A few decades ago, there was no machine mold era. All the products were hand-finished, which was extremely time-consuming. Nowadays, various molds are popular, and the same products can be quickly and mass-produced as soon as they start making a mold. Rapid plastic forming technology: low cost, high quality, good 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 we really realize its advantages. Therefore, the research focus of rapid prototyping technology has gradually shifted from the development of original processes and equipment to the rapid manufacturing of metal molds. 12. Compared with traditional mold manufacturing technology, the rapid manufacturing cycle of rapid mold manufacturing and Production costs are greatly reduced, especially for the production of small batches and prototypes. In this paper, using the LOM prototype in rapid prototyping technology, combined with silicone rubber conversion and precision casting technology, the rapid manufacturing technology of zinc-based alloy plastic mold was developed, and the automobile lampshade mold was fabricated by using this technology.

The rapid manufacturing process of zinc-based alloy plastic molds is from the computer 3D design of plastic products to the rapid manufacturing process of zinc-based alloy molds.

The prototype of the product prototype and the silicone rubber mold obtained from the plastic product was fabricated on the HRP-3 thin-layer laminate manufacturing (LOM) rapid prototyping machine developed by Huazhong University of Science and Technology. Firstly, the product is 3D-designed with Pro-E or UG software, and then layered information is generated in layers along the height direction and input into the rapid prototyping machine. The LOM machine cuts the paper coated with hot melt according to the profile information of the part, and pastes it layer by layer with a heating roller. Finally, the waste is removed and processed to obtain a paper prototype of the product. Compared with other rapid prototyping methods, LOM prototypes have a faster forming speed, lower material cost, and higher forming accuracy and surface quality after processing. The surface-treated paper type was placed as a master mold of a silicone rubber mold and placed in a mold frame. The imported non-shrinking silicone rubber and the hardener are mixed, poured into the mold frame, and placed in a vacuum injection molding machine for defoaming treatment.

After being taken out, it was left at room temperature for 24 hours, and the silicone rubber was completely hardened, and the silicone rubber was cut according to a predetermined parting surface to obtain a silicone rubber mold of the product. The silicone rubber mold had the same shape as the metal mold finally required. Due to its good elasticity, it is very suitable as an intermediate transition mold for making metal molds. The silicone rubber mold has excellent reproducibility, can replicate very fine parts of the prototype, and reduces the loss of shape and dimensional accuracy during the intermediate conversion process, which is more advantageous for the mold making of plastic products. The strength of silicone rubber can completely withstand the molding operation of the later casting process, and the demoulding is very easy. It can be used as a master mold to manufacture multiple pairs of molds, and is an ideal material for intermediate molds for casting.

The rapid casting mold of the mold is the most critical part of the rapid mold making process. On the one hand, the mold should maintain the shape, precision and surface quality of the prototype to the utmost; on the other hand, it must meet the requirements of the subsequent casting process and reduce The probability of casting defects occurring, thereby improving the quality of the casting. The process of rapid casting in combination with a silicone rubber mold includes a gypsum type, a ceramic type, a transfer coating method, and the like. Ceramic castings are costly and complex, and difficult to master. The gypsum type is easy to manufacture, but the strength of the mold is low, the solidification speed of the molten metal is slow, and casting defects such as shrinkage cavities are likely to occur.

Therefore, we use the transfer coating method for rapid mold conversion, the process principle.

The process principle of the transfer coating method is to spray a layer of self-hardening transfer coating on the surface of the silicone rubber mold, and the glass sand is quickly filled on the back surface of the coating and compacted before the coating is hardened, and CO2 hardening is blown. After a certain period of time, after the coating is hardened, the silicone rubber mold is taken out to obtain a transfer coating mold which replicates the surface characteristics of the silicone rubber mold, and the mold can be used for pouring the molten metal after drying. Since the coating is transferred from the silicone rubber mold surface to the sand surface after the mold is released, the coating does not occupy the effective space of the cavity, so the dimensional accuracy of the mold is greatly improved, and the surface shape of the prototype can be accurately reproduced after the coating prepared by the fine powder material is hardened. , precision and surface roughness are better. The coating is applied only to a thin layer on the surface in contact with the mold, so that the production cost of the mold is lowered, the productivity is improved, and the strength of the mold is high and the process is greatly simplified as compared with the gypsum type and the ceramic type.

The transfer coating consists of a refractory powder, a binder, and a hardener. The aggregate of the coating is made up of several refractory powders with different particle sizes and different compositions to achieve the best coating density and surface roughness and gas permeability of the mold. At the same time, adding appropriate amount of graphite powder to improve the mold release performance, Reduce surface cracks in the mold. The binder is made of self-hardening water glass, and an appropriate amount of water is added to make the coating performance and brushing performance of the coating better.

Spray a thin layer of release agent on the surface of the silicone rubber mold. Calculate the required amount of paint, spray the paint evenly on the surface of the silicone rubber mold with a spray gun, then quickly fill the water glass sand on the coating, and then compact the CO2 to cure. After half an hour to an hour, the coating hardens to a sufficient strength to release the mold, and the coating can be smoothly transferred from the silicone rubber mold surface to the sand surface, and the mold is easy to mold and the surface is smooth. Then, it can be combined and poured in an oven for 80 hours at 80~120c.

