Cure cycle modification for efficient vacuum bag only prepreg process

Abstract

Over the past decade, increasing use of continuous fiber-reinforced polymer composites has created a demand for manufacturing methods with lower costs, higher production rates, and improved processing efficiency. To meet the growing demands, vacuum bag only (VBO) prepreg processing has been proposed and implemented in industrial settings. However, in the absence of high consolidation pressure, VBO prepreg must undergo compaction for longer durations during cure and requires use of more elaborate processing schemes to conform to complex geometries. The main objective of our cure cycle modification was to reduce overall manufacturing time for more efficient processing, while maintaining robust part quality. This study demonstrates the effect of cure cycle on formability and part quality of three complex-shaped composite structures, a bulkhead, fuselage, and I section frame, featuring drop-offs, corners and sandwich areas consisting of less than 10 plies. Three different cure cycles were chosen: Reference cure cycle 1, Modified I and Modified II cure cycle. The reference was modified based on resin cure kinetics/viscosity modeling results and “effective flow number” to shorten the overall cure cycle time while maintaining robust part quality. To compare the quality of manufactured parts, destructive test and digestion method were used. For the bulkhead parts, Modified I was proven to be more effective in meeting the commercially acceptable part criteria (void content, ply wrinkle, resin ridge, and surface resin starvation), whereas Reference failed to meet the requirements, showing pervasive presence of porosity in drop-offs, corners and sandwich areas. The fuselage and I section frame parts produced with Modified I and Modified II were shown to meet the part quality requirements, with slight improvements in surface quality observed with the Modified II method.

Keywords

Cure kinetics vacuum bag only viscosity aircraft composite part composite prepreg cure cycle modification flow number

Introduction

Over the past decade, increasing use of continuous fiber-reinforced polymer (CFRP) composites has created a demand for manufacturing methods with lower costs, higher production rates, and improved processing efficiency. The demand for CFRPs has been constantly growing across aerospace, automotive, marine, and wind energy industries as the composite materials offer advantageous mechanical properties such as light weight, high specific strength/stiffness, fatigue life and improved corrosion resistance compared to conventional materials.^1–4 Traditionally, primary composite structures are manufactured using autoclaves (pressurized ovens), where prepregs—fiber bed reinforcements pre-impregnated with catalyzed yet uncured resin—are consolidated under high pressure and temperature. Consolidation under high pressure suppresses porosity and ensures robust part quality, but prohibitive equipment and operating cost, slow production rate, and inflexible processing environment hinder extensive use of autoclaves. Consequently, the CFRP composite material paradigm has shifted from autoclave processing to Out-of-Autoclave (OoA) processing.^5–7 Among many OoA processing methods, Vacuum Bag Only (VBO) processing method has gained the most interest by offering energy efficiency using only an atmospheric pressure differential (1 atm) for part consolidation.⁵ However, in the absence of high consolidation pressure, VBO prepreg must undergo compaction for longer durations during cure and requires use of more elaborate processing schemes to conform to complex geometries. It has been demonstrated that advanced VBO prepregs such as CYCOM 5320-1 (Solvay) can be successfully used to manufacture large aerospace structures.^8–10 The prepreg has excellent hot/wet properties, mechanical properites equivalent to autoclave and flexible cure cycles. The Tg (wet glass transition temperature) is 163°C and the 0° tension according to ASTM D 3039 is 2565 MPa.¹¹ The VBO prepreg manufacturer (Solvay) recommends two different cure cycles. The Reference cure cycle involves a room temperature vacuum hold of 16 hours or more to evacuate gases trapped within the laminate during layup, followed by an initial cure at 121°C and post-cure at 177°C.¹¹ A more recent Modified I replaces long room temperature vacuum dwell with a shorter yet higher temperature debulk step.⁸ Completion of a single cure cycle takes over 22 and 8 hours for Reference and Modified I respectively, both of which are significantly longer than typical autoclave processing times. In this work, we investigate modification of VBO prepreg Reference by reducing the overall cure cycle time while maintaining high part quality. The Reference was modified based on resin cure kinetics/viscosity modeling results and “effective flow number,” previously defined in Kim and Nutt.¹² The quality of the manufactured parts was assessed with destructive test and digestion method to demonstrate that parts fabricated with modified cure cycle yield robust quality despite the reduced processing time.

