THIN WALL AND SUPERIOR SURFACE QUALITY PROCESSING METHOD OF FIBER REINFORCED THERMOPLASTIC FOR COSMETIC APPLICATIONS

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1 THIN WALL AND SUPERIOR SURFACE QUALITY PROCESSING METHOD OF FIBER REINFORCED THERMOPLASTIC FOR COSMETIC APPLICATIONS José Feigenblum, Julien Fritsch, Matt Boulanger RocTool Abstract It is now confirmed that in order to obtain a top surface quality of a thin wall thermoplastic composite component, a heat and cool (H&C) technique is required to allow a good transfer of the preheated or non preheated material into the mold. The induction heating capabilities allow high heat molding of the tool to obtain a resin rich surface of the final part and avoid forming issues or surface defects, also like cycle time and energy consumption are acceptable, this can explain why this technology is used for mass production in the electronic market. It will be discussed some design rules describing key points as, steel selection, inductive integration, thermal expansion, H&C performance and energy consumption. Finally, we will share few examples of tool design and cycle time associated, and also some trend for large parts in order to propose an out-of-autoclave process. Introduction RocTool s activity is principally directed towards the development of heat and cool processes, which improve the transformation of plastic and composite materials. RocTool s philosophy, by means of its heating processes, is orientated towards effective heating and targeted for a specific part of the mold. Due to injection of fast and localized energy, rapid cooling is made possible. In fact, only fast heating, limiting the effects of thermal dispersion in the mould, enables rapid cooling and therefore, thermal cycling of a mould that is fully compatible with the industrial sector. For this challenge to succeed, electromagnetic induction has been used and mastered within the company RocTool (RT). This technique possesses two obvious attributes: the possibility to supply a high level of power and a precise control of where the injection of this energy will be localized. The ultimate aim, for all industrial specialists is to obtain a thermal cycling which respects the balance between the level of energy to be injected and the cycle time. Based on this postulate, the historic RT process, the Cage System [1], responds in practically every way to this definition, by directly heating the molding part of the tool. However this process is based, as will be later described, on a concept of an open mould where the induced currents circulate directly on the surface of the mould. Thus, in the context of the transformation of composite materials loaded with carbon fiber (CFRT Carbon Fiber Reinforced Thermoplastic), where moreover, shear edges are needed, the advantage of a new solution has become obvious.

2 3iTech Technology This new configuration [2] proposes an integration of the heating elements and the inductors as well as heating of the mould by induction (Fig.1). However, compared with heating cartridges there is: - no power limit - no thermal resistance between the heat source and the tool cavities - no limit with size (up to several meters) - no limit with complex shape (use of a flexible inductor) - orientation of the heat flux possible (mainly toward the molding surface) Molding surface Inductor network (red) Cooling network (grey) Figure 1: 3iTech Technology details of a composite mould Unlike heating cartridges, inductors are supplied with energy from an induction generator, hence the possibility of injecting high levels of power that can reach several tens of kw s. Therefore the inductor, made out of copper, will be made to the required length, taking into account the shape of the mould to be heated. The inductor is brought into action from a distance along the wall of the cavity which encircles it, with the aid of a magnetic field to directly produce the induced currents. Induction description With the aim of producing a heating effect combining effectiveness and homogeneity, all types of combinations can be developed in a non-exhaustive manner between the turn section and the cavity section. A first lever would be to reduce the air section (from a few mms to less than one mm) between the inductor and its own cavity in order to increase the inductive output. Moreover, tubes or wires (most often in copper) either round, square or even rectangular ranging from a few millimeters up to a few centimeters, will be used. Other solutions are the use of tape, braids or even materials obtained by a plasma deposit, the objective being, in all its

