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Design and analysis of composites manufacturing tooling for rapid heating and cooling
Journal of Manufacturing Processes 36 (2018) 487–495
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Design and analysis of composites manufacturing tooling for rapid heating and cooling
T
⁎
Daniel F. Walczyk , Meagan A. Lettko Center for Automation Technologies and Systems, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY, 12180, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Composites manufacturing Rapid heating Rapid cooling Design analysis Specialized elastomeric tooling
Simple analytical models for designing rapid heating (via electric resistance or fluid flow in conformal channels) and cooling (via fluid flow in conformal channels) capabilities for composites manufacturing tooling are presented and demonstrated. The electric resistance heating and conformal channel cooling models are implemented for design purposes through spreadsheets and experimentally validated for a test mold designed for the Specialized Elastomeric Tooling out-of-autoclave process. Considering the modeling assumptions made, the rapid heating model provides very reasonable estimates of time versus mold surface temperature behavior. However, the rapid cooling model over-predicts the cooling time, although sensitivity to flow conditions (laminar vs. turbulent) is quite evident. Both models allow a designer to quickly vary design parameters to determine how production performance (e.g., cycle time) is affected. Finally, the model for rapid heating via conformal channels is demonstrated for a test mold. The high flow rates of heating oil required in this example as predicted by the model demonstrate the compromises a designer must make in tooling development.
1. Introduction There is significant interest within the composites manufacturing community for production tooling that has rapid heating and/or cooling capabilities to reduce cycle time while not adversely affecting temperature uniformity and control. Examples of commercial products that address this need include ÉireComposites’ patented MECHTool (Mold Efficient Cooling and Heating) [1], Regloplas Corporations temperature control units for pressurized water or thermal oil heating sources used for autoclave tools [2], RocTool’s Light Induction Heating (LIT) technology [3] and LaminaHeat’s conformable heating films and blankets used for composites tooling [4]. Computer-Aided Engineering (CAE) software, such as Finite Element Analysis, provide powerful capabilities for designing composites manufacturing tooling that rapidly heat up and/or cool down, but many designers lack these resources or, if available, are hesitant to use them due to steep learning curves. Although sacrifices are made in predictive accuracy, analytical methods based on reasonable modeling approximations arguably provide engineers with a simpler and more intuitive approach to thermal performance design for tooling and can serve as a solid foundation for computational modeling efforts if optimization is required. This paper presents and experimentally verifies analytical design methods for (1) rapid heating using either embedded electric resistance
⁎
heating elements or heated fluid flowing through conformal channels and (2) rapid cooling using fluid flowing through conformal channels. Details of these methods, based on unit-cell and lumped parameter modeling approaches from Refs. [5,6], are provided along with a design examples to demonstrate how the models can be implemented through a spreadsheet. 2. Example process Rapid heating and cooling design/analysis is demonstrated for a relatively new out-of-autoclave (OOA) process used to consolidate/cure thermoset and thermoplastic composite laminates and sandwich structures called Specialized Elastomeric Tooling (SET). The patented [7] and commercialized [8] SET process involves the use of a custom engineered match tool set to mimic the conditions of an autoclave, that is, provide uniform pressure and temperature over the composite workpiece. As shown in Fig. 1, the SET tool set consists of a thin-walled metal curing mold with evenly spaced conformal heating and cooling channels/elements to provide uniform temperature control at the mold’s surface in contact with the composite part's A-side. The composite part is sandwiched between the curing mold and an insulative compression mold (e.g., made of tooling board) covered with a variable-thickness SET mask, typically high-temperature silicone. The SET
Corresponding author. E-mail addresses: [email protected] (D.F. Walczyk), [email protected] (M.A. Lettko).
https://doi.org/10.1016/j.jmapro.2018.10.039 Received 17 July 2018; Received in revised form 29 October 2018; Accepted 31 October 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 36 (2018) 487–495
D.F. Walczyk, M.A. Lettko
Fig. 1. Schematic of the SET Process.
similar to the aforementioned methodology proposed by Yoo and Walczyk [5] and Xu et al. [6].
mask and compression mold shape are engineered using structural Finite Element Analysis (FEA) modeling and a proprietary algorithm to provide uniform pressure over the composite layup at the required process temperature when a particular compression load is applied at the mold’s center of pressure. From a SET process point of view, rapid heating allows the tool to reach process temperature quickly when production is started and in between cycles. This reduces overall cycle time. Rapid cooling is important after consolidation for handling thermoplastic composite parts made from prepreg to ensure that the demolding temperature is well below the resin’s glass transition temperature. For thermoset composite layups made from prepreg, rapid cooling also helps demold the part from the tool after consolidation/curing due to differences in tool and part coefficients of thermal expansion (CTE).
3.1. Rapid heating As previously mentioned, two methods of rapid heating are considered in this section: electric resistance and fluid flow through a conformal channel. 3.1.1. Electric resistance heating The lumped parameter model equation for rapid heating is based on the energy conservation requirement expressed as Eq. (1). Since no energy is generated within the mold itself (E˙ g = 0 ) and heat transfer from the tooling surfaces is considered negligible due to rapid heat up (E˙ out = 0 ), Eq. (1) reduces to E˙ in = E˙ st as reflected in Fig. 2(a). This relationship can be expressed in terms of material properties, inputs, and time changing quantities as
3. Design theory A lumped parameter model is used to predict rapid heating with an electric resistance heater (Fig. 2(a)), while one-dimensional (1-D) heat transfer models for individual cells (i.e. section of a curing tool defined by half the channel spacing distance, w, on each side of a conformal channel’s center line) are used for conformal channel heating (Fig. 2(b)) and conformal channel cooling (Fig. 2(c)). Because uniform heat-up or cool-down across the uniform temperature mold surface is assumed, the boundaries between adjacent cells are considered to be adiabatic. The model equations are based on the energy conservation requirement expressed on a rate basis per unit length of channel:
E˙ in + E˙ g − E˙ out = E˙ st
Pin = mcm
ΔT T (t ) − Tmo = ρm Vcm m Δt t
(2)
Solving Eq. (2) for Tm(t), the current mold temperature, yields
Tm (t ) =
Pin t + Tm0 ρm Vcm
(3)
where Pin is the maximum cartridge heater power output, t is time, ρm is the curing mold material density, V = cell volume = wtm L as defined in Fig. 2(a), L = cell length (usually mold length in direction of parallel heating elements), cm is the mold material specific heat capacity, and Tm0 is the mold starting temperature. This is a very simplistic model that neglects any heat losses due to natural convection from the tool edges, conduction to the insulation or SET mask, and geometrical nonuniformity due to cooling channels and cartridge heaters.
