Plastic mold rapid prototyping technology is a major breakthrough in manufacturing technology in the past 20 years, but only by forming the rapid production capability of practical parts can it truly exert its advantages. Therefore, the research focus of rapid prototyping technology has gradually shifted to the rapid manufacturing of metal molds from the original process and equipment development research. Compared with traditional mold manufacturing technology, the production cycle and production cost of rapid mold manufacturing based on rapid prototyping technology Both are greatly reduced, especially for the production of small batches and trial parts with more advantages and vitality.
The process of rapid manufacturing of zinc-based alloy plastic molds from computer 3D design of plastic products to the process of rapid manufacturing of zinc-based alloy molds.
Prototypes of products and silicone rubber limb molds. Prototypes for obtaining plastic products were manufactured on the HRP-3 thin-layer laminated manufacturing (LOM) rapid prototyping machine developed by Huazhong University of Science and Technology. First use Pro-E or UG software to design the product in 3D, then layer by layer along the height direction to generate slice information, and input into the rapid prototyping machine. The LOM machine cuts the paper coated with hot melt glue according to the layer profile information of the part, and pastes it layer by layer with a heating roller. Finally, the waste material is removed and processed to obtain a paper prototype of the product. Compared with other rapid prototyping methods, the LOM prototype has a fast forming speed, low material cost, and high forming accuracy and surface quality after processing.
The surface-treated paper type is used as the master mold of the silicone rubber mold and placed in the mold frame. The imported non-shrinkage silicone rubber and hardener are mixed and poured into the mold frame, put into the vacuum injection molding machine for defoaming treatment.
After taking it out, it is left at room temperature for 24 hours. After the silicone rubber is completely hardened, the silicone rubber is cut according to the preset parting surface to obtain the silicone rubber mold of the product. The silicone rubber mold has the same shape as the metal mold to be finally obtained. Because of its good elasticity, it is very suitable as an intermediate transition mold for making metal molds. Silicone rubber molds have good reproducibility and can replicate very fine parts of the prototype, reducing the loss of shape and dimensional accuracy during the intermediate conversion process, which is more beneficial to the production of plastic product molds. Silicone rubber has the strength to withstand the molding operation of the later casting process, and it is very easy to demold. It can be used as a master mold to make multiple molds. It is an ideal material for intermediate molds for casting.
The rapid casting of the mold is the most critical part of the rapid manufacturing process of the mold. On the one hand, the mold must maximize the shape, accuracy and surface quality of the prototype; on the other hand, it must meet the requirements of the subsequent casting process and reduce The probability of casting defects, thereby improving the quality of castings. The processes for rapid casting with the silicone rubber mold include gypsum type, ceramic type, and transfer coating method. Ceramic mold casting has high cost and complicated process, which is difficult to master. The production of gypsum mold is convenient, but the strength of the mold is low, the solidification speed of the molten metal is slow, and casting defects such as shrinkage are prone to occur.
Therefore, we adopt the transfer coating method for rapid mold conversion, and its process principle.
The process principle of transfer coating molding is to spray a layer of self-hardening transfer coating on the surface of the silicone rubber mold. Before the coating is hardened, quickly fill the back of the coating with water glass sand and compact it, and blow CO2 to harden it. After a certain period of time, after the coating is hardened, the silicone rubber mold is taken out to obtain a transfer coating casting mold that replicates the surface characteristics of the silicone rubber mold, and the casting mold can be used for pouring molten metal after drying. Since the coating is transferred from the surface of the silicone rubber mold to the surface of the sand mold after the mold is lifted, the coating does not occupy the effective space of the cavity, so the dimensional accuracy of the mold is greatly improved, and the coating made of fine powder can accurately replicate the surface shape of the prototype after hardening , Accuracy and surface roughness are better. The coating is only a thin layer on the surface in contact with the mold, so the production cost of the mold is reduced and the productivity is improved. Compared with the gypsum and ceramic molds, the mold has high strength and the process is greatly simplified.
The transfer coating consists of refractory powder, binder and hardener. The aggregate of the coating is composed of several refractory powders with different particle sizes and different compositions to achieve the best coating density and the surface roughness and breathability of the mold. At the same time, the appropriate amount of graphite powder is added to improve the mold release performance, Reduce mold surface cracks. The self-hardening water glass is used as the binder, and the appropriate amount of water is added to make the coating performance and brushing performance better.
Spray a thin layer of release agent on the surface of the silicone rubber mold. Calculate the amount of paint needed, spray the paint evenly on the surface of the silicone rubber mold with a spray gun, then quickly fill the coating with water glass sand, and blow CO2 to harden it after compaction. After half an hour to an hour, the coating can be lifted after hardening to a sufficient strength. At this time, the coating can be smoothly transferred from the surface of the silicone rubber mold to the surface of the sand mold. The mold is easy to lift and the surface is smooth. Then bake for 80 hours in the oven at 80 ~ 120c for a few hours to cast.
