Introduction
Many engine developers are now using rapid prototyping methods to produce engine prototypes. This method omits the process of making molds, greatly shortening the engine development cycle. A set of major engine components, including cylinder blocks, cylinder heads, intake and exhaust pipes, etc., can be manufactured within a few weeks for later test research. If the design needs to be changed, only the CAD data needs to be changed, and the modified prototype can be produced in a very short time for further test and evaluation.
1.Rapid Prototyping Technology
Rapid Prototyping (RP) technology is a general term for group technologies that have been developed in recent years to quickly produce samples or parts directly based on CAD models. Different from traditional manufacturing methods, rapid prototyping starts from the CAD geometric model of the part and forms a solid part by layering and discretizing the material using software and data prototyping systems. It uses laser beams or other methods to pile up materials. Since it converts complex three-dimensional manufacturing into a series of two-dimensional manufacturing superpositions, it can generate almost any complex parts without molds and tools, greatly improving production efficiency and manufacturing flexibility.
The basic principle of rapid prototyping is discrete additive manufacturing. No matter how complex the shape and inner cavity of a part are, it can be cut with a set of parallel planes to obtain a series of sufficiently thin slices. These thin slices can be approximately regarded as two-dimensional part models. These thin slices can be gradually produced using different technologies, and a complete part can be obtained by stacking these slices according to certain rules. There are many mature rapid prototyping technologies at present, and here we only introduce the selective laser sintering (SLS) method that is most suitable for the production of engine parts.
Selective Laser Sintering (SLS) uses the heat provided by an infrared laser beam to melt thermoplastic materials to form three-dimensional parts. A thin layer of thermoplastic material is spread on the processing surface through a powder roller, and then the cross-sectional shape of the part is scanned on the powder surface by a laser. For amorphous materials, the laser scans the area where the powder softens, and the whole body is bonded together at the contact points to form a solid structure. This process is called melting or sintering; for crystalline materials, the heat of the laser melts the powder to form a liquid, which hardens into a solid after cooling.
At the beginning of the process, a very thin layer (100μm~250μm) of thermal energy powder is evenly spread on the work platform. The auxiliary heating device heats it to a temperature below the melting point. On the uniform powder surface, the computer controls the laser to scan according to the information of the current layer of the part. The powder is sintered to form a solid in the place where the laser scans. The place where the laser does not scan is still powder, which can be used as a support for the next layer and can be removed after the molding is completed. After the previous layer is completed, the molding piston drops one layer, the powder supply piston rises, and the powder is moved from the powder supply piston to the molding piston by the powder spreading roller. After the powder is flattened, the next layer can be scanned. Repeat this auxiliary powder and selected area sintering process until the last layer, and a three-dimensional entity is produced.
One of the biggest features of selective laser sintering is that the molding process has nothing to do with the complexity, so it is particularly suitable for components such as engine cylinder blocks, cylinder heads, intake and exhaust pipes with extremely complex internal structures; another important feature is that it can be formed with a wide range of materials, especially casting resin sand and expendable investment materials. Therefore, by combining with casting technology, engine components can be quickly cast.
2.Rapid Casting Technology
Although rapid prototyping technology effectively solves the molding problem of complex parts, due to the limitations of rapid prototyping process and molding materials, it is difficult for parts obtained through rapid prototyping to be consistent with the materials of the actual final parts, and the various performance indicators of the parts cannot meet the requirements of the final parts. Therefore, the three-dimensional entities directly obtained through RP technology can generally only be used as visual models and assembly models, but cannot be used as functional prototypes for various tests and usage inspections.
Quick Casting or Rapid Casting technology is a technology that effectively combines rapid prototyping with traditional casting technology to quickly manufacture complex metal parts. Half of the engine’s cylinder block and cylinder head are casting products. Using rapid casting technology, engine products with consistent materials and close performance to the final product can be obtained in a very short time for testing and inspection.
There are two ways to achieve rapid casting using selective laser sintering: one is to directly sinter the heat-cured resin sand for casting by laser, and then obtain the casting by sand casting; the other is to directly sinter the disappearing resin sand or wax powder by laser, and then obtain the casting by precision casting. The common feature of these two methods is that mold manufacturing is omitted, so if used for single-piece small-batch production, the production cycle is greatly shortened.
