WEEK 14 MOLDING AND CASTING

•G•R•O•U•P• •A•S•S•I•G•N•M•E•N•T•

All group assignments can be found on the official website of FAB LAB BENFICA

In the lab, we have two types of silicone that have already passed their shelf life. We are waiting for the new batch to arrive, but the delivery is currently delayed. In the meantime, we will try using the old ones and see how it goes. We have: Smooth-On Smooth-Sil 940 and another unidentified tin-cure silicone with a 5% catalyst (labeled "katali7"—the label is difficult to read).

I created this mold with Mayku FormBox MAYKU Later in my Individual assignment I’ll explain the process more detailed.

Safety Data Sheet Review

1. Unknown Silicone (with "katali7" Catalyst)

Type: Likely a Tin-cure silicone rubber (condensation-cure)
Shelf life: extended!
Mix Ratio: Estimated 95A:5B by weight (based on labeling)
Handling: Less sensitive to contamination, faster to set
Observation: Part B (catalyst) comes in a small bottle; curing occurs rapidly after mixing
Limitations: Lacks detailed product documentation; not food-safe; shorter mold lifespan compared to platinum silicones

2. Smooth-On Smooth-Sil 940 Smooth-Sil 940 Safety Data Sheet

Type: Platinum-cure silicone rubber (addition-cure)
Shelf life: extended!
Mix Ratio: 1A:1B ratio 1:10
Pot Life: ~30–40 minutes
Cure Time: ~24 hours at room temperature
Hardness: Shore A40
Tear Strength & Elongation: High tear resistance and flexibility
Features: Food-safe once cured, low shrinkage, great for detailed molds
Handling: Requires clean tools and surfaces, as contamination (especially with sulfur) may inhibit curing

The battery of my scale died so I had to improvise 🙂

I think this silicon might not work at all, but let’s see.

Test Casts

Both silicones should be de degassed to get rid of the bubbles. Since I don't have a chamber I used vibration of the polishing machine to shake the molds to bring some of the bubbles to the surface

I created sample casts using both silicones in identical test molds and in the top one I mixed rest of both of them together with no specific proportion to see what will happen.

Key differences observed:

The unknown silicone cured quickly, resulting in a clean, flexible white cast. It was easy to demold, though some visible bubbles were present.
Smooth-Sil 940 mixed well, producing a smooth pinkish mixture (with Part B being a reddish liquid) and showed fewer bubbles during mixing. However, it did not cure even after 24 hours and remained liquid.
Both silicones were likely past their shelf life, which may have affected their performance, especially in the case of Smooth-Sil 940.
A mixture of the unknown silicone and Smooth-Sil 940 also failed to cure, which was expected, since Smooth-Sil itself did not harden at all.

UPDATE (new silicon arrived)

R PRO 30 - Silicone rubber mould making 1:1, ideal for high hardness molds

PRODUCT FOR PROFESSIONAL USE datasheet

R PRO 30 is a 1:1 addition silicone rubber with high resistance to tearing.
HIGH PERFORMANCE: R PRO 30 mold silicone faithfully copies even the smallest details allowing you to make 20% more castings than other silicone rubbers and the molds have no linear shrinkage. It is a very fluid rubber that guarantees easy mixing and does not need to be degassed with vacuum machines.
HARDNESS: The catalysed rubber allows you to make soft (hardness 30 ShoreA) and very elastic molds with a high resistance to tearing, the processing time is 45 minutes while the hardening time is 3 hours
MOLDS PERFORMANCE: The physical life of the mold depends on how it is used (cast materials, frequency, etc.). Casting abrasive materials such as concrete can quickly erode mold detail, while casting non-abrasive materials (wax) does not affect mold detail.

For my test casts, I added some blue pigment powder just to see what would happen, without measuring any specific proportion.

The untouched mixture will be used for my individual assignment cast, which you can check out later on this page.

Test Casts V2

R PRO 30

is really hard and not sticky at all. It captures fine details perfectly and didn’t show any air bubbles.

The only flaw I noticed was due to not mixing the pigment powder thoroughly, some undissolved clumps left visible stains., but that’s on me.

•I•N•D•I•V•I•D•U•A•L• •A•S•S•I•G•N•M•E•N•T•

MODEL

For my individual assignment I’ll be doing the button cover that didn’t work out in the week of 3D printing.

I created a new 3D printed model so I can use it with the Mayku Form Box I mentioned in the group assignment.

The paw shape is like inverted pyramid, which means the draft will be a problem. That’s why I divided it into smaller pieces that will be possible to slide out of the form

And lastly I created another piece that will make to hollow part of the cast.

I started with printing the bottom part and I forgot to include the suction holes, so I needed to add them manually:

The top part came out pretty nice.

You can see that parts are separated. I’ll still keep them attached for now. but later I will cut them off and keep them together like a kids puzzle.

MAYKU

The Mayku FormBox is a desktop vacuum former that creates molds and shapes using heat and vacuum. Here's a short step-by-step process:

Place the material – Insert a thermoplastic sheet into the FormBox’s frame.
Heat the sheet – Lower the frame over the built-in heater until the plastic becomes soft and pliable.
Prepare the mold/object – Place the object you want to form over on the forming bed.
Form the mold – Lower the softened sheet over the object and activate the vacuum; the sheet wraps tightly around the object.
Cool and remove – Allow the plastic to cool, then lift and remove the newly formed mold.

Each sheet has their own properties so you just simply need to follow the instruction on the attached paper. You can always adjust settings by increasing heating time or heat level itself

I was hoping the edges in the bottom will be more defined… Not even mentioning the groove I created in not marked at all. I decide to add more suction holes and increase the heat level to 5,5

There’s a mini difference but nothing significant… I’ll try the other sheet:

The effect is still not sharp. I’ll go extreme this time and instead of 1min and level 5 of heat I’ll do 2,5 min and level 6. Let’s see what happens

The sheet is stretching a lot…

Fail :( and the edges are still not sharp, even with such melted sheet…

Apparently, you can’t reuse the sheet but I was curious to try it :D

Again wrinkcles and not much sharper then before…

Ok I think that’s enough of testing the bottom mold. Time for the top one:

FINAL MOLD A

The effect is still now what I wanted exactly. Maybe the square I created for easy placement of 2 molds together is not letting the plastic sheet stretch enough… I’ll cut it out:

FINAL MOLD B

Non of the solution gave the effect that I really wanted, but I think the cast from the MOLD A should work for my Cat’s button project.

FEELING UP MOLD A

I should have made some air holes, but with a bit of shaking and moving, I managed to get rid of all the trapped air. I'll keep it under weight so it doesn't shift.

CAST FROM MOLD A

Here’s the final cast with the new silicon that we just got:

FILES

CAT’S PAW MODEL

PAW_SILICONE.stl

MODELS FOR MAKING MOLDS

OUTSIDE:

PAW_MOLD.stl

INSIDE:

INSIDE.stl

•C•H•E•C•K•L•I•S•T•

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.

Learning outcomes:

Design appropriate objects within the limitations of your process
Demonstrate workflows used in mold design, construction and casting

Have you answered these questions?

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