14. Mass-Produce by Casting¶

This week I learnt how to mill molds and cast them with different materials. I learnt how to effectively design molds, successfully fabricate them, and safely cast them.

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This week's assignments (Apr 23 - Apr 30):

Group assignment:
- Review the safety data sheets for each of our 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 I'll be using, produce it with a smooth surface finish that does not show the production process, and use it to cast parts.

Groupwork 1: Making test casts¶

This week, we were able to try casting all the materials at FabLab Kamakura. Through this, I got an idea of the range of materials on offer, and some of the key parameters when choosing materials (such as texture, flexibiliy, color, etc).

Also, by going through the datasheets, I understood the potential toxicity of certain materials, and how important it is to read the datasheets carefully and thoroughly.

Link to Group Documentation

Groupwork 2: Milling vs Printing for Mold-making¶

Since we didn’t have time to compare milling vs printing during groupwork, I did a comparison individually. I need to try making 3D printed molds another time.

Process	Milling	Printing
Precision	◎ More precise finish	△ The finish is not as detailed and rough around the edges, generally requiring post-processing. But recently there are higher definition printers (e.g., 12K Stereolithography/SLA)
3D Design	Not able to do undercuts	Able to do undercuts
Materials	◎ More variety, such as Metals, Plastics, Composites	△ Fewer options
Ability to iterate	△ Not as easy, as milling takes time and has less room for error	〇 More suited for rapid prototyping
Speed	△ Depends, but generally slower	〇 Faster if small in size
Cost	〇 Generally less (but needs upfront investment of milling machine)	〇 Can be less, if in low volume
Other		Limited by printer size. Potentially less waste.

Design a mold to cast something with a smooth surface finish¶

For my individual assignment, I wanted to try making a mold that can mass-produce the spice bottle cases.
My plan was to cast it using EcoFlex, a type of flexible, stretchy material, so it could be versatile for a variety of spice bottle sizes.
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Tools and Materials I Used:

Machine for milling: Roland SRM-20
Software for generating milling toolpaths: MODELA Player 4
Endmill:
→ 1/8 inch (3.18 mm) long 4FL SE, for rough cut and 1st finishing cut
→ 1/16 inch 2FL BN, for 2nd finishing cut
Material of mold: Blue Wax (Ferris File-A-Wax), size: 147 x 88 x 37mm
Material of final cast: EcoFlex 30

1. Research & Planning¶

Our task for this week was to design an appropriate object within the limitations of our lab.
The mold/cast making workflow depends on 2 factors; 1.whether the final product is hard or soft, and 2.how many sides need to be cast.

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Due to limited time we can use the lab, and the limited power of our SRM milling machine, we were advised to mill a very small 1-sided mold. Trying to achieve my original goal with these restrictions helped me understood the limits of 1-sided casts and of milling.

In addition, I also later discovered below limitations:
- Very long time to mill using the SRM machine - reinforcing the fact that we can only make very small and simple shapes.
- Thickness of the Endmill - final structure has to be significantly thicker than the endmill (which is 3.18mm), and with fillets to avoid corners.
- Milling’s inability to do undercuts - this limits the design significantly

Because of these limitations, I was not able to achieve my initial goals, but I decided to try answer below questions through this week’s assignment;
- The qualities of lab’s flexible material “Ecoflex”: how well does it stretch? How suitable is it for my project (in terms of aesthetics, durability, etc)?
- The benefits of milling over 3D printing What method should I ultimately consider for the final product?

2. Designing the Mold¶

I used Fusion 360 to design the 3D mold.
・Apparently, the emphasis on “With a smooth surface finish” was added for the first time this year, but this was not relevant for me as I was only making a 1-sided cast.

Steps for designing the mold;

First, I need to decide whether I am doing a soft cast or hard cast.
As explained earlier, on a hard mold such as wax, only soft/flexible material can be cast. Since my final cast material was soft, I only needed to mill a negative of my final cast on the wax.
If I was doing a hard cast, I would use the wax mold to cast a flexible mold (using soft material such as Silicone Rubber), then use this mold to cast the final hard object.
I started by modelling the final object on Fusion.
I then drew a cube as the wax, and used the “Combine” feature to cut out the negative of the final object inside the cube.
I made a fillet to make sure the mold qualifies for the assessment criteria (It needs to be a proper 3D shape, with a curve, not 2.5D).
I also made sure to make all the gaps a little wider than the endmill. I later found out that this was still too narrow and had to go back and expand them by a couple more millimeters to avoid the endmill jamming.

The final mold model, to export as an STL file

3. Generating the CAM Toolpath¶

Milling the mold involves at least 2 stages; roughing and finishing.
1. Roughing - to remove large amounts of stock material quickly, and
2. Finishing - to remove small amounts of material, to achieve smoother surface.

I decided to make 3 milling toolpaths; 1 for roughing, 1 for finishing with a square endmill (⅛inch (3.18 mm) long 4FL SE), and final one for finishing with a round endmill (1/16inch 2FL BN).

