In the Material Extrusion process (MEX) the main limiting factor is the printing speed, as is the case in many other Additive Manufacturing (AM) processes. In order to enhance the printing speed, systematic experimental investigations are necessary to understand the material dependent processes in the nozzle. In MEX, the filament partially melts inside of a heated hot end and the flow is induced by the feeding force of the solid filament part, acting as a stamp on the molten polymer. In a previous work by Wolff et al., Computational Fluid Dynamics (CFD) simulations of the MEX hot end were performed based on experimental studies using the experimental maximum feeding rate as the upper boundary condition. It was depicted that the numerical model can accurately predict the feeding force and can represent the change between process windows as the speed increases. To calculate the maximum feeding rate, the heat transfer conditions in the hot end must be considered. In this study, various techniques for determining the upper limit of feeding rates are evaluated for acrylonitrile-butadiene styrene (ABS) and polypropylene (PP) materials with different nozzle geometries. When the solid filament cone reaches the contraction zone with glass transition respectively melting temperature at maximum feeding rates, the results match well with the experimental data. Ultimately, it is possible to quantify the heat transfer in the nozzle through numerical modelling of the filament feeding force.

The co-kneader is a single-screw extruder, which performs an oscillating motion in extrusion direction additionally to rotating. The four rows of kneading pins along the cylinder walls combined with the interrupted screw flights make a high mixing possible, while enabling gentle processing at manageable shear forces and exact temperature control. Although the co-kneader is used for a wide variety of material systems as polyvinylchloride or polyamide and especially for special applications in the industry, its process design is mainly based on experimental trials. Therefore, an analytical model which allows a description of polymer melting and flow in the kneader was developed by Rudloff et al. In this previous work satisfactory simulation results could be achieved when using a retaining ring at the end of the melting zone which causes material backlog. This enables to define the melting start at the begin of the first mixing element and to assume a fully filled melting zone as well as apply dispersed melting models. However, if the co-kneader is operated without a retaining ring, the pellets can pass through the melting zone and only plastic deformation occurs, until the melt film is formed further towards the die by heat conduction. In this paper, a first model approach of plastic energy dissipation (PED) for Co-Kneaders is developed to specify the start of melting more precisely for all configurations. In the model, the energy dissipation was formulated as a function of granulate geometry and weight, mechanical material parameters and degree of granulate deformation. This allows to extend the existing dispersive melting model to include the melting contribution from PED.

In the research work presented here, an integrating sphere demonstrator which is suitable for the non-destructive determination of the degree of cross-linking or curing and has the potential for use as an at-line device for in-process quality assurance was assembled and explored. The measurement system allows the analysis of absorption and scattering coefficients of materials independently by means of absolute optical spectroscopy. The two optical parameters showed a good correlation with the degree of cross-linking of cross-linked polyethylene (PE-X) and the degree of curing of different thermosets and adhesives, each of which was determined using different reference methods (wet chemical analysis, differential scanning calorimetry (DSC), and dielectric analysis (DEA)). The results show that different PE-X materials can be distinguished well by their absorption and scattering in the visual (VIS) and near-infrared (NIR) wavelength range, respectively, and conclusions on their degree of cross-linking are possible. Also, the curing of resins can be monitored based on the absorption. In addition, Raman spectroscopy was used to achieve a better understanding of the material changes during the cross-linking of the materials. It also showed a good suitability for monitoring the curing processes in thermosets. In summary, the new method can be used to determine the crucial parameters of these industrial important material types and fulfils the great demand for fast, non-destructive testing, which can be carried out during the process or on the finished product.

Material extrusion-based additive manufacturing, strictly speaking of the Fused Filament Fabrication (FFF), is characterized by using a thermoplastic polymer in form of a solid filament as a built material. The filament partially melts inside of a heated hotend and is subsequently extruded under the pressure generated by the filament feeding force of the feeding rollers. The processes in the heated extrusion channel are complex and difficult to model, as the melt properties are a nonlinear function of temperature and shear rate. Moreover, there is a phase transition from solid filament to liquid melt. In this paper, the required feeding force and resulting extrudate temperature at the nozzle outlet at different feeding rates, liquefier temperatures and nozzle geometries of two acrylonitrile-butadiene styrene (ABS) and polypropylene (PP) materials were investigated. For a better understanding of the melting process in the hotend, the degree of melting of the strands was determined via dead-stop experiments at selected operating points. Two process windows could be identified: At low feeding rates, the feeding force increases linearly with increasing velocity, and at a certain point, the force increases rapidly and fluctuates. The melting investigations showed that this is related to the increasingly unmolten material reaching the nozzle outlet. The semi-crystalline PP showed a smaller processing window for stable flow compared to ABS and lower extrudate temperatures due to its differentiated cp progression at the phase transition. Based on the experimental studies, Computational Fluid Dynamics (CFD) simulations were performed to predict the pressure and temperature distributions inside the channel. For modelling the shear-dependent viscosity the Carreau model was used, while the temperature dependency was described by a jump function with a solid viscosity of 10(6) Pas. It is shown that the numerical model can predict the feeding force with good accuracy and represent the change between process windows at increasing speeds.

