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Conference Proceedings
Optimisation of the cellular structure of chemically foamed rubber profiles
Foamed rubber profiles have found widespread industrial applications. Their cellular structure offers savings potential with respect to component weight, material consumption and costs. The foam structure is generated using chemical blowing agents, which are added to the rubber mixture before processing. Foaming and vulcanisation occur simultaneously in the curing unit. Both processes are adjusted by modification of the rubber mixture, which is currently state of the art for the calibration of the foam quality. There are little systematic approaches to an optimisation of the cellular structure during processing. In a current research project, correlations between processing parameters, polymer structure and foam structure of chemically foamed rubber profiles are systematically analysed and evaluated. A lab scale pin barrel extruder is used for production of chemically foamed rubber profiles with an EPDM-based rubber compound and Azodicarbonamide (ADC) - based blowing agents. In first experiments, the thermal stress of the rubber compound during processing and the extrusion speed have been identified as influencing factors on the cellular structure. A high thermal stress caused by increased processing temperatures leads to a faster scorch of the compound and thus to finer cellular structures with smaller average cell sizes and higher cell densities. With increasing extrusion speed, a finer cellular structure is formed.
A methodical analysis of influencing factors on the cellular structure allows an optimisation of the foamed profiles in terms of their cellular structure and will lead to a guideline for processing. Primary objective of further investigations is the analysis of influencing factors of the curing equipment used on the profile quality.
Physical foaming of silicone rubber profiles
Silicone rubber profiles are used in an increasing amount of applications, for example in sealing or damping technologies. A cellular structure of the silicone rubber profile enhances the damping properties and leads to a reduction of component weight, material consumption and cost at the same time. For production of cellular rubber profiles, the use of chemical blowing agents is state of the art. However, physical blowing agents such as nitrogen offer ecological, economical and procedural advantages. So far, an application of a physical foaming process on silicone rubber has been difficult due to the sorption properties of the material.
In a current research project, a continuous foaming process for silicone rubber is developed using a modified lab scale extrusion plant. A constant foaming process was realised in first experiments. The silicone rubber profiles are analysed in terms of cellular structure, surface quality and density. Experimental investigations show that the amount of blowing agent injected has a significant influence on density, foam structure and surface roughness. The use of a nucleating agent improves the cellular structures in terms of a smaller average cell size and increased homogeneity. Besides, the foam structure can be influenced by the pressure in the extrusion die. Primary objective of further investigations is the analysis of the influence of silicone rubber material properties on the profile quality.
Elastomer Projectile Injection Technique: Manufacturing of Fiber-reinforced Elastomeric Hollow Bodies
Injection molding is the most important process to manufacture complex polymer parts. Today, however the injection molding of elastomers almost entirely produces solid workpieces. Functionalized complex hollow components, e. g. for the conduction of medias, are often manufactured in costly multistep processes. The projectile injection technique offers a new approach to easily produce elastomers in a one-step process into complex hollow parts.
In addition to the well-known process variations of the gas (GIT) and water injection technology (WIT), the projectile injection technology (PIT) provides special advantages for the production of elastomeric continuous fiber reinforced hollow bodies, especially considering the residual wall thickness. When using the PIT, a projectile is initially placed on an injector. The melt is injected into the mold cavity afterwards. Subsequently a fluid is then injected into the component interior. The PIT uses the projectile for the actual melt displacement. It is driven by means of the fluid through the device and thereby forms the interior cavity.
Because of the fundamentally different mechanism of melt displacement and cavity formation when using the PIT in comparison to GIT or WIT, in thermoplastic processing standard materials which do not need to be specially modified for FIT can be used. So far, the PIT has not been used for the manufacturing of elastomeric hollow bodies. The target of ongoing investigations is to clarify whether or not the PIT is suitable for elastomeric processing.
