FDM Matchplate Patterns for Green Sand Casting

Transcription

1 FDM Matchplate Patterns for Green Sand Casting Sand casting is a cost effective and efficient process for small-lot production, and yet, when using automated equipment, it is an effective manufacturing process for high-volume manufacturing. The sand casting process is relatively simple, and the production of the sand molds and cast metal parts is relatively quick. However, the fabrication of the patterns to produce the sand molds can be time consuming and labor intensive. There are several types of sand casting materials and several approaches to mold making. This document addresses green (wet) sand casting with a flask-less mold-making process that uses matchplate patterns. Substituting matchplates manufactured with the FDM process for those machined from metal will reduce pattern development time to expedite the receipt of prototype or production sand-cast parts. Since FDM is an automated, unattended process, sand casting foundries also increase overall efficiency and productivity while reducing labor costs. An additional benefit is that there is no change in tool design, the tool making process or the casting process. CONTENTS 1. OVERVIEW TRADITIONAL PROCESS OVERVIEW PATTERN DESIGN FILE PREPARATION MATERIALS MATCHPLATE PREPARATION MOLD MAKING AND CASTING TOOLS & SUPPLIES RECAP - CRITICAL SUCCESS FACTORS... 8 Original Date: 04/19/2011 Page 1 of 8 Rev. Date: NC MAKE YOUR IDEAS REAL.

2 1. OVERVIEW 1.1. Application: For green sand casting, FDM produces the patterns that form the sand molds Proven to work with: Flask-less methods that use matchtplate patterns Likely to work with: Flask method. Cope and drag patterns or loose patterns. Figure 1: Window hardware manufactured with FDM matchplate. Dry, air-set and no-bake sand processes FDM is a best fit when: Applications: Low-volume production or custom manufacturing. àà Matchplates have completed 400 cycles, using ABS material, with no sign of wear or damage. Higher cycle counts are possible but have not been tested. Prototyping, proof of concept and process refinement. Compaction pressure below 1,700 psi (11.7 MPa). Higher pressures may be possible but have not been tested. Casting design. Moderate to high complexity. Draw depths < 1 in. (25.4 mm). àà Larger draws possible if feature thickness is > 0.5 in. (12.7 mm). Figure 2: Machined aluminum matchplate. Figure 3: Flask-less, green-sand cope. 2. TRADITIONAL PROCESS OVERVIEW 2.1. The steps in the traditional investment casting process are: Make pattern (Figure 2). Options include matchplate, cope and drag, or loose patterns Make green sand mold (Figure 3). Cope side. Compact sand against pattern. Drag side. Compact sand against pattern. Original Date: 04/19/2011 Page 2 of 8 Rev. Date: NC

3 Assemble mold combine cope and drag sides Pour metal (Figure 4). Cool Remove sand (Figure 5) Cut off gates and runners Finish to specification. Figure 4: Casting metal into green-sand mold FDM adjustments Substitute FDM matchplate for machined pattern No other design or process alterations are necessary. 3. PATTERN DESIGN The matchplate combines an FDM insert with a pre-fabricated metal blank. The FDM insert provides the sand impression for the part, gates and runners. The aluminum blank, which has pockets to hold the FDM insert, provides a rigid structure that prevents damage. Figure 5: Vibratory bed for sand removal Insert design. The design configuration for part, gates and runners is the same as that used when making matchplates conventionally. Begin with the CAD file for the cast part (Figure 6) and scale it to compensate for shrinkage Split pattern. Figure 6: CAD model (front and rear views shown) for window handle Next, split the pattern along the parting line. Separate the two halves by an amount equal to the thickness of the matchplate blank (Figures 7 and 8). Then copy the pattern to create the desired number of impressions per molding cycle. Finally, add the gate and runner system (Figure 9) Create mounting flange. The last step is to extend a flange around the insert that matches the size of the pocket that is machined into the matchplate blank. On the flange, incorporate bolt holes for attachment. Spacing of 1 to 2 inches (25 to 50 mm) is sufficient. Figure 7: Split the CAD model along the parting line and separate the halves by the thickness of the matchplate blank. Figure 8: Side view of matchplate pattern showing allowance for matchplate blank. Original Date: 04/19/2011 Page 3 of 8 Rev. Date: NC