The main components of the zinc-based alloy used to cast the metal mold are: Al 8.20% aluminum, 51% A-Cu master alloy and pure magnesium. The zinc-based alloy is prepared by melting in a resistance furnace. The preheating temperature of the mold is 120C and the casting temperature of the zinc-based alloy is 495 ~ 505C. After the casting is solidified, the pouring riser is cleaned and removed, and the required zinc base can be obtained after polishing Alloy plastic mold. A zinc-based alloy mold for automobile plastic lampshades was produced by this process.
Discuss the manufacturing process of zinc-based alloy plastic molds including four process links: ① LOM prototype of the computer three-dimensional model of the mold; ② LOM prototype ※ silicone rubber mold; ③ silicone rubber mold ※ transfer coating mold; ④ transfer coating mold ※ zinc-based alloy Mould.
Each conversion process will cause loss of the dimensional accuracy of the mold. The dimensional error from the three-dimensional model to the mold is the comprehensive result of the errors generated by each link in the entire process.
The production of LOM paper shapes involves the data processing of CAD solid modeling, the process of making prototypes of LOM machines, and the post-processing of prototypes.
3D model slicing and computer data processing, as well as the influence of laser head movement and laser spot diameter on the dimensional accuracy during the paper making process can be solved by software compensation, but the stepped rough surface formed by laminated paper requires surface grinding And processing to obtain a prototype with a smooth surface. Therefore, the error from the three-dimensional model to the paper shape is mainly generated during the final finishing and surface treatment of the paper shape.
The change in size of the LOM prototype silicone rubber mold is very small. Due to the elasticity, it shows a slight expansion trend. Since the transfer coating mold is taken after the surface of the silicone rubber mold is hardened, the accuracy is high, and the refractory powder used in the coating is stable in thermal physical properties under a suitable particle size distribution, but the gradual loss of water during the baking process will lead to the overall size Shrinkage.
The effect of zinc-based alloy casting on the accuracy of the mold is mainly due to the shrinking size. The free linear shrinkage of the zinc-based alloy used in the test is about 1.1%, but the actual size shrinkage of the casting is affected by the solidification shrinkage in the mold, and its changes are also related to the casting shape, wall thickness and pouring temperature. Conditions. Through the measurement of the dimensional accuracy in the process, it is found that the dimensional change rate of the alloy casting process is much greater than other links, and the most significant impact on the accuracy of the mold. Most of the dimensional changes in the mold manufacturing process are caused by the solidification shrinkage of the zinc-based alloy.
According to the test results and analysis, understand the dimensional accuracy change of each link in the rapid manufacturing process of zinc-based alloy molds, summarize and predict, and feedback the dimensional correction amount to the design of the computer 3D model, you can control the dimensional accuracy of the mold. Obtain high-precision zinc-based alloy molds.
Moulds made of zinc-based alloys have good comprehensive performance, and have certain advantages in terms of mold making cycle, cost price, mold performance, manufacturing process and environmental protection. Compared with similar steel molds, mold making cycle and cost price It must be reduced by 30% to 40%, which is a fast and economical molding material. In addition, zinc-based alloy molds also have the following advantages: ① low melting temperature, simple casting, and high-precision castings can be obtained in a short time; ② good alloy fluidity and good reflection performance, easy to obtain smooth cavity and complex cavity ; ③ Good processing performance, better than aluminum alloy, less working time than steel mold; ④ Material can be remelted and reused, low mold material cost, meet the needs of sustainable development. The traditional ZAS zinc-based alloy mold alloy composition is 4% Al, 3% Cu is hypoeutectic structure. It is composed of zinc-rich n-phase, binary and ternary eutectic. n The amount of compatible aluminum is small, the solid solution strengthening effect is poor, and it is difficult for the alloy to obtain high mechanical properties. Aiming at the defects of low mechanical strength and poor heat and wear resistance of ZAS zinc-based alloy die alloys, a new type of zinc-based alloy die alloys has been developed. The composition is :.
5% ~ 9.5% Al, 6.8% ~ 7.5% Cu. When the aluminum content is greater than 5% (Zn-Al eutectic point), the structure of the zinc-aluminum alloy is composed of aluminum-rich a phase and eutectic structure. The amount of a phase soluble aluminum is high, the solid solution strengthening effect is good, and the mechanical properties of the alloy are also Improved. Therefore, this new alloy not only has higher strength and better heat resistance, but also has better casting process performance. The mechanical properties of the alloy are: tensile strength 330 ~ 410MPa, hardness 135 characteristics of rapid prototyping technology It is “fast” corresponding to the short time of zinc-based alloy mold making. The combination of the two can greatly shorten the production time of plastic molds and greatly accelerate the time for new plastic products to market. In the case of suitable choice of mold types and working conditions, zinc-based alloy mold materials can achieve rapid conversion from rapid prototyping to rapid molds, greatly improving the company’s ability to respond quickly to the market.
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