Mould-integrated heating technology for efficient and appropriate processing of fibre-reinforced thermoplastics

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Procedia CIRP 00 (2017) 000–000 Procedia CIRP 85 (2019) 130–137

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2nd CIRP Conference on Composite Material Parts Manufacturing 2nd CIRP Conference on Composite Parts Manufacturing (CIRP-CCMPM 2019) CIRP Conference onMaterial Composite Material Parts 2nd2nd CIRP Conference on Composite Material PartsManufacturing Manufacturing

Mould-integrated28th heating technology for and appropriate processing of Mould-integrated heating technology forefficient efficient and appropriate processing CIRP Design Conference, May 2018, Nantes, France fibre-reinforced thermoplastics of fibre-reinforced thermoplastics a a a Jan P. Beuscher ’*, Raphael Schnurr , Felix Gabriel , Markus KUhna, Klausarchitecture Drodera A newJanmethodology to analyze the functional and physical of a,∗ a a a a a,∗ Raphael Schnurra , Felix Gabriela , Markus K¨ a , Klaus Dr¨ Beuscher Jan P. P.Technische Beuscher Raphael Schnurr , Felix , Markus K¨u uhn hn , KlausGermany Dr¨o oder dera Universitat,, Braunschweig, Institute of Machine ToolsGabriel and Production Technology (IWF), Braunschweig, existingTechnische products for an assembly oriented product family identification Technische Universit¨ Universit¨a att Braunschweig, Braunschweig, Institute Institute of of Machine Machine Tools Tools and and Production Production Technology Technology (IWF), (IWF), Braunschweig, Braunschweig, Germany Germany a

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Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat Abstract Abstract Abstract École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France Automated large-scale of of hybrid structures is a iskey requirement to bring lightweight design design into high volume industrialindustrial applications. Promising Automated production hybrid structures requirement to lightweight into high applications. Automated large-scale large-scaleproduction production of hybrid structures is aa key key requirement to bring bring lightweight design into high volume volume industrial applications. material combinations for hybrid structures consist of short consist and continuous fibre reinforced plastics and metals. plastics The aimand of this studyThe is toaim develop economic Promising material combinations for hybrid structures of short and continuous fibre reinforced metals. of this study material hybrid structures consist of short and continuous fibre reinforced plastics and metals. The aim of this study *Promising Corresponding author.combinations Tel.: +33 3 87for 37 54 30; E-mail address: [email protected] strategies for economic automatedstrategies final-shape manufacturing processes manufacturing for these structures. This paper addresses the development of a mould -integrated heating is to develop for automated final-shape processes for these structures. This paper addresses the development is to develop economic strategies for automated final-shape manufacturing processes for these structures. This paper addresses the development of of for efficient andtechnology appropriate processing of continuous fibre-reinforcedofthermoplastics. Due to challenges in ensuring Due requiredchallenges processing aatechnology mould-integrated mould-integrated heating heating technology for for efficient efficient and and appropriate appropriate processing processing of continuous continuous fibre-reinforced fibre-reinforced thermoplastics. thermoplastics. Due to to challenges parameters during manufacturing, inefficient process chains dominate the composite process industry. The most critical parameter is the temperature of the composite in in ensuring ensuring required required processing processing parameters parameters during during manufacturing, manufacturing, inefficient inefficient process chains chains dominate dominate the the composite composite industry. industry. The The most most critical critical during forming and over moulding. To enable such complex process chains, the material must conventionally be heated high above its melting to balance parameter is is the the temperature temperature of of the the composite composite during during forming forming and and over over moulding. moulding. To To enable enable such such complex complex process process chains, chains, the the point material must Abstract parameter material must temperature losses during subsequent process steps. In this to study, conventional processes andduring heating strategies are investigated to evaluate their potential of a conventionally conventionally be be heated heated high high above above its its melting melting point point to balance balance temperature temperature losses losses during subsequent subsequent process process steps. steps. In In this this study, study, conventional conventional mould-integrated heatingstrategies technology. Asinvestigated reference, external infraredtheir radiation heating is The mould-integrated heating makesAs use of transparent areas and are evaluate potential aa used. mould-integrated heating external processes and heating heating strategies the are trend investigated tomore evaluate their potential of mould-integrated heating technology. As reference, reference, external Inprocesses today’s business environment, towardsto product variety and of customization is unbroken. Duetechnology. to this development, the need of within the cavity to enable in-mould infra-red radiation. The main benefit of this technology is the increase of energy efficiency due to isothermal moulding infrared radiation heating is used. used. The The mould-integrated heating makes useproducts of transparent transparent areas within within theTo cavity to enable enable in-mould infra-red infrared heating is mould-integrated heating makes use of areas the cavity to in-mould infra-red agile and radiation reconfigurable production systems emerged to cope with various and product families. design and optimize production processes. The main benefit of this technology is the increase of energy efficiency due to isothermal moulding processes. radiation. radiation. thisoptimal technology is thematches, increaseproduct of energy efficiency due toare isothermal processes. systems as The wellmain as tobenefit chooseofthe product analysis methods needed. moulding Indeed, most of the known methods aim to

