Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding

Transcription

1 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding B. Middleton WMG, University of Warwick, Coventry, CV4 7AL, UK Summary Thermoplastics and thermosets have very different temperature requirements which pose a problem when processing them together in multi-material parts. This paper examines the time available for the cure of a thermoset material in a thermoplastic injection moulding environment. It uses thermal equations to form a basic physical model of the temperature profile during the cooling stage within an injection moulding cycle. The model explores the different cooling times provided by using different thermoplastic substrates and the heat available from each of these to cure the thermoset component during a standard cycle. Thermal analysis of commercially available thermoset materials was undertaken and compared to the model results to determine if cure could be obtained inmould. It was found that the properties and processing conditions of PA66 provided the greatest heat exposure to the thermoset material; it was also found that temperatures within the mould are high enough to initiate cure however only within the proximity of the thermoplastic thermoset boundary. A result of this effect is to produce a novel in-to-out cure mechanism that produces a thermoplastic-thermoset-thermoplastic structure with the potential application as a barrier layer. 1. Introduction Developments into the processing of plastics mean that increasingly complex parts can be routinely produced. More recently, advances in the plastics industry are driven by environmental issues such as CO 2 emissions, energy consumption, sustainability, recycling waste and volatile emissions [1-4]. Injection moulding is one of the most important processing methods of thermoplastic parts, and like many other areas of plastics manufacturing, there Bethany.Middleton@warwick.ac.uk Smithers Rapra Technology, 2011 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

2 B. Middleton has been and continues to be research into developing the process further. Earlier research focussed on the benefits of producing multi-material parts utilising the properties of two distinct materials [5-7]; newer research focuses on removing post mould processes. Processes such as in-mould decoration, in-mould veneering and in-mould painting all remove post processing stages and as such result in savings in both cost and energy consumption (therefore CO 2 emissions) [8-10]. Developing a method for incorporating painting into the processing of a part would reduce the need for plastic paint shops [11]. In-mould painting by injection moulding could be carried out by spraying a powder coating in an open tool and then moulding onto it, however this would require masking large areas, resulting in a slow, messy process. A recent in-mould painting development involves modifying a mould tool to house a paint introduction system; this means the powder is applied in a closed mould and eliminates the problems mentioned earlier [11-13]. Powder coatings are usually thermosets as they are more durable; so when in-mould painting using a thermoset coating and a thermoplastic substrate there is a conflict with the moulding temperature requirements of the two materials. Ideally, the hot thermoplastic substrate would supply enough heat to cure the thin layer of thermoset coating and produce a complete painted in-mould. 1.1 Aims and Scope The overall aim of the paper is to determine whether in-mould painting, using a thermoplastic-thermoset material combination, can deliver a painted, moulded part in one stage with a fully cured film coating. This will be achieved by creating a model of the temperature changes occurring within the mould during a cycle; then using the model, a comparison of different materials and processing conditions can be made. In addition, thermal analysis of currently available thermoset materials will be undertaken. The results of both will be compared to see if adequate cure temperatures are achieved and maintained within the moulding cycle. 2. The Problem For the development of this model, the initial problem can be simplified to: A thermoplastic part moulded with one side coated by a thermoset film (see Figure 1) Moulded using the latest technique in in-mould painting 86 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

3 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding ABS used as the thermoplastic, a tough engineering plastic commonly injection moulded A processing temperature of 200 C and a tool temperature of 50 C; which are appropriate for ABS The thermoset film coating to be used is an unsaturated polyester; many such coatings are commercially available The part would be a small, sample moulding, dimensions approximately 7.5 cm x 7.5 cm x 0.3 cm, including a 50 μm layer of thermoset It is recognised that the model itself is not original work; however the novelty occurs in the application of it to an in-mould painting technique using a thermoplastic-thermoset material combination. The following assumptions have been made for this simple initial analysis. The effects of the boundary interfaces have been ignored; this is because the purpose of the model was to calculate the temperature and time available for thermoset curing in order to estimate working heat exposure times rather than develop an accurate and complex model of the system. However, the importance of considering boundary layer heat transfer coefficients in more complex models is acknowledged [14]. Due to the low thickness of the thermoset layer a further assumption was that by the time of thermoplastic injection the thermoset had reached the isothermal temperature of the mould tool; in the example given in Figure 2, this means a temperature of 50 C. One final assumption made was to ignore the use of geometric correction factors; in the research literature they vary widely [15, 16]. Again, these would need to be taken into account for more accurate modelling. Whilst the Figure 1. A representation of a single side painted sample injection moulded part Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

