuge

Mahlen

Part 2 of the Series Deals with Basic Methods for Evaluating a Screw Geometry

To be able to carry out the practical test in a resource-saving manner, the designers use simulation tools to assess the screw geometry. What is important here is explained using a 3-zone screw as an example.

Once the basic layout of a plasticizing unit has been determined and the appropriate injection unit has been selected, the development of the screw geometry enters its decisive phase. To be able to carry out the practical test in a resource-saving manner, the designers use simulation tools to assess the screw geometry. What is important here is explained using a 3-zone screw as an example.

The basic design of a plasticizing unit and the correct choice of the injection unit were discussed in the first part of this series. The way to determine the required screw diameter on the basis of the shot volume was also explained. By applying the formula for the mean residence time, the utilization rate and the thermal material load can be estimated; the latter needs to be kept low to achieve high end product quality. The maximum injection pressure and the available screw torque are additional key variables for successful injection molding production.

These considerations (see <a ext="tango:{45D59CEA-6DC5-4DBA-A9B4-CA287D7F7E2F}">Kunststoffe international 10/2020, pp. 27-29</a>) form the basis for the choice of the barrel-and-screw combination and also the starting point for further optimizations. In the second part of this series of articles, the basic methods for simulative assessment of the geometry of a given screw are presented – using the example of a 3-zone screw.

Before All Experiments Comes  the Simulation

The first question to be answered is what objectives should be pursued in developing a screw geometry. Often the goal can be clearly defined, such as increasing the flow rate, reducing the melt temperature, improving the quality of the blend, etc. The requirements become more complex as soon as the desired results are only indirectly linked to the screw geometry, or when they can be attributed to several causes, for example, when it is desired to reduce the formation of plaques, or when the wear behavior and conveying stability need to be improved.

Such multiple demands on screw aggregates often conflict with each other. Careful balancing of the layout is necessary to resolve such conflicts between several different objectives.

It has become common practice to optimize the geometry of a screw by way of simulation before the first tests are carried out with real experimental screws. With PSI/REX, Wittmann Battenfeld GmbH, Kottingbrunn, Austria, has a special software at its disposal for calculating the screw design. This software is ultra-modern and subject to continuous updates by targeted research carried out at Paderborn University, Germany.

While using the computer to calculate the screw geometry, the geometry can be varied extremely flexibly, and the resulting change can be immediately visualized on the screen. By running systematically through a pre-defined series of tests, it is possible to analyze the emerging trends. Finally, the results of all calculations are combined and compared. From the sum of this information, the corresponding screw geometry is developed and further optimized down to the last detail – until the desired result comes into view.

Only then are experimental screws produced in original size and used in practical tests. Depending on the complexity of the task, several different experimental screws may be used to approach the objective from various angles. If these tests prove successful, the optimization process is completed. Where there is still room for improvement, the development loop is re-run.

Model Case for the Optimization fo the Screw Geometry

Next, the parameters of a standard 3-zone geometry shall be discussed, and their influence on the manufacturing process shall be illustrated by an example. In order to give a full description of such a geometry in terms of process technology, the following parameters must be known (Fig. 1).

From the number of geometry parameters for only a relatively simple standard 3-zone screw, it is already apparent that there is basically a multitude of possible variants even for this type of screw. In the case of more complex geometries, such as those found in barrier screws, screws with shearing and mixing sections or shearing/mixing screws, the number of geometry parameters is many times higher. Starting basically from the recommendations available in the relevant professional literature [1], the optimization of the geometry for a screw with a diameter of 50 mm (DSC) is calculated below as an example.

It is assumed that the length of the feed zone is 50 % of the total length of the screw and the lengths of the compression zone and metering zone should each be 25 % of its total length. We set the feed zone depth at 0.1 D, i. e. 5 mm. The flight depth ratio between the feed zone and the metering zone should be 2. The L/D ratio is assumed to be 22.

A variety of different calculations can be performed for a screw with these pre-defined parameters. The present discussion focuses on the melt throughput, the pressure curve or pressure build-up capacity and the melting process.

