Exercise: Mold Design Workshop (in multiple parts)

This extended exercise is a hands-on practical introduction to designing and fabricating silicone parts. It introduces essential mold design techniques and practical fabrication of silicone parts.

A critical lesson of this exercise is that each development iteration involves a long critical path: design review, STL file approval, 3D printing, print cleaning, casting and curing, bonding and curing. Each of these steps can introduce significant delay; it is important to plan your schedule to keep the process moving.

Learning Objectives

  1. Use 3D CAD to design silicone elastomer parts with internal cavities.

  2. Use 3D CAD for open mold cavity design.

  3. Use 3D printing to fabricate molds for silicone casting.

  4. Use proper lab procedure to mix and pour silicone to fabricate cast parts.

  5. Fabricate silicone part by bonding multiple cast pieces.

  6. Test and evaluate elastomer response to air or fluid actuation.

Deliverables

Each of the three parts of the exercise will have a similar deliverable in the form of a blog post to be written and subsequently updated with additional documentation.

Please write and update a blog post including:

  1. CAD design. (This will usually be posted early for design review.)

    1. Uploaded zip of CAD files.

    2. CAD renderings of silicone and mold parts.

    3. Comments on design rationale as needed.

    4. Computed part volume and material selection (for material estimation).

  2. Results documentation. (Please update your post to include outcomes.)

    1. Brief comments on successes and problems.

    2. Photograph and/or video of the final result, as appropriate.

Part A: Open Mold Design

The first phase of the exercise involves designing a part and mold for casting by the instructor. Please post the CAD files and images by each deadline to allow for review prior to approval for 3D printing. It may take some iteration to produce a valid mold design.

You will then submit your part for printing, and your instructor will mix and cast silicone elastomer. You will demold and bond your own parts.

The primary reference example was designed using SolidWorks and is documented as Design Example: Open Molded Soft Silicone Actuator.

Procedures

  1. The first step is to prepare a draft of a 3D-printable mold suitable for open molding (i.e. single part mold). Please design your mold block to fit within an 80 mm square and your total part volume no more than 50 cc. This will limit our usage of both 3D printer and silicone resources while developing an understanding of the process.

  2. It is highly recommended to design your parts in SolidWorks or similar parametric CAD system. The following steps will help you create a part and mold design.

    1. Model an actuator part to be fabricated in silicone which includes internal fluid actuation cavities. Please be sure to set your model units to millimeters. The outer faces must be flat as they will be formed by the open top of each mold. The cavities can be shaped as desired to allow expansion during air or water actuation. It is recommended to leave a 5mm round port into which to bond a short leader tube.

    2. The part will be fabricated in two sections and bonded together. The first step in creating the dual-cavity mold is to choose a splitting plane and apply a ‘Split’ operation to divide the actuator part into two separate derived parts. The geometry of each featured face exposed by the split should be concave with no overhangs, as they will be each formed by a mold. This usually implies splitting along the centerline of the round inflation port.

    3. The dual-cavity mold will be defined by subtracting each derived part half from a mold block. The first step is to create a solid mold part, typically a rectangular block larger than the two parts halves side by side.

    4. Create an assembly from the mold. Add both derived part halves to the assembly and position them with the outer flat faces coincident with the mold top and the body inside the mold block. E.g., the ‘inner’ face of each half will be facing downward into the mold block, and the flat ‘outer’ face will be on the surface. Add mates for precision alignment.

    5. Edit the mold part within the assembly context and add a Cavity feature to subtract both part halves.

    6. Open the mold part separately, inspect, and export as a STL for fabrication.

  3. For the first deadline of Part A, please have your instructor check the design and fix obvious problems before you submit it for 3D printing. Please post your design files as a single zip file and one or more visual renderings from CAD.

  4. After the design is approved, please submit the mold STL file for 3D printing using Skylab. General 3D printing instructions follow:

    1. Log into https://skylab.ideate.cmu.edu/ with your Andrew identity.

    2. Upload your STL file to the ‘Start Order’ page (usually the default view). Millimeter units are preferred.

    3. Wait a minute or two for the analysis.

    4. Select the best orientation of the six offered (usually the least support material). A reasonable price for this print is $5 to $10. If it is more, please check with me.

    5. Set resolution to “Detail 0.005IN”. This will help ensure the surfaces are properly closed.

    6. Set Density to Sparse.

    7. Add note in comment field: Please charge to 16-480 Creative Soft Robotics.

    8. Select ‘Proceed with Order’ when ready.

    9. You won’t receive a completion notice, it will simply change status.

    10. Parts involving support material will need to to spend time in the dissolution bath in HL A5.

    11. More info: IDeATe 3D Printing Guide, https://resources.ideate.cmu.edu/equipment/3d-printers/stratasys-cb3d/

    12. Please let your instructor know when the mold is ready.

  5. After you submit your completed 3D printed mold, your instructor will fill all molds in a single large batch.

  6. In the following class, you will then demold your silicone parts and learn to bond them together using a small quantity of additional silicone.

Part B: Open Mold Refinement

The second mold and casting iteration should constitute a substantive iteration on the first design. The specifics will vary by project. This might include:

  1. Fixing mold failures: conceptual errors, overly fragile geometry, problems with demolding.

  2. Fixing 3D printing problems.

  3. Fixing bonding failures: improving geometry for alignment, bonding precision.

  4. Including an alignment jig.

  5. Improving the internal cavity design for fluid flow.

The second iteration will also be cast as a group, but individually demolded and bonded.

Part C: Open Mold Fabrication

The third mold iteration should constitute a substantive iteration on the second design. The specifics will vary by project. This might include:

  1. Additional steps similar to Part B.

  2. Scaling up the mold to a larger geometry.

  3. Testing a two-part mold design for fully featuring the surface.

You will be responsible for all fabrication steps for this iteration: casting, demolding, bonding.

You will also be free to choose any of the silicone elastomers offered as part of your course allocation; please see Silicone Rubber Procedure for specifics

References