4 B. Middleton Figure 2. A diagram showing a cross-section of the thermoset-thermoplastic combination in mould simplicity of the model is therefore acknowledged, the author has found it extremely useful in exploring this process and its limits. Given the confusion and variance of cooling models available in the literature, to aid the reader, a greater than needed breakdown of the equations used has been provided. The cooling problem can be considered by a straightforward derivation of cooling times utilised in injection moulding production for obtaining ejection times [16]. From Figure 2, the initial problem is assumed to be: A 50 μm thickness of polyester thermoset powder coats the inside of an injection moulding tool at 50 C, the mould cavity has internal dimensions of 7.5 x 7.5 x 0.3 cm 3. An ABS substrate is injected at 200 C onto it, giving a moulding of total thickness 3 mm. From this statement, the values required for the model can be determined; values associated with the part dimensions, the processing conditions and the properties of the materials used. 3. The Model Table 1 gives the abbreviations of terms used in the equations used to create the model. An explanation of how the model was constructed follows. Equation (1) - Equation (5) were used to calculate how the temperature across 88 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

5 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Table 1. Terms used in equations Term Thickness of polymer Surface area of polymer Mass of polymer Polymer specific heat Polymer thermal conductivity Initial temperature of polymer Temperature of polymer at time, t Thickness of film Surface area of film Mass of film Film specific heat Film thermal conductivity Temperature of film at interface at time, t Film temperature at distance, x from interface at time, t Temperature of mould Time Distance across film from interface Symbol X P A P m p c P k P T P(0) T P(t) X f A f m f c f k f T f(t,0) T f(t,x) T M t x the thermoset film changed as the part cooled within the mould. The model starts with a cooling equation (Equation (1)); this shows how the temperature of the polymer at the surface changes over time. Equation (1) The surface polymer temperature as a function of time T P(t) = ( T P(0) T M )e k p A p m p c p X p t + T M (1) The temperature of the polymer can then be used in the conduction formula to give the rate that heat energy is transferred to the film (Equation (2)): Equation (2). The rate of heat transfer to the film as a function of the polymer temperature dq f dt = k f A f X f ( T P(t) T M ) (2) Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

6 B. Middleton Equation (3) shows how the rate of energy transferred to the film is related to the temperature gradient (over small increments) across the film; this equation can be rearranged in the form of Equation (4) which then in turn integrates with respect to x, to give Equation (5). Equation (3). The rate of energy transfer to the film as a function of the temperature gradient across the film dq f dt = k f A f dt f dx (3) Equation (4). The temperature gradient across the film as a function of the rate of energy transfer to the film dt f dx = dq f dt 1 k f A f (4) Equation (5). The temperature of the film (at time, t and distance x into the film) as a function of the energy transferred to the film and the temperature of the polymer. T f( t,x ) = dq f(t) dt x + T k f A P(t) f I (5) The equations can be manipulated further to give an overall solution to the differential problem (Equation (6)); the solution is a linear correlation across the depth of the film, the slope of which decreases over time. Equation (6). The temperature of the film (at time, t and distance x into the film) as a function of the mould temperature and the temperature of the polymer at time, t. T f( = x t,x ) X T T P(t) M f ( ) + T P(t) II (6) I T P(t) is the integration constant since at x = 0 (the thermoset-thermoplastic boundary), T f(t,x) is equal to T P(t). II Equation 2 is substituted into Equation 5 to give, T f(t,x) = k f A f cancel out to give equation 6. k A f f X ( T T P(t) M) x f k f A f + T P(t) )The terms 90 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

