12. Moulding and Casting¶
For this week we had to:
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Group assignment:
- Review the safety data sheets for each of your molding and casting materials
- Make and compare test casts with each of them
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Individual assignment:
- Design a mold around the stock and tooling that you’ll be using, mill it (rough cut + (at least) three-axis finish cut), and use it to cast parts.
Group assignment¶
This week at Fab Lab Armenia we welcomed an interesting guest - Ashot Margaryan who is a graduate of Fab Academy. He currently works in the steel industry and has extensive knowledge of various casting methods.
You can see our group assignment in detail here.
Ashot shared his experience and knowledge with us, talking about the creation of molds and demonstrating different types and technologies of casting. He also discussed the process of smelting various metals and emphasized the importance of observing safety measures. In addition, he invited us to visit his factory.
Safety¶
We have discussed in detail the importance of wearing goggles and gloves and avoiding direct contact with resin or rubber components when working with molding and casting materials. When working with liquid resin or rubber, it is important to follow the manufacturer’s instructions, including the correct mixing ratio and cure time.
Failure to follow these instructions may result in a hazardous or ineffective end product. It is also necessary to properly dispose of leftover materials and thoroughly clean the work area after use to prevent accidental exposure to materials and ensure the safety of others.
Negative mold casting¶
During our group task, we had the opportunity to work with different materials:
- rubber PMC 121/30 DRY
- Mold Max60
- Modeling clay
Mold casting¶
As part of the group work, Ashot Margaryan, a former student of the Fab Academy, decided to demonstrate the process of casting an aluminum alloy into a plaster mold.
Ashot melted aluminum in a special furnace capable of heating up to about 1000°C, although the melting point of aluminum is about 660°C.
The results were impressive: we got a cast aluminum alloy mold.
Individual assignment¶
I want to express my gratitude to the former student of Fab Academy Ashot Margaryan. He has been immensely helpful to me this week. Thanks to him, I learned how to organize the process of obtaining the mold and casting, anticipate potential difficulties in advance, and find solutions for them ahead of time.
Since up to this point I had not had to generate G-code in Fusion360, I decided that I would do my project entirely on it, both the design process and G-code generation.
Coin design¶
At the beginning, you need to draw the end result.
I had DXF logo files for FabLab Armenia Dilijan and Fab Academy. Fusion360 has the ability to export to Sketch. To do this, go to INSERT -> Insert DXF
Then select the file and the plane in which we want to insert the Sketch
In the next tab, you can move the Sketch
Then we get into the properties of the imported Sketch
If we want to leave it as it is, then click on Finish Sketch and you’re done.
In this sketch, I was missing the outside of the coin. Since I’m going to leave room for filling on the side, for these reasons, I want to make the coin not round, but 12 charcoal. To do this, go to CREATE -> Polygon -> Circumscribed Polygon (I want the 12-gon to be inscribed in a circle with a radius I set).
Then you need to specify the radius of the circle, the number of edges and choose the direction (in my case, the snap worked)
3D model of the coin¶
The next step, I will squeeze out my coin. I select the entire area and click on the Extrude tool. This tool also has the ability to specify the taper angle. This feature is very useful to me, because in order for the hardened material to come out of the mold easily, it is necessary to add an angle of inclination. In my comparison, Distance I specify -5 mm (-, because I want it to stretch in the opposite direction), and Taper Angle I specify 3 deg ..
Now let’s stretch the edge of the coin. Distance I will set 1 mm and Taper Angle I will set -3 deg..
With the same parameters, let’s extrude the logo
Now let’s round off the edges of the coin edging and the logo using the Fillet tool.
By rounding the parts of the coin I need, the following result will come out.
Now it’s time for the other side of the coin. I will throw in the sketch in the same way, I will make extrusion and rounding. You will get the following result.
Obtaining a 3D model of a casting mold¶
Once the 3D model of the coin is ready, I can proceed to create the negative mold.
First, I will draw a square around the coin, with sides matching the size of the wax piece from which I want to create the negative mold. Then, I will draw another square to provide thickness to the mold walls. Next, in the corners, I will draw circles to mark the future connection points for the final molds that will be created through the casting of plaster into the negative mold.
Then, using the Extrude tool, I will raise the walls with a 5-degree deviation angle to ensure that the pouring can easily come out of the mold without any hindrances.
Using the same tool, I will mark the outer contour and give it thickness by extruding it in the opposite direction.
Then, using the same tool, I will create holes in the areas where the future connections will be, but I will only use two holes.
Now, let’s add a place for material pouring as well.
To ensure proper casting, two openings are required: one for pouring the molten liquid and another for air and gases to escape. These openings should be located at the top. However, considering the design of the coin, it was not practical to create two separate openings. Therefore, I made one larger opening that can serve both as a pouring channel and an outlet for air and gases. However, as a precautionary measure, I will create small openings closer to the top on the final plaster mold.
Then, I will copy the created part of the mold and use it as a basis to create the mold for the reverse side of the coin.
Generating G-code¶
To generate G-code, you need to navigate to the MANUFACTURE section.
First, create a New Setup:
Then, set the zero coordinate position:
For roughing operations, there are two options in the 3D machining section: Adaptive Clearing and Pocket Clearing.
Since I’ll be machining wax, it would be logical to choose Pocket Clearing. Adaptive Clearing is mainly used for metal. However, for the sake of learning, I can select Adaptive Clearing for roughing and observe the machining process.
