Multi-Component Reaction Injection Moulding (MC-RIM): a Processing Technology for the Manufacturing of Multi-Component Polyurethane Mouldingsa

Macromol. Mater. Eng. 2000, 284/285, 1–7 1

Full Paper: Multi-component injection moulding technologies are gaining constantly in importance for thermoplastics processing, in particular the gas-assisted injection moulding (GIM) technique. These multi-component moulding concepts could also offer a great potential of application to the manufacture of polyurethane (PU) parts. However, due to the significant differences between the material behaviour of thermoplastics and reactive PU systems with their coupled chemical and physical processes the transference of the experiences gathered in thermoplastics injection moulding has to be regarded critically. Therefore, focussing on the gas-assisted reaction injection moulding of hollow parts, designated as GRIM technique, numerous experimental investigations concerning the most significant process parameters gas delay time, mould temperature and gas pressure has been performed. The Representative voluminous GRIM-mouldings. main results of these process investigations are reported.

Multi-component reaction injection moulding (MC-RIM): A processing technology for the manufacturing of multi-component polyurethane mouldingsa

E. Haberstroh, I. Kleba* Institut fu¨r Kunststoffverarbeitung, RWTH Aachen, D-52056 Aachen, Germany

Introduction subject of this article. The processing technology under Multi-component injection moulding techniques are well investigation is rather related to the sandwich and gas- established for the processing of thermoplastics. In general assisted injection moulding process. they can be subdivided in processing techniques which The sandwich moulding or co-injection process, respec- combine two or more thermoplastics with each other and tively, is characterised by a short shot of the cavity with a techniques where a gas as second component is used. Con- first thermoplastic component. Afterwards a second ther- cerning the former it can be further distinguished between moplastic melt is injected into its centre such that the final the connecting injection moulding technique and the sand- part has a sandwich construction. The sandwich moulding wich moulding process. Processing techniques for polyur- techniques show advantages, for example, concerning the ethanes (PU) which are similar to the connecting injection manufacturing of moulded parts with good mechanical moulding technique are already in series production. The properties and simultaneously good surface quality by most known application of such a processing technique is combining a short fibre reinforced core material with an the manufacturing of flexible multi-section foams for non-reinforced skin material. Furthermore rigid/soft com- automotive seat cushions.[1, 2] This so-called dual hardness binations enable technical parts with a soft-touch feel. process is used to combine two different kinds of PU foam However, the most widely disseminated material combina- systems or formulations in one moulding process to realise tion in industrial practice is a foamed core and a compact a soft seat of bearing and less flexible supporting side skin to reduce raw material consumption and weight. zones. However, this kind of processing techniques is not Another interesting multi-component technique for thermoplastics is Gas-assisted Injection Moulding (GIM) a Presented at the 20th Plastics Technology Colloquium, IKV which allows to manufacture hollow parts or mouldings Aachen, Germany, March 22–24, 2000. with partial hollow areas. In this case after the short shot a

Macromol. Mater. Eng. 2000, 284/285 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1438-7492/2000/0112–0001$17.50+.50/0 2 E. Haberstroh, I. Kleba