Basic Assumptions and  Throughput Behavior

Further assumptions include the metering stroke of 85 mm and the cycle time of 35 s. The back pressure is set at 80 bar. To simulate moderate and realistic metering conditions for the material polystyrene (grade:PS 454N; manufacturer: Styrolution), a circumferential screw speed of 300 mm/s is assumed. The barrel temperature profile is slightly rising from the filling opening to the antechamber. All calculations are carried out for the 50 mm screw position (Figs. 2 and 3).

For the previously selected cycle parameters, the average metering performance is calculated at about 12.49 g/s for the present screw geometry. This means that the machine transports 12.49 g/s in the metering phase and thus takes about 12.7 s to plasticize 158 g of material. With a residual cooling time of more than 12.7 s, the machine can start a new metering stroke on time. But if plasticizing takes longer than the residual cooling time, the timing of metering impacts the total cycle time and thus reduces productivity.

The total output of 16,25 kg/h determines the amount of material consumption in the course of production. Since the screw does not dose during most of the cycle time, this output falls below the figure suggested by the average metering performance. The total output is the decisive parameter in dimensioning auxiliary equipment (dryers, material loaders, etc.).

Pressure Build-up Capacity and Melting Process Curves

During the metering phase, the pressure inside the screw channel increases from the feed opening to the back pressure in the antechamber. Depending on the screw geometry, there may be one or more pressure peaks in between.

Figure 2 shows the pressure curve over the screw length. In this particular case, the pressure curve begins to rise at about L/D = 2 and reaches the peak pressure of about 160 bar at about L/D = 14.25. In the last zone of the screw, the metering zone, the pressure drops continuously up to the check valve.

The melting process (MP) is visualized via two curves (Fig. 3): the solid bed width (green) is shown for the corresponding screw channel section, and the proportion of molten material (red) during the metering process. In addition, the development of these two parameters towards the end of the cycle is illustrated (in orange and beige).

From the results, it can be concluded that this melting process promises good melting of the material, since the proportion of melt has already reached 100 % at about L/D = 8 (proportion of melt MP = 1). In other words, the solid bed width has been reduced to 0.

It can be seen that even with a “simple” 3-zone screw, the designer has a large number of geometric parameters at his disposal to achieve the development goal. With the simulation software, the actual experimental effort can be reduced significantly. However, the results of the simulation cannot replace the practical experiments, since not all boundary conditions can be considered or are known. In a closed environment they show trends and possible need for action.

[1] Johannaber, F.; und Michaeli, W.: Handbuch Spritzgießen. Carl Hanser Verlag, München 2014

[2] Gornik, C.: Neue Erkenntnisse zur Plastifiziereinheit von Spritzgußmaschinen basierend auf experimentellen Untersuchungen, Dissertation, Montanuniversität Leoben 2007

The screw geometry is finely optimized in several calculation steps until the desired result is achieved. Subsequently, test screws are manufactured in original size and tested in practical tests with various material types

Fig. 1. Schematic diagram of a 3-zone screw with number of screw flights i = 1

Fig. 2. Pressure curve along the screw at 50 mm stroke position

Fig. 3. Melting process curves for the screw at stroke position 50 mm towards the end of the cycle

Name - Hauptartikel

Filipp Pühringer heads the Process Engineering Development Department at Wittmann Battenfeld GmbH in Kottingbrunn, Austria.

Part 1 of the series was devoted to the basic design of a plasticizing unit and the correct choice of injection unit and was published in Kunststoffe international (<a ext="tango:{45D59CEA-6DC5-4DBA-A9B4-CA287D7F7E2F}">issue 10/2020, p. 27–29</a>). In the 3rd part of this series of articles, the calculation results will be analyzed, and first steps towards optimization of the geometry will be outlined. It will appear in one of the next issues.

