Soluble Cores & Mandrels for Hollow Composite Parts

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Kory Scott
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1 Soluble Cores & Mandrels for Hollow Composite Parts There are many manufacturing options for composite part production. Composites are made by winding, wrapping, molding and laying-up various combinations of material fibers and resin systems. Using molds, bucks, patterns, cores and mandrels, composite parts are manufactured in all shapes, sizes and configurations. Independent of the process, hollow composite parts present a unique manufacturing challenge. Straight-pull tube shapes and straight-walled cavities are easily addressed, but any configuration that traps the pattern, core or mandrel needs an alternative solution. Options include extractable cores made of eutectic salt, ceramic or urethane. For low-volume manufacturing, there are two additional alternatives. The first is to lay-up two halves of the hollow piece and join them in another operation. The second option is to lay-up the part on the inside of a clamshell tool. Both alternatives require a tooling pattern (plug), tooling lay-up and part lay-up with curing cycles between each step. The newest alterative for hollow composite parts is FDM soluble cores. Composite is wrapped directly on a core that is dissolved away after the part has cured. This approach eliminates all tooling, which makes one-day delivery possible and design revisions practical. Also, wrapping composite around a core, versus pressing it into a cavity, makes the process simpler and less labor intensive. CONTENTS 1. OVERVIEW TRADITIONAL PROCESS OVERVIEW COMPOSITE SYSTEM COMPATIBILITY CORE DESIGN FILE PREPARATION MATERIALS POST PROCESSING PART PRODUCTION TOOLS & SUPPLIES RECAP - CRITICAL SUCCESS FACTORS...10 Original Date: 04/19/2011 Page 1 of 10 Rev. Date: NC MAKE YOUR IDEAS REAL.

1. OVERVIEW 1.1. Application: Composite parts are manufactured in many ways.

as an air duct, that would be made from clamshell tooling.

The pattern is then dissolved to produce the hollow, composite part.

2.1. Small quantities (often less than 100 parts) of high-value components with short

2. Part characteristics: Figure 2 Complex, intricate design. Seamless construction.

Curing temperatures below 200 F (93 C). Consolidation pressures below 50 psi (345 kpa).

The process is NOT compatible with: Figure 3: Seamless construction with no wrinkles.

Dimension machines (material inversion is not supported). 2.

The steps in the conventional approach to making hollow, composite parts are: Figure 4:

2 1. OVERVIEW 1.1. Application: Composite parts are manufactured in many ways. This application guide addresses production of composite lay-ups for hollow parts, such as an air duct, that would be made from clamshell tooling. FDM produces a soluble pattern on which the composite is wrapped. The pattern is then dissolved to produce the hollow, composite part. Figure 1 & 2: Composite brake duct manufactured from soluble FDM core FDM is a best fit when: Small quantities (often less than 100 parts) of high-value components with short lead times. Functional prototypes for evaluation. Low-volume (custom) manufacturing Part characteristics: Figure 2 Complex, intricate design. Seamless construction. Consistent, wrinkle-free walls Resin systems: Epoxy resin composite structures. Curing temperatures below 200 F (93 C). Consolidation pressures below 50 psi (345 kpa) The process is NOT compatible with: Figure 3: Seamless construction with no wrinkles. Polyester resin systems. Geometries with restrictive internal flow characteristics. Dimension machines (material inversion is not supported). 2. TRADITIONAL PROCESS OVERVIEW 2.1. The steps in the conventional approach to making hollow, composite parts are: Figure 4: Pattern for laying up clamshell tooling Make pattern (Figure 4) Make clamshell tool Lay-up composite material for side A (Figure 5) and cure Lay-up side B and cure Cure tool Assemble clamshell. Figure 5: Pattern and first side of tool. Original Date: 04/19/2011 Page 2 of 10 Rev. Date: NC

2.1.3. Produce composite part. 2.1.3.1. Feed composite strips through tool (Figure 6). Press into cavity. 2.1.3.2. Consolidate resin. 2.1.3.3. Cure composite part. 2.1.3.4.

