Week 14 Moulding and Casting

This page documents the group assignment for week 14 of Högni, Jóhannes and Ólöf.

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

Overview and approach

We did the group assignments in two different locations and tried different manufacturing methods and materials.

Högni and Jóhannes worked at Fab Lab Ísafjörður and documented about it.

Ólöf worked at Fab Lab Austurland and documented about it.

Tips

Neil gave us some tips in the lecture.

Use protective clothing, gloves and glasses.
Important to sheer and absolutely not to scoop, because scooping can add airbubbles to the material.
Pour in a thin bead. It can be a good idea to paint the first layer of resin and then the other layers will stick to the first layer.
Wait for all the bubbles to come out.
A poor mix will not get warm and will not harden completely.
If you are working with big moulds the heat can damage the mould.
Remember to slightly angle all faces (draft angle) for easy release.
Hydrostone and silicone will not stick but check if you need a release agent if you are using another materials.
Test the materials before beginning.
Caution! If there is a warning that says the material is not suitable for home use, we should not use them.
Caution! Read the safety precautions on the datasheet.
Let material set before disposing.

Some tips from Adrián:

Add an offset at the top of the mould for easy removal.
First do a rough milling and then use a finish stepover.
Make sure that the endmill is long enough not to collide with the walls.
Use a ball nose.
You can recycle the wax shavings.
Use 3D milling settings in Fab Mods or Fab Modules
Remember that you need a hole to pour in and also a hole/holes for air to excape.

Fab Lab Ísafjörður

Silicone ZA 22

Jóhannes uses Silicone ZA 22 in blue from Zhermack.

Specifications:

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

ZA 22 Safety Data Sheet

Precautions and personal safety

This type of silicone is a relatively harmless material, as it isn't labeled with hazard pictograms. However, it is important to read and understand the safety recommendation that include:

Avoiding skin contact
No food contact
Adequate ventilation
Eye protection
Gloves, working cloth

For the mould, Jóhannes uses MB2001 tooling board from polyurethane we have in our lab.

Jóhannes made a test cast with the material to check if it will set okay and behaves as expected.

Here is the setup with the scale, spatula, mixing bowl and gloves.

Casting setup

Next I measured 15 g of each both liquids in the mixing bowl.

Measuring

After stirring for a while a got a uniform liquid, but I could immediatly see, that there are air bubbles inside.

Mixed

I decided to not pour into any mould, but just let it cure in the mixing bowl. After around 15 min I checked and it was still liquid, but propably more viscose than earlier.

After seven hours I checked next and it looked quite solid. When trying to remove it however, I discovered some sticky liquid in the bottom of the bowl. Maybe, this was caused by unsufficient mixing or it would have profited from a few more hours.

Result

Info

For the actual casting, I'm going to mix even more, use a vacuum chamber to remove air bubbles and give it more time for curing.

Machined mould form polyurethane

Jóhannes made a three-piece mould (two symetric halves with core). The two halves were machined in the Shopbot from polyurethane block.

He also incorporated 3d-printed pieces into the cast.

Here is a picture of the setup:

Three piece mould

The quality of the mould isn't perfect. There can be seen some milling marks and it should be possible to achieve better results. Maybe it would be wise to slow down the machine and fix the mould in a location of the bed, that would vibrate less and with some heavy mass (thick wood or metal plate?) underneath.

Fab Lab Austurland

Ólöf used Zhermack HT 33 Transparent, Jesmonite AC100 and Acrystal Prima.

Zhermack Silicone HT 33

Zhermack HT 33 Transparent

You can read the datasheet for Zhermack HT 33 Transparent here.

The HT 33 is a bicomponent addition RTV silicon rubber, you mix base and catalyst and it sets at room temperature. When vulcanized it has a high chemical resistance to addressive components of certain types of resin. It has high resistance to wear and tear and can be used for casting fine details. It is heat resistant and withstands aging well. This material has high anti-stick effect.

The mixing ratio is 1:1

Specifications:

Hardness: 33 Shore A
Working time: 20-22 min
Curing time: 3 hours
Tear strength: 16 N/mm
Viscosity: 7000-8000 cP

Precautions and personal safety

Zhermack HT 33 Transparent is not classified as hazardous but you should only use it in a well ventilated area. When working with the material it is adviced to use protective clothing, glasses and mask. Keep material away from food.

Acrystal Prima

You can read the datasheet for Acrystal Prima here.

According to this information, Acrystal is based on natural mineral crystals and aqueous acrylic resins. It uses water as solvent has low exotherm which means that it doesn't generate much heat when setting.

It bicomponent and is easy to work with. It can be used internally but if it is used externally, a protective film is necessary. It has many advantages. It is non-toxic, has low odour, the moulds do not need to be cleaned and since water is used as solvent, tools can be cleaned with water. When it sets it has low expansion, it is fire resistant and very durable. It can be used for fine details.

The mixing ratio is by weight 1:2,5 1 kg Acrystal Prima liquids 2,5 kg Basic Crystal powders

Specifications:

Hardness: 81-83 Shore D
Working time: 8-10 min
Curing time: Can be released from mould around 20 minutes to 2 hours, depending on size. Fully cured after 72 hours
Compressive strength: 25-30 mPa (oven dried)
Bending strenght: 18 mPa

Pot life is only 8-10 minutes, so you have to work fast after mixing the liquid with the powder. Demoulding can be done after 20-100 minutes.