(1)
where E˙ in is the rate of input energy to the unit cell per unit length, E˙ g is the rate of energy generated within the unit cell per unit length, E˙ out is the rate of energy expended from the unit cell per unit length and E˙ st is the rate of energy stored to the unit cell per unit length. The resulting mold temperature equations can be used to predict heat transfer performance versus time for these specific situations using an approach
3.1.2. Conformal channel heating The unit cell model for conformal channel heating, shown in
Fig. 2. Model schematics for (a) electric resistance heating, (b) conformal channel heating and (c) conformal channel cooling. 488
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Fig. 3. Cross-sectional schematic of the curing mold with rapid heating (via electric resistance heaters) and cooling (via conformal cooling channels) that was used to validate the thermal lumped parameter and unit-cell design approaches for SET tooling.
coefficient, based on the Dittus-Boelter equation, is
Fig. 2(b), is fully developed in Ref. [5], and the useful equations are included in this section. The general solution to mold temperature, Tm(t), versus time is
g3
Tm (t ) =
g3 ⎞
+ ⎛⎜Tm0 − ⎟ g2 ⎠ ⎝
g2
hc =
g − 2 t e g1
( )
g1 = ρm 2lm wcm g2 = g3 =
( +(
hc πDTc 2
ha lm km
Bi =
)
+ 1 + 2ha w
ha lm hc πD 2km
)
+ 2ha w T∞,
g1 = ρm lm wcm hc πDkm w h πD k w + m ⎞ 2lm ⎛ c lm ⎠ ⎝ 2
hc πD 2
(hc πD)2
⎤ ⎥ Tc
h πD k w + m ⎞⎥ 4⎛ c lm ⎠ ⎦ ⎝ 2
(6)
where km, w, lm, hc, D and Tc are the same as previously defined. 3.3. Convective heat transfer coefficient at fluid channel interface From Ref. [14]; if ReD < 2,300, then channel flow is fully developed laminar and hc, based on Newton’s Law of Cooling, is
hc =
4.36kf D
lm h c km
(9)
Although design should proceed fabrication and testing for composites tooling used in production, the process was done in reverse for this study (i.e., tool was designed from experience then validated with theory) so that experimentally measured thermal performance could be compared with design theory predictions. Specifically, a 2.54 × 30.5 × 30.5 cm (1”thick×12”×12”) aluminum mold size was chosen for this experimental investigation, since the geometry is representative of a typical small SET tool. The thermal management concept considered for a SET curing mold in this case is to evenly space parallel cooling channels and cartridge heaters along the flat tool’s midline as shown in Fig. 3. Final design specifics include four 0.64 cm (0.25”) diameter cartridge heaters spaced 7.62 cm (3.0”) apart and eight 0.635 cm (0.25”) diameter cooling channels evenly spaced 3.81 cm (1.5”) apart. This same configuration can be applied to a more complex tool shape using the Profiled Edge Laminae (PEL) tooling approach [9–13], shown schematically in Fig. 4, or a similar additive manufacturing method. The set-up used for rapid heating and rapid cooling validation experiments (Fig. 5(a)), consists of (from bottom to top): two stacked 2.5 cm-thick rigid fiberglass insulation panels the same size as the mold to approximate an adiabatic surface; the SET curing mold itself with eight embedded conformal cooling channels and four heating channels (6.4mm∅×305 mm long Watlow cartridge heater inserted in each); a 0.159 cm (0.063”) thick woven carbon/epoxy panel to represent a standard composite layup; a 1.27 cm (0.5”) thick layer of high-temperature silicone rubber to serve as the SET mask (also approximates another adiabatic boundary); a rigid 1.27 cm thick steel plate to maintain uniform pressure over the silicone mask; and finally dead weights to provide pressure to maintain contact between the mask and composite layup. A typical consolidation pressure of 670 kPa, for example, was not applied, since it would have required 65 kN (14.5 kips) of force and would have had a negligible effect on the tool's heat
As with Section 3.1, the unit cell model for conformal channel cooling, shown in Fig. 2(c), is fully developed in Ref. [5], and the useful equations are included in this section. Heat input, E˙ in = Q′, for the general case is entirely from the top surface, although it is assumed to be zero when a previously heated SET curing mold needs to be cooled down quickly. Eq. (4) describes the mold temperature profile versus time, and relevant constants for this equation are
( )−
(8)
4. Experimental setup
3.2. Rapid cooling – conformal channel cooling
⎡ g3 = Q′ + ⎢ ⎢ ⎣
(0.023ReD0.8 Pr n )
For Bi > > 1, the temperature gradient across the solid (Tm (t ) − Ta (t ) ) is much larger than that between the conformal channel wall and the thermal fluid (Ta (t ) − Tc ) . For Bi < < 1 in situations such as a thinwalled tool with high km, the opposite is true.
(5)
where lm is the distance between the outer surface and the conformal channel, km is the thermal conductivity of the mold material, hc is the convective heat transfer coefficient between the heating fluid and the conformal channel wall, D is the hydraulic diameter of the conformal channel (diameter for a circular cross section), ha is the effective convection heat transfer coefficient between ambient air and the upper and lower cell surfaces (free convection in this case), and T∞ is the temperature of ambient air.
g2 =
D
where ReD is the Reynolds number for flow in a circular tube of diameter D, Pr is the Prandtl number, and n = 0.3 for cooling or 0.4 for heating. Depending on values for km, hc and lm, temperature gradients within the mold for either the rapid heating (Eqs. (4) and (5)) or rapid cooling (Eqs. (4) and (6)) situations are a strong function of the Biot number (Bi):
(4)
It should be noted that Eq. A3 in this reference has a typographical error, so it is corrected here and presented as Eq. (4). In this case, Tm0 is the starting temperature of the mold in absolute units (i.e. Kelvin or Rankine). Constants for this equation are
hc πD 2
kf
(7)
where kf is thermal conductivity of the cooling fluid. If ReD > 10,000, then channel flow is fully developed turbulent and the heat transfer 489
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D.F. Walczyk, M.A. Lettko
Fig. 4. Illustrations of (a) an unclamped PEL tool, (b) fixturing (e.g., for cutting) and registration scheme for individual lamina, and (c) section view showing piecewise continuous nature of a PEL tool.
by attaching a water hose with pressure gauge to a standard faucet, adjusting the flow valve to achieve the desired flowrate (measured by recording time to fill a specific container volume) and recording the corresponding pressure. Water flows to a plenum which, in turn, feeds each of the cooling lines by an equal length hose. Flow was restricted in some hoses so that flowrate in all eight channels is uniform. Exhaust water simply flows into a sink. A similar set-up to that of Fig. 6 was used for the rapid heating validation experiments, except that the temperature controller was replaced with a variable power supply to the cartridge heaters. As shown in Fig. 7, power to the heaters is provided with a Variac Model TDGC2KM variable voltage regulator. Voltage and current at the outlet are measured using a standard voltmeter and Fluke Model 322 clamp meter, respectively. Temperature at the four mold surface locations
transfer characteristics. Temperature control was achieved using a simple Watlow controller/relay arrangement that reads temperature with a thermocouple located in the middle of the curing mold in contact with the mold surface and provides power to the cartridge heaters accordingly (Fig. 6) to maintain setpoint temperature. Mold temperature vs. time is measured using a four thermocouples positioned at corners of a 15.2 cm (6”) square centered on the mold plate to avoid being above a cartridge heater or cooling channel. Thermocouple electrical leads are run through channels milled into the composite layup, as shown in Fig. 5(b). Temperature vs. time at the four thermocouple locations (TC #1, TC #2, TC #3, TC #4) are monitored and recorded using a 4channel Omega Model HH309A Data Logger Thermometer. As shown in Fig. 6, flowrate in the cooling channels is maintained
Fig. 5. (a) Stack of components for rapid heating/cooling validation experiments and (b) top view of carbon/epoxy layup with slots milled to accommodate thermocouple leads. 490
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(Fig. 7(b)) is monitored by the Omega datalogger. 5. Experimental results and design theory validation Since the experimental tool configuration only incorporates electric resistance heating and conformal channel cooling, only the design theory presented in Sections 3.1.1 and 3.2 are validated in this section. Although not pertinent to experimental validation, typical rapid heating and cooling specifications used in industry were chosen as target values. The mold is to heat up from 20 °C (room temperature) to 130 °C (typical molding temperature) in 1 min or less for part curing and also cool from 130 °C down to 70 °C (typical demolding temperature) in 1 min or less for part demolding. These targets correspond to a temperature ramp-up rate of 1.83 °C/s and ramp-down rate of 1.0 °C/s. Fig. 6. Experimental set-up for rapid cooling design theory validation.