analyze product or one product onB.V. the physical level. Different product families, however, may differ largely in terms of the number and 2019aThe Authors. Published by family Elsevier ©© 2020 The Authors. Published by Elsevier B.V. ccPeer-review  2019 The Authors. Published B.V. under responsibility of by theElsevier scientific committee oflicense the 2nd(http://creativecommons.org/licenses/by-nc-nd/4.0/) CIRP Composite product Material Parts Manufacturing.  2019 The Authors. Published by Elsevier B.V. nature components. This impedes efficient comparison andConference choice of on appropriate family combinations for the production This isofan open access articlefact under the CCanBY-NC-ND Peer-review under responsibility of scientific committee on Material Parts Peer-review under responsibility of the scientific committee of the 2nd CIRP Conference on Composite Material Parts Manufacturing. system. A new methodology is proposed to analyze existing of products viewConference of their functional and physical architecture. The aim is to cluster responsibility of the theProcessing; scientific committee of the the 2nd 2ndinCIRP CIRP Conference on Composite Composite Material Parts Manufacturing. Manufacturing. Keywords: Injection; Moulding; Composite; Temperature; these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable Keywords: Injection; Moulding; Composite; Processing; Temperature; Keywords: Injection; Moulding; Composite; Processing; Temperature; assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the similarity between product families by providing design support to both, production system planners and product designers. An illustrative 1. Introduction technologies [7]. In In order order to reach this goal, close 1. Introduction Introduction technologies [7]. order to to reach reach this thisgoal, goal,aaaclose closecooperation cooperation 1. example of a nail-clipper is used to explain the proposed methodology. An technologies industrial case[7]. study on two product families of steeringcooperation columns of between material suppliers, machine manufacturers, toolmakbetween material suppliers, machine manufacturers, toolmakers and between material suppliers, machine manufacturers, toolmakHybrid structures made of carried different materials are industrial replacing thyssenkrupp Presta France is then to give a first of the proposed approach. Hybrid made of are Hybrid structures structures made of different differentoutmaterials materials are replacing replacing evaluation ers and applicators is necessary, as conventional solutions are applicators is necessary, as conventional solutions are often not ers and applicators is necessary, as conventional solutions are conventional lightweight construction strategies further ©conventional 2017 The Authors. Published by Elsevier B.V. for enabling conventional lightweight lightweight construction construction strategies strategies for for enabling enabling often not appropriate [1]. appropriate [1]. often not appropriate [1]. Peer-review underpotential, responsibility the scientific committee of the 28th weight saving sinceofeconomic production is gaining in CIRP Design Conference 2018. further further weight weight saving saving potential, potential, since since economic economic production production is is This research work handles with the challenge ofofproviding providing This research research work work handles handles with with the the challenge challenge of providing This importance in equal measure withmeasure its scopewith of additional functions gaining in importance in equal its scope of addigaining in importance inmethod; equal Family measure with its scope of addimandatory processing temperatures of thermoplastic prepregs mandatory processing temperatures of thermoplastic prepregs within Keywords: Assembly; Design identification mandatory processing temperatures of thermoplastic prepregs [1, 2, 3]. The combination of material classes, e.g.material metals, polymers, tional tional functions functions [1, [1, 2, 2, 3]. 3]. The The combination combination of of material classes, classes, within a combined thermoforming and overmoulding process. a combined thermoforming and overmoulding process. It focusses on within a combined thermoforming and overmoulding process. ceramics or polymers, composites causes improved properties of improved individual e.g. e.g. metals, metals, polymers, ceramics ceramics or or composites composites causes causes improved It focusses on a mould integrated heating technology, as introa mould integrated heating technology, as introduced in [8, 9] It focusses on a mould integrated heating technology, as introadvantages ofindividual single materials. With this approach various properties properties of of individual advantages advantages of of single single materials. materials. With With duced in in [8, [8, 9] 9] duced requirements such as mechanical, thermal, electrical, etc. can be met this approach various requirements such as mechanical, 1.this Introduction approach various requirements such as mechanical, therther- of the product range and characteristics manufactured and/or in an ideal compromise with weight and costs [4]. 2. Processing of hybrid structures mal, mal, electrical, electrical, etc. etc. can can be be met met in in an an ideal ideal compromise compromise with with assembled in this system. In this context, the main challenge in Fibre-reinforced plastics (FRP) have a high potential because of 2. of hybrid structures weight and costs [4]. Due and to costs the [4]. fast development in the domain of modelling and analysis now notmost onlyefficient to cope with single 2. Processing Processing of hybrid structures weight Injection moulding is is one of the technologies for their mechanical strength and low density. Due to highpotential material costs Fibre-reinforced plastics (FRP) have a high becommunication and plastics an ongoing digitization product range or families,it Fibre-reinforced (FRP)trend have of a high potential and be- products, processinga limited polymers. Even though theexisting process product is discontinuous, and processing requirements its application isdensity. still notDue economical. Injection moulding is one oneand of the thecompare most efficient efficient technologies cause mechanical and to Injection is of most digitalization, enterprises facingDue important also to be moulding able analyze products to define cause of of their their manufacturing mechanical strength strength and low loware density. to high high but enables short cycletotimes down to to a few seconds. Thetechnologies mould takes FRP with thermoset matrix require a high amount of manual for processing processing polymers. polymers. Even Even though though the the process process is is discondisconmaterial costs and processing requirements its application is for material costs and processing its aapplication challenges in today’s market requirements environments: continuingis new It can be observed that of classical existing overproduct the tasksfamilies. of shaping, cooling and ejecting the manufactured operations during processing andthermoset curing reactions. Thermoplastics tinuous, it it enables enables short short cycle cycle times times down down to to aa few few seconds. seconds. still FRP matrix aa high tinuous, still not not economical. economical. FRP with with thermoset matrix require require high tendency towards reduction of product development times and product families are regrouped in function of clients or features. part [10]. Thermoforming is an industrial process to drape, form and FRP prepregs, on the other hand, are provided in ready-to-process The mould mould takes takes over over the the tasks tasks of of shaping, shaping, cooling cooling and and ejectejectamount of manual operations during processing and curing reThe amount ofproduct manuallifecycles. operationsInduring processing curing re- However, shortened addition, there is and an increasing assembly prepregs orientedand product areand hardly to find. shape thermoplastic sheetsfamilies using heat pressure [11]. conditionThermoplastics and are thereforeFRP betterprepregs, suited for on series production. [5, are 6] ing of of the the manufactured manufactured part part [10]. [10]. Thermoforming Thermoforming is is an an indusindusactions. the other hand, ing actions. of Thermoplastics FRP prepregs, on thetime other hand, are The material has to family be heated above the heating temperature, usually demand customization, being at the same in a global On the product level, products differ mainly in two The success of hybrid structures depends onare the development and trial process process to to drape, drape, form form and and shape shape thermoplastic thermoplastic prepregs prepregs provided ready-to-process condition and better trial provided in in with ready-to-process condition and world. are therefore therefore better main over characteristics: 40 K above the melting temperature but stillandunder the competition competitors all over the This trend, (i) the number of components (ii) the availability of efficient and automated production and sheets sheets using using heat heat and and pressure pressure [11]. [11]. The The material material has has to to be suited for series production. [5, 6] and decomposition temperature depending onelectrical, the matrix electronical). material [12].be suited is for inducing series production. [5, 6] which the development from macro to micro type of components (e.g. mechanical, heated above the the heating temperature, usually over 40 40 K K above The structures depends developheated above temperature, usually over Combining bothheating single processes provides the potential to above reduce The success success of hybrid structures depends on the developmarkets, results of in hybrid diminished lot sizes due on to the augmenting Classical methodologies considering mainly single products the melting melting temperature temperature but but still still under under the the decomposition decomposition temtemment and availability of efficient and automated production the 1 showsproduct a fully integrated ment and availability of efficient and automated production product varieties (high-volume to low-volume production) [1]. orredundant solitary,operations. already Fig. existing families analyze the

