The application of resin grinding wheel has penetrated into all industrial fields, and the amount used in our civilian and home decoration is amazing. With the surge in the domestic resin grinding wheel DIY market, the performance indicators of resin grinding wheel have been greatly improved. At the same time, it is related to the manufacture of resin grinding wheel. The technological progress of China, especially the automation and intelligence of production equipment, is developing at an alarming rate. In the future, whoever has the advanced technology first will gain the opportunity for rapid development.
The technological progress of resin grinding wheel will depend more on more accurate and scientific understanding of resin grinding wheel manufacturing technology in the future.
It can be said that resin abrasive disc, especially cutting/grinding wheel , are still in a period of rapid development in 3 to 5 years. More than 80% of the global resin grinding wheel production capacity is concentrated in China. In the next 3 to 5 years, only a few high-end products will be produced in European and American countries, and more well-known foreign grinding companies will compete for market share of resin grinding wheels through joint ventures, mergers, acquisitions and other channels. The competition in the resin grinding wheel manufacturing industry will be more fierce, and reshuffle is inevitable.
Phenolic resin is an important and most common binder with the widest range of applications, the largest amount of use, and a variety of properties. The selection of the appropriate phenolic resin is directly related to the performance of the resin grinding wheel we require, and the degree of performance. which is related to the process conditions of the resin grinding wheel manufacturing process.
First, the problem of raw materials
Resin grinding wheel is a complex system composed of a variety of main and auxiliary raw materials.The main materials are abrasives and binders, and the auxiliary materials include fillers, reinforcing materials, and colorants.
1. Issues that should be paid attention to in terms of abrasives
At present, the quality of abrasives on the market is uneven, mainly as follows:
1) The chemical composition of the abrasive is often qualified, but the physical properties are poor; it is mainly manifested in the gap between the bulk density of the abrasive and foreign countries.
2) The particle size composition of the abrasive is chaotic, which is quite different from the standard provisions; it is mainly manifested in the difference between the basic particle content of the same particle size abrasive and foreign countries.
2. In terms of binders, mainly phenolic resins:
1) Resin quality stability, especially the level of free phenol content in the resin. Excessive free phenol content will accelerate the resin cracking after heating and hardening of the resin grinding wheel and affect the strength of the grinding wheel;
2) The mixing uniformity of resin powder and urotropine, urotropine is used as the hardener of resin, if the amount of urotropine added is insufficient, the resin hardening is not complete, which will affect the strength and hardness of the grinding wheel; if the content is too high, the excessive Urotropine is not combined with the resin, and decomposes and volatilizes during the hardening process, which increases the pores of the grinding wheel and reduces its strength and hardness.
3) The particle size (fineness) of the binder is too coarse or too fine
It is generally believed that the particle size of the binder should be fine, which is conducive to the uniform distribution of the binder. If the particle size is too coarse, the molding material is not easy to mix evenly, which affects the hardness and strength of the grinding wheel. Even for coarse-grained resin flake cutting disc and cymbal-shaped grinding wheel, the particle size of the selected binder (resin powder) should be finer than 320#.
However, if the particle size of the phenolic resin powder is too fine, it will bring difficulties to the mixture, and it is difficult to achieve the uniformity of the mixture, thereby affecting the cutting and grinding performance of the resin grinding wheel.
Second, resin grinding wheel manufacturing process problems:
Resin grinding wheel manufacturing is an industrial product with strong craftsmanship, and there are many problems in compounding, molding, hardening, processing and other processes.
1. Mixing process: The key to mixing is uniformity. The molding material should achieve: all components are evenly distributed and loose, but no obvious powder leakage should occur. It must be ensured that the material is evenly spread and has good molding performance.
2. Forming process: At present, most of the domestic rotary feeding mechanisms are used. However, due to the limitations of the rotary feeding mechanism, many manufacturing enterprises and some enterprise employees cannot operate the essentials, the mixture is unevenly distributed, and the shape and position tolerances and static balance exceed Difference.
