Week 14 - Moulding and Casting

Please refer to the group page to read about the group assignment.

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

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.

Approach

This week I would like to continue with my plan for the first final project idea (that was replaced in week 11), because it might be still relevant. The goal is to cast a flexible joint from silicone and incorporate 3d-printed connection parts in the casting.

The material I'm planning to use is Silicone ZA 22 from Zhermack. The reason for choosing this that I felt it should give a decent amount of flexibility, but will (hopefully) be strong enough for my application. Silicone will withstand UV-light and should not deteriorate for a couple of years.

Specifications:

• Hardness: 22 Shore A
• Working time: 15 min
• Curing time: 1,5 h
• Tearing strength: 20 N/mm
• Viscosity: 4500 cP

Safety Data Sheet

For the mould, I'm using MB2001 tooling board from polyurethane we have in our lab.

I'm going to mill the mould in the Shopbot using Fusion CAM.

Design

First I designed the parts on both side of the joint. I found a thick-walled Ø 50 mm HDPE tube and started from there. The lower and the upper connector are quite similar, but the lower one has mounting option for a BNO085 breakout board.

The detailed view shows, how the casting will reach around the six "briges" in the 3d-printed part and hopefully joining the two parts together in radial and axial direction.

I need the center hole for the sensor wires to run through and also to make the joint more flexible. I will therefore need to find a suitable core for the mould.

Next I drew a cuboid around the joint and substracted both the joint and connecting pieces from it and made added indexing holes for Ø 6 mm dovel pins.

Then I split the mould and moved to the Manufacturing workbench in Fusion.

Casting considerations

I thought a while about what orientation would be best for this cast. I see three option.

1. Horizontal orientation
   • adding in- and outlet to each end
   • runner and vent will need to be cut off / might leave a mark
2. Vertical orientation - using the center hole for pouring
   • requires punching through the runner after moulding
   • no visible marks
   • might encounter issues with air inclusion
3. Angled orientation
   • might use either of the above or a mix
   • more complicated setup

I think my first try will be the horizontal option and I will add holes for pouring and venting in the plastic ends. In that way, I won't "damage" the mould and leave marks on the joint itself.

Note

According to Fusion my casting will have a volume of 122,5 ml, which will result in a weight of 140,9 g.

Milling

I set up the tools and the stock in Fusion.

My first operation will be drilling for the dovel pins and using these holes to clamp the workpiece down.

Afterwards I can face the block with a 10 mm end mill and use it to remove as much material as possible.

I will also go around the edges to make them straight.

After the second tool change I'm going to use a 1/4'' ball end mill to mill the shape. First rough with leaving 0,5 mm stock and then one fine finishing pass with 0,01 mm tolerance.

Here is a simulation of the five milling operations.

And here is the 3D model viewer.

Here is the stock clamped down for drilling.

Then I screwed the stock down using the indexing holes and started the facing.

Next was the rough milling of the shape.

Changing to the ball end mill:

Prior to finishing:

After finishing:

The tools used:

Because there were some imperfections in the surface, I decided to use 320 sand paper to smoothen it a bit more. I think this might be vibration marks, because the Shopbot isn't quite stiff enough or also a bit worn out.

Here you can see the mould, where I placed the plastic end pieces and the core (a plastic tube). The dovel pins are also inserted.

Now it's time for moulding! I made a test with the material and documented in the group assignment about it.

Casting

First Attempt

I started with preparing the vacuum chamber setup and tested it. Everything seemed to work fine.

I checked if the mold will fit in the chamber with clamps.

Then I prepared the mold and the core with mold release.

Next I carefully measured and mixed the two components in the mixing bowl and put the bowl into the vacuum chamber.

Pump failure and chaos

After a short while, the vacuum pump stalled and wouldn't start again. That wasn't an ideal moment for the pump to stop, so I called my instructor and together we tried to figure out what was going on. Eventually he went to get another pump and because 15 min already had passed, I started pouring the silicone into the mold. However, it turned out to be too viscos for pouring into a 5 mm hole. We tried to have the other vacuum pump suck the silicone into the mold, which worked okay, but time ran away. In the end we had to give up, knowing that this wouldn't be a perfect cast.

Here is the broken vacuum pump, the chamber with the mold and the funnel still full of silicone and the teporarily connected vacuum pump.

After curing and demolding here is the result:

It is actually quite strong and yet flexible, so I think no changes are required to mold

Learning

The important learning from this first try is the high viscosity of the silicone. It would be best to inject it into the mould next time. Otherwise the mold seems to be okay and the vacuum pump also magically started to work again.

Second Attempt

I started with the same preparation as for the first attempt.

After mixing the ingredients, I put the bowl into the vacuum chamber and was suprised to see the liquid "boiling" for at least five minutes. There was obviously a lot of air in the silicone. I couldn't wait for all the air to bubble out, but aborted the process and continued with the casting.

For that I had put the mold at a slight angle and injected the silicone with a 60 ml syringe. Because of the viscosity it requires some patience especially, when filling the syringe, but it worked pretty well.

I stopped when the silicone started to pour out of the vent.

Here is the result:

You can still see some imperfections and small air bubbles, but otherwise it turned out very good.

One thing to consider next time is to put the bottles with silicone components into the vacuum chamber and degas them properly before mixing. That might help to get rid of more air.

Further development

Because I want to continue developing an anemometer with the casted joint, I created a seperate page for documenting the further steps.

Anemometer