Plastics will stay green. Quite literally. The Special in this issue (from page 7) deals with color and design of plastics.

We talked to pigment manufacturers and others about the color trends they are expecting in the coming year. And we can already reveal the following: in 2021, too, we can look forward to many products in a rich green.

This is mainly due to the continued high status that many consumers attach to environmental protection. Of course, companies producing sustainable products want to tell their customers about it. And what could convey this message better than green? Ultimately, it is no longer enough just to design an article offering the customer added value. The product also has to reflect the consumer’s attitudes and ideals. And the choice of the right color often plays the crucial role in this.

But apart from giving an optical indication of “green” products, pigments also contribute to the more sustainable use of plastics. Suitable pigments prevent degradation of polymers by increasing their light protection. Light and weather-resistant pigments offer high color stability and thereby retain the perfect appearance of products. This increases their longevity.

Pigments thus offer processors additional opportunities to use plastics in a more environmentally friendly way. We can hope that industry will actually make use of these and other capabilities in the interests of greater sustainability. Then, the plastics industry will be able to give consumers an optical signal of progress in environmental protection. After all, it is not jut a matter of concrete technical improvements but also their presentation. And the color green is certainly not the worst messenger for this.

Editorial Streifinger

Streifinger, Florian

Trend Report on the Coloration of Plastics

Plastics processors have a wide range of colors at their disposal to color their products. Choosing the right color can ultimately be the key to success. But colors are just as subject to trends as materials and designs. Kunststoffe asked manufacturers of pigments, masterbatches, and colorants which colors would be particularly popular in the coming year. We were also able to learn what impact the corona crisis was having on color selection and how the demand for increased recycling and greater sustainability was changing the coloration of plastics.

Plastics processors have a wide range of colors at their disposal to color their products. Choosing the right color can ultimately be the key to success. But colors are just as subject to trends as materials and designs. Kunststoffe asked manufacturers of pigments, masterbatches, and colorants which colors would be particularly popular in the coming year.

Appearance is the first thing about a product that strikes the customer’s eye. Visual impression often influences the purchasing decision even before information on the technical data or contents of a package is absorbed. This makes choosing the right colors very important. Colors send signals, create feelings, produce emotions, and awaken memories. They also assist branding by associating the name of a product or a company with a specific color. The magenta of Deutsche Telekom is one of the most prominent examples here. It is equally important for companies to choose the right colors for their products.

Like other materials, colors are subject to trends. They often follow societal developments and reflect consumer wants. The growing demand for better environmental protection has been manifested in recent years in an increase in the use of green shades for packaging and products, because these create an association with a more sustainable lifestyle. Kunststoffe asked manufacturers of pigments, masterbatches and colorants what the trend colors would be in the coming year. We also wanted to know what impact the corona crisis was having on color selection and how the demand for more sustainable use of plastics and greater recycling was influencing their product development.

The trend for colors associated with nature and a healthy environment is set to continue in the coming year. Manufacturers are agreed on this. Green and blue shades in particular will remain in demand. This is due to the continuing very high environmental awareness. Dr. Dietmar Mäder, Global Head of Plastics Marketing & Technical Service at Eckart GmbH, Hartenstein, Germany, believes that the present corona crisis is also accelerating this trend: “Since many people are currently spending a great deal of time at home because of Covid-19, nature is acquiring greater importance. This has an influence on color selection. We are seeing a rise in green and blue shades, for example moss tones reminiscent of a woodland walk, pale blue inspired by the sky, and dark blue conjuring up the sea,” he explained.

The uneasy global situation, not just on account of Covid-19 but also because of the numerous conflicts such as the trade war between the USA and China and the unresolved relationship between the EU and the UK, is giving rise to general uncertainty among consumers. For this reason, according to the manufacturers, the popular colors are those that stand for reliability and solidity, while at the same time radiating warmth, friendliness, and humanity. “In 2021, warm orange and yellow will feature prominently. The global problems will give rise to a color range that represents stability and appears authentic but also enhances the atmosphere,” predicted Judith van Vliet, Senior Color Designer at Clariant ColorWorks EMEA. According to Lydie Tuchbreiter, Commercial Industry Manager North America at BASF Colors & Effects, blue – as the color of the sky and the sea – will also be very much in demand in future on account of the difficult global situation. “Blue is frequently described as a reassuring, relaxing, authentic color. It is popular for household items, decorative articles, and sports equipment, particularly during unsettled times of crisis,” she explained.

Another interesting aspect is touched on by Mäder. Contrary to the current assumption that in uncertain times consumers would tend to buy mainly cheap articles in order to save money, he is instead seeing the opposite trend: the purchase of high-quality products as a mood brightener. “Since the current daily diet of news places an emotional stress on consumers, they want products that express quality. To satisfy this need, glossy, brilliant colors that radiate quality are in demand,” Mäder explained. According to Mäder, this expression of high quality can be achieved through colors with a metallic luster, such as warm copper tones or silver. These colors will therefore be very popular in 2021. Irrespective of the current economic and political situation, Mäder sees a general trend towards articles with a high-quality look. This applies both to packaging and to the products themselves. Consumers are prepared to spend more money on high-quality, long-lasting articles. The proverbial “cheaper the better mentality” belongs to the past. Packaging and products will therefore cost more to produce, which will also enable the use of more expensive pigments and colorants.