5.1. Electric resistance heating Based on Eq. (3), the power required to raise the tool temperature by the desired 130 °C – 20 °C = 110 °C within 60 s is 2800 W per heater (4 × 2800 = 11,200 W = 11.2 kW for entire mold). This was well beyond the capacity of the AC power supply used. From Table 1 for five rapid heating experimental runs, the maximum power output for the combination of four embedded cartridge heaters and power supply was 1125 W total or 281 W per heater. The lumped parameter model embedded in a spreadsheet (Fig. 8) predicts a 110 °C temperature rise time of 600 s (10 min) at this power level according to the temperature vs. time plot in Fig. 9 (labeled ‘model’). All material properties and parameter values for the experimental setup and run are shown in the spreadsheet cells. Five rapid heating experiments were conducted where the curing mold plate at near room temperature, Tmo (varied between 21.4 and 29.4 °C), was raised to approximately Tf = 130 °C by applying the power levels provided in Table 1. The temperature vs. time curve for Test #1 is also plotted in Fig. 9 (note linear temperature rise similar to model), and data for all five tests is provided in Table 2. Using the Δtemperature/Δtime slope, m, for each experimental test, the estimated time for a 110 °C temperature rise is 799 s (13.3 min) on average. The model prediction of 600 s is 25% less than this experimentally derived value. Given all of the simplifications made with the lumped parameter model, such as neglecting natural convection and conduction losses, this analytical approach provides very reasonable estimates of heating time for a SET curing mold and can be used for design purposes. To check the steady-state temperature uniformity of the electrically heated tool, the curing mold surface was uncovered (i.e. SET mask and deadweight removed), heated up under open-loop control to approximately 130 °C (measured with thermocouple probe at center of plate), and thermally imaged with a FLIR A320 camera. The colored temperature plot is shown in Fig. 10. Despite some minor areas of non-
Fig. 7. Experimental set-up for rapid heating design theory validation. Table 1 Maximum AC power supply output for rapid heating experiments. Point in Heating Cycle
Start
Middle
End
I (amps) V (volts) P (watts) I (amps) V (volts) P (watts) I (amps) V (volts) P (watts)
Experimental Test # 1
2
3
4
5
8.21 140 1149 8.06 140 1128 8.05 140.1 1128
8.13 140.1 1139 8.09 140.1 1133 8.03 140.1 1125
8.08 140 1131 8.05 140.3 1129 8.04 140.4 1129
8.07 138.8 1120 7.94 138.6 1101 7.97 138.3 1106
8.09 139 1126 8.03 138.9 1115 8.01 139 1113
Fig. 8. Electric resistance heating model embedded in a spreadsheet. 491
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Fig. 9. Predicted rapid heating temperature response of the SET curing mold based on a unit cell analysis approach and thermocouple readings for Test #1 at the lower power level (1.13 kW total).
uniformity, attributable to a roughed up mold surface resulting in different thermal emissivities, the surface temperature is quite uniform. This is undoubtedly due to aluminum’s high thermal conductivity.
Table 2 Rapid heating experimental data and estimated time for 130–20 = 110 °C temperature rise. Test #
1 2 3 4 5
Avg. Start Temp., Tmo (°C)
Avg. End Temp., Tf (°C)
ΔT = Tf − Tmo (°C)
22 29.4 31.5 21.4 27.9
130.4 130.4 130.4 129.3 130.6
108.4 101 98.9 107.9 102.7
Δt (s)
m=
ΔT Δt
t=
110℃ m
(s)
5.2. Conformal channel cooling
(°C/s)
759 750 723 780 753
0.1428 0.1347 0.1368 0.1383 0.1364 Avg.
Similar to the previous section, the goal of the rapid cooling experiments was to validate theoretical predictions used in design by heating the aluminum mold plate to 130 °C and allowing it to reach steady state, shutting off power to the cartridge heaters, cooling the plate below 70 °C by flowing 10 °C tap water at the desired flowrate, and concurrently measuring temperature at four locations on the curing mold surface. Prior to running the cool-down experiments, volumetric flow rate was measured five times by timing water flow into a bucket,
770.3 816.6 804.1 795.4 806.5 799
Fig. 10. Thermal IR plot of curing mold at approximately 130 °C with temperature scale on the right hand side. 492
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TC #1 and TC #2, and less so among the experimental runs. Referring to Fig. 5(b), one would expect thermocouples on the inlet side (TC #1 and TC #2) to show faster cooling time, since water at the outlet side is inherently warmer than the inlet side as it picks up heat. The conformal cooling channel analysis consisted of embedding Eqs. (4), (6)-(8) into a spreadsheet, shown in Fig. 12. All material properties, parameter values and modeling assumptions (e.g., Q’ = 0) for the experimental setup and run are included in the spreadsheet cells. The tool is predicted to drop the required 130 °C–70 °C = 60 °C within 160 s, as shown in Fig. 11, which is 219% longer than the experimentally measured average time of 50.2 s. For all of the simplifications made with the unit cell approach model and approximations with estimating convective heat transfer coefficients, this analytical approach over-predicts the required cooling time in this particular case. Comparing the value of hc for laminar and turbulent flow in Fig. 13 illustrates how sensitive the analysis is to flow conditions. It should also be noted that the Biot number from Eqn. 9 is 0.08 in the case (i.e. Bi < < 1) suggesting that the temperature gradient across the mold is small.
Table 3 Data for determining cooling mass flow rate. At 7 kPa (1 psi) Gauge Pressure Test #
Time, t (s)
Total Water Discharged (8 tubes), mw (g)
Mass Flow Rate / Tube, m˙ t = mw 8t (g/s)
1 2 3 4 5
9.63 9.6 10.01 9.92 10.09
925.9 887.9 888.9 825.4 873.4 Average
12.02 11.56 11.10 10.40 10.82 11.18
Table 4 Times (s) for SET curing mold plate to drop from 130 °C to 70 °C during rapid cooling experiments. Thermocouple # Test #
1
2
3
4
Test Avgs.
1 2 3 4 5 Thermocouple Avgs.