perature depending depending on the the matrix matrix material material [12]. [12]. perature To cope with this augmenting variety as well as to be able to product structure on on a physical level (components level) which Combining both both single single processes processes provides provides the the potential potential to to Corresponding author. Tel.: +49-531-391-65045 ; fax: +49-531-391-5842. ∗∗* Corresponding author. Tel.: +49-531-391-65045 ; fax: +49-531-391-5842. Combining Corresponding author. Tel.: +49-531-391-65045 ; fax:in+49-531-391-5842. identify possible optimization potentials the existing causes difficulties regarding an efficient definition and cc 2019 2212-8271  2019 The Authors. Authors. Published Published by by(Jan Elsevier B.V. Email address: [email protected] P. Beuscher) Email address: [email protected] (Jan P. P. Beuscher) Beuscher) reduce redundant operations. Fig. 11 shows aa fully integrated 2212-8271  The Elsevier B.V. reduce redundant operations. Fig. shows fully integrated Email address: [email protected] (Jan production system, it is important to have a precise knowledge comparison of Material different families. Addressing this Peer-review under responsibility ofthethe thescientific scientific committee of the the 2nd CIRP Conference on Composite Composite Material Partsproduct Manufacturing. Peer-review under responsibility ofof committee of the 2nd CIRP Conference on Composite Material Parts Manufacturing. Peer-review under responsibility scientific committee of 2nd CIRP Conference on Parts Manufacturing. 2212-8271 © 2020 The Authors. Published by Elsevier B.V. This is an©open article Published under theby CC BY-NC-ND 2212-8271 2017access The Authors. Elsevier B.V. license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of scientific the scientific committee theCIRP 2nd Design CIRP Conference on Composite Material Parts Manufacturing. Peer-review under responsibility of the committee of the of 28th Conference 2018. 10.1016/j.procir.2019.09.038



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Jan. P. Beuscher / Procedia CIRP 00 (2019) 000–000

Material supply

Handling

Preforming

Transferring

Thermoforming

2

Injection moulding

Removal

FRTP plastics

metal Fig. 1. Route of an integrated processchain of combined processing of different materials.

vertical closing injection moulding machines are used in combination with IR-heating devices placed outside of the mould, respectively outside of the machine. This setup of external IRheating devices implicates the need of overheating the FRTP to compensate temperature losses during transferring and inserting. Depending on the polymer matrix of the FRTP decomposition and degradation can take place. Therefore, in the paper the heating processes of external and internal IR-heating devices are investigated with regard to the process dynamics, the accuracy of the achievement of the temperature target value and the impact of the temperature to the bonding strength. The investigation is carried out under the restriction of one material combination, namely injection moulded plastics and the FRTP.

