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Fab Academy 2020
15. Molding and Casting

Assignment :

Jump to Group Assignment section below

Week 15 learning journey :

This week topic sounded interesting and fun, and it will be my first time doing molding and casting.

Problem I faced this week :
The CNC machine in the SP Fab lab broke down and I am not able to use, it having some issue with the tool changer. So I faced some delay and had to look for another cnc machine.

I will be making a "two part mold" to cast out a smiley face ball.

Learning for this week:


Some basic info :

3D Printing vs molding and casting :

It is possible to 3D-print the mold, however you would need to do some post processing of the surface, as a typical 3D-printed mold is not going to have a smooth 'surface finish' because it might contain tiny jagged edges along its surface (or steps along a slope). To have a better and smoother surface finish, we would need to machine the mold. Example LEGO® brick is created using molding and casting and not 3D print.

2D, 2.5D vs 3D toolpath :

We have been using cnc machining over the last few weeks; example to mill our PCB boards as well as in "making something big" week.

A toolpath tells the cutting machine what to cut, the direction to move and how fast to move, how many passes, etc.

The term 2D, 2.5D and 3D describe the cutter's movement along an axes.

A 2D machining cuts in two directions (x and y).
A 2.5D machining cuts up and down as well (x,y, z), however it can only cut in two directions at the same time. The cuting will require the cutting tool to move in z-axis, but it is just moving up and down, tend to be straight movements in and out of the material. Example you can add grooves or flat pocket and most milling falls in this category.
A 3D machining is able to cuts in three direction at the same time. Example machining of sculptured surfaces required in molds or of a fully contoured 3D shape.

Reference :
- Lydia Sloan Cline, 2018, Make: Fusion 360 for Makers, Maker Media, Inc. CA.
- Shopbot : What is CNC Anyway?

Molding and Casting

Molding is a process of creating a 'hollow' container with a desired shape of the 3D model/object. This hollow container is call a 'mold' (or 'mould') and will be use to shape the casting materials and thereby we are able to create duplicate copies of the model/object from the mold. Molds are negative forms that are used to shape casting materials.

Mold can be a
- Simple one part mold – for models with a flat back and no undercuts
- Flexible block mold – for 3D models with undercuts
- Two part mold – for models with challenging shapes and undercuts

Casting is the process of pouring the liquid casting material (example metal, water, plastic, etc) into the mold while the material is still in its molten form and then allowed to solidify. The solidified part is also known as a 'casting', which is ejected or broken out of the mold to complete the process.

Curing is a chemical process to make the casting material toughen (or harden) by cross-linking of polymer chains, to give the final casting piece more strength.

Example liquid epoxy resin are typically mixed/cured by the use of catalyst, often called 'hardener'. There will be a chemical reaction which generate heat causing the resin to harden.

Colorant like color dye or pigment is added to the casting resin to give the final product a color. Usually one to three drops.

Reference :
- Instructables: Mold making & casting basics
- Wikipedia: Molding
- Wikipedia: Casting
- Wikipedia: Curing (chemistry)
- Castin'Craft Ideal Book by Casey Carlton



Getting started : "Designing the mold around the tool!"

The first step is to

I am using an old 3-axis CNC Machine in the FabSTUDIO, comes with a EAS (made in Germany) 3–axis stepper motor control unit MC 400/C/3 axis and a milling table dimension of around 300mm(x-axis) x 400mm(y-axis) and z-axis height of around 100mm.


The "cutting length" of the ball nose end mill is around 20mm. The cutting diameter and length will influence my design with regards to maximum depth of cut, the narrowest gap between objects as well as the smallest features that I can design.



Molding and Casting Material

I am using EasyMold™ Silicone Rubber (Liquid) for the mold and for the casting material I be using 'FastCast™ Urethane Casting Resin'. Both are manufactured by ETI (Environmental Technology Inc.).

