In the fall of 1917, a friend invited me to participate in the "smart guy" competition with him, since he himself had already taken part, and it was impossible for the second time. Under this grant, de jure, 0.5Mrub is given for two years without any reporting on expenditures, although de facto, 4/5 of the total amount is on hand. It just so happened that with this money it is customary to buy a self-run carriage. I wanted otherwise and agreed with the condition to spend money on the development of the workshop and, at least, buy a milling machine. As I understand it, this was not part of my colleague's plans and the conversation did not come up in the future. Later I found out that another proxy was found. This fact hurt my pride, and I decided that I would have my own grant with machine tools and equipment. It remained to find a project in which there would be something dragged into the subject,and then CR620 / 724, which had been stretching for about five years at that time, turned up just as well. At that time, I had ready-made models of the 724 cm³ cylinder and the accompanying small things, and in addition, the ability to implement it on my own - 3D printing, casting technology, a working gating system for such complex castings and a lathe. I decided that the grant will go towards indirect expenses for the project, and will be done mainly at my own expense. If I won, I got tight deadlines, with which I have a problem. The application passed. The need to show some pieces of iron at the final stage of project selection forced me to accelerate.I decided that the grant will go towards indirect expenses for the project, and will be done mainly at my own expense. If I won, I got tight deadlines, with which I have a problem. The application passed. The need to show some pieces of iron at the final stage of project selection forced me to accelerate.I decided that the grant will go towards indirect expenses for the project, and will be done mainly at my own expense. If I won, I got tight deadlines, with which I have a problem. The application passed. The need to show some pieces of iron at the final stage of project selection forced me to accelerate.
The exposition has been given and now it is time to go directly to the technical part.
Foundry Tool Making Process
Casting methods are both reusable and disposable. Model equipment is needed to obtain casting molds. The metal is poured into casting molds, which are also reusable (metal, mainly when using injection molding). The casting mold consists of an external mold and an internal core, the task of which is to form cavities in the casting. In this project, ground casting was used, that is, in disposable molds, consisting of a mixture of minerals.
To use reusable model equipment, it is necessary in the casting design process to take into account the problem of extracting mold elements from the model equipment (reusable model equipment of a similar cylinder, rod (far right) and casting (center) are shown in the main picture of the article)... Often, for example, the crankcase of a motorcycle engine and its covers, to obtain casting molds, four parts of the model equipment are enough (two half-molds for the outer surface and two for the core). However, for the cylinder of a two-stroke engine, due to the complex geometry of the internal cavities, a set of about two dozen model tooling parts is needed. And since within the framework of this work the goal is not to obtain many identical castings, it was decided to use disposable model equipment.
In the previous step, a model of the cylinder was obtained as a finished part. The casting is a finished part with a gating system and a machining allowance. Above the castings were made to feed the casting with the melt during crystallization, this allows you to shift the area of shrinkage from the casting to the gating system, as well as to ensure the removal of displaced air.
Cylinder casting model
Investment casting was used to obtain the casting. In this case, the model tooling is a model of a casting made of a low-melting material (casting wax or a suitable plastic). In this project, PLA (polylactide) plastic was used, since, unlike many others, it has an obvious melting point, after which its viscosity decreases sharply, which allows it to easily flow out of the mold, in contrast to ABS, which is not sufficiently low viscosity up to at least 300 ° C. When calcined, the plastic residues burn out with the formation of a small amount of solid waste, which, unlike ABS plastic, is not adhered to the walls of the mold and is easily blown out with compressed air. When ABS is heated to 700-800 ° C without good oxygen access, solids are formed, adhered to the form,which then spoil the casting.
For casting using this technology, the gas permeability of the casting mold is important, because it still contains substances that pass into the gas phase when heated to the melt temperature. In the case of poor gas permeability of the material of the casting mold, gas bubbles form in the casting, which is one of the most common causes of defects in the foundry business.
The plastic model of the cylinder casting was produced by 3D printing. However, the size of the printable area of the printer is smaller than the size of the casting, which is why it had to be broken down into its constituent elements. All other parts were made using the same process.
Partitioning a Cylinder Casting Model for 3D Printing
Making a model of the engine cylinder and its casting molds
The model was printed with a 250μm PLA layer and consisted of twenty parts. All parts of the casting model were printed with 10% coverage. The components of the model were assembled on centering pins and glued, and the joints between them were sealed with foundry wax. Without sealing the joints on the casting, the gaps between them will be repeated, which are difficult to clean from the remains of the molding sand and are stress concentrators. Technological holes were made in the model in the cooling jacket. These holes create additional supports for the weak, due to its geometry, the core of the cooling jacket. Otherwise, during pouring, it can split from uneven heating and its fragments will be carried away by the flow of the melt, which has already led to the rejection of similar castings. Any damage to the molds during pouring generates loose particles that always spoil the casting.Therefore, it is better to make technological holes in convenient places than then figure out how to weld a defect somewhere inside.