process chain, as introduced in [13]. This integrated process chain makes use of individual advantages of each material. Stacking and material supply are efficient due to the flat geometry of the semi-finished products. During handling and first feeding to the preforming process conventional gripping technology can be used, but from this stage the main challenge occurs. Between the process steps of preforming and consolidating after injection mould the FRTP must be heated above the melting temperature to allow shaping and bonding operations. In this stages the inserted metal supports the process due to its robust and consistent behaviour. It supports the molten FRTP during transferring the preform to the mould and reduces the temperature loss because of gripping and convection. Additionaly, the high temperature of the FRTP has to be kept until wetting with the plastic melt to guarantee a sufficient bonding strength. Combined processes of thermoforming and injection mould, occasionally with small metallic inserts, are available and distributed by machine manufacturers. These industrial processes offer short cycle times and robust production systems for less complex component geometries. Usually horizontal or vertical closing injection moulding machines are used in with heating technologies because of requirements of processed materials or quality factors. There are three types of heating approaches for those manufacturing processes:

3. Experimental setup 3.1. Material and processing properties The experimental investigations are carried out with FRTP samples, which composite laminates consist of a polyamide 6 matrix and continuously woven glass fibre fabrics with a thickness of 2.0 mm. The laminate has a glass fibre content of 66 % . The melting temperature of polyamide 6 is above 220 ◦ C and its thermal decomposition temperature at least above 300 ◦ C (Fig. 5). Despite its high specific heat capacity of around 2, 700 K Jkg in molten state, the absolute heat capacity is still lower than the one of metal. However, the most disadvantageous property of polyamide towards metals regarding the heat management is the thermal conductivity, which is several times smaller and complicates a homogenous heating of the thermoplastic matrix [14]. This explains the importance of a precise and gentle heating. The properties are shown in overview in Table 1. The short fibre-reinforced injection moulding compound (SFRC) used for this investigation has a polyamide 6.6 base material and a short glass fibre content of 50 % . Beneficial to this material combination is the difference of the melting

1. Basic mould tempering: Internal heating concept, that keeps the mould at a constant temperature and appears indirect and isothermal to inserted materials 2. Dynamic mould tempering: Approach to improve processing and final part properties, internal heating, indirect, variothermal (varying mould temperature) 3. Material tempering: to process required temperature, external heating, direct, adapted material heating The basic mould tempering (isothermal) is necessary for high efficient processes and focussed on the heat transfer from material to the mould. Variothermal mould tempering causes long cycle times and a high energy demand. Both approaches act indirectly, thus not directly on the material. Material tempering is a efficient strategy to heat directly but has negative impacts on the material quality. Therefore, the most advantageous heating strategy located in a common solution space. This strategy must operate directly, internally, locally and materially gently. Combined processes of thermoforming and injection moulding, occasionally with small metallic inserts, are available and distributed by machine manufacturers. These industrial processes offer short cycle times and robust production systems for less complex component geometries. Usually horizontal or

Table 1. Physical properties of polyamide 6. [14]

2

Property of polyamide 6

Symbol

Value

Melting temperature Decomposition temperature Specific heat capacity (melt) Specific heat capacity (solid) Thermal conductivity (melt) Density at 20 ◦ C

Tm Td cm cs λm ρ

220 − 260 ◦ C > 300 ◦ C 2, 680 − 2, 730 K Jkg 1, 400 K Jkg 0.23 mWK g 1.06 − 1.16 cm3

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optimize the heating process towards short cycle times trying to avoid material overheating and damaging. It is described in ideal form with the following differential equation:    t de(t) (1) . e(τ)dτ + Kd u(t) = K p e(t) + Ki dt 0

Table 2. Physical properties of polyamide 6.6. [14] Property of polyamide 6.6

Symbol

Value

Melting temperature Decomposition temperature Specific heat capacity (melt) Specific heat capacity (solid) Thermal conductivity (melt) Density at 20 ◦ C

Tm Td cm cs λm ρ

250 − 270 ◦ C > 340 ◦ C 2, 750 K Jkg 1, 700 K Jkg 0.23 − 0.25 mWK g 1.05 − 1.14 cm3

3.3. Heating technologies As promising heating technology for economic processes IR-heating systems have been selected for this investigation. IR radiation is largely absorbed in polymers, so that short heating times and high efficiency are achieved. In order to meet the requirements of large-scale production even the ratio of space to power, the possibility of appropriate material heating and a robust applicability (prevention of adhesion, reproducibility, market availability) must be ensured. IR radiation can be used for both dynamic mould tempering [16][17] and material tempering [18]. Though IR radiation can be used for mould tempering, because of its bad cycle times and eco-behaviour only material tempering is part of this investigation. Two settings of IR radiation are investigated. In both settings the sample or material is inserted to the mould at the beginning of the process. The sample is threaded onto spring-guided pins and positioned in a defined distance of 2 mm to the mould surface to avoid an early heat loss. The first setting is a external IR radiator field, shown in Fig. 3. It uses a pneumatic lift (Fig. 2) to place the radiator in front of the sample and to clear the machine room after heating. In Fig. 3 a sensor-equipped mould is used to measure the temperature of the samples inserted with a distance to the surface. The IR radiator is placed with distance of 30 mm between sample and glowing spiral of the radiator. The second setting represents the integration of IR radiators into the mould and is explained below.