Molding material : EasyMold™ Silicone Rubber (Liquid)
EasyMold Silicone Rubber is a easy to use, odor free, non-toxic, two component silicone rubber. ~ www.eti-usa.com

Working Time : 45 minutes to 1 hour of working time at 70° F / 21° C.
Mix Ratio : 1 : 1
Curing Time : 24 hours
   

Casting material : FastCast™ Urethane Casting Resin
"FastCast™ is a two-component clear urethane casting resin, which turns white once set. Easy to use, 1:1 ratio and is odorless. Ideal for casting, quick prototyping, miniatures, models, taxidermy and more. FastCast™ sets to a hard cast in approximately 10 minutes at 70°F. Once cured, FastCast can be drilled, sanded, shaped and painted."" ~ www.eti-usa.com

Working Time : maximum 2 minutes of working time at 70° F / 21°C. Mix for 2 minutes and pour immediately into mold
Mix Ratio : 1 : 1
Curing Time : 10 to 15 minutes (cooler room temperature or high humidity might take longer)
   

Video Tutorial on how to use Easy Mold™ Silicone Rubber on Youtube :

Source: How to Make a Block Mold Using Easy Mold Silicone Rubber @ https://www.youtube.com/watch?v=cImkZzJeiA0

Video Tutorial on how to use FastCast™ Urethane Casting Resinon Youtube :

Source: How to Use FastCast Urethane Casting Resin @ https://www.eti-usa.com/how-to-use-fastcast-urethane-casting-resin-video



Group Assignment : Reading the Safety Data Sheet (SDS)

A Safety Data Sheet (SDS), or previously known as Material Safety Data Sheet (MSDS) or product safety data sheet (PSDS), is a document which is usually prepared by the manufacturer to help you learn more about the substance/chemical/product pertaining to the safe use of the substance/chemical/product or occupational safety and health aspect.

The manufacturer, ETI, had posed the 'SDS' for all the their product range on their website, including the EasyMold and FastCast.



Safety Data sheet for :
- EasyMold™ Silicone Rubber Part A
- EasyMold™ Silicone Rubber Part B
- FastCast™ Urethane Casting Resin Part A
- FastCast™ Urethane Casting Resin Part B

The SDS contain the following sections :
1. Product and Company Identification
2. Composition and Information on Ingredients
3. Hazard Classifications
4. First Aid Measures
5. Fire Fighting Measures
6. Accidental Release Measures
7. Handling and Storage
8. Exposure Controls and Personal Protection
9. Physical and Chemical Properties
10. Stability and Reactivity
11. Toxicological Information
12. Ecological Information
13. Disposal Considerations
14. Transportation Information
15. Regulatory Information
16. Other Information


SDS of EasyMold™ Silicone Rubber (Liquid) :
I looked through the SDS. As a Safety Officer and a In-charge of my own workshop, the first thing I would look at is the header title, to check that the product code (or product name) in the SDS header matches exactly the product that I be using.

Hazards :
Next is to look for any information regarding the physical hazards of the product. Here, there is no acute or chronic health effects, however product might cause slight eye irritation if some of the product comes in contact with the eye.
Skin contact: No acute toxic skin effects are expected.
Inhalation: Inhalation is not expected due to low vapor pressure.

First Aid measure :
After contact with the skin: Wipe off excess material with cloth or paper. Use a waterless hand cleaner to remove as much of the remaining material as possible. Wash with soap and water.
After contact with the eyes: If contact with eyes, immediately hold eyelids apart and flush with plenty of water for at least 15 min.

  

Handling and Storage :
To me, this section is the second most important piece of information to look out for. How the product can be store in the Fab Lab and how to handle it. Example you might not want to trigger any unwanted chemical reaction. I have two hazardous substance storage cabinet in my workshop, I will typically store them inside to keep them away from other users.
- Open and handle container with care
- Ensure adequate ventilation
- Do not store with basic substances (e.g. alkalis, ammonia, amines), oxidizing agents, strong acids
- Protect against moisture

Personal Protection :
Hand protection: Recommendation: Any liquid-tight rubber or vinyl gloves.
Eye protection: Recommendation: Safety glasses with side shields.
  


SDS of FastCast™ Urethane Casting Resin :
The Casting Resin is a much more harzardous than the molding material and need to be taken very seriously.

Hazards :
Harmful if inhaled.
Causes skin irritation.
Causes serious eye irritation.
May cause allergy or asthma symptoms or breathing difficulties if inhaled.
May cause an allergic skin reaction.
May cause respiratory irritation.
May cause damage to organs (Respiratory System) through prolonged or repeated exposure.
  
Handling and Storage :
Do not breathe mist or vapor.
Avoid contact with eyes, skin, and clothing.
Avoid prolonged exposure.
Use only outdoors or in a well-ventilated area.
Wear appropriate personal protective equipment.
Wash thoroughly after handling.
Observe good industrial hygiene practices.
Personal Protection :
Wear safety glasses or goggles
Wear chemical resistance gloves
   


In addition to the info provided in the SDS, some Do's and Don'ts of casting :

Reference :
Castin'Craft Ideal Book by Casey Carlton' on ETI website.