Cross-linking is a cost-effective method to improve the thermal, chemical and mechanical material properties of thermoplastics and expand their range of application. Cross-linked polyethylene (PE-X), used for water pipes or surgical implants, is a well-known example. In addition, thermosets, which are important in the context of the energy revolution and e-mobility, cure by cross-linking. The degree of cross-linking or curing of these materials has a major impact on various material properties and thus needs to be controlled. To date, only destructive and time-consuming laboratory tests are used to determine this crucial parameter. Accordingly, there is a great demand for fast, non-destructive testing, which can be carried out during the process or on the finished product. Absolute optical spectroscopy, where absorption and scattering coefficients of a sample can be determined separately, offers a promising approach to solve this problem. In the research work presented here a demonstrator based on an integrating sphere was used to investigate various PE-X materials and different thermoset resins. Wet chemical analysis, differential scanning calorimetry (DSC) as well as dielectric analysis (DEA) served as references. The results show that different PE-X materials can be well distinguished by their absorption and scattering in the visual (VIS) and near-infrared (NIR) wavelength range, respectively. Moreover, conclusions on their degree of cross-linking are possible. Similarly, the curing of the resins can be monitored based on the absorption. The assembled integrating sphere demonstrator is suitable for non-destructive determination of the degree of cross-linking or curing and has the potential for use as an at-line device for in-process quality assurance.

In industrial production, adhesives are often the preferred joining agent of choice. However, there is currently no standard method for non-destructive monitoring of the adhesive bond quality. In order to solve this issue, the use of single-sided NMR is suitable. This work demonstrates exemplarily for an adhesive that the NMR signal correlates very well with the curing. Thus, it is possible to create process models, which allow the monitoring of adhesive curing.

ZusammenfassungDer Ruf nach einer lückenlosen Qualitätskontrolle wird auch in der Kunststoffaufbereitung und -

Tensile strength of adhesive bonds with three different commercial epoxy resins and laser sintered polyamide 12 (PA 12) has been evaluated by means of Centrifugal Adhesion Testing Technology (CATT, LUMiFrac Adhesion Analyzer). Different post treatment states (as received and chemically smoothed) and pre-treatment states (as received and atmospheric plasma) of the PA 12 surface have been considered. It has been shown that the epoxy resins infiltrate the surface porosity of the laser sintered material, which leads to a special fracture pattern. Tensile strength of the adhesive bonds has been significantly improved after atmospheric plasma pretreatment. Chemical smoothing had a neglicable effect on the tensile strength of the tested specimens, but lead to a change in fracture pattern, especially without atmosphric plasma pretreatment.

Non-destructive testing (NDT) is a growing field in quality monitoring in the plastics industry, as it offers the possibility to lower the expenses for quality assurance without lowering the quality level. Passive thermography is being established as a method for automatic on-line monitoring of the weld quality in joined plastics. Active thermography is an established method in quality assurance of composite materials, but further experience is necessary in the field of polymer joints. In this work, passive and active thermography have been evaluated for their sensitivity to common defects known in weld joints and adhesive bonds of polymers.

The degree of cross-linking and curing is one of the most important values concerning the quality of cross-linked polyethylene (PE-X) and the functionality of adhesives and resin-based components. Up to now, the measurement of this property has mostly been time-consuming and usually destructive. Within the shown work the feasibility of single-sided nuclear magnetic resonance (NMR) for the non-destructive determination of the degree of cross-linking and curing as process monitoring was investigated. First results indicate the possibility of distinguishing between PE-X samples with different degrees of cross-linking. The homogeneity of the samples and the curing kinetics of adhesives can also be monitored. The measurements show good agreement with reference tests (wet chemical analysis, differential scanning calorimetry, dielectric analysis). Furthermore, the influence of sample temperature on the characteristic relaxation times can be observed.

We demonstrated an efficient surface pretreatment with excimer laser at 193 nm, which could be successfully used as a surface preparation tool to improve the adhesion and mechanical properties of bonded polymer joints. The effect of the ultraviolet (UV) light can be used both as a method for increasing the specific surface area improving the mechanical adhesion and as a method for activating the polymer surface improving the physical adhesion. Mechanical tests and studies on the failure of bonded polycarbonate (PC) and polyetheretherketone (PEEK) samples showed a significant increase in the quality of bonded connections by using pretreatment with UV laser. As a result of the studies, the bond strength of the PC samples was increased by about 32% using a PB437 UV curing adhesive. By using Loctite 9466 applied to PEEK samples, a strength increase of about 36% was achieved.

The existing wetting methods for the determination of acid-base properties on solid surfaces are discussed. Striving for a better understanding of the adhesive polymer interactions in adhesively joined polymers, the methods of Berger and van Oss-Chaudhury-Good were found as the most suitable methods for the investigation of wetting on solid polymer surfaces. Methods of nonlinear systems by Della Volpe and Siboni were adapted and evaluated on plastic surfaces. In the context of these investigations various data of the surface free energy as well as its components have been identified for a number of polymer surfaces by application of spatial equation solutions.

We report nondestructive measurements of the properties of two-component epoxy adhesives at terahertz wavelengths using a transmissive time-domain spectroscopy system. The results show that the different epoxies have measurably different THz properties, that the changes which occur during the curing process can be monitored by measurements of their refractive index and absorption at terahertz wavelengths, and that this technique is sensitive enough to record changes in the optical material parameters during the postcuring process.