Group Assignment

Review the safety data sheets for each of your molding and casting materials

In this case materials available are:

  1. Silicone RTV -20 (shore) (bi-component products - base and catalyst)
    • Not a hazardous substance
    • In case of Inhalation: just move to fresh air
    • In case of Eye Contact: flush eyes with plenty of water
    • In case of Skin Contact: wash thoroughly with soap and water.
    • Special hazards arising from the substance or mixture: None known

    Please for safety data sheet visitthis Web Page.

  2. DryStone Casting Media
    • Not a hazardous substance
    • Prevention: observe good industrial practices

    Please for safety data sheet visitthis Web Page.

Compare printing vs machining molds

First, it is neccesary to decide about the design of the figure/object to fabricate. In our case, as a group we decide to test something related with our group machine, to fabricate a pumpink. This because it could allow us to test rounded surfaces that could be fabricated each half with different technique. One half with 3D printing and the other half with CNC milling

Object Designing

At first we considered a desing with a 80mm radio (see figure a) and produce two halfs negative molds one part for CNC miling and and the other one for 3D printing process. The idea was to reproduce one figure with two different source of positive molds, so the comparison could be easier.
However, we did not notice that for setting figure's dimensions, we need to considered CNC miling machine's volume. This because the endmill have a limitation regarding milling height (25mm maximun). Thus we have to change punkin's radius to 50 mm (see figure b)

Then, we also get confused and generated possitive molds for milling and printing. That would be useful only if we manted to reproduce the mold directly form a flexible material. However, the object will have to be reproduce from cement, so we have to desing again

Because we need to machine two negative molds that will cast a complete figure, each of them need to have alignned pins to allow a modls fitting. Hence, we considered four (4) pins for each mold part, and to secure fitting, with 0.25mm tolerance.

You can see in the image, that tolerance was consider at bottom of the pin (height) and for diameter. Thus, fitting between male-female pins will no present any problems.

Final design consideration have to be focus on negative-molds walls:

  1. You must consider 5mm distance to the walls, to avoid thin molds walls, that couldn't hold the molding process.
  2. In the case of wax blocks, minimun wall widht has also to be 5mm, to avoid any wax fracture
  3. In the case of 3d printed negative-mold that is not a limitation so we consider 2mm width

    4. Negative-Molds 3D Printing

    For the 3D printing we use a Bambu Lab P1S, it took 90 minutes printing in high quality. You can appreciate printing configuration in the following picture.

    Negative-Molds CNC Milling

    For the CNC samll scale milling we use Modela MDX20 and we follow this process

    1. Open FabModules program
    2. Set XY Axis originSee image below

    3. Manually calibrate Z Axis
    4. Load .stl file.
    5. Set position in our case we inverted the image in -Z axis
    6. Press make png buttom
    7. Press Make path
    8. Press Make mrl
    9. Send fileThe image below shows settings before milling

    The image below shows the final negative molds in both technologies

    Molds Fabrication Process

    Once we have both molds (3D printed and Milled) we iniciate the process to obtain both molds following this steps:

    1. First we started with silicone components mixing. Proportions were 100a / 3b, we weighted (100gr + 3gr)

    2. We place the mix in both negative molds and tried to extract bubbles by hand

    3. We wait for around 30 minutes to retire the finals molds. For direct observation we notice that the printing version obtain better resolution

      4. Figure Production

      To obtain the final figure we follow this process:

      1. First we started with cement mixing, The proportion were 100a / 20 (cement + water), we weighted (100gr + 20gr)

      2. Drying process took 1 hour to retire the finals molds.
      3. For direct observation we notice that the printing version obtain better resolution, as you can see at the image below. The bottom part was obtain from CNC Milling mold (a). The top part was obtain form 3D printing (b).

        4. We compare some features of the processes in order to conclude which one presented better results

        Criterion CNC Miling 3D Printing
        Machining Time Roughing: 35 min
        Finising: 45 min        		 Printing time: 90 mins
        Suface Finish Medium quality with 1/8" end-mill (can achieve very smooth using smaller diameter) High quality using 0.90 layer height (may require post-processing if use larger layer heights)
        Design Complexity Limited by the accessibility of cutting tools and the geometry of the machine Allows complex geometries without additional cost, ideal for molds with fine details or intricate internal structures

        Please you can find editable (.prt) file and machining (.stl) files for 3D printing and CNC milling here.

        Individual Assignment

        Selecting the figure to fabricate

        I select to fabricate a figurine base on lego figures. I based my design on a web site in order to maintain some relative dimensions. This web site show some dimensions reference to fabricate a wood-made figure, you can visit it here.

        The first process was 2D design based on a figure dimension reference, as you can see in the following picture

        Then, I need to create the volumetric form using extrude and revolve function on Fusion, like the show below

        Finally, I split the figure (creating components), created the volume that represent wax dimensions, and relocate both figures into the two boxes and create "."stl" files to machine.

        You can download desing files here.

        Mold Machining

        For the CNC small scale milling we need to differenciate two processes that need to be developed within a waterfall schema, meaning that exist interdependancy between them.

        Rough Milling Finishing Milling
        Low quality surface finish Quality of the surface finish is the top priority
        Roughing end mills don't need cutters with very sharp edges, taking high chip load Finishing end mills need sharper cutters, allowing low chip load to achieve a smoother finish.
        Roughing process will be machined with 1/8" long-singled square endmill Finishing process will be machined with 1/8" ball-nose endmill
        End Mill overlaping could be set lower (aproximately 20%) End Mill overlaping need be set close to 100% to improve the surface quality

        For both (rough and finish) CNC small scale milling processes I used Modela MDX20 and followed this process

        1. Open FabModules program
        2. Set XY Axis originSee image below
        3. Manually calibrate Z Axis
        4. Load .stl file.
        5. Set position in our case we inverted the image in -Z axis
        6. Press make png buttom

        7. Overlapingsettings need to be stablished (25% for rough nd 60 for finish)
        8. Press Make path
        9. Press Make mrl
        10. Send fileThe image below shows settings before milling

        The image shows the first machining that took 103 minutes

        This picture shows second machining stage that took 101 minutes

        Mold and Figure Production

        Once we have both molds (3D printed and Milled) we iniciate the process to obtain both molds following this steps:

        1. First I started with silicone components mixing. Proportions were 100a / 3b, we weighted (120gr + 3.6gr)
        2. Placed the mix in both negative molds and tried to extract bubbles by hand
        3. Waited for around 30 minutes to retire the finals molds
        4. Then, prepared cement-water mixing (120 gr + 24 gr)
        5. Poured cement mix into the molds, and wait for 1 hour

          6. Reflections

          Somethings to highligh for this procees is:

          • The fab modules program only allows to use 1/8 long end mill. Thus, our molds need to considered depthness regarding endmill height
          • We need to repeat the milling process twice. First, for roughing process with 1/8" long-singled square endmill (a) and the other one for finishing with 1/8" ball-nose endmill (b)

          • Using 3D printing will be advantageous versus CNC milling to produce molds depending of 3D printing quality that we have available. In our case be printed two 3D printed molds, one using BCN printer and the other using Bambu Lab. Due to quality be use the last one to obtain better final result
          • we need to be very careful with silicone components mixing proportions requeriments, to avoid reworks due to rapid hardiness