gaseous second component is injected into the centre area cing parameters of a stable multi-component process is of the thermoplastic melt. This gas injection technique is the viscosity development within the pre-injected skin state of the art for thermoplastics and well established in component. For thermoplastics the change of viscosity several productions. It is one of the most promising special during the moulding process is mainly influenced by injection moulding technologies for processing thermo- cooling effects of a hot melt in a cooled mould. This soli- plastics and continues to gain market shares. For example, dification behaviour leads to a lower viscosity and hence between 1997 and 1998 a growth in use of 10% by North flow resistance in the middle area of the cavity so that American moulders has been stated.[3] Once limited to due to the principle of minimal energy consumption – thick-walled mouldings, nowadays this processing tech- assuming a lower viscosity of the core component in case nology is applied to manufacture a broad range of plastic of the sandwich moulding technique – a centred propaga- products including complex and thin-walled parts. Con- tion of the core material is preferred as desired. In the cerning the course of the process and the quality-relevant opposite the moulding of reactive PU systems is influ- process parameters it can be fallen back on comprehensive enced by an exothermic chemical reaction coupled with practical experiences as well as on numerous experimental physical processes. Thus, the rheological behaviour of and numerical studies, e.g., ref.[4–11] PU systems is depending on temperature but also on the As its main advantages, raw material and weight reduc- degree of conversion. This so-called rheokinetic or tion at simultaneously good mechanical properties as well chemo-rheological material behaviour leads to rather as shorter cycle times can be cited. Moreover, this injec- complex viscosity profiles inside the mould depending on tion moulding technique allows a reduction of energy con- time, mould temperature and the reaction kinetics of the sumption due to the comparably low pressure requirement PU system. and a minimisation of warp and orientation phenomena as Moreover, PU systems are generally characterised by a well as the compensation of sink marks due to the possibi- significant lower viscosity level in the first stage of the lity to apply an internal holding pressure through the gas reaction compared to thermoplastic melts. Since it is channels. Moreover, recent investigations have shown that obvious that the realisation of a, for instance, stable gas the gas injection technology is capable to realise parts with bubble propagation without a gas breakthrough is easier in functional hollow spaces, e.g., media pipes. This could be higher viscous liquids the PU GIM process can be demonstrated for injection moulding of thermoplastics as expected to be more difficult to control. This underlines well as hot-curing liquid silicon rubber (LSR).[12, 13] the significance of the pre-reaction phase. One conse- The mentioned advantages of the multi-component quence of the lower viscosity is that a short shot and injec- technologies already indicate the great potential of appli- tion of the core component against the gravity is preferred cation of such a moulding technique for the processing of as will be described later. reactive polyurethane (PU) systems – in particular taking Against this background Knipp has proposed a proces- into account that polyurethanes are known for their broad sing concept which is based on the use of an accumula- range of material properties which can be varied from soft tor.[14] This accumulator serves as a pre-reaction chamber and elastic to hard and stiff by choosing different initial which allows the skin component to reach a sufficient visc- components and compounding materials. Hence, the com- osity level before it is injected into the cavity. Moreover, bination of these material advantages with the processing the use of PU systems which show a marked thermoplastic potential of the multi-component techniques could not material behaviour during their curing reaction is sug- only lead to an improvement of existing PU processes and gested to achieve similar cooling effects as for the proces- products but also to interesting new PU applications which sing of thermoplastics. Based on this processing concept cannot be realised with one single PU system. Against this numerous experimental investigations have been per- background the development of a multi-component reac- formed for both a gaseous core component and a second tion injection moulding technique (MC-RIM) for polyur- reactive PU mixture as core material. In accordance with ethanes is subject of a current research project. the designations for the multi-component techniques for thermoplastics these two process variants of the MC-RIM concept were named Sandwich-RIM and Gas-assisted Pre-considerations concerning the PU specific (G)RIM. In the following this article mainly focuses on the material behaviour development efforts concerning the GRIM technique. Due to the significant differences in the material behaviour during the moulding process between thermoplastics and reactive polyurethane systems the transference of the experiences gathered in injection moulding of ther- Pre-investigations and description of the moplastics has to be regarded critically. This becomes process more obvious on closer inspection of the rheological In pre-investigations the feasibility of the GRIM proces- material behaviour. One of the most important influen- sing concept could be proved for a stocky tubular part Multi-component reaction injection moulding (MC-RIM) ... 3

