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.


Each of the 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 individually 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: Silicone Actuator Design

Designing a fluidic actuator 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 an internal cavity which can be filled with either air or water. The internal shape may be contoured into chambers to create specific inflation shapes.

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 design the part on paper and provide a detailed set of dimensioned drawings.

For this exercise we will be using a pair of open single-part molds to create two halves of the part which are then bonded together. This is a stringent design constraint, since the top surface of the rubber in the open mold will be flat. Generally this leads to a part design with two parallel flat surfaces on the outside, a well-defined splitting plane parallel to those faces, and internal cavities which are convex when viewed from that plane.

A reference example of such a part is available as Design Example: Open Molded Soft Silicone Actuator. You are welcome to use the CAD files as a reference, but please design your own part. The sample is a simplified pneumatic-net actuator based on a Whitesides lab paper [R25].

The main deliverable should be a blog post with rendered images showing the geometry.

Added after the fact:

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.

Please keep your parts small. The absolute limit in any one dimension is 110 mm in order for the larger mold block to fit within the 120 mm limit of the Stratasys 3D printer. The recommended limit is to fit within a 70 mm square.

For fluidic actuation, please leave a 5mm round port into which to bond a short leader tube.

Part B: Open Mold Design

We will be fabricating your actuator part by casting each half separately in a open single-part mold and then bonding the halves together. 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 Actuator. Please review the following details:

  1. An open mold can only form one side of the casting. For this exercise, the formed surface of each half is the interior in order to create and shape the cavity. The outer surfaces of the part will be flat.

  2. The CAD procedure is to separate the desired part into two halves. Each half is then subtracted from a mold blank, cavity-side down, flat-side up.

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


  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 absolute limit in any one dimension for the mold block is 120 mm in order to fit within the 120 mm limit of the Stratasys 3D printer. The recommended limit is to fit within a 80 mm square.

  4. Please print with a fine resolution; this has the best chance of printing as a watertight surface. The silicone is viscous but will penetrate even small holes and leak out into the hollow 3D-print.

  5. 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.

SolidWorks Workflow:

  1. 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.

  2. 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.

  3. 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.

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

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

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 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. (The mold body doesn’t need to be strong).

    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.

  3. Most parts following will these guidelines will not require support material as they will not have any overhanging features. Nevertheless, please visit the output queue early to inspect your part. If it does have support which needs to be dissolved, this may take several hours in the hot washing bath, located in HL A5. Please be sure to allow sufficient time for washing and subsequently drying your part before casting. The wash bath is hot and alkaline; please wear the heavy rubber gloves when adding or retrieving parts.

In-class Casting Workshop

Please arrive to the casting workshop class with a ready set of 3D molds. We will mix a common batch of silicone, degas it, then distribute it for you to pour into your molds.