Background

The heat of reaction resulting from epoxy-amine reaction measured from DSC (Differential Scanning Calorimeter) can be further analyzed to get the degree of cure evolution. First, the cure rate relates to the measured heat flow proportionately and can be expressed as below,

\frac{d α}{d t} = \frac{1}{Δ H_{T}} \frac{d H}{d t}

(1)

where α is the degree of cure, dα/dt is the cure rate and dH/dt is the measured heat flow.^12–14 Then, α is just an integral of dα/dt over time. In this work, a phenomenological cure kinetics model developed by Kim et al.¹ and Kratz et al.,¹⁵ which captures complex epoxy cure reaction mechanism and out-time effect, was used to predict degree of cure as a function of cure temperature and time:

\frac{d α}{d t} = \sum_{i = 1, 3} w_{i} K_{i} α^{m_{i}} {(1 - α)}^{n_{i}} + \sum_{j = 2, 4} w_{j} \frac{K_{j} α^{m_{j}} {(1 - α)}^{n_{j}}}{1 + \exp (D_{j} (α - (α_{C 0, j} + α_{C T, j} T)))}

(2)

K_{n} = A_{n} \exp (- \frac{E_{A, n}}{R T}) where n = i, j

(3)

where K_n is the Arrhenius temperature dependent term, A_n Arrhenius constant, E_A,n activation energy, m_i and n_i reaction order-based fitting constants, D_j diffusion constant, T temperature, α_C0 critical degree of cure at absolute zero, and α_CT increase in critical degree of cure with temperature. The parameters are shown in Table 1 and 2.

The viscosity (η) of resin depends on both degree of cure and temperature. The resin infiltration plays an important factor in manufacturing a quality part. Generally, the resin viscosity can be divided into three distinctive phases (viscous fluid, gel and solid) as a function of cure states. In viscous phase, the viscosity decreases, or resin flow increases with temperature. During this phase, the fiber bed is infiltrated with resin, and air or volatiles can be suppressed and evacuated. In the gel phase, the resin viscosity increases dramatically with temperature, and the flow stops. In solid phase, further crosslinking takes place which locks up the resin network, giving the highest achievable glass transition temperature.¹⁶ Again, a phenomenological viscosity evolution model from Kim et al.¹ was used to predict temperature and degree of cure dependent viscosity profile:

η = w_{1} η_{1} + w_{2} η_{2} {(\frac{α_{g e l}}{α_{g e l} - α})}^{A + B α^{d} + C α^{e}}

(4)

η_{i} = A_{η_{i}} \exp (- \frac{E_{η_{i}}}{R T}) where i = 1, 2

(5)

where w₁ and w₂ are out-time dependent weight factors, η_i Arrhenius dependent viscosity component, A_ηi Arrhenius constant, E_ηi viscosity activation energy, α_gel degree of cure at gelation, and A, B, C, d, and e fitting constants.

In a following work, Kim et al.¹² defined “effective flow number (N_Fl,eff),” a variable controlling flow level, obtained by integrating inverse of viscosity profile from 0 to t_gel (time to gelation):

N_{F l, e f f} = \int_{0}^{t_{g e l}} η {(t)}^{- 1} d t

(6)

where higher N_Fl,eff indicates greater resin flow. A cure cycle featuring higher “effective flow number” promotes resin infiltration into fiber tows, eliminating micro-voids. Macro-voids (voids within resin-rich regions) removal, on the other hand, is facilitated by longer flow time at moderate flow level. The objective of our cure cycle modification is to reduce overall processing time while keeping macro-voids content low and maintaining “effective flow number” comparable to that of Reference and Modified I.

About heat transfer, we manufactured a 48-ply thick laminate using the same materials and procedures, and used six thermocouples (simultaneously) to monitor process temperatures during the cure cycle. One thermocouple was placed directly onto the tool (TC1), three in-between the laminate (after every 12 plies—TC2, 3, and 4), one at the top of the laminate (TC5), and one above the breather (TC6, as a control to track oven temperature). The temperature difference between TC1 and TC5 remained below 5°C (1–3°C at most times) throughout the cure cycle, while the difference was almost negligible between TC1 and TC2. However, during the temperature ramp steps, we observed that both the tool and the laminate temperatures (TC1-5) lagged from the oven temperature (TC6), resulting in deviations from model predictions for degree of cure and resin viscosity. Introducing the intermediate dwells to the conventional process cycle decreased the temperature gaps between the dwells, and thus significantly reduced the extent of temperature lags. In our future work, we plan to address the effects of cure cycle modification (or intermediate dwells) on mitigating temperature lags and improving cure model accuracy.