When you select the operation type, a menu will open on the right:
Clicking on Select in the TOOL section will open a pop-up window where you can choose the type of milling cutter to be used:
After selecting the tool type, another pop-up window will appear where you need to specify the physical parameters of the milling cutter:
After specifying the parameters, click Accept. I have set the Spindle Speed to 5000 rpm, which is low since it will be machining wax, and the Cutting Feedrate to 1000 mm/min:
After clicking OK, a new operation will be created, which can be seen visually:
Next, using the Circular operation from the 2D section, mark the locations for the fixtures and create holes using the same 6mm milling cutter:
Then, choose the Contour operation from the 3D section:
Create a new tool, “1/8 inch:
After clicking OK, a new operation will be created. This accounts for the roughing operation, and now it serves as the finishing operation:
For the finishing operation, select Parallel for the front side of the coin:
However, since the text and logo are very small, to ensure good machining, it is necessary to use a 1/64 inch milling cutter for machining specific areas:
I have modified three types of operations for machining:
Now let’s see how the operations work together:
Once all the operations are created, it’s time to save them as G-codes. However, the program has generated the operations in a general order, and now they need to be saved in a format compatible with the CNC Roland SRM-30 machine.
Let’s start saving them in order. I have two operations with a 6mm diameter end mill. To save them, select the operations and click on the NC Program button:
If you couldn’t find the desired “Post” in the list, you’ll need to download it from the AUTODESK website. Download two files.
In the pop-up window, choose the “Post” option and select “Roland ISO/ roland iso”:
Then specify the file name and choose the folder where you want to save it:
After clicking the “Post” button, the G-code will be saved with the specified name in the chosen folder.
Repeat the above process for the operations associated with other end mill sizes.
Milling process¶
After obtaining the G-codes, you can proceed to work on the CNC Roland SRM-30 machine.
Since I need to work with three types of end mills, I will set the Z-axis zero point at the same location. Placing it at different positions may introduce errors due to the material not being perfectly flat.
Let’s observe the Adaptive Milling process with a 6mm end mill:
You can notice that the plunge process occurs very slowly, and once the desired depth is reached, the end mill accelerates.
After completing the operation with the 6mm end mill, I changed to a 1/8 inch end mill for the finishing pass:
Then, I replaced the end mill with a 1/64 inch one and performed the finishing pass in the lettering areas. You can see the result of the operation:
Getting rid of some hairs¶
After milling on a CNC machine, there may be some imperfections in the wax mold in the form of tiny hairs. To remove them, all you need to do is use a hairdryer.
Creating a finishing mold from plaster¶
Because working with plaster is relatively easy, does not require safety precautions, is heat-resistant, and also dries quickly, I decided to create a plaster mold for pouring babbitt.
To prevent the plaster from sticking to the walls of the wax mold and to make it easy to remove, I first applied a layer of petroleum jelly to the mold. I also placed metal rods in predetermined locations.
In order to easily remove the plaster casting from the mold after it had hardened, I inserted screws into the mold in areas that would not affect the accuracy of the plaster form.
To prepare the plaster mixture, I started by pouring water into a cup and gradually adding the plaster while mixing it. To ensure thorough mixing, I used an attachment on a power drill and mixed the solution.
To eliminate air bubbles, I used a table with a mounted whetstone machine. By placing the mold on the table and turning on the whetstone machine, I was able to release the solution from any trapped bubbles.
Afterward, I poured the plaster mixture into the mold.
However, there were still some bubbles in the poured solution. To eliminate them, I placed the mold filled with the solution on the table with the mounted whetstone machine and turned on the machine.
After approximately 20 minutes from pouring the plaster, the mixture gained enough strength to be removed from the mold. First, I removed the metal rods.
Then, using the screws that were previously inserted, I carefully extracted the plaster form from the wax mold. After removing the assembly screws, I obtained the following result:
Pinning the form¶
After a few hours: after mold strengthening and moisture loss, you can start the casting process. To fix the form, I will use the places left in advance and through these places I will fix the bolts with nuts.
Then I fix the mold with a vise to ensure stability and for the reason that the intended casting site is in the corner of the mold. Also, basalt slabs were placed on the bottom of the mold on the table to avoid damage to the table:
Melting babbitt¶
In our laboratory there are babbitt ingots.
Let’s cut out a piece with a desktop jigsaw.
After we place it in a special heat-resistant mug and then we place the mug in the oven. We set the temperature on the oven to 400 degrees Celsius.
The recommended potting temperature for babbitt is around 270 degrees Celsius. When reaching about 370 degrees Celsius, you can remove the mug with a special tool.
Since I do not have the professional skills of a caster, in order to avoid a quick cooling, I heat above the required value.
Filling the babbitt into the mold¶
During pouring, it is necessary to use goggles, a mask and special heat-resistant gloves. And take a mug with melted babbitt using tongs.
I took it out of the oven and put it on a basalt tile.
After melting, slag appears on the surface, which I removed with a teaspoon:
After removing the slag, you can already pour it into the mold:
Let’s watch the casting process in a fast paced video:
Plaster mold removal¶
After a couple of minutes, the form quickly hardens and it is already possible to start removing the manet from the form.
The frozen babbitt did not stick to the mold, and therefore it can be removed from the mold very easily.
After demolding, the coin has some roughness, but the roughness is very soft.
Coin machining¶
After being removed from the mold, there is an extra part on the coin that was intended to provide casting.
Let’s cut out the excess part with a desktop jigsaw.
Finishing¶
To make the coin smooth, it is necessary to work it out with sandpaper a little.
Let’s look at the final view from different angles:
Conclusion¶
This week has been quite unusual, and it required a lot of work to achieve the final result. We had to create two molds: a negative mold and a final mold, in order to obtain the desired outcome. I have become very interested in this technology and I see its potential in producing a large quantity of parts. On one hand, creating a single mold can be time-consuming, but if it will used for mass production of parts, the labor and extensive work will be justified.