The demand for longer-lasting products is also confirmed by other colorant manufacturers. It comes from a desire on the part of many consumers to move away from disposable articles towards more sustainable plastics products. However, this poses some problems for manufacturers. If products are to be used for longer, they must also look good for longer. The stability of the colorants used therefore takes on greater importance. Pigments and colorants with, for example, high light and weathering resistance are required. “Extending the service life of consumer goods made from plastic is an important contribution to sustainability,” explained Philippe Lazerme, Head of Marketing for the Business Unit Pigments at the Clariant subsidiary Colorants International. “Besides the technical properties of a product, appearance also plays an important role. To address this need, we supply our PV-Fast range of pigments with high light and weathering resistance. These allow articles to stay attractive for longer. The light-protective effect of pigments should also not be forgotten.”

In making this last point, Lazerme is referring to an additional benefit that pigments and colorants can offer in relation to extending the service life of plastics products. Various polymers degrade on exposure to light, for example UV light. This degradation process can be prevented with, for, example, special UV light-absorbing pigments. Such additional properties could become even more relevant in future. Sami Palanisami, Global Marketing Manager Plastic Colorants at Milliken Europe, questions whether, in view of the demand for greater sustainability, the coloration of plastics will continue to be driven by purely esthetic considerations or whether the colorants used will not also have to offer additional advantages for the products or components. “There are applications in which colorants are used solely to improve the visual appearance of the plastics. This could be called into question in future from the viewpoint of sustainability,” he said.

There is complete unanimity among manufacturers that black plastics, particularly those colored with carbon black, continue to pose a major problem for recycling. To recycle plastics waste efficiently, the waste must be segregated by material. To do this, most sorting machines use near-infrared sensors (NIR). However, carbon black absorbs NIR light, which means that plastics colored with it cannot be detected and differentiated by the sorting machines. To improve the recyclability of black plastics, many manufacturers have developed products intended to replace carbon black. For example, BASF has launched the black pigment Sicopal Black K 0098 FK and Clariant the three pigments Graphtol Black CLN, Solvaperm Black PCR, and Polysynthren Black H.

Another problem is seen to be the grey or brown undertone frequently present in recycled, colored plastics. Plastics processors must take this into account in coloration. In coloring recyclates, it is often not possible to use the same formulation as for virgin material. In addition, the undertone of the recyclates varies between different batches. To achieve uniform coloration of products over time, the colorant formulation must be tailored to the particular recyclate. Assistance can be provided here by software tools that calculate the effect of the recyclate undertone on the final color. In this way, unsuitable material can be separated out or the necessary correction to the colorant formulation calculated. Clariant, for example, offers such a tool.

Colors inspired by nature will also be very much in demand in2021. They give products a more sustainable appearance

Fig. 1. Metallic luster creates the impression of high quality. The corona crisis is accelerating the consumer demand trend for products with a high-quality appearances

Fig. 2. Since carbon black continues to pose significant problems in plastics recycling, many colorant manufacturers are now offering alternative black pigments for the coloration of plastics

Trendbericht - Hauptartikel

Pigmente

Kunststoff Werkstoffe

Additive

Pigmente, bunt

Masterbatches

Farbstoffe und Pigmente

Kunststoffe

Photonik

Optik

Recycling

Umweltmanagement

Farbkonzentrate (Batches)

Farbstoffe

Effektpigmente

Kunststoff-Recycling

Kunststoffverarbeitung: Verfahren

Kunststoff-Aufbereitung

Farben, flüssig

Exciting Design instead of  Exciting Manufacturing

A 3D-Printed Nozzle and Modified Dosing Method Deliver Reproducible Marbling  in a Robust Process

Marbling is often a real eye-catcher in plastic components. However, the effect is difficult to create and there are some pitfalls when it comes to reproducibility. Novel nozzles for injection molding machines in combination with a special dosing technology allow marbled molded parts to be produced far more easily in a much more robust process.

Marbled molded parts have been around since the early days of plastics when, for example, combs or buttons bearing a marbled pattern were produced. In some cases, the marbling happened unintentionally because homogenization in the plunger injection molding machine had still not been perfected.

With the advent of screw injection molding machines, not only plasticizing capacity has increased, but above all significantly better melt homogenization has become possible, paving the way for colored molded parts without any color streaks on their surfaces. In addition, mixing elements are sometimes deliberately installed in injection units in order that mixtures of uncolored pellets and masterbatches may be converted into streak-free molded parts.

Now, marbling effects are created either mechanically with the aid of special marbling screws or by means of two-component injection molding machines. In the latter, two molding compounds of different color are injected into one cavity, usually by sequentially opening and closing valves which are located either inside the mold or between the machine nozzle and the mold. However, the production of reproducibly marbled parts in two-component injection molding machines is relatively costly, as two injection units are required and have to be operated.

Since parts with marbled surfaces often look very attractive and unique, they are still in demand, even though the manufacturing process is complex. A process by which they might be produced reliably and reproducibly without major technical outlay would therefore appear desirable. An ability to specifically influence the marbling patterns would also give designers additional freedom over the visual design. BASF SE, Ludwigshafen, Germany, has developed one such process that allows marbled molded parts to be produced with little outlay.

Marbled molded parts typically have patterned visible surfaces bearing such effects as color shading, color streaks, lines and texture. The formation of the effect varies with the proportions of the different colors and the base color of the part. Although marbled molded parts manufactured in a production run are not absolutely identical, they look essentially very similar. Some of the patterns achieved with this technology are reminiscent of the appearance of natural materials, such as marble or wood. For high-contrast, clearly defined marbling patterns to be achieved, the admixed color batch must not be completely homogenized. At the same time, though, all the pellets must be completely melted and homogenized in order that the finished molded parts may possess the specified properties.