7 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Table 2 and Table 3 show the values used for the modelling of the initial problem. Table 2. The values of terms used that remained constant; values associated with geometry and the film material Film: Unsaturated Polyester Term Symbol Value Commonly Used Units SI Units Thickness of polymer X P 3 mm m Surface Area of polymer A P 120 cm m 2 Thickness of film X f 50 μm m Mass of film m f 1 g kg Surface Area of film A f 60 cm m 2 Film specific heat c f 1.2 [17] kj kg -1 K J kg -1 K -1 Film thermal conductivity k f 0.7 [17] W m -1 K W m -1 K -1 Table 3. Values of terms that were variable; values associated with the polymer substrate and the moulding conditions Polymer Substrate: ABS; Film: Polyester Term Symbol Value Commonly Used Units SI Units Mass of polymer m p 24.1 g kg Initial temperature of polymer T P(0) 200 C K Polymer specific heat c P 1.3 [17] kj kg -1 K J kg -1 K -1 Polymer thermal conductivity k P 0.18 [17] W m -1 K W m -1 K -1 Temperature of mould T M 50 C K A spreadsheet was created using the previously mentioned thermal relationships along with the data from Table 2 and Table 3; the results were as follows. The cooling of the injected thermoplastic was calculated using Equation (1), this data was then fed into Equation (2) in the spreadsheet and the results of which, the energy rate transfer curve, can be seen in Figure 3. Once the rate of heat transfer was established, Equation (5) was used to obtain the temperature gradient across the thickness of the thermoset film at any given time; the example given in Figure 4 shows the initial temperature gradient, at t = 0 s after injection. It is therefore possible to plot a graph showing the temperature gradient across the film at different times. This is shown for a model based on the Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

8 B. Middleton Figure 3. A graph showing the rate of energy transfer from the ABS substrate to the thermoset film over time Figure 4. The temperature profile across the depth of the thermoset film. 0 μm is the thermoplastic-thermoset interface and 50 μm is the thermoset-tool interface initial problem of ABS injected at 200 C with a mould tool temperature of 50 C in Figure 5. What is important to interpret from Figure 5 is the time period for which cure temperatures are maintained for and to what depths through the film. Over 92 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

9 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Figure 5. A graph showing how the temperature profile across the depth of the thermoset film changes over time half of the film is exposed to temperatures above 100 C for 15 s (as indicated by the shaded area in Figure 5); but by 1 minute, cooling has occurred to such an extent that only the film close to the thermoplastic interface remains above 80 C. If 100 C was the temperature required to initiate a curing reaction in the thermoset film, the exposure time to allow the occurrence of cross linking would be very short on thermoset timescales, especially as curing reactions are usually thought of in tens of minutes rather than seconds. Figure 4 and Figure 5 also highlight a further issue with this model in that the film surface contacting the mould tool never rises above 50 C; this is not the case in practice. Despite that, this estimation of exposure time is extremely useful for comparing the performance of both the thermoset and thermoplastic materials. In order to expose more of the film to higher temperatures for longer, the effect of raising the temperature of the ABS melt to 230 C was examined. Figure 6 shows the thermal model using the same materials but with the ABS injected at a temperature 30 C higher. Assuming as before a 100 C initiation temperature for the thermoset cure reaction is needed, the window for curing Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

10 B. Middleton Figure 6. A graph showing how the temperature profile across the depth of the thermoset film changes over time following the injection of ABS thermoplastic at 230 C is now opened to 30 s to a depth of around 22 μm into the film (again, this is indicated by a shading on Figure 6). Further variations of the model with different substrate materials, processing and tool temperatures were carried out with the variables shown in Table 4. These initial chosen temperatures reflect a general processing temperature of each of the selected thermoplastics. The substrate materials that were modelled are: Nylon 6 (PA6), Nylon 66 (PA 66), polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Each of the different substrate thermoplastics was modelled and data gathered regarding the temperature profile across the film at different times; graphs for each substrate material were plotted (similarly to Figure 5 and Figure 6). Regularly spaced heat exposure times were selected in order to compare the performance of each substrate material within the system; the selected heat exposure times were the arbitrary values: 100 C maintained for 30 s, 100 C maintained for 60 s and 150 C maintained for 30 s. The graphs were interpreted in order to determine to what depths these heat exposure times were maintained for using the different substrates. The results are shown in Figure Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

11 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Table 4. Values used to model different substrate materials Film: Polyester Term Symbol Values for different substrate materials PA 6 PA66 PBT PET Units Mass of polymer m p g Initial temperature of polymer T P(0) C Polymer specific heat c P 1.7 [17] 1.7 [17] 1.3 [17] 1.05 [17] kj kg -1 K -1 Polymer thermal conductivity k P 0.29 [17] 0.23 [17] 0.21 [17] 0.24 [17] W m -1 K -1 Temperature of mould T M C Figure 7. A graph comparing the film depth penetrations of heat exposure times using different substrates It is clear to see from Figure 7 that PA66 provides the best penetration depths for the heat exposure times selected and therefore is most likely to provide adequate heat for cure initiation within the mould. The data was examined further; for each individual substrate material, post injection times of 0 minute, 1 minute, 2 minutes and 3 minutes were selected, allowing a comparison of the temperature profiles across the thermoset film when injecting different substrates to be plotted. Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

