Plaster Precision Casting Molds Application of New Technolog

Guangdong Provincial Machinery Research Institute Chen new

  [ Abstract ] This paper describes the plaster mold casting zinc alloy , the application of silicone rubber molding , vacuum casting and solidification and other new technologies , so that the overall complexity of the mold can shape and mold casting . Optimize mold structure , improve precision molds , mold cavity surface roughness of Ral.2 ~ 1.7μm, the dimensional accuracy of 100 soil 0.06mm, casting finished 100%.
Keywords: plaster casting molds
  An Introduction
  1950s , plaster casting technology has been in the United States , the Soviet Union and other countries studied systematically . Since the 1980s the development of electronics and plastics industries , and promote the rapid development of technology , widely used in zinc, aluminum or copper alloy castings and plastic molds, shorten the molding cycle time and reduce costs. Because plaster poor thermal conductivity , low permeability and brittleness to complex mold casting difficult , generally vacuum and low pressure casting to be successful. We use silicone rubber molding , vacuum casting and solidification of new technologies such as high-quality zinc alloy die casting , the process shown in Figure 1 .
  Second, the mold and molding process characteristics
  1 . Mold characteristics
  The mold material from the powder (A), gypsum (B) composed by a certain percentage with water , and after the slurry formulated for modeling. The mold powder has a certain permeability and reduce shrinkage ; gypsum mainly from the binding action , the mold to ensure adequate strength . The main characteristics of the mold is thermal conductivity, strength and shrinkage ratio γ0 σ strength σ, γ , and shrinkage gypsum powder with a weight ratio (A / B) the relationship shown in Figure 2 , Figure 3. Where σ1 and γ1, representatives without the baking mold wet strength and shrinkage ; σ2, γ2 represents baked dry strength and mold shrinkage. From Figure 2 , Figure 3 shows : σ1, σ2 all with A / B value increases and reduces , γ1γ2 also with the A / B value increases the value close to zero . When the A / B value is equal to about 4 , the mold is still enough strength , mold cracking does not occur , there is conducive to obtaining an excellent mold . 
  2 . Molding process
  Size is not strictly required for products that can be directly used product for the mother mold casting. The precise product size requirements , consider shrinkage alloys , re-processed to make a master pattern . If the product complex structure, fine pattern , no draft angle or with notches requires the use of non- shrinkage, good elasticity and good reproducibility of silicone rubber molding . Modeling process shown in Figure 4 . In Figure 4a , A is said female mold with a recess at the site , the use of silicone rubber can be easily achieved integrally molded shape, without parting modeling. To solve the difficult problem of complex mold shapes . Because the process of the mold cavity surface made smooth , and can reproduce the real shape of the master model , and thus a mold with the casting mold , the type of controlling surface roughness up Ral.2 ~ 1.7μm, dimensional accuracy up to 100 ± 0.06mm.
  Third, the baking mold process
  Gypsum has α -and β -two, all belong to hemihydrate gypsum (CaSO4 · l/2H2O), add water after they become gypsum (CaSO4 · 2H2O). Gypsum at different temperatures crystalline state transition occurs , accompanied by a volumetric shrinkage . About 120 ℃ , by the (CaSO4 · 2H2O) becomes (CaSO4 · l/2H2O); at 164 ℃ or more, by (CaSO4 · l/2H2O) becomes CaSO4, namely anhydrite . From this , to make the mold completely dehydrated , the minimum baking temperature must be above 170 ℃ .
  In the baking , the impact of the main factors mold thermal cracking and shrinkage . Therefore, to improve thermal conductivity and reduce shrinkage cracking is the main measure to prevent mold . Material composition in the mold , the powder is much superior than the thermal conductivity of gypsum and high temperature under a certain expansion . Therefore, when a larger proportion of the powder and gypsum to four times , the mold is greatly improved thermal conductivity , and the expansion of the gypsum powder is almost offset the shrinkage , the mold shrinkage rate is close to zero integrated . For this reason , rapid baking process can be used to bake the mold , the process is as follows : that is, the wet continuous casting directly from room temperature to about 200 ℃ quickly and at the same temperature for 5 to 10 hours , the mold has been completely dehydrated , mold cracking does not occur , fully meet the requirements of casting process . The baking process and is now commonly used heating and insulation ladder step by step baking process compared to , can greatly shorten the drying -type cycles and save energy.
  Fourth, the casting process
  1 . Vacuum casting process and apparatus
  Known gypsum mold poor breathability . For complex shape, with a narrow trench fine pattern and a mold is concerned, in the atmosphere by gravity casting process is very difficult to be able to succeed . Should generally vacuum , vacuum or low-pressure casting process to be successful , but large investment in equipment .
  The study analysis, as long as the casting mold cavity portion capable of a certain negative pressure , that is, low degree of vacuum as the vacuum casting process can get the same effect . For this reason , we have successfully developed a negative pressure shown in Figure 5 pouring devices. The device is simple in structure , small footprint, low investment , and easy to operate. A negative pressure generating part of the mold cavity as follows : When the mold is filled with liquid metal , the entire mold was frame plate 1 , 3, and the negative pressure liquid alloy platform seal box 4 ( see Figure 5 ) . When starting the vacuum evacuation device , the mold by means of the gas permeability of the mold itself , is extracted from the suction hole . Parts of the mold cavity to form negative pressure . General control pressure 100 ~ 200mmHg, zinc alloy casting temperature 480 ~ 500 ℃.
  2 . Solidification process
  In the atmosphere , using gravity casting process, the casting parts prone to shrinkage Heavy trap , shrinkage and porosity and other defects. To overcome these drawbacks, according to the principle of metal solidification . Liquid alloy during solidification is always to maintain a stable temperature gradient . May be selected for the Heavy parts of the casting with the heat generated by the heating means continuously heating the parts shown in Figure 5b, so that the parts of a high temperature liquid alloy remain until the last solidified so far. Then , the farther away from the site of the lower temperature of the liquid alloy , the alloy also the first part of the liquid solidified alloys Los temperature gradient along the direction of the solidification temperature . Simultaneously during solidification , often to replenish the bath liquid alloy , the molten pool surface is always maintained at a certain height . When the pool is sufficiently reduced to a circle , the heating was stopped , the end of the solidification process .
  With the zinc alloy die casting process is no gate , no shrinking trap , no shrinkage and no loose organizational structure is dense alloy , casting yield up to 100% .
  V. Conclusions
  This article introduced silicone rubber molding , rapid baking mold , vacuum casting and solidification of new technologies and device applications , has made the following main technical and economic effects: ( 1 ) allows to achieve the overall complexity of the mold cavity mold modeling and casting , optimizing mold structure ( 2 ) sprueless available , no risers , no sink trap , no shrinkage , no loose and dense alloy casting mold , the cavity surface roughness up Ral.2 ~ 1.7/μm, dimensional accuracy up to 100 ± 0.06mm and casting yield up to 100% ; ( 3 ) new technologies and device structure is simple, practical , effective , small investment , short production cycle and low cost, ease of production promote the application .