Initial trials conducted by BASF on molded parts revealed that the patterns differed according to whether they were produced with a needle-valve or an open nozzle. The number of flights of a screw tip was even discernible on the parts’ surfaces. The patterns arise from the separating and merging of melt flows in the channels of the plasticizing unit. This insight prompted a decision to rethink the nozzle of the injection molding machine and to employ different melt channel geometries in the nozzle cap. For this purpose, a recess for accommodating interchangeable inserts was created in a standard nozzle cap. As a result, the channel geometry in the nozzle can be changed to create all manner of patterns (Fig. 1). A symmetrical arrangement of straight channels, for example, results in symmetrical patterns.

Marbling proves to be particularly effective in the case of flat molded parts. But it is also possible to create three-dimensional articles that possess highly attractive marbled surfaces (Fig. 2). The marbling pattern is determined by the nozzle design and the mold’s gating system and the subsequent spreading of the melt. Thus, it is also influenced by the location of the injection point and the filling behavior of the melt in the direction of flow during injection into the cavity. Figure 1 shows the many possibilities for symmetrical parts that are gated centrally. Attractive patterns may also be achieved by using molds equipped with hot runners and tunnel gates (Fig. 3). The resulting molded parts are not centrally gated and so the pattern of marbling streaks is completely different from those shown in Figure 1.

Additive manufacturing processes offer a very high degree of design freedom over the production of nozzle inserts. In particular, they can be used to create channel geometries that cause the melt to swirl as it flows into the mold (Fig. 4). This affords a way of producing mirror-image patterns and rotationally symmetrical patterns. The swirling of the melt is clearly visible in the pattern shown in the Title figure and Figure 1. Such patterns were previously not feasible with conventional processes.

The nozzles can be produced by additive processes such as laser welding and layer fusion processes, such as fused deposition modeling (FDM) and fused filament fabrication (FFF). Catamold pellets from BASF, for example, can be used to manufacture the nozzle and nozzle inserts. Here, a filament made of Ultraform, a polyoxymethylene (POM) from BASF that contains metal powder, is printed by the FFF method, followed by debinding and sintering. The final step of the process entails adjustment work such as fitting the printed insert into the nozzle cap.

The marbled parts can be produced on standard injection molding machines fitted with just one plasticizing unit. All that is necessary is to synchronize the dosing of the material with the injection molding cycle. The materials are fed as usual into the existing feed opening in the hopper area of the plasticizing barrel. Conventional, only slightly modified dosing devices fitted with two hoppers are used (supplied, e. g., by motan GmbH, Konstanz, Germany). The base material, which is usually mixed with a color batch, is supplied in the conventional way, while a further high-contrast color batch, e. g. black, is fed into the plasticizing unit separately with great precision.

As mentioned, the process can be carried out on commercial injection molding machines without the need for specialized equipment. Ideally, the sole requirement on the injection unit is that it meets certain specifications, e. g. a low ratio of shot volume to barrel size. The injection molding process itself is not different from usual, and the process parameters are normally the same. The various influencing factors on marbling are summarized in Table 1.

Correct dosing and the combination of “contrast” batches and coloring component are also important in the production of marbled molded parts. The choice of color combination is key to a successful marbling outcome. If the contrast batch is overdosed, the coloring component very soon loses its brilliance.

Ultimately, mold design, part geometry and the nozzle used in the machine and in the hot runner determine the resulting pattern. In the course of conducting its development work, BASF tested molds fitted with conventional sprues as well as with hot runners. In both cases, attractive patterns were obtained. The finish of the mold cavity’s surface has an additional influence on the patterns’ appearance. Whereas highly polished surfaces accentuate the contrasts intensely, textured surfaces lead to an unobtrusive, matt appearance (Fig. 5), as is also the case when glass fiber-reinforced materials are processed.

Marbled molded parts are chiefly used for decorative purposes in the consumer goods sector. They serve as household objects, e.  g. bowls, plates, cups, handles, and covers, as well as laptop lids and smartphone covers, as fashion accessories, e. g. buttons, as decorative elements in vehicle interiors, e. g. caps and covers, and as parts for toys. For many of these applications, the polymers should at least have a food contact (FC) classification, as is the case for Ultradur B4520 FC, a polybutylene terephthalate (PBT) supplied by BASF. In contrast, various polyamides (PA) such as the Ultramid Deep Gloss or Ultramid Vision family from BASF are suitable for automotive interiors.

With the aid of the process presented here, it is possible to reproducibly manufacture marbled molded parts in different patterns on standard injection molding machines fitted with novel nozzle inserts and special dosing technology. Aside from PBT and PA, the process lends itself to other thermoplastics that are currently undergoing testing. BASF offers its customers not only suitability tests on existing molds and machines, but also support with nozzle design and 3D printing of pilot series, with the selection of materials and colorants and process control, as well as assistance with implementation into mass production. Mold provings can be carried out on the converter’s premises or at any time in BASF SE’s pilot plant on the customer’s own molds.