Results of the experimental process investigations

Procedure, set-up and quality criteria Based on the findings of the pre-investigations the GRIM process has been investigated in detail and optimised for rod shaped mouldings with circular and rectangular cross-sections using the same non-foaming, very slow reacting PU system – an experimental product of Bayer AG, Leverkusen (Germany) – which is characterised by a marked thermoplastic material behaviour during the poly- Fig. 1. Course of process of the Gas-assisted Reaction Injec- merisation reaction. As gaseous core medium compressed tion Moulding (GRIM) technique. air has been used. Dosing and mixing of the single components of the PU system were performed manually and the injection of the mixture into the mould was done geometry (D L 36 mm, L = 144 mm). However, a suffi- using a pressure pot or a plunger injection unit, respec- cient part quality, that is a constant wall thickness of the tively. The degree of mould filling was found to have no PU skin layer over the length of the gas bubble, could not significant influence on the moulding process. For the material under investigation the degree of mould filling be achieved in this early stage of the process develop- L ment. Nevertheless, it was shown that the proposed accu- which prevents a gas breakthrough was found to be FG mulator is not necessary; on the contrary a pre-reaction of 65%. Against this background this value was assessed for the PU mixture within the mould for a certain period of all further investigations. The geometrical boundary conditions have been varied by using a modular mould time – the so-called (gas) delay time, td – was found to be essential to realise a hollow PU part. Moreover, the (Fig. 2) and different mould inserts which allow to manu- pre-investigations have indicated that due to the low vis- facture rod shaped parts with a diameter of D = 5–30 mm cosity of PU a gas injection in direction of the gas bubble (circular cross-section) and an edge width of B = 40– propagation and against the gravity is preferred to enable 60 mm (rectangular cross-section). The length of the parts a stable mould filling process. was kept constant with L = 500 mm. The resulting single processing steps of the GRIM technique are schematically shown in Fig. 1. In accordance with the GIM process for thermoplastics the first step is a short shot with a reactive PU mixture, characterised by the degree of mould filling FG. After the (gas) delay time td, which allows the necessary pre-reaction of the PU mixture and the generation of an adequate viscosity level, the gas is injected into the core area of the PU mixture. During this process the gas bubble propagates inside the centre of the mixture while due to the fountain flow at the flow front the PU skin material is displaced from the inside to the boundary area of the cavity. After the cavity is filled volumetric Fig. 2. Modular test mould with exchangeable mould inserts. a gas holding pressure is applied until an appropriate degree of curing is reached. After this holding pressure The main focus of the experimental investigation of the time, th, the gas pressure is released and the part can be GRIM technique was the identification of the influence of demoulded. the mould temperature cm and the gas delay time td on the For the Sandwich-RIM technique the course of the pro- moulding process and the gas bubble structure. Moreover, cess is quite similar. Also for this process the pre-reaction the influence of the gas injection pressure was analysed. phase after the short shot during the delay time maintains As the main quality criterion the thickness of the PU skin its importance. After the delay time the second PU system of the final hollow article – the so-called residual wall is injected instead of the gas. In the following both PU sys- thickness, r – at different cross sections over the length (or tems are cured at least until the skin material reaches its height) of the gas bubble was determined. Furthermore, dimensional stability. Certainly, a direct application of an the specific residual wall thickness rspec., which is defined internal gas holding pressure is not possible with this tech- as the ratio of residual wall thickness and radius of the part nique. However, this could be realised by using a foam R = D/2, as well as the means r9 and r spec.9 have been used to system as core material. characterise the part quality. 4 E. Haberstroh, I. Kleba

Influence of the gas injection pressure The investigations concerning the variation of the gas injection pressure (0.1–1 MPa) or the velocity of gas bubble propagation, respectively, have shown that in the entire pressure range a stable gas bubble propagation is possible. However, at a pressure level of 0.1–0.3 MPa a comparably large deviation from the mean residual wall

thickness was observed while for pgas A 0.3 MPa the parts show a more homogeneous residual wall thickness over the entire length of the gas bubble with no significant influence of the pressure on the mean residual wall thickness. This means that a higher gas pressure or a faster gas bubble propagation improves the part quality. Against this background for all further investigations a gas pressure of 0.5 MPa was applied.

Influence of the (gas) delay time Fig. 3 shows the specific residual wall thickness as a function of moulding diameter D (circular cross-section) for different delay times as well as the appropriate maxi-

