12. Molding and casting¶
This week I worked on defining my final project idea and started to getting used to the documentation process.
Group Assignment:
- [x]Review the safety data sheets for each of your molding and casting materials
- [x]Make and compare test casts with each of them
- [x]Compare printing vs milling molds
Individual assignment:
- [x]Design a mold around the process you’ll be using, produce it with a smooth surface finish, and use it to cast parts.
Group Assignment¶
Here is teh group assginment link:
- Group Assignment on Machine Week
Mold Silicone Materials in the Lab¶
How to use mold silicone:
1. First mix the silicone and curing agent in a ratio of 100:2 and stir evenly.
2. When making the mold, first paint a thin layer on the master mold and wait until no bubbles are generated before pouring more material.
3. Ways to reduce bubbles in silicone molds:
- Notes on the use of curing agent: A smaller proportion of curing agent can extend the curing time.
- Notes on stirring materials: The best mixing time is 5 minutes. It is best to stir in the same direction to avoid air bubbles.
4. The shelf life of mold silicone is generally 3 to 6 months, and the best use time is 3 months, otherwise there will be oil leakage.
Precautions:
- It needs to be used on a clean and dust-free operating surface.
- Remember to wear gloves when operating to avoid dripping into the eyes, mouth and nose.
AB Glue Materials in the Lab¶
During the mold and casting process, we will use AB glue as the material for the final casting model.
The use of AB glue is relatively simple, just mix the two glues in a 1:1 ratio.
Precautions:
1. The mixing ratio of AB glue should be relatively accurate, reaching a 1:1 ratio.
2. Be careful not to let AB glue come into contact with your skin.
3. AB glue has a pungent smell, so remember to wear a mask.
Small Digital Scale¶
In order to more accurately measure the weight of the materials needed for molding casting, we used a small digital electronic scale.
1. High-precision measurement: This digital gram scale can perform precise measurements with an accuracy of 0.01 grams.
2. Clear display: The electronic scale is equipped with an LCD screen that clearly displays the measurement numbers, making it easy to read the measurement results.
Comparison of 3D printing and CNC Milling¶
Both 3D printing and CNC milling are ways of making molds, but the manufacturing methods and principles of the two are slightly different.
Advantages of 3D printing:
- The processing technology of additive manufacturing can better save the materials required to make molds and has lower costs.
- It can quickly restore some model details, and has corresponding advantages in restoring the details of tiny assembly structures.
Disadvantages of 3D printing:
- Due to the printing accuracy limitation of 3D printing marks, layered textures will appear on the surface of the mold, and post-processing is required to restore the detailed surface.
- 3D printing cannot be used as a mold for some productions that require high-temperature materials.
Advantages of CNC Milling:
- Traditional machine tool processing can make the surface treatment of workpieces and molds smoother.
Disadvantages of CNC Milling:
- Subtractive manufacturing processes waste more material.
- Only suitable for manufacturing larger or simpler geometrically shaped workpieces and molds. The degree of detail restoration of complex workpieces is poor.
Individual Assignment¶
Idea of the molding and casting¶
When I was thinking about what exactly I needed to mold and cast, the first thing that came to my mind was Chinese copper coins and bronze ware.
So I checked a lot of information and found that ancient Chinese copper coins were actually a casting product in ancient times. Craftsmen first made a mother coin as the first coin, and then made molds based on this mother coin to create more copper coins.
So I also decided to make a copper coin to give it a try. I looked up the dimensions of the coins and started my design.
Design and 3D Priting the mold¶
1st design & 3d printing mold¶
- Because I wanted to design and restore the original shape of the copper coin, but found that Traditional Chinese fonts could not be input into onshape, so I had to design it myself. I used a lot of complex lines to restore the appearance of Traditional Chinese characters. At this time, I also paid special attention to the need for mirroring of fonts. After the design was completed, I directly exported the design file to STL format for subsequent 3D printing.
- When doing 3D printing, I used Bambu Lab’s 3D printer. In order to make the printing effect better, I also specially changed the printing nozzle to a thinner one and used a 0.2mm printing nozzle for 3D printing.
- After I printed it at a ratio of 1:1, I found that the printed size was still too small, which resulted in incomplete display of the font and also could not satisfy subsequent production. So I enlarged it to 1x the size and re-3D printed it.
- When I first started designing, I originally hoped to design a mold directly, and only need to pour the filling material into it to get a final complete copper coin model. I overlooked the most serious point. When I poured the hard copper coins in, it was difficult to take out the model from the hard mold. So I had to redesign it.
2nd design & 3d printing mold¶
After absorbing the experience of the first design and processing, I made modifications. I designed the design of the copper coin itself on the front, and modified the font to read “Open Source Hardware” in Simplified Chinese. I still wanted to process it at a 1:1 ratio size. , so I got the final result.
Manufacturing the Mold¶
- Prepare the preparations and materials.
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Find a suitable container and mix the model silicone and curing agent in a ratio of 100:2.
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Let the mixed material stand for a while and gently pour it into the mold I printed by 3D.
4. After waiting for successful curing, take out the mold.
5. After waiting for 24 hours, it was finally time to take off the silicone mold. But I found that because the curing agent added before was relatively small, it took a long time for the silicone mold to solidify, which was longer than I expected. In addition, due to uneven stirring, the curing agent and silicone were not fully mixed. I found that when I took off the silicone mold, there was a part of the silicone that had not solidified. But in comparison, the two molds can still be used.
Casting my coin¶
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Prepare the preparation work and materials, first I need to take out the AB glue used to make the model.
Note: When using, try to mix at a ratio of 1:1, otherwise it will affect the coagulation effect.
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Using an electronic scale, accurately weighed and prepared the AB glue, I found a clean mixing container. First put the container on the electronic scale, and then pour in glue A and B of similar weight. Here my container weight is 2.07 grams, glue A uses 2.88 (=4.95-2.07) grams, glue B uses 2.95 (=7.9-4.95) grams.
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I used a bamboo skewer to fully stir the weighed A.B glue, and stir it for as long as possible to make it evenly mixed.
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Then pour the mixed glue evenly into the mold. Here I poured my silicone mold, and also poured some glue into the 3D printed model I had printed in advance.
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The last thing is to wait for a while for the glue to solidify before I can remove my final model.
But I made a serious mistake because I did not spray the release agent in advance when pouring the silicone and AB glue. As a result, the 3D printed mold could not be successfully removed after filling with the AB glue. As a result, I had to make a new mold and make it again.
Hero shot¶
I am quite satisfied with the final product. The silicone mold will not be damaged due to too small details. The model I made shows the fonts relatively realistically.
For the evaluation guidelines state. I have explained the difference between 3D printing and CNC molding in my homework. The reasons why the surface of my coin is not so smooth are as follows.
1. For the molding method: When comparing CNC molding and 3D printing to make molds, after comparison, I hope to restore the fine font display on the coin. This part is currently impossible to be completed by the CNC machine in our laboratory, so I can only choose 3D printing to make the mold.
2. For the post-processing process of the mold: Due to the small size of the mold, the diameter of the coin is only 24 mm, and the thinnest printing layer of the 3d printer is 0.2mm, so it is inevitable that some printing marks are produced on the surface of the mold. If the mold is to be processed more carefully, I should pre-process the surface of the mold. This part of the work is because the laboratory lacks materials such as filling clay, so the surface texture of the coins I finally cast can be seen.
3. For the processing of coins after casting: Because I use single-sided casting, when importing casting materials, it is necessary to be a little more than the mold itself, so there is some excess casting material overflow on the back of the cast coin that needs to be processed.