52 50 48 47 43 48
49 47 46 45 43 46
52 53 50 52 49 51.2
56 55 55 58 53 55.4
52.3 51.3 49.8 50.5 47
6. Design example To investigate different options available for the SET curing mold besides electric resistance heating and demonstrate the design theory in Section 3.1.2, rapid heating with hot engine oil at 160 °C flowing through the conformal channels shown in Fig. 3 is considered for the SET experimental setup. The conformal heating channel analysis consists of embedding design equations from Ref. [5] – specifically Eqs. (4) and (5) – and predictive equations for convective heat transfer for internal fluid flow from Ref. [14] into a spreadsheet and iterating on flowrate until manufacturing requirements are met, i.e. raising mold temperature from 20 °C to 130 °C in 60 s. A printout of the spreadsheet is shown in Fig. 13, and a plot of temperature versus time is shown in Fig. 14. Final heating conditions consist of the eight 0.635 cm (0.25”) diameter conformal channels evenly spaced 3.81 cm (1.5”) apart with heated engine oil (160 °C, as previously mentioned) flowing at 18.9 L/ min per channel for a total of 151 L/min for all eight channels in a parallel arrangement. This volumetric flow rate of oil for such a small mold is highly impractical. It should be noted that although power requirements for an oil heating system will be similar to an electric resistance heating system, oil heating may provide better temperature control than a purely electric one.
weighing the water collected, and calculating the mass flow rate. Using the data shown in Table 3, the average volumetric flow rate per tube used in the simulation is:
⎛ 1.667 × 10−5 g 1 mL 60 s 1L ⎞⎜ ⎞⎛ ⎞⎛ v˙t = m˙ t ρ = ⎛11.18 ⎞ ⎛ L w s ⎠ ⎝ 1 cm3 ⎠ ⎝ 1 min ⎠ ⎝ 1000 mL ⎠ ⎜ ⎝ 1 min ⎝ m3 = 1.12 × 10−5 s
m3 s
⎞ ⎟⎟ ⎠
A typical set of temperature vs. time curves for the four thermocouples in contact with the mold surface (Run #1) is shown in Fig. 12. After the temperatures reach steady state at around 130 °C and rapid cooling begins, there is an exponential temperature decay. The elapsed times for the cooled mold to drop by 60 °C are provided in Table 4. The grand average time from this data for the SET curing mold to drop from 130 °C to 70 °C is 50.2 s. There is significant variation among the thermocouples, i.e. cool down for TC #3 and TC #4 taking longer than
Fig. 11. Predicted cooling temperature response of the SET curing mold based on a unit cell analysis approach and thermocouple readings for Test #1. 493
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Fig. 12. Conformal channel cooling model embedded in a spreadsheet.
Fig. 13. Spreadsheet used to analyze the SET curing mold’s heating system consisting of conformal channels with hot oil flowing through them.
7. Conclusions
and heating times are, respectively, 69% more and 25% less than model predictions. This heating result is considered by the authors to be very reasonable for design purposes, since convective and conductive heat losses in the model are neglected. The cooling result is quite far off, although it is quite clear how sensitive the analysis is to flow conditions by comparing convective heat transfer coefficients for laminar and turbulent flow. To further demonstrate the usefulness of this modeling approach, process requirements (i.e. fluid flow rate and temperature) for rapid heating with hot oil are determined as an alternative to electric resistance heating. From a practical side, the analysis shows: (1) how effective conformal channels with reasonable coolant flow rates are for rapid cooling; and (2) the extremely high power requirements and flow rates
The process of using simple analytical models to design composites manufacturing tooling for rapid heating and cooling is described in this paper and benefits of this approach are discussed. A unit-cell approach based on Refs. [5,6] for rapid cooling and a lumped parameter model for rapid heating are proposed for designing a temperature-controlled curing mold located within a tooling stack for the SET process. The basic design concept considered in this case consists of parallel conformal cooling channels interspersed with electric resistance heating elements, all located at the mold’s mid-plane. Validation experiments conducted on a 2.5 × 30.5 × 30.5 cm aluminum mold plate with embedded cooling and heating channels show that actual rapid cooling 494
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Automation Technologies and Systems, particularly Mr. Kenneth Myer, for prototyping support; and (2) Vistex Composites, LLC for providing process design specifications for tooling. References [1] Gardiner G. Patented MECHTool system. 2012 Accessed 16 July 2018 https://www. compositesworld.com/articles/patented-mechtool-system/. [2] Regloplas AG. News. 4.0. Individual. Controlled. Best practice. 2017 Accessed 16 July 2018 http://www.regloplas.com/us/. [3] Roctool. Heat & cool for composites processing. 2018 Accessed 16 July 2018 http:// www.roctool.com/technology/composite-processing/. [4] LaminaHeat. Putting heat where it’s needed. 2018 Accessed 2 October 2018 http:// www.laminaheat.com/en/. [5] Yoo S, Walczyk DF. A preliminary study of sealing and heat transfer performance of conformal channels and cooling fins in laminated tooling. J Manuf Sci Eng 2006;129(2):388–99. https://doi.org/10.1115/1.2515522. [6] Xu X, Sachs E, Allen S. The design of conformal cooling channels in injection molding tooling. Polym Eng Sci 2001;41(7):1265–79. https://doi.org/10.1002/pen. 10827. [7] Walczyk D, Hoffman C, Righi M, De S, Kuppers J. Consolidating and curing of thermoset composite parts by pressing between a heated rigid mold and customized rubber-faced mold. US Patent 8,511,362, issued 20 August 2013. [8] Vixtex Composites. Technology. 2018 Accessed 16 July 2018 https://www. vistexcomposites.com/. [9] Walczyk DF, Yoo S. Design and fabrication of a laminated thermoforming tool with enhanced functionality. J Manuf Process 2009;11(1):8–18. https://doi.org/10. 1016/j.jmapro.2009.04.003. [10] Yoo S, Walczyk D. An adaptive slicing algorithm for profiled edge laminae tooling. Int J Precis Eng Manuf 2007;8(3):64–71. [11] Williams RE, Walczyk DF, Hoang TD. Using abrasive flow machining to finish and seal conformal channels in laminated tooling. Rapid Prototyp J 2007;13(2):64–75. https://doi.org/10.1108/13552540710736740. [12] Yoo S, Walczyk DF. An advanced cutting trajectory algorithm for laminated tooling. Rapid Prototyp J 2005;11(4):199–213. https://doi.org/10.1108/ 13552540510612893. [13] Yoo S, Walczyk DF. Advanced design and development of profiled edge lamination tooling. J Manuf Process 2005;7(2):162–73. https://doi.org/10.1016/S15266125(05)70093-1. [14] Bergman TL, Lavine AS, Incropera FP, DeWitt DP. Fundamentals of heat and mass transfer. 7th ed. New York: John Wiley & Sons; 2011.