4 3.2. Considerations Deep draws. If the matchplate has areas with draws deeper than 1 in. (25 mm), review the features in the deep-draw region. If there are any thin-walled features, increase their widths to 0.5 in. (12.7 mm) to prevent damage during sand compaction Multiple inserts. For matchplates with two inserts, machine two pockets in the metal blank. Separate the pockets with a rib of material that is wide enough to resist bending during sand compaction (Figure 12). Figure 9: Section view of pattern. Note that runner system is excluded (see 3.3.2) Options Matchplates split patterns. Split patterns can replace a single, two-sided insert (Figure 13). Follow the instructions (above), but produce two separate CAD files. Each split pattern will have a flat side that flush mounts to a solid base on a matchplate blank. Figure 10: Matchplate insert (cope side) with 8 window handles Gate and runner components. If alterations to the mold configurations are likely, extract the gates and runners from the insert and design them as separate components (Figures 14 and 15) that will also be made with FDM. This approach isolates the redesign and rebuilding process to just the gate or runner, which reduces the time and expense when refining the mold design. Figure 11: Matchplate insert (drag side) Export STL file. After completing the pattern design, export the CAD model as an STL file. Ensure that settings, such as chord height, deviation and angle will produce a fine mesh (small facets). This will minimize postprocessing efforts and preserve accuracy. 4. FILE PREPARATION 4.1. In Insight, open the STL file for the pattern (Figure 16). Figure 12: Matchplate blank incorporates a thick rib between inserts for stiffening Orient pattern. When orienting the pattern, consider both the surface finish and total process time (Figure 17). Fine-grained sand will capture surface imperfections, so smooth surfaces are needed. While an orientation with a low Z height may decrease build time, it may increase the labor needed to post process the pattern. Figure 13: Split-pattern matchplate. FDM patterns mount flush to each side of blank. Original Date: 04/19/2011 Page 4 of 8 Rev. Date: NC

5 Slice the file. Select a Slice height (Modeler > Setup) that balances surface smoothness, feature resolution and build time. For moderate-sized patterns, in. (0.25 mm) is suggested. For larger patterns, in. (0.50 mm) may be advisable. After making Slice height selection, slice the file (Figure 18) Part interior style. Set Part interior style to Solid-normal. (Modeler > Setup) Figure 14: CAD models of interchangeable runner systems for casting refinement. When making sand molds with automated systems, a solid fill style is recommended. This style maximizes the pattern s resistance to compressive forces, which will reduce the possibility of insert damage when compacting sand. If hand-ramming sand molds, Sparse or Sparse-double dense may be considered since the compaction forces are lower. Note, however, that there has been no testing with these styles, so there is no data and no recommendations for their use Visible surface style. Figure 15: Making gating and runner system as a separate piece, it may be interchanged with alternate configurations.. Set Visible surface style to Enhanced mode. (Modeler > Setup) Enhanced mode use small rasters for the toolpaths of external, visible surfaces. Internally, it uses thicker rasters. This style improves surface finish, which decreases the time needed for surface smoothing, while decreasing build time. Enhanced mode allows adjust of the raster widths, but the default settings are suitable for sand casting patterns Create toolpaths and supports. Figure 16: Open the pattern s STL file in Insight. 5. MATERIALS Any FDM material can be used for green sand matchplates. However the most commonly used materials are: ABS ABS-M30 ULTEM Recommended materials. Figure 17: Orient the pattern with consideration for both surface finish and total process time. Considering the performance of ABS-M30 and the number of finish options available, it is the recommended FDM material for matchplate production (Figure 19). Foundries have reported production of 400 molds with ABS-M30 matchplates with no wear or damage. Replace ABS with ULTEM 9085 if the application requires a sparse fill and compaction forces are in the mid to high range. Use ULTEM 9085 with a solid fill for applications where the compaction pressures approach (or exceed) the upper threshold of 1,700 psi (11.7 MPa). Figure 18: Specify slice height and slice file. Original Date: 04/19/2011 Page 5 of 8 Rev. Date: NC

