Sheet Bulk Metal Forming

The scientific objective is the analysis of a process combination for the manufacturing of toothed, thin-walled functional elements from process-optimized semi-finished products. By using an upsetting, orbital forming as well as a modified flexible rolling process, tailored blanks with flow-optimized properties are manufactured. In this context, a defined material flow control plays a decisive role for the forming of different geometries. In order to pursue a holistic approach, the process limits as well as the potential of the individual processes are identified. With the application of such blanks, geometrically complex components with functional elements such as gears or carriers are formed in a new combined deep-drawing and upsetting process. The aim is to reduce the number of steps in the process chain and thus save energy and resources. Thereby, different levels of functional integration are taken into account with regard to the geometry and arrangement of the functional elements, but also combinations of both.

The rising demands on forming processes regarding the functional integration of components and lightweight construction pose a challenge to conventional processes. In that context, the application of bulk forming operations on sheet metal semi-finished products - also referred to as sheet-bulk metal forming (SBMF) - is a suitable approach to meet these challenges. One of the most relevant industrial forming processes is the cold extrusion, which enables a high material and energy efficiency. Furthermore, extruded components show beneficial mechanical properties, a high surface quality and can be produced with a near net-shape geometry. However, the application of cold forming processes for SBMF is limited due to an insufficient die filling and high tool loads. In order to meet these challenges and overcome resulting limitations, it is necessary to enhance the process understanding. This is achieved by establishing two different reference processes (forward and lateral extrusion) for the manufacturing of functional integrated geared components and the basic analysis of the occurring material flow and tool load in sheet-bulk metal forming processes. Through a combination of numerical and experimental analysis, similarities and differences between both processes are investigated. It is shown that design parameters, tribological conditions, as well as process parameters, substantially influence the process result, regarding achievable accuracy and tool load. The gained process understanding is used to identify limitations and to derive measures to expand the process boundaries.

The technology of incremental sheet-bulk metal forming (iSBMF) provides a resource-efficient approach for the near-net-shape manufacture of functional components featuring a load-adapted shape. For this reason, consecutive edge thickening and gear forming processes are experimentally developed, numerically investigated and analytically described. Due to the flexibility of the incremental procedure, different load paths can manufacture identical geometries. Concurrent, the strain hardening behavior of the workpiece material depends on the load path. This research focuses a targeted adjustment of the mechanical properties within the component by a variation of the load path. Moreover, the kinematic-depending geometrical accuracy of iSBMF gear elements is determined. As the mechanical adjustability is limited to the strain hardening potential of the material, an additional hybrid approach is investigated. This approach enables the grading of mechanical properties by layering sheets with different strength and strain hardening behavior. Moreover, a reduced averaged strength of the stack reduces the tool load appearing on the filigree forming tools. Moreover, a significant load reduction is examined when an electrical current is introduced in the forming zone. This force reduction provides the basis for the iSBMF of high strength materials. Thus, the conflict between a load reduction and the preservation of strain hardening is analyzed.

Shorter production times, associated with sheet-bulk metal forming (SBMF), require a demand-adapted monitoring and measurement of the formed geometries. In the present work, a multi-scale and multi-sensor setup was developed to meet these requirements. The difficulty in measuring workpieces formed during SBMF are highly reflective areas with varying demands on the measurement resolution of the functional elements. With the developed measurement setup and the integrated sensors used, the functional elements can be measured with sufficient resolution. The registration of the sensors is done with a specially constructed calibration plate. A parallel kinematic hexapod positioning unit provides six degrees of freedom to precisely position the workpiece in front of the individual measurement sensors. The hexapod is therefore registered in a global coordinate system with the new calibration plate. After the calibration, the measured datasets can be merged together automatically. The rigid connection of the calibration plate to the hexapod enables a more demand-oriented verification of the registration between the individual measurements of the sensors. For overexposure due to directly reflecting areas, it is possible to estimate the quality of each reconstructed data point. A deflectometry system, which is attached to the existing fringe projection sensor, makes it possible to detect strongly directionally reflecting areas and thus provide a holistic reconstruction of the workpiece. The influence of remaining lubricant on the workpiece is simulated by a developed thin film thickness standard. This is to adapt the measuring procedure for reconstructions under harsh conditions in industrial environment. This standard allows the measurement of thin films with the existing optical measurement systems in a range <100 µm. The effects of thin layers of lubricant on the result of the fringe projection measurements show an increasing impact on the measured dataset with increasing layer thickness. With the developed measurement setup, the investigations on the lubricant used and reflective properties, the formed workpieces can be measured at different scale ranges.

