Week 14, Molding and Casting¶

Assignments¶

Group assignment¶

Review the safety data sheets for each of your molding and casting materials
Make and compare test casts with each of them
Compare printing vs milling molds

Seee

Individual assignment¶

Design a mold around the process you’ll be using, produce it with a smooth surface finish that does not show the production process, and use it to cast parts.

Group work¶

See the group page

Individual work¶

For this week, I wanted to explore how molding could be the basis for making printing templates. In many ways, I am interested in how technology can be used in artistic ways. In some ways, I built from my work in Computer Aided Design week. For the vinyl cutting portion of that week, I made a t-shirt with a star pattern on a raised arm. I wanted to use that same image, and make a stamp to print that image.

As a constraint on my design, I went to a local craft store and bought a blue inkpad. The pad is 70x45mm, so my stamp will be within that dimension. The starting point is the clenched fist SVG file from the Computer Aided Design week.

Mold design¶

I utilized Autodesk Fusion to design my mold. Having learned to use Fusion in previous weeks, the process was relatively straight forward. Particularly because I could leverage the SVG created in week 2. (Side note: I am certainly gaining confidence and skills week to week. I try to use knowledge and experience from previous weeks as I move onto a given week’s assignment.)

To start, I created a sketch, which I will turn into a block to hold the stamp. In fusion, I made a rectangle that was oversized compared to my final stamp size, and I started with a sketch of 105x75mm. As I will be using a wax block for the mold, I had selected a block in the lab, and this size was available on a partially used block.

Using the extrude command (shortcut: E), I turned the retangle into a block, making this dimension 30mm. My design will be relatively shallow, and the block I will use is 38mm thick. The image below also contains the sketch which will be the first cut into the mold. The size of this sketch is 70x45mm, which is the desired size. As stated above, this is the size of the ink pad to be used with the final stamp.

Extruded block with sketch for start of mold

With this sketch, I cut into the block by 3mm. I didn’t need or want the final stamp to be very thick, and chose three 3mm. To make the cut, select the sketch (made on the top face), and then use the Extrude command, but tell the distance to be -3mm as a cut into the block body. This creates the mold outline.

Then, I needed to bring in my main pattern. From the “insert” menu, there is an option for “import SVG”. I used that. The imported SVG paths needed to be aligned within the cut box, and also scaled to fix in the box. I also paid special attention to assure that I wouldn’t end up with the desired stamp reversed. The molding will reverse the image, and then the act of printing will reverse it again. Therefore, I didn’t need to create a negative for my imported SVG, as the native orientation is the one desired.

The imported SVG is a sketch on the mold design. I wanted to cut out this pattern, as it will end up being raised from the molded silicon. To make the cut, I needed to select each separate area of the path. In the design, the fingers have gaps between the main arm drawing. I therefore selected all the fingers and the arm for the drawing sketch (holding shift while selecting). I then used the extrude command again and used a negative dimension of 1mm, which will cut into the block. This resulted in the final design for the block.

The mold file for the design.

With the design finished, it was time to move onto making the mold. It was exported as an STL file, and transferred to computer connected to the ShopBot.

Making mold¶

To make the mold, I will be using the Shopbot in the lab. The process for using the Shopbot is built from what I did in Computer controlled machining week. Important details on working with the Shopbot are found in that page, as well as making paths in VCarve. There certainly is an important difference in this week, as this is a 3d design to be cut on the Shopbot.

For the cutting, I will be using 2 bits. The bits I am choosing are smaller ones in the lab, as my stamp is not large, and there are fine details in the design.

For the first pass, I will be using a 3mm milling bit to remove the bulk of the material. This bit has a 1/4 inch shank and the cutting portion is 3mm in diameter. For this bit, the bottom is flat.

For the finishing pass, I used the smallest available bit in the lab. I found a v-shaped bit with a rounded tip. For this bit, the tapper starts at 1/4 inch, going down to 1/64 diameter at the rounded tip. The angle of the V is 6.2 degrees, so it is a shallow V shape.

For both of these bits, it is important to tell VCarve the specifics of the bit. The 3mm bit is a frequently used bit in the lab, and choosing that bit has presets already available for spindle speed and the movement rate. However, for the finishing bit, there were no presets available. The Fablab Speed and Feeds Calculator is a valuable tool to make the calculations.

To calculate the speed for the finishing bit, I put in the tool diameter (I used 1/4 at the top), and the number of flutes (which is 3 for this bit). Then, there are values needed for surface speed and chip load. Those both can be looked up from the tables at the bottom of the page. Since I am cutting wax, I used the lookup values for that material. The speed is 200 ft/min for wax, and I used a chip load of 0.005 inches. This value was taken from the 1/8 inch tool diameter, as I am only using the narrow portion of this bit.

The calculator then reported values for the bit. I then used 3000 rpm, a feed rate of 46 and a stepover of 0.1125. These values are used in VCarve as I defined the bit there.

In Vcarve, I started by saying the work piece is 105x75mm with a thickness of 30mm. I told VCarve that the Z axis zero point is on the top of the work piece. I then created paths for the two bits. For the milling bit, I used the rough cut option. This is to remove the bulk of the material. The plunge depth is set at 1/2 the diameter of the bit (or 1.5mm). For the finishing bit, the finishing cut option was used to make the paths. These paths were exported as G-code for import into the ShopBot control software.

Then, the first 3mm milling bit was inserted into the ShopBot, and the warm-up routine run. Then, the bit was aligned on the block so that the cut would fit the available area. The X/Y was zeroed, and also the Z axis was zeroed. Then, this first cutting path was loaded and the ShopBot run.

Note that the wax block was firmly affixed to the spoil board, using wooden pieces on all four sides. These were specifically placed to be away from the cutting area.

It is important to remain with the ShopBot while being run. I continued to monitor the run as it was going.

Gcode instructions and paths visible for running ShopBot

Then, the bit was changed to the V finishing bit, and the Z axis zeroed (X/Y zero remaining the same). The finishing path was loaded and run.

Looking at the cut mold, there are clear horizontal lines. The final product needs to not show how it was constructed.

Therefore, I made an additional finishing path. For this one, I lowered the offset to 0.05, and also changed the path to work in spiral around the mold. This was run on the ShopBot, and resulted in a smooth mold (see image in the casting section below).

To make the mold, I used silicon. This is a 2 part mixture, with 1:1 of the silicon and activator. I only needed a small volume, so I took 23g of each. I gently stirred the components with a popsicle stick and then poured it into the mold.

Making pressure plate¶

While the silicon needed to set overnight, I made a pressure plate for the stamp. I needed this to create even pressure when printing. This is a simple rectangle with a handle.

To start, I made a block of 75x30x3mm in Fusion. I also made a perpendicular sketch for the outline of the handle. This is only a half side which I will rotate.

Then, to make the handle, the sketch was rotated.

This was printed on a lab Prusa 3d print with PLA.

Files for the Pressure plate.

Assembling and use¶

I then had all the pieces needed to make the stamp.

After setting overnight, the silicon cast was cut from the mold. I cut around the edges to remove the small overflow.

Here are all the pieces. The pressure plate and extracted cast can be seen. The mold can also be seen, and the finishing is smooth after the 2nd finishing pass.

The cast was attached to the pressure plate with 2 sided carpet tape, and is ready for use.

The stamp was pushed into the ink pad and then stamped onto a paper. In the image, you can see two attempts. The lighter one is my first attempt. I didn’t push hard enough, and not as much ink was transferred. I repeated with more force, and I got a very nice image from my stamp.

This was a very enjoyable week, and I found a way to utilize 3d molding as part of a process with a final artistic output.