Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments

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1 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments S. Kiatmanaroj, V. Goodship and G.F. Smith Advanced Technology Centre, Warwick Manufacturing Group, University of Warwick, Coventry, CV4 7JL, UK Received: 16 September 2003 Accepted: 28 November 2003 ABSTRACT Thermoset co-injection moulding is a novel technology that might be an alternative for thermoset recycling. This paper describes some experiments on the sandwich moulding of two polyesters, a powder coating and a BMC using a new special design thermoset co-injection machine and manifold system. The first thermoset sandwich moulding has been made and a number of initial moulding problems are presented and discussed. 1. INTRODUCTION Sandwich injection moulding technology is one of the multi-material injection moulding processes. The machine is capable of producing two-material plastic components by injecting the polymer melts from two separated barrels in a suitable order. Generally, skin material is injected first and then followed by core. Once the skin melt touches the mould wall, a frozen layer is formed, which allows the core melt to flow through. Some more skin is injected again to seal the moulding. By doing this, the skin will encapsulate the core completely at the end of the process cycle. Hence, the moulding will appear as if it is made of only one material. This technology is very successful in the plastic market since a lower cost component can be made of a cheaper or recycled material covered with a virgin one. A soft feel surface moulding injected with a stronger core material or a smooth surface part with foam core are also possible (1). Another interesting application is in-mould coating plastic. By using a thermoplastic paint as skin, colour coated mouldings can be produced without being sent to the paint line (2). Since the technology was invented, many research papers have been published. Most of them focussed on how to control the skin and core distribution and how to get adequate interfacial adhesion between the skin and core layers by looking at the effect of processing parameters such as viscosity ratio, injection time and speed and the amount of compatibilisers (1,3-11). The simulation works Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

2 S. Kiatmanaroj, V. Goodship and G.F. Smith used that information to predict the formation and shape of the sandwich mouldings. It was successful for some simple geometry moulds, but not for the more complicated ones (7,12-14). As mentioned previously, the technology is very successful today, but is limited to thermoplastics. However, the idea of sandwich injection moulding is also possible for thermosets, even though it might be more difficult since controlling temperature and cross-linking may be a problem. The use of recycled materials as the sandwich s core is very interesting in terms of thermoset recycling. Apart from the two recycling methods that have already been used, which are (i) mixing some regrind material with the pure polymer (15-17) and (ii) reusing the glass fibre obtained by combustion of the polymer in a fluidised bed (18), we now have the sandwich technique as a third choice. The technique has the advantage that a greater percentage of recyclate can be used in the core without leading to a strength problem, since there is almost no effect on impact strength if the core content varies (19). Another useful application of the sandwich technology is to coat a glass-filled thermoset with a smooth material or paint. As with the thermoplastic case, a coloured component can be made in one cycle without the need for a paint line. At present, no research work has been done on injecting two thermoset materials together. Hunold (20) presented the first conference paper on the sandwich injection moulding of different types of material, thermoset and thermoplastic, and thermoset and rubber. The first sandwich moulding was a combination of a phenolic skin and a polyethylene core. Nevertheless, there were some heat-related problems, which require more development using improved compounds, mould and machine technology. To co-injection mould thermosets, their temperature, curing rate and flow properties are very important. This study presents a new manifold system designed for the co-injection of thermoset materials and for overcoming the problems experienced in making a thermoset sandwich moulding. The result is analysed by considering the system and the materials properties. 2. EXPERIMENTAL 2.1 Machine and tooling A Battenfeld thermoset co-injection moulding machine, BA2000/ BK equipped with the UNILOG 9000C control system has been especially 172 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004