3. Hardening process: This is the most critical production process. There are two factors that are likely to cause quality problems. One is that the temperature difference between the curing oven or the firing kiln is too large; the other is the improper method of loading the furnace. The temperature difference of the hardening furnace and kiln is too large, which causes the hardening of the resin grinding wheel to deviate from the set hardening specification (hardening curve), so that the curing (hardening) quality of the grinding wheel cannot be fully guaranteed.
4. Processing procedures (mainly large diameter and high thickness resin grinding wheels): resin grinding wheels, especially large diameter and high thickness grinding wheels, or grinding wheels with strict dimensional tolerances, should be processed after hardening to ensure the geometric size and shape and position tolerances of the grinding wheel.
5. The production formula and process parameters are rigidly copied.
Third, the problem of equipment and tooling
1. Equipment problems: Production equipment is an important material condition for grinding wheel production. Some enterprises lack equipment, lack good mixers, and have small tonnage specifications of hydraulic presses. At the same time, equipment maintenance is poor, and product quality is not easy to guarantee.
2. Tooling problem: Some small-scale resin grinding wheel enterprises have weak tooling management and no good storage place, and they are piled up at the production site.
Fourth, inspection equipment problems:
1. Raw material inspection, process inspection, and factory inspection are important contents for resin grinding wheel manufacturers to implement effective inspection and control, but there are few manufacturers that truly implement standardized management and standardized management. The main problems are incomplete system, unimplemented functions, lax management, lack of inspection equipment and lack of control, and poor professional quality of inspection personnel.
2. Incomplete configuration of inspection equipment
The inspection equipment is not complete, and the inspection equipment for resin grinding wheels has three major pieces: namely, rotary strength testing machine; sandblasting hardness machine (or Rockwell hardness machine)—except for resin slices and grinding discs; static balancer (diamond grinding wheel also needs Dynamic balancer), and others should include the end face of the thin grinding wheel, the radial runout tester, the inner diameter gauge, the vernier caliper, the grinding wheel cutting performance testing machine, etc. In addition, inspection equipment lacks due care and maintenance.砂轮择及
The most commonly used wetting agent in the manufacture of resin grinding wheels is phenolic resin liquid. When mixing with a normal resin liquid ratio, if the dryness and wetness of the material are difficult to control, wetting agents such as furfural and dibutyl ester are used.
Because furfural has good compatibility with liquid phenolic resin, most manufacturers are still using it. However, furfural is highly corrosive, irritating odor, and harms the health of operators. Therefore, many manufacturers are looking for more suitable alternative products.
In order to meet the needs of customers, after nearly a year of development, new phenolic resin liquids BR-19500 and BR-1950A have been successfully developed. This resin liquid can solve the looseness of the mixture, and the mixture is moderately humid. The lower loosening effect is very good; at the same time, the mixture is parked for a short time, and the bulk density of the mixture tends to be stable after 1 hour; more importantly, the particle shape of the mixture is uniform, and the resin and auxiliary materials cover the abrasive particles. .
Curing of phenolic resins relies on a heating reaction to complete hardening (curing). During the hardening, the heating rate is very critical.
1. Hardening mechanism of phenolic resin
Accurate definition: The resin obtained by addition and appropriate polycondensation reaction of phenol and formaldehyde in the synthesis reaction equipment is usually a mixed system of oligomers with low molecular weight and various methylol phenols, although Novolaks and Resoles use As mentioned in the previous section, there are differences in structure.
Phenolic resin has excellent performance only after forming a cross-network (or body type) structure, including mechanical properties, electrical insulation properties, chemical stability, thermal stability, etc.
The hardening of phenolic resin is the process of transforming it into a network structure, showing two stages of gelation and complete curing. This transformation is not only a physical process, but also a chemical process. Therefore, the hardening of phenolic resin is by no means solidification in the general sense that the melt is cooled below the melting point, but in the concept of polymer chemistry, the cross-linking (cure) of linear (branched) molecules into network molecules leads to the loss of soluble, soluble and soluble molecules. Melt solidification.
After the phenolic resin is hardened, while obtaining excellent physical properties, it loses its solubility and meltability, and no longer has processability. Therefore, its curing process must be completed in the use or molding process of plastics, paint coatings and various engineering materials composed of phenolic resins (Novolaks or Resoles) as binders.