3 configurations, to be better adapted to the size of the mould and to its shape, as well as to the required power. The almost complete connection along the length of the inductor makes it possible to achieve 80 to 90 % output. In all cases, the parameter of the frequency will not be relevant; the skin depth deduced from Maxwell s equations [3] (where the eddy currents are located providing a Joule effect heating) will most often be much less than one mm. As described below (Fig.2), the shape of the groove can influence the efficiency of the heating, providing a non isotropic heat flux in order to concentrate mainly the heating close to the moulding surface. Figure 2: Electromagnetic skin depth representation with various form of groove Mold parameters For practically all the moulds, the central parameter of this system remains the distance of the inductor on the molding surface. In fact, even if this system remains a system of heating by induction, the energy input to the molding surface will be provided by thermal conduction (Fig.3). The definition of this key parameter, in fact a thermal resistance, which will control the diffusion through the meniscus, will be: = d / λ With d the distance between the cavity and the molding surface (m) With λ the thermal conductivity of the steel (W/m.K)

4 d Figure 3: Inductor configuration In this way, it will always be an aim to reduce the thermal resistance, either by bringing the cavity closer to the molding surface, or by using material with a high thermal conductivity. There as well, it will be necessary to choose a material making it possible to respect the balance between a good mechanical strength, a good resistance against corrosion and thermal fatigue (due to the thermal cycling), or even one that can be easily CNC machined, providing a cost for the realization according to the intended application (Plastic Injection, Thermo-Compression, Resin Transfer Molding, ). As has been previously described, the electrical power that can be delivered to the inductor is limitless, therefore unlike the heating processes obtained by steam or pressurized water, where it is difficult for the source temperature to reach more 170 C, here other parameters can be used such as the distance between the cavity and the molding surface or even the thermal conductivity. Indeed, using inductive heating, we can t ignore the electromagnetic output of the alloy used (Tab.I), even if a good thermal conductivity will offer a better efficiency for the diffusion of the heat flux for both stages (heating and cooling) through the mould. Table I: Mould materials: trade-off between inductive and thermal efficiency Mould material Aluminum / S / P / H11 Stainless Steel / Stavax Electrical resistivity e µohm.cm Relative permeability µr Inductive efficiency (Heat) f ~( e x µr) 1/2 Thermal conductivity W/m.K Specific Heat (J/Kg.K) Density (Kg/m3) Thermal diffusivity (Cool) (10e-6 m²/s)

5 Heat and Cool configurations As for any process of rapid heating, it must at the same time allow rapid cooling. To do this, the heating phase must be rapid and optimized, in order to avoid having too much energy being dissipated afterwards. Fig. 4 shows examples of several possible Heat & Cool configurations, suitable for different processes and levels of associated temperatures to be reached. Figure 4: Different Heat (red)/cool(blue) Configurations In particular, two different embodiments (corresponding to configuration I & II respectively) are compared below. Geometry 1 (Fig. 5) uses a cooling network close to the molding surface and an inductor network placed just after the cooling channel. With geometry 2, we switch the inductor network and the cooling channel. Cooling and heating networks use drilled cavities. Geometry 1 Molding Surface Geometry 2 Figure 5: Geometry 1&2 Cooling resp. Heating close to the molding surface The following study will describe a Heat and Cool cycle from 60 ⁰C to 200 ⁰C (140 F to 390 F) and cooling back to 60 ⁰C. We compared two different steels: H11 steel with an inductive output of 80 % and a thermal conductivity of 27 W/m.K and P20 steel with an inductive output of 70 % and a thermal conductivity of 34 W/m.K (Tab. II). Table II: Different heating/cooling times for different geometries and material combinations Geometry 1 Geometry 2 Geometry 1 Geometry 2 Cooling close to molding surface H11 Heating close to molding surface H11 Cooling close to molding surface P20 Heating close to molding surface P20 Heating time (s) Impregnation time (s) Cooling time (s) Total cycle time (s)