Marbling leads to attractively designed molded plastic parts, but is not so easy to implement in production

Table 1. Influencing factors on the marbling

Fig. 1. All kinds of marbling effects can be achieved, depending on the channel geometry employed

Fig. 2. Not only flat molded parts but also attractive three-dimensional articles can be produced with a marbling effect. Correct dosing and the combination of “contrast” batches and coloring component during production are also important

Fig. 3. Molds fitted with hot runners and tunnel sprues can be used to produce very attractive patterns. As these molds are  not injected from a central point, the marbling streaks vary in their pattern 

Fig. 4. Nozzle inserts made by 3D printing offer designers huge freedom of design. Flow-optimized channel geometries in the nozzles can be used to make the melt swirl on its way into the mold. This also supports the creation of rotationally symmetrical patterns

Fig. 5. The surface finish of the mold cavity influences the appearance of the samples.  Highly polished surfaces strongly accentuate contrasts. Textured surfaces leads to a matt appearance

BASF - Hauptartikel

Dr. Reinhard Jakobi is Head of Performance Materials Processing at BASF SE, Ludwigshafen, Germany; <a href="mailto:Reinhard.jakobi@basf.com">Reinhard.jakobi@basf.com</a>;

Angelika Homes and Susanne Zeiher work in the Performance Materials Processing Department.

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Spritzgießwerkzeuge

Kunststoffverarbeitung: Maschinen und Komponenten

Werkzeuge für die Kunststoffverarbeitung

Dienstleistungen

Werkzeug-, Formen- und Modellbau

Additive Fertigung

Porous Nanostructures as Replacement for White Pigments

An innovative process allows to fabricate thin, white polymer surfaces without white pigments. For that, the polymer is foamed with supercritical CO2 resulting in a fine structure of nanobubbles. Film thicknesses of a few 10 µm are sufficient to obtain a perfect white surface.

For all kinds of consumer goods, a bright white surface is frequently desired to convey purity and cleanliness. Titanium dioxide (TiO2) is the most commonly applied white pigment. It is used in many industrial branches and gives plastic packaging, coatings and paints, as well as food and cosmetics, an opaque and brilliant whiteness [1, 2]. However, rising raw material costs, the labor-intensive extraction and large carbon footprint, as well as the cost-intensive disposal of partly harmful by-products, motivate to replace TiO2 in white surfaces.

In recent years, there has also been growing concern that TiO2 could be harmful to human health and the environment. In particular nanometer-sized TiO2 particles are repeatedly criticized [3]. In France, these concerns have led to a ban on TiO2 as a food additive [4]. In addition, in the long term, TiO2 could pose a similar problem to that of microplastics, as chemically inert TiO2 particles could be released finally from the embedding medium and subsequently accumulate in the environment.

Nevertheless, there are many good reasons to use TiO2 as a white pigment [2]. Some authors and engineers do not see an alternative to this material (see Kunststoffe international 6–7/2019, [5]). Due to their high optical refractive index, TiO2 particles scatter light particularly effective, since a strong contrast of the optical density the matrix (polymer, binder, etc.) is achieved. This enables a very effective scattering of all wavelengths of visible light at the interface between medium and TiO2 particles, resulting in a white color impression and good opacity.

However, a look at nature shows that the classical industrial approach to use small particles with high optical density as scattering centers is not the only possible way to effectively scatter light. In nature, there are several examples of white materials where a different scattering mechanism is utilized. Air-filled cavities instead of pigment particles act as scattering centers.

The best-known example of this principle in biophotonics is the white beetle Cyphochilus insulanus (Title figure), which originates from Southeast Asia [6, 7]. Its white color probably serves as a camouflage, since it lives in a shady environment on a suitably colored fungus [8]. The upper side of its body is almost completely covered with white scales, which are about 60 µm wide and 200 µm long. The scales are porous inside and consist of a network of fibrous structures (Fig. 1). Despite their small thickness of less than 10 µm, the scales scatter light of any wavelength almost perfectly. This effect has been intensively investigated by various groups in recent years and served as a model for the development of scattering materials [9–12].

The example of the beetle Cyphochilus illustrates very well the difference between natural and man-made structures. While engineers often develop solutions using materials consisting of many different chemical elements, the natural ones are usually limited to a single basic material with a rich, complex three-dimensional structure. One can be only amazed by mechanical, optical or physico-chemical properties of such natural solutions. Biomimetics is concerned with understanding, abstracting and imitating the phenomena of nature in order to make them technically usable. Therefore, it often leads to promising approaches which might never have been found in any other way.

The above described effect found in the beetle’s scales can also be reproduced in man-made materials, for example, by introducing in their volume very small, air-filled cavities. This effect can be observed in everyday life by watching soapsuds or sea spray. Light is scattered at the interface between the pore and the surrounding medium such as water or polymer. The high optical density contrast results from the low optical density of air compared to the surrounding medium. The main trick is to make these bubbles tiny enough to enable a technical use as a coating. When the pore size is reduced to a few hundred nanometers, all wavelengths of visible light are scattered effectively, so that even thin layers of the porous material appear white.

Researchers at the Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany, have developed a process for the production of such coatings. For this purpose, polymers are foamed with the aid of supercritical CO2 (Fig. 2). As a non-toxic, inert and non-flammable gas, CO2 has become very important not only for foaming, but for various other applications, for example for polymer blending and simplified polymer modification processes (with CO2 as the carrier as well as the swelling agent) or for the production of polymeric particles [14, 17]. This is mainly because it acts as a plasticizer for polymers with good CO2 solubility. Under the influence of CO2 the glass transition temperature of these polymers drops significantly [15]. Due to the good diffusion properties, the gas is often used in the supercritical state at a pressure of over 73.75 bar and a temperature of over 31 °C. This aggregate state lies between liquid and gas state.

In the process developed at KIT (Fig. 2), the polymer is first saturated with CO2 under constant temperature and pressure conditions, typically until the gas concentration in the sample reaches the solubility limit. While CO2 diffuses into the polymer, its glass transition temperature decreases. Under certain conditions, the glass transition temperature is even lower than room temperature [16]. The polymer swells and also becomes viscous if its temperature is higher than the current glass transition temperature.