Ultrasonic devulcanization of tire rubber of different particle sizes in twin-screw extruder
The present study is devoted to ultrasonic devulcanization of tire rubber particles of 10 and 30 meshes by means of a new ultrasonic twin-screw extruder. Ultrasonic amplitude and devulcanization temperature were varied at a fixed frequency of 40 kHz. The die pressure and ultrasonic power consumption during devulcanization were recorded. Degree of devulcanization was investigated by measuring the crosslink density, gel fraction and revulcanization behavior. Rubber of 30 mesh exhibited a lower die pressure and higher degree of devulcanization than those of rubber of 10 mesh. Due to the higher level of devulcanization and lower viscosity of devulcanized rubbers at higher amplitudes, the temperature of devulcanized rubbers at the die was reduced with an increase of the ultrasonic amplitude. Cole-Cole plots, crosslink density and gel fraction of devulcanized and revulcanized rubbers, revulcanization behavior, and modulus of revulcanizates separated in two distinct groups based on the level of devulcanization and effect on molecular structure of devulcanized rubber. Revulcanizates with a greater degree of devulcanization exhibited higher elongation at break, while those with a lower degree of devulcanization exhibited higher strength and modulus. Revulcanizates of rubber of 30 mesh exhibited a consistently higher elongation at break. The normalized gel fraction versus normalized crosslink density was described by a unique function independent of the processing conditions and rubber particle size.
Effect of Molecular Weight on Viscosity and Impact Toughness of Polyoxymethylene with applications in Powder Injection Molding
Polyoxymethylene (POM) is considered a high performance engineering polymer with many applications primarily in the automotive industry. Currently, POM has also found uses in powder injection molding (PIM) technology, where it acts as a carrier medium for metal or ceramic powders during an injection molding process, it is later removed during the debinding step and a solid metallic or ceramic piece is obtained after sintering. The main advantage of using POM in PIM technology is the faster debinding process compare to polyolefin-based feedstock, since POM sublimates into its monomer directly when exposed to an acid vapour. During the process of PIM, the binder has two contradictory requirements: viscosity should be as low as possible when in the molten state, but mechanical properties in the solid state, like toughness, should be as high as possible. One way to lower the viscosity is to use POM with lower molecular weights. It was observed that POM’s viscosity increases with average molecular weight (MW) at a faster rate than impact toughness and it is suggested that a MW of around 40000 g/mol provides the most appropriate combination of strength and fluidity.
Particle free ultrasonic welding through infrared preheating
Ultrasonic welding of thermoplastics is a well known and well established industrial process. One of the advantages of this joining process is the extremely short welding time. A disadvantage is the tendency of formation of loose particles during the welding process. Especially in the field of automotive and medical technology there are high demands to the quality and appearance of joined parts. Besides the weld strength the optical appearance of the weld seam and the contamination of the production area can affect the choice of the joining technology. In order to remain competitive the ultrasonic welding process has to be improved.
Investigations at the institute of plastic processing show, that the formation of loose particles during the welding process can be minimized by making use of an infrared preheating.
Because of the preheating the first phase of the ultrasonic welding process - when the particles are abraded – can be shortened or even avoided entirely.
New plasticizing concept for micro injection molding
The rapid developments in microsystems technology over the last decades have led to an increased demand for micro components in various areas of everyday life. Due to their complexity and low component masses additional requirements must be considered for the injection molding of micro components made of thermoplastics. Especially the reproducible plasticizing of the required amount of material is still a major challenge for conventional plasticizing units. Hence these units are not designed for a micro-oriented production, the required micro component qualities can only be achieved at the cost of an increased material consumption.
In recent years, micro injection molding machines that are specifically designed for the production of micro-molded parts have been developed. These machines often separate the injection from the plasticizing by utilizing a separate injection system. This system is adapted in its dimensions to the small injection volumes and thus permits a better overall resolution of the injection process while reducing the material throughput at the same time. These Systems, however, show procedural disadvantages. Besides the increased technological complexity of these systems, the desired homogenizing of commonly used three-section screws is not achieved.
This poster presents a new plasticizing concept developed by the Institute of Plastics Processing (IKV), Aachen, Germany, in corporation with ARBURG GmbH + Co KG, Loßburg, Germany. The concept, the so-called ‘inverted plastication’, is based on the kinematic reversal of the screw flights’ arrangement. It is characterized by the position of the screw flights, which are attached to the inside of the plasticizing cylinder. This also includes the feed section which provides sufficient flight depths for standard granulate. The injection piston is mounted coaxially within the cylinder. Due to the lack of the screw flights the injection piston is exposed to lower mechanical stresses and therefore featu
Numerical Investigation of the Effect of Micro-Damage on the Fibre Longitudinal Compressive Strength
For the reliable design of statically or dynamically loaded lightweight structures made of fibre reinforced plastics (FRP), a wide knowledge of the material-specific failure behaviour is necessary. Depending on their loading conditions laminates made of FRP fail by one of the macroscopic failure modes named fibre fracture, inter-fibre fracture or delamination. When structures made of FRPs are designed to be used in load bearing applications the evaluation of their load carrying capacity in compression parallel to fibre direction is of primary interest. For this purpose, a vast number of research investigations, whose main objective is linked to determining the compression strength of a structure out of FRPs, has been carried out. Influencing factors, which have been considered, are fibre properties, fibre volume content, non-linear matrix properties, interface properties, residual stresses, fibre misalignment and ply waviness.