Fig. 14. Predicted temperature response of the SET curing mold rapid heated with hot oil based on a unit-cell analysis approach.
required for rapid heating of a metal mold using electric-resistance heaters and fluid flow through conformal channels, respectively. The latter result may help explain a composites industry research thrust to replace metal production tooling with ultrathin composites tooling that has embedded heating meshes. Funding This work was supported by the Center for Automation Technologies and Systems, a New York State-designated Center for Advanced Technology, and by Vistex Composites, LLC through the National Science Foundation SBIR Phase II program [grant number 1534709]. Acknowledgements The authors would like to acknowledge: (1) Rensselaer’s Center for
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Design and analysis of composites manufacturing tooling for rapid heating and cooling
T
⁎
Daniel F. Walczyk , Meagan A. Lettko Center for Automation Technologies and Systems, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY, 12180, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Composites manufacturing Rapid heating Rapid cooling Design analysis Specialized elastomeric tooling
Simple analytical models for designing rapid heating (via electric resistance or fluid flow in conformal channels) and cooling (via fluid flow in conformal channels) capabilities for composites manufacturing tooling are presented and demonstrated. The electric resistance heating and conformal channel cooling models are implemented for design purposes through spreadsheets and experimentally validated for a test mold designed for the Specialized Elastomeric Tooling out-of-autoclave process. Considering the modeling assumptions made, the rapid heating model provides very reasonable estimates of time versus mold surface temperature behavior. However, the rapid cooling model over-predicts the cooling time, although sensitivity to flow conditions (laminar vs. turbulent) is quite evident. Both models allow a designer to quickly vary design parameters to determine how production performance (e.g., cycle time) is affected. Finally, the model for rapid heating via conformal channels is demonstrated for a test mold. The high flow rates of heating oil required in this example as predicted by the model demonstrate the compromises a designer must make in tooling development.
1. Introduction There is significant interest within the composites manufacturing community for production tooling that has rapid heating and/or cooling capabilities to reduce cycle time while not adversely affecting temperature uniformity and control. Examples of commercial products that address this need include ÉireComposites’ patented MECHTool (Mold Efficient Cooling and Heating) [1], Regloplas Corporations temperature control units for pressurized water or thermal oil heating sources used for autoclave tools [2], RocTool’s Light Induction Heating (LIT) technology [3] and LaminaHeat’s conformable heating films and blankets used for composites tooling [4]. Computer-Aided Engineering (CAE) software, such as Finite Element Analysis, provide powerful capabilities for designing composites manufacturing tooling that rapidly heat up and/or cool down, but many designers lack these resources or, if available, are hesitant to use them due to steep learning curves. Although sacrifices are made in predictive accuracy, analytical methods based on reasonable modeling approximations arguably provide engineers with a simpler and more intuitive approach to thermal performance design for tooling and can serve as a solid foundation for computational modeling efforts if optimization is required. This paper presents and experimentally verifies analytical design methods for (1) rapid heating using either embedded electric resistance
⁎
heating elements or heated fluid flowing through conformal channels and (2) rapid cooling using fluid flowing through conformal channels. Details of these methods, based on unit-cell and lumped parameter modeling approaches from Refs. [5,6], are provided along with a design examples to demonstrate how the models can be implemented through a spreadsheet. 2. Example process Rapid heating and cooling design/analysis is demonstrated for a relatively new out-of-autoclave (OOA) process used to consolidate/cure thermoset and thermoplastic composite laminates and sandwich structures called Specialized Elastomeric Tooling (SET). The patented [7] and commercialized [8] SET process involves the use of a custom engineered match tool set to mimic the conditions of an autoclave, that is, provide uniform pressure and temperature over the composite workpiece. As shown in Fig. 1, the SET tool set consists of a thin-walled metal curing mold with evenly spaced conformal heating and cooling channels/elements to provide uniform temperature control at the mold’s surface in contact with the composite part's A-side. The composite part is sandwiched between the curing mold and an insulative compression mold (e.g., made of tooling board) covered with a variable-thickness SET mask, typically high-temperature silicone. The SET
Corresponding author. E-mail addresses: [email protected] (D.F. Walczyk), [email protected] (M.A. Lettko).
https://doi.org/10.1016/j.jmapro.2018.10.039 Received 17 July 2018; Received in revised form 29 October 2018; Accepted 31 October 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
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Fig. 1. Schematic of the SET Process.
similar to the aforementioned methodology proposed by Yoo and Walczyk [5] and Xu et al. [6].
mask and compression mold shape are engineered using structural Finite Element Analysis (FEA) modeling and a proprietary algorithm to provide uniform pressure over the composite layup at the required process temperature when a particular compression load is applied at the mold’s center of pressure. From a SET process point of view, rapid heating allows the tool to reach process temperature quickly when production is started and in between cycles. This reduces overall cycle time. Rapid cooling is important after consolidation for handling thermoplastic composite parts made from prepreg to ensure that the demolding temperature is well below the resin’s glass transition temperature. For thermoset composite layups made from prepreg, rapid cooling also helps demold the part from the tool after consolidation/curing due to differences in tool and part coefficients of thermal expansion (CTE).
3.1. Rapid heating As previously mentioned, two methods of rapid heating are considered in this section: electric resistance and fluid flow through a conformal channel. 3.1.1. Electric resistance heating The lumped parameter model equation for rapid heating is based on the energy conservation requirement expressed as Eq. (1). Since no energy is generated within the mold itself (E˙ g = 0 ) and heat transfer from the tooling surfaces is considered negligible due to rapid heat up (E˙ out = 0 ), Eq. (1) reduces to E˙ in = E˙ st as reflected in Fig. 2(a). This relationship can be expressed in terms of material properties, inputs, and time changing quantities as
3. Design theory A lumped parameter model is used to predict rapid heating with an electric resistance heater (Fig. 2(a)), while one-dimensional (1-D) heat transfer models for individual cells (i.e. section of a curing tool defined by half the channel spacing distance, w, on each side of a conformal channel’s center line) are used for conformal channel heating (Fig. 2(b)) and conformal channel cooling (Fig. 2(c)). Because uniform heat-up or cool-down across the uniform temperature mold surface is assumed, the boundaries between adjacent cells are considered to be adiabatic. The model equations are based on the energy conservation requirement expressed on a rate basis per unit length of channel:
E˙ in + E˙ g − E˙ out = E˙ st
Pin = mcm
ΔT T (t ) − Tmo = ρm Vcm m Δt t
(2)
Solving Eq. (2) for Tm(t), the current mold temperature, yields
Tm (t ) =
Pin t + Tm0 ρm Vcm
(3)
where Pin is the maximum cartridge heater power output, t is time, ρm is the curing mold material density, V = cell volume = wtm L as defined in Fig. 2(a), L = cell length (usually mold length in direction of parallel heating elements), cm is the mold material specific heat capacity, and Tm0 is the mold starting temperature. This is a very simplistic model that neglects any heat losses due to natural convection from the tool edges, conduction to the insulation or SET mask, and geometrical nonuniformity due to cooling channels and cartridge heaters.