In the next step, a sudden drop in pressure induces thermodynamic instability. This leads to a supersaturation of the polymer, whose solubility for CO2 is much lower under the new conditions. As a result of the supersaturation, nucleation occurs, and for further growth the nuclei must reach a critical size. Now pore growth can take place by diffusion of gas molecules from the polymer matrix into the newly formed nuclei.

While the gas diffuses from the polymer matrix into the growing pores and partly out of the material, the glass transition temperature increases again. The pore structure solidifies when the glass transition temperature has risen to the actual temperature of the sample [17–19]. Due to the foamed porous structure the polymer turns white as shown in Figure 2 by the example of a previously transparent PMMA film. The pore size and density can be controlled by the process parameters and other process modifications. For example, a higher temperature ensures longer time for pore growth and thus larger pores, while a reduction in pressure moderates the pore density.

The optical spectra in Figure 3 show how effectively light is scattered at the nanopores. For these examples, thin PMMA films with different, precisely defined layer thicknesses, were applied to a glass substrate and foamed with supercritical CO2 with optimized parameters. The films were then optically analyzed. The reflection of these films in the visible range is almost constant and increases with the thickness of the foamed PMMA. An effective layer thickness of 9 µm is sufficient to achieve a reflection of approx. 60 %. The effect increases with layer thickness and at a thickness of 79 µm significantly more than 90 % of the incident light is reflected. The inset in the figure shows the inner structure of the film with an effective layer of 9 µm, recorded by scanning electron microscopy (SEM). The pores have diameters of several 100 nm and are distributed irregularly in the polymer.

The right graph in Figure 3 compares the scattering of the foamed PMMA films with white paper, so-called “photonic glass”, and the scales of the Cyphochilius beetle. The transmission of light is plotted as a function of the inverse material thickness. This so-called “Ohm’s law for light” gives an indication of the mean free path length of light and thus allows a simple comparison between different materials [7, 11]. With this method, the curve for white paper has the steepest slope, while the curve for “photonic glass” is much flatter. The scales of the white beetle enable the most effective scattering in our comparison and the corresponding curve is very flat, since the mean free path of light scattering is only about 1.47 µm. The foamed PMMA films perform slightly worse than “photonic glass”, but much better than white paper. For the human eye, a film of only 10 µm thickness already appears perfectly white.

The presented process successfully avoids the need for potentially harmful pigments by creating porous polymeric structures. Just like the bubbles of shaving or bath foam, the structure of these also scatters light, making the material appear white. The technique is inexpensive and suits not only for PMMA but also for various thermoplastics such as thermoplastic polyurethanes (TPU) and biodegradable polylactides (PLA).

The thin polymer films are very flexible and light. Although nanostructuring changes the elastic properties of the material, the films are mechanically stable enough [20] to be thermoformed and applied as a coating to various surfaces [11].

Foaming with supercritical CO2 does not leave any residues of solvents, as the process of generating a pore structure is purely physical. Accordingly, there is great potential in applications where solvent residues are to be avoided and where TiO2 alternatives are particularly in demand, for example in food and cosmetics packaging. The increasing demand for efficient recycling of products is another advantage, as the inserted pores are much easier to reverse than embedded particles. If the polymer is heated accordingly, the nanobubbles disappear without residue. Furthermore, the localized heating above the material’s glass transition temperature allows the white films to become locally transparent again and can be beneficial for ink-free stamping or counterfeit protection.

Since many white consumer goods from packaging to furniture surfaces are made of polymers or polymeric coatings with added TiO2, there is a wide range of applications for the foamed films. In addition, there is a growing willingness to replace TiO2 particles. On the one hand this is desired by end users and on the other hand by manufacturers expecting further legal restrictions for TiO2 usage in the future.

However, the foamed material is not a 1:1 replacement for TiO2 or other white pigments. It requires new approaches to apply the foamed polymer as a film. Instead of using complete polymer coatings or foils, foamed particles could also be introduced into a plastic matrix. Figure 4 shows that particles can also be foamed: an originally transparent particle of PMMA appears white after foaming. This technique can also be used to whiten materials that cannot be foamed directly.

1. Weir, A., et al.: Titanium Dioxide Nanoparticles in Food and Personal Care Products. Environmental Science & Technology 46 (2012) 4: p. 2242–2250

2. Winkler, J.: Titan Dioxide – Production, Properties and Effective Usuage. 2nd Revised Edition. Vincentz Network, Hannover 2013

3. Nau, K. and H.F. Krug, Sichere Nanomaterialien?! Physik Journal 15 (2017): P. 29–34

4. Frankreich verbietet umstrittenen Weißmacher Titandioxid. 2019; Quelle: https://www.spiegel.de/gesundheit/ernaehrung/titandioxid-frankreich-verbietet-weissmacher-in-lebensmitteln-a-1263406.html; https://www.legifrance.gouv.fr/eli/arrete/2019/4/17/ECOC1911549A/jo/texte

5. Schlotter, U. and I. Sartorius: An Irreplaceable Additive – Use and Recovery of TiO2 in Plastics Products. Kunststoffe international 6–7 (2019): p. 6–8

6. Vukusic, P., B. Hallam, and J. Noyes: Brilliant Whiteness in Ultrathin Beetle Scales. Science 315 (2007): p. 348

7. Burresi, M., et al.: Bright-White Beetle Scales Optimise Multiple Scattering of Light. Scientific Reports 4 (2014): P. 6075