12 B. Middleton Graphs comparing the differences in heat available through the thickness of the film provided by using different thermoplastic substrate materials are shown in Figure 8 and Figure 9; these show the temperature gradients across the film at 1 minute and 3 minutes after injection respectively. Figure 8 and Figure 9 indicate that using a thermoplastic substrate of PA66 gives the highest temperature profile across the thermoset film at any time during the cooling cycle. This is partly due to PA66 having the advantage of being able to be injected at a high temperature and ejected at a higher temperature than many other thermoplastics and also the fact that PA 66 has a high specific heat value, 1.7kJ.kg -1.K -1 [17]. The high specific heat value means that the material cools more slowly because it requires more energy transfer to reduce the temperature by 1 Kelvin than lower specific heat materials such as PET (this can be seen by comparing the insert graphs in Figure 9). This is also illustrated by looking at Figure 10 and Figure 11, which show the changing temperature profile across the thermoset film during the polymer cooling when injected with PA 66 (Figure 10) and PET (Figure 11) at 280 C in a tool temperature of 85 C. For PA 66, 100 C is maintained for one minute 37 µm into the depth of the film, for PET there is still a window of one minute where this temperature is maintained, however only to a depth of 28 μm (also confirmed in Figure 7). Figure 8. A comparison of the different temperature profiles across the thermoset film 1 minute after the injection of different thermoplastic substrates, the legend shows both the substrate and the injection temperature 96 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

13 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Figure 9. A comparison of the different temperature profiles across the thermoset film 3 minutes after the injection of different thermoplastic substrates, the legend shows both the substrate and the injection temperature. The inserts show the cooling curves of (A) PA66 and (B) PET; despite been injected under the same conditions, PET cools more quickly Figure 10. The changing temperature profile across the thermoset film when injected with PA66 at 280 C in a tool of temperature 85 C Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

14 B. Middleton Figure 11. The changing temperature profile across the thermoset film when injected with PET at 280 C in a tool of temperature 85 C 4. Suitability of Current Thermoset Coatings With the knowledge of the temperature profiles within the injection moulding cycle, it was necessary to find if a thermoset material could cure within this process window. Success depends on sourcing both a fast and low temperature cure. Commercial powder coatings are as yet not available specifically for the newest developments in in-mould painting. Commonly available powder coatings have a much longer curing requirement; for example, commercial unsaturated polyester thermosets such as Interpon 310 Silver and Interpon 600 (Akzo Nobel, Netherlands) have cure regimes of up to 20 minutes at 200 C and 8 minutes at 210 C respectively [18]. Three different samples of fast curing unsaturated polyester thermosets have been obtained (for propriety reasons the materials will be referred to as Samples 1, 2 and 3) and thermally analysed to test their suitability to the in-mould painting process. Each of the polyester powder coatings was tested using DMTA and DSC (see Figure 12 for an example of combined DMTA / DSC scans). The details of the tests are as follows: 98 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

15 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Figure 12. Thermal analysis of thermoset sample 2 DMTA Process: - Triton Technologies Tritec 2000 DMA machine - Powder pocket (to hold the powder sample) mg of sample - Dual cantilever mode - 2 C/min ramp rate DSC Process: - Polymer Laboratories PL STA 1500 machine - Disposable aluminium crucible mg of sample - Ramp rate of 10 C/min Scans performed on the DSC and DMTA were analysed and the average thermal behaviours of the samples are summarised in Table 5. The gel point was determined by extrapolating the endothermic trough on the DSC scan and by calculating the midpoint of the sharp decrease in the storage modulus on the DMTA scan. The cure onset temperature was determined by the point at which the storage modulus increased on the DMTA scan and the beginning of the exothermic cure peak on the DSC scan. Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