mum and minimum values (cm = 608C). It can be seen

that at a very long delay time of td = 360 s and for larger part diameters (D F 15 mm) the deviations from the Fig. 3. Influence of the delay time on the specific residual wall mean residual wall thickness are small. In contrast to this thickness for different moulding diameters (cm = 608C, pgas = 0.5 for smaller moulding diameters a gas injection even at MPa). higher pressure levels was impossible. This can be attributed to the fact that the ratio of cavity surface to cavity volume increases with decreasing moulding diameter. As wall thickness over the length of the gas bubble for some a consequence the PU mixture experiences a faster representative samples. Once more this manner of repre- increase of viscosity over the entire cross-section due to sentation illustrates the constancy of the residual wall an increasing influence of the mould temperature on the thickness over the entire length of the gas bubble for a curing rate such that the PU mixture looses its flowability. wide range of the investigated gas delay times. Remark- Moreover, it could be observed that for D = 8 mm a gas able is the exceptional good part quality for moulding dia- delay time of 300 s already leads to a very slow gas bub- meters of D f 10 mm at a short delay time of three min- ble propagation and to a significant decrease of the resi- utes. However, for the voluminous part with a diameter of dual wall thickness from the beginning to the upper end 30 mm the formation of a skin material accumulation in of the gas bubble. Therefore, the upper limit of the pro- the bottom area of the part (xL L 0–200 mm), which con- cessing window in time is reached. tains a secondary gas bubble, can be observed at short At a medium delay time of 270 s a good gas bubble delay times. Moreover, a significant decrease of the resi- structure with a small deviation from the mean residual dual wall thickness from the bottom of the main gas bubble wall thickness can be observed for all investigated cross- to the top end can be seen. These phenomena will be sections (Fig. 3, middle diagram). A similar observation explained in detail later on. was made for a short delay time of 180 s and moulding dia- Fig. 5 shows the mean residual wall thickness as a func- meters smaller than 15 mm (Fig. 3, lower diagram). For tion of gas delay time for the different moulding diameters moulding diameters larger than 15 mm and short delay under investigation. It can be seen that for DB F 12 mm the times a significant increase of deviation of the residual mean residual wall thickness shows the tendency to wall thickness with increasing moulding diameters can be decrease with increasing delay time while for smaller observed. This phenomenon can be attributed to a too short cross-sections the residual wall thickness seems to be con- pre-reaction phase and hence too low viscosity of the mix- stant. The phenomenon of decrease of wall thickness with ture when the gas is injected. In this case the lower limit of increasing delay times is a significant difference compared the processing window in time is reached. to the processing of thermoplastics where due to the pro- A better visualisation of the quality of the hollow parts gressive cooling and solidification of the melt from the allows Fig. 4 which shows the distribution of the residual outside to the inside the opposite effect can be observed. Multi-component reaction injection moulding (MC-RIM) ... 5

Fig. 6. Influence of delay time on the residual wall thickness for voluminous mouldings (cm = 608C, pgas = 0.5 MPa, width of moulding B = 40 mm).

already mentioned skin material accumulation in the bottom area of the part in front of the gas nozzle containing a secondary gas bubble at a very short delay time of 225 s. This phenomenon can be attributed to the increasing influence of gravity. It can be expected that during and after the gas injection process the PU mixture, which has a rela- tively low viscosity due to the short pre-reaction phase, flows back to the bottom of the part and in front of the gas injection gate. On the one hand this back flow leads to the generation of the mentioned secondary gas bubbles during Fig. 4. Distribution of the residual wall thickness over the the gas injection process. On the other hand, in the first length of the gas bubble for different delay times and moulding stage of the holding pressure period the back flow due to c 8 diameters ( m = 60 C, pgas = 0.5 MPa). the still low viscosity level leads to the observed accumulation of skin material. The mass flow rate of this back flow phenomenon is conversely proportional to the viscosity but proportional to wall thickness to the power of three. Hence, at the same viscosity parts with a larger cross-section show the tendency to flow back earlier than thinner mouldings. As a consequence for voluminous mouldings a longer delay time and hence higher viscosity level is necessary to reach a homogeneous gas bubble structure. In this case an increase of delay time of 45 s is already sufficient to prevent the back flow effect. To sum up it can be said that at long delay times the processing window in time is limited by an upper viscosity level and the resulting flow resistance. For smaller cross- sections the upper limit of the processing window concern- Fig. 5. Influence of the gas delay time on the mean residual ing the delay time is reached at shorter delay times. This c 8 wall thickness for different moulding diameters ( m = 60 C, pgas can be explained by the strong influence of the mould tem- = 0.5 MPa). perature on the reaction kinetics and the change of viscosity for thinner parts. In contrast to this the lower limit is Hence, the reason for this finding has to be attributed to effected by an increasing influence of gravity with the PU specific rheokinetic material behaviour. decreasing viscosity level. This process behaviour confirms Fig. 6 which shows the mean residual wall thickness and its minimal and maximal deviation versus the gas delay time for the rectangular tubular part with an edge width of B = 40 mm. With an Influence of the mould temperature increasing delay time a decrease of the mean residual wall Similar to the delay time the mould temperature cm has a thickness as well as of the deviation can be observed. significant influence on the reaction kinetics, the tem- Moreover, the photography shown in Fig. 6 illustrates the perature development and hence on the viscosity rise. 6 E. Haberstroh, I. Kleba

Fig. 7. Mean residual wall thickness as a function of moulding diameter for different mould temperatures (td = 270 s, pgas = 0.5 Fig. 8. Local extension of the gas bubble for voluminous MPa). mouldings at lower mould temperatures (rectangular cross-section, B = 40 mm, td = 265 s, pgas = 0.5 MPa). Fig. 7 shows the mean residual wall thickness as a function of moulding diameter for two different mould temperatures of 20 and 608C and a medium delay time of

