Latest revision as of 12:27, 7 June 2023

ABSTRACT: The transformation processes used for thermoplastic composites, have been increasing their complexity, while incorporating new high demanding materials. From the aeronautic and medical sector, with their restrictions and specific demanding needs. The PAEK materials have been promoted and deeply applied due to their high temperature stability and their chemical resistance. In this research, the work has focussed on the use of the reinforced 40%CF PEEK process through injection moulding. In these over injection moulding processes the joint between 40%CF PEEK and other materials such as Aluminium, or reinforced PA and PEEK has been studied. As well as the adhesion between them and the variation generated on the mechanical properties. It has been observed that this joint is far better between thermoplastic materials than between metals and thermoplastic, this is due to the fact that even tough the PEEK has high processing temperatures, these are not relevant chemically speaking for metals such as Aluminium. Limiting the forces of the interphase to the physical and mechanical interaction between both materials.

Nevertheless, in the case of the thermoplastic over injected parts several parameters have been observed to be affecting the final properties of the joint. Firstly, the over injected angle, obtaining higher mechanical properties as the angle increases as well as the contact surface. And the effect of the roughness of the overinjected part, having these a direct impact on the final mechanical properties.

Keywords: Overinjection; PEEK; Carbon fibres;

RESUMEN: Los procesos de transformación de composites termoplásticos, han ido aumentando su complejidad a la vez que los nuevos materiales. Desde el sector aeronáutico y médico, con sus estrictas y especificas necesidades, se ha promovido el desarrollo de materiales de muy alta temperatura y resistencia química, como son los materiales de PAEK. Este trabajo se ha enfocado en el uso de PEEK reforzado con 40% de fibra de carbono para hacer procesos de sobre inyección. En estos procesos de sobre inyección se ha trabajado en la unión del PEEK+40%CF con otros materiales metálicos como es el aluminio, y otros materiales plásticos como es el propio PEEK con fibra y PA con fibra. De esta forma se ha estudiado el comportamiento y la adhesión entre los diversos materiales y la variación de las propiedades mecánicas. Se ha observado en el estudio la unión más sencilla se da entre materiales plásticos, dado que pese a las altas temperaturas de inyección del PEEK esta sigue siendo muy baja para afectar y entrelazar con la estructura del Al a nivel molecular y su interacción se limita a las interacciones mecánicas y físicas de la geometría de la pieza.

En el caso de los materiales plásticos se ha observado como los tratamientos superficiales en las superficies sobre inyectadas afectan directamente a las propiedades mecánicas obtenidas. Viendo un efecto directo en el ángulo de unión entre materiales, implicando mayor ángulo y superficie de contacto mejores propiedades. Y el efecto de los tratamientos superficiales sobre las propiedades de tracción e impacto de las piezas finalmente producidas.

Palabras clave: Sobre inyección; PEEK; Fibras de carbono;

1. Introducción

In the twentieth century many different developments have launched the quality of life and the life expectancy over the roof. These developments have allowed us to travel, communicate and most importantly to provide solutions to problems all around the glove. The developments of all these points in our life has been guided by many different sectors, such as construction, transportation, food, health, aerospace….[1] Nevertheless, it is key to remember that the activities and advances observed in these sectors are only possible thanks to the developments achieved in other transversal activities. In this sense the material science has experienced a unprecedent boost thanks to the development of plastics. Plastics have played a fundamental key role. They are present in all the aforementioned sectors and have enable to distribute good quality products all over the world.

In this sense, it is possible to say that plastics, and in particular thermoplastics have been essential for our everyday development and for the enhancement of our lives. However, there are many different types of plastic materials and although all of them are facing new challenges, it is important to differentiate between their different natures, qualities and capacities. As depending on these properties will depend their future and their capacity to overcome the new worldwide trending situation.