moving direction

injection unit external IR radiator mould mounting plate Fig. 2. Injection mould machine ENGEL Victory spex used for this investigation. A test bench simulates the conditions of this system.

points. The melting temperature of this compound is between 250 − 270 ◦ C and its thermal decomposition temperature is above 340 ◦ C. Typical processing temperatures for injection moulding are between 280 − 305 ◦ C [14]. Therefore, the temperature gap between the FRTP and the IMC can be used to decrease the temperature requirements to FRTP. 3.2. Machine setup and test bench A horizontal closing injection moulding machine of type ENGEL Victory spex with a clamping force of 1.200 kN and a maximum swept volume of 154 cm3 is used. The injection moulding machine has an interface that allows for external process control in combination with different heating technologies and temperature measurement (Fig. 2). The process development takes place on a test bench in a laboratory with standard climate to ensure reproducible data. The test bench is a model of the original system and has identical connectivity, automation and mechanical interfaces. Thus both have similar mould mounting plates and mould changing systems so that the same equipment (mould, external/internal IR radiator, process controller) can be examined in both settings. The investigated heating process, with regard to both the injection moulding machine and the test bench, are controlled by the same programmable logic controller (PLC). It can either communicate with the injection moulding machine to integrate the heating process into the program sequence of the injection process or it can control the test bench. The PLC is capable of acquiring and processing signals, thus temperature values of the samples can be measured and IR radiators can be controlled. A proportional-integral-derivative controller (PID) is used to

3

3.4. Mould setup The functional demonstrator of the mould-integrated heating technology uses conventional double-pipe IR radiators (Fig. 4). The key to this approach can be found in the creation of transparent or translucent areas in the cavity. These allow the use of

FRTP sample

IR radiator

mould

thermocouple

Fig. 3. Setting of external IR radiator to heat FRTP.

3



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4. Process investigation

the radiators without complying with the requirements of process influences (temperature, pressure, etc.). For this purpose, appropriate materials must be identified that can withstand the process loads during thermoforming or injection moulding. Materials that are suitable for this purpose include glass, glassceramics or ceramics that have a high degree of transmission in the spectral range of infrared radiation. Fig. 4 shows a functional demonstrator of a research mould with integrated IR radiators with irradiation windows, that are realized with transparent Mg-Al-spinell ceramics. The mould has two separate irradiation areas to allow a multiple application with different sample cavities, e.g. rip-pull-off, lap-shear and bending samples. Between both areas the cold sprue is positioned. A ventilation system allows a convectional cooling of the IR radiators. A basic mould tempering systems is designed for temperature levels up to 160◦ C. The target temperature of this mould is related to the material properties used for injection moulding. Although, the functional demonstrator of the mouldintegrated heating technology serves the entire mould surface for heating, this scenario is simplified and suits to normalised geometries of the used test specimen. Future investigations have to assess whether extensive applications over entire mould surfaces are appropriate. Even limitations of complex geometries depends on technological conditions of manufacturing of the transparent materials, that are not discussed in this work.

The process investigation using a single-sided heating aims at a knowledge gain of efficient and appropriate processing of FRTP and transferring it to process strategies of mouldintegrated heating. Therefore, this investigation includes several steps. First the FRTP is investigated to gain material knowledge of its thermal behaviour. In the next step IR heating strategies and IR wavelengths are investigated to understand its impact of the heat distribution through the thickness of the FRTP and to derive parameters to set the PID controlled heating process. Finally this setting is transferred to the mould-integrated system. Therefore, the following investigations shall be carried out:

guide rail ventilation cavity face sprue transparent ceramic inserts conventional double tube IR-radiator

A

133 4

basic mould tempering A-A

A Fig. 4. Concept of mould-integrated IR radiators using irradiation windows of transparent ceramics.

4

1. Thermal analysis of FRTP The thermogravimetry (TG) is used to understand the thermal behaviour of the polymer matrix. TG is a method to analyse polymers under varying temperature environments and uses a precision balance to detect the change of weight and the mass fraction of elements of the investigated sample. The standard method uses defined atmospheres in different temperature areas to avoid additional chemical reactions with oxygen. The heating rate during the measurement effects possible chemical reactions [15], but does not have a relevant influence on the material examined here. To develop gentle process technologies the polymer behaviour during manufacturing and heating processes have to investigated. One important characteristic is the decomposition. This analysis is done under O2 atmosphere and a constant heating rate of 10 K/s. Another method, used with support of NETZSCH, is the laser flash analysis (LFA) to measure thermal diffusivity of FRTP. This method uses a laser flash to generate an energy impact at one surface of a sample and to measure the resulting temperature increase and time delay at the other side of the sample. The material is investigated in a temperature range between 20 ◦ C and 200 ◦ C at five temperature points. 2. Comparison of different wavelength areas of IR The investigation of heating and set parameters for the PID controller and the comparison of this set of parameters of different IR wavelengths are done in the test bench introduced before. The experimental setup (Fig. 3) uses thin film thermocouples to measure the temperature at both sides of the sample. The distance of the radiators to the sample and of the sample to the mould surface are equal at all time. 3. Heating behaviour of mould-integrated IR radiators In the same experimental setup the external radiators are replaced with mould-integrated IR radiators. 4. Evaluation of the impact of direct material heating in comparison to mould heating Subsequently, the impact of direct material heating has to be evaluated in comparison to a basic mould heating. Therefore, using the described experimental setup hybrid lap-shear samples are produced for the characteristic investigation of the bond strength. In this case,

