Mold is the key factor in producing qualified parts. Quality of mold directly affects quality and cost of various products. A pair of molds must go through molding design, raw material selection, billet making, machining, heat treatment, inspection and other aspects from beginning of molding design to delivery. Problems in each link will have a negative impact on quality of mold, lighter to reduce service life, heavy to make mold scrapped and can not be used.
Quality of mold mainly includes following aspects:
1. Quality of parts: Produce qualified parts, dimensions, roughness and internal quality of parts meet design requirements of drawings;
2, Service life: Under premise of ensuring quality of parts, number of parts produced by mold can be completed;
3. Maintenance and use of mold: whether operation is convenient, whether maintenance is easy, etc.;
Quality of mold directly affects user's use. Good quality mold can ensure quality of parts produced by user, and ensure that user can complete production of parts on time and in quality. Poor quality molds can not guarantee quality of parts, and molds fail prematurely, which not only affects quality of parts, but also increases production cost of products. It also affects delivery of parts on time and causes losses to users. For plastic injection molding manufacturer, production of mold that can not ensure quality of user, enterprise will lose customer and directly affect survival of plastic injection molding manufacturer.
Purpose of mold failure analysis is to analyze multi-disciplinary analysis of mold from molding design to use, aim at failure of mold appearance and internal factors, find out cause of failure, identify economic responsibility, formulate solutions to prevent similar failures from happening again and continuously improve quality of mold.
Role of mold failure analysis in production of molds is very important, which involves production information and influencing factors of production process. Results of failure analysis are fed back to mold design, manufacturing, use, maintenance and other departments, and then improvement measures are applied to molding design, manufacture and use of mold, which is the most effective way to prevent similar failures.

 

Method and steps of mold failure analysis are briefly described in typical failure mode-fracture:

First, scene of accident should be protected; analyst should enter site as soon as possible to check location and form of mold failure; ask about use condition of production equipment, operation, mold failure process and count actual service life of mold. During investigation, care should be taken to collect all fracture fragments in order to determine main fracture and perform fracture analysis.
Care should be taken to protect fracture from cleanliness and freshness when collecting broken pieces. For clean fractures, they should be placed in desiccator for protection immediately; for fractures contaminated with sludge, fractures should be washed with gasoline, acetone (or chloroform, benzene, etc.) and absolute ethanol, and dried with hot air, then it is placed in the dryer; for fracture with corrosion products, it can be directly placed in the dryer without removing corrosion products.

By reading relevant technical data and test reports, checking same batch of raw materials, and inquiring about production personnel, it is verified whether all links in manufacturing meet technical requirements of relevant standards, design and process. Survey contents are generally: material condition, forging quality, cutting and grinding quality, EDM and wire cutting quality, heat treatment and surface treatment quality, assembly quality, etc.
Chemical composition and metallurgical quality of moulding materials were reviewed by chemical composition analysis, mechanical properties measurement, metallographic analysis, and non-destructive testing.
Investigation of service history of mold is mainly to check mold operation record, adjustment and maintenance records, ask operator about use conditions of mold, whether to operate according to regulations and whether there are abnormal phenomena.

Working conditions of mold mainly include stress state of mold and working environment. Stress conditions include load properties such as static load, impact load or cyclic load; load type such as tensile, compressive, torsional, bending, etc.; stress distribution, stress concentration, maximum stress magnitude and location, fracture location stress state and stress magnitude, etc. Working environment includes operating temperature level, range of change and magnitude of thermal stress; type, content and corrosiveness of environmental medium; tissue type of molded part material, stability of tissue, size and distribution of tissue stress.
Fracture condition of mold mainly includes degree of plastic deformation at fracture, orientation, position and surface condition of fracture, and relationship between fracture and mold structure.
According to working conditions and environment of mold, combined with macroscopic analysis of fracture condition and characteristics, nature and type of fracture can be preliminarily determined.
To get a right judgment, you must find out where main break is. When mold breaks into multiple pieces, all pieces should be assembled according to original shape of mold, and degree of adhesion should be observed. The worst adhesion and largest gap is the earliest fracture. According to form of main fracture, amount of plastic deformation at fracture can be measured by visual observation or a measuring tool to determine whether it is ductile fracture or brittle fracture.
According to orientation of main fracture, type of load causing fracture of mold and actual stress state can be analyzed. For example, brittle fracture is always perpendicular to direction in which maximum normal stress acts, and flush fracture is always parallel to direction in which maximum shear stress acts. When fracture originates from gap or stress concentration of mold profile structure, it indicates that notch effect and stress concentration have a great influence on fracture.
In addition, according to difference of oxidation color of fracture, working temperature of mold can be roughly analyzed; According to whether there is corrosion product in fracture, it can be determined whether working medium of mold is corrosive.

Fracture analysis is to analyze macroscopic morphology and microscopic morphology of fracture. Results of analysis can provide an important basis for further determining nature, type and cause of fracture.
Macroscopic analysis of fracture is to analyze morphology of fracture with naked eye, a magnifying glass or a low power stereo microscope. It is used to determine: nature of fracture, ie rapid fracture, fatigue fracture or stress corrosion fracture; type of fracture, such as whether rapid fracture is tough or brittle, location of fracture source and direction of fracture; stress magnitude, stress concentration, and fatigue life at fatigue fracture.
Microscopic analysis of fracture is to analyze microscopic morphology of fracture using a high-power scanning electron microscope or a transmission electron microscope. It is a deepening and necessary complement to macro fracture analysis. It is used to analyze properties of microscopic fractures, namely microscopic toughness or microscopic brittle fracture, mechanism of microscopic fracture, and influence of microstructure on fracture. In addition, it can be used to estimate fatigue crack macroscopic rate and fatigue life.
For stress corrosion cracking, fracture is often attached with corrosion products. In order to avoid interference of corrosion product on fracture analysis, composition and phase structure of corrosion products on the fracture should be analyzed first, then chemically or electrochemically removed, and then fracture analysis.
When there are material defects and processing defects in mold, cracks are often generated at the defects and expanded. Therefore, relationship between crack source and fracture path and various defects should be analyzed to determine effect of defects on fracture. Common defects include non-metallic inclusions, carbide segregation, surface microcracks, overheating, overburning, and insufficient tempering. Fracture fragments can be selected. At fracture source and fracture extension zone, metallographic samples are prepared perpendicular to fracture section, and defects and microstructure at the fracture are observed by optical or electron microscopy.