13. Molding and Casting: Angel, Noah, Kathryn, Jenna¶
Milling - Noah¶
Part of the group work was to compare different methods of making molds. Our lab has 2 methods, 3d printing and milling, and everybody choose to 3d print their individual mold. For the group work, I milled out my own mold out of machining wax, which has a good guide for the othermill here.
File Preparation¶
For some reason, importing stl’s onto bantam tools software didn’t work, so I had to find a different method. For this, I remembered what Dr. Gershenfeld had said a long time ago about Mods. Learning mods took way too long, as searching for mods, or even mods community edition usually just gave video game mods. I eventually found this fab academy page for electronics production that described how to use mods to make PCB’s with a specific milling machine my lab doesn’t use. Through trial and error, I eventually found that on the mods page
right click -> programs -> open program -> G-code -> mill 3d stl.
After loading that program, I loaded my stl file. Be careful when exporting that stl from whatever software you use that it is a Binary STL using inches as units. If not, it will make 1mm equal to an inch and the website will crash trying to compile such a large model. I took that STL and put it into the
After importing the file, I went to the Mill raster 3d block and clicked calculate. in that block, theres also a really important setting called “stepover”, which would be easier to understand if it was just called “quality”. Basically, the lower it is, the longer your mill will take, but the better it will look.
After calculating with a stepover of .5 (rough pass) and .2(finishing pass), I took the files it
Random file fix¶
Because im using the program for general milling machines, it didn’t follow some standards for othermill. the main problem was that it added in G54 to the start of the gcode. this changed offsets, but is Not supported on the othermill machines. In order to fix that, I simply deleted that line from the gcode files it exported. If you dont delete that, the othermill software gives you a giant warning about it not milling where you want it, then it won’t mill where you want it.
Othermill¶
After getting the good gcode files, I sent them to the computer connected to the mill. I loading in a 1/8th ballnose for both passes, as my lab doesn’t have 1/8th straight for the roughing pass. In the software itself, I only changed offsets to make sure that it milled on my wax, then started the mill. The rough pass took 10 minutes a 1.5 inch square basically, and the finishing took twice as long to cut with twice the detail.
Post Milling¶
After milling, I spent a bit sanding down the face. Due to the fact that we didn’t have that straight bit, there were very small but visible channels going down the machine.
Molding¶
For the mold, we followed the same processes we did for our individual mold. We did end up using a different material, just because it was already out for another persons mold. We also used the quick release spray to create a layer between the wax and the molding material, which would make getting it out easier.
Milling + Molding result¶
Milling Advantages¶
- Wider range of materials: When using 3d printing, your limited to almost always plastics, but resins on a sls are also possible.
- Accuracy: A well tuned milling machine can surpass even the best accuracy of a 3d printer.
- Surface Finish: Thanks to the ability to due a finishing pass, you can make something on a milling machine that doesn’t require any manual post processing
Milling Mold Material Datasheet Analysis - Noah¶
We used PMC™-746 for the material for the mold
Technical Datasheet¶
The technical datasheet is basically how to use it, and its special features that will make you want to buy this over the competition
The key points from this are
- 2:1 mix ratio (weight and volume)
- 15 minute pot life (usable time)
- 16 hour cure time (16 hours to hardened)
- casts must be sealed
- mix well
- comes out with a clear amber color
Safety Datasheet¶
According to the safety datasheet, this is one horrible evil material. On top of the trifecta of touch, breathing, and ingestion all causing irratation, it also contains a known carcinogen (cancer causer).
The key points on what to do from the safety sheet are:
- Avoid breathing vapors.
- Wash with soap and water thoroughly after handling.
- Do not eat, drink or smoke when using this product.
- Use only outdoors or in a well-ventilated area.
- Keep out of reach of children (obviously).
and the key points on the dangers are
- bad if swallowed, in contact with skin, or inhaled
- causes serious skin and eye irratation
- may cause respitorary irritation
- “suspected carcinogenicity” (suspected to cause cancer)
Milling Material Datasheet Analysis - Kathryn¶
For the machining wax, I referred to this source of information.
Technical¶
Its main features including being non-abrasive, self-lubricating and pretty user-friendly. It is ideal for producing accurate reproductions.
As for machining guidelines,
- Spindle Speed: 3,000 RPM
- Feed Rate: 100 in./min
- Finishing: Spindle Speed: 8,000–10,000 RPM
- Feed Rate: 40–100 in./min
Data obtained using a ½” ball end mill.
In terms of physical properties
| Property | Value |
|---|---|
| Specific Gravity | 0.94 |
| Ash Content | 0.010% |
| Hardness (Shore D) | 43 |
| Softening Point | 157°F |
| Volumetric Shrinkage | 7% |
| Viscosity @ 160°F (CPS) | 30 |
Safety¶
-
The wax has a high flash point of 575°F (302°C), which means it is stable under normal machining conditions but should not be exposed to open flames or excessive heat.
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The specific ingredients are listed as proprietary, which is common for such products. However, this means that in case of overheating, fumes may be released, so adequate ventilation is recommended during use.