Figure 6: Pull composite strips through tool and press against walls. 2.2.2. Add process of dissolving core. 3. COMPOSITE SYSTEM COMPATIBILITY 3.1.

The solution that dissolves the core attacks and weakens the polyester resin. Figure 7: Cured composite duct with clamshell tool. 3.2.

These results are geometry and lay-up dependent. Surface bagging. Minimizes core loads by applying consolidation pressure only to the skin of the core, which makes core damage less likely.

3 Produce composite part Feed composite strips through tool (Figure 6). Press into cavity Consolidate resin Cure composite part Open mold and remove part (Figure 7) Trim part FDM adjustments Replace pattern and clamshell tooling with soluble pattern (Figure 8). Figure 6: Pull composite strips through tool and press against walls Add process of dissolving core. 3. COMPOSITE SYSTEM COMPATIBILITY 3.1. Resin systems: FDM soluble cores are compatible with epoxy resin systems. They are not compatible with polyester resin systems. The solution that dissolves the core attacks and weakens the polyester resin. Figure 7: Cured composite duct with clamshell tool Consolidation methods: FDM soluble cores are compatible with the following consolidation methods: Vacuum/autoclave. Has been tested up to 200 F (93 C) and 50 psi (345 kpa). These results are geometry and lay-up dependent. Surface bagging. Minimizes core loads by applying consolidation pressure only to the skin of the core, which makes core damage less likely. Envelope bagging (Figure 9). Simplest method, but it can impart a considerable load to the core. If used, ensure that the core will have the necessary structural strength by adjusting toolpath parameters in Insight. Shrink tape. May produce significant compressive loads that may damage the core. So, care must be taken to find a tape that matches curing temperature while minimizing pressure. Figure 8: FDM soluble core (right) eliminates pattern and tooling. Figure 9: Envelope bagging is one of the consolidation methods compatible with FDM soluble cores. Original Date: 04/19/2011 Page 3 of 10 Rev. Date: NC

4 3.3. Curing processes: FDM soluble cores are compatible with the following curing methods: Thermal sets. Maximum curing temperature is 200 F (93 C). Less than 180 F is recommended since core strength is improved and thermal expansion is decreased. Two-part resins. The room-temperature cure preserves core strength and minimizes thermal expansion. Recommended for large, complex parts where maximum core strength is needed. Microwave, E-Beam and X-Ray. Limited temperatures associated with these resin-curing systems will maximize core strength and minimize thermal expansion. 4. CORE DESIGN 4.1. Create solid, 3D model. To begin the process, a CAD model of the core is needed. This CAD model represents the hollow area of the composite part Open the CAD model for part (Figure 10). Using the interior surfaces of the composite part, create a model for the core. This model will fill the air space within the part (Figures 11 and 12). Figure 10: Start with the CAD model of the final product (brake duct) Export STL file Adjust settings, such as chord height, for the STL file so that small facets are produced. This will create smooth surfaces that require less post processing for the soluble core CTE compensation (optional). The coefficient of thermal expansion (CTE) for the FDM soluble material is substantially higher than that of the composite resins. Compensation may be desirable if: The application requires tight tolerances. Figure 11: CAD model of brake duct and the soluble core (white). The curing temperature is high. The core may be damaged as it tries to expand if the composite material restricts that expansion SR30 soluble support material. SR30 s CTE is 6.5E-5 in/(in- F) [11.7E-5 mm/(mm- C)] Figure 12: Soluble core model. Export as an STL file. Original Date: 04/19/2011 Page 4 of 10 Rev. Date: NC

Shrink the CAD model of the core by the difference between SR30 s CTE and that of the composite material.

1E-6 mm/(mm- C)] 6.5E-5 in/(in- F) ) [11.7E-5 mm/(mm- C)] 10 in. X 125 F X ( 6.5E-5 in/(in- F)- 0.6E-6 in/(in- F)) = 0.0805 in. 254 mm X 52 C X (11.7E-5 mm/(mm- C) - 1.1E-6 mm/(mm- C)) = 1.53 mm 5.

This orientation promotes fluid flow along the channels created by the sparse interior fill created in the next step.