Within the scope of the subproject “Dynamic Process Forces” of the Collaborative Research Centre TCRC 73, a sheet-bulk metal forming (SBMF) process was developed to produce a demonstrator component with internal and external gearing from a sheet metal blank in three production steps. In the first step, a cup is deep-drawn. A hole is shear-cut in the centre of the cup and the cup frame is displaced into a tooth cavity by a compression punch so that the external gearing is formed. In a subsequent second step, an internal gearing is formed by an upsetting punch. A recalibration of the internal gearing takes place in a third process step. In the course of the project, methods for a better control of the material flow and for increasing component quality are investigated in order to extend the limits of sheet-bulk metal forming. These methods include the installation of superimposed oscillation in the main force flow of the forming machine and the use of structured and coated tools, the so-called tailored surfaces.

In order to support the design of sheet-bulk metal formed parts, a self-learning engineering assistance system (SLASSY) was developed during the course of the project. SLASSY can help in product development of sheet-bulk metal forming (SBMF) mainly in two ways. On the one hand, it facilitates the knowledge transfer from manufacturing experts back to product developers and on the other hand, it enables product developers to optimize their part designs based on the metamodels created from simulation and experimental parameter studies. The key scientific findings are the generalized workflow for creating these metamodels, which allows the creation of knowledge very soon during the development of a new manufacturing technology like SBMF. Based on this, metamodels allow the automatic generation of design alternatives, which are also valuable and manufacturable. Potential applications for this self-learning engineering assistance system can be found in every manufacturing-related industry, since it enables product developers to keep up with the development of a new or newly introduced manufacturing technology in their respective field. The methods developed can be applied to all fields, where simulations and experimental studies are applied to find new knowledge in a given manufacturing domain.

In this article investigations on micromilling of hardened tool steels as well as developments for an efficient process design are presented. In the course of this, suitable parameter ranges as well as process instructions are given, with which stable micromilling processes of hardened tool steels can be achieved. Besides the analysis of the milling process, a developed simulation technique is presented, which can be used to design and optimize the machining process. For further optimization, the cutting edge preparation of micromilling tools by wet abrasive jet machining and millpolishing and its influence on the manufacturing results are discussed. The production and application of functional surface structures in forming processes is additionally examined. Approaches and developed software are presented, with which machining programs for large-area structures as well as free-formed component surfaces can be generated with minimal effort within the CAD/CAM environment. Besides the derived process chain and tools for an efficient generation of machining programs, the performance and wear behavior of the surface structures is considered.

In sheet-bulk metal forming (SBMF), the high geometric complexity of workpieces and differently dimensioned functional elements within the forming tool can lead to an insufficient form filling of cavities. One approach to optimize the SBMF process are functional surface structures, which can be used to influence the contact situation between forming tool and workpiece. Therefore, the aim of this project was to create defined surface structures with specific roughness properties on forming tools for SBMF. This was realized by a milling process with intentionally induced regenerative tool vibrations and high-feed (HF) milling. For this purpose, the basics for a simulation environment were first created, within which the process parameter settings for the generation of the occurring vibration patterns were defined. The effects of the surface structures thus generated were investigated and validated both theoretically and experimentally. The results were transferred to free-form surfaces with application for the SBMF tools. Subsequently, the proceedings were extended for HF milling which proved to be advantageous to micro milling in terms of tool life, process efficiency and control. The application of quasi-deterministic patterns with different tribological properties of the surface depending on the desired local surface characteristics and the process parameter values was enabled.