(1)
where E˙ in is the rate of input energy to the unit cell per unit length, E˙ g is the rate of energy generated within the unit cell per unit length, E˙ out is the rate of energy expended from the unit cell per unit length and E˙ st is the rate of energy stored to the unit cell per unit length. The resulting mold temperature equations can be used to predict heat transfer performance versus time for these specific situations using an approach
3.1.2. Conformal channel heating The unit cell model for conformal channel heating, shown in
Fig. 2. Model schematics for (a) electric resistance heating, (b) conformal channel heating and (c) conformal channel cooling. 488
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Fig. 3. Cross-sectional schematic of the curing mold with rapid heating (via electric resistance heaters) and cooling (via conformal cooling channels) that was used to validate the thermal lumped parameter and unit-cell design approaches for SET tooling.
coefficient, based on the Dittus-Boelter equation, is
Fig. 2(b), is fully developed in Ref. [5], and the useful equations are included in this section. The general solution to mold temperature, Tm(t), versus time is
g3
Tm (t ) =
g3 ⎞
+ ⎛⎜Tm0 − ⎟ g2 ⎠ ⎝
g2
hc =
g − 2 t e g1
( )
g1 = ρm 2lm wcm g2 = g3 =
( +(
hc πDTc 2
ha lm km
Bi =
)
+ 1 + 2ha w
ha lm hc πD 2km
)
+ 2ha w T∞,
g1 = ρm lm wcm hc πDkm w h πD k w + m ⎞ 2lm ⎛ c lm ⎠ ⎝ 2
hc πD 2
(hc πD)2
⎤ ⎥ Tc
h πD k w + m ⎞⎥ 4⎛ c lm ⎠ ⎦ ⎝ 2
(6)
where km, w, lm, hc, D and Tc are the same as previously defined. 3.3. Convective heat transfer coefficient at fluid channel interface From Ref. [14]; if ReD < 2,300, then channel flow is fully developed laminar and hc, based on Newton’s Law of Cooling, is
hc =
4.36kf D
lm h c km
(9)
Although design should proceed fabrication and testing for composites tooling used in production, the process was done in reverse for this study (i.e., tool was designed from experience then validated with theory) so that experimentally measured thermal performance could be compared with design theory predictions. Specifically, a 2.54 × 30.5 × 30.5 cm (1”thick×12”×12”) aluminum mold size was chosen for this experimental investigation, since the geometry is representative of a typical small SET tool. The thermal management concept considered for a SET curing mold in this case is to evenly space parallel cooling channels and cartridge heaters along the flat tool’s midline as shown in Fig. 3. Final design specifics include four 0.64 cm (0.25”) diameter cartridge heaters spaced 7.62 cm (3.0”) apart and eight 0.635 cm (0.25”) diameter cooling channels evenly spaced 3.81 cm (1.5”) apart. This same configuration can be applied to a more complex tool shape using the Profiled Edge Laminae (PEL) tooling approach [9–13], shown schematically in Fig. 4, or a similar additive manufacturing method. The set-up used for rapid heating and rapid cooling validation experiments (Fig. 5(a)), consists of (from bottom to top): two stacked 2.5 cm-thick rigid fiberglass insulation panels the same size as the mold to approximate an adiabatic surface; the SET curing mold itself with eight embedded conformal cooling channels and four heating channels (6.4mm∅×305 mm long Watlow cartridge heater inserted in each); a 0.159 cm (0.063”) thick woven carbon/epoxy panel to represent a standard composite layup; a 1.27 cm (0.5”) thick layer of high-temperature silicone rubber to serve as the SET mask (also approximates another adiabatic boundary); a rigid 1.27 cm thick steel plate to maintain uniform pressure over the silicone mask; and finally dead weights to provide pressure to maintain contact between the mask and composite layup. A typical consolidation pressure of 670 kPa, for example, was not applied, since it would have required 65 kN (14.5 kips) of force and would have had a negligible effect on the tool's heat
As with Section 3.1, the unit cell model for conformal channel cooling, shown in Fig. 2(c), is fully developed in Ref. [5], and the useful equations are included in this section. Heat input, E˙ in = Q′, for the general case is entirely from the top surface, although it is assumed to be zero when a previously heated SET curing mold needs to be cooled down quickly. Eq. (4) describes the mold temperature profile versus time, and relevant constants for this equation are
( )−
(8)
4. Experimental setup
3.2. Rapid cooling – conformal channel cooling
⎡ g3 = Q′ + ⎢ ⎢ ⎣
(0.023ReD0.8 Pr n )
For Bi > > 1, the temperature gradient across the solid (Tm (t ) − Ta (t ) ) is much larger than that between the conformal channel wall and the thermal fluid (Ta (t ) − Tc ) . For Bi < < 1 in situations such as a thinwalled tool with high km, the opposite is true.
(5)
where lm is the distance between the outer surface and the conformal channel, km is the thermal conductivity of the mold material, hc is the convective heat transfer coefficient between the heating fluid and the conformal channel wall, D is the hydraulic diameter of the conformal channel (diameter for a circular cross section), ha is the effective convection heat transfer coefficient between ambient air and the upper and lower cell surfaces (free convection in this case), and T∞ is the temperature of ambient air.
g2 =
D
where ReD is the Reynolds number for flow in a circular tube of diameter D, Pr is the Prandtl number, and n = 0.3 for cooling or 0.4 for heating. Depending on values for km, hc and lm, temperature gradients within the mold for either the rapid heating (Eqs. (4) and (5)) or rapid cooling (Eqs. (4) and (6)) situations are a strong function of the Biot number (Bi):
(4)
It should be noted that Eq. A3 in this reference has a typographical error, so it is corrected here and presented as Eq. (4). In this case, Tm0 is the starting temperature of the mold in absolute units (i.e. Kelvin or Rankine). Constants for this equation are
hc πD 2
kf
(7)
where kf is thermal conductivity of the cooling fluid. If ReD > 10,000, then channel flow is fully developed turbulent and the heat transfer 489
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Fig. 4. Illustrations of (a) an unclamped PEL tool, (b) fixturing (e.g., for cutting) and registration scheme for individual lamina, and (c) section view showing piecewise continuous nature of a PEL tool.
by attaching a water hose with pressure gauge to a standard faucet, adjusting the flow valve to achieve the desired flowrate (measured by recording time to fill a specific container volume) and recording the corresponding pressure. Water flows to a plenum which, in turn, feeds each of the cooling lines by an equal length hose. Flow was restricted in some hoses so that flowrate in all eight channels is uniform. Exhaust water simply flows into a sink. A similar set-up to that of Fig. 6 was used for the rapid heating validation experiments, except that the temperature controller was replaced with a variable power supply to the cartridge heaters. As shown in Fig. 7, power to the heaters is provided with a Variac Model TDGC2KM variable voltage regulator. Voltage and current at the outlet are measured using a standard voltmeter and Fluke Model 322 clamp meter, respectively. Temperature at the four mold surface locations
transfer characteristics. Temperature control was achieved using a simple Watlow controller/relay arrangement that reads temperature with a thermocouple located in the middle of the curing mold in contact with the mold surface and provides power to the cartridge heaters accordingly (Fig. 6) to maintain setpoint temperature. Mold temperature vs. time is measured using a four thermocouples positioned at corners of a 15.2 cm (6”) square centered on the mold plate to avoid being above a cartridge heater or cooling channel. Thermocouple electrical leads are run through channels milled into the composite layup, as shown in Fig. 5(b). Temperature vs. time at the four thermocouple locations (TC #1, TC #2, TC #3, TC #4) are monitored and recorded using a 4channel Omega Model HH309A Data Logger Thermometer. As shown in Fig. 6, flowrate in the cooling channels is maintained
Fig. 5. (a) Stack of components for rapid heating/cooling validation experiments and (b) top view of carbon/epoxy layup with slots milled to accommodate thermocouple leads. 490
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(Fig. 7(b)) is monitored by the Omega datalogger. 5. Experimental results and design theory validation Since the experimental tool configuration only incorporates electric resistance heating and conformal channel cooling, only the design theory presented in Sections 3.1.1 and 3.2 are validated in this section. Although not pertinent to experimental validation, typical rapid heating and cooling specifications used in industry were chosen as target values. The mold is to heat up from 20 °C (room temperature) to 130 °C (typical molding temperature) in 1 min or less for part curing and also cool from 130 °C down to 70 °C (typical demolding temperature) in 1 min or less for part demolding. These targets correspond to a temperature ramp-up rate of 1.83 °C/s and ramp-down rate of 1.0 °C/s. Fig. 6. Experimental set-up for rapid cooling design theory validation.
