Industrial materials：The quality foundation of medium density fiberboard is the grinding of fibers in the refiner mill. The thermal grinding process is a very important process in the entire production process, and the key to affecting the performance and potential of the refiner mill lies in the grinding of discs. It can be said that the grinding of discs is the most active and active factor in the refiner mill.

Overview:

The grinding disc of a refiner mill is a component that directly separates fibers.

Development background:

Medium density fiberboard is one of the major inventions in the forest industry in the 20th century, and its excellent performance holds an irreplaceable position compared to other types of boards. Its development speed will still be at the forefront in the first half of the 21st century, and the refiner mill grinding plate, as the core component of the refiner mill, has also been rapidly developed. The development of thermal grinding discs began in the mid-1930s and has a history of nearly 70 years to this day. The grinding discs of the first generation of refiner mills were formed by grinding teeth on a cast disc. In the early 1950s, after the introduction of grinding discs installed on grinding discs, the tooth profile of the grinding discs was directly cast and produced and processed by professional casting companies.

Mechanism of action

Fiber is the quality foundation of medium density fiberboard, and the refiner grinding process is a very important process in the entire production process. The key to affecting the performance and potential of the refiner mill lies in the grinding disc, which can be said to be the most active and active factor in the refiner mill. The grinding discs of the refiner mill, while separating fibers, also cause wear and significant wear. During the entire service life of the refiner mill, the cost of replacing the grinding disc even exceeds the purchase cost of the refiner mill. Therefore, understanding the wear mechanism of the grinding disc is crucial for designing, manufacturing, and selecting the grinding disc correctly.

Wearing mechanism

Grinding disc wear has both commonalities and particularities of general metal wear, mainly being more complex, difficult to observe and predict, and difficult to simulate and reproduce. The wear of grinding discs is not only related to the characteristics of fiber raw materials, but also to the characteristics and quantity of fiber inclusions, as well as to many factors such as the parameters of the refiner grinding process and the operating accuracy of the thermal mill. Fiber separation is completed in a high-temperature, high-pressure, high-speed, and completely enclosed system, and it is very difficult to observe the wear of fiber separation and grinding discs. SUNDS, a Swedish company, once tried a transparent grinding chamber body grinder to observe the fiber breaking process, but the effect was not ideal. Due to the large number of parameters in the hot grinding process and the complex and variable working conditions, some people have designed a grinding disc wear testing machine to study the wear situation of the grinding disc. However, due to the large number of variables, simulating the working conditions of the grinding disc is far from the actual situation with complex changes. Therefore, the research and analysis of grinding disc wear is limited. There are many factors that affect the wear of grinding discs, and it is difficult to observe and test, which brings many difficulties to the study of grinding disc wear. Therefore, establishing a precise and simple correspondence between grinding discs and wear resistance has not yet been successful.

Related research and development institutions

Andritz AG, Austria

China Nantong Huayan Foundry Co., Ltd

China Engineered wood Machinery Factory

Finnish Metso Company

Material quality

Since the Swedes invented the refiner grinding method in the 1940s, the grinding plates of the refiner mill have experienced more than half a century of development. The change in the material of its grinding discs is a development process from cast iron to cast steel, from a single metal to an alloy, from a single high hardness to comprehensive technical characteristics, from a single resistance to mechanical wear to simultaneous corrosion wear, from heat treatment to alloying to improve performance. By comparing the wear resistance of stainless steel grinding discs, multi alloy grinding discs, and high chromium cast iron grinding discs, the results show that multi alloy grinding discs and high chromium cast iron grinding discs have better wear resistance than stainless steel grinding discs. There have been no successful cases of plastic and polymer material grinding applications that have been studied in the past.

Function Introduction

The hot grinding process is a very important process in fiberboard production. Electricity consumption accounts for 35-45%, and heat consumption accounts for 15-20%. The operation status of the refiner mill has a significant impact on the quality and production cost of fiberboard.

Technical requirements for grinding discs

Gear grinding: 

Grinding teeth is the grinding part of the grinding disc that separates fibers, and it is also the most important part of the grinding disc. There are many types of grinding teeth, which can be divided into two categories based on the direction of grinding teeth: oblique teeth, zigzag teeth (single tooth), and straight teeth (double tooth).

The design basis for grinding teeth mainly includes fiber usage, quality, production requirements, fiber raw material characteristics, grinding disc rotation direction, and refiner grinding process parameters.

The design of grinding teeth for hard fiberboard, soft fiberboard, and medium density fiberboard is completely different. However, regardless of the type of grinding teeth, the requirements for strength, wear resistance, impact resistance, regrinding, self sharpening, and good quality and high yield of separated fibers are consistent.

The classification form of grinding disc tooth profile does not exist in isolation, but often appears in combination in actual production according to needs.

HY Cut fin (TM) series

---Suitable for free beating, quickly reducing the wet weight of long fibers under low power consumption, which is beneficial for improving uniformity.

HY Broom fin (TM) broom fin series

---Suitable for slightly sticky beating, with a strong effect on fiber splitting and brooming, which is conducive to improving the beating degree.

HY Soft fin (TM) series

---Suitable for short fiber pulp, with weak cutting effect, maintaining fiber length and reducing wet weight loss.

HY Ease fin (TM) release fin series

---Suitable for multiple types of slurries, with strong dispersing and pumping capabilities, reducing power consumption and improving yield.

Casting quality:

Although cast iron and cast steel with the addition of multiple alloying elements can improve their performance, their casting performance deteriorates and increases with the increase of alloy content. Grinding discs are thin-walled, complex in shape, and castings with significant changes in cross-sectional dimensions, making casting difficult. However, due to the operation requirements of the grinding disc, no Casting defect are allowed in the grinding part, otherwise the grinding disc of any high-quality material will lose its use value.

Machining accuracy:

A high-precision grinding disc is an important condition for ensuring efficient fiber separation and improving the wear resistance of the grinding disc. The thickness of the same set of grinding discs, the flatness of the installation base surface and tooth top plane, and the parallelism between the two are all required to be very high, equivalent to Appendix B of GB/T1184-1996 "Form and Position Tolerances"; And the regulations for levels 6 and 4 in Bs. The roughness of the above planes also needs to be finely machined to Ral six.

Balancing Precision:

In the rotor composed of the refiner mill spindle, rotating disc, grinding disc, and coupling, the shape symmetry of the grinding disc structure is poor, the cross-section is complex, the variation is large, and the assembly error is also more prominent than other components. Therefore, the impact on the dynamic and static balance of the rotor is the greatest, so the requirements for the dynamic and static balance of the grinding disc are particularly important.

Service cycle of grinding discs:

The service life (durability) of a grinding disc refers to the cumulative effective working time from the time a new grinding disc is put into use to its failure. The sign of grinding disc failure is that the grinding disc has lost its specified function and cannot achieve the expected effect. If it continues to be used, it will endanger safety or be uneconomical.

There is no unified standard for determining the failure of grinding discs. Generally, the basis for replacing grinding discs is a significant increase in the current of the main motor of the thermal mill, a significant increase in coarse fibers, a decrease in fiber quality, and a decrease in fiber production.

Broken grinding discs or cracked grinding teeth are considered abnormal failures.

Selection of grinding discs:

The selection of grinding discs should consider numerous influencing factors and be more in line with the actual production situation. It is not advisable to only pursue any single indicator of the grinding disc. High quality grinding discs should be wear-resistant, impact resistant, cavitation resistant, with good overall performance, long service life, good casting quality, high processing accuracy, stable operation, good pulp fiber morphology, high yield, and low power consumption.