Nitrile rubber (NBR) is mainly used to prepare rubber products. As the temperature increases and the acrylonitrile content increases, the NBR thermoplasticity increases. The thermoplastic thermoplastic compound has good fluidity and is suitable for the preparation of molded products. The molded rubber product is prepared by charging a certain shape and quality of the rubber compound into a mold, vulcanizing it on a flat vulcanizing machine, and then demoulding and trimming. The quality of molded rubber products is generally determined by the compound formulation and processing technology. This paper mainly discusses the process factors that affect the quality of NBR molded products.
1· Common quality defects in NBR molded products
The purpose of molding is to impart certain shape and physical properties to the rubber article and to avoid surface defects. The common quality defects of NBR molded products are as follows: tearing at the parting line, flow marks on the surface, unevenness, blooming, lifting or rolling, heavy skin cracking, air bubbles inside, excessive shrinkage, and demoulding A crack is created.
2·Influencing factors of NBR molded products quality
Preparation of NBR molded products must pay attention to factors such as rubber compound preparation, mold design and vulcanization conditions.
2.1 Preparation of rubber compound
2.1.1 Mixing process
Poor dispersion of the compounding agent affects the physical properties of the NBR molded article. For example, sulfur has a small solubility in NBR and is not easily dispersed, which often causes peeling of the tearing off or whitening of the surface when the rubber product is released from the mold, and sulfur particles can be seen when the vulcanized rubber is stretched. The uneven dispersion of the plasticizer causes the rubber to delaminate before molding, generates bubbles, and reduces the vulcanization properties of the peroxide compound. Impurities in the compounding agent can cause loose rubber and appearance defects in precision products. Insufficient rubber compound should be re-mixed in time. The scorch rubber is difficult to flow during the molding vulcanization process, and the scorching of the rubber material may cause defects such as lack of compaction, delamination, curling and twisting of the product.
2.1.2 Compound viscosity
The fluidity of the compound in the mold cavity depends on the viscosity of the rubber compound. The viscosity of the NBR compound is usually determined by the plasticizer and filler. It is difficult to reduce the viscosity of the compound by using a plasticizer or prolonging the mastication time. The Mooney viscosity [ML(1+4) 100 °C] of the NBR compound used for the molding method is 35 to 130. If the viscosity of the rubber is small and the vulcanization pressure is insufficient, the product will generate bubbles during vulcanization; if the viscosity of the rubber is large, it is not easy to fill the mold during vulcanization, and the product is liable to cause lack of glue or marks.
2.1.3 Film isolation
In order to prevent adhesion, the NBR compound can be coated with a release agent on the surface of the film, but the stearic acid powder cannot be used. Because stearic acid is insoluble in NBR, it will act as an excessive lubrication, which will affect the molding of the rubber. When bonding. When the NBR compound is combined with the metal core, the powder release agent cannot be used, and the adhesion of the rubber to the metal contact surface must be ensured.
2.1.4 Rubber material parking
After mixing the NBR compound, it should be parked for 16 hours before molding. The optimal time for parking is 24 h. If the rubber is not parked, bubbles and spots will appear in the product; the parking time is too short, which will affect the physical properties of the product; if the parking time is too long, it will seriously affect the adhesion between the rubber and the metal material.
2.1.5 Size of compound
The size of the rubber compound is not exactly the same as the size of the corresponding cavity. The quality of the rubber compound should be larger than that of the rubber compound during molding. When the molding is over, the rubber (or overflow) will double the air in the cavity and maintain the vulcanization pressure. effect. The amount of overflow should be strictly controlled according to the size of the product, the molding method and the mold structure. If the amount of glue is not enough to fill the cavity, it will cause lack of glue; if the amount of glue is too much, it will hinder the mold from closing and cause the product to be out of specification. For the general flat vulcanizing machine, the compound of the separate mold should be packed with 15% to 20%, and the rubber pressed into the separate mold should be installed with 5% to 10%, and the rubber for the press-in mold should be loaded more. 2% or so.
2.2 Mold design
NBR is widely used in the manufacture of O-rings, U-rings, seals and gaskets. When the dimensional tolerance of the product exceeds the specified tolerance range, its service life will be significantly shortened, and even a few minutes will be scrapped. Therefore, the mold design must ensure that the product size is stable.
