Causes of plastic mold failure

Plastic molds are various molds and tools used in industrial production to obtain desired products by methods such as injection molding, blow molding, extrusion, die-casting or forging molding, smelting, and stamping. Mold failure will occur during use, and the factors that affect plastic mold failure can be classified into three points.

1. Unreasonable structural design causes failure.

Sharp corners (where the stress concentration is more than ten times higher than the average stress) and stress concentration caused by excessive cross-sectional changes are often the root causes of early failure of many molds. And in the process of heat treatment and quenching, sharp corners cause residual tensile stress and shorten the life of the die.

2. Failure caused by poor mold material quality.

Internal defects of mold materials, such as looseness, shrinkage, segregation of inclusions, uneven distribution of carbides, original surface defects (such as oxidation, decarburization, folding, scars, etc.) affect the performance of steel; excessive inclusions cause failure; surface decarburization Causes failure; uneven distribution of carbides causes failure.

3. Improper machining of the mold.

(1) Tool marks during cutting. In the machining process, the cavity part of the mold or the rounded part of the punch often leaves a knife mark locally due to the deep penetration, which causes severe stress concentration. When quenching is performed, the stress concentration part is very easy to produce micro crack.

(2) Failure caused by electrical machining. When the mold is subjected to electrical machining, a large amount of heat is generated due to the electric discharge, which will heat the processed part of the mold to a high temperature, causing changes in the structure and forming the so-called abnormal electrical machining layer. On the surface of the abnormal layer, due to the high temperature melting and then solidifying quickly, the layer is white under the microscope with many fine cracks inside. The area under the white layer is quenched, which is called the quenched layer, and the inside is weakened due to the influence of heat. , The temperature is not high, only tempering occurs, called the tempered layer. The melting and resolidification layer has a high hardness of 610-740HB with a thickness of about 30μm, and the quenching layer has a hardness of 400-500HB and a thickness of 20μm. Tempering is a high-temperature tempering, with a softer structure, with a hardness of 380-400HB and a thickness of 10μm.

4. The mold heat treatment process is inappropriate. Improper selection of heat treatment process parameters such as heating temperature, holding time, cooling speed, etc. will all become mold failure factors.

(1) Heating rate. Plastic mold steel contains more carbon and alloy elements and has poor thermal conductivity. Therefore, the heating speed should not be too fast and should be carried out slowly to prevent deformation and cracking of the mold. When heating and quenching in an air furnace, in order to prevent oxidation and decarburization, packing protection heating is used. At this time, the heating speed should not be too fast, and the heat transmission should be slow. In this way, there will be no large thermal stress and it is safer. If the mold heating speed is fast and the heat transmission is fast, great thermal stress will be generated inside and outside the mold. If it is not properly controlled, it is easy to produce deformation or cracks, which must be prevented by using residual heat or slowing down the acceleration of heating.

(2) The influence of oxidation and decarburization. Mold quenching is carried out at high temperature, if not strictly controlled, the surface is easy to oxidize and decarburize. In addition, after the surface of the mold is decarburized, due to the difference in the structure of the inner and outer layers, large structural stress appears during cooling, which leads to quenching cracks.