Plastic Mold Failure Causes | How to Choose Injection Mold Materials


The materials and heat treatment used in the mold are the main factors affecting the service life, so how to choose materials for making plastic injection molds? And what other factors affect its use?

Causes of Injection Mold Failures

1) Wear and corrosion of cavity surface

The plastic melt flows in the mold cavity under a certain pressure, and the solidified plastic parts are pulled out of the mold, which will cause friction and wear on the molding surface of the mold. The fundamental cause of plastic mold wear failure is the friction between the mold and materials. However, the specific form and process of wear are related to many factors, such as the pressure, temperature, material deformation speed, and lubrication condition of the die in the working process. When the materials and heat treatment used in the plastic mold is unreasonable, the surface hardness of the cavity of the plastic mold is low and the wear resistance is poor, especially when solid materials are used to enter the cavity of the plastic mold, it will aggravate the wear of the cavity surface. In addition, during the process, chlorine, fluorine, and other components are decomposed into corrosive gas by heat, which causes corrosion and wear on the cavity surface of the plastic mold, leading to failure. If there is wear damage at the same time, the coating or other protective layer on the cavity surface will be damaged, which will promote the corrosion process. The cross action of the two kinds of damages accelerates the corrosion wear failure.


2) Pressure and heat 

Plastic deformation failure can be caused by the pressure and heat on the surface of the plastic mold cavity, especially when the small mold is working on the large tonnage equipment, it is more likely to produce overload plastic deformation. The material used for plastic mold has insufficient strength and toughness, and low deformation resistance another reason for plastic deformation failure is that the hardening layer on the surface of the mold cavity is too thin, the deformation resistance is insufficient, or the working temperature is higher than the tempering temperature, resulting in phase change softening, which leads to early failure of the mold.


3) Stress

The main reason for fracture is the structural stress and thermal stress caused by structure and temperature difference, or insufficient tempering, which transforms the residual austenite into martensite at the service temperature, causing local volume expansion, and the organizational stress generated in the mold.

Selection of Injection Mold Materials

So how to choose mold materials to reduce die failure and prolong die life? Materials for plastic mold should have the following properties.

1) Heat-resistant

With the introduction of a high-speed molding machine, plastic items operate more quickly. As the forming temperature is between 200 °C and 350 °C, if the plastic fluidity is weak and the forming speed is quick, the forming surface temperature of the mold will quickly surpass 400 °C. The die steel must have a strong heat resistance in order to ensure precision and little deformation of the tool.


2) Wear-resistant

In order to increase the flexibility of plastics, it is frequently required to use inorganic components such as glass fiber. Due to the fact that the addition of additives drastically affects the fluidity of plastics and causes the wear of molds, high wear resistance is necessary.


3) Outstanding machinability

Most plastic molding dies also require cutting and fitter repair in addition to EDM. In order to extend the useful life of cutting tools, the work hardening during the cutting process is kept to a minimum. In order to prevent die deformation from affecting precision, it is hoped that the amount of residual stress during milling may be kept to a minimum.


4) Highly thermally stable

The shape of plastic injection mold parts is frequently complex and difficult to process after quenching, thus wherever possible, thermally stable materials should be chosen.


5) Mirror processing effectiveness

The surface of the cavity is smooth, the surface of the mold must be polished to a mirror finish, and the surface roughness must be less than Ra0.4 m. To maintain the appearance of plastic pressings and assist demolding.


6) Heat treatment effectiveness

In general, 52.3% of mold failure accidents are caused by heat treatment, hence heat treatment plays a significant part in the entire mold production process. The quality of the heat treatment procedure has a substantial effect on the quality of the mold. In general, the heat treatment deformation must be modest, the quenching temperature range must be broad, and the overheating sensitivity must be low, with better hardenability and hardenability being a must.


7) Corrosion resistance

During the process of forming, corrosive gas may be emitted and degraded into corrosive gas, causing the die to corrode. Occasionally, the die may be damaged by corrosion at the air flow channel opening; therefore, the die steel must have excellent corrosion resistance.