Sheet-bulk metal forming processes are commonly characterized by temporally and locally varying stress conditions, likely to affect material flow during forming and therefore the geometrical accuracy of formed parts. Local modifications of friction conditions can contribute to control the flow of material and simultaneously to reduce process forces and tool wear. This chapter discusses the systematical development and evaluation of amorphous carbon coatings aiming to adapt tribological conditions and to increase wear resistance of coated forming tools. Considering requirements and boundary conditions of forming processes, especially tungsten-modified amorphous carbon coatings demonstrated their beneficial tribological-mechanical behavior in high loaded sliding contacts due to low adhesion to steel. Coating development based on design of experiments revealed wide ranges of adjustable frictional behavior and mechanical properties contributing to local tribological conditions by the application of masking techniques. The coatings proved their operational performance in application-oriented forming processes, ranging from friction tests to simulated and experimental forming operations. Multilayer strategies increased the coatings’ toughness and their resistance to wear and fatigue. These benefits, along with an additional experimentally verified sensory function for wear condition monitoring, may provide an individual and improved service life of amorphous carbon coated forming tools.

Tribological conditions in sheet-bulk metal forming (SBMF) represent a unique stress and load profile. However, a fundamental understanding of the involved mechanisms and interactions is crucial in order to ensure the performance of the tools. Within the presented chapter new approaches by using friction and wear optimized thin films, processed by physical vapor deposition (PVD), as well as bionic-inspired surface structures are introduced for adapting the tribological conditions for SBMF. The scope of the investigations is based on the development of PVD coatings by adapting their chemical composition and mechanical profile using magnetron-sputtering technology. Additionally, the tribological coatings can be used for a near-net shaped deposition on structured surfaces to enhance their performance and improve their wear resistance for forming applications. It was found, that a consideration of the performed pre-treatments in relation to the roughness profile as well as the residual stresses in the subsurface are crucial for well-adherent PVD-coatings. Therefore, a fundamental understanding of the effects and interactions of Cr-based nitride coatings is generated and presented. With respect to the real forming applications, the developed surface modifications resulted in a reduction of the forming forces and a simultaneously improved form-filling by a directed material flow.

Sheet-bulk metal forming, as a novel and innovative production process, represents a possible approach for the realization of present and future requirements in the context of production technology through the combination of different, forming processes. Due to the metrological requirements of modern, digital and largely automated production plants, the development of new types of measuring instruments for use in industrial environments is driven. Within the scope of the TCRC 73, a new type of compact and flexible endoscopic 3D sensor was developed in subproject B6, enabling measurements of stressed structures inside a sheet-bulk metal forming plant. This serves the purpose of preventing failure and obtaining a better understanding of the process. This contribution provides a description of the optical design, the principal components, sensor head configurations, as well as a description of algorithms and models and demonstrates them by reference to applications.

Complex forming parts that include functional elements can be produced by means of sheet-bulk metal forming within one process step. Adding functional elements to a workpiece design can lead to design asymmetry. As a result, horizontal forces appear during the forming process, leading to horizontal displacements of the press ram. Such displacements lead to process inaccuracies. In this report, the appearance of press ram displacements is discussed. Studies were carried out to determine the amplitude of possible displacements. The impact of horizontal forces on the manufactured workpiece quality was investigated. A numerical study that featured both, the manufacturing process as well as a multibody-model of the press has been conducted to increase the accuracy of the process simulation. In order to avoid horizontal displacements of the press ram, electromagnetic solenoids were designed and built. Experiments were carried out that show the increased process accuracy by compensating press ram displacements by means of counteracting forces. Additionally, a reduction of tool wear by superimposing horizontal oscillations to the forming process was detected in an experimental approach. A concept for a tool, which adds the horizontal degrees of freedom to the framework of the press bed, is presented as well.

In sheet-bulk metal forming (SBMF), locally varying high tensile and compressive loads occur during the forming process. A superposition of the load stresses with manufacturing related residual stresses in the subsurfaces can increase the fatigue of functionally relevant tool areas. The form grinding process, as one of the last and quality-determining manufacturing steps, can be used to adapt the subsurface properties specifically to the load scenarios and stress states during the forming process. Therefore, the aim of the subproject B8 - Grinding Strategies for Local and Stress Orientated Subsurface Modification of Sheet-Bulk Metal Forming Tools - of the TCRC73 is the process safe application of locally adapted residual stresses in order to increase the service life of the forming tool. For this goal, fundamental relationships between grinding with toric tools and the subsurface properties of forming tools were investigated. The residual stress state is significantly determined by the three process parameters grinding strategy, feed velocity and CBN grain size. An empirical model for the prediction of residual stresses was derived from the main influencing process parameters. Other process parameters, such as the depth of cut and the path distance, which decisively determine the contact surface, have a subordinate role in the formation of surface modifications. The process reliability of the grinding process depends on the wear behavior of the tools. Grinding tools with a large grain diameter show a favorable wear behavior. In addition, the dressing process of the tools has a significant influence on the wear behavior. Furthermore, the grinding process as one of the last process steps can be used to improve the adhesion of ceramic Physical Vapour Deposition (PVD) coatings by an optimized pre-treatment and thus to substitute an additional nitriding process.