16 B. Middleton Table 5. A summary of the thermal properties of 5 commercially available polyester thermoset powder coatings Sample No. Colour Type Av. Gel Point ( C) Av. Cure Onset ( C) 1 White Polyester Brown Polyester Brown Polyester From Table 5 it can be seen that the most preferable coating material was Sample 2; with a gel temperature of 52 C, a film will form on the mould tool and a cure onset is at 110 C is low enough to be initiated within the mould cycle. If a substrate of PA66 is used it can be seen that from Figure 10 this gives a curing window of about 1 minute into 28 μm into the film; however, curing will occur as a one side cure, due to temperature differences on either side of the material. The thermoset material adjacent to the mould tool would remain uncured because, as shown by the model, the outside of the layer (thermoset-tool interface) does not reach sufficient temperature to cure; only material closest to the injected substrate reaches cure temperatures (this is illustrated in Figure 13). It is important to note that the thermoset cure reaction during inmould painting is proceeding in the opposite direction to that of a normal paint cure (as highlighted in Figure 14). In one case, cure initiates from the Figure 13. A representation showing the penetration depth of the cure temperature reducing as the PA66 substrate cools over time 100 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

17 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding Figure 14. Diagrams showing the reverse direction of the in-mould cure; a, shows the cure direction of a traditional injection moulded thermoset part; b, shows the cure direction of a post cured thermoset coated part and c, show the cure direction of an inmould painted part outside and perpetuates inwards (using mould tool heat or an external heat source) as opposed to that produced by injection moulding which initiates cure from the substrate side (from the inside towards the outer surface). The effect of this may cause quality problems as the exterior surface of the moulded part would be uncured. Following the moulding process, post curing could be carried out once the paint is applied, for example using UV or IR radiation. Alternatively mould tool heating methods could be employed to rapidly cycle the mould through higher temperatures; induction heating [19] is one such technology that could be used, however further discussion is beyond the scope of this paper. Due to the reverse direction of cure using an in-mould painting technique a novel sandwich structure is produced; a thermoplastic substrate lies below a thin thermoset layer which in turn is below a thermoplastic top layer (an uncured thermoset material is still thermoplastic in its behaviour). This is shown in Figure 15. Although as mentioned this is not preferable for a painting process, this could be utilised for other purposes, perhaps where a tough thermoset barrier layer is required below the surface. The nature of the thermoplastic-thermoset interface created within the film is of further interest; at this stage it is unclear whether the boundary is discrete, whereby either side the material is greatly different, or the change is gradual and crosslinking has occurred to varying degrees throughout the depth of the film. Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

18 B. Middleton Figure 15. Novel thermoplastic-thermoset-thermoplastic sandwich structure produced by the in-to-out cure mechanism; two possible variations in structure are shown. A distinct interface within the film is shown above and below indicates a gradual change Mouldings have been produced in order to compare to the results of the model but a method for testing cure below the surface needs to be developed. These samples will be analysed and the findings reported in a further paper. This research shows that already available powder coatings would have cure initiated within a moulding cycle but it is unlikely that the cross linking reaction will be complete due to the short time periods that sufficiently high temperatures are maintained for; an ideal thermoset powder coating for an in-mould painting process would have the following properties: 1. It must soften in the tool temperature range of the substrate: in this case this has been a range of C 2. Cure should initiate around 100 C (preferably lower) 3. The cure reaction should take approximately 1 minute 4. It must adhere to the substrate 5. It must release from the metal tool Note as the model shows, the thermoplastic material will remain at high temperature ( 200 C) for only a short period of time and maintain lower temperature at (~80 C) for much longer due to the thermoplastic substrate cooling rapidly. This means a lower curing temperature would give a longer time period over which the reaction could take place. However, a very fast cure time would be beneficial to fast processing. 102 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

19 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding When selecting a coating material it is useful to have an overview of the temperature changes that the thermoset will undergo during processing; using cooling equations a coating temperature profile has been constructed from before moulding until cooling back to room temperature (Figure 16); these calculations also confirmed the assumption that the thermoset material would reach the tool temperature by the time of injection (a 50 μm thick film would reach 84.5 C from 22 C in 0.33 s). It is important to note that this profile will only be experienced by the thermoset material at the thermoset-thermoplastic boundary; from the model it was shown that the thermoset material adjacent to the mould would remain at the same temperature as the tool during the cycle. Therefore thermoset material in between these two interfaces would go through varying temperature changes. Looking more closely at the model data detailing the cooling of PA66, it shows that the material at the substrate boundary is subjected to the cure onset temperature (or higher) for 99 s. Figure 16. The temperature changes undergone by the thermoset material adjacent to the injected PA66 substrate 5. Other Considerations for In-Mould Painting Translating thermal analysis cure data such as this into useful process conditions is problematic due to the inherent differences in the heating regimes involved. Whilst thermal scans provide incremental temperature increases, Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