3 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments designed and built for this project. It consists of a horizontal stuffer-feed injection unit suitable for bulk moulding compounds and a vertical hopperfeed injection unit for granular materials. Between the machine injection unit and mould, a new manifold system (shown in Figure 1) has been installed to work as a gate channel where the two injected polymers meet at the mould sprue. PDE cartridge heaters and type-j thermocouples in both halves of the manifold are used to control the operating temperature. A central sprue, tray shape mould of dimensions 312 x 242 x 2.5 mm was used in this experiment. It provides a good flat surface with bending curves at its four sides. As with the manifold, heaters were also placed inside both mould halves in order to heat the materials during the curing process, which causes the plastics to cross-link. Insulation plates are placed between the mould and the machine to prevent heat transfer from the mould, thus keeping the mould temperature in the desired range. Around the sprue area are water jackets, which help reduce the heat transfer from the material in the mould cavity to the manifold part. 2.2 Materials The materials used in this experiment were a general grade bulk moulding compound, BMC G7B 5580 from BIP Ltd as a core material and a black powder coating, Interpon PM F from Akzo Nobel as a skin material. Figure 1. a) The manifold for 2K thermoset injection moulding machine. Both halves have been placed between the injection unit and the heated mould. b) The cross-section of the manifold nozzle Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

4 S. Kiatmanaroj, V. Goodship and G.F. Smith The resin used in the BMC was a glass filled unsaturated polyester with a density of 1.73 g/cm 3. This material is normally injection or compression moulded to produce engineering and electrical components. Interpon is an unsaturated polyester in-mould coating, normally used with BMC in compression moulding technology. Its density is 1.25 g/cm Gel time and melt flow index measurement The gel time and flow ability of Interpon had been examined prior to the injection moulding experiment with BMC, to establish its processing window. 5 g of powder coating was put onto a hot plate and levelled to around 1 mm thickness. With variable temperature from C, the gel time was measured from the time that the material started to melt until it turned to gel, which was determined by a metal stick. Uncured material would show plastic behaviour and could be picked up without breakage. Figure 2 shows the effects of temperature on the material gel time. The gel time of Interpon decreases dramatically as the temperature increases. At a very high temperature, more than 120 C, the melt takes only few seconds to turn into gel. The three temperatures, 80, 85 and 90 C were selected from the gel time curve since the material could be in liquid state for what was considered a reasonable processing window of seconds. The material Figure 2. The effect of temperature to the powder coating gel time 174 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004

5 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments was then put into a Ray-Ran 4MPCA melt flow indexer to examine the material s flow characteristics. By selecting a multi-slicing mode, the MFI value was measured ten times at ten different points, giving an average MFI value and the standard deviation. Table 1 shows the effect of temperature on the material MFI value. Table 1. The powder coating melt flow index (g/10 min) and standard deviation at three temperatures T emp ( C) MFI STD At the higher temperatures the material tends to flow more easily, as is shown by the higher MFI numbers, but the time the material can stay in a liquid state is shorter. The material tends to cure more easily at the higher temperatures too, as shown by an increase in the standard deviation value. A melt temperature of around C was selected as the temperature window of the skin material. 2.4 Injection moulding of BMC and Interpon To start, it was decided to mould BMC alone to set up a suitable manifold temperature and sprue temperature. The molten BMC would be injected from the horizontal barrel through the manifold runner and then flow into the mould, while the top runner was blocked to prevent the melt escaping from the system during injection. The manifold system was initially designed to operate at a sufficiently high temperature to cure the whole runner after the mould had completely filled, and then the part would be removed at the same time as ejecting the component from the mould. Several tests showed that the plasticised polymer tended to cure in the runner and sprue before it could flow into the mould, since the sections there are too small in comparison with the component (Figure 3). The problem would be worse when the top runner was in use, because of a smaller runner diameter. Consequently, the material in the manifold part was preserved in its molten state during the moulding cycle by setting the manifold temperature to 80 C. Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

