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Week 13 Molding and Casting

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.

Review the safety data sheets

For this exploration we focussed on 3 different resins,

Smoothon OOMOO 30 Medium softness easy to use tin cure silicone rubber

SmoothOn Smooth-Cast 305 urethane

Dr. Resin Epoxy

We reviewed the data sheets for all of these before we did our test pours.

David and Jeremy reviewing MSDS together.

SmoothOn OOMOO 30

OOMOO MSDS

The OOMOO series is a safe product that is easy to use. There were no special handling requirements or specific hazards mentioned in the data sheet. “This chemical is not considered hazardous by the 2012 OSHA Hazard Communication Standard (29 CFR 1910.1200).” and “The product contains no substances which at their given concentration, are considered to be hazardous to health.” So it was deemed safe to pour in the lab with just rubber gloves.

SmoothOn Smooth-Cast 305

Smooth-Cast 305 MSDS

Unlike the OOMOO, the Smooth-Cast had a number of stated potential hazards needs more care during handling. Some of the potential hazards listed are:

H313 - May be harmful in contact with skin.

H315 - Causes skin irritation. H317 - May cause an allergic skin reaction.

H319 - Causes serious eye irritation. Page 1 / 10 FG-4021A Revision date 13-Jan-2025

H332 - Harmful if inhaled.

H334 - May cause allergy or asthma symptoms or breathing difficulties if inhaled.

H335 - May cause respiratory irritation.

H351 - Suspected of causing cancer.

H373 - May cause damage to organs through prolonged or repeated exposure.

From this data sheet we knew we had to use it in a well ventilated space. Fortunately, the air exchange in our lab is quite good, so we felt safe to used it in small volumes in the lab.

Dr. Resin Epoxy

This resin was something Jeremy had purchased a while ago and the MSDS could not be located online and the paper version was already discarded. Therefore, we reviewed the MSDS from a similar product from Smooth On,

Smooth ON Epoxys MSDS

This MSDS recommended using gloves and wearing long sleeves while handling and to avoid skin contact.

Test Casts

We did a test cast for each of the resins.

For the epoxy and the the urethane, we poured them in a mold that Jeremy and his daughter had made with OOMOO silicone about a year ago. It is a mold of Greek themed key chains that are a fun test. Since the OOMOO would stick to the OOMOO mold, we pourded the OOMOO into a wooden cavity witha 3D printed UNCC C logo.

Mold of greek themed trinkets used for expoxy and urethane.

SmoothOn OOMOO 30

For the mold, we hot glued a UNCC C logo keychain upside down in a piece of scrap wood that was leftover from “Make Something Big” week. We took the 3D print, put hot glue on the ‘C’ and bonded it to the wood to make the simple mold.

Putting hot glue on a 3D printed C logo keychain.

Finished mold after bonding it to the wood cavity.

Once the mold was ready, we poured the resin. The OOMOO mixed and poured easily. It has a 1:1 mix ratio by volume and each part is a different color (one pink, one blue) so when they mix, it turns purple and it is easy to see if there are streaks of unmixed areas.

Mixing the OOMOO.

Pouring the OOMOO. Note the purple color of a well mixed resin.

It took about 6 hours before it set up well enough to review. Once it was hardened, we pulled the 3D print out of the cavity leaving the C in the silicone. As the C was bonded to the wood, we did not get a good surface on the C. In retrospect, we should have bonded the 3d Print on the other side and put it in a deeper cavity so that it would look more like the Greek trinket mold from above.

Result of the test cast.

SmoothOn Smooth-Cast 305

We used the silicone Greek trinket mold to test this resin. Since the 305 had a 100:90 by weight mix ratio, we procured a scale to make sure we mixed it in the correct ratio. We donned gloves and covered our scale with cardboard. Then we poured 300g of part A into one cup. Then we poured 270g of part B in another cup. Then we combined and stirred.

Measuring out the 305

Mixing the 305

Then we poured the 305 into the mold. It was easy to pour we had no problems. For the second trinket, we added some purple dye to give it some color and it mixed in just fine.