The material for the casting molds was sifted sand with a fraction of 0.63 mm mixed with sodium water glass in a ratio of 4/1. The sand must be carefully tamped down and special attention must be paid to the channels in the casting. Before burying the model with filled channels in the molding sand, it is advisable to remove a small layer of the mixture from the channel outlets - it has time to start hardening in air and after that the casting model can crack along these surfaces, which would not be desirable. After the completion of molding, to harden the mixture, it must be blown through the punctures with carbon dioxide. CO₂ breaks the bond between sodium oxide and silicon oxide in water glass and SiO₂ bonds the grains of sand together. When blowing out, a characteristic crunch is heard, which indicates that the hardening process is underway. Each puncture requires about 1/2 minute of carbon dioxide supply.It is especially important to blow through the rod well.
For the manufacture of casting molds, the casting model was installed in the flasks and the free space in the flask and inside the model was filled with a mixture of sand and sodium water glass. The same mixture was used to make a sprue and a plate increasing the height of the risers. All the components of the mold were assembled and lined with bricks with a gap.
Assembled casting model The
sprue was made inclined at an angle of 45 ° to the vertical.
Checking
mold mates In the next step, the plastic from the mold was melted, and it was calcined and blown with compressed air to remove the crumbling sand with ash. After the previous operations, the mold was left in the furnace, since unnecessary heating cycles can lead to its cracking, which can lead to the separation of a part of the core, the consequences of which are described above.
Calcined casting mold
Execution of engine cylinder casting and its machining
Silumin grade AK9ch was used for filling, since it has good strength and fluidity, low casting shrinkage and is the classic choice for the manufacture of cylinders and engine blocks with cast iron liners. The ingots were divided into small fragments that fit into the crucible and melted in a melting furnace (the furnace is homemade, I can illuminate the structure) . When the melt warmed up to a temperature 20 ° C lower than the pouring temperature, a casting mold heated to 300 ° C was removed from the furnace, which remained heated from calcination, and installed in a prepared brick formwork.
The space between the mold and the formwork was filled with sand to prevent the melt from escaping from possible cracks, and the joints of the mold components were sealed with molding sand. The casting was performed at a melt temperature of 740 ° C.
Alas, there are no fill photos.
Cylinder casting
I must admit that the problem of removing the rods is significant. With complex geometries and a strong mixture, it can be insoluble. When designing a casting, you should think in advance how and how to crawl inside. To extract the rod, I also used the technological holes of the casting, which strengthened it. Without them, most likely, the rod would not have been removed. The bulk of the material was extracted with a screwdriver with a drill from a perforator. The drill is tungsten carbide and therefore does not blunt against sand. Where it is impossible to crawl with a drill, you can use a thick steel wire or a fluffed steel cable, rotated by a screwdriver. After the channels have become through, the remains of the molding sand lend themselves well to high pressure washing.Nevertheless, even after all the manipulations in the cooling jacket, pieces of the molding mixture remained on the walls and during the processing they no-no, but fell out.
Instead of a conclusion
The project will use the bottom of the engine crankcase Honda CR500. The crankcase has been modified to allow widening of the purge passages, a larger liner skirt, and transfer of studs. The design of the cylinder included the possibility of increasing the piston stroke up to 95mm in the new crankcase, due to fear of damage to the native crankcase.
With a model casting and a semi-finished 620cc cylinder, I did win a grant, it was in November 17th. The casting of 724 cm³ was made only in March. It took a little more than a day of continuous work for calcining the mold and casting. In my opinion, casting differs from, for example, welding in that it is an irreversible process - if there is a problem somewhere, then you cannot go back a step, but only to the very beginning. It was especially scary with the disposable plastic model. The most exciting moment is the breaking of molds, which is comparable to opening a New Year's gift in childhood, only the stakes are much higher. Fortunately, the casting, although not as planned, turned out to be satisfactory the first time. What a relief it was! Now it was necessary to process it and make all sorts of mating parts.
Past parts:
Part zero: Own 2-stroke motor. CR620
Part One: An 8-year-long project - I would have known, I would never have gotten involved: my 2-stroke motor
UPD: Added description of the rods extraction process, 17 Aug. 20g.