6 S. Kiatmanaroj, V. Goodship and G.F. Smith Moulding Manifold runner Figure 3. A side view of the tray moulding and manifold runner To separate the manifold from the high temperature mould, water jackets were placed around the mould sprue and the temperature was varied from 90 down to 70 C (this temperature is the outlet temperature of the cooling water), where a full moulding could be made without gate freeze-off. The machine was set as shown in Table 2 for the BMC according to the supplier s recommendation. Figure 4 shows the complete moulding made of BMC. The best temperature for moulding this material was around 74 C. Using the settings shown in Table 2 both BMC and Interpon were injected into the mould to make the first thermoset sandwich moulding. The skin and core metering strokes were set at 80 and 50 mm respectively, and the switch-over point was at 30 mm. The sandwich moulding of the two thermosets is shown in Figure 5. Only half of the mould could be filled and core material broke through at the centre. With the two materials moulded together, the melts were cured, hardening and blocking the flow before the entire mould could be filled. It was uncertain where the melt started to cure and whether the powder coating affected the flow properties of the BMC. 176 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004

7 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments Figure 4. The component made of BMC a) b) Figure 5. a) The 2K thermoset moulding. b) The sandwich moulding and the complete tray Table 2. Settings for sandwich injection moulding of BMC and Interpon Settings/Material MC G7B 5580 ( 21) B Interpon* M ould Temperature ( C) B arrel temperature ( C ) 35/ 35 (front/rear) 80/ 60 (front/rear) Screw rotational speed (rpm) Back pressure (bar) < 3 0 Injection speed (mm/sec) * The material melt temperature is around C under this setting condition Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

8 S. Kiatmanaroj, V. Goodship and G.F. Smith Interpon was single-injected from the top barrel using the same condition applied to the above dual injection moulding, in order to study its flow in the mould. The result is shown in Figure 6. The amount of Interpon material that could flow into the mould was even lower than for the BMC and the sandwich mouldings. Compared with the result in Figure 5, it was the BMC melt that helped push more Interpon into the mould before the gate was frozen off. In other words, Interpon retarded the flow of BMC in the sandwich moulding. The difficulty of injecting the Interpon might be caused by the material properties themselves or by the design of the machine system. The next two sections focus on how these factors might affect the moulding. 2.5 Effect of material properties Gel times of the two materials were compared and the results are shown in Figure 7. This shows how fast those materials reacted to the temperature changing. At a temperature above 90 C the BMC tended to cure faster than the Interpon, while the opposite happened at lower temperatures. All temperature settings of the manifold and mould tools were set at around 80 C or higher, which means there was a greater chance that the BMC could turn solid, rather than Interpon. In reality, the Interpon was cured first and caused the short shot problem already described in the previous section. So, Figure 6. The moulding made of Interpon 178 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004

9 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments Figure 7. Gel time curves of BMC and Interpon other factors apart from the temperature must have affected the cross-linking reaction during the moulding. In Figure 8 the viscosities of both materials were compared. The relationship between the viscosity and shear rate of Interpon at 85 C was measured using a Rosand rheometer RH7, while the viscosity of BMC was determined by an in-line viscosity measurement, provided by the supplier (22). The viscosity of Interpon was higher than that of the BMC at any shear rate, i.e. Interpon was more viscous and so more difficult to inject at the same settings as those used for BMC. At the same injection speed and the same volume, more shear friction can be generated at the tool wall and produce excess heat. This heat might be a factor in activating the curing reaction. 2.6 Effect of the manifold system The manifold was designed to be a gate for the two materials to flow through and form a sandwich layer prior to entering the mould sprue. The complete plastic part, including the manifold, is shown in Figure 9. The skin runner was designed to be quite long, as there was a design limitation Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

10 S. Kiatmanaroj, V. Goodship and G.F. Smith Figure 8 The relationship of viscosity and shear rate of BMC and Interpon Mould Skin runner (Interpon) Core runner (BMC) Manifold Figure 9. The complete moulding part including the component and the manifold runners 180 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004