6 Temperature ( C) Fig. 6 shows as expected that cooling close to the surface (geometry 1/conf. II) allows rapid cooling and heating close to the surface (geometry 2/conf. I) enables fast heating, for both H11 and P20 steel. Tab.II & Fig. 6 show that better inductive efficiency of H11 steel results in faster heating and better thermal efficiency of P20 steel provides faster cooling. In short, even if P20 can achieve better cycle time, the energy consumption will be higher every cycle, and furthermore, the power unit required will be bigger Figure 6. Heat and cool cycle curves cooling close to the MD Heating close to the MD time (s) Thermal expansion Even if an efficient heating is provided by designing a heating network as close as possible to the molding surface (Fig. 7a), taking care to the mechanical strength of the mold, for large part (higher than a 17 laptop housing) and moreover for high temperature (over 300 C), an unbalanced temperature distribution generates, in addition with an expected thermal expansion, a bending of the tool. Figure 7: Heating configuration a) one side heating b) two side heating Figure 8: Thermal expansion and bending effect So, a first proposal will be to double the heating networks (Fig. 7b), in order to balance the thermal gradient through the overall thickness of the tool (Fig. 8), but this needs more power and more machining. By this way, another way can be to put heating and cooling networks alternately on the same line (Fig. 9), this line being defined by the neutral axis of the block to be heated. Even if this configuration is fully dedicated to 2D tooling configurations, this will show the importance to optimize the thickness of the tool to be heated.

7 Also, it will be very important to keep the control of the thermal expansion, by blocking the center of the tool, and providing a way for the tool to expand freely following the plane of the tool base. In fact all around the mold, we need to pay attention by avoiding any constraints which can prevent the thermal expansion. Figure 9: H&C on the same level Bending control Composite application As far as thermo-compression is concerned, the cycle time is always an objective. However, it will be acceptable in particular cases, and depending on the dimensions of the mould and of the temperature to be attained (which could reach up to 400 C) and with the concern of optimizing the power to be supplied, to target heating times of a minute. Thermoplastics systematically need a time for maintaining them at the required temperature in order to enable the fibers to become impregnated by the thermoplastic matrix. Contrary to the RHCM (Rapid Heat Cycle Molding) type process, on the molding surface with an all or nothing system (hot steam or cold water), it will be possible, in this case, to reach high temperatures, here where the aeronautic applications are numerous as in the transformation of PPS or PEEK (respectively C and C). But what is more, with a new potential for the injection of the energy, with a time period which can be modulated, in order to ensure good precision when controlling a stage, for example, using a regulator. Finally in a dissociable way, the two sides of the mould need to be heated, commonly called the core and the cavity. Therefore, several possibilities are opened up. - A generator supplying the core and the cavity connected in serial - A generator supplying the core and the cavity connected in parallel

8 - Two generators supplying respectively the core and the cavity There will be a very marked advantage in this configuration with regard to the possibilities of modulating the power on the sides of the mould with different thermal masses and with different thermal behavior over the time (Fig. 10). However, the investment required is higher, even if he will provide a strong flexibility in order to control the process. By this way, it is proposed to use a new embodiment requiring only one generator and allowing a fine tuning of the power in both sides of the tool. As described (Fig.11), a coil, connected in serial to one side of the tool, play like a by-pass, and adjusting the inductance of the coil you can change the relative impedance of both sides of the tool, allowing different level of current, generating different level of heating. This setting is adjustable before the starting of the process, but next step will be to be able to adjust dynamically this value, this will make sense when we want to drive long curing time with huge thermal difference between core and cavity. Figure 10: Picture of a RTM mould, 2D tool configuration Figure 11: Parallel coil (140) allowing various temperatures with only one generator