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5. Results and discussion

processing temperatures should be as low as possible and holding times as short as possible. As a compromise a lower process temperature can be applied if longer holding or processing times a necessary. The results of the LFA show a decreasing thermal diffusivity (Fig. 6) due to molecular oscillations. The thermal conductivity, that is important to distribute the heat over its thickness, is almost constant over the temperature range with a slight increase at 200 ◦ C. That means for a gentle heating strategy a constant heat flux through the material can be provided as soon loss effects (e.g. convection) are compensated.

5.1. Heating parameters of polyamide 6

5.2. Comparison of different wavelength areas of IR

The thermal analysis using TG shows an interesting behaviour of the polymer matrix depending at the temperature. The results of several TG-methods of FRTP are shown in Fig. 5. Applying the standard method (Tstart = 20 ◦ C, Tend = 650◦ C) a decomposition temperature at about Tdecom = 300.5 ◦ C can be detected and literature value can be approved (Tab. 1). An investigation of different heating rates in order to evaluate realistic heating processes show no dependence on the thermal behaviour of the PA6. The graph of the standard method shows a straight progression and weight loss beginning with the evaporation of water in the material. Approximately 80 ◦ C above the melting point at about 220 ◦ C a rapid increase of the weight loss can be detected. This point marks the beginning of the decompostion process of the matrix that continues until 550 ◦ C. A further focus on the special applications of heating processes allow adapted methods, that work in a temperature range between 25 ◦ C and 290 ◦ C, respectively 250 ◦ C. In both cases a temperature level below decomposition temperature is targeted with a holding time of 30 min to investigate heating processes that keep a temperature level for several minutes. The graphs of the analysis results in Fig. 5 show, that a beginning decomposition takes place at temperature of 290 ◦ C within in a short time. Although this damage is less severe than that of the standard method it causes loss of weight of over 10 %. In comparison to that, the method with a target temperature of 250 ◦ C causes less damage over of that time period. For the temperature management of the FRTP during processing can be considered that

Experimental investigations to identify parameters of the PID controller for heating FRTP have to provide a fast heating process with a small overshoot of the response. The investigations are carried out in the described setup of the external IR field and the signal of the radiator power is transmitted through the PID controller in the PLC. The parameters of the equation (1) consist of three terms, that are set wit a proportional gain constant K p , an integral gain constant Ki and a derivative gain constant Kd . To reach a fast heating process this process used just proportional gain constant and set the others to zero. As a result the control behaviour is strongly oscillating and is weakly damped. This investigations used a set point of 260 ◦ C to guarantee homogeneous melting at the surface and a sufficient heat transfer through the material. The comparison of different IR radiator wavelength areas are shown in Fig. 7 and Fig. 8. Short-wave IR radiation couples close to the surface in to the material. Therefore, a short heating time is required to reach the melting temperature. In this setting about 12 s are necessary to reach 220 ◦ C if the controller goals at 260 ◦ C. This target temperature is needed to provide a heat transfer within the material to reach the melting temperature through the thickness of the sample. In comparison to short-wave radiation a setup with fast-medium-wave radiation is shown in Fig. 8. Short-wave radiation enters deeper into the material so that low heat transfer rates in polymers can be compensated (cf. [19]). However, this effect ordinarily has to be compensated with lower heating rates on the surface. Neverthe-

start of decomposition

90

450

80

300

70

150

60

O2 atmosphere, 10 K/min 0

15

30

45

60

0,30

5

0,25

4 3 2 1

0

0

time [min]

Spec. Heat Thermal Diffusivity Thermal Conductivity

1,2 1,0

0,20

0,8

0,15

0,6

0,10

0,4

0,05

0,2

0,00

0

40

80

120

160

200

thermal conductivity [W/(m*K)]

600

spec. heat [J/g*K)]

100

6

thermal diffusivity [mm²/s]

method 25°C - 250°C method 25°C - 290°C method 20°C - 650°C

temperature [°C]

mass fraction / tg [%]

norm specimen are taken according to DIN EN 1465 to investigate the tensile shear strength. For this purpose, for each shot two samples of FRTP with the dimensions of 90 mm x 90 mm x 2.0 mm are placed in the cavity and are overmoulded over a length of 12.5 mm. After conditioning the samples in accordance with standard DIN EN ISO 291, they are destructively examined in tensile testing machine.

5

0,0

temperature [°C]

Fig. 5. Thermal analysis of polyamide 6 using Thermogravimetry (TG).

Fig. 6. Thermal analysis of polyamide 6 using laser flash analysis (LFA).