Cure cycle modification

Traditionally, structural composites for aircraft parts begin as layers of prepregs or carbon fiber beds pre-impregnated with uncured resin. These layers of prepregs are stacked on a tool to form a laminate of different geometry to meet end-use application, vacuum bagged, and placed in an autoclave for processing. In the autoclave process, additional pressure is applied to these layers of prepregs while vacuum is being pulled in the bag. The applied pressure conforms the laminate to the shape of the tool and suppresses porosity by maintaining volatiles in liquid state, which is the main source of defect in the prepreg-based manufactured parts. Autoclave manufacturing is robust and well understood. However, it incurs high capital investment on equipment acquisition and operations cost. To mediate cost inefficiency, a new generation of Out-of-Autoclave (OoA) processing method has been introduced, targeting production of autoclave quality parts using VBO consolidation. As shown in Figure 1

, VBO prepreg offers an extra evacuation channel comprising of a partially impregnated microstructure to facilitate the volatiles removal during processing. Therefore, in the VBO processing, the resin behavior encompassing cure and flow becomes more important than it did in the autoclave case, which requires a different approach to manufacture quality parts.

Reference method

Reference, developed mainly for autoclave processing, consists of a vacuum hold at room temperature (RT) for 16 hours, followed by a ramp of approximately 1.5°C/min and 2 hours of hold at 121°C. After the initial dwell, the temperature is once again raised at a rate of 2.0°C/min to 177°C and held constant for another two hours for post-cure (Figure 2

(a)). The prolonged vacuum hold for 16 hours at RT helps to remove the internal air entrapped between the tool surface and laminate as well as in inter-ply region without applying additional pressure. Using phenomenological cure kinetics and viscosity models in a following work, Kim and Nutt,¹² the effective flow number (N_Fl,eff) of Reference was predicted to be 582.1 Pa⁻¹.

Modified I method

Recently, the manufacturer has released a new cure cycle to reduce the total processing time by replacing the extended RT vacuum hold. It was found that a 2 hour dwell at 60°C can achieve comparable volatiles removal to that obtained from the 16 hour vacuum hold at RT (Figure 2(b)). The main reason for introducing the dwell at 60°C instead of RT is to shorten the cure cycle time while securing enough resin viscosity to avoid breakdown of volatiles evacuation channels. The rapid evolution of resin viscosity at a temperature higher than 60°C is expected to collapse the evacuation channel even before the removal of volatiles. In addition, the major source of volatiles in the VBO resin is known to be water, and Figure 3

shows that 60°C is well above the boiling point of water at the vacuum level (28 inHg or 71 cmHg).⁸ Therefore, the vacuum holds at 60°C for 2 hours can potentially facilitate moisture and air removal compared to original 16 hour vacuum hold at RT. The phenomenological models¹² predicted N_Fl,eff of 582.6 Pa⁻¹ for Modified I, which was almost the same as that of Reference.

Modified II method

The Modified II has been developed to reduce the processing time and cost while maintaining the required part quality. The cure cycles of the existing methods take a long time to complete where Reference takes at least 22 hours and Modified I over 8 hours. A longer processing time increases the operation cost and serve as a production bottleneck. To increase process efficiency, Modified I needs to be modified to meet both required part quality and cost.

