2.2 Main parameters of mold design
(1) The gap between the clearance punch and the left and right die should be appropriate. If it is too small, it will collide or bite due to the assembly error of the punch and the die; if it is too large, the alloy melt will be ejected through the gap, causing an accident; or the longitudinal burrs may be generated in the gap to reduce the pressing effect and hinder the unloading. material. A reasonable gap is related to the pressurization start time, pressurization speed, pressure magnitude, workpiece size, and metal material. According to the actual production experience, the single side gap is 0.1 mm.
(2) The mold release alloy melt solidifies into a casting under the pressure of the punch, and is tightly wrapped on the punch and the ejector insert after cooling. In order to facilitate the release of the punch and the ejector insert, a 3° draft angle is provided on the punch and the ram insert. Since the shape of the casting is round and divided into two left and right concave molds, as long as the right concave mold is moved to the right by a certain distance, the casting is easily taken out from the left concave mold, so that it is not necessary to set the draft angle.
(3) After the exhaust and the two left and right concave molds are completely closed, the alloy melt is slowly poured into the cavity, and the gas in the cavity can be substantially discharged. At the time of extrusion casting, a small amount of gas remaining in the guide portion of the punch is discharged through the gap between the punch and the die.
(4) Squeeze casting of mold material is carried out under a certain pressure and a certain temperature, and there is no scouring of molten metal like a die-casting mold. The working pressure is higher than that during die casting, and only requires a certain compressive strength of the mold at high temperatures. In addition, in order to prevent thermal fatigue cracks on the surface of the mold that is in contact with the molten alloy, the left and right female molds, punches and ram inserts are made of 3Cr2W8V alloy mold steel. After heat treatment, the hardness is HRC48-52, and the surface of the cavity is soft nitrogen. Processing.
3 Process parameters of extrusion casting
Squeeze casting is a combination of casting and forging. The production process is: melting of alloy, preparation of mold (cleaning, preheating, spraying lubricant), casting of metal, pressurization of liquid metal, maintenance of pressure, pressure Removal and removal of castings, etc.
In order to ensure the quality of castings, the process parameters [1~2] must be selected reasonably.
(1) The specific pressure has a direct impact on the physical and mechanical properties of the casting, casting defects, microstructure, segregation, melting point and phase equilibrium. Therefore, it is important to determine the unit pressure necessary for forming. If the specific pressure is too small, the surface and internal quality of the casting cannot reach the technical index; if the specific pressure is too large, the improvement of the performance is not very obvious, and the mold is easily damaged, and a device with a large mold clamping force is required. The extrusion casting test was carried out on a 2 000 kN hydraulic press. The test proves that the specific pressure suitable for the extrusion casting of the aluminum alloy wheel should be selected within the range of 50-60 MPa.
(2) Pressurization start time From the results of the wheel extrusion casting test, the interval between the start of pressurization is too long, and the strength and elongation of the casting are lowered. The starting pressurization time is 3 to 5 s, which is more suitable.
(3) Pressurization speed Squeeze casting requires a certain pressurization speed, and if possible, it is better to pressurize it faster. With a high pressurization speed, the punch can quickly apply pressure to the metal for ease of forming, crystallization, and plastic deformation. However, it should not be too fast, otherwise it will cause splashes and eddy currents on the surface of some alloy melts, causing defects in the castings, and excessive alloy melt flowing out in the gap between the convex and concave molds to form longitudinal burrs that are difficult to remove. . Therefore, the punch must be slowly pressed into the liquid metal. Since the hydraulic press used has a slower feed feed rate, it is pressed at the speed of the work stroke.
(4) Holding time The pressure holding time mainly depends on the thickness of the casting. Under the conditions of forming and crystal solidification, the holding time is short. However, if the holding time is too short, shrinkage holes are likely to occur inside the casting. If the holding time is too long, the production cycle will be prolonged, the deformation resistance will be increased, and the service life of the mold will be reduced.
Considering the wall thickness of the wheel, the dwell time of the extrusion casting is about 12 s.
(5) If the mold preheating temperature mold is not preheated, the alloy melt will solidify quickly after being injected into the cavity, which will not be too late; but the preheating temperature should not be too high, otherwise it will prolong the holding time and reduce the productivity. Also not conducive to spraying lubricant. The preheating temperature of the wheel die casting mold is 200 to 300 ° C, and is usually heated by a kerosene torch.
(6) Alloy pouring temperature Pouring temperature is too high or too low, which has a significant effect on alloy forming. Too low, the alloy is very easy to solidify, the required unit pressure is large; too high, easy to produce shrinkage. It must be noted that the casting temperature of the extruded alloy is higher than the sand casting temperature. It is generally desirable to control the casting temperature to a relatively low value because it is desirable to eliminate porosity, shrinkage, and porosity during extrusion casting. When the pouring temperature is low, the gas easily escapes from the inside of the alloy melt, and is rarely left in the metal, and the pores are easily eliminated. In addition, the chance of shrinkage formation can be reduced, and at the same time, due to the lower pouring temperature, the metal overflows less and the burr can be reduced. For the casting of the wheel, the casting temperature is 720 ~ 740 ° C is most suitable.
(7) The role of the lubricant lubricant is to protect the mold, improve the surface quality of the casting and facilitate the removal of the casting from the mold. The oil graphite lubricant, that is, 5% of 200-300 mesh graphite powder is added to 95% of the oil, and the mixture is evenly stirred. Sprayed on the surface of the mold cavity with a spray gun, the thickness of which is 0.05-0.1 mm. Excessive thickness will affect the surface quality of the casting.
(8) After cooling and extrusion casting, the mold should be demolished immediately, so the casting temperature of the casting is higher. In order to prevent cracks from forming at the interface between the thin wall and the thick wall when the high temperature casting is air-cooled, the casting after the mold is discharged into the sand pile immediately, and then the air cooling is taken out when it is cooled to below 150 °C.
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