Due to the local application of bulk forming operations to sheet metal, often in combination with sheet metal forming, the tribology of sheet-bulk metal forming (SBMF) is characterised by more widely varying tribological loads than in conventional forming. Local loads, which are characteristic for bulk forming or sheet metal forming, occur. This complicates the numerical modelling of SBMF processes. Consequently, a half-space model is investigated that simulates the workpiece-tool contact under varying loads. A central challenge of SBMF is the limited geometrical accuracy of components because of a partly imperfect material flow. Hence, the potential of tailored tribological systems for material flow control by local adaptation of friction is investigated. It is numerically and experimentally shown that the material flow in SBMF can be controlled by local adaption of friction, thus improving the component accuracy regarding the die filling significantly. By using the half-space model, the functional relations of adaptions of tool- and workpiece surface were investigated and suitable tailored tribological systems are experimentally applied in SBMF processes. This shows that tailored friction systems have the potential to extend process limits through a material flow control in SBMF.

The simulation of polycrystalline materials provides detailed insight into the material characteristic. Sheet-bulk metal forming is a complex process that needs comprehensive information about the formed metallic material. Further, transient hardening and Bauschinger effects make this process even more challenging. In order to accurately predict the forming process and the final shape of the formed part under these circumstances, one needs to consider sophisticated elastoplastic material models. Plastic deformation is based on a microscopic length scale phenomenon that involves the dislocation activities within the microstructure. Therefore, a physically motivated dislocation density-based material model is developed to consider the effect of plastic deformation for polycrystalline materials. However, the resolution of the material at a microscopic length scale quickly leads to limitations regarding computation time and cost. Due to the high geometrical resolution, it is impossible to simulate large geometries and resolve the complex plastic transformation at the micro-level within the entire domain. Therefore, based on insights gained with representative volume element simulations of the microstructure an effective plasticity model is developed as well. The effective material model can be applied in coarse scale simulations. It can also provide an accurate mechanical response under non-proportional loading while considering isotropic, as well as kinematic hardening. Additionally, this effective material model can be easily extended to anisotropic yield functions. Both length-scale models are used to validate the mechanical response of ferritic steels under cyclic loading.

In this contribution we consider inverse mechanical problems in terms of parameter identification and shape optimization. The fundamental material behavior is thereby modelled with an elasto-plastic constitutive law based on the logarithmic strain space, considering anisotropic yield and kinematic hardening. The identification of the constitutive material parameters is based on the virtual fields method (VFM) minimizing the gap between external and internal virtual work. By using a strategy with relation to a stress sensitivity analysis, the virtual fields can be obtained automatically. A specifically designed cruciform specimen, which produces heterogeneous deformation states, is used with a biaxial testing machine. For the shape optimization, a Newton iteration step is deduced to iteratively minimize the differences between desired and deformed shape of a forming simulation. The presented inverse, node-based algorithm covers a wide range of applications, since all requirements of a forming process are fulfilled. The method is demonstrated by means of a backward extrusion process.

Sheet-bulk forming processes are applied to manufacture complex components with intricate shape elements or with large variations in wall thickness from sheet metals. Accumulated plastic strains achieved in sheet-bulk metal forming are substantially larger than in conventional sheet metal forming. Differing from sheet forming, the stress state is three-dimensional for these processes due to the thick sheets and process kinematics. Due to these specific process conditions, conventional methods to predict failure in sheet forming such as forming limit curves are not sufficient. Thus, process analysis as well as characterisation of microstructural and mechanical properties for a prediction of properties affecting failure of formed components require other methods. Application of constitutive models for damage computation allows predicting the onset of failure during forming operations. Moreover, even before failure, the mechanical properties, i.e. the elastic stiffness of components, are affected by the evolution of voids. Previous research did not focus on the comparison of different model strategies with respect to the accuracy of predictions and the necessary strategy for parameter identification and validation. This contribution demonstrates that a Gurson-type model, which relied on high-resolution microstructural data, provided the best prediction of failure for a local indentation and sheet upsetting. Suited preparation methods were developed to analyse small voids in the nanometre range. A novel fracture criterion is shown to offer the best compromise of identification effort, implementation effort and accuracy. The assessment of the effect of void evolution on component properties is an important aspect. Different non-destructive methods were validated based on measurements of resonance frequency and propagation velocity. A quantitative relation between the measured void area fraction and the elastic properties was established for components relevant for sheet-bulk metal forming. A testing procedure to determine the performance of components under elevated strain rates was evaluated and the prediction capacity of different modelling approaches with respect to the strain rate sensitivity was compared.