For those plastic or polymeric structures widely used in packaging and everyday products their capacity to be recyclable will determine their future. However, it is not the exact same situation for other polymeric materials. Each polymeric structure can be differentiated depending on their key properties; which in some cases might be barrier properties, mechanical properties, thermal properties, viscosity… Depending on the properties of the material it can be used in some applications or others. Although the plastic industry as mentioned before has been blooming since the last century it is still working on new polymeric structures that can provide new materials and provide new solutions for current unreachable markets. In this frame, some of the last materials developed are the PAEK polymeric family (Polyaryletherketone), which mechanical structure can be seen in Figure 1.

These family of materials is very characteristic and their properties are mainly related with their stability [2]. They show high thermal stability along with chemical stability, it is these two key properties that made them of high interest for sectors such as the aero and the medical [3]. They can be used in solutions that require high stability, like the medical in which biocompatible solutions must be applied in order to not affect the body. These biocompatible materials are those that do not interact with the water based solutions in the organism. The high thermal stability provides a perfect framework to use the PAEK materials in the aeronautic industry, with a Tg above 140ᵒC.

This stability of the polymer is intrinsic to its chemical configuration. However, these as all other properties of the plastic matrixes can be improved through the application of different additives and fillers. In the case of the PAEK and other matrixes that have been used in the research showed in this article, the selected filler are fibres; carbon fibres and glass fibres. The presence of different additives as reinforcements of the thermoplastic matrix makes of our material a composite [4]. As it is a combination of several materials in order to obtain a third material which properties goes beyond the sum of the independent properties. Thanks to these solutions it is possible for a material to fulfil a wider variety of requirements.

Although, the capacity of the materials to be used in many different sectors is something really interesting. It is also very important to keep in mind the necessity of the materials to be process with the proper techniques, enabling the fabrication of a final product. There are many different processing technologies for thermoplastic materials such as film blowing, casting, thermoforming, Sheet Mould Compression, Bulk Mould Compression and injection moulding. All these processing technologies are based on the same statements that the thermoplastics can be melted and reshaped. However, not all are used for the same purposes, while film blowing and casting are for planar structures.

Injection moulding is used for complex geometries and high production rates due to the high rate production capacity of the machines [5]. As all technologies, injection moulding has some drawbacks such as the production and price of the mould. However, this is easily overcome by the high production rates achieved. Making out of injection moulding the most efficient manufacturing technology for the production of thermoplastic parts.

All technologies used nowadays in the thermoplastic industry are perfectly optimized and are producing parts at an industrial rate. Nevertheless, they are all still going beyond their own benchmark and incorporating new technologies in order to offer more and allow improvements in the processing. In the case of injection moulding, the manufacturing technology in which this research is focus, one of the last developments has been the use of several materials in a single process, the over injection technology [6]. This technology itself it is not new as the injection of plastic over mats and textiles has been done for many years. But in this case the over injection overlaps two different thermoplastic materials.

In this research, the focus is placed in the over injection processing of different high value thermoplastic materials and the characteristics and properties that must be provided to the geometry in order to generate a good quality interphase between PAEK materials reinforced with CF and other thermoplastic matrixes. Besides, the work developed and showed within the article tackles the over injection of PEAK on aluminium parts and the properties and difficulties seen in those cases. Simulating in this sense the stages usually seen in the over injection process. First substituting partially, the metallic structure by plastic and secondly substituting it completely in different materials.

Normally, this over injection process and the different contractions and heat transference properties of the material imply that the over injection of different materials implies a weak point in the interphase. In this study the interphase has been geometrically modified in order to observe the changes in the interphase properties of several materials depending on the angle of incision and the surface roughness.

2. Materials and methods

2.1. Materials

PEEK grade (PEEK90HMF40) was purchased from Victrex (Lancashire, UK), the Aluminium was purchased from CTM (Zaragoza, Spain) and finally the PA6 (LUMID PA6 30GF) was also acquired from LG Chem (Seul, South Korea).

2.2. Methods

For the injection moulding of specimens a BOY 25 injection machine was used, with an aluminum mould that enable to provide the shape mentioned in the ISO 179 for impact testing and the ISO 527 for tensile tests. In this case, due to the size of the injection machines the specimens manufactured and used in this research were the 1BA.

