Molding and casting

My only experience molding and casting are from a long time ago. As a kid I would cast small objects with pre made molds and plaster of paris. I have really been looking forward to finally making my own mold and make something fun. I wasn’t really sure on what to make at the start of this week but I really wanted to figure out some limits of the process. After the the class on wednesday I got home and realised that time was going to be the biggest challenge this week. So to get things started I decided to design a simple shape to get my head around the whole 3-step process. Milling wax, pouring a mould, casting a object. I was ready to mill on friday and pour my mould on saturday (and) monday – and since I had other obligations on tuesday the week was already at it’s end. So with some creative time management I made it work but unfortunatly wasn’t able to find the limits of the process.

During the group assignment this week I awakened my inner hypochondriac by reading datasheets of the materials we are using. I wasn’t surprisded that these were all chemicals but they really come with some serious warnings. Also I was quite surprised that all of these materials have been animal tested. This is not very surprising ofcourse but also not very motivating to me to use these materials. I’d like to find out more about more friendly materials.

Tools used this week

Hardware
Shopbot

Software
Fusion 360 (CAD and CAM)

Tools
3 mm end mill (25mm cutting length / 2 flutes)

Materials
Machinable wax blue
Smooth on – PMC-770
Smooth on – Vytaflex 50
Smooth on – release agent


Group assignment

For the group assignment this week we sat around a huge table with different Smooth-On products. Each of us got the Technical bulletin and Safety sheets and started reading and sharing our discoveries.

The Technical bulletin reads quite quickly. It gives an overview of the technical properties of the material. It tells you the mixing ratio’s and it has a table that compares it to the other products in it’s range. Also it will let you know important stuff like how to keep your moulds in a good shape, how to pour or how to make your material even stronger. Also this is where the personal protection equipment is described for the mixing. Most materials require this.

In the Safety sheets you can read everything about the material. Most products consist of two parts, part A and B, each come with their own safety sheets. From the materials we investigated it seems common that one of these materials is relatively safe and the other more hazardous. When using these mateials check for the following:

Check this in the data sheet!!

  1. Check your material with the datasheet. It should have matching SDS numbers.
  2. Read chapter 2 carefully and be aware of these hazzards. Identify here what PPE should be used.
  3. Chapter 4 and 5 describe first aid measures and fire fighting measures. Check if you are able to do this in your lab if needed.
  4. Chapter 6 should be read in case of spilling the material – basically use common sense!
  5. Handeling and storage is explained in chapter 7 – wash your hands after handling these materials, keep containers clean and store them properly.
  6. Chapter 8.2 describes what PPE should be used, take this very seriously.
  7. Chapter 9 describes the apperance of the material, when this is out of the ordinary don’t use it.

Local regulations are required to document all materials, amounts and safety sheets. Also chemicals should be stored in containers to prevent materials dripping on floors etc. This is also described in the data sheets.

Materials

PMC 121/30Vytaflex 50Smooth-Cast 300Smooth-Cast 305Smooth-Cast 310
MaterialUrethane rubberUrethane rubberPlasticPlasticPlastic
Pot life30 min60 min3 min7 min15 – 20 min
Cure time16 hr16 hr10 min30 min3 hr – 4 hr
ColorClear AmberOff-WhiteWhiteWhiteWhite
Stiffness30 A50 ARigidRigidRigid
Mixing by volume1A:1B1A:1B1A:1B1A:1B1A:1B
Mixing by weight1A:1B1A:1B100A:90B100A:90B100A:90B
Release agent?YesYesYesYesYes
Saftey sheetSafety sheetSafety sheetSafety sheetSafety sheet

For the group assigment we looked at the different materials. They are easily split up into three different kinds: rubber, plastics and silicones. Rubbers and silicones are great to cast molds from since they are flexible. Usually silicones are a lot more flexible than rubbers but they do tear more easily. Plastics are very rigid and brittle and are only good for casting objects.

The Smooth Cast 300, 305, 310 plastics are nearly identical plastics. They have the same color (white) and similar rigidity. The biggest difference is the pot life and cure time. These are all very thin when pouring. These are super easy to mix together since they are super thin and don’t trap any air bubbles.

The rubbers biggest differentiator is the shore. When casting a simple mold with few details it makes sense to use a higher shore, when casting complex parts you might require a bit more flexibility from your mold, a lower shore. When mixing rubber the liquid is quite thick and easily traps air bubbles. Carefully stir the material and put in a vacuum pot to remove air bubbles.

Our instructor Henk demonstrated good strategies for milling wax. Since the material is relatively soft it is possible to mill at high speeds (on our machine 30mm/s works well as a feed speed). The material can be mounted using double sided tape underneath, additionally you can add four wooden scrap parts around it to make it completley stuck. Since our blocks are not completely square it’s important to pick a flat side or take the time to surface one side to get good adhesion. The milling process consists of two steps, roughing and finishing. Roughing can be done with a larger milling bit, as large as the design allows, for us it was 5mm. Go for big step overs (48%) and large pass depths (width of milling bit). For the finishing pass do the opposite, keep the same speeds but make small step overs (10% or less, less is better). There is only one pass for the finishing job.