An additional influence factor which has not yet been discussed in literature is the influence of the load history. Shear loading and loading transverse to fibre direction lead to microscopic damages – so called micro-cracks – which accumulate in fibre-reinforced plastics at increasing static load or cyclic loading conditions long before the first macroscopic damage occurs. Furthermore, they influence the compression strength parallel to the fibre direction. In this poster results of a numerical model will be presented. The model allows for the investigation of the effect of micro-cracks on the fibre longitudinal compressive strength. The micro-cracks are introduced in form of fibre/matrix debonding as well as matrix cracking. The results show, that micro-damage highly affects the fibre longitudinal strength properties, depending on the extent and the location of the damage.
Testing of a pancake die in coextrusion blow molding
Increasing demands on plastic components, increasing cost pressure and the demand of a higher efficiency of the production lines lead, inter alia, to an ongoing development of mold technologies. The high requirements for blow molded hollow articles are met by using coextrusion technology. The core of a coextrusion blow molding machine is the die head. The main tasks of a die head are to divert the flow direction of the melt, to form a parison and to bring together the different polymer melts.
Common die heads are spiral mandrel dies, side-fed dies or spider-type mandrel dies. In current research activities at IKV the mold concept of a pancake die (stack die), which is already established in blown film extrusion, is tested in the coextrusion blow molding process. The focus of the research is on short material and color change times, low production costs, flexible applications of the mold as well as on the product quality. To simplify a possible industrial implementation, advantages and disadvantages of the die head concerning the extrusion of the parison, the wall thickness distribution and the properties of the hollow articles are worked out in comparison with conventional die heads. Practical testing of the mold is accompanied by the simulation of the flow process.
The Effects of Various Injection Molding Mechanisms on Birefringence Distribution for 7 Inch Light Guide Plate
As the adoption of injection molding technology increases, injected-molded optical products require higher dimensional accuracy and optical stability than ever before. Recently, many alternative injection molding techniques have been adopted to increase the stability of optical and dimensional characteristics such as injection/compression molding or rapid heating cycle molding. In the present study we have focused on the optical anisotropy, i. e. birefringence as a significant factor which affects the function of many optical components. Four different molding methods, i.e., conventional injection molding(CIM), injection/compression molding(ICM), rapid heat cycle molding (RHCM) and rapid injection/compression molding(RICM=ICM+RHCM) were chosen to investigate the optical anisotropy of 7 inch LGP by examining the gap-wise and in-plane distribution of birefringence and extinction angle. Gap-wise birefringence was measured at every 5 mm following the center line of flow direction from gate to the end of part by a polarizing microscopy and in-plane birefringence was evaluated under the polariscope optical setup. As a result, for the cases of CIM and RHCM-only the maximum value of in-plane birefringence was about -1.0 x 10-4 near the gate and decreases to almost zero, which is general behavior in injection-molded parts. On the contrary, for the cases of ICM and RICM the maximum birefringence was less than -0.5 x 10-5 near the gate, which is less than half of CIM and RHCM-only. And, for the gapwise distribution of birefringence, two extra birefringence peaks near the center region showed the effect of packing pressure, which came from the extra flow during packing stage in CIM. In RHCM, those two inner peak values were reduced because of relaxation of molecular orientation at rather high temperature. Furthermore, in ICM, quite constant distribution of birefringence of -2.75 x 10-5 could be found over the whole region except the wall. For the combination of compression and
Influence of excitation type and layer structure on barrier and elongation properties of SiOx–based multilayer CVD barrier coatings
Plasma processes constantly gain importance in the field of plastics processing. For instance, microwave (MW) enhanced plasma polymerization of silicon organic precursors is one of the most effective techniques to create permeation barriers (SiOx-coating) for plastics. These layers have extremely low permeation coefficients to several media. For the deposition of high barrier coatings on flexible substrates it is necessary that the barrier effect is maintained even under high strain. Unfortunately for some applications silicon oxide barrier coatings on flexible polymers are likely to fail at low strain levels.
