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Influence of mold structure design

Update:10-06-2020
Summary:

Some mold materials and steel materials are very good, […]

Some mold materials and steel materials are very good, often because the mold structure design is unreasonable, such as thin edges, sharp corners, grooves, abrupt steps, thick and thin disparity, etc., causing large deformation of the mold after heat treatment.
1. Causes of deformation
Due to the uneven thickness or sharp rounded corners of the mold, the thermal stress and tissue stress between the parts of the mold are different during quenching, which leads to the difference in volume expansion of each part and causes the mold to deform after quenching.
2. Precaution
When designing the mold, in order to meet the actual production needs, the thickness of the mold should be minimized and the structure asymmetry should be minimized. At the junction of the thickness of the mold, structural design such as smooth transition should be adopted as much as possible. According to the deformation rule of the mold, the processing allowance is reserved, and the mold will not be scrapped due to the deformation of the mold after quenching. For molds with particularly complicated shapes, in order to make the cooling uniform during quenching, a combined structure can be used.

three. Mold manufacturing process and the influence of residual stress
It is often found in the factory that some molds with complex shapes and high precision require large deformation after heat treatment. After careful investigation, it was found that the mold did not undergo any pre-heat treatment during the mechanical processing and final heat treatment stage.
1. Cause of deformation
The residual stress during machining and the stress after quenching are superimposed, which increases the deformation of the mold after heat treatment.
2. Precaution
(1) After roughing and before semi-finishing, a stress relief annealing should be carried out, that is, (630-680) ℃×(3-4)h furnace cooling to below 500 ℃, air cooling, or 400 ℃×(2- 3) h stress relief treatment.
(2) Reduce the quenching temperature and reduce the residual stress after quenching.
(3) Using quenching oil 170oC oil-out air cooling (stage quenching).
(4) The isothermal quenching process can reduce the residual stress of quenching.
Using the above measures can reduce the residual stress of the die after quenching and reduce the deformation of the die.

Four. Influence of heat treatment and heating process
Effect of heating speed
Mold deformation after heat treatment is generally considered to be caused by cooling, which is incorrect. Molds, especially complex molds, the correctness of the processing technology often has a greater impact on the deformation of the mold. It can be clearly seen from the comparison of the heating process of some molds that the faster the heating speed, the larger the deformation.
(1) Cause of deformation
Any metal must expand when heated, because when the steel is heated, the uneven temperature of each part (that is, the uneven heating) in the same mold will inevitably cause inconsistency in the expansion of each part in the mold, resulting in heating due to heating. Uneven internal stress. At temperatures below the phase transition point of steel, uneven heating mainly produces thermal stress, and uneven heating beyond the phase transition temperature will also produce unequalities in the transformation of the tissue, resulting in both structural stress. Therefore, the faster the heating rate, the greater the temperature difference between the surface of the mold and the center, the greater the stress, and the greater the deformation of the mold after heat treatment.
(2) Preventive measures
The complex mold should be heated slowly when it is heated below the phase transition point. In general, the deformation of the vacuum heat treatment of the mold is much smaller than that of the salt bath furnace. Using preheating, one-time preheating (550-620oC) can be used for low alloy steel molds; two-time preheating (550-620oC and 800-850oC) should be used for high alloy steel molds.

 
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