9 Moreover, as specified previously, this inductive process preferably heats the molding surface by a conductive effect, therefore, here where the main applications propose configurations of a CFRT material type (most frequently connected with PA12, PA6, TPU, PC, PPS or PEEK), the notions of shear edges will be dealt with in an identical way to a traditional process, without the risk of an electrical short-circuit between the core and the cavity, although they are both supplied with induced currents, the inductors being, in this configuration, embedded and shielded by the mould himself. Process analysis (Plate tool for Automotive applications characterizations) Part dimensions: dim. 300x300mm - thick. 1 to 5mm (Fig. 12) Steel selection: Cavities H11 ( =27 W/m.K) Base plate P20 ( =34 W/m.K) CFRT: PA6/CF Generator: 200kW (both sides connected in serial) Cooling: Fully turbulent flow rate during cooling step - No water during the heating and impregnation steps Figure 12: Tool layout

10 Parameters The heat cycle is defined as follow: t= 0 to 900s: Heating from 50 C to 220 C (120 F to 430 F) - Heating rate ~115 C/min t=90 to 120s: Overshoot and impregnation at 240 C t=120 to 240s: Cooling from 240 C to 50 C Total cycle time: 4min The level of power is defined as follow: Electrical Power for cavity: 90 kw Electrical Power for core: 90 kw Total power delivered by the generator: 180 kw Energy consumption for one cycle: 4.5 kw.h The cooling is defined as follow: Flow rate per channel for cavity: 10 l/min, T_water= 20 C (70 F) Flow rate per channel for core: 10 l/min, T_water= 20 C (70 F) Base plate is thermo regulated at 70 C (160 F) Mold performance The cycle time is strongly correlated with the power used and the temperature variation applied on the mold, we can see below the temperature evolution. Figure 13: Temperature evolution on the tool surface The above graph (Fig. 13) represents the temperature evolution over the molding surface on both core and cavity surfaces, we can see a temperature distribution less than +/- 10 C, which is largely acceptable for CFRT products. For this plate, the energy consumption is

11 about 4.5 kwh for one part and the total cycle time lower than 4 minutes. According to the given heating time, even if the power required is important, the energy consumption is still acceptable, and the good cycle time explains why this technology can be dedicated to the automotive industry. However, as explained previously, Heat & Cool performance is nothing without any control of the tool behavior. Also, integrating some specific requirements in order to allow a strong effect of the cycling of the thermal phenomenon, it s possible to define a tool with a limited deflection (acceptable for CFRT processing (Fig. 14)), thus, it will be always a good point, by limiting stress or others, to increase the life time of the tool. Figure 14: 3D Thermo mechanical analysis and deflection along Z axis [4] Conclusion With a solid basis, because they were well mastered during the study phase (simple 3D inductive phenomenon), all the applications must make it possible to answer to the diverse applications proposed, such as for an injection of TP and thermo-compression for any materials even if they have a high melting point. All the RT development efforts will be orientated in a way such to enable an optimization of two more main sectors: - An improvement in the output and the cycle times (thermal and magnetic mould properties, configuration of inductors, etc) in order to limit the cost for the energy; - An improvement in the conception and the realization of the mould, in order to limit the cost for the tooling. With regard to the process, a real answer will be, developing new materials that are able to be transformable at low pressure, low temperature and at short impregnation times is a key factor for future industrial processes, allowing process using light tooling (as nickel shell for instance) and vacuum bagging, as already proposed for an Out Of Autoclave (OOA) process, without any press or heavy machine to process the CFRT. Finally, for certain applications, the limitation of the shaping of CFRT sheets dedicated to very complex shapes, as 90 draft angles, bosses, ribs, even if we can obtain high mechanical

12 strength with these weaved materials, is quite impossible. It s why, asking for a second process (as TP injection for instance), more often the market is requiring a combination of a composite shell (less complex so less expensive to process) with an overmolding step (Fig. 15), most of the time for design consideration. Figure 15: Overmoulded CFRT insert with PC References 1. [Cage System patent WO 2008/ ] 2. [3iTech patent WO 2006/136743] 3. [James Clerk Maxwell; A treatise of Electricity and Magnetism, Gauthier-Villars, vol. I (1885) & vol. II (1887). 4. [Comsol Multiphysics -