8. Wilts, B.D., et al.: Evolutionary-Optimized Photonic Network Structure in White Beetle Wing Scales. Advanced Materials. 30 (2018) 19: p. e1702057

9. Hallam, B.T., A.G. Hiorns, and P. Vukusic: Developing optical efficiency through optimized coating structure: biomimetic inspiration from white beetles. Applied Optics 17 (2009): p. 3243–3249

10. Akerlind, C., et al.: Scattering and polarization properties of the scarab beetle Cyphochilus insulanus cuticle. Applied Optics 54 (2015) 19: p. 6037–6045

11. Syurik, J., et al.: Bio-inspired, large scale, highly-scattering films for nanoparticle-alternative white surfaces. Scientific Reports 7 (2017): p. 46637

12. Syurik, J., et al.: Bio-inspired Highly Scattering Networks via Polymer Phase Separation. Advanced Functional Materials 28 (2018) 24: p. 1706901

13. US Patent 4,473,665: Microcellular closed cell foams and their method of manufacture (1984) Martini-Vvedensky, J.E., N.P. Suh, and F.A. Waldman

14. Hentze, H. and M. Antonietti: Porous polymers and resins for biotechnological and biomedical applications. Journal of Biotechnology 90 (2002) 1: P.p. 27–53

15. Chiou, J.S., J.W. Barlow, and D.R. Paul: Plasticization of Glassy-Polymers by CO2. Journal of Applied Polymer Science 30 (1985) 6: p. 2633–2642

16. Goel, S.K. and E.J. Beckman: Generation of Microcellular Polymeric Foams Using Supercritical Carbon-Dioxide. 1. Effect of Pressure and Temperature on Nucleation. Polymer Engineering and Science 34 (1994) 14: p. 1137–1147

17. Tomasko, D.L., et al.: A review of CO2 applications in the processing of polymers. Industrial & Engineering Chemistry Research 42(2003) 25: p. 6431–6456

18. Costeux, S.: CO2-Blown Nanocellular Foams. Journal of Applied Polymer Science 131(2014) 23: p. 2633–2642

19. Okolieocha, C., et al.: Microcellular to nanocellular polymer foams: Progress (2004–2015) and future directions – A review. European Polymer Journal 73 (2015): p. 500–519

20. Syurik, J., et al.: Bio-inspired micro-to-nanoporous polymers with tunable stiffness. Beilstein Journal of Nanotechnology 8 (2017): p. 906–914.

Fig. 1. The scanning electron microscopy image shows the inside of a scale of the white beetle Cyphochilus insulanus. The porous structure consists of chitin and scatters light so well that a layer thickness of only 10 µm is sufficient to make it appear white

Fig. 2. Foaming with supercritical CO2 enables the creation of nano-sized pores in polymers such as PMMA. This means that the actually transparent material turns white without adding pigments due to the structure alone

Source: Luisa Borgmann, KIT, graphic: © Hanser

Fig. 3. The graph on the left shows the reflection as a function of wavelength for different film thicknesses. The SEM image shows the inner structure of an approx. 10 µm thick foamed PMMA film. The graph on the right compares the foamed PMMA films with other white materials

Source: Julia Syurik, KIT, graphic: © Hanser

Fig. 4. The process is also suitable for foaming PMMA particles. In the future, these particles will be further tested for their suitability as pigments

Source: Luisa Borgmann, KIT, graphic:  © Hanser

The white beetle Cyphochilus insulanus is almost completely covered with white scales

KIT - Hauptartikel

<ul><li>Coloration solely caused by nanostructuring without any pigments.</li><li> A tiny layer thickness of about 10 µm is sufficient to achieve a high scattering.</li><li>The polymeric material remains pure, which facilitates recycling.</li></ul>

To-Dos till Readiness for Series Production

<ul><li> To up-scale the demonstrated technology according to manufacturer’s needs, for example, for larger films.</li><li>The foamed material cannot directly replace other white pigments in existing production processes. New approaches are required, to apply it as a coating or to add it as a new type of pigment particles into a matrix.</li></ul>

Luisa Borgmann, M.Sc., has been working on the further development of the method in her PhD at KIT since 2019.

Dr. Julia Syurik developed the presented method during her time at KIT from 2012 to 2017.

Prof. Dr. Hendrik Hölscher has been head of the Department of Bionic Surfaces at KIT since 2008; <a href="mailto:hendrik.hoelscher@kit.edu">hendrik.hoelscher@kit.edu</a>

It is a pleasure to thank Gabriele Wiegand and Siegbert Johnsen from KIT for their continuous support of this project.

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Inline Quality Monitoring and  Data-Based Quality Prediction

Successfully Managing Digital Transformation with Automation and Artificial Intelligence

Maintaining competitiveness while producing in Germany requires not only cost-efficient production, but also high-quality products. Vorwerk Elektrowerke is accepting this challenge. In collaboration with the Institute of Product Engineering at the University of Duisburg-Essen, Germany, the company is testing a 100 % inline quality monitoring system as well as a quality prediction using machine learning.

The Vorwerk Elektrowerke GmbH & Co. KG manufactures high-quality household appliances that are known worldwide under brand names like “Thermomix” and “Kobold”. Since these are primarily located in the upper price segment, customers have correspondingly high demands on product quality. These are reflected in the quality requirements that apply to the molded parts used. These are produced in three shifts at the Wuppertal plant in Germany, the largest production site of the Vorwerk Group, mainly by injection molding.