First, the initial dwell temperature was modified to 55 ± 6°C, which was still above the boiling point of water under full vacuum (Figure 3). Lowering the temperature of initial dwell by 6°C may not contribute significantly to energy savings, but such modification still allows easier handling and process monitoring for the operator. Second, the original 2 hour isothermal dwell at 121°C was removed from the cure cycle. In a typical autoclave process, this stage allows volatile suppression while maintaining low viscosity (high resin flow) by having additional pressure placed on the bag. On the other hand, VBO process, with limited pressure available during vacuum consolidation, is not as effective on suppressing volatiles in the low viscosity region as in the case of autoclave. Instead of having 121°C dwell, which is neither effective on energy savings nor on volatiles removal, a slow ramp was applied from the initial dwell temperature to the post-cure temperature (177°C). Potential defects such as voids can be eliminated during the first dwell stage at 55 ± 6°C. The temperature, degree of cure, viscosity, and inverse viscosity evolution profiles of Modified II, predicted by the phenomenological models, are shown in Figure 4

. The predicted N_Fl,eff value (area under the inverse viscosity curve) of Modified II was 680.5 Pa⁻¹, which was approximately 17% higher than those of Reference and Modified I. Hence, based on effective flow number analysis, the Modified II was expected to exhibit improved resin infiltration into the fiber tows compared to both Reference and Modified I. Table 3

Table 1.

Parameters for cure kinetics models.¹

A₁	$\frac{E_{1}}{R}$	m₁	n₁	A₂	$\frac{E_{2}}{R}$
1.48E + 07	1.02E + 04	0.17	19.3	8.3E + 04	8.54E + 03
m₂	n₂	D₂	$α_{c 0}$	$α_{c T}$
0.70	0.87	97.4	−1.6	5.7E-03
A₃	$\frac{E_{3}}{R}$	m₃	n₃	A₄	$\frac{E_{4}}{R}$
6.39E + 07	8.94E + 03	1.65	16.6	9.8E + 04	7.1E + 03
m₄	n₄	D₄	$α_{c 0}$	$α_{c T}$
1.66	3.9	63.3	−0.60	3.0E-03

*w₁ = −5.34 × 10⁻³t³+ 2.40 × 10⁻¹t² – 2.67t + 1.

w₂ = −2.74 × 10⁻⁵t³+ 2.24 x 10⁻³t² – 0.05t + 1.

₃ = 3.21 × 10⁻⁵t³ – 1.62 x 10⁻³t²+ 2.74t + 1.

₄ = 1.

Table 2.

Parameters for viscosity models.¹

$A_{η_{1}}$	$\frac{E_{η_{1}}}{R}$	$A_{η_{2}}$	$\frac{E_{η_{2}}}{R}$	$α_{g e l}$
4.52E-09	7.59E + 03	1.73E-14	1.24E + 04	0.66
A	B	C	d	e
14.1E-06	53.7	−44.96	−0.13	−0.11

w₁ = 1.

₂ = 2.9 × 10⁻⁵t³ – 2.3 × 10⁻³t²+ 2.8 × 10⁻²t + 1.

Table 3.

Comparison of cure cycles processing parameters.

Cure cycle	Reference	Modified I	Modified II
Vacuum level	Min. 28 inHg	Min. 28 inHg	Min. 28 inHg
Pre-processing step (Debulk)	16 hr RT vacuum	60°C dwell (2 hr)	55 °C dwell (2 hr)
Cure cycle	121°C (2 hr) ⇒ 177 °C (2 hr)	121°C (2 hr) ⇒ 177 °C (2 hr)	177°C (2 hr)
Ramp rate	2 ∼ 3°C/min	2 ∼ 3°C/min	1 ∼ 2°C /min
Total processing time	>22 hr	>8 hr	>6.5 hr
Effective flow number (N_Fl,eff)	582.1 Pa⁻¹	582.6 Pa⁻¹	680.5 Pa⁻¹

summarizes the processing parameters of Reference, Modified I and Modified II.

Effect of pre-processing condition

Bulkhead (demonstrator I)

Bulkhead, one of the major structural parts in aircraft, was selected as the first demonstrator part to compare the effectiveness of Reference and Modified I, because the part features complex configurations such as contours, corners, drop-offs and honeycomb core area consisting of less than 10 plies (Figure 5

). The dimension of the demonstrator part was 0.8 m by 0.3 m. CYCOM 5320-1 T650 PW (plain weave) from Solvay was used to manufacture this demonstrator part. The main areas of interest for porosity analysis were corners, drop-offs, and core areas.