The main factor affecting the dimensional accuracy of the product is the shrinkage of the rubber. In the case of small dimensional tolerances of the product, the mold design must first consider the shrinkage of the rubber. The reason for the shrinkage of the rubber is that the coefficient of thermal expansion of the rubber is greater than the coefficient of thermal expansion of the metal mold material.
The dimensional tolerance of NBR products is generally from 0.5% to 1.0%. The allowable tolerances for mold sizes of molded rubber products in the United States vary with the product. The general product mold dimensional tolerance is 0. 127 mm, the special precision product mold dimensional tolerance is 0. 005 mm; the rubber product's processing dimensional tolerance depends on the rubber type, formulation, and size. And the change of the shape changes, the solid product processing dimensional tolerance is 0. 254 ~ 0. 635 mm, the sponge product processing dimensional tolerance is 0. 254 ~ 1. 520 mm. High-frequency preheating of the rubber before vulcanization can effectively reduce the internal deformation of the rubber; after cooling, the shrinkage of the rubber exceeds the cavity, and the degree of shrinkage varies with the composition of the rubber (especially the type and amount of filler). The change is related to the internal stress before the molding of the rubber. The effect of the filler on the shrinkage of the NBR molded article is shown in Table 1.
The cause of the overflow can be attributed to insufficient or uneven vulcanization pressure, too small escaping holes and too large cavity area. According to the design principle, the escaping volume should be 15 to 20 times the volume of the cavity to accommodate the overflow. If the overflow is required to be small, the stress between the cavity and the escape hole is small. In the molds that overlap the cavity, the distribution and thickness of the overflow vary greatly depending on the vulcanization pressure. Generally, the thermal tear strength of NBR is smaller than that of other synthetic rubbers, so the vulcanization pressure of NBR products should be appropriate, and attention should be paid to the fact that the finished product does not form a reverse cut when it is discharged. In addition, the flow tank of the side should be as close as possible to the end of the cavity 0.4~3.2 mm.
2.3 Vulcanization conditions
2.3.1 Vulcanization temperature
The molding vulcanization temperature of the NBR compound is usually 140 to 200 ° C, and the specific vulcanization temperature depends on the physical properties, size and molding method of the product. Low-temperature vulcanization can reduce the quality of the product caused by scorch and mold contamination, and can also obtain better physical properties of the product. Large product parts require a relatively flat vulcanization process, and the vulcanization time can be adjusted to optimize the physical properties of the product at different vulcanization temperatures.
The NBR compound can be effectively preheated by a dielectric method. This preheating method enables the rubber to uniformly heat up before molding, accelerates the vulcanization speed, and overcomes problems such as uneven vulcanization and skew of thick products. The skew of the product is caused by the continued vulcanization of the central portion of the product after vulcanization.
The NBR compound is vulcanized at a high temperature, and each operation step needs to be carried out quickly. For example, the rubber compound can be held in a cavity of 153 ° C for 2 minutes and then pressurized and vulcanized. In addition, the use of high temperature vulcanization, mold scale will also increase, thereby increasing the number of cleaning. Mold scale is generally caused by the accumulation of swarf and release agent.
2.3.2 Vulcanization pressure
The vulcanization pressure is very important for the quality of NBR products. If the pressure changes during vulcanization, the physical properties of the product will decrease, so the vulcanization pressure should be stable. NBR compound vulcanization pressure is generally
1.4~3.4 MPa.
The finished product overflows too much to cause waste. This problem is often attributed to the mold design, which is actually caused by insufficient vulcanization pressure. Insufficient pressure during molding, often resulting in pores, external bubbles and deformation; excessive pressure may cause distortion.
The NBR product can be segmented and vulcanized, that is, the NBR compound is first vulcanized and shaped in a flat vulcanizing machine, and then subjected to two-stage vulcanization.
2.3.3 Curing time
The cure time is the time at which the compound is vulcanized in the mold to achieve optimum performance at a certain temperature and pressure. When the vulcanization time is too long, cracks may occur when the product is demolded, and it can be solved by low-temperature vulcanization for a long time; the vulcanization time is too short, the product is less sulfur, and it is easy to tear at the parting line, and the vulcanization temperature can be used to solve the problem.
3. Conclusion
The factors affecting the quality of NBR molded products are mutually restrictive. When NBR molded products produce quality problems, not only the formulation factors should be adjusted, but also the reasons for mixing rubber preparation, mold design and vulcanization conditions should be sought, and improvement measures should be proposed to ensure product quality and yield.