Functional components manufactured by sheet-bulk metal forming will commonly be exposed to cyclic loading during operation. Due to the cold forming during sheet-bulk metal forming, work-hardening occurs and ductile damage is induced in the form of voids in the microstructure. To predict the influence of specific processing parameters on the components’ properties and their fatigue life, a fracture mechanics based fatigue life model was employed. Specifically, the evolution of ductile damage was analyzed and cyclic fatigue experiments as well as crack propagation experiments were carried out for different material conditions. Regarding ductile damage, the development of small to medium sized voids could be observed for an increasing degree of deformation. The fatigue model allows inferring the crack length by inverse calculation. It could be shown that the calculated initial crack lengths correspond well with the determined defect size caused by ductile damage. The parameterized fatigue model allows estimating the fatigue life of sheet-bulk metal formed components manufactured by various processing routes and exposed to different load cases and thus enables a fatigue life related process design.

Friction characteristics and wear of sheet-bulk metal forming tools featuring microstructured surfaces are simulated on the mesoscale. Computational homogenisation is used for surface cut-outs on which the structures are geometrically resolved and where friction laws model lower scale roughness. Two approaches are presented for the modelling of wear. A dissipation based Archard model is implemented in a Python postprocessor geometry-update scheme using Abaqus/Standard as Finite Element solver. Sinusoidal surface structures exhibit anisotropic adaption of friction coefficients which is preserved throughout severe surface wear. Bionic isotropic structures feature quasi-isotropic friction adaption, where the sliding direction along the edges of the structure experiences a faster wear evolution. Experimental comparisons show good qualitative agreement, although more investigations are required. Results can provide load-dependent coefficients for macro-simulations as well as insights on precise characteristics. A mechanism-based approach uses the Particle-Finite-Element Method towards the simulation of particle abrasion. The shape detection method is detailed along with a focus on the Contact Domain Method with examples showing the ability to model material separation. Inelastic material behaviour and more robust contact approximations have to be included to allow for wear simulations. First studies on lubrication propose the use of the viscoelastic Norton-model for simulating drawing grease.

The efficient process of orbital forming can be used to manufacture functional components with a varying sheet thickness profile. It is characterized by a tumbling movement of the die and counterpunch, thus reducing the contact area between work piece and tool as well as the maximum forming force significantly. Within the project A1 of the Transregional Collaborative Research Centre (TCRC 73), the process of orbital forming has been investigated fundamentally. The control of the material flow to increase the maximum form filling could be identified as major challenge. The objective of this transfer project is the enlargement of the forming limits in comparison to conventional orbital forming by developing and applying tailor-made process and heat treatment strategies for industrial relevant steel and aluminum alloys. For the purpose of a transfer to industrial application, a demonstrator geometry with a varying sheet thickness profile is defined. Main aspects of the investigation are the resulting geometrical and mechanical properties and the possibility to enlarge the forming limits. The final geometric shape is realized by applying different cutting operations. An industrial application is evaluated by assembling the demonstrator to a near-series clutch disc and analyzing the functional behavior.

In order to meet the increasing requirements on modern production, the innovative process class of sheet-bulk metal forming has proved to be suitable. Particularly lightweight construction and functional integration lead to complex shaped functional elements. In addition to this, the widespread use of high-strength materials also results in challenges in the production of such components by forming. This leads to demanding forming processes, in which the die filling is often insufficient due to unintended material flow. In this respect, tailored surfaces offer one possibility of controlling the material flow and thus increasing component quality. This concept refers to the local adaptation of the tribological system during the forming of the components. Within this research project it is the objective to transfer the process knowledge that has been generated in the Transregional Collaborative Research Centre 73 (TCRC73) to the industrial environment. This involves investigating the impact of the requirements and influences of mass production on the efficacy and service life of the tailored surface. For this purpose, based on a finely ground reference surface, the effects of abrasive blasting and laser beam polishing as surface modifications of tool steels are investigated in a laboratory and industrial scale. Emphasis is placed on the formation of the modified surfaces, the effects on the tribological operational behaviour and the impact on the fatigue behaviour of tool steels.