Results of experiments (reference vs. Modified I)

The quality of the bulkhead demonstrator parts manufactured using Reference and Modified I were evaluated through porosity analysis. The main difference between the Reference and Modified I were debulk temperature and time: 16 hours at RT and 2 hours at 60°C for Reference and Modified I respectively. To observe the internal defects such as wrinkle, porosity and void, 21 different specimens were sectioned from each demonstrator part (Figures 6

and 7

) and examined. Both Reference and Modified I parts exhibited no wrinkles throughout the flat, cornered, and drop-off regions, yet the overall void content was significantly higher for the bulkhead part fabricated with Reference. In particular, macro-voids in corner area was observed only in Reference part. A digestion method was used to measure the overall part void content according to EN 2564.¹⁷ Three test samples were sectioned from the same locations of each part, whose porosity analysis result is shown in Table 4

Table 4.

Void contents in bulkhead demonstrator part.

	Reference	Modified I
No. 1	2.78%	1.54%
No. 2	3.87%	1.19%
No. 3	3.52%	0.76%
Avg.	3.39%	1.16%

. The average void content of Reference part was three times higher than that of Modified I part.

The surface porosity on the tool side (8 locations in Figure 5(b)) was observed (Figure 8

) and calculated with DinoCapture 2.0 program (Table 5

Table 5.

Surface porosity in demonstrator I.

	Reference	Modified I
No. 1	6.1%	1.9%
No. 2	7.5%	1.2%
No. 3	4.9%	1.0%
No. 4	12.0%	0.9%
No. 5	16.8%	0%
No. 6	14.0%	0%
No. 7	1.0%	5.7%
No. 8	4.7%	0%
Avg.	8.4%	1.3%

). The measurement of surface porosity showed that Modified I (1.3%) was more effective than Reference (8.4%) in evacuating the air entrapped between the tool side and surface of the part:

S P = \frac{\sum^{​} A_{i n d}}{Unit Area} \times 100 %

(7)

where SP is surface porosity, A_ind is area of individual porosity (mm²) and unit area is 35 mm².

Both internal and surface porosity analysis results demonstrate that parts produced with Modified I feature considerably lower void contents than Reference parts. The viscosity model predicts that the resin viscosity decreases by two orders of magnitude when the temperature is raised from RT to 60°C. Therefore, introducing a pre-processing step at an elevated temperature of 60°C provided more time for volatiles to escape under moderate flow level condition, thereby facilitating macro-voids removal, compared to having a prolonged RT debulk stage. Based on the degree of cure simulation results, obtained from Raven software (https://www.convergent.ca/products/raven-simulation-software) using phenomenological cure kinetics model, the CYCOM 5320-1 resin exhibits rapid increase in degree of cure starting around 93°C. The 2 hour pre-processing step at 60°C, which is well below the onset of degree of cure rise, allowed voids to escape prior to breakdown of evacuation channels. Cured ply thickness data of bulkhead parts are shown in Table 6

Table 6.

Thickness variation in demonstrator I.

	Reference	Modified I
4ply, CPT 0.032in	0.033 ± 0.001 in (3% STDEV)	0.032 ± 0.0005in (0.4% STDEV)
8ply, CPT 0.064in	0.066 ± 0.001 in (2.5% STDEV)	0.065 ± 0.001in (1.9% STDEV)

Effect of cure temperature

Demonstrators (demonstrator II and III)

A complex-shaped fuselage with multiple honeycomb cores was chosen as the second demonstrator part, measuring 2 m in length (Figure 9

). The part was manufactured with Solvay CYCOM 5320-1 toughened epoxy resin prepreg of two different carbon fiber bed architectures: T650 PW and UD (unidirectional). To manufacture high quality parts, it was critical to secure sufficient resin flow during processing especially because the part included multiple cores. A careful design of cure cycle was necessary to prevent the resin from infiltrating into the honeycomb cores and to minimize surface resin starvation.

The last demonstrator part fabricated was I section frame (Figure 10

). The manufactured frame was over 2 m in length, again fabricated with two different versions of CYCOM 5320-1 prepregs: T650 PW and UD. A closed mold was used to make the “I” shape. Due to having thick closed mold, it was relatively more difficult to evacuate the entrapped air once the void had formed. Furthermore, the air entrapped between the tool surface and the preform could also cause surface pin holes. Both Modified I and Modified II were used to manufacture this demonstrator part, whose internal and surface quality was then evaluated.