Spritzgießen

Spritzgießmaschinen, allgemein

Spritzgießmaschinen

Schnecken und Plastifiziersysteme

Efficient and Precise Heating up of Thermoplastic Composites

Infrared heating technology has become established as the standard in the processing of thermoplastic composite materials. The focus is on process control. On the one hand, it is important that the heating process is fast, on the other hand, the material must not be overheated. It is also important to achieve excellent temperature homogeneity. Simultaneous processing of blanks with different thicknesses is a particular challenge.

Infrared heating technology has become established as the standard in the processing of thermoplastic composite materials. It makes it possible to heat flat organo sheets of up to about 3 mm thickness within the cycle times characteristic for injection molding processing. The focus is on process control. On the one hand, it is important that the heating process is fast, on the other hand, the material must not be overheated. It is also important to achieve excellent temperature homogeneity. Simultaneous processing of blanks with different thicknesses is a particular challenge.

There are two main reasons why thermoplastic composites are continuing to gain in importance, especially in lightweight automotive design. Firstly, the consistent thermoplastic approach makes it possible to efficiently integrate the forming and functionalization of fiber-reinforced components, which reduces unit costs. Secondly, the use of exclusively thermoplastic polymers makes it easier to develop recycling strategies. Returning composite components to the material loop at the end of their service life is one of the major tasks in the development of new vehicle concepts.

In the established one-step process for the production of thermoplastic composite components, the materials in the form of plate-shaped blanks are located in magazines. They are removed from the magazines by a robot and conveyed to an infrared (IR) oven. After heating, the now malleable composite material is picked up again by the robot and deposited in the mold of the injection molding machine. It is reshaped and reconsolidated by the mold closing movement; immediately afterwards, the desired detail geometries are added by injection molding. After completing the cooling phase, which is required above all for the injection-molded areas, the ready-to-fit composite part is removed from the mold. One of the most important advantages of thermoplastic composites is that the cycle time for producing a technical component is only 40 to 80 s. A thermoset composite solution would require at least a few minutes for this.

The sequence from bringing in the material into the mold to overmolding is particularly time-critical. As soon as the heated composite blank is removed from the IR oven, it begins to cool down. However, to overmold with detailed geometries, there must still be sufficient heat in the material to achieve good adhesion between the composite material and the injected thermoplastic.

Transporting the heated composite blank, known as hot handling, needs to be professionally planned, tested and implemented with the aid of suitable automation solutions. The design and positioning of the IR oven are also crucial. Engel Austria GmbH, Schwertberg, Austria, offers IR ovens, developed and produced in-house, in both horizontal and vertical orientation with optional single-sided or double-sided heating panels in various designs.

For thin materials, a vertical IR oven positioned on the stationary mold mounting platen of the injection molding machine often offers advantages. The thinner the material, the faster it cools down. The spatial proximity of the IR oven and the mold shortens the move paths and reduces the handling time accordingly. Single-sided IR ovens are often sufficient for heating up thin materials with wall thicknesses of up to 1mm. In addition, the single-sided design of the vertical IR oven facilitates accessibility and rapid removal of the heated blank.

Efficient Heating of Thick  Semi-Finished Products

The fact that infrared radiation, which has a high energy density, is excellently suited for heating up thermoplastic composite semi-finished products is attributable to the structure of the plate-shaped materials. The blanks typically consist of several individual layers, between which there are no cavities after stacking and consolidation. This ensures that the heat is conducted rapidly from the surface to the core. As infrared radiation acts quickly and is, at the same time, a zero-contact method, preforms of various thicknesses can be heated up to well above the melting point of the matrix polymer without causing adhesion problems.

The depth to which the electrons are excited depends on the wavelength of the IR radiation and the absorption behavior of the material. Typical thermoplastic matrix materials have relevant absorption bands in the range of a wavelength of 2 to 3 µm [1]. Medium-wave infrared radiators achieve a high radiation intensity in this range, which is why they are first choice for heating up thermoplastic composites. This allows a penetration depth of up to 0.5 mm to be achieved. Rapid heating occurs in this layer just below the surface.