Precautions and personal safety

Acrystal Prima has one hazard pictogram. It can cause allergic skin reaction. People should avoid breathing in mist. It is advized to wear protective gloves, protective clothing, eye protection and face protection.

Acrystal Prima protective wearing

Jesmonite AC 100

You can read the datasheet for Jesmonite AC100 here.

Jesmonite AC100 is a water-based bicomponent, acrylic polymer/mineral resin system. It can be used internally and externally, but needs a sealer if used externally. It is very heat resistant and strong material.

The mixing ratio is 2,5:1 by weight 2,5 kg of base 1 kg of liquids

Specifications:

Hardness: 81-83 Shore D
Working time: 8-10 min
Pot life: 8-15 min
Curing time: 15-20 min
Compressive strength: 25-30 MPa
Tensil strenght: 25-35 MPa
Bending strenght: 15-20 MPa
Viscosity: <500 mPa/s

Precautions and personal safety

Jesmonite AC100 is not considered hazardous but inhalation of vapour or mist can cause nausea, headache and irritation of throat, lungs and nose. For that reason it is advised to use a mask when working with this material. Prolongued or repeated contact with skin or eyes can cause irritation.

Jesmonite AC100 using guide

Here you can find a user guide for Jesmonite AC100.

Small tests

I began by making small tests with all materials. I also wanted to see how the Zhermach Silicone ZA 22, that Jóhannes tested, would come out with a 3D printed mould. But first, the small tests:

I measured the wight of the material to make sure that the mixing ratio was correct.

I wore protective clothing, glasses, mask and gloves.

Here you can see how these four tests came out.

I think they all looked okay except for the Zhermack Silicone HT 33. I don't have a vacuum chamber so the test is full of tiny bubbles. They are very small so I am not sure if it will affect the final outcome or not, but here is a close-up of the test.

3D printed moulds

I wanted to see how 3D printed moulds would work. I made a simple mould in Fusion. You can read about the design process in more detail on my website here. I used the fillet option to make the outlines of the mould soft, so that it would be easier to release the cast from the mould but. I wanted to see if sharp lines and small details on the cast would turn out okay, so I did not use the fillet option on the object's outline.

Then I used the Bambu Lab X1 Carbon and printed two moulds in pink PLA. When the printing was done, I used water to measure the quantity needed. I could see that I had made the moulds unecessarily big, which means that I use more material than necessary.

Using the 3D printed moulds to cast silicone moulds

I used the moulds to cast two silicone moulds. I used the Zhermack HT 33 Transparent and the Zhermack ZA 22 in blue. I wasn't sure if the moulds would hold the silicone so I let them set with a bowl placed under them.

Very hard to release casts from moulds

When I wanted to remove the casts from the moulds, I ran into problems. It was very hard to remove them. I tried to use my fingers but that was impossible. I knew that I should not use sharp objects so I decided to use a plastic brush to help me. I began with the Zhermack ZA 22 in blue, which is softer with 22 Shore A. This took time but I managed to release it without damage.

I should have used a taper angle when designing the mould. It was not enough to use a fillet to soften the corners.

Then I tried to get the Zhermack HT 33 Transparent out of the mould. It was even harder. I really thought that I wouldn't be able to do it, but it worked after quite some time of hard work and to my surprise it wasn't damaged.

Here you can see the 3D printed moulds and the cast silicone moulds:

As you can see in the image below, the lines from the 3D printed moulds are quite apparent. This is a big minus.

Using the silicone moulds to cast small objects

The final step was to see how the silicone moulds would work for casting small objects.

I added Jesmonite colours to the two sets of Jesmonite AC100 that I casted. The cast objects came easily loose from both the Zhermack HT 33 Transparent and the Zhermack ZA 22 in blue.

Bubbles in Jesmonite AC100 and Acrystal Prima

Here you can see the final cast parts. The two on the left are made of Acrystal Prima. The pink and the blue ones are Jesmonite AC100. There are small bubbles in all these casts, even though I tried my best to pour the material from good height, let it run in a thin line and tried to let the material run into cavities without closing air in. I also tapped the moulds against the table for some time after pouring the material in. One more thing; When I designed the mould I thought about whether to make the Fab Lab logo stand out or be engraved. In the final casts I can see that it would have been much better to make the logo stand out in the final cast, so I should have had the logo stand out in the Fusion design mould.

Conclusions

3D printed mould works very well but the lines from the 3D printed object are visible on the cast.
Using a taper angle will make it easier to release a cast from a mould.
Machined molds generally give a better surface finish, but require more design and fabrication time.
You have to think carefully about how you are going to release your cast object from the mould.
It is hard to get rid of bubbles if you don't have a vacuum chamber. Even with a vacuum chamber, removing air bubbles might require longer time than you have before the material starts curing.
Check the viscosity of the material before designing the mold and think about how it will flow in all corners.
Using a syringe and injecting the material might help filling complex or closed moulds.
You have to work fast with materials that have a short pot time/working time.
Make moulds that do not use more material than needed.
Create a step at the top of the mould. This thinner edge makes it easier to get the cast out of the mould.
Be careful when you pour the material into the mould. Bubbles can easily get trapped in the cast.