One possible approach to overcome the poor elongation properties and to avoid a loss in barrier properties under strain poses the deposition of multilayer stacks. The main goal is to prevent crack formation and crack propagation through the entire multilayer stack by incorporating decoupling intermediate layers. Besides MW-excited plasmas also capacitive coupled plasmas (CCP) may be used.
In this study polyethylene terephthalate (PET) films are coated using different multilayer setups. Besides the excitation type for the deposition of the incorporated layers also the order of the stack forming layers is varied. Tensile tests as well as oxygen permeation measurements are carried out in order to identify the influence on barrier and elongation properties.
Influence of layer material and structure on barrier and elongation properties of SiOx-based multilayer CVD barrier coatings
Plasma processes constantly gain importance in the field of plastics processing. Due to their macromolecular structure plastics do not offer sufficient barrier functionality against oxygen and water vapor permeation, which is a key demand in a variety of applications. A common solution in plastics processing is the deposition of thin silicon oxide layers (SiOx) using microwave (MW) excited plasma processes. Unfortunately for some applications silicon oxide barrier coatings on flexible polymers are likely to fail at low strain levels.
One possible approach to overcome the poor elongation properties and to avoid a loss in barrier properties under strain poses the deposition of multilayer stacks. The main goal is to prevent crack formation and crack propagation through the entire multilayer stack.
In this study polyethylene terephthalate (PET) films are coated in a roll-to-roll process using different multilayer setups. Besides the material of the incorporated layers (silicon oxide / hydrocarbon) also the order of the stack forming layers is varied. Tensile tests as well as oxygen permeation measurements are carried out in order to identify the influence on barrier and elongation properties.
Optimizing the CO2 footprint through defined usage of recyclates.
Plastics are an indispensable part of daily life no longer. The CO2 balance of a plastic component is improved by using recycled materials, since the provision of the recyclate is energetically less costly than the production and delivery of new products. These relationships, particularly in response to a defined use of recycled materials in plastic parts have not yet been extensively studied. Our experiments showed that the mechanical properties of plastics, especially fiberreinforced, can be predicted when using recycled materials. The program we designed to perform this calculation has a CO2 accounting for a variety of arbitrary recyclate shares offered. This shows clearly how much CO2 eq. can be saved by recycling.
Blow head design and optimization
Major sectors with high demands and specifications for polymer products are packaging and automotive. Due to the complexity of polymeric materials and the high specifications regarding the product quality and e. g. homogeneity of wall thicknesses, a key issue is the rheological design of the extrusion die that is used for the primary forming of the polymer melt. Usually, numerical die design approaches (e. g. based on computational fluid dynamics) are time consuming, costly, tie down manpower and highly depends on the experience and training of the responsible engineers.
Applying a holistic approach based on the analogy between electrical engineering (voltage, current, resistance) and hydrodynamics (pressure drop, volume flow, flow resistance) offers a promising way to achieve good die design results very time efficient. In order to describe flow phenomena the control volume approach (also referred to as network theory) is used and a simulation model for complex multi-level extrusion dies is implemented. Interdependencies between different levels of the extrusion die are taken into account. The approach aims for a fast and automatic flow calculation. The results of the flow simulation are compared against user specifications and a quality value is computed that describes the quality of the design. This value is used for optimization techniques tin order to develop a smart and time-efficient way to find optimal solution for complex multi-level extrusion blow heads.
Phase Inversion- Assisted Synthesis of Electrospun Nanoporous Polycaprolactone (PCL) Fibers for Protein Adsorption
Impregnation of desirable biological moieties can significantly enhance the biocompatibility of an electrospun scaffold. Nanopores provide additional impregnation or attachment sites for target bio-molecules on scaffold surface. In the present work, a combination of vapor and non-solvent- induced phase inversion processes was used to create 20 - 200 nm sized pores on PCL fibers. 24-hour adsorption studies, performed with Collagen-I protein, showed a 2.5 fold increase in protein retention of porous fibers over their non-porous counterparts, thus exhibiting efficacy of the high surface porosity. The pore size distributions can be tailored by controlling key parameters i.e. relative humidity and solvent/non-solvent ratio to enhance the loading of target molecule.