270 s. As mentioned above for cm = 608C the gas bubble shows a good quality with a low deviation of the residual wall thickness from the mean for each of the investigated cross-sections. A similar good part quality could be achieved for mould temperatures of 45 and 758C. In contrast to this at a marked low mould temperature of 208C just the mouldings with a diameter of D f 12 mm have a comparably good quality. For larger moulding diameters significant fluctuations of the residual wall thickness can be observed, especially for the largest cross-section of D = 30 mm. In this case an uneven gas bubble structure in the bottom area of the part can be seen. Moreover, a sig- Fig. 9. Influence of the mould temperature on the mean resid- nificant decrease of the residual wall thickness over the ual wall thickness for different moulding diameters (td = 270 s, p = 0.5 MPa). length of the gas bubble was observed. These phenomena Gas can also be attributed to the above described back flow effect due to a slower reaction and lower viscosity level under these mould temperature conditions. However, in progressive displacement of the higher viscosity core this case the hindrance of the curing rate and viscosity material, this “fresher” mixture is continuously cleared increase is restricted to the outer zone of the part or cav- away due to the shear flow resulting in the observed gas ity, respectively. bubble extension when it passes this area. In the opposite For the larger rectangular cross-sections a further inter- at higher mould temperatures a more homogeneous mix- esting effect could be observed. As can be seen in Fig. 8 at ture can be expected since the mould temperature was low mould temperatures the moulded parts show a local found to be in the range of the temperature level in the decrease of residual wall thickness in the height of the middle area of the residual part. Hence, in this case the

mixture level of the short shot (xL L 325 mm). This exten- extension phenomenon is absent. sion of the gas bubble decreases with increasing mould A further result of the process investigations concerning temperatures and is absent for mould temperatures higher the influence of mould temperature is that below a mould- than 608C. An explanation for this effect is that due to the ing diameter of 15 mm the mean residual wall thickness thermal conditions – low mould and therefore air tempera- seems to be independent of the mould temperature while ture on top of the mixture level, exothermic heat genera- for D F 15 mm an increase of the residual wall thickness tion in the middle area of the moulding – the material close with the mould temperature can be observed (Fig. 9). This to the mixture level has a comparably low temperature behaviour can be attributed to the faster reaction and visc- which results in a slower viscosity rise as a consequence of osity growth in the outer zone of the cavity while for smal- a slower reaction compared to the material in the middle ler cross-sections the mould temperature is influencing the area of the residual part. At the beginning of the gas injec- development of viscosity over the entire cross-section tion process this low viscosity mixture is displaced to the resulting in more homogeneous properties of the PU mix- mould walls. During the gas bubble propagation and the ture. Multi-component reaction injection moulding (MC-RIM) ... 7

Conclusions introduction of air to the initial components. Moreover, this processing concept could allow to manufacture parts The process investigations have shown that for various with functional hollows, e.g., parts with media flow chan- cross-section dimensions the realisation of a gas injection nels or cable ducts. process for reactive PU systems – designated as Gas- The potential of this technology is not yet exhausted. assisted (G)RIM – is possible. Depending on the proces- For example, the combination of the GRIM concept and sing parameters an astonishing good part quality can be the Sandwich-RIM technique, which was also realised by obtained. Fig. 10 shows two examples of representative experimental investigations with a rather promising result, voluminous GRIM-mouldings which where manufac- is thinkable. However, the future has to show to what tured under ideal process conditions. As the main influen- extent the ongoing investigations can prove the practical cing process parameters the gas delay time and the mould benefit of the MC-RIM technique. temperature were identified. This can be attributed to the fact that both of them have a significant influence on the reaction kinetics and hence the viscosity increase. Against this background a detailed analysis of the rheoki- Acknowledgement: The investigations set out in this report netic material behaviour during the pre-reaction phase is received financial, material and advisory support from Bayer of high importance and subject of the current investiga- AG, Leverkusen (Germany), to whom we extend our special tions. thanks. For the provision of machines and machine parts as well as for the advisory support we would like to express our special thanks to Hennecke GmbH, Sankt Augustin (Germany). For material support we would like to thank ACMOS Chemie GmbH, ISL-Chemie GmbH and RheinChemie GmbH (Germany).

Received: September 13, 2000

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