In this way specimens for impact and tensile were manufactured through standard injection moulding conditions.

In the case of the 30% GF reinforced PA6 the profile temperature for the screw was going from 235ᵒC to 255ᵒC at the die, the mould’s temperature was also controlled and set at 75ᵒC. Before using the PA6, the material was dried for 4 hours at 100ᵒC.

On the other hand, for the 40%CF reinforced PEEK, the temperature profile was going from 340ᵒC to 370ᵒC and the moulds temperature was set at 200ᵒC. Besides, the material was dried at 150ᵒC for 3 hours.

After having injected all different PA6 and PEEK specimens, they were cut at different angles in order to have half a specimen with certain angle (Figure 2). Once the specimens were cut and treated with the proper sandpaper (40 grains/ cm²; 80 grains/ cm²; 120 grains/cm²). They were process in the same way as the pellets, drying the material for 4 hours in the case of the PA6 and 3 hours for the PEEK.

During the over injection process, the specimens were carefully placed in the mould (Figure 3), and kept in the mould until they reached an equilibrium with the metal of the mould. Once the specimen, was at the proper temperature the over injection was carried out.

Finally, to characterize and test the mechanical properties of the generated specimen two different mechanical characterizations were carried out.

On one side, charpy impact. For this test the ISO 179 as mentioned above. Besides, the equipment used for the characterization was the model PI-25 from Metrotec (Gipuzkoa, Spain) with tools for trials from 0.5 to 25 J. As the interphase between the two materials is placed in the centre of the specimen, no notch is machined, therefore following the ISO 179-1/1eU. In each case the 5 specimens were measured in order to provide an average value.

On the other side, the tensile tests were carried out following the ISO 527 with the specimen 1BA, defined in the ISO. In this case the tensile testing machine was a Z 2.5 10kN ProLine MPMS S0206 from Zwick GmbH & Co. (KG, Ulm, Germany). In each case the 5 specimens were measured in order to provide an average value.

However, it is also important to remark that in some cases there was no possibility of performing a proper trial, due to the weak interaction between materials. In this sense, different stages were determined and can be observed in the table below and were used as reference if the specimen broke before the proper measurement on the specific machines.

Table 1. Processing steps between the injection and the proper characterization

3. Results and discussion

The results obtained in the over injection of the CF reinforced PAEK can be easily differentiated depending on the nature and characteristics of the material that is over injected.

3.1. PAEK and Aluminium

For the over injection of the aluminium with CF reinforced PAEK, the aluminium was heated to the same temperature of the mould, this was achieved by leaving the part in the mould until the equilibrium temperature was reached. At this point the injected was carried out.

Results as observable in Table 2 were not at all satisfactory, not been able to eject a sample from the mould with good quality in most of the scenarios. In most cases the ejection was not suitable due to the position of the ejectors in the mould, forcing the interphase to hold the structure of the specimen.

However, it was possible to observe a simple difference among the injection. Those samples in which the aluminium had an angle of 60º, although very fragile, were able to be extracted from the mould. Besides, those in which the surface of the metal was treated with a sand scraper of 80 grains/cm² were even able to go to the characterization machines. However, they broke so easily in the trials that it was not possible to obtain good value.

Table 2. Moment when the aluminium over injected with PAEK specimen broke

In this case, the angle and the surface of the over injected part are affecting the mechanical properties of the final specimen. The main factor associated with the difference in the mechanical properties observe is the shear and the time for the application of it during the injection moulding.

While in the case of the 0º the melted material directly contacts an homogeneous and flat surface, there is no time for a proper bonding, as this time and the contact surface are increased the final results improved, enabling a stronger joint between both parts.

3.2. PAEK over PAEK

In the case of the PEAK material overinjected with PEAK the situation is quite different as in the aluminum-PAEK. In this case as observable in all samples with the different angles and roughness were measured.