How to generate the milling Toolpath;

Import the STL file to MODELA Player 4, set the model size, orientation and origin.
Press “Create a New Process”.
Define the milling area.
By defining this, we can limit the Milling machine’s working area, significantly decreasing the time to mill
Set Feed & Speed. The material we used was Blue Wax, so …
Set Maximum roughting Stepdown to 3 millimeters
Set the Fine Stepdown to 3mm and clicked OK
Save the G-Code.

4. Producing the Mold¶

We were given a block of wax (Blue Wax, Ferris File-A-Wax, 147 x 88 x 37mm) to mill our mold.

Steps for milling the mold:

To mark the middle origin; draw 2 diagonal lines on the wax surface.
Fix the wax to the bed of milling machine.
Attach the endmill.
Set the XY and Z Origins.
Select the file (Start with the Rough Cut G-code)
Start machining.
Press pause from time to time to clean up the accumulated chips.
Once rough cut is complete, repeat the same for Finish cut

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It took me nearly a whole day to mill and I run out of time before I could complete it. This was because I made the cutting area only slightly larger than endmill diameter, and the wax would accumulate on the endmill and get stuck in the gap, causing an error that aborted and restarted the whole process. I found myself stopping the machine every few seconds to remove the wax, so I decided to go back to my CAD design to expand the gap by 2mm. This solved the problem but made my final object much thicker than I hoped.
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5. Casting¶

I decided to make the cast with the very stretchy EcoFlex 30 (Link to Data Sheet).

Ecoflex™ rubbers are versatile silicones. It’s very soft, very strong and can stretch many times its original size without tearing, rebounding to its original form without distortion. They are water white translucent and can be color pigmented. Thinners can further lower the viscosity. Suitable for a variety of applications, e.g., prosthetic appliances, special effects (especially in animatronics where repetitive motion is required), etc.
Thy are mixed 1A:1B (weight/volume) and cured at room temperature with negligible shrinkage.

EcoFlex 30	TB (Technical Bulletin)
Viscosity	3,000 cps
Timing	Pot Life: 45 min, Cure Time: 4 hours
Material strength	Shore Hardness: 00-30, Tensile Strength: 200 psi Elongation at break: 900%, Die B Tear Strength: 38 pli
Shrinkage	< .001 in./in.

⚠️How I produced the cast safely;
According to the datashseet, it is a non-hazardous material, but basic protection measures such as use of PPE was recommended.
PPE: Wear gloves, apron, mask (note that they need to be certain materials; check the SDS).
Room: Large well-ventilated room. Cover table-tops with newspaper.
Practices: Wash hands well with soap in case of contact with materials.

☝️Precautions for avoiding bubbles:
In order to get good quality mold, we must not stir in too much air while mixing, otherwise air bubbles will remain in the cast.

When stirring: Ensure through mixing to cure them uniformly (while also trying to avoid air bubbles).
Pot life: Each material has its pot life (time it takes harden), ranging from as short as a minute to half an hour. Always check the material’s pot life before using it.
Painting: Before pouring, apply thin layer of mold material to the model surface. (Thinner the better, especially for detailed molds.)
Pouring: Slowly pour onto one corner of the mold, and let the mixture flow in at the edge of mold to the center.
Vacuum degassing can also be used, though it was not available at FabLab Kamakura.

6. Results¶

Because I did a 2nd finishing mill with round endmill, the surface came out very smooth. The details also came out quite nicely.
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They fit well on different size spice bottles, but it looks too thick.
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Reflections:¶

This week I learnt how to design appropriate molds for milling, and the workflows in mold design, production and casting.
I could understand why Molding and Casting is an ideal way of mass-producing identical objects.

I learnt about the different mold-making/casting methods, including;
→ Difference in workflow depending on whether the final product is hard or soft.
→ What it means by 1-sided cast, and 2 or more sided casts.
→ Considerations when making 2 or more sided casts.
→ Pros and cons of milling vs 3D-printing molds.

Going back to my initial questions, the Ecoflex material turned out to be much stretchier than I expected (more than I need it to be), and perhaps too delicate for long-term use in the kitchen. I would love to try out other materials but unfortunately casting materials are much harder to come by in Japan.

For the future, when I have more time, would like to try…

Make my own Bio-Materials
Design a multi-sided mold that can cast the ideal sleeve of my spice bottles.
Produce 3D-printed casts

Useful links:¶

Assignment Checklist:¶

Linked to the group assignment page and reflected on my individual page what I have learned
Reviewed the safety data sheets for each of my molding and casting materials, then made and compared test casts with each of them
Documented how I designed and created my 3D mold, including machine settings
Ensured my mold has smooth surface finish, that does not show the production process (by postprocessing if necessary)
Shown how I safely made my mold and cast the parts
Described problems and how I fixed them
Included my design files and ‘hero shot’ of the mold and the final object