The industrial manufacturing of functional components featuring a load-adapted shape primarily occurs by cutting or joining processes. These processes lead to inferior material efficiency and mechanical properties. The technology of incremental sheet-bulk metal forming (iSBMF) avoids these disadvantages but presents challenges towards a controllable material flow and tool life. Controlling both aspects is essential for the industrial application of this new class of processes. To solve the remaining challenges in iSBMF, the process is enhanced by thermal grading. For this approach, inductive and conductive heating units thermally control the local yield stress σ_f during the process. Based on numerical investigations of the enhanced process an analytical model is developed. This model enables a process understanding and a tremendously accelerated process design. The numerical and analytical modelling present the thermal grading to be very beneficial for solving the challenges in iSBMF. In contrast, experimental feasibility of the required thermal profile is identified as the major challenge.

Within the framework of the transfer project T05, the results from the sub-project A7 concerning a superimposed oscillation forming process will be transferred to an industry-oriented axial forming process of the company Felss Systems GmbH. Recursive axial forming serves as the reference model process. Here a gear geometry is axially formed on a hollow shaft by superimposed oscillation in a low frequency range around 15 Hz and in a high amplitude range around a few millimeters. The aim of this project is to gain a deeper knowledge of the influence of a more powerful oscillating device on achievable improvement in mould-filling, in a reduction of forming forces, in an increase in surface quality and in shorter process times. For this purpose, the quality characteristics of the semi-finished workpieces are first determined. A tool system for the integration of the oscillating device into the axial forming process of the company Felss Systems was designed, developed and manufactured. In order to show the comparison between the powerful superimposed oscillation and the conventional recursive axial forming, the superimposed oscillation experiments are both conducted by means of the model process on laboratory level and by industrial recursive axial forming. The evaluation and comparison of the two processes will be conducted with the focus on the project by means of surface inspection, analysis of the form-filling and concentricity to prove and quantify the potential of the TCRC73 technology for industrial processes.

The technology of sheet-bulk metal forming (SBMF) offers the possibility of manufacturing metal parts from a flat semi-finished product with functional secondary forming elements. Therefore, this technology provides the advantages of a resource-efficient production by shortening the process chain, saving materials and reducing the production costs. However, high contact normal stresses arise in SBMF-processes, leading to increased tool wear. The project T06 deals with the further development of numerical wear calculation for processes with high contact normal stresses. The numerical wear modelling is performed using the adapted friction law according to Archard, based on the friction shear stress. This model is applied exemplarily for the wear calculation in an industry-oriented process based on an extrusion process of the company Fischerwerke GmbH. An experimental setup is developed which simulates the industrial application at laboratory level. A hydraulically working oscillating device is installed in this test stand. The influence of a superimposed oscillation on the process characteristics of the Fischer process is investigated. The focus is on a reduction of the average forming force and a related reduction of the tool loads.

The increasing demand for complex components with filigree secondary functional elements promotes the application of new process technologies to extend the process limits of sheet-bulk metal forming (SBMF). The filling of cavities poses great challenges for manufacturing with sufficient quality. In cold forming, a considerable potential could be observed regarding mould filling, through a local adaptation of friction properties by surface structuring. In this study the transferability to hot sheet-bulk metal forming, which offers specific advantages due to thermal support, is to be investigated. The machinability of a hardened (53 HRC) hot work tool steel (HWS) AISI H11 by micro- and high feed milling is investigated related to tool wear and surface quality. Functional surface structures are applied on dies and adapted within the scope of hot sheet-bulk metal forming. Subsequent a developed hot ring compression test is to be used for tribological investigation of the structure-dependent material flow. In addition, an increase in the wear resistance of the structures by wet abrasive jet machining is to be focused on. Finally, the improvement of the surface modifications by introducing selected structures into a prototype tool is to be evaluated under real operating conditions with regard to their durability and mould filling.