5 Shrink the CAD model of the core by the difference between SR30 s CTE and that of the composite material. Example: Temperature delta: 125 F (Cure=200 F, ambient=75 F) 52 C (Cure=93 C, ambient=41 C) Core length: Composite CTE: SR30 CTE: Compensation: 10 in. (254 mm) 0.6E-6 in/(in- F) ) [1.1E-6 mm/(mm- C)] 6.5E-5 in/(in- F) ) [11.7E-5 mm/(mm- C)] 10 in. X 125 F X ( 6.5E-5 in/(in- F)- 0.6E-6 in/(in- F)) = in. 254 mm X 52 C X (11.7E-5 mm/(mm- C) - 1.1E-6 mm/(mm- C)) = 1.53 mm 5. FILE PREPARATION 5.1. Import the STL file into Insight and orient. Align the ends of the core along the Z-axis (Figure 13). This orientation promotes fluid flow along the channels created by the sparse interior fill created in the next step. Providing these pathways accelerates the dissolving process that removes the core from the composite part (section 8.4). If the composite part has only one opening for the internal cavity, orientation is not as important. Yet, it is still advisable to orient it such that the soluble support solutions will have access to the greatest surface area when the composite part is immersed. Figure 13: Align the pattern with its length along the Z axis to promote flow through its sparse fill Slice part (Figure 14) Define toolpaths. The goal for tool path setup is to decrease the amount of material in the core so that it dissolves faster. At the same time, the core must maintain structural integrity while exposed to the heat and pressure of the composite lay-up and curing processes. To achieve this, the core is constructed with a modified Sparse-double dense Part interior fill style (Figure 15) Toolpaths > Setup. Set Part interior style to Sparse - double dense. Open Advance parameters. Set Part interior depth to in. (1.52 mm). Set Part sparse fill air gap to in. (6.35 mm). Set Sparse solid layers such that the capping layers have the same thickness as the side walls. Value = Part interior depth slice thickness Optimized settings. Figure 14: Sliced file ready for toolpath adjustments. Figure 15: The toolpaths will create a mostly hollow interior that dissolves quickly, Original Date: 04/19/2011 Page 5 of 10 Rev. Date: NC

For the vast majority of composite parts, the toolpath settings (above) will provide satisfactory results.

In either case, adjustments are made to Part sparse fill air gap, Part interior depth and Part raster width (Figure 18).

If adjustments are excessive, the core surface may dip/depress between rasters or the core may collapse. Figure 16: Top view of soluble core toolpaths.

6 For the vast majority of composite parts, the toolpath settings (above) will provide satisfactory results. If desired, these may be adjusted to decrease build time and material consumption or improve strength. In either case, adjustments are made to Part sparse fill air gap, Part interior depth and Part raster width (Figure 18). For faster builds and lower material consumption: Increase air gap, decrease interior depth and/or decrease raster width. Caution: consider the process temperatures and pressures. If adjustments are excessive, the core surface may dip/depress between rasters or the core may collapse. Figure 16: Top view of soluble core toolpaths. For greater strength: Decrease air gap, increase interior depth and/or increase raster width. Caution: these adjustments will increase the volume of support material, which will increase the time needed to dissolve the core Remove top & bottom surfaces (optional). Figure 17: The toolpath uses Sparse double dense with three contours around the perimeter. The capping surfaces on the top and bottom of the core, assuming the part is oriented as described above, are undesirable. They may be removed manually (by hand) from the pattern or eliminated from the FDM part (if these surfaces are roughly parallel to the build platform) (Figure 19). To eliminate them from the part requires a custom group. Create a custom group for the solid layers (Toolpaths > Custom groups > New). Adjust toolpath settings (Toolpath parameters). Set Contours to depth to in. (1.52 mm). Set Raster to raster air gap to 0.25 in. (6.35 mm). Figure 18: Toolpath setting to optimize for build speed, material consumption or strength. Check Align rasters and Double rasters boxes. Apply the custom group to all capping layers. Select layers and click Add in the custom group window. Figure 19: Use custom groups to remove the solid layers on the top and bottom of the core. Original Date: 04/19/2011 Page 6 of 10 Rev. Date: NC