Production at the high-wage Germany generally requires continuous innovations in the field of process technology in order for the company to remain competitive in the long term, both in terms of quality and cost aspects. Within this context, Vorwerk has decided to work together with the Institute for Product Engineering (IPE) at the University of Duisburg-Essen to investigate the added value of optical 100 % inline quality assessment and direct prediction of molded part quality based on process data for injection molding production.

The first step was to select a suitable molded part. The choice fell on the filter cover of the Kobold VK 200 vacuum cleaner (Title figure). This is made of acrylonitrile-butadiene-styrene (ABS) and has a sealing lip made of a thermoplastic elastomer (TPE). The filter cover (Fig. 1) is produced on a hydraulic two-component injection molding machine (type: 500– 2000–180-CX Z; manufacturer: KraussMaffei Technologies GmbH, Munich, Germany).

In addition to the assembly capability and an optically perfect surface of the molded part, the sealing effect of the sealing lip is of great importance. Otherwise, the function of the vacuum cleaner would be affected, and the customer would be bothered by unwanted background noise when air flows through the sealing area. Two things are necessary to ensure the sealing effect:

<ul><li>The complete forming of the sealing lip and</li><li>the accurate, circumferential position of the TPE contour.</li></ul>

The latter is directly related to the molded part contour of the hard component, which is represented by two width dimensions and should be the focus of this investigation.

Inline Assessment of the  Molding Contour

A digital camera system (type: CV-X; manufacturer: Keyence Deutschland GmbH, Neu-Isenburg, Germany) was used for the trial (Fig. 2). As no color information is required for recording the molding contour, a monochrome sensor came to use. In order to be able to map the variation in width dimensions sufficiently accurately, the highest-resolution sensor in this model series was chosen, with 21 megapixels. The entire camera system was integrated into the existing process both in terms of hardware and software by Mabri.Vision GmbH, Aachen, Germany.

The molded parts are photographed immediately after removal, while they are still in the robot’s grip. Therefore, the handling robot sends a trigger signal to the camera control. Afterwards, the molded part is fed to the assembly line via an automatic conveyor system. Over an investigation period of around four weeks, more than 22,000 molded parts were produced, and their quality parameters recorded as described above.

Overall, the system proved to be very robust. It is designed in such a way that it automatically starts when the robot is switched on and records the quality of all parts produced from then on. The molded part dimensions extracted from the recorded images were defined once, then automatically recorded and provided with a time stamp. For trouble-free further processing, it is therefore important to synchronize the times of the injection molding machine and the camera system. This can be ensured in the case of permanent use by connecting the system to a time server through connection to the company network.

Within the scope of optical quality monitoring, two width dimensions are recorded for fundamental testing. These are determined as the maximum perpendicular distance from the middle partition of the molded part to the outer contour and are referred to in the following as dimension 1 and dimension 2. The total width of the molded part is the sum of the two individual dimensions. By recording the individual dimensions, statements can be made about possible asymmetrical warpage of the part in addition to the overall width, which is equally relevant to quality.

In addition to the inline quality monitoring described above, the project team investigated whether the quality parameters under consideration can also be predicted indirectly – i. e. without measurement – from the process data stored in the actual data log of the injection molding machine. For this purpose, methods from the field of machine learning are used, which is a sub-area of artificial intelligence. Although this topic has already been investigated in the past [1–4], available applications [5] have so far hardly been able to establish themselves in industry.

Machine Learning for the Prediction  of Quality Features

Based on many years of experience in the analysis of injection molding process data, the IPE researches the various aspects relevant for molded part quality prediction. The aim is to create a holistic system that performs all necessary data processing steps automatically [6] and therefore enables the user to handle the system easily (Fig. 3). The procedure comprises in detail:

<ul><li> Data pre-processing,</li><li> feature construction and selection,</li><li>modeling and hyperparameter optimization,</li><li>model selection and quality prediction with the best model, as well as</li><li>model adaption in case of changed process conditions.</li></ul>

The data preparation (standardization, etc.) required for proper processing is followed by the construction and selection of the process features used as input for the quality prediction, the first central step for a successful prediction [7]. Own investigations [8] have shown that a filter-based approach with sequential forward selection as search strategy and correlation-based feature selection [9] provides the best results in injection molding.

Seven “state of the art” learning algorithms from the field of supervised learning are used for the actual modelling. In addition to well-known algorithms such as artificial neural networks, e. g. Support vector machines and Gaussian process regression are included. These are trained in parallel as part of the overall system and compete against each other – only the model that achieves the best performance on the validation data is finally used for the prediction.

To ensure that the individual learning algorithms reach their maximum performance, the respective hyperparameters that control the model structure and learning process are automatically optimized during modeling. Through production, the relationships between process and quality parameters can change, which reduces the quality of the prediction. This can be recognized by methods which can detect so-called concept drift, which also were implemented. The IPE was able to design the prediction system in such a way that operation on a desktop PC of the current generation is possible with reasonable computing effort.

To evaluate the quality prediction, a design of experiments (DOE) covering 75 injection molding cycles was carried out in addition to the approximately 22,000 cycles from series production. The following machine setting parameters, selected based on experience, were varied within the scope of the DOE:

<ul><li>Injection speed of the hard component (ABS),</li><li>holding pressure level of the hard component,</li><li> cooling water temperature,</li><li>cooling time.</li></ul>

Because of the lower number of cycles in the DOE, it was possible not only to assess the quality features with the inline camera, but to manually measure with a gauge specially manufactured for this molded part, which is also used in normal serial production. This allows a comparison of how well the quality data generated with both measuring methods are suitable for modelling. It must be considered that the shrinkage of the molded parts is not yet completed at the time of recording with the camera system, so that there is a deviation from the final dimensions.