To make sand molds by rapid prototyping, first of all, a combined sand mold model should be designed according to the three-dimensional CAD blank model of the parts. In order to be close to the future mass production process, the sand mold model should be kept consistent with the sand mold model made by the mold as much as possible. After the various parts of the sand mold model are converted into the processing files of the rapid prototyping equipment through the layering processing of the software, laser sintering can be carried out. The resin sand used for molding is very similar to the commonly used thermosetting resin sand, except that there are more stringent indicators for the sand particle size distribution and morphology, resin composition and surface treatment. The layer thickness during molding is generally 0.2mm, and the accuracy can be controlled within ±0.25mm. Due to the high speed of laser scanning, the resin cannot be completely cured during molding. After molding, the unsintered floating sand is removed, and the sand mold is generally placed in a heating box for secondary curing. The sand mold after secondary curing can achieve the same performance as the sand mold made by the core shooting machine. Since most of the engine parts are cast by sand casting, rapid sand casting has become the most common and effective method for engine prototype trial production.
The method of rapid investment casting is to use 50-80μm disappearing resin powder or wax powder as raw material, directly layer the three-dimensional CAD blank model of the part, and then use laser to directly sinter the powder layer by layer into a precision investment mold consistent with the part blank, and then directly cast the investment mold through a gypsum mold or a ceramic shell to obtain the required casting. The parts made by rapid precision casting have good surface quality and high precision. There is no need to design sand molds and other steps, and the process is relatively simple. At the same time, since the volume of the part blank is often smaller than the volume of the sand mold, the direct molding of the investment mold with SLS is faster than the molding of the sand mold, and the cost is relatively low. However, investment casting is generally more suitable for thin-walled parts. For thick-walled parts, shrinkage and other casting defects often occur due to slow cooling. Therefore, this method is generally used for relatively thin-walled parts such as intake pipes in the production of engine parts. The casting investment mold of the engine intake pipe is directly sintered with disappearing resin powder.
3.Application Cases
Aluminum alloy intake manifold parts made by rapid precision casting. It takes only 10 days from receiving the 3D CAD data of the parts to completing the blank, including 1 day for rapid prototyping of the parts melt mold, 7 days for investment casting, and 2 days for other post-processing and inspection. The intake duct is an extremely important component of the engine. It is composed of complex free-form surfaces, which has a very important impact on improving the intake efficiency and the combustion process. In the design process of the engine, it is necessary to conduct air duct tests on different intake duct schemes. The traditional method is to use a dozen or dozens of air duct wood molds or plaster molds to process and then cast the air duct into sand molds. After testing the air duct to find out the deficiencies, the model must be modified again. This repetitive process is time-consuming and labor-intensive, and the accuracy is difficult to guarantee. The rapid prototyping method can provide a set of CAD data of different surfaces at one time, and a set of intake duct parts can be obtained at the same time through rapid casting. After testing, a comprehensive set of data is obtained, so as to screen out the best air duct scheme, which greatly speeds up the development speed.
Compared with the automobile intake pipe, the cylinder block and cylinder head of the engine have more complex structures, but the biggest advantage of rapid prototyping is that it has nothing to do with the complexity. The more complex the parts, the more suitable they are for rapid prototyping. Since the internal structure of the cylinder block and cylinder head is complex and the wall thickness is relatively thick, the best way to make these parts is rapid sand casting. A set of aluminum castings of cylinder blocks and cylinder heads obtained by rapid sand casting. The average production cycle of such parts is about 2-3 weeks. Since the casting process is extremely similar to the final production process, the dimensional accuracy and mechanical properties of the parts are highly comparable to the final product parts. Therefore, the cylinder block and cylinder head cast by rapid sand casting can be directly used for various evaluation tests of the engine, such as flow analysis of the airway and cooling performance test of the waterway.
The combination of rapid prototyping and casting technology can be effectively applied to the rapid manufacturing of prototypes during the engine design and development stage, which helps to ensure product development speed, improve product development quality, greatly reduce development costs, and promote early product market entry.