Test Casts

Before you dive right in to make the final mold or cast the pieces, it is a good idea to make test casts with both the molding and casting material that you be using. These small test casts help me understand the characteristics of the silicon rubber and casting rasin, as well as by doing a few times of castings provided me with valuable expriences before my final cast.

I had carried out some simple testing :


For EasyMold™ Silicone Rubber (Liquid) :
1) My workspace for the molding and casting with old newspapers and A2-size white recycled drawing paper. I had bought some small plastic containers for making the test casts


2) Using a measurement cup to mixed 'Part A' and 'Part B' of the EasyMold™ Silicone Rubber into the various mix ratios that I wanted.


3) Poured the Silicone Rubber molding material into the various small containers.


4) I allowed all the silicone rubber to cure accordingly to their timings under normal room temperature of around 21°C.


Test cast 1 (mix ratio) :
Photo below showing test casts of different mix ratio :
(Part A 0.5 : Part B 1),   (Part A 1: Part B 1),   and (Part A 1 : Part B 0.5).

The first and second samples feel the same and offers the same rigidness and hardness.
The third sample (A 1: B 0.5) feel a bit soft and you can easily twist it. I looked back at the SDS and relasied that both Part A and B contain the same chemical, but Part B has an additional chemical compound (Polydimethyl hydrogenmethyl siloxane) inside. In simple explaination, what it mean is that you can mix with a little portion of Part A and still have rougly the same strength. But Part B might be the bonding agent, if you mix less amount of Part B, your mold will becomes softer, which is not good if you are looking for a rigid mold.

Test cast 2 (cure Time) :
Photo below showing test casts of different curing timings (17 hours,  20 hours,  24 hours).

I am hoping to see a softer or even a wet surface on the sample that only had cured 17 hours. But all three samples seem the same to me. All three are very rigid and hard, cannot eaily twist.

Perhaps I should cut the first test cast into half to have a look at the inside to see if it is still wet and soft on the inside...


For FastCast™ Urethane Casting Resin :
Here I dripped two drops of the yellow color dye into a Part A solution and mixed it first before adding Part B. Then I cast another piece with 1:1 ratio of Part A and Part B without any additional color dye.


Working-time : Under room temperature of around 32°C, the working time for Urethane Casting Resin is around 30 seconds to 40 seconds after your mixed Part A and B together and start stiring.

Stiring the mixture is very important for the casting rasin, if you did not stir enough of it will, some portion of the mix will not be able to solidify, you end up with liquid portion here and there (along the edge/corner in particular). Stiring have to be fast but gentle, do not cause any wave as it will result in bubbles. Around 30 seconds, you can pour the casting rasin. If you starts to feel the 'heat' from the mixture, it will be a bit late, but you can still hurry to pour it into the mold. Problem is it already starting to solidify while you pour.

The cast pieces on the right look a bit darker, which had two drops of the colour dye added. But not that obvious.

I relaised that the yellow color dye that I added is for transparent epoxy casing rasin. Therefore there is not much colour changes.



Designing the 3D model : Smiley Face Ball (for 3 axis machining)

I designed a small "Smiley Face Ball" of 40mm in diameter in Fusion 360 and attempt to experiment on making a two-part mold.

1) Drawing the design. I created the negative mold using 'combine and cut' feature in Fusion 360, then drawn a 'funnel' to the 'cast piece' for easy pouring of the casting rasin.
2) I created second 'mold' for the top piece for the two part mold. For the eyes, the bottom (end) of the hole is a rounded curvature.

3) My mold for the mold.
4) So far so good from the look at the modeling.

5) I decided to use 'Cut3D' to generate the 3D toolpath as it is very easy to use.
6) A simulation of the roughing toolpath as well as the finishing toolpath.


Creating the mold

I am using the recycled foam board availabe in my workshop for my first modling and casting as the machining wax block is very expensive in our country. Therefore the surface finish is not that good.

1) I used the 'hot wire cutter' to cut out some recycled pieces of 2-inch (around 50mm thick) foam block from student's past project.
2) Using NC-EAS(Y) to mill the mold. Loading of the 'Roughing' toolpath first.

3) Roughing cutting. You can see the stepped contour.
4) Finishing cutting. Finer carved out contour now.