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tmelt = 12 s

surface temperatur [°C]

360

toverheat

sponse behaviour have to be choosen. Further more a reduction of the set point is a important factor to prevent the material of thermal damage. However, this experimental investigation demonstrates the potential of cycle time reduction by the use of mould-integrated IR radiators. A typical process cycle of an over-moulding of FRTP with the mentioned injection moulding machine takes approximately 30 to 33 s. If an external heating system is used, the total cycle time will increase over 40 % to at least 42 s based on the results in Fig. 7. In comparison to this cycle time, a mould-integrated IR heating system does not effect the based cycle time at all, because of its overlap with process steps like the mould closing (Fig. 10). Finally, the results of the investigation of lap-shear samples of over-moulded FRTP confirm a strong dependency of the shear strength to the temperature of the mould as discussed before. The mould heats the samples conductively within a short time to its temperature. With an increasing temperature the shear strength increases from 5 MPa to over 12 MPa. A even stronger impact shows an active heating using IR radiators. It causes high shear strength of 13 MPa to 15 MPa at mould temperatures of 100 ◦ C to 140 ◦ C (Fig. 11).

front side

300

set point, 260°C

240 180 melting temperature, 220°C

120 60 0

0

10

20

30

40

50

60

time [s] Fig. 7. Heating process using short-wave IR radiation

tmelt = 21,2 s

surface temperatur [°C]

360

toverheat

front side back side

300

135 6

set point, 260°C

240 180 melting temperature, 220°C

120

6. Conclusion

60 0

0

10

20

30

40

50

The success of processing hybrid structures and their properties depends on reliable manufacturing technologies. In combined injection moulding and thermoforming, a major focus applies on the temperature conditions of the FRTP during processing. It must be ensured that a minimum heating time is achieved in order to provide an economic process and minimal heating temperature in order to avoid material damage and excessive energy consumption. Therefore, a mould-integrated heating technology for efficient and appropriate processing was developed. To set a robust process an investigation of thermal material properties was done. Furthermore, different infrared wavelength areas were examined to select the right radiators for individual applications.

60

time [s] Fig. 8. Heating process using fast-medium-wave IR radiation

less, the heating time of fast-medium-wave radiators increase up to 21.2 s. The second curve in Fig. 8 represents the temperature at the back side of the sample. The diagram only shows the relevant period for industrial heating processes, but in contrast to the investigation using short-wave IR radiation the backside temperature reaches the melting point after 85 s. 5.3. Internal heating processes

360

surface temperatur [°C]

The approach of mould-integrated IR radiators shows advantages in single-sided heating processes. Adapting this concept to experimental setup used before with identical distances between radiators and sample, heating power and PID settings a significant reduction of the heating time can be achieved. The results of this investigation are shown in Fig. 9. Within 2.5 s the target temperature of 220 ◦ C is passed. The heating behaviour shows a higher dynamic than the external heating concepts. This is shown by high gradients and higher values of the overshoot amplitudes. The overshoot exceeds clearly the decomposition temperature for several times. A material damage occurs at the surface of the sample. It can therefore be determined that the reduction of disturbing factors, e.g. convection, requires an adapted PID tuning for this heating technology. The time resolution and the dynamic of the controller has to be increased and sensors with a hight re-

tmelt = 2,5 s

toverheat

front side

300

set point, 260°C

240 180 melting temperature, 220°C

120 60 0

0

10

20

30

40

50

60

time [s] Fig. 9. Heating process using short-wave IR radiation in an in-mould heating concept

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136

Mould integrated heating (2.3 s) External heating (12 s)

material parameters. The authors are responsible for the contents of this publication.

Mould closing (4.5 s) Injection (5.4 s)

12 s

Packing (3.9 s)

Ejection, et al (1.2 s) Residual cooling (9 s)

References

Dosing (9 s)