6 6. MATCHPLATE PREPARATION 6.1. Remove supports. Following the FDM build, begin the matchplate preparation by removing the support structures. These may be removed manually, or if soluble supports are used, by dissolving them Smooth molding surfaces. If using ABS for insert production, there are two surface smoothing options: solvent smoothing or sanding. For all other materials, sand surfaces to the desired smoothness. Figure 19: ABS-M30 is one of three recommended materials for green sand casting Sanding (Figure 20). Begin with an initial sanding using 120- to 320-grit sand paper. Next, fill in all depressions and layer lines using body filler, glazing putty or spot putty. After the filler has dried, sand all surfaces to the desired smoothness Solvent smoothing (ABS materials only). Figure 20: After support removal, smooth pattern surfaces. If available, use the Finishing Touch Smoothing Station to prepare the pattern. This is a labor-free process that can be completed in less than five minutes. It yields a smooth surface finish while preserving dimensional accuracy. Alternatively, apply a solvent to the pattern to smooth the surfaces. These may be brushed onto the pattern, or the pattern can be dipped into them. Avoid pooling and limit the duration of exposure since the solvent will begin to degrade feature details and dimensional accuracy. With either method, allow the pattern to set for two hours before doing any other pattern processing. This gives the pattern enough time for its surfaces to harden. Before putting the pattern into service, allow it to rest for at least 24 hours. Figure 21: Mount (bolt) FDM insert to machined matchplate blank Assemble matchplate. Mount the FDM insert on the metal matchplate blank (Figures 21 and 22). Bolt the insert to the blank using firm, but not excessive, torque. Excessive force may cause the FDM insert to crack. Optionally, place a metal bushing in each bolt hole to prevent cracking. Figure 22: Complete matchplate (drag side shown) with two pattern inserts and removable gate/ runner system Apply face coat (optional). Borrowing a practice from the wood-pattern days, apply a face coat to the FDM inserts (Figure 23). Use a material like Master Foundry- Kote to seal the pores of the insert to avoid release agent from wicking into the surface and prevent sand from sticking to it. These materials also provide abrasion resistance. To apply, follow manufacturer s instruction. Figure 23: Apply a face coat to seal pores and provide added abrasion resistance. Original Date: 04/19/2011 Page 6 of 8 Rev. Date: NC

7 6.5. Apply release agent. The last step before mold making is to apply a release agent to the matchplate. Use any material that is compatible with the sand casting process. 7. MOLD MAKING AND CASTING Once the matchplate has been prepared, there are no alterations to either the mold making or casting processes. Figure 24: Mount matchplate in automated molding machine Mold making. Mount matchplate in automated molding machine (Figures 24 and 25). As noted earlier, recommended pressures are below 1,700 psi (11.7 MPa) for an FDM matchplate with solid fill. If using sparse fill options, maximum pressures will be lower. The molding machine will produce the cope and drag sides of the mold (Figure 26) and assemble them (Figure 27). If ripping occurs when sand pulls off of the cope or drag as the matchplate is extracted (re) apply the face-coat material. Figure 25: Matchplate in automated molding machine Casting. Complete production of sand-cast parts by: Casting the metal alloy into the sand mold (Figure 28). Breaking out castings from sand mold (Figure 29). Cutting off gates (Figure 30). Finishing to specification (Figure 31). Figure 26: Cope side of sand mold for window handle. 8. TOOLS & SUPPLIES 8.1. Required items: None beyond those used in any green sand casting operation Optional items: Master Foundry-Kote. Finishing Touch Smoothing Station (Figure 32). Solvents (MEK, Acetone, Micro-Mark s SAME STUFF, Weld-On #3 and similar). Figure 27: Complete green-sand molds for window handle Sources Foundry-Kote: Kindt- Collins Company ( Finishing Touch Smoothing Station: Stratasys, Inc. Figure 28: Cast metal alloy. Original Date: 04/19/2011 Page 7 of 8 Rev. Date: NC

8 9. RECAP - CRITICAL SUCCESS FACTORS Good FDM practices. Use matchplate blank with FDM insert. Increase wall thickness in deep draws regions. Follow standard sand casting guidelines. Figure 29: Break out castings from sand mold. To obtain files for the sample tool, or to obtain more information on this application, contact: Stratasys Application Engineering (toll free) (local or international) ApplicationSupport@Stratasys.com Figure 30: Cut off gates. Figure 31: Raw window handle casting ready for finishing. Figure 32: Finishing Touch Smoothing Station. Figure 33: Finished product window handle with latch plate. Original Date: 04/19/2011 Page 8 of 8 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-SandCasting-04-11