13. Produce many identical objects by Molding & Casting¶

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This week I learnt how to mill molds and cast them with different materials. In particular…

I understood the range of materials on offer at FabLab Kamakura, and what are some of the main parameters when choosing materials (such as texture, flexibiliy, color, etc).
The potential toxicity of some materials, and the standard Safety Precautions.
The mold-making workflow, and some best practices for designing and producing effective molds.
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 printing, as method for producing molds.

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¶

Link to Group Documentation

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.

Groupwork 2: Milling vs Printing¶

Since we didn’t have time to compare printing vs milling, I did a comparison by myself;

Process	Milling	Printing
Precision	◎ More precise	△ Rough around the edges and generally requires post-processing. But recently there are higher definition (Stereolithography/SLA) printers (e.g., 12K)
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 for the sleeve of my spice bottle.
My hope was to cast it using a type of flexible, stretchy material, allowing it to be versatile for a variety of spice bottle sizes.
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Materials Used:

For making the mold: Blue Wax (Ferris File-A-Wax), size: 147 x 88 x 37mm
For casting the final product: EcoFlex 30

Tools used:

Machine for milling: SRM-20
CAM Software: 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

1. Research & Planning¶

Our task for this week was to design an appropriate object within the limitations of our lab.
Due to the limited time we can use the lab, and the power limitation of our SRM milling machine, we were advised to do a very small 1-sided cast.
Trying to achieve my goal above with these limitations helped me understood the limitations of 1-sided casts and of milling.
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Some of the limitations of milling in our lab that I discovered were:
- Long time taken to mill using the SRM machine - therefore difficult to make large or complicated shapes.
- Thickness of the Endmill - final structure needs to be thicker than the endmill, and with fillets to avoid corners. Since the endmill we use for rough cut is 1/8 inch (3.18mm), the final object has to be at least 4mm thick.
- Milling’s inability to do undercuts - as endmills cannot reach underneath structures, limiting the design significantly

Because of these limitations, I was not able to achieve my initial goals, but I decided to try answer these questions through this week’s assignment;
- The qualities of lab’s flexible material “Ecoflex” - such as how well it stretches, how suitable it is for my project (in terms of aesthetics, durability, etc).
- The benefits of milling and printing, and what method I should ultimately use for the final product.

2. Designing the Mold¶

I used Fusion 360 to design the mold.
* Apparently, the emphasis on “With a smooth surface finish” was added for the first time this year, but I did not need to consider it this time as I am only making a 1-sided cast.

Steps for designing the mold;

First, decide whether I am doing a soft cast or hard cast. On a hard mold, 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 flexible mold (using soft material such as Silicone Rubber), then use that as the mold for the final hard cast.
I started by modelling the final object on Fusion
I then modelled the Wax cube, and used the “Combine” feature to cut out the final object from the wax 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 at least the endmill diameter. I later had to go back and expand this by a couple more millimeters to make the milling process less troublesome.
Finally, I exported the model as an STL file.

3. Setting CAM Toolpath¶

Milling the mold involves at least 2 steps, so at least 2 toolpaths need to be generated.
- Roughing Toolpath - to remove the large amount of stock material quickly, and
- Finishing Toolpath - to remove small amounts of material, and to achieve smooth surface.

I used the MODELA Player 4 to produce 3 toolpaths; 1 for roughing, 1 for finishing using square endmill (⅛inch (3.18 mm) long 4FL SE), and final one for finishing using round endmill (1/16inch 2FL BN).

Steps for generating Toolpath;

Import the STL file, 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.
Estimated Time was…

4. Producing the Mold¶

We were given a block of wax (Blue Wax (Ferris File-A-Wax), size: 147 x 88 x 37mm) to mill the mold. As I mentioned before, since my final cast material is going to be soft, I only needed to mill a negative mold on my wax.

Steps for milling the mold:

Get the origin: Mark the paraffin wax block with two diagonal lines, to set the middle of the block as the origin. 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 which would abort and restart the whole process. So I had to stop every few seconds to remove the wax. I ended up having to tweak my CAD design to expand the gap by 2mm which helped somewhat but made my final object too thick.
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5. Casting parts¶

I used EcoFlex 30.

Ecoflex™ rubbers are platinum-catalyzed silicones that are versatile and easy to use. Cured rubber is very soft, very strong and very “stretchy”, stretching many times its original size without tearing and will rebound to its original form without distortion.
Ecoflex™ rubbers are water white translucent and can be color pigmented with Silc Pig™ pigments for creating a variety of color effects. You can also add Smooth-On’s Silicone Thinner™ to further lower the viscosity. THI-VEX™ silicone thickener can be added by weight to Ecoflex™ silicones for brushable applications. They are suitable for a variety of applications including making prosthetic appliances, cushioning for orthotics and special effects applications (especially in animatronics where repetitive motion is required). Ecoflex™ rubbers are mixed 1A:1B by weight or volume and cured at room temperature with negligible shrinkage. Low viscosity ensures easy mixing and de-airing.

EcoFlex 30	Description	TB (Technical Bulletin)
Viscosity		3,000 cps
Specific Gravity		1.07
Specific Volume		26.0
Pot Life		45 min
Cure Time		4 hours
Shore Hardness		00-30
Tensile Strength		200 psi
100% modulus		10 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 certain types of gloves don’t work for the materials; check the SDS).
Room: Large well-ventilated room. Cover the table-tops with newspaper.
Practices: Wash hands well with soap in case of contact with materials.

Precautions for avoiding bubbles:
For best mold quality, we must not stir in too much air while mixing, otherwise air bubbles will remain in the cast.
Before pouring, we should brush the interior of the mold with a layer of rubber, and while pouring, we should pour slowly and let the material flow in from the edge to the center.

Stirring: Two-part materials have to be mixed thoroughly, otherwise they will not be cured uniformly. You have to mix them completely, but try to avoid producing air bubbles.
Pot life: Although you have to mix completely, each material has its pot life, 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
Vacuum degassing can be used. This is not available in our Lab.

6. Results¶

Because I made finishing cuts with a 1/16 inch round endmill, the surface came out very smooth. The details also came out nicely.
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Regarding my initial questions;
- The qualities of our flexible material “Ecoflex” - Very nice and stretchy, but too delicate for long-term use in the kitchen.
- The benefits of milling and printing, and what method I should ultimately use for the final product - I still need to experiment more with both methods, to evaluate the most effective and least labor-intensive method

Reflections:¶

This week I learnt to…
- Design appropriate objects within the limitations of my process
- Demonstrate workflows used in mold design, construction and casting

How did it turn out?
Describe problems and how I fixed them

I could understand why Molding and Casting is a smart way of mass-producing identical objects. Since I am still in the prototyping stage, I don’t necessarily need to mass-produce with this approach

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

Try produce a multi-sided mold that can cast the ideal sleeve of my spice bottles.
Try Bio-Materials
Learn how to choose the right endmills (Possibly useful link)

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