After the finishing pass the results were acceptable enough for us to cast the wax mold. There are still a lot of lines visible in the wax. This is because we chose production speed based on available time (large milling bit and large step-overs). The wax doesn’t need to be sprayed with a release agent according to the supplier. We chose Vytaflex 50 for the casting, this is a two component material that needs to be mixed 1:1 based on volume. After the mixing we put it in a pressure chamber to remove airbubbles. After putting on the lid the compressor is switched on and the air is being removed. Once the process is finished slowely let the air flow back into the chamber. If you do this too quickly you need to start over. We slowely poored the mix into the mold with a thin stream.


Individual assignment

This week I designed a simple geometric shape, a sphericon, to cast. My objective was to use a similar registration principle in my mold as was shown in the recitation (Neils spinner) and to create a double sided mold with a hole for the pouring and a air vent. A 3 step process was used to create the final object. First (designing and) milling the wax, second casting the mould, thrithly casting the final object.

Step 1

Desiging the wax mold

I started with my design, the thing I want in the end, a sphericon. Than I added two ‘vents’ for the pouring and airing. The object was sliced in half and both halfs put next to each other.

Around both halfs a hexagon is drawn for the registration (same size!). One will be used for a negative space the other one for a positive. To prevent them from not matching I rounded off all corners with a 3 mm radius. The ‘right’ half gets a negative space hexagonal space around it so for now it’s finished. The ‘left’ half gets a positive hexagonal space with a height of 10 mm. This height should be enough for the two parts to stick together.

Next the left half gets a negative shape around it to accomodate the ‘right’ half of the mold. I gave it a flat side so it can stand by itself during the casting with the vents pointing upwards. Lastly I drew a block the size of my stock and made sure everything would fit. All shapes are added and/or substracted to the stock. As a final touch there is a small connection between the left and right side. This will allow the molding material to level out.


Milling the wax mold

For the CAM I chose to use Fusion. Since the wax needs to be milled in 3D to time becomes a factor. My design has a very clear 3D part (the sphericon) but also some clearly more 2D parts (positive and negative hexagon). To mill this in 3D using Vectric it wouldn’t use much intelligence so Fusion was a better choice. I have used a few alternatives in the past so I am quite familiar with adapting to new CAM software. It was my first time using Fusion for CAM and I was quite happily surprised. Fusion CAM has quite a steep learning curve but this is quickly forgiven once you find out how powerfull it is at generating clever paths and setting a infinite amount of settings. With the Shopbot post processor already build into the software files can easily be generated on any computer and send to the machine. This can safe a lot of time since it can be done on your own machine.


Step 1

In Fusion go to the Manufacture menu. This brings up the manufacturing enviroment, from here we can directly create, view, change and export toolpaths. For this project I needed to create a single setup since it can be milled in one go. In the top bar create a new setup. Here you can check your stock size, since I already drew this in my desgin it inmediatly showed the correct size. Also this is the place where you set the zero for your toolpath. I did this selecting a point on the bounding box.

Step 2

Now we can choose toolpath operations. For this job 3D toolpath operations are needed. I chose several just to try them out. In particular I liked the adaptive clearing one – it is really fast and gives a nice finish. If you hover over the different options they are explained. Once you have the operation you want to do select the milling bit. Since this was my first time doing this I had to add my own milling bits into Fusion.

Step 3

After all the operations are chosen and settings are checked we can simulate the job. This is where I found out that I didn’t have the correct zero set. So I went back to the job setup and changed it.

Step 4

Once everything was fine I exported the post processing files from the menu. This can be done one by one or as a batch. Make sure you have the correct post processor selected. You can do this in the menu Actions > Post process or by right clicking on the operation on the left hand side.

Step 5

Now it’s time for milling. I put my material in the machine just like we did for the group assignment, I put in the milling bit, zeroed the machine and pressed start. After the roughing job I realized that I milled away my zero so I couldn’t change my milling bit anymore. Luckily I had a milling bit that was small enough to do both roughing and finishing.

Casting

After the mold was ready for casting I decided to make a mold from rubber I still at at home (PMC 770). I skimmed (big mistake) through the datasheet and mixed a batch. I poored it into the mold and the next day it came out perfectly. At the waag I sprayed it with release agent and poored Smooth cast 305 into it. When it was time to demold I was surprised that it had turned into a brick that wouldn’t open up again. It was completely solid.

I went back to the data sheet and saw that the mold need to be prepared with a seeling agent before the release agent. Fortunatly I had poored twice and had a spare one but unfortunatly there was no sealing agent available.

For the second attempt I used Vytaflex, and this time I really read the datasheet. It just needed a release agent. I removed all air bubbles in the pressure chamber and poored it into my mold. The next day I sprayed my mold with release agend and mixed another batch of Smooth Cast 305. Within 30 minutes this was cured and my sphericon came out beautifully. Unfortunatly the registration wasn’t perfect so there was quite a bit of flashing to clean up. I did this with scissors and some sand paper. I think the registration failed a bit since the mold wasn’t properly held together, next time I would use two pieces of wood and elastic bands to keep it tighter closed. Overall pretty happy! The sphericon rolls in a really entertaining way.

Vytaflex 50Smooth-Cast 305
MaterialUrethane rubberPlastic
Pot life60 min7 min
Cure time16 hr30 min
ColorOff-WhiteWhite
Stiffness50 ARigid
Mixing by volume1A:1B1A:1B
Mixing by weight1A:1B100A:90B
Release agent?YesYes


Files

Sphericon Mold
CNC setup sheet

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