[1] L¨assig, R., Eisenhut, M., Mathias, A., Schulte, R.T., Peters, F., K¨uhmann, T., Waldmann, T. and Begemann, W. (2012): Serienproduktion von hochfesten Faserverbundbauteilen: Perspektiven f¨ur den deutschen Maschinen- und Anlagenbau, Roland Berger Strategy Consultants and VDMA. [2] Dr¨oder, K., Herrmann, C., Raatz, A., Große, T. and Sch¨onemann, M., L¨ochte, C. (2014): Symbiosis of plastics and metals: integrated manufacturing of functional lightweight structures in high-volume production,Kunststoffe im Automobilbau, VDI-Verlag, D¨usseldorf. [3] Henning, F.; Weidenmann, K.; Bader, B. (2011): Hybride Werkstoffverbunde. In: Handbuch Leichtbau: Methoden, Werkstoffe, Fertigung. Henning, F., Moeller, E., Eds. Hanser, M¨unchen. [4] Bader B., T¨urck, E., Vietor, T. (2019): Multimaterial Design. A Current Overview of the used Potential in Automotive Industries. In: Technologies for economical and functional lightweight design. Zukunftstechnologien f¨ur den multifunktionalen Leichtbau, Dr¨oder, K., Vietor, T., Eds., Springer Vieweg, Berlin, Heidelberg. [5] Brecher, C.; Emonts, M.; Kermer-Meyer, A.; Janssen, H.; Werner, D. (2014): Herstellung von belastungsoptimierten thermoplastischen Faserverbundbauteilen, in: Leichtbau-Technologien im Automobilbau, Siebenpfeiffer, W. Eds., p. 70-75, Springer Vieweg, Wiesbaden. [6] Nestler, D.J. (2014): Beitrag zum Thema Verbundwerkstoffe - Werkstoffverbunde: Status quo und Forschungsans¨atze, Univ.-Verl. and mv Monsenstein und Vannerdat, Technische Universit¨at Chemnitz, Chemnitz. [7] Eickenbusch, H.; Krauss, O.; Hrsg.Verein Deutscher Ingenieure e.V., VDI-Gesellschaft Materials Engineering (2014): Werkstoffinnovationen f¨ur nachhaltige Mobilitaet und Energieversorgung: Studie; D¨usseldorf [8] Beuscher, J.P., Schnurr, R., M¨uller, A., K¨uhn, M. and Dr¨oder, K. (2017): Introduction of an in-mould infrared heating device for processing thermoplastic fibre-reinforced preforms and manufacturing hybrid components. In: 21st International Conference on Composite Materials (ICCM), Xi’an (China). [9] Beuscher, J.P., Schnurr, R., M¨uller, A., K¨uhn, M. and Dr¨oder, K. (2018): Process Developement for Manufacturing Hybrid Components using an InMould Infrared Heating Device. In: 18th European Conference on Composite Materials (ECCM), June, 24th-28th 2018, Athens (Greece). [10] Johannaber, F.; Michaeli, W. (2014): Handbuch Spritzgießen, doi=10.3139/9783446440982, Hanser, M¨unchen (Germany). [11] Klein, P. (2009): Fundamentals of Plastics Thermoforming. Morgan & Claypool Publischers, Ohio (USA). [12] Behrens, B.-A.; Raatz, A.; H¨ubner, S.; Bonk, C.; Bohne, F.; Bruns, C.; Micke-Camuz, M. (2017): Automated Stamp Forming of Continuous Fiber Reinforced Thermoplastics for Complex Shell Geometries. In: Procedia CIRP, Vol. 66, pp. 113-118. [13] Dr¨oder, K.; Schnurr, R.; Beuscher, J.P.; Lippky, K.; M¨uller, A.; K¨uhn, M.; Dietrich, F. and Dilger, K. (2016): An integrative approach towards improved processability and product properties in automated manufacturing of hybrid components. At: 2. Internationale Konferenz Euro Hybrid - Materials and Structures, pp. 188–193, Kaiserslautern (Germany). [14] Wypych, G. (2012): Handbok of Polymers. ChemTec Publishing, Toronto (Canada). [15] Wagner, M. (2018): Thermal Analysis in Practice - Fundamental Aspects. Hanser Publications, Munich, Cincinnati (Germany/USA). [16] Chang, P.-C. and Hwang, S.-J. (2006): Experimental investigation of infrared rapid surface heating for injection molding, Journal of Applied Polymer Science, vol. 102, no. 4, pp. 3704-3713. [17] Yu, M.-C. and Young, W.-B. and Hsu, P.-M. (2007): Micro-injection molding with the infrared assisted mold heating system, Materials Science and Engineering: A, vol. 460-461, pp. 288-295. [18] Dˇzalto, J. (2017): Entwicklung eines großserientauglichen Aufheizprozesses f¨ur naturfaserverst¨arkte Kunststoffe, Dissertation, Technische Universit¨at Kaiserslautern (Germany).

Based on 30 s cycle time

Fig. 10. Reduction potential of the cycle time using mould-integrated IR radiators. According to [10].

The investigations revealed the decomposition behaviour of FRTP in the range between the melting and above the decomposition temperature. An important observation is that thermal decomposition also occurs at temperatures below the temperature limit as a function of time. Short-wave and fast-medium-wave IR radiation is suitable for heating FRTP. This examination demonstrated two relevant effects. First, short-wave radiation couples directly into the surface of the FRTP. That is the reason for high heating rates and short process cycles. Second, fast-medium-wave radiation enters deeper into the material. That behaviour causes lower heating rates at the surface, but a faster heating through the material’s thickness. An mould-integrated IR radiators minimizes loss effects, e.g. convection, and decreases the heating time rapidly. A continuing challenge is the heating through the material’s thickness, that has to be investigated by optimizing the PID controller of the system. Acknowledgements This research and development project was funded by the German Federal Ministry of Education and Research (BMBF) within the Forschungscampus ”Open Hybrid LabFactory” and managed by the Project Management Agency Karlsruhe (PTKA) under research funding scheme 02PQ5100 (ProVorPlus ). Many thanks applies to NETZSCH¨ GERATEBAU GMBH for supporting investigations of thermal

Shear strength [MPa]

20

IR heating conduction

16 12 8 4 0

80

100

120

7

140

Mould temperature [°C] Fig. 11. Lap-shear strength depending on mould temperature and active heating.

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[19] Gaab, L. (2014): Optimierung der Heizprozesse von CFK- und GFKStrukturen mit Infrarot-Strahlung, in: Leichtbau-Technologien im Automobilbau, Siebenpfeiffer, W. Eds., p. 23-28, Springer Vieweg, Wiesbaden.

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