With thicker organo sheets in particular, it can happen that the core is not yet sufficiently heated although the surface has reached its target temperature. The heat required to melt the core area can only be transferred into the material by conductive heating. Plastics and glass fibers are relatively poor heat conductors, which is why soaking the core area takes some time. To ensure that the core area has reached the required temperature, the surface is kept at the set temperature for a defined, empirically determined time.

Infrared radiation is the most efficient method for heating up organo sheets. In terms of heat conduction, the layer near the surface is a kind of internal heat source that no longer has to overcome thermal transfer resistance, as is the case in pure contact or convection heating. Especially compared to convection heating, infrared heating achieves significantly shorter heating times. In general, IR ovens are suitable for all relevant matrix polymers, including the high temperature plastics PAI, PPS, PEKK, and PEEK.

Highest Temperature Homogeneity with Optimum Material Protection

The challenge in heating with infrared radiation is to achieve an optimum combination of high efficiency and good temperature homogeneity without degrading the material due to local overheating. The target or setpoint temperature, which is usually well above the melting temperature, may at most only be slightly exceeded. To ensure this, the surface temperature is monitored using pyrometers during the entire heating process. The measured data are fed back into process control. Figure 1 shows an example of the temperature curve for a single measuring position during the heating and soaking phase for a semi-crystalline thermoplastic, such as polypropylene or polyamide.

Virtually linear progressions until the crystallite melting point is reached are characteristic of this. The heating rate is determined by the power per unit area of the radiator, the absorption coefficient, and the heat capacity and thermal conductivity of the thermoplastic composite. The heating rate decreases when it approaches the crystallite melting point – for polyamide 6 this is approx. 223 °C. Part of the supplied energy is used for melting the spherulites. Once the matrix material has melted, the heating rate increases again and even becomes more dynamic than below the crystallite melting point. The molten polymer has a lower density and heat capacity and responds accordingly with faster heating. Close to the desired temperature, the control for the soaking phase kicks in and limits the further temperature increase to prevent overshooting.

The slightly irregular curve around the crystallite melting point is due to the processes involved in melting the thermoplastic matrix material. Due to the temperature distribution over the material’s cross section and the temperature-dependent absorption coefficient, the effects are spread over a larger temperature and time range.

In order to obtain a completely linear heating process, the heating rate can be limited to a certain value and promoted to a control parameter. However, it is in the interest of process efficiency and also of material protection to fully utilize the heating power of the IR radiators in the heat-up phase as long as no significant local temperature deviations and also no overheating of the composite surface are detected.

Controlling a Large Variety of Operating States

IR ovens might be set up to provide optimum temperature homogeneity for a specific operating point, for example, when it is loaded with 70 % of the nominal payload area in the form of a rectangle. However, since the real operating point usually deviates significantly, this is not meaningful. A system with a very good basic characteristic in terms of thermal homogeneity is more useful. The software of the Engel IR ovens is therefore capable of optimally controlling a large variety of operating states and influencing variables, which enables excellent thermal homogeneity over a wide range of applications.

The IR oven design principle assumes a full load, i. e., the use of the entire radiator area minus the defined edge width. The IR oven shown in Figure 2, for example, has a radiator area of 1250 x 750 mm and a nominal payload area of 1100 x 600 mm. Radiators that are not required can be switched off individually. In addition, several radiators are usually combined into groups and the whole group of radiators is controlled on the basis of a specific temperature measurement.

The possible causes of thermal inhomogeneities are complex. Among other factors, the influence of the control zones on each other, extend and shape of load in the payload area, the ambient conditions, and the transient behavior of the IR oven all play a role.

Precise control of the IR heating process is an essential part of process control in the manufacture of thermoplastic composite parts. In Engel system solutions, the IR oven control is therefore integrated into the injection molding machine control unit. This allows the process parameters and measured values of the IR oven to be documented along with the production data from the injection molding process.