11 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments in moving the top barrel down to the sprue position. Its diameter was 6 mm (to reduce the plastic scrap from the injection moulding) while the core runner was 6.5 mm diameter to match the nozzle of the horizontal injection unit, which was easily reached from the back. Consider the manifold geometry. The skin has to flow to a longer and smaller runner than the core. At the same injection rate, this generates more shear friction. Especially when a viscous material like Interpon is injected through the system, more frictional heat is likely to be produced. The effect this has on the curing of the material in the mould can be seen from Figure 6. During the single injection moulding, the Interpon was cured almost as soon as it touched the mould wall, even though the system temperature was kept as low at around 80 C, which should have provided enough time for the material to be able to flow in and fill the mould. In contrast, the BMC could fully fill the mould under the same settings. More Interpon could be injected in the sandwich moulding, as shown in Figure 5. The improvement was achieved because the BMC was injected within one second of the Interpon injection. This retarded the Interpon melt cure, and together with the very low inlet temperature of the BMC, helped reduce the Interpon temperature so that the skin was still able to flow in. To increase the amount of Interpon in the mould it is necessary to decrease the melt temperature, or more precisely, to counter any effects that cause the melt temperature to rise. The two solutions that might be considered are (a) reducing internal shear friction by reducing the injection speed and (b) modifying the manifold runner and cooling the melt down by reducing the manifold temperature. The latter needs to be carefully adjusted as too low a temperature causes the material viscosity to increase. 2.7 Breaking through at the moulding s centre In Figure 5, looking at the middle front of the component where it is opposite the mould sprue, there was a breakthrough of the core material. At the back of the moulding, the core was fully encapsulated. The same moulding was sectioned and checked to see whether the core material could penetrate to the skin or whether it had accumulated only at the centre. The cross section is shown in Figure 10. The moulding shows a sandwich structure at the far end of the disk, with the skin layer getting thicker away from the centre, and it is thicker at the back than the front. When injection moulding with a tool such as this, the direction of Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

12 S. Kiatmanaroj, V. Goodship and G.F. Smith Sandwich area Core breakthrough Sandwich area Figure 10. The cross-section of the 2K thermoset moulding flow changes suddenly from horizontal to vertical. This results in very high pressure and shear at the moving half surface, especially opposite the gate. In order to fill the mould, the new material melt from the barrel pushes the previous material outwards and replaces it. Generally the previous material melt will leave a thin film, called a frozen layer, on the mould surface. This film is thicker in the case of thermoplastics, as the material can form the layer as soon as it touches the cold wall, while with thermosets it is molten. This results in a lower viscosity when it enters the hot mould, and then it becomes solid because of the cross-linking reaction. If the speed of injection is too high, there will be no time for the material to form that layer. When two thermosets are injected, the molecular flow is more complex, since the melt properties are a function of both materials. The core breakthrough at the centre shows that the skin (Interpon) did not have enough time to form a solid layer. At the time when the new melt was injected into the mould, the BMC melt swept out the skin. The moulding shows that the skin was pushed outward because of fibre abrasion and the higher density of the BMC melt. It was not until the cross-linking took effect that the skin could form the solid layer, when it had covered around half of the flow path. In order to provide more time for the skin to form the layer, the act of starting the core injection should be delayed. This can be done by increasing the delay time or reducing the switch-over distance. However, the core speed may have to be increased to force the core to penetrate into the skin as far as the edge of the moulding. Another possible way to increase the sandwich area on the moulding s surface might be to move the gate position to one of the edges of the moulding, as shown in Figure 11. The molecular flow is similar to that in the bottom half of the central gate moulding, but more uniform. However, more shear stress and cavity pressure may occur and the melt might be cured before the mould could be fully filled. These factors need to be carefully considered. 182 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004

13 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments Orientation at skin Scale (120 mm) Figure 11. The molecule orientation of a polymer at skin in a side gate mould (23) 3. CONCLUSION This paper describes some initial experiments on the sandwich injection moulding of two thermoset polyesters, and the problems are presented. Prior to the test run on the machine, the powder coating was investigated in terms of gel time and flow behaviour in order to optimise the temperature settings for the co-injection machine. The BMC was injected solely to set up the manifold system. The manifold temperature was chosen on the principle that the plasticised polymer was still in its molten state during all injection cycles. All information gained from those tests was used in determining the machine s initial settings for the moulding. The first thermoset sandwich component was produced. Two problems found from the experiment were (a) short shot resulted from pre-curing of the skin and (b) breakthrough of the core component at the position opposite the mould sprue. It was shown that the short shot problem was caused by the powder coating being pre-cured during injection. The excess heat generated in the system especially in manifold runner was the main cause. Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