7 5.4. Switch model and support materials. The core is the model in FDM terms, but it needs to be built with the soluble support material. To do this, use a new function that was made available in Insight 7.1 (Figure 20). Go to Toolpaths > Output file setup. Check Invert build materials box. This toolpath option causes the model to be built with support material and the support to be built with model material. 6. MATERIALS Figure 20: In output file setup, check the invert build materials box to make the core from soluble support material The recommended FDM material is SR30. SR (soluble release) 30 has been used exclusively for all testing and trials of the soluble core process. It is recommended because of it durability when exposed to elevated temperatures and pressures. SR20, an alternative soluble support material, may be viable. However, there is no test data or experience on which to build. 7. POST PROCESSING After building the soluble core in a Fortus machine, prepare it for the composite lay-up process Detach support structure Smooth surface (optional). If surface roughness on the interior walls of the composite part is undesirable, smooth the surfaces of the soluble core. The core may be hand-sanded with 120- to 220-grit sandpaper (Figure 21). Optionally, a grit/soda blaster, loaded with walnut shells or baking soda, may be used Seal surfaces. If the composite resin impinges on the surface of the soluble core, it will not dissolve completely. When this happens, the composite part will have an irregular lining of undesired material. Figure 21: Sand the soluble core to the desired level of smoothness. For resin transfer composites, sealing is mandatory. Consolidation pressure drives the liquid resin into any surface pores. For pre-pregs, sealing is often unnecessary, but this is dependent on the complexity of the part Sealant options: Water-soluble sealant. Release-coated shrink tape. Release bagging system. Original Date: 04/19/2011 Page 7 of 10 Rev. Date: NC

The water-soluble sealant is brushed onto the surface of the core.

Prior to wrapping the core with composite material, apply a release agent (Figure 22). Use only water-based products or carnauba wax.

Wrap composite material around the soluble core to the desired thickness (Figure 23). Trim excess prior to curing (Figure 24).

3 C) or a pressure of 50 psi (345 kpa). Figure 23: Wrap composite cloth around core. If using a composite with a higher cure temperature, use a two-stage approach.

Figure 24: Trim to remove excess composite cloth. After the composite part has cured, remove the core by dissolving it. 8.4.1.

8 The water-soluble sealant is brushed onto the surface of the core. The two release options will seal the surfaces while acting as the release agent that allows the core to separate from the cured composite. 8. PART PRODUCTION 8.1. Apply release agent. Prior to wrapping the core with composite material, apply a release agent (Figure 22). Use only water-based products or carnauba wax. If release-coated shrink tape or release a bagging system is used as the sealing method, a release agent is not needed. Figure 22: Coat core with a water-based release agent Composite layup. Wrap composite material around the soluble core to the desired thickness (Figure 23). Trim excess prior to curing (Figure 24). Then apply the materials suited for the selected consolidation method (Figures 25 and 26) Composite curing. As previously noted, do not exceed a curing temperature of 200 F (93.3 C) or a pressure of 50 psi (345 kpa). Figure 23: Wrap composite cloth around core. If using a composite with a higher cure temperature, use a two-stage approach. Perform an initial cure at a lower temperature. Remove the core (section 8.4). Complete the cure at the higher temperature Core removal. Figure 24: Trim to remove excess composite cloth. After the composite part has cured, remove the core by dissolving it To accelerate the dissolving cycle: If capping layers were not removed in the file preparation step (5.3.3), break open the ends of the core to expose the sparse fill structure to the dissolving solution. One option is to drill several holes through the capping layer with a large diameter bit. Next, manually extract as much of the core as possible (optional). Figure 25: Wrap part with release ply Dissolve core. Figure 26: Add breather cloth over release ply. Original Date: 04/19/2011 Page 8 of 10 Rev. Date: NC

Immerse the composite part in a heated, soluble support removal bath (Figure 30). Do not use ultrasonic tanks because they are ineffective at core removal.

Common configurations that require increased circulation are long passages as diameter decreases the length requiring additional flow also decreases and

Figure 27: Consolidate composite (surface bagging shown). Monitor the process and remove the composite part when the core has completely dissolved (Figure 31).