Reaching Targets Simultaneously with All Control Zones

Due to the multitude of possible causes for thermal inhomogeneities, each control zone (CZ) has a different heating characteristic in practical applications. In an experiment, thermal inhomogeneities were specifically induced (Fig. 3). It was demonstrated that this results in clear differences in the heat-up rate. The desired temperature is reached at different times and stabilization of the surface temperature at the desired temperature is delayed. This situation served as the starting point for automatic heating behavior optimization.

The improved behavior is shown in Figure 4. The control strategy relies on comparisons between the current actual temperatures in the respective control zones. The heating curves now show a uniform, almost linear progression. Stabilization in the area of the temperature setpoint has improved significantly compared to the initial situation.

In the case of a control zone that reaches its temperature setpoint early at full radiator power, the radiator power must be reduced early in the heating phase. Automatic reciprocal optimization and coordination of the control zones means that the radiator power of all affected control zones is continuously adjusted to keep the deviations within an adjustable narrow tolerance band. This results in a uniform heating rate for all control zones and all control zones reach the temperature setpoint at the same time.

What is of particular importance here is that the process control strategy does not require a lead control zone. The values measured by all the pyrometers are permanently considered and a highly dynamic heating process is implemented in all control zones; the process is adapted to the specific behavior of the matrix material during heating, melting and further heating of the material in the melt phase.

When the surface temperature approaches the temperature setpoint, further granular adjustment during process control must prevent the surface temperature from overshooting. High temperature consistency in the soaking phase ensures that the target temperature in the core area is reached at a precisely predictable time. Heating up of the thermoplastic composite can therefore be terminated after a defined, and shortest possible duration; this in turn contributes towards protecting the material.

Especially in the soaking phase, it is crucial to keep temperature deviations low to avoid local overheating and degradation of the material. For this purpose, the control parameters must be adjusted in the best possible way. For example, if the material, the blank size or the thickness of the material is changed, the control parameters for the soaking phase also need to be readjusted. A separate measuring cycle can be utilized to automatically optimize the control parameters. Doing so delivers information on the existing control behavior and based on these, the control parameters are recalculated and adjusted. This results in the excellent control quality shown in Figure 5.

Heating up Semi-Finished Products of Different Thicknesses Together

The Title figure shows the fully automated production cell for a door structure component made of three organo sheets of different thickness. The blanks with thicknesses of 1.0 and 2.5 mm are heated together in a horizontal IR oven (Fig. 6). Under standard conditions the thinner organo sheet would heat up far faster than the thicker one. However, limiting the maximum permissible temperature deviation between the control zones ensures a uniform and even temperature profile. The two blanks reach the temperature setpoint simultaneously. A single-sided oven with a vertically arrangement is used for the 0.6 mm organo sheet. The main focus here is on particularly fast hot handling, because the very thin composite blank cools down very quickly. The three blanks are brought together in the mold. Within a cycle time of 70 s, they are processed to create a composite part with load optimized design.

In combination with automation, infrared heating technology is key to optimum processing of thermoplastic composite materials. Innovative strategies for controlling the heating processes make it possible to achieve rapid and highly precise heating processes in order to process organo sheets of different thicknesses in a single step. The machine operator is supported by automatic optimization routines for fine tuning. Software-supported determination of suitable control parameters leads to very constant surface temperatures during the heating phase. External influences are automatically eliminated by control actions before they lead to critical process deviations.

1. Ehrenstein G. W., Handbuch Kunststoff-Verbindungstechnik, Hanser, 2004, p. 223

Three organo sheets of different thickness are heated in two IR ovens in the production of a door structural part

Fig. 1. The surface temperature is monitored using pyrometers during the entire heating process. A double-sided IR oven is equipped with six pyrometers as standard equipment. The picture shows an example of the temperature curve at one measuring position during the heating phase and the soaking phase

Fig. 2. A double-sided horizontal IR oven with 1100 x 600 mm payload area: the radiators are combined into groups to allow for control on the basis of a specific temperature measurement

Fig. 3. In the experiment, thermal inhomogeneities were specifically induced (initial situation). The resulting heating curves were then optimized by automatic process control (CZ = control zone)