Crystalline Structure of Blends of Isotactic Propylene-1-Hexene Copolymers Revealed by WAXS/SAXS Techniques
Blends of a miscible pair of propylene-1-hexene (PH) copolymers with 11 and 21 mol% of 1- hexene have been studied in reference to their polymorphic behavior, kinetics and crystal structure using in situ WAXS and SAXS analysis. PH21 crystallizes in a trigonal packing in the whole range of undercooling, while PH11 develops monoclinic crystallites (at low undercooling) or the mesomorphic form (at high undercooling). The level of crystallinity increases from 17 to 25% and scales directly with increasing content of PH21. WAXS analysis indicated that while the content of trigonal phase decreases with increasing PH11, the rate of formation of trigonal phase in the whole range of undercooling increases with addition of PH11, which as a pure component does not form trigonal phase. The unexpected enhanced kinetics of formation of trigonal phase with blending is attributed to the increasing composition of 1-hexene in the melt during evolution of the monoclinic phase in the first stage of isothermal crystallization of the blend.
Novel Prototype to Study the Effects of Helical Spiral Flow on in-vitro Biodegradation of Polymers for Bioimplants
There are currently no tests to determine degradation rates and characteristics of bioabsorbable materials that come in direct contact with blood. Blood follows a helical flow pattern resulting in conditions not simulated in current degradation tests. This is of concern to the degradation of stents because certain stent designs have the potential to liberate fragments large enough to induce strokes or other detrimental health concerns. A novel prototype designed to simulate in vivo conditions including flow rates, pressures, temperatures, and flow characteristics was designed and built. This system was designed with a fast change testing chamber to allow sample removal and different configurations to simulate different sized arteries and cardiovascular health levels. The critical consideration for the testing chamber was the silicone artificial artery to simulate helical flow found in blood vessels. The effects of this flow pattern were compared to laminar and turbulent flow patterns and optimal conditions found to best simulate actual body conditions. Degradation of polymers was characterized with weight loss of the sample, visual inspection via camera, and observed fragment size running through meshes to indicate size.
Manufacturing Induced Curvature of Carbon Fiber Laminates: Experimental Observation and Model Validation
Carbon fiber composites are used frequently in a wide variety of industries; such as automotive, aerospace and sports equipment, primarily due to their large strength to weight ratios. The design and manufacturing of such parts, as well as the final part's performance, create engineering difficulties as compared with alternative materials and processes. As a carbon fiber composite is manufactured, strains are formed due to the curing kinetics of the resin matrix and thermal effects caused by a mismatch in the coefficient of thermal expansion between the carbon fibers and the resin matrix. This work compares the curvature of an un-balanced (cross-ply) laminate with the curvature of a balanced laminate. The experimental results are compared with a finite element structural and coupled thermal-structural analysis which incorporates micromechanical theories to predict the stiffness and coefficient of thermal expansion of a lamina from the constitutive properties of the fiber and the resin matrix. The experimental and modeling results show qualitative and quantitative agreement.
Toughening of Polylactide with Pre-heat Treated Natural Rubber
Both polylactide (PLA) and natural rubber (NR) are biocompatible and biodegradable polymers. PLA possesses high strength and modulus but low toughness, while NR exhibits excellent elasticity and ductility. In view of their complementary properties, NR seems an ideal candidate to toughen PLA. To the best of our knowledge, PLA blends showed increased ductility only when more than 10 wt% rubber was added. This study demonstrates a significant improvement in the toughness of PLA by melt blending PLA with pre-heated NR. SEM studies showed that the rubber phase was uniformly dispersed in the PLA matrix. With as little as 1 wt% NR, the elongation at break and tensile toughness of PLA/NR blend were significantly improved over those of neat PLA (207% vs. 16% and 83 MJ/m3 vs. 9 MJ/m3, respectively) without loss in tensile modulus and stress. In addition, by blending in PLA with 20 wt% NR, samples obtained did not even break in the notched Charpy impact test. FTIR spectrum indicated that carbonyl groups were generated in NR chains after hot shearing and led to enhanced compatibility between PLA and NR, which accounted for the improved toughness
Recycling of PLA
Polylactide (PLA) is a bioplastic which has a high potential for packaging applications. Due to a high raw material prize and a limited availability the usage of PLA is limited apart from some niche products at the moment. Nevertheless, the number of applications is increasing.
At the Institute of Plastic Processing (IKV) the recycling behavior of PLA is evaluated. Recycling helps to cut the raw material consumption and lowers material costs. Additionally, it improves the ecological balance.
Following the industrial praxis different recycling strategies are analyzed. This paper gives a review about the multiple processing of PLA and the processing with melt degassing.
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