In which I printed my very first 3D scanned object.
Design a 3D mold, machine it, and cast parts from it.
- Design a test mould for my final project
What I Learned
- How to do 3D modeling in Houdini
- How to use the Modela MDX 40A
- How to make a one-part mould
- How to melt machinable wax
- How to use SRP Player
- How to read a Materials Safety Datasheet (MSD)
- How to make a Vario 40
- How to cast in Flexicast
Tooling, Materials & Speeds
- Modela MDX 40A
- 1/8" square mill bit for rough cut
- 1/32" square mill bit for final pass
- Feed Rate: 720 mm/min
- Speindle: 9000 rpm
- Vario40 (120 g)
- Flexicast (80 g)
This week's "Moulding and Casting" assignment sounds simple enough until you start having to think of the object you want to make as a positive and negative. I found it a bit confusing having to mill an object and then mould it and then cast it, but it makes sense if you want to make a unique design from scratch, rather than replicating an existing object.
For this assignment, I wanted to create a mould for my Final Project,
To accomplish this, I tried to extend on the 3D design of a lotus I modelled in Rhino in Week 05. In that design, the lotus was a bowl, but for my Final Project, I would need it to be a solid shape.
First, I tried to model it in Rhino, but it was difficult to create a complex pattern without Grasshopper, and I was unable to turn all these petal surfaces into a solid object that could be milled.
So I try a new 3D tool called Houdini. This tool was recommended to me by my partner who is a VFX artist at Weta Digital. He uses Houdini to create FX shots for films and there is a free non-commercial version for Mac that I was able to download and use.
Houdini is based on procedural modelling, which is similar to (or the same as) parametric modelling - using parameters and relationships to create geometry rather than creating individual shapes and surfaces. It was significantly more intuitive to use than Rhino. Even though I didn’t have any previous experience using this software, I was able to create the shape that I wanted (with the guidance of my partner) that was airtight (passed Netfabb).
Unfortunately, Houdini does not work on my Macbook Pro because I need to install a new graphics card. I will definitely look into this because it’s such a fun tool. I would probably only use it to create organic shapes, and continue to use Rhino to create mechanical parts.
How to create a 3D Flower in Houdini
- Create petal shape
- Duplicate petal and rotate (parameters - number of petals, the * of rotation)
- Extrude (parameters - depth of surfaces and spacing between the layers of petals)
- Group - to create a solid, airtight base
- Knife function - to trim the bottom
- Fine tuning - to adjust the depth of the base, the spacing between the layers of petals
- Subdivide function - to round the edges
After every step, I would group unshared edges to ensure ensure geometry is airtight. This additional step paid off when I checked the final object in Netfab and found that it was pretty much ready to print.
I showed my model to Wendy, and realized that I had unconsciously limited the aesthetic of my design based on some inaccurate assumptions I made about the MDX. While it’s true that I have to consider the diameter of the drill bit and the shaft, as well as the drill-able length of the bit, I didn’t necessarily have to provide that amount of spacing between each layer of petals or individual petals. The MDZ is capable of fine engraving using a 1/32” bit. With this size bit, I would be limited to 3.2 mm width cut and 15 mm depth cut.
She showed me in SRP how I could set-up my design so that I would use a 1/8” bit for roughing, and 1/32” for engraving. She also gave me some advice on how to construct the platform (leaving room to re-set the Z axis when I change bits) and to leave enough room for the locator points.
I went back into Houdini and adjusted the model to remove the spacing between the layers of petals and individual petals. The effect looked more like a mandala which I really liked.
I tried to import the object into Rhino to build the base of the wax mould, and I had to convert the mesh that was exported from Houdini into a NURBS polysurface by watching this
So I tried to build the base in Netfabb, but found out that I couldn’t merge the objects without paying $299 for a non-commercial license.
I also tried to build the base in 123D Design, but it gave me a warning that the objects I was merging had too many faces. I tried to merge them anyway, and it crashed. I merged all the primitives of the base and left the object in embedded in the base for now.
Finally, I went back into Houdini and designed the base as an extrusion to my object. This ensures that it is a closed, water-tight object, but it is very difficult to be precise in measurements in Houdini. I would not recommend using Houdini if you want to create something with precise measurements. But anyway, it did work after much tweaking.
How to Create a Machinable Wax Mold
- Turn on extraction
- Turn on the hot plate (medium high heat - you don't want the wax the burn)
- Place wax into pot on top of hot plate (make sure you have enough wax to fill your mould because you can't add more once you've started pouring)
- Stay by the wax as it's melting (it's tempting to walk away and let it melt, but it's not safe)
- Do NOT stir the wax - it creates bubbles (I like to pop the bubbles on the surface with the knife)
- When all the wax has melted (you can check by poking carefully with a metal knife), pour the wax into the mould very quickly (if you pour too slow, the wax starts to get cold creating layers)
- If your mould leaks, stop pouring immediately and clean up the mess!
How to set-up on SRP
- Engrave a small X at the centre of your block of machinable wax
- Use hot glue to mount the wax block on the machine
- Save the STL of your object and base on the local desktop
- Set the dimensions of your mould
- Define the kind of cut that you want to make
- Select the drill bit you'll use for the rough cut (1/8")
- Select the drill bit you'll use for the finishing cut (1/32")
- You can choose to give a smaller cutting area for the finishing cut to speed up the machining time (I only needed the the finishing cut on the top part of my object)
Before opening up a jar of toxic materials, here's a checklist to stay safe:
- Read the Materials Safety Datasheet (MSD) - you can access it on the computer at the lab, and there is also a binder of them in the 3D workshop
- Put on your safety glasses
- Wear a air filter mask - a dust mask will do nothing - try to put it on before entering the chamber
- Wear latex gloves
- Turn on the extraction
- Bring pen and paper and a calculator - for the maths
- Use a scale
- Lots of paper towels
- Newspaper to cover the work surface
- Take photos
A couple of things you should think about before casting:
- Mix ratio - it says so right on the bottles - this tells you how much to pour of each material, based on the total volume you need - try not waste
- Bubbles - using a vaccuum chamber is most effective, but trickierfor less viscous materials like silicone
- Pot Life - says so right on the bottle. This tells you how long you have to mix and pour before it starts to cure.
- Curing Time - this tells you how long to wait before you can see the fruits of your labour.