Results of experiments (Modified I vs. Modified II)

The cross-sectional images of fuselage and I section frame demonstrator parts, manufactured with Modified I and Modified II, are shown in Figures 11

to 14

. To evaluate the part quality, internal defects such as wrinkle, porosity and void were inspected according to the engineering requirements. For the fuselage configuration, no wrinkle or void was observed from both Modified I and Modified II parts. However, for the I section frame configuration, the porosity level increased in both parts, because non-uniform heat distribution of the thick closed mold disturbed resin flow and air evacuation. The Modified II was shown to yield enhanced part surface quality (Figure 15

). To compare the void contents, the same digestion method test was performed (Tables 7

Table 7.

Void contents in demonstrator II.

	Modified I	Modified II
No. 1	0.63%	0.77%
No. 2	0.19%	0.11%
No. 3	1.55%	1.75%
Avg.	0.79%	0.88%

and 8

Table 8.

Void contents in demonstrator III.

	Modified I	Modified II
No. 1	1.72%	2.87%
No. 2	2.32%	2.14%
No. 3	1.99%	1.67%
Avg.	2.01%	2.23%

). The void contents results show that both cure cycles were effective in meeting the part quality requirements.

We modified the Modified I by eliminating the 2 hour isothermal dwell step at 121°C. In the case of using autoclave, where high pressure is applied, macro-voids can be effectively evacuated throughout the entire 4 hours of pre-processing (2 hours at 60°C) and isothermal dwell (2 hours at 121°C) steps. However, in the case of using VBO processing, where the available pressure is limited by vacuum consolidation, having 2 hours of low-viscosity dwell step at 121°C may not be efficient for removing macro-voids. Hence, we modified the cure cycle based on the assumption that macro-voids can practically be removed to the maximum extent during the 60°C pre-processing step. Despite the shortened processing time, the phenomenological models predicted that the effective flow number of Modified II was 17% higher than that of Modified I. The major concern with Modified II was that removing the 2 hour dwell at 121°C may lead to greater macro-voids content. However, the porosity analysis demonstrated that both Modified I and Modified II parts meet part quality requirements, with only minimal increase in porosity level observed in parts manufactured with Modified II.

Physics and mechanical property

Using Reference, Modified I and Modified II, flat panels were manufactured for degree of cure and glass transition temperature (T_g) measurements (Tables 9

Table 9.

Degree of cure data.

	Reference	Modified I	Modified II
Degree of cure (%)	94.3%	95.4%	98.8%

and 10

Table 10.

Glass transition temperature (T_g) data.

	Reference	Modified I	Modified II
T_g (°C)	199.9	201.8	195.7

) DiBenedetto equation¹⁸,¹⁹ empirically describes the relationship between T_g and degree of cure:

\frac{T_{g} - T_{g 0}}{T_{g \infty} - T_{g 0}} = \frac{λ α}{1 - (1 - λ) α}

(8)

where T_g0 is the glass transition temperature of the uncured resin (α = 0), T_g_∞ is the glass transition temperature of the fully cured resin (α = 1), and λ is a structure-dependent parameter value. The total heat of reaction of uncured CYCOM 5320-1 resin was measured with DSC (183.9 J/g). The residual heat of cure of each laminate was also obtained from DSC, which was then subtracted from the total heat of reaction to determine the final degree of cure. The glass transition temperatures of the laminates were measured with DMA (Dynamic Mechanical Analyzer). The degree of cure and T_g data demonstrate that all three laminates virtually went to full cure, with Modified II part showing the highest degree of cure.

Figure 16

shows the tensile test results for the parts produced with three cure cycles (ASTM 3039). From each laminate, three specimens were taken for the tensile test. For observing the relationship between degree of cure and mechanical property, shear or compression tests may be more suitable. Yet, tensile test was chosen instead in this study to concentrate on a tensile property that depends on resin quality based on constant of fiber characteristics. The laminate manufactured with Modified II, which showed the highest final degree of cure, exhibited the highest tensile strength.

Conclusion

In this study, the Reference cure cycle of VBO process from manufacturer was modified based on phenomenological cure kinetics/viscosity models and “effective flow number.” The main objective of our cure cycle modification was to reduce overall manufacturing time for more efficient processing, while maintaining robust part quality. Considering that VBO process has limited pressure availability, the two-hour isothermal dwell step at 121°C was removed from Modified I. Despite the shortened processing time, the effective flow number, which represents process flow quantity or level, of Modified II was 17% higher than those of Reference and Modified I.