Table 3. Values for the rupture of over injected PAEK-PAEK specimens by impact and tensile mechanisms

In the case of the PAEK, the situation is completely different to the conditions of the aluminium. For the PAEK, the injection temperature and the Mould temperature over the Tg of the PAEK makes possible to have a joint that goes beyond the mere physical union between the materials. As the shear rate is increased higher mechanical properties are found in the interphase/joint as the two different parts connect at polymeric level. Again, as observe in the Aluminium the 60º angle is the most efficient angle of all as it offers the highest level of shear rate and therefore the best union.

For the comparison between the different roughness applied to the sold PAEK, it is clearly that the behaviour is completely different for the young module and for the impact resistance. While in the impact resistance, the energy absorbed by the specimen is decreased with the application of different surface treatments. For the young module, they improve the mechanical properties of the interphase. In fact, the effect of the roughness improve the values obtained for the young module placing them at the same level than the injected reference. Demonstrating that a roughness of 80 grains/cm² increased the shear rate and therefore generates enough interaction between the different polymeric chains that is almost comparable with the properties of a single shoot specimen.

On the other side, samples with very low grains/cm² caused a very limited effect in both types of specimens at 30º and 0º, being these samples the only ones that broke even before performing the injection process.

3.3. PAEK and PA

In the case of the PAEK and PA combination two different strategies were tested, on one side the PEAK was injected over the solid PA and on the other it was the PA the material that was over injected on the solid PAEK.

In the latter strategy, the results were similar to the ones mentioned for the over injected aluminium. The PAEK was not altered either by the temperature of the Mould during the injection moulding, neither by the melted PA that entered in contact with the PAEK.

Table 4. Moment when the PAEK over injected with PA specimen broke

For the first strategy in which the solid PA materials are over injected with PAEK. In this case as the mould is placed at the processing temperatures of the PAEK (180ᵒC), which over the Tg of the PA (below 100 ᵒC) and the injection temperature of PAEK is set at 370ᵒC. This implies that the PA6 material is not only deformed but also melted by the heat of the PAEK, in consequence the generated interphase consisting in PEAK and PA material is not affected by the roughness provided to the PA by the sandpaper. As the PA in the interphase is melted and completely deformed by the pressure of the injection moulding (Figure 5).

The idea of melting the PA through the heat of the mould and the PAEK has proven to be a suitable solution that enable to produce interphases with higher young module than the PA (The young module of the interphase is mayor or equal to the PA’s module). The joint itself does not elongate and therefore all the specimens break just after the measurement of the Young module. As observable in Table 5, the angles of the over injected parts show the same behaviour than in the previous materials.

Table 5. Moment when the PA over injected with PAEK specimen broke

It is observed, in this sense that the over injection of the PAEK on the PA generates an interphase with high mechanical results. However, this is a very brittle part. Demonstrating that even though the energy needed to deform the part is high, the specimens break at low elongation rates. In some cases, even below 0,5%.

4. Conclusions

The over injection process as any injection process depends highly on the injection process, the mould and the geometry of the part. In this case, it is clear that the results obtained and the performance and measurement of the specimens manufactured could have been improved by the ejection system (Plate ejection instead of punctual), avoid the formation of a plastic casting through the addition of a heating system in that part of the mould, and of course through the adaptation of the geometry of the part so a higher surface is over injected.

These points are key for the over injection of materials and their importance has been demonstrated in all the different cases studied.

Regarding the roughness and the angle for the over injection, two completely different conclusions can be obtained from all the cases demonstrated:

1- The roughness of the part, do not impact the interphase of the over injected materials in a significant way

2- The angle in which the material come into contact and therefore the length of the surface has a direct impact on the mechanical properties.

Finally, it has been seen that the order in which the material are injected has also a direct impact. Being key that the temperature of the second material injected is over the Tg and even melting point of the over injected material. As the other way around the joint is so weak that it is mandatory to have a mechanical anchor between materials.

The research demonstrated in this article has shown a first step in the over injection process. However, further studies should be done with materials with more similar thermal properties. This point has not only been seen in the case of the Al and the PA over PAEK, but it has been possible to see it also in the PAEK over PA. Where depending on the injection speed and injection conditions have been able to degrade the PA material and avoid completely a good adherence.

5. Bibliography

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