Exercise: Mold Design Workshop

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 post-processing, and casting 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 tendon actuation.

Deliverables

Please individually create a Google Shared Drive folder including:

CAD design:

  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).

After fabrication, please update your folder to include outcomes:

  1. Brief comments on successes and problems.

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

Silicone Flexure Design

Designing a flexure is both a practical objective for learning mold design technique as well as potentially a useful resource for projects.

The first part of our workflow is designing a part to be fabricated using soft silicone rubber which includes flexure hinges and tendon pathways.

SolidWorks is the preferred CAD software, but you may choose an alternate of your preference if it supports Boolean operations sufficient for synthesizing the mold geometry. If you have no 3D CAD experience, please let me know and I will provide extra tutorial as needed.

For this exercise we will be using an open single-part mold to create all the geometry. This is a stringent design constraint, since the top surface of the rubber in the open mold will be flat, and the sides cannot include features which would lock the part in the mold.

A reference example of such a part is available as Design Example: Open Molded Soft Silicone Flexure. You are welcome to use the CAD files as a reference, but please design your own part.

In your Google Drive upload, please include rendered images showing the part and mold geometry.

Please use model units of millimeters.

Please limit your total part volume to no more than 50 cc. Note that this is the volume of solid material, not including any open cavities. SolidWorks will report the part volume as part of Mass Properties; other CAD system have similar functions.

Open Mold Design

We will be fabricating your actuator part by casting silicone into the open single-part mold. In this phase of the exercise, you will complete the design and fabrication of 3D-printed molds.

The principal reference for this exercise is Design Example: Open Molded Soft Silicone Flexure. Please review the following details:

  1. An open mold can only form one side of the casting.

  2. In the mold assembly, the part is subtracted from a mold blank, flat-side up.

  3. The mold surface cannot have overhangs which would lock the part in place.

Guidelines:

  1. Please use model units of millimeters.

  2. Please calculate and note your part volume: we will need this for estimating material needs.

  3. The silicone will conform to small feature sizes, but very thin walls less than a millimeter or two can break while demolding. Tiny pockets can also trap air while casting and create defects.

  4. Please use the SolidWorks Flexure Design Workflow as a reference for the CAD steps.

Approval and Printing:

  1. 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.

  2. After the design is approved, please submit the mold STL file for 3D printing using the Mosaic Array interface and the submission link provided with the assignment. Current instructions can be found in IDeATe 3D Printing Guide.

In-class Casting Workshop

Please arrive to the casting workshop class with a mold ready. We will mix a common batch of silicone, degas it, then distribute it for you to pour into your molds.

References