It should be noted that modelling is generally also possible based on historical process and quality data. The former is continuously recorded in any case by the injection molding machine. However, the condition is, that the quality data is assigned to the exact process cycle.

The coefficient of determination is used to evaluate the models. This can achieve values between 0 % and 100 % and indicates the degree of overall variance of the quality feature described by the model. The results show that a quality prediction with coefficients of determination of up to 98.8  % is generally very well possible (Fig. 4).

In the DOE, the model performance based on the camera data with coefficients of determination between 95.7 % for dimension 1 and 98.8 % for the total width are slightly above the model performance on the basis of the gauge data with 87.8 % for dimension 1 to 93.6 % for the total width. The coefficients of determination for the prediction of dimension 2 lie in between. However, all these model performances can be classified as satisfactory.

In serial production, however, the coefficients of determination of the two partial widths (dimension 1 and dimension 2) are significantly below the coefficient of determination of the overall width. The latter has a very high model performance of 90.0 %. As a further analysis showed, the low coefficients of determination of the part’s partial widths (12.6 % and 18.1 %) are due to an insufficient accuracy of the camera system when recording the position of the middle rib, used as a reference. This occurs when the alignment of the part on the gripper is subject to fluctuations around the vertical axis and could be remedied by adjusting the measuring system.

Since the errors of the partial widths cancel each other out exactly, the overall width is unaffected. In the case of the DOE, this problem has hardly any negative effect, due to the variation of the machine settings, the part’s real variation outweighs the measurement uncertainty. Overall, it can be stated that modelling is also possible on the basis of data from series production, as long as there is sufficient measurement accuracy.

All in all, the conditions for the future use of the technologies investigated are very good, says plant manager Dr.-Ing. Markus Koch (Fig. 5): “Competitiveness in Germany as a high-wage location can be achieved if the autonomy times of manufacturing processes are high and therefore employee deployment is low. If this succeeds with low reject rates, as is the case with permanent quality monitoring, the duty is fulfilled. The freestyle is achieved when process adjustments can be made depending on quality deviations without the need for manual intervention.”

With the investigation of the inline camera system and the model-based molded part quality prediction, Vorwerk Elektrowerke decided to actively shape the digital transformation and thus secure its competitiveness in the long term. The camera system proved to be easy to operate and very robust over the test period. The testing of the quality prognosis system was also successful. The high model performance shows that it is possible to make a good prediction of the quality characteristics of molded parts. However, this requires a high accuracy of the quality data used in the learning phase.

1 Häussler, J.: Eine Qualitätssicherungsstrategie für die Kunststoffverarbeitung auf der Basis künstlicher Neuronaler Netzwerke. PhD-Dissertation, Universität-Gesamthochschule Essen 1994

2 Vaculik, R.: Regelung der Formteilqualität beim Spritzgießen auf Basis statistischer Prozessmodelle. PhD-Dissertation, RWTH Aachen 1996

3 Al-Haj Mustafa, M.: Modellbasierte Ansätze zur Qualitätsregelung beim Kunststoffspritzgießen. PhD-Dissertation, Universität-Gesamthochschule Essen 2000

4 Schnerr, O.: Automatisierung der Online-Qualitätsüberwachung beim Kunststoffspritzgießen. PhD-Dissertation, RWTH Aachen, 2000

5 Liedl, P.; Haag, G.; Müller, H. et al.: Spitzenqualität mit kurzen Zyklen. Kunststoffe 100 (2010) 2, pp. 38–40

6 Schulze Struchtrup, A.; Kvaktun, D.; Schiffers R.: A Holistic Approach to Part Quality Prediction in Injection Molding Based on Machine Learning. In: Hopmann, C.; Dahlmann, R. (Eds.): Advances in Polymer Processing 2020. Springer Vieweg, Berlin, Heidelberg 2020

7 Guyon, I.; Elisseef, A.: An Introduction to feature extraction. In: Guyon, I.; Gunn, S.; Nikravesh, M. et al. (Eds.): Feature extraction – foundations and applications. Springer Verlag, Berlin, Heidelberg 2006

Bild 5. Continuous quality monitoring as a must, autonomous process control as freestyle – plant manager Dr.-Ing. Markus Koch is satisfied with the investigations

Customers who choose the Kobold VK 200 vacuum cleaner expect top quality. To ensure this, efforts for the introduction of Industry 4.0 measures are taken in the production plant in Wuppertal, Germany

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Fig. 1. The contour of the molded part is crucial for the functionality of the sealing lip (TPE) of the filter cover

Bild 2. The CV-X camera system used has a monochrome sensor with a resolution of 21 megapixels

Bild 3. The automatic model generation relates to only little effort for the user: once the learning data has been provided, everything else runs automatically

Bild 4. The achieved model performances show that a prediction of the quality characteristics based on the process data is well possible. The requirements for this are precise quality data 

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The Vorwerk & Co. KG was founded in Wuppertal, Germany, in 1883 and has developed from a carpet factory into a broadly-based international group of companies in the course of its more than 130-year history. Vorwerk’s core business comprises the development, production and sale of high-quality household products. Vorwerk Elektrowerke is part of the family-owned company that employs a total of more than 12,300 permanent staff. As the largest production location of the group with its headquarters in Wuppertal, Vorwerk Elektrowerke manufactures high-quality household appliances such as the multifunctional Thermomix food processor or the Kobold vacuum cleaner.