Through porosity analysis of Reference and Modified I bulkhead parts, it was concluded that replacing the prolonged RT debulk step with a pre-processing step at an elevated temperature of 60°C, where resin viscosity was predicted to be two orders of magnitude lower than it was at RT, facilitated macro-voids removal. Two other types of demonstrator parts—fuselage and I section frame—were manufactured using Modified I and Modified II. The major concern with Modified II was that reduced processing (flow) time may lead to increased macro-voids content. However, the internal porosity analysis results showed that both parts manufactured with the two cure cycles met engineering part quality requirements, demonstrating that 60°C pre-processing step was sufficient for macro-voids removal. Finally, flat laminates made with Modified II featured the highest degree of cure and tensile strength. Still, parts made with VBO process have relatively higher void content compared to those made with autoclave process. Hence, future work will address further optimization of VBO process cure cycle.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Dong-Keun Hyun

Jung Hwan Shin

References

Kim

Centea

Nutt

. Out-time effects on cure kinetics and viscosity for an out-of-autoclave (OOA) prepreg: modelling and monitoring. Compos Sci Technol 2014; 100: 63–69.

Kim

, et al. Design optimization of double-array bolted joints in cylindrical composite structures. IJASS 2016; 17: 332–340.

Hinrichsen

Bautista

. The challenge of reducing both airframe weight and manufacturing cost. Air Space Europe 2001; 3: 119–121.

Kumar

M-G

Roy

, et al. AFP mandrel development for composite aircraft fuselage skin. IJASS 2014; 15: 32–43.

Centea

Grunenfelder

Nutt

. A review of out-of-autoclave prepregs – material properties, process phenomena, and manufacturing considerations. Compos Part A Appl Sci Manuf 2015; 70: 132–154.

Hamill

Centea

Nutt

. Surface porosity during vacuum bag-only prepreg processing: causes and mitigation strategies. Compos Part A Appl Sci Manuf 2015; 75: 1–10.

Centea

Nutt

. Manufacturing cost relationships for vacuum bag-only prepreg processing. J Compos Mater 2016; 50: 2305–2321.

Ridgard C. Process selection and optimization for out of autoclave prepreg structures. In: CAMX 2015 Conference Proceedings, 26-29 October 2015, pp. 1550–1561.

Bond G, Hahn G and Thomas J. Non-Autoclave manufacturing technology: demonstration of applications. In: Defense manufacturers conference, Anaheim Convention Center Anaheim CA, 11 November 2011.

10.

Bond G and Hahn G. Non autoclave prepreg manufacturing for primary aerospace structure. In: 41st International Society for the Advancement of Material and Process Engineering (SAMPE) technical conference, Wichita, Kansas, 19-22 October 2009.

11.

Cytec Engineered Materials. CYCOM 5320-1 Information Sheet.

12.

Kim

Nutt

. Processability of DDS isomers cured epoxy resin: effects of amine/epoxy ratio, humidity and out-time. Polym Eng Sci 2018; 58: 1530–1538.

13.

Kazils

Partridge

. Exploring equivalence of information from dielectric and calorimetric measurements of thermoset cure—a model for the relationship between curing temperature, degree of cure and electrical impedance. Polymer 2005; 45: 5868–5878.

14.

Hwang

. A study of cure kinetics by the use of dynamic differential scanning calorimetry. Compos Sci Technol 2002; 62: 29–40.

15.

Kratz

Hsiao

Fernlund

, et al. Thermal models for MTM45-1 and cycom 5320 out-of-autoclave prepreg resins. J Compos Mater 2013; 47: 341–352.

16.

Hubert

Poursartip

. A Review of Flow and Compaction Modelling Relevant to Thermoset Matrix Laminate Processing. J Reinf Plast Compos 1998; 17: 286–318.

17.

EN. 2564, Carbon fibre laminates – determination of the fibre, resin and void contents, July 1998.

18.

DiBenedetto

. Prediction of the glass transition temperature of polymers: A model based on the principle of corresponding states. J Polym Sci Part B: Polym Phys 1987; 25: 1949.

19.

Nielsen

. Crosslinking effect on physical properties of polymers. J Macromol Sci Part C: Polym Rev 1969; 3: 69–103.