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The relationship between die casting quality and mold

Update:23-02-2021
Summary:

Mold is the main tool for die casting. Therefore, when […]

Mold is the main tool for die casting. Therefore, when designing the mold, try to make the overall structure of the mold and the structure of the mold parts reasonable, easy to manufacture, easy to use, safe and reliable. To make the mold not deform during die-casting, the molten metal flows stably in the mold, so that the casting can be cooled uniformly, and the die-casting can be fully automatic without failure. In addition, appropriate mold materials should be selected reasonably according to production batches and material conditions.

1. The structure of the mold and mold parts should be reasonable

From the point of view of strength, the mold parts are designed as a whole, strong and durable, not easy to be damaged or deformed during use. However, if the shape of the die-casting part is complicated, and the mold parts are also complicated, the mold processing will be difficult and the processing accuracy will not be high. If the mold parts are made as a combined type, the processing is greatly simplified, high processing accuracy is easily obtained, and high-quality die castings can be obtained.

2. Determination of the number of cavities

To determine the number of cavities, the equipment capacity, the difficulty of mold processing, the production batch size, and the precision requirements of castings should be considered. Especially for multi-cavity molds, due to the difficulty of mold processing and large dimensional accuracy errors, the runner configuration is not easy to achieve a balance, and the performance of each cavity casting is inconsistent. When the die-casting parts require high precision and complex geometric shapes, the prohibited words are moulded one cavity at a time. Small castings depend on the situation.

3. Design of gating system

The gating system is not only a channel for filling the die-casting mold with liquid metal, but also regulating the flow rate and pressure transmission of the melt, as well as the exhaust conditions, and the thermal stability of the die-casting mold. Therefore, the design of the pouring system must analyze the structural characteristics of the casting, Technical requirements, alloy types and characteristics, and the type and characteristics of die casting machines must also be considered, so that a reasonable pouring system can be designed.

4. Exhaust system design

The mold should be equipped with overflow grooves and exhaust channels with sufficient overflow range, which is very important to ensure product quality. People often ignore the phenomenon that the overflow channel is blocked prematurely by the incoming molten metal. It is necessary to adopt a reasonable structure to make the molten metal flow into the deeper part of the overflow tank first to ensure that the exhaust hole is always open for the longest time. In addition, the overflow tank should be equipped with ejector rods to remove the metal in the overflow tank.

5. Mold temperature

The temperature of the die casting mold is an important factor affecting the quality of the casting. Improper mold temperature not only affects the internal and external quality of die castings (such as porosity, shrinkage, looseness, mucosa, coarse grains and other defects in the casting), but also affects the dimensional accuracy of the casting, and even deforms the casting, causing cracks in the die casting mold and causing the casting The surface is formed with network burrs that are difficult to remove, which affects the appearance quality of die castings. Taking aluminum alloy as an example, the alloy temperature is poured into the mold at 670-710°C. In the long-term production practice, it is concluded that the best temperature of the mold should be controlled at 40% of the casting temperature. The temperature of the aluminum alloy die-casting mold is 230-280℃, and the mold temperature within this range is conducive to obtaining high-quality and high-yield castings.

Molds generally do not use gas or electric heating, but use preheating and cooling devices, which use oil as a medium as required to preheat and cool the mold.

6. Determination of the size of molded parts

When calculating the size of die-casting parts, the shrinkage rate of the selected die-casting material must be in line with reality, otherwise it will lead to unqualified products. If necessary, after the actual measurement of the test mold, the size of the die casting can be calculated. For high-precision products, even the thermal expansion of the die-casting parts and the impact of the product's storage and use environment on the dimensional accuracy of the product should be taken into account.

7. Determination of the position of the parting surface

The position of the parting surface will affect mold processing, exhaust, product demoulding, etc. Usually, the parting surface will leave a trace on the product, which affects the surface quality and dimensional accuracy of the product. Therefore, when designing the position of the parting surface, in addition to considering the problems of product release, mold processing, and exhaust, the parting surface position can be placed in a place where the product surface quality is not high or the dimensional accuracy is not high.

8. The mold cannot be deformed

Often due to the unreasonable mold structure or improper selection of mold materials, the mold is cracked and deformed during use, and the product is unqualified. Therefore, when designing the mold, appropriate measures must be taken to ensure the quality of the product.

Generally, during die casting, the pressure in the mold is 70-100 MPa. In order to prevent the mold from deforming and displacing, the cavity must be sufficiently thick, and the plate and backing plate for installing the core must be sufficiently thick. If necessary, a support pad can be added under the backing plate. The core and cavity should be installed reliably, the side roughness of the core and the mounting hole should be appropriate, and the roughness should not be too low. The through-hole core should be fixed on both sides to prevent the product from being thick on one side and thin on the other. For the core of the blind hole on the product, it is also necessary to find a way from the position, quantity and core reinforcement of the feed inlet to make the core force balanced.

For the die-casting mold, the strength of the cavity and the backing plate can be checked. Check the strength and stiffness of the cavity wall thickness, and check the stiffness of the backing plate. In addition to taking certain guarantee measures on the mold structure, mold materials with small deformation and good strength must be selected. In addition, the gap between the mold guide post and the guide sleeve, or the wear of the guide post and guide sleeve during use will affect the quality of the product. Especially for products with high dimensional accuracy, in order to ensure product accuracy, the movable and fixed mold cone matching parts can be set on the parting surface, or 2-4 positioning rods can be set at appropriate places around the cavity for positioning and enhancement. , In order to prevent the dislocation of the moving and fixed molds, which is more important for large-scale and mass production molds. 

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