The main components of the zinc-based alloy used to cast the metal mold are: Al8.20% aluminum, 51% A-Cu master alloy and pure magnesium. Melting and formulating zinc-based alloy in electric 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 grinding and polishing. Alloy plastic mold. The zinc-based alloy mold of automobile plastic lampshade was fabricated by this process.

Discussion The production process of zinc-based alloy plastic mold includes 4 process steps: 1 computer 3D model LOM prototype; 2LOM prototype ※ silicone rubber mold; 3 silicone rubber mold ※ transfer coating mold; 4 transfer coating mold ※ zinc-based alloy Mold.

Each transformation process will cause loss of the dimensional accuracy of the mold. The dimensional error from the three-dimensional model to the mold is a comprehensive result of the errors produced by the various processes throughout the process.

The production of LOM paper type involves the data processing of CAD solid modeling, the prototyping process of LOM machine, and the post-processing of prototype.

3D model slicing and computer data processing, as well as the movement of the laser head during the paper making process and the effect of the laser spot diameter on the dimensional accuracy can be solved by software compensation, but the stepped rough surface formed by the laminated paper needs to be surface-polished. And processing to obtain a smooth surface prototype. Therefore, the error of the three-dimensional model to the paper type is mainly generated in the post-finishing and surface treatment of the paper type.

The change in the size of the LOM prototype silicone rubber mold process is very small, and it tends to slightly expand due to the elasticity. The transfer coating mold adopts the mold after the surface hardening of the silicone rubber mold, and the precision is high, and the thermal physical properties of the refractory powder used in the coating are stable under the proper particle size distribution, but the gradual loss of water during the baking process leads to the overall size. A small amount of contraction.

The influence of zinc-based alloy casting on the accuracy of the mold is mainly due to the shrinkage of the size. The free line shrinkage of the zinc-based alloy used in the test is about 1. 1%, but the actual dimensional shrinkage of the casting is affected by the solidification shrinkage in the mold, and the change is also related to the shape, wall thickness and casting temperature of the casting. It is related to casting 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 larger than other links, and the influence on the precision of the mold is most significant. Most of the dimensional changes during the mold making process are caused by the solidification shrinkage of the zinc-based alloy.

According to the test results and analysis, understand the dimensional accuracy of each link in the rapid manufacturing process of zinc-based alloy molds, summarize and predict, and feed back the dimensional correction to the design of the computer 3D model to control the dimensional accuracy of the mold. A high-precision zinc-based alloy mold is obtained.

The mold made of zinc-based alloy has good comprehensive performance, and has certain advantages in the molding cycle, cost price, mold performance, production process and environmental protection. Compared with similar steel molds, the molding cycle and cost price It is a 30% to 40% reduction, which is a fast and economical molding material. In addition, the zinc-based alloy mold has the following advantages: 1 low melting temperature, simple casting, high precision casting in a short time; 2 good alloy fluidity, good reproducibility, easy to obtain smooth cavity and complex cavity 3, good processing performance, better than aluminum alloy, less than steel molds; 4 materials can be remelted and reused, mold material costs are low, in line with the needs of sustainable development. The conventional ZAS zinc-based alloy mold alloy has a composition of 4% Al and 3% Cu as a hypoeutectic structure. It consists of zinc-rich n-phase, binary and ternary eutectic. The amount of n-phase dissolved aluminum is small, the solid solution strengthening effect is poor, and it is difficult to obtain high mechanical properties of the alloy. Aiming at the defects of low mechanical strength and poor heat resistance and wear resistance of ZAS zinc-based alloy mold alloy, a new zinc-based alloy mold alloy was developed with the following composition: 5%~9.5%Al, 6.8%~7.5%Cu. From the phase diagram, when the aluminum content is more than 5% (Zn-Al eutectic point), the microstructure of the zinc-aluminum alloy consists of aluminum-rich a phase and eutectic structure. The amount of aluminum dissolved in a phase is high, the solid solution strengthening effect is good, and the mechanical properties of the alloy are also improved. Therefore, this new alloy has both high strength and good heat resistance, and has good casting process performance. The mechanical properties of the alloy are: tensile strength 330~410MPa, hardness 135 rapid prototyping technology It is a "fast" corresponding to the short molding time of zinc-based alloys. The combination of the two can greatly shorten the production time of plastic molds, and greatly accelerate the time for new plastic products to be introduced to the market. With the choice of mold type and working conditions, zinc-based alloy mold materials can realize rapid conversion from rapid prototyping to rapid mold, which greatly improves the rapid response capability of enterprises to the market.

Conclusion: This plastic mold rapid manufacturing technology developed with LOM prototype combined with precision casting technology is very suitable for the rapid production of small batch prototypes and sample parts, and can also be used to replace individual damaged parts of the product, such as the car made in this paper. The headlight cover is to meet the urgent needs of the car repair department, saving time, material and financial resources. However, this process is not suitable for a mold that requires precise forming, and a plastic mold having a fine portion needs to be manufactured in combination with other processes such as machining.

The demand for product manufacturing in the future market will show a rapid, diverse and small-volume development trend. The rapid prototyping technology combined with zinc-based alloys to rapidly produce plastic molds will have great potential for development, and its market prospects are very broad.

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