. 9. TOOLS & SUPPLIES 9.1. Required items: 9.2. Sources: Tools and materials for composite lay-up (Figure 33). FDM SR30 soluble support material.

Flexible release bagging system (E.g., Stretchlon). Figure 28: Cure composite part below 200 F (93.3 C). Figure 29: Heated, soluble support removal tank. 9.2.1.

9 Immerse the composite part in a heated, soluble support removal bath (Figure 30). Do not use ultrasonic tanks because they are ineffective at core removal. Note that good circulation of the fluid is important. In some circumstances, an auxiliary pump, or similar, may be needed to increase the rate of circulation. Common configurations that require increased circulation are long passages as diameter decreases the length requiring additional flow also decreases and passages with multiple or tight bends. When using auxiliary flow, route it directly to the soluble core. Figure 27: Consolidate composite (surface bagging shown). Monitor the process and remove the composite part when the core has completely dissolved (Figure 31). Complete the process with a thorough rinsing to remove all of the soluble support solution (Figure 32). 9. TOOLS & SUPPLIES 9.1. Required items: 9.2. Sources: Tools and materials for composite lay-up (Figure 33). FDM SR30 soluble support material. Soluble support solution and heated agitation tank. Release material: Release agent (E.g., Frekote 700-NC). Release-coated shrink tape (E.g., Dunstone or 3M). Flexible release bagging system (E.g., Stretchlon). Figure 28: Cure composite part below 200 F (93.3 C). Figure 29: Heated, soluble support removal tank Frekote: SR30 and soluble solution: Stratasys, Inc Bagging systems: Figure 30: Place part in heated tank and allow core to dissolve. Figure 31: Remove composite part from tank after core has dissolved. Original Date: 04/19/2011 Page 9 of 10 Rev. Date: NC

10. RECAP - CRITICAL SUCCESS FACTORS 10.1. Composite material: Compatible resin system and curing cycle. 10.2. Soluble core: Sparse-double dense interior fill style. Invert build materials. 10.3.

To obtain a file for the sample part, or to obtain more information on this application, contact: Stratasys Application Engineering 1-855-693-0073 (toll free) +1 952-294-3888 (local or international)

Date: NC Stratasys Incorporated 7665 Commerce Way, Eden Prairie, MN 55344 +1 888 480 3548 (US Toll Free) +1 952 937 3000 (Intl) +1 952 937 0070 (Fax) www.stratasys.com info@stratasys.

com 2011 Stratasys Inc. All rights reserved.

10 10. RECAP - CRITICAL SUCCESS FACTORS Composite material: Compatible resin system and curing cycle Soluble core: Sparse-double dense interior fill style. Invert build materials Core removal: Figure 32: Thoroughly wash part to remove dissolving solution. Thorough application of sealer and/or release agent. Heated solution with good circulation. To obtain a file for the sample part, or to obtain more information on this application, contact: Stratasys Application Engineering (toll free) (local or international) ApplicationSupport@Stratasys.com Figure 33: Composite layup materials. Figures 34 & 35: Final product, a composite brake duct, is ready for use. Figure 35 Original Date: 04/19/2011 Page 10 of 10 Rev. Date: NC Stratasys Incorporated 7665 Commerce Way, Eden Prairie, MN (US Toll Free) (Intl) (Fax) info@stratasys.com ISO 9001:2008 Certified Stratasys GmbH Weismüllerstrasse 27, Frankfurt am Main Germany (Tel) (Fax) europe@stratasys.com 2011 Stratasys Inc. All rights reserved. Stratasys, Fortus, Dimension, uprint and FDM are registered trademarks and Real Parts, Fortus 360mc, Fortus 400mc, Fortus 900mc, Finishing Touch, Insight, Control Center and FDM TEAM are trademarks of Stratasys Inc., registered in the United States and other countries. *ULTEM 9085 is a trademark of SABIC Innovative Plastics IP BV. All other trademarks are the property of their respective owners. Product specifications subject to change without notice. Printed in the USA. SYSS-TAG-SolubleCores-04-11

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