14 S. Kiatmanaroj, V. Goodship and G.F. Smith The core breakthrough was the result of incorrect injection time and high pressure at the position opposite the gate. A longer core injection delay time or a shorter switch over distance will allow enough time for the skin to form a solid layer and cover the core. The simulation from Moldflow showed that changing the sprue position might help to achieve a more uniform flow. However, more experiments are needed to investigate this aspect further. A number of potential problems with co-injection moulding of thermosets have been highlighted by this work. It has been shown that balancing the shear rate, cure time and change in viscosity during co-injection are vital for successful thermoset sandwich moulding to be achieved. REFERENCES 1. R. Sélden, Journal of Injection Moulding Technology, 1 (4), (1997), G.F. Smith and R.A. Easterlow, Plastics, Rubber and Composites Processing and Applications, 25, 3, (1996), J.L. White and H.B. Dee, Polymer Engineering and Science, 14, 3, (1974), J.L White and B.L. Lee, Polymer Engineering and Science, 15, 7, (July 1975), P. Somnuk and G.F. Smith, SPE ANTEC 95, , Boston USA, (1995). 6. S.S. Young, J.L. White, E.S. Clark and Y. Oyanagi, SPE ANTEC, , New York USA, (1980). 7. D.J. Lee, A.I. Isayev and J.L. White, SPE ANTEC 98, , Atlanta USA, (1998). 8. R.Sélden, Polymer Engineering and Science, 40, 5, (May 2000), A. Derdouri, A. Garcia-Réjon, K.T. Nguyen and Y. Simard, SPE ANTEC 99, , New York USA, (1999). 10. W. Rungseesantivanon, PhD Thesis, University of Warwick Coventry UK, (December 2000). 11. V. Goodship and K. Kirwan, Plastics, Rubber and Composites, 30, 1, (2001), L.S. Turng, V.W. Wang and K.K. Wang, ASME (Heat and Mass Transfer in Solidification Processing), 25, (1991), S.C. Chen, K.F. Hsu, K.S. Hsu and M.C. Jeng, SPE ANTEC 93, 82-86, New Orleans USA, (1993). 14. G. Schlatter, J.F. Agassant, A. Davidoff and M. Vincent, Polymer Engineering and Science, 39, 1, (1999), Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004

15 Sandwich Injection Moulding of Thermosetting Materials. Part I: Initial Experiments 15. J. Petterson and P. Nilsson, Journal of Thermoplastic Composite Materials, 7, (January 1994), M. Gruskiewicz and K. Butler, Advances in Automotive Plastic Components and Technology, 104, (1995), L.A. Dzeskiewicz and J.D. Ralston, ANTEC 94, , San Francisco USA, J.R. Kennerley, N.J. Fenwick, S.J. Pickering and C.D. Rudd, SPE ANTEC 96, , Indianapolis USA, (1996). 19. R.C. Donovan, K.S. Rabe, W.K. Mammel and H.A. Lord, Polymer Engineering and Science, 15, 11, (1975), Dietrich Hunold, SPE Thermoset Topical Conference: Thermoset for The Molder, Research Triangle Park, NC, (12-14 March 2001). 21. BIP Limited, Product Data Beetle BMC Type G7B 5580, (Jan 2001). 22. M. Rides, In-line Measurement of Shear Viscosity Using an Injection Moulding Machine, National Physical Laboratory, UK, (1993). Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3,

16 S. Kiatmanaroj, V. Goodship and G.F. Smith 186 Progress in Rubber, Plastics and Recycling Technology, Vol. 20, No. 3, 2004