5) Calculate the mold volume using water. Total volume is around 185ml. Add 3%~5% to cater for any spill or some amount sticking on the stirring stick and mixing cup.
6) Spray release agent on the surface and allow it to dry for at least 45 minutes before molding. I applied two layers. The surface a bit of oily.

7) Mix Part A and Part B 1:1 ratio. I filled the holes of the eyes and mouth first before pouring the rest over the model. Take note of the 45 Minute to 1 hour working time, and 24 hour curing time.
8) Shake the mold for a few minutes to allow bubbles to escape out. Allow mold to cure for 24 hours before demolding

9) My finished mold box and the silicon mold.


Casting :

1) Calculate the volume needed for my 3D model from Fusion 360. It is around 32 ml.
2) Spray 'mold release' agent on surface of the silicon mold and allow it to dry for around 45 minutes. I used a wide transparent tape to secure the mold together. This is because I found that by doing this way will secure the mold tightly together and the tapes cover all around the mold to protect from any spillage. The tape does not stick on to the silicon mold and will be easily removed after casting.

3) I marked out on a small plastic cup the required ammount needed with a black marker pen. Poured the Part A into another larger plastic cup. Added a few drop (around 12 drops) of the yellow dye to Part A and stir to give it a bit of colour. Then I added and mixed Part B with the Part A accordingly based on 1:1 ratio and stired for 30 seconds.
4) Pouring the FastCast™ Urethane Casting Resin Part A and Part B into the silicon mold. I pinched the top of the cup for easy pouring. Do not pick up, touch or move the mold once you have poured. Curing will take about 15 minutes to 25 minutes. Issue faced : I am pouring the casting rasin down the narrow funnel of the mold slowly, as well as taking photo, I didn't realised that half-way thought that the casting rasin started to solidify and changed color.
5) After it cured, the cast piece should drop out of the mold easily. The mold can be reused to cast again and again. From the front, my 'smiley face' model look round.
6) From the side, I had to use fine sandpaper to sand away the proturting portion at the joint. My 'smiley face' model should look round like a ball, but it now looked like an egg... I relised that the wall between the front and the back is just 3mm, this might be the reason, as the casting material tend to heat up and expand a bit.

7) I tried the casting again, now using two pieces of wood panels to straighten the front and the back of the silicon mold.
8) My finished Smiley Face Ball, which now look rounded from the front as well as from the side.


Reflection :

Molding and casting is fun! However it is not as simple and straight forward when I first imagined. You need to do a bit of practises to get use to the process.

3-axis machining :
This week I had learned the important lesson of machining a 3D object, what is roughing cut and what is finishing cutting. Which is quite useful and cool, as I now understand how to machine 3D object in the future.

3D Mold design :
For the design of my mold, I should cater for much thicker wall; like say 10mm instead of 3mm. 3mm is too thin.

Molding :
Next time, I would consider trying out a wax block for my mold box or look for better material. The foam block that I am using is old and filed with cavities within, perhaps I should have paint it over.

Casting :
For casting, you have to mixed the Part A and Part B thoroughly all within 30 seconds, before it starts to harden. You have to test it out a few times as working time also include the pouring into the mold time.

When stiring, it have to be in a steady movement to avoid forming of bubbles. Instead of using a thin wooden stick to mix, next time I might use a brush for mixing as it be more efficient.

Next time, I will consider using a air vacuum machine to remove the bubbles. It is very important to ensure that there are no bubbles formed inside the silicon mold as well as during casting.



Files :

My 3D model and mold design in Fusion 360 :
Smiley Face Ball (for molding).f3d

Cut3D file :
Cut3D front mold.v3d
Cut3D back mold.v3d



References :

Lydia Sloan Cline, 2018, Make: Fusion 360 for Makers, Maker Media, Inc. CA.
Shopbot : What is CNC Anyway?
Instructables: Mold making & casting basics
Wikipedia: Molding
Wikipedia: Casting
Wikipedia: Curing (chemistry)
Smooth-On: Basics of Mold Making
Product Design Online : Tutorials on Fusion 360
Fusion 360: Mold Making Tips
Autodesk Fusion 360 - Mold Creation
Lydia Sloan Cline, 2018, Make: Fusion 360 for Makers, Maker Media, Inc. CA.
Casey Carlton, 1982, Castin'Craft Ideal Book by Casey Carlton, Field Landings, CA