The hardening process exhibits the following characteristics:
(1) The structural factors (composition, molecular weight, reactive functionality, etc.) of the resin before hardening are significantly affected; (2) The hardening reaction is significantly affected by catalysts, curing agents, resin PH values, etc.;
(3) The hardening process has thermal effect;
(4) The hardening rate is significantly affected by temperature and pressure;
(5) By-products (such as water, formaldehyde, etc.) are produced during the hardening process;
(6) The hardening reaction is an irreversible process.
2. Hardening conditions of phenolic resin
The phenolic resin powder must be heated to a fluid state to form a binder bridge. However, if the heating rate is too fast, the grinding wheel will deform. Therefore, an appropriate hardening curve must be adjusted to ensure that the binder is cured and the grinding wheel is not deformed.
As mentioned in 1, a certain amount of volatiles will be released during the hardening process, and the pressure generated will deform the grinding wheel when the conditions are mature.
This pressure must be offset by the external pressure or the strength of the bond bridge itself, and it is also possible to keep the volatiles inside the wheel without destroying the shape of the wheel without affecting the performance of the wheel. It can make the grinding wheel made of phenolic resin bond bridge resin harden rapidly.
At the same time, the formation speed of the binder bridge mainly depends on the hardening rate of the phenolic resin, and is closely related to the type of the wetting agent (containing the resin liquid) and the coordination and reaction of the resin powder. Therefore, the type of phenolic resin and the hardening rate must be considered when designing the hardening curve.
3. Setting of hardening curve
In resin grinding wheel manufacturing technology, there are two basic hardening curves, and all other hardening curves are the transformation of these two curves.
1) Short curve (rapid heating curve)
The heating rate is fast, more than 10 ℃ per hour. The total curve time is 20 to 24 hours.This curve is often used to harden the phenolic resin liquid as a wetting agent, and a formula resin grinding wheel with a certain amount of gypsum filler added to the grinding wheel formula.
The characteristics of this curve are: rapid hardening, and the flow length of the phenolic resin powder used is short.
2) Long curve (slow heating curve)
It is suitable for resin grinding wheels with good binder fluidity (medium density, using low viscosity resin liquid as wetting agent, etc.) The hardening temperature is between 160 and 190°C, and the total length of the curve is 40 to 60 hours.
When we design the hardening curve, we must carefully consider the heating rate, length, maximum temperature and other details of the curve, and must be verified by many tests.
The principles that must be followed in the design of the hardening curve:
For large-diameter grinding wheels and high-thickness grinding wheels, the curve should be held for a long time at the highest temperature during hardening to ensure complete hardening;
When it is required that the grinding wheel is not completely hardened, the maximum temperature of the curve needs to be adjusted lower;
When the high-hardness cold-pressed grinding wheel is hardened, the holding time in the 105-120°C section must be extended so that the volatiles can be completely released;
The grinding wheel made of resin powder with extremely short flow length should heat up faster during hardening to ensure that the binder has sufficient fluidity, so as to obtain the best product performance.
Short curve, low temperature hardening, can produce “soft” grinding wheel;
On the contrary, when producing “hard” grinding wheels, the maximum temperature is lower than 160°C, the strength of the grinding wheel is low, the grinding effect is weak, and the grinding wheel is soft; when the maximum temperature is higher than 210°C, the hardening is excessive, and the produced grinding wheel is brittle and easy to break.
It is very difficult and of little use to accurately detect the degree of hardening of resin grinding wheels.
The acetone extraction method used by individual manufacturers to detect the degree of hardening was later found to be very different from the grinding effect of the grinding wheel. Clearly, the resin is completely insoluble in acetone before it has hardened enough to make a usable wheel.
A more scientific method is to test the tensile strength and flexural strength of the resin grinding wheel sample block (“8” block), but it does not match the actual performance of the grinding wheel, so it is of little significance.
The more accurate fact is: after long-term production practice and exploration, many grinding wheel manufacturers have developed a set of reasonable hardening curves, which should still be adhered to.
