training task: filling a 3D printed mold with metal
This is a laboratory work on the subject of "Modern production technologies". The task is to make a 3d printed figure up to 10 * 10 cm in size on a 3D printer from a PLA filament, which we will use to make a gypsum mold, into which we will later pour aluminum. The task is to try the metal casting method in practice.
in order to save time, I used a 3D model of the tooth (since the task is not to model from scratch, but to practice). If I have some extra time, I'll try to model something on my own.
using Orcaslicer v 2.2. to model the profits, I used the create primitive function in the slicer. they are 3 cones. also, to get out the leftovers, I also used the function of creating primitive models in the slicer - these are 3 cylinders. for better printing, the cylinders are slightly recessed into the plane of the table - this is necessary in order to increase the contact area of the part with the first layer of the printing table.
I also installed tree support (in auto mode, it suited me) to improve the printing of overhangs.
FIlament - PLA Geeetech (purple). I chose this filament because it is the most trouble-free in my personal FDM printing practice. The teacher recommended using transparent PLA, but I don't think it will be a big problem.
3d printer - Flashforge 5m pro
Printing in action:
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the filament was open and I hadn't dried it beforehand. I hope that this will not affect the print quality much. printing is done from the Creality thermal camera, the temperature is set to 50 degrees Celsius for PLA.
Creality Space Pi Plus (filament dryer)
Unfortunately the main model of the tooth was not printed - in the morning I found noodles instead of it - the rest of the primitive models and the roots of the tooth itself turned out.
since the main task is to practice metal casting, the deadline for the laboratory work itself came up and I no longer had time to print something again, I decided to take a ready-made model - a soldier of the Chinese tin army. This model is larger than the ones that should be suitable for my task, but its shape is suitable for casting metal.
Its about 19+cm height
other parts from wrong printing I decided to use it to create casting channels.
in order to connect these parts to the model, I decided to use a lighter. Since PLA melts at low enough temperatures, it won't be difficult for me for a long time.
Initially, I wanted to use a plastic bottle as a mold for casting plaster (1,5 l).
to correct the bottle, to make it easier to pour plaster into it, I used scissors.
The initial fitting was fine with me and I decided to attach the metal casting parts to the model by preheating them with a lighter.
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previously, I fitted the model to the bottle again and cut out the holes with scissors to accommodate the injection channels.
In order not to ruin the model, I attached the first of the channels to the model pad.
However, I soon realized that it was not worth using the front part of the model for this, since the rest of the injection molding channels would have to be placed on the chest and face of the figure, and I took it off.
then I installed 2 injection channels on the back of the model and tried it on in the bottle again.
in order to increase the area of the injection channels when pouring plaster, I used ordinary adhesive tape.
at this point, it became clear that the bottle was not very appropriate for this task, and I decided to look for another container. The class time was limited, so I had to choose from the materials at hand. So, I took an empty cardboard box from the filament
there was another problem: it was necessary to fix the model in the box so that it would be stationary during the casting and during its curing. I thought I could solve this if I positioned the model slightly at an angle so that it would lock into the box without rotation or displacement (it later turned out that the idea was not the most successful).
then I got ready to mix the plaster. It was decided to use the proportions that the teacher suggested to us - 5 parts gypsum, 2 parts water, 1 part sand (by weight).
Scales were used to measure the amount of material, gypsum, sand, and water were poured into a plastic container. I had to stir the mixture using improvised materials - a plastic measuring spoon (not in the photo)
to avoid spilling plaster from the box, it was decided to wrap it with duct tape.
for additional sealing, I tried using a plastic bag from under the filament, but it got stuck on the bottom third of the box, so I had to cut that part off. which didn't fit
then I added a trash bag.
It seemed to me that the figure touches the walls in 2 places, and in order not to spoil the edges of the figure during filling, I laid 2 cut-off pieces from the plastic bag from the filament that I cut earlier.
I used a vibrating table to fill the box with plaster. it is necessary for the plaster to be evenly distributed inside the box and along the figure, as well as for air bubbles to escape.
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it seemed to me that the proportions for making the gypsum mixture that the teacher provided to us were not quite correct: the mixture turns out to be too liquid. this increases fragility and a high risk of blistering. therefore, starting with the next portion of the mixture (it took several of them for such a volume), I mixed the mixture by eye, adding more gypsum to it so that the consistency of the mixture resembled sour cream (as in a dental laboratory in dentistry).
Important notes: for a better filling of the mixture, it is better to pour it over the wall of the box. It is very important to avoid lumps (given the lack of a kneading tool, it was not easy). it also turned out that the tape and bags around the box were not superfluous - the excess plaster poured into the bag, but the space around remained clean.
I came to check the condition of the model. my goal was to check the model and, if everything is in order, then put it in the oven for a day or two to dry the plaster faster and move on to the next stage as soon as possible. I want to speed up, because I already understand that this is a rough draft of the project, which will allow me to better understand the principles of metal casting in order to make it right again.
this is a view of the plaster on top of the box
visually, everything was not bad, so I decided to check how solidified the plaster was (using my fingers)
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The plaster was able to hold its shape to the touch, but it was quite wet. so the idea of heating it up for additional drying seems more and more relevant. I was somewhat surprised that the plaster had not dried in 3 days. I compare this plaster with dental casts, which, depending on the type, would definitely be ready for use in 2-3 hours.