Pouring the urethane into the mold.

The 305 cured quickly, but we let it sit for about 6 hours before demolding anyway. The pulled out of the silicone mold with no issue and made detailed and tough parts.

The hardened trinkets.

Dr. Resin Epoxy

We used the silicone Greek trinket for the epoxy as well. The epoxy was a 1:1 by volume mix ratio, so there was no need to use the scale. We measured 1 ounce of part A and 1 ounce of Part B. Then we mixed them together and mixed them with a popsicle stick. We also added a little bit of green dye to give it some color. Once it was mixed sufficiently, it was poured into the mold.

Pouring the epoxy resin into the mixing cups.

Pouring the epoxy resin into the mold.

The epoxy took a long time to cure. After about 7 hours in the mold it was still soft. One was demolded and it stretched out and lost its shape. So we left it in the lab over the weekend to cure more. We pulled them out on Monday morning and they were cured and pulled out of the mold nicely.

The resulting epoxy trinkets. The one on the left was demolded too soon and has distortion.

Compare printing vs milling molds

We made molds of the UNC Charlotte C logo to compare printed vs milled molds.

3D Printed Mold

We used SolidWorks to design a simple mold of the C logo. We made it about 60mm tall and 10mm deep. We drafted with 1 degree of taper. Then we subtracted it from a solid rectangle to creat the cavity. Then we shelled the bottom of the mold to reduce material usage and added a wall around the top to hold in any overrun from the pouring.

CAD of the C mold

This was then printed on a Prusa MK4S using matte PLA material.

3D printed mold.

SolidWorks File

STL File

Milled Mold

We used the same geometry to make a machined mold from wax. We started by exporting a dxf of the cut file from Solidworks and imported it into VCarve software. Then we created 2 pocketing routines with a 1/8” end mill. The first pocket was to get down 5mm for a trough for the overrun. Then we we made the second pocket for the C shape that would be the actual part that we wanted to create.

DXF used for toolpath

Image of VCarve toolpath setup.

Then we placed a piece of machinable wax into our Roland SRM-20 machine using double sided tape. We zeroed all 3 axes and loaded the cut file generated from VCarve. Then we let the machine do the cutting to finish the mold.

Placing our machinable wax into the milling machine.

Finished wax mold.

Casting

For the casting we opted for the Smooth-Cast 305 urethane. We chose this as it was the fastest setting of our suite of materials and David wanted to try it for his individual project molds.

We started by brushing mold release into each of our molds.

Brushing on the release.

Since Smooth-Cast is 1:1 by volume, we poured 2 ounces of each part into one of our mixing cups and stirred it for about 2-3 minutes to make sure it was homogeneous. Since the resin is clear it was a bit hard to tell when it was fully mixed, but there were some streaks and light cloudiness where you could tell there was still an interface between the two parts. Then we added about 6 drops of purple dye to the mix and poured it into our molds.

Mixing the 305 resin.

Pouring the resin.

Then we let the molds set. The pot life is 7 minutes and cure time is 30 minutes. After about 10 minutes the resin was visually kicking and showing some non-translucent areas.

The 305 material in the mold starting to set.

After about an hour we demolded the parts. We started with the 3D printed mold and it was clear very early in the process that we were going to have to destroy the mold to get it out. So we cut the mold away with side cutters and pliers. It took a while as even with the mold release, it stuck to the mold. Eventually we got it out but not with out breaking off the legs of the C.

Similarly, the wax mold had to be cut apart too. We used a band saw to cut close to the mold and were able to chisel it out once the walls were thinned out. The wax part was able to be released without any breakage.

Demolding the wax mold after cutting with a band saw.

Once they were out we were able to compare the quality of the casts. They both showed machining marks from the 3D printing and maching process, and the quality was very similar (if I had not broken the 3D print). The wax mold did transmit some of the color from the wax to the part, which was unexpected.

Comparison of the two parts.

Since the quality is similar, 3D print molds seem like a better option as they are faster to print and can be reproduced easily if they are damaged. If we do rigid molds again, we will be sure to make them in such a way that they can be demolded more easily.