We reviewed key information to look for on safety data sheets.
Proper PEE starts with
gloves to protect your hands
lab coat to protect clothes
and depending on how nasty the material is, could include something more serious like a respirator and/or eye protection.
always check the safety data sheet to see what level of PPE is required for the material you’re working with!!
In general it’s a good rule of thumb to mix chemicals in a room with good ventilation.
Group Exercise: Making a mold
As a group we went through the process of picking a two-part polymer mold material and using the data sheet to understand important concepts for working with materials such as mixing ratio (by volume or by weight), pot life, and demolding time, and curing time. Also make note of the expiration date - many of these materials don’t have a long shelf life and should be used quickly once opened. Materials that are too old won’t cure well.
We chose to make a mold with the pink food-safe material:
Mixing:
to determine how much material to mix so you have enough to fill your mold but not so much that you have a lot of waste, you can pour water into the mold and then measure how much water it took
measure part A into your mixing cup and then zero and measure part B into the same cup (if trying to conserve materials and not waste another cup). Pot life timer starts as soon as the two parts come into contact with each other
mix vigorously, using shearing motion, for longer than you think you need. Make sure to scrape the sides, bottom, and corners of the container in the process of mixing
save the cup with remnants of the material so you can tell how the curing process is going
When materials are very viscous they can capture lots of air bubbles in the process of mixing, which can affect the quality and structural integrity of the resulting mold and/or part. In that case, you can use a vacuum chamber to de-air the material, either prior to pouring into the mold or after you’ve poured into the mold (but if you want to de-gas after you pour into the mold, make sure the walls of the mold are high enough because the material will kindof bubble up as the bubbles are pulled to the surface, and could overflow into the vacuum chamber if you’re not careful).
Describing how to use the vacuum chamber to remove bubbles from the mold material
Pouring:
pour into the lowest point of the mold
pour from up high, in a thin bead - this will help to break air bubbles trapped in the mixture
can tap or vibrate the mold to further dislodge or pop any air bubbles which may be trapped
make sure the part is left on a flat surface to cure uninterrupted
Group Exercise: Using the mold to cast a part
The next day we returned to find our mold cured. We demolded it from the wax to see how it turned out.
We went on to make test casts using a few different materials, from other molds available in the lab.
3D printing vs machined molds (additive vs subtractive manufacturing processes)
There’s a push to explore 3D printed molds (but with a high-resolution process such as SLA resin, or FDM with some sort of surface finishing process to remove the layer lines). Parts fabricated additively using SLA come off the printer with a great surface finish, and result in very little material waste compared to that generated by subtractive manufacturing (more on that later). However, resin is known to act as a cure inhibitor and in tests performed in our lab as well as others, casts from SLA-printed molds haven’t come out very well. There’s definitely room to optimize this workflow, potentially through use of a coating or barrier between the 3D printed resin and the cast material.
You can machine molds from various materials and of various sizes, depending on your use case. Wax is a common machining material for small-scale molds which can be fabricated on a little CNC. Foam, wood, machineable plastic, etc can be machined on a large format CNC to make larger molds.
Individual Assignment
I continued with the theme of hearts this week and made a food safe mold with which I made a chocolate heart!
Design
I designed my mold in Fusion 360 to be fabricated via 3-axis milling on the CNC.
I found a low-poly royalty free heart 3D model that I liked and imported into Fusion 360, then scaled it in Z to make it a bit higher until I was satisfied with the shape. I modeled my stock material, the wax block I would use to machine my mold, and subtracted a cavity in which the heart would sit in order to create the negative mold.
Mold design considerations:
Make sure the wax walls are >= 6mm thick or you risk them breaking during the machining process
no overhangs (or you won’t be able to remove the casting from the mold)
slight draft angle of the walls & filleting/chamfering at the corners will make it easier to remove the casting from the mold
make sure to leave enough space around the edges of your shape and the wall of the cavity that the walls of the resulting mold will hold their shape
make sure to leave enough space at the top of the cavity for the resulting mold to have some structural integrity
Finished mold in Fusion360:
CAM for the CNC
We use Modela Player 4 to create the CAM paths to run on the Roland SRM-20 CNCs we have in the lab.
Import STL
File > Open
Confirm STL position/orientation; set origin
Create New Process: Roughing job
a. Select Roughing process
b. Set tool as the 1/8" flat endmill
c. Set the Partial cutting area
d. Tool path: contour, up cut
e. Cutting in amount = layer height. no more than 1/2 the tool diameter. larger stepdown means a faster job
Create New Process: Finishing job, X scan
a. Select Finishing process
b. Set tool as 1/8" ball nose endmill
c. Set Partial cutting area
X scan toolpath previewpreview finishing job
(Optional) Create New Process: Finishing job, Y scan
If desired, you can prepare and run a second finishing job which is 90 degrees offset from the first finishing job, so scan lines in the Y direction rather than the X direction. I didn’t actually run the Y scan finishing job because I was satisfied after the X scan finishing job, but I went through the process of creating the process.
Set Y scan linesY scan toolpath preview
Save/Export jobs
Note: another option for the CAM would have been to use mods:
Milling the wax
I milled my wax block on the Roland SRM-20.
Sticking the wax block to the CNC with lots of doublesided tape
Rough cut w/ flat endmill
1/8" flat endmill
Finishing cut w/ ball nose endmill
1/8" ball nose endmill
1:1 mix ratio of Part A:Part B by weight or volume.
Working time: 30 minutes
Demolding time: 3 hours
Curing time: 24 hours
Assemble materials
Assembling my materials
Determine how much volume I need to fill the wax mold
Water test
I found I would need around 120mL of Easyl-940 to fill my mold.
Measuring
for 120mL of material total, I need 60mL each of part A and part B
Measuring part A
Measuring part B
Mixing
I know from the data sheet I have 30 minutes of working life
I opted not to use the vacuum container to de-gas; although I could see bubbles I knew many of them would be released when I poured the material into the wax.
Pouring
I poured from high above the mold in a thin bead, into the lowest part of the mold
I tapped the wax containing my mold material against the table to hopefully release any bubbles from the surface of the wax
Curing
I let my mold sit overnight to cure on a flat table
Demolding
In the morning, I used some compressed air (thanks Adai!) to remove the cured silicone mold from the wax block.
Post-cure
The final step of preparing my mold was post-curing it in a 100C oven for 4 hours to make it truly food safe. Then I washed it well with dishsoap, and it was ready for use.
Test Casts
I did test casts with first water (ice) and then chocolate.
Ice:
worked alright with ice, but would have been nicer if the ice was completely clear
the water didn’t fill the mold completely, probably due to the hydrophilic nature of the silicone
It was a fun week! This was a relatively simple project but it was useful to go through the steps of designing and creating a 3D mold using digital fabrication.