Thus, you should ensure adequate ventilation when machining or melting the wax to prevent inhalation of any fumes that may be released upon heating, use appropriate temperature controls to prevent the wax from reaching temperatures near its flash point, and use standard PPE such as safety glasses and gloves are recommended to prevent physical injury during machining.
Comparison between Casting Materials - Angel Fang¶
Example of Soft Cast¶
2nd Version of Gear Belt¶
We decided to use 3D printer to make a hard mold. I followed this tutorial made a one-part mold.
Then we printed out the design and make it as our mold. Then we used Sorta Clear 37 to make the cast.
Unfortunately, since it was hard mold, it was really hard to take the belt out. It turned out that the belt was broken and we could not use it anymore. So we decided to gave up on making belt using molding and casting and turned to electrical tape instead.
I learned that for this kind of mold, it would be better to use two-part mold to take thecastout intact.
Example of Hard Cast¶
Soap Dish¶
For my individual assignment I decided to make a soap dish since my mom requires me to make one.
I first designed it in Fusion360.
I used the 0.08mm extra fine to decrease the layer height.
Then, I used the same material which is Smooth-Sil 940 to make the mold. I brought my own container. However it was too big so molding materials would be wasted. So I found a container that can barely fit my cast.
Then I mixed the Part A and Part B and put my mold at the bottom of the container and poured the molding material in.
However, after 10 minutes, I saw my cast was floating up. So I had to stand there, using a chopstick to hold the cast down. Later, Mr. Dubick told me thatI can hot glue my cast down so it would not float.
The next day, I took out the mold from the container. It was really hard to take it out since there was little gap between the mold and the walls of the container. But with the help of Ms.Horstman and Stuart Christhilf， I manged to took the mold out and took out the cast from the mold.
Next, I decided to use Smoooth Cast 65d to make my cast. Once I mixed the Part A and Part B together, the mixture began to warm up. Right after I poured the mixture into my mold, it staretd to harden.
It took only 2 minutes max to get it harden.
Then I took my final product out carefully.
I did not do anything for solving the bubble issue and this material seemed to not have any issue like that so my soap dish turned out pretty well.
Then my soap dish is completed!
The difference between soft and hard cast¶
For the Smoooth Cast 65d, it was really to cure. It took only 2 minutes max to cure the whole thing. However, because of its fastness, it is really hard to do any modification after pouring the misture in. Unlike resin, you are able to pour coloring into the resin to make different colors. For the Smooth Cast 65d, you cannot do that. It is a good choice if you don’t care baout the color and just want to get something done really quick.
For the Sorta Clear 37, it takes much longer time to cure. It usually takes a day to be fully dry. Since the cast is gonna be soft, it is really hard to take the cast out from a hard mold. So you have to make two part mold for this casting material. Hoever, it is really flexible and is a good option when you need to make soft things.
Datasheet Analysis - Kathryn¶
SORTA-Clear 37 (Silicone Rubber) SORTA-Clear 37 is platinum-cure silicone. It has a 25-minute pot life and cures in about 4 hours at room temperature. It’s medium-firm with a Shore A hardness of 37, which means it’s flexible but not super soft. It’s certified food-safe and skin-safe, so you can use it for chocolate molds or wearable items. It mixes 1:1 by volume, so no scale is needed, and it has very low shrinkage. You should avoid using latex gloves or anything with sulfur nearby, as those can inhibit the silicone’s ability to cure. Vinyl gloves are safe. Also, make sure your space is well-ventilated and wear gloves and eye protection when mixing. Users should operate in a well-ventilated area to avoid inhaling vapors or mists. Vinyl gloves are recommended, as latex gloves can inhibit the curing process . Additionally, wearing safety glasses and long sleeves can help minimize contamination risks.
Smooth-Cast 65D (Semi-Rigid Urethane Resin) Smooth-Cast 65D is an opaque, fast-curing resin that’s tougher than most plastic. It’s often used for things like props, prototypes, or casting solid parts in molds. It mixes easily at a 1:1 ratio by volume and has a super short pot life of just 2.5 minutes, so you need to work fast. It sets up solid in about 10–15 minutes. With a Shore D hardness of 65, it’s semi-rigid — not rubbery but not brittle either. It’s moisture-sensitive, so make sure your workspace is dry and not humid, and mix quickly to avoid bubbles. It gives off a lot of heat while curing, so definitely wear gloves, safety goggles, and work in a well-ventilated area (or use a respirator for large casts). Smooth-Cast 65D is not food or skin-safe, and you should not use it in any application that involves body contact or ingestion. Due to its chemical composition, it’s essential to handle this material with care. Users should work in a well-ventilated area to avoid inhaling fumes, which can be harmful. Wearing protective gear, including safety glasses, long sleeves, and rubber gloves, is recommended to prevent skin and eye contact . It’s also important to note that the resin is moisture-sensitive; exposure to atmospheric moisture can affect the curing process and the quality of the final product.
Applying a suitable release agent to molds can facilitate demolding and prevent the resin from sticking to surfaces . After curing, castings may be hot to the touch and could cause skin irritation; allowing them to cool to room temperature before handling is prudent. Proper storage of the resin in a warm environment (around 73°F or 23°C) and sealing containers tightly after use will help maintain its usability and prevent degradation .