20 B. Middleton the thermoset powder almost instantaneously hits the tool temperature on impact and then is exposed to a rapid heat ramping on the other side of the thermoset film during polymer injection. In addition, this is without considering any other complex shear, mixing, pressure, cure reaction energy or other such interactions that may be occurring at the polymer interfaces. Whilst monitoring thermoset curing rates is the subject of many research papers in their own right [20-25], the newest in-mould painting system provides a different heating problem to those normally encountered. Other problems facing the in-mould painting process is the surface finish of the cured film. In order to obtain a good surface finish it is essential that, as well as curing, the thermoset gels prior to thermoplastic injection phase; this means a thermoset with a short gel time and low gel temperature would be preferable. The temperatures of both the injection moulding tool and the incoming polymer materials are critical in the success of this process. Insufficient initial tool temperatures can fail to melt the thermoset powder, this means that during injection, the powder film fails to form sufficient strength prior to injection and is swept along with the polymer. The result is visible lines of powder flow which mirror the lines of polymer filling orientation. This is an effect also seen with thermosets that do not gel quickly enough. So the mould temperature must be controlled very carefully, it must be high enough to gel the thermoset but be low enough to enable the ejection of a stable part. 6. Conclusions It has been shown that achieving an in-mould thermoset cure is theoretically possible during a thermoplastic moulding cycle. Analysis reveals that commercially relevant thermoset cure initiation temperatures are achieved within the mould cycle, but it is unlikely that the reaction would complete during the timescale of a thermoplastic injection. The most suitable materials for the process studied here were PA66 thermoplastic at 280 C, injected onto a gelled low temperature cure film with the mould tool at 85 C. Using this regime, cure temperatures can be maintained for 99 seconds at the filmsubstrate interface, for 60 seconds at a depth of 28 µm into the film and is achieved instantaneously at a depth of 44 µm. To achieve cure throughout the complete depth of the film, a high temperature will be necessary on the plastic/mould tool interface to ensure sufficient penetration of the cure reaction. This has implications regarding the temperature at which the part can be ejected and overall cycle time. To overcome this, the tool would need to be modified with more efficient heating and cooling systems; alternatively 104 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2, 2011

21 Modelling Cure Times for Thermoset-Thermoplastic Injection Moulding the process could be successful using a specially developed powder coating, with the requirements stated earlier to encourage the cure reaction. This in-mould painting technique has application in other areas and if developed further could produce a distinct cured, thermoset layer under the surface of a thermoplastic material. References 1. M. Villalobos, A. Awojulu, T. Greeley, G. Turco and G. Deeter, Energy, 31 (2006) 15, C.. Williams and M.. Hillmyer, Polymer Reviews, 48 (2008) 1, T. Katsura and H. Sasaki, Packaging Technology and Science, 14 (2001) 3, J. Yu and L.X.L. Chen, Environmental Science & Technology, 42 (2008) 18, D.A. Messaoud, B. Sanchagrin and A. Derdouri, Polymer Composites, 26 (2005), V. Goodship and J.C. Love, Rapra Review Reports, 13 (2002) P. Hong-Seok, L. Gyu-Bong and D.B.H. Anh, International Forum on Strategic Technology, (2007), K. Makenji, V. Goodship, S. Buckley and C. Evered, Progress in Rubber Plastics Recycling Technology, 22 (2006), S.-C. Chen, M.-H. Li, S.-T. Huang and Y.-C. Wang, International Communications in Heat and Mass Transfer, 37 (2010) 5, C.A. Puentes, O.I. Okoli and Y.B. Park, Composites Part A: Applied Science and Manufacturing, 40 (2009) 4, V. Goodship and G.F. Smith, International Journal of Environmental Technology and Management, 8 (2008) 4, V. Goodship, C. Lobjoit, I. Dargue and G.F. Smith, Progress in Rubber, Plastics and Recycling Technology, 22 (2006) 3, V. Goodship, N. Cook, I. Dargue, C. Lobjoit, K. Makenji and G.F. Smith, Plastics Rubber and Composites, 36 (2007) 1, A. Dawson, M. Rides, C.R.G. Allen and J.M. Urquhart, Polymer Testing, 27 (2008) 5, J.Z. Liang and J.N. Ness, Proceedings of the International Conference on Manufacturing Automation, (1992), Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 2,

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