-  Week 12  +

molding and casting

Tasks

  • Design a 3D mold, machine it and \cast parts from it
  • Demonstrate workflows used in mold design, construction and casting

    • Concept and Design Development

    How to stack books properly on a desk?
    I have a habit of keeping some quick references on my desk, especially novels that I am currently reading. I usually face problems in stacking them properly aligned to each other. If I stack them inclined to the wall, they tend to loose its form atfer some time and become skewed in shape. And in case they are stacked perpendicular to the desk surface the tend to fall back.Its quite evident in the following images.

    Solution
    To solve the problem, I decided to cast a small books stacking stand. The idea is to have books stacked at an angle of 20 degrees to the desk surface and align to each other. And a support at an angle of 110 degrees desk (90 degrees from the surface which is 20 degrees from the desk surface). Sketch below gives an idea of what I imagined.

    Drawing

    At first, I decided to draw 2D profile of the stand I wanted to cast. I thought of deriving the profile using Golden Ratio. I used the tradition geometric method to draw it in AutoCad. The golden rectangle I drew has its shorter dimension of 10 cms. Below screenshot shows the rectangle in orange.

    Now, to get longer surface to accomodate more books I added additional length of 6.18 cm to it to maintain the ratio. It was achieved my extending the arc further on the other side. It can be seen in the screenshot below.

    Finally, what I get is seen as shaded area in the image below. So the shaded part is what I will be casting. It is the face of the mold. It is what I will see in elevation too.

    Now, to achieve inside inclined surfaces to be exactly perpendicular to each other I had to make some modifications highlighted in red in the image below. This modification for inside cut allowed me to accomodate commensurate radius of the drill bit I will be using for milling. Also, pulling apart curve A and B by 20 mm would help me to remove the mold easily.

    Giving some depth to the profile is the next step. To do so, I used extrusion command in rhino. The depth/ thickness of the books stand could be anything between 3cms to 5cms. I choose to make mold for the higher limit, so that I could cast any thickness upto 5cms. 3D file below gives a fair idea of how it will look eventually.

    Molding Casting 3D by fabish on Sketchfab

    Process 1-CNC Machining

    Toolpath

    First thing that I did was, I opened the 3D model in PartWorks, and the initiated the process of setting up the parameters for CNC milling. In Orientate and Size Model section, I defined the correct orientation of the model to be milling. Then in the material size and margin section I defined the origin point, the depth of the model from the top surface and the bottom surface of the model. Then came the three toolpaths- roughing, finishing and cut out. I created only Roughing toolpath and finishing toolpath as I wanted to keep the base for creating base for rubber mold, when it is poured. Following are the parameters I gave:

    Roughing Toolpath

  • spindle speed - 13000 rpm
  • milling bit - 1/4" stright Cut
  • feed rate - 1.5 inches/sec
  • plunge rate - 1.5 inches/sec
  • stepover - 35%

    • Finishing Toolpath: Key parameters in the finishing toolpath are-

  • spindle speed - 9000 rpm
  • milling bit - 1/4" ball nose
  • feed rate - 2.0 inches/sec
  • plunge rate - 2.0 inches/sec
  • stepover - 10%

    • Preview Machining: Here I did the simulation of roughing and finishing toolpath to ensure everything is in place.

    Save Toolpaths: Here I saved the toolpaths in .sbp format which is good for milling. The estimated projected time it showed was 3hrs 7mins.

    Milling

    For the purpose of 3D milling, I decided to create my own material for milling such a deep (in z-axis) object. As getting a solid piece of wood would be very expensive, I settled for some scrape MDF we have in our lab. I used it to create the milling material. Then I mounted it on the sacrifice board and hit the start button for milling to happen. But, in just one go, I realised that the milling has some issues as there were horizontal mill grooves that started to form. I immediately pressed stopped button. Later I realised that the orientation of the material was wrong. Since, its a cuboid and not a cube I should have ensured that the material is mounted in the right orientation. I corrected it and started the milling again. This time it came out exatly what I wanted. The slit could be seen in the video below on the top surface.

    11mm tk MDF x 6no.s are milled separately in CNC and glued together to create the milling material.

    The video is showing the milling of the mold in progress.

    Process 2-Molding

    To make the rubber mold, the first step is to zero the scale after putting the glass I will be using for the mixture. I am using Vytaflex 40 Urethane Rubber compound to make rubber mold, which actully is a negative mold.

    Below is the mold with removable boundary, that would be required later for removing the mold easily. I applied grease to the mold to avoid any bonding between MDF and rubber compound mix.

    Next, in the following images it can be seen that I mixed Part A and Part B of the compound by weight in 1:1 Ratio.

    Then I mixed both part A and Part B well to get even consistency.

    Then I poured the mixture into the CNC-milled mold. It is important to avoid no air bubbles are formed. After that I allowed it to settle for 16hrs for it to attain its actual form, strength and elasticity.

    This is the final mold that came out very smoothly.

    Process 3-Casting

    Once the mold is in place, numerous castings could be produced. Basically casting is the final object/ product. I am using Smooth Cast 300 Liquid plastic.

    Now to make a cast I poured liquid plastic in it. Part A to Part B should be in the ratio of 100:90. Following two images shows me pouring plastic compound.

    This is a realtime video demonstrating how fast it takes liquid plastic to settle and hardens.

    Then I allowed it to settle for as few minutes and it was done.

    This image shows all parts, CNC milled mold, rubber mold and final Cast.

    Final Object

    Links

    Week 12 Files
    Sketchfab
    PartWorks
    Shopbot Tutorial