5.1. Electric resistance heating Based on Eq. (3), the power required to raise the tool temperature by the desired 130 °C – 20 °C = 110 °C within 60 s is 2800 W per heater (4 × 2800 = 11,200 W = 11.2 kW for entire mold). This was well beyond the capacity of the AC power supply used. From Table 1 for five rapid heating experimental runs, the maximum power output for the combination of four embedded cartridge heaters and power supply was 1125 W total or 281 W per heater. The lumped parameter model embedded in a spreadsheet (Fig. 8) predicts a 110 °C temperature rise time of 600 s (10 min) at this power level according to the temperature vs. time plot in Fig. 9 (labeled ‘model’). All material properties and parameter values for the experimental setup and run are shown in the spreadsheet cells. Five rapid heating experiments were conducted where the curing mold plate at near room temperature, Tmo (varied between 21.4 and 29.4 °C), was raised to approximately Tf = 130 °C by applying the power levels provided in Table 1. The temperature vs. time curve for Test #1 is also plotted in Fig. 9 (note linear temperature rise similar to model), and data for all five tests is provided in Table 2. Using the Δtemperature/Δtime slope, m, for each experimental test, the estimated time for a 110 °C temperature rise is 799 s (13.3 min) on average. The model prediction of 600 s is 25% less than this experimentally derived value. Given all of the simplifications made with the lumped parameter model, such as neglecting natural convection and conduction losses, this analytical approach provides very reasonable estimates of heating time for a SET curing mold and can be used for design purposes. To check the steady-state temperature uniformity of the electrically heated tool, the curing mold surface was uncovered (i.e. SET mask and deadweight removed), heated up under open-loop control to approximately 130 °C (measured with thermocouple probe at center of plate), and thermally imaged with a FLIR A320 camera. The colored temperature plot is shown in Fig. 10. Despite some minor areas of non-
Fig. 7. Experimental set-up for rapid heating design theory validation. Table 1 Maximum AC power supply output for rapid heating experiments. Point in Heating Cycle
Start
Middle
End
I (amps) V (volts) P (watts) I (amps) V (volts) P (watts) I (amps) V (volts) P (watts)
Experimental Test # 1
2
3
4
5
8.21 140 1149 8.06 140 1128 8.05 140.1 1128
8.13 140.1 1139 8.09 140.1 1133 8.03 140.1 1125
8.08 140 1131 8.05 140.3 1129 8.04 140.4 1129
8.07 138.8 1120 7.94 138.6 1101 7.97 138.3 1106
8.09 139 1126 8.03 138.9 1115 8.01 139 1113
Fig. 8. Electric resistance heating model embedded in a spreadsheet. 491
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Fig. 9. Predicted rapid heating temperature response of the SET curing mold based on a unit cell analysis approach and thermocouple readings for Test #1 at the lower power level (1.13 kW total).
uniformity, attributable to a roughed up mold surface resulting in different thermal emissivities, the surface temperature is quite uniform. This is undoubtedly due to aluminum’s high thermal conductivity.
Table 2 Rapid heating experimental data and estimated time for 130–20 = 110 °C temperature rise. Test #
1 2 3 4 5
Avg. Start Temp., Tmo (°C)
Avg. End Temp., Tf (°C)
ΔT = Tf − Tmo (°C)
22 29.4 31.5 21.4 27.9
130.4 130.4 130.4 129.3 130.6
108.4 101 98.9 107.9 102.7
Δt (s)
m=
ΔT Δt
t=
110℃ m
(s)
5.2. Conformal channel cooling
(°C/s)
759 750 723 780 753
0.1428 0.1347 0.1368 0.1383 0.1364 Avg.
Similar to the previous section, the goal of the rapid cooling experiments was to validate theoretical predictions used in design by heating the aluminum mold plate to 130 °C and allowing it to reach steady state, shutting off power to the cartridge heaters, cooling the plate below 70 °C by flowing 10 °C tap water at the desired flowrate, and concurrently measuring temperature at four locations on the curing mold surface. Prior to running the cool-down experiments, volumetric flow rate was measured five times by timing water flow into a bucket,
770.3 816.6 804.1 795.4 806.5 799
Fig. 10. Thermal IR plot of curing mold at approximately 130 °C with temperature scale on the right hand side. 492
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TC #1 and TC #2, and less so among the experimental runs. Referring to Fig. 5(b), one would expect thermocouples on the inlet side (TC #1 and TC #2) to show faster cooling time, since water at the outlet side is inherently warmer than the inlet side as it picks up heat. The conformal cooling channel analysis consisted of embedding Eqs. (4), (6)-(8) into a spreadsheet, shown in Fig. 12. All material properties, parameter values and modeling assumptions (e.g., Q’ = 0) for the experimental setup and run are included in the spreadsheet cells. The tool is predicted to drop the required 130 °C–70 °C = 60 °C within 160 s, as shown in Fig. 11, which is 219% longer than the experimentally measured average time of 50.2 s. For all of the simplifications made with the unit cell approach model and approximations with estimating convective heat transfer coefficients, this analytical approach over-predicts the required cooling time in this particular case. Comparing the value of hc for laminar and turbulent flow in Fig. 13 illustrates how sensitive the analysis is to flow conditions. It should also be noted that the Biot number from Eqn. 9 is 0.08 in the case (i.e. Bi < < 1) suggesting that the temperature gradient across the mold is small.
Table 3 Data for determining cooling mass flow rate. At 7 kPa (1 psi) Gauge Pressure Test #
Time, t (s)
Total Water Discharged (8 tubes), mw (g)
Mass Flow Rate / Tube, m˙ t = mw 8t (g/s)
1 2 3 4 5
9.63 9.6 10.01 9.92 10.09
925.9 887.9 888.9 825.4 873.4 Average
12.02 11.56 11.10 10.40 10.82 11.18
Table 4 Times (s) for SET curing mold plate to drop from 130 °C to 70 °C during rapid cooling experiments. Thermocouple # Test #
1
2
3
4
Test Avgs.
1 2 3 4 5 Thermocouple Avgs.
