Color Stability, Chemico-Physical and Optical Features of the Most Common PETG and PU Based Orthodontic Aligners for Clear Aligner Therapy

It is difficult to find research papers collecting comparative results about characterization studies of clear aligners. Therefore, the aim of this paper is to provide the first comparative analysis of most commercial clear aligners, in terms of their stability towards intra-oral staining agents, their physicochemical and optical properties, as well as their water absorption behavior. Five types of aligners, characterized by different techniques, are considered: Erkodur, Essix Plastic, Ghost Aligner, Zendura, and Invisalign. The obtained results show that clear aligners are made up of PETG, semi rigid PU, and a mixture of PU and PETG, with different degrees of crystallinity which affect the transparency of each aligner. In particular, the PETG-based materials reveal the highest value of short-range order and the highest properties in terms of transparency in the visible range. After 14 days of immersion into red wine and coffee, PETG and PU-based aligners reveal a perceivable change in color (NBS values from 1.5 to 3), corresponding to a loss of transparency due to the deposition of impurities on the surface. These results are particularly marked for Invisalign, showing changes towards other colors (NBS up to 35), probably due to the thermoforming process which led to the formation of a wrinkled surface entrapping the impurities.

Experimental Investigation of In-Plane Shear Behaviour of Thermoplastic Fibre-Reinforced Composites under Thermoforming Process Conditions

In order to meet environmental regulations and achieve resource efficiency in the series production of vehicles, recyclable polymer composites with a high strength-to-weight ratio are increasingly being used as materials for structural components. Particularly with thermoplastic fibre-reinforced polymers or organo-sheets, the advantage lies in the tailored mechanical properties of the final component by adapting the orientation of fibres based on the direction of loads. These components produced by thermoforming organo-sheets also offer a cost benefit and short cycle times. During the thermoforming process, the shear behaviour of the organo-sheet is the most dominant and determines the mechanical properties and quality of the resulting component. However, the current standard for characterising the shear behaviour of organo-sheets does not consider the strain and cooling rates inherent in the thermoforming process. This research investigates the influence of thermoforming process parameters on the shear behaviour of organo-sheets with a new methodology combining DSC and DMA experiments. During the thermoforming process, the transition of the matrix material from a molten state to a solid state is dictated by the crystallisation kinetics and their dependence on heating and cooling rates. Thus, non-isothermal DSC scans, which correspond to a temperature cycle in a thermoforming process, are used in the DSC experiments to establish the relationship between the recrystallisation temperature of the organo-sheet material and the cooling/heating rates in the thermoforming process. In order to achieve thermoforming-process-relevant cooling rates, fast scanning calorimetry (Flash DSC) is used in addition to conventional DSC measurements. DMA experiments carried out with 45° fibre orientation show that the recrystallisation temperature consequently influences the shear storage modulus of the organo-sheet. The results from DSC measurements show a shift of recrystallisation temperatures to lower temperatures as the cooling rate increases. The combined analysis of results from the DSC and DMA experiments supports the findings and shows the influence of the process temperature, cooling rate and strain rate on the recrystallisation temperature and, in turn, the shear behaviour of organo-sheets. Thus, a recommendation for establishing a new standard for characterising the shear behaviour of organo-sheets is made.

Interface-dominated mechanical behavior of CF/PEKK composites according to different heating rate during thermoforming process

While thermoplastic polymers exhibit several desirable properties, their applicability is limited by their high viscosity and extreme processing conditions. To overcome these limitations, in this study, we used the thermoforming process to produce carbon fiber/polyetherketoneketone (CF/PEKK) laminates, which were pre-made through an oven-based consolidation process using prepregs. The laminates were produced at three different heating rates ([Formula: see text]C/min, [Formula: see text]C/min and [Formula: see text]C/min). The laminates produced at the heating rate of [Formula: see text]C/min showed improved interlaminar shear strength, [Formula: see text] tensile strength, and average interlaminar fracture toughness. On the other hand, heating at rates higher than [Formula: see text]C/min increased the initiation value of the interlaminar fracture toughness ([Formula: see text] but resulted in nonuniform composites of poor quality. This is because increasing the heating rate reduced the uniformity of the heat distribution with the laminates, resulting in the polymer molecules exhibiting different binding rates and thus nonuniform cross-linking. Thus, the proposed method is a suitable one for producing high-quality thermoplastic composites.

Design and simulation of components of vacuum forming machine using household vacuum cleaner

Plastic products ranging from toothbrushes to smartphones are an inseparable commodity in daily human life and their impact cannot be underestimated. This paper aims to design and simulate the vacuum forming process using readily available materials in context of Nepal. Vacuum forming process is a thermoforming process where the heated plastic sheet derives the shape of the mold through the application of vacuum and is used to make packaging products and other household products. Simulations were done to find out the optimum distance between the plastic sheet and the heater, arrangement of the wire in the heater, load bearing capacity of the design and the flow of vacuum in the arrangement. Nichrome wire coiled as heater coil is used as the heating material and laid in a spiral path with the plastic sheet 35mm below provided the best heating results and 1800W vacuum cleaner provided the necessary pressure of 85-90kPa and velocities of 100- 115m/s while the steel posts provided adequate strength.

Simulation of the Thermoforming Process of Glass Fiber Reinforced Polymeric Components: Investigation of the Combined Effect of the Crosshead Speed and Material Temperature

Thermoplastic based composite materials are increasingly gaining the interest of many engineering sectors, among them the automotive. Their unique features, resulted by the thermoplastic matrix characteristics, such as their recyclability and their formability have given new perspectives in their use. Among the most promising fabrication methods of thermoplastic composite components is the thermoforming process, the press forming of a heated semi-finalized composite plate. This method, although requires a quite simple working station and can be implemented in mass production, demonstrates a series of disadvantages on the quality of the product. Among them, the variation of the thickness, formation of wrinkles and overall undesired deformations are considered as defects that decrease the quality not only from the esthetical but also from the structural point of view. In the present work, a numerical analysis of the thermoforming process is conducted when applied to a box-shaped geometry. As an input for the material behavior during the process, mechanical tests are conducted at elevated temperatures. The flat and curved critical zones of the component are identified and an analysis of the effect of the temperature and the crosshead speed of the molds on the thickness distribution are examined as well as the overall residual stress field. The results indicate a strong dependency of the quality of the product by these parameters of the process.