The next step is to remove the cardboard box and bags to prepare the model for moving to the oven. I did this step with a stationery knife and ordinary scissors.
Unfortunately, after I removed the box and packages, defects were found - part of the box was sealed in plaster and part of the model was not well filled with plaster. in order not to damage the model when trying to remove the plaster and not waste time on re-pouring, I decided to simply add more plaster to these areas - since this is a training (rough) project, some defects are quite acceptable.
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It should be noted the reason why it will be necessary to cover the cardboard box with plaster, which is immured in it: since we will place the blank in an oven for heating at 500 degrees Celsius to burn out the model from pla, there is a danger of open fire. According to the teacher, the PLA will not burn (I will pay attention to checking the answer to this question later). so I need to cover the cardboard part with plaster to avoid air access during heating in the oven and avoid the appearance of an open fire (smoke detectors are installed in the laboratory and this can lead to a number of problems).
to do this, I also diluted the gypsum mixture to cover these areas.
what I should have done: considering that the areas that need to be covered with plaster are on different sides of the model, I should have applied plaster on one side one at a time, waited for it to harden for at least an hour, then turned the model over to the other side and dealt with the next area.
Why didn't I do that?: it takes too much time. so I took a lot of plaster and decided to put it on all the parts at once. yes, I want to collect all the possible problems that you can commit by repeating this - and maybe it will save you from mistakes.
Now the shape looks like an Easter cake.
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it looks pretty good
Since the plaster was already dry, I decided to burn the PLA out of the mold using an oven
The temperature is 500 degrees Celsius. Exposure time is 1 hour
Moving the model to the oven
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Closed oven
The burning in the furnace took place within 1 hour. Then the oven was turned off. It seems to have remained closed for 6 days. I can't be completely sure, because other people have been coming into the lab all this time. But since the model remained in its place in the oven, I can assume
The process of removing the mold from the oven
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The form looks good enough. There are several shallow cracks and one of medium (?) depth. The teacher said that there was no problem, everything turned out fine and you can fill it with metal.
Using my fingers, I checked how smooth the surface of the plaster was in the area of the injection channels. The surface was smooth. Considering that I selected the consistency and proportions of the gypsum mixture "by eye", based on the experience of manufacturing gypsum dental dental models, it seems successful on the outer surface of the mold. After pouring the metal, I will be able to understand how well it turned out to reflect the surface of the model itself with plaster. We agreed with the teacher to pour aluminum into the mold in a couple of days.
it's time to fill the mold with aluminum. The temperature of the furnace in which aluminum was melted was 780 degrees Celsius. this is about 100 degrees above the melting point. before the procedure, the oven was heated for 2-3 hours.
first, we checked the heating of the furnace
due to the high temperature of the molten metal, the teacher did not allow me to do this part on my own. All I had to do was watch from the sidelines and film the process.
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The following items were used in the course of the work:
protective gloves
protective helmet
tongs with an elongated handle
There is also a special lab coat.
During all the procedures, it was necessary to move away from our teacher by several meters in order to avoid metal burns if something went wrong.
A tiled floor acted as a surface resistant to contact with molten metal. The mold was mounted on a plaster stand.
first, the tongs will get a mold with molten metal, the metal is stirred
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Then, using tongs, the metal mold is placed above the gypsum mold and the casting process begins carefully.
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The metal mold is then moved to a special metal mold where the unused metal will be drained.
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the metal is carefully poured into the mold so that it cools down as soon as possible. ingots are prepared from this mold for subsequent casting.
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the mold itself is turned UPSIDE down and placed UPSIDE down.
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the mold is made of gypsum with poured aluminum (photo 2 minutes after the casting process). you can pay attention to the crack that appeared on the wide side of the mold. my colleagues said that immediately after casting, it began to increase right in front of my eyes, but after observing this, I did not find such an effect. however, its appearance is still associated with temperature treatment. perhaps it really increased immediately after the casting process, which may be due to the occurrence of temperature stresses.
a video showing what the cooling metal looks like (about 2-3 minutes after casting). note the change in aluminum color to yellow. apparently, this is due to oxidation
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then the form is moved with the help of forceps to a place where people walk less often. it takes about 4 hours for the metal to cool down (information from the teacher)
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According to the plan, I want to open the form the next day and see what happened.
here is a view of the mold after casting the metal after 24 hours. cracks are visible. Several parts of the plaster fell off while I was putting the mold on the table.
I used a hammer to carefully separate the plaster from the metal.
The mold itself was placed in a tight garbage bag to less pollute the workplace.
the gypsum removal process
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the process of plaster removal (imitation of a first-person view)
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the type of mold at the stage of gypsum removal. pay attention to the cracks. I noticed that it is better to knock with a hammer so that the cracks of the frame open. to do this, there is no need to knock on them all at once.
It's almost done
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done
Overall, we managed to display the form well. However, not all the details turned out well. In the following pictures I will show the defects on the model.
video with the finished model