52 50 48 47 43 48
49 47 46 45 43 46
52 53 50 52 49 51.2
56 55 55 58 53 55.4
52.3 51.3 49.8 50.5 47
6. Design example To investigate different options available for the SET curing mold besides electric resistance heating and demonstrate the design theory in Section 3.1.2, rapid heating with hot engine oil at 160 °C flowing through the conformal channels shown in Fig. 3 is considered for the SET experimental setup. The conformal heating channel analysis consists of embedding design equations from Ref. [5] – specifically Eqs. (4) and (5) – and predictive equations for convective heat transfer for internal fluid flow from Ref. [14] into a spreadsheet and iterating on flowrate until manufacturing requirements are met, i.e. raising mold temperature from 20 °C to 130 °C in 60 s. A printout of the spreadsheet is shown in Fig. 13, and a plot of temperature versus time is shown in Fig. 14. Final heating conditions consist of the eight 0.635 cm (0.25”) diameter conformal channels evenly spaced 3.81 cm (1.5”) apart with heated engine oil (160 °C, as previously mentioned) flowing at 18.9 L/ min per channel for a total of 151 L/min for all eight channels in a parallel arrangement. This volumetric flow rate of oil for such a small mold is highly impractical. It should be noted that although power requirements for an oil heating system will be similar to an electric resistance heating system, oil heating may provide better temperature control than a purely electric one.
weighing the water collected, and calculating the mass flow rate. Using the data shown in Table 3, the average volumetric flow rate per tube used in the simulation is:
⎛ 1.667 × 10−5 g 1 mL 60 s 1L ⎞⎜ ⎞⎛ ⎞⎛ v˙t = m˙ t ρ = ⎛11.18 ⎞ ⎛ L w s ⎠ ⎝ 1 cm3 ⎠ ⎝ 1 min ⎠ ⎝ 1000 mL ⎠ ⎜ ⎝ 1 min ⎝ m3 = 1.12 × 10−5 s
m3 s
⎞ ⎟⎟ ⎠
A typical set of temperature vs. time curves for the four thermocouples in contact with the mold surface (Run #1) is shown in Fig. 12. After the temperatures reach steady state at around 130 °C and rapid cooling begins, there is an exponential temperature decay. The elapsed times for the cooled mold to drop by 60 °C are provided in Table 4. The grand average time from this data for the SET curing mold to drop from 130 °C to 70 °C is 50.2 s. There is significant variation among the thermocouples, i.e. cool down for TC #3 and TC #4 taking longer than
Fig. 11. Predicted cooling temperature response of the SET curing mold based on a unit cell analysis approach and thermocouple readings for Test #1. 493
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Fig. 12. Conformal channel cooling model embedded in a spreadsheet.
Fig. 13. Spreadsheet used to analyze the SET curing mold’s heating system consisting of conformal channels with hot oil flowing through them.
7. Conclusions
and heating times are, respectively, 69% more and 25% less than model predictions. This heating result is considered by the authors to be very reasonable for design purposes, since convective and conductive heat losses in the model are neglected. The cooling result is quite far off, although it is quite clear how sensitive the analysis is to flow conditions by comparing convective heat transfer coefficients for laminar and turbulent flow. To further demonstrate the usefulness of this modeling approach, process requirements (i.e. fluid flow rate and temperature) for rapid heating with hot oil are determined as an alternative to electric resistance heating. From a practical side, the analysis shows: (1) how effective conformal channels with reasonable coolant flow rates are for rapid cooling; and (2) the extremely high power requirements and flow rates
The process of using simple analytical models to design composites manufacturing tooling for rapid heating and cooling is described in this paper and benefits of this approach are discussed. A unit-cell approach based on Refs. [5,6] for rapid cooling and a lumped parameter model for rapid heating are proposed for designing a temperature-controlled curing mold located within a tooling stack for the SET process. The basic design concept considered in this case consists of parallel conformal cooling channels interspersed with electric resistance heating elements, all located at the mold’s mid-plane. Validation experiments conducted on a 2.5 × 30.5 × 30.5 cm aluminum mold plate with embedded cooling and heating channels show that actual rapid cooling 494
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Automation Technologies and Systems, particularly Mr. Kenneth Myer, for prototyping support; and (2) Vistex Composites, LLC for providing process design specifications for tooling. References [1] Gardiner G. Patented MECHTool system. 2012 Accessed 16 July 2018 https://www. compositesworld.com/articles/patented-mechtool-system/. [2] Regloplas AG. News. 4.0. Individual. Controlled. Best practice. 2017 Accessed 16 July 2018 http://www.regloplas.com/us/. [3] Roctool. Heat & cool for composites processing. 2018 Accessed 16 July 2018 http:// www.roctool.com/technology/composite-processing/. [4] LaminaHeat. Putting heat where it’s needed. 2018 Accessed 2 October 2018 http:// www.laminaheat.com/en/. [5] Yoo S, Walczyk DF. A preliminary study of sealing and heat transfer performance of conformal channels and cooling fins in laminated tooling. J Manuf Sci Eng 2006;129(2):388–99. https://doi.org/10.1115/1.2515522. [6] Xu X, Sachs E, Allen S. The design of conformal cooling channels in injection molding tooling. Polym Eng Sci 2001;41(7):1265–79. https://doi.org/10.1002/pen. 10827. [7] Walczyk D, Hoffman C, Righi M, De S, Kuppers J. Consolidating and curing of thermoset composite parts by pressing between a heated rigid mold and customized rubber-faced mold. US Patent 8,511,362, issued 20 August 2013. [8] Vixtex Composites. Technology. 2018 Accessed 16 July 2018 https://www. vistexcomposites.com/. [9] Walczyk DF, Yoo S. Design and fabrication of a laminated thermoforming tool with enhanced functionality. J Manuf Process 2009;11(1):8–18. https://doi.org/10. 1016/j.jmapro.2009.04.003. [10] Yoo S, Walczyk D. An adaptive slicing algorithm for profiled edge laminae tooling. Int J Precis Eng Manuf 2007;8(3):64–71. [11] Williams RE, Walczyk DF, Hoang TD. Using abrasive flow machining to finish and seal conformal channels in laminated tooling. Rapid Prototyp J 2007;13(2):64–75. https://doi.org/10.1108/13552540710736740. [12] Yoo S, Walczyk DF. An advanced cutting trajectory algorithm for laminated tooling. Rapid Prototyp J 2005;11(4):199–213. https://doi.org/10.1108/ 13552540510612893. [13] Yoo S, Walczyk DF. Advanced design and development of profiled edge lamination tooling. J Manuf Process 2005;7(2):162–73. https://doi.org/10.1016/S15266125(05)70093-1. [14] Bergman TL, Lavine AS, Incropera FP, DeWitt DP. Fundamentals of heat and mass transfer. 7th ed. New York: John Wiley & Sons; 2011.
Fig. 14. Predicted temperature response of the SET curing mold rapid heated with hot oil based on a unit-cell analysis approach.
required for rapid heating of a metal mold using electric-resistance heaters and fluid flow through conformal channels, respectively. The latter result may help explain a composites industry research thrust to replace metal production tooling with ultrathin composites tooling that has embedded heating meshes. Funding This work was supported by the Center for Automation Technologies and Systems, a New York State-designated Center for Advanced Technology, and by Vistex Composites, LLC through the National Science Foundation SBIR Phase II program [grant number 1534709]. Acknowledgements The authors would like to acknowledge: (1) Rensselaer’s Center for
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