Abstract: Analyze the process influencing factors of the quality of nitrile rubber (NBR) molded products. The main process factors of quality defects in NBR molded products are: compounding process, compound viscosity, compound storage time, compound size, mold structure and dimensional tolerances, curing temperature, curing pressure and curing time. Taking targeted measures can effectively solve the quality problems of NBR molded products.
NBR is mainly used in the preparation of rubber products. As the temperature increases and the acrylonitrile content increases, the NBR thermoplasticity increases. Good thermoplastic materials have good fluidity and are suitable for the preparation of molded products. Molded rubber products are made by mixing a certain shape and quality of mixed rubber into a mold, vulcanizing it on a flat plate vulcanizing machine, and then demolding and trimming. The quality of molded rubber products is generally determined by the rubber formula and processing technology. This article mainly discusses the process factors that affect the quality of NBR molded products.
1. Common quality defects of NBR molded products
The purpose of molding is to make rubber products have certain shape and physical properties, and avoid surface defects. Common quality defects of NBR molded products are as follows: tearing at the parting line, flow marks on the surface, non-smoothness, frosting, lifting or rolling, heavy skin cracks, internal air bubbles, excessive shrinkage in size, and demolding A nick is created.
2 · Factors affecting the quality of NBR molded products
The preparation of NBR molded products must pay attention to factors such as compounding rubber preparation, mold design and curing conditions.
2.1 Preparation of compound rubber
2.1.1 Mixing process
不良 Poor dispersion of compounding agents will affect the physical properties of NBR molded products. For example, sulfur yellow has low solubility in NBR, and it is not easy to disperse. It often leads to peeling or whitening of the surface when the rubber product is demolded, and sulfur particles can be seen when the vulcanizate is stretched. The uneven dispersion of plasticizers will cause the compound to delaminate before molding, generate air bubbles, and reduce the vulcanization performance of the peroxide compound. Impurities in the compounding agent can cause loose rubber compounds and appearance defects of precision products. Inadequately mixed compounds should be re-mixed in a timely manner. Scorched rubber material is difficult to flow during the molding vulcanization process. Scorching of the rubber material can cause defects such as compactness, delamination, curling and distortion.
2.1.2 Rubber viscosity
The fluidity of the rubber compound in the cavity depends on the viscosity of the compounded rubber. The viscosity of the NBR compound is usually determined by the plasticizer and filler. It is difficult to reduce the viscosity of the rubber using plasticizers or prolonging the plasticizing time. The Mooney viscosity [ML (1 + 4) 100 ° C] of the NBR rubber for the molding method is 35 to 130. If the viscosity of the rubber material is small and the vulcanization pressure is not enough, the product will produce bubbles during vulcanization; if the rubber viscosity is large, it is not easy to fill the mold during vulcanization, and the product is likely to cause rubber shortage or wrinkles.
2.1.3 Film isolation
In order to prevent blocking, a release agent can be coated on the surface of the film when the NBR compound rubber is lowered, but stearic acid powders cannot be used because stearic acid is insoluble in NBR and will play an excessive lubrication role, which will affect the rubber during molding. When bonding. When the NBR compound is combined with a metal core, a powder release agent cannot be used, and the viscosity of the contact surface between the compound and the metal must be ensured.
2.1.4 Rubber compound parking
NBR rubber should be left for 16 hours after molding and then molded. The optimal time for parking is 24 hours. If the rubber material is not parked, bubbles and spots will appear on the product; if the parking time is too short, it will affect the physical properties of the product; if the parking time is too long, it will seriously affect the adhesion of the rubber material to the metal material.
2.1.5 Rubber size
The size of the rubber compound cannot be exactly the same as the size of the corresponding cavity. The quality of the rubber compound during molding should be greater than the rubber part of the product. The overflow of the rubber (or overflow edge) during molding will play a dual role in expelling the air in the mold cavity and maintaining the vulcanization pressure. effect. The amount of flashover should be strictly controlled according to the product size, molding method and mold structure. If the amount of glue is not enough to fill the mold cavity, it will cause lack of glue; if the amount of glue is too much, it will prevent the mold from closing and cause the product to be out of specification. For the general flat vulcanizing machine, the rubber compound of the separate mold should be filled by 15% -20%, the rubber compound pressed into the separate mold should be filled by 5% -10%, and the rubber compound of the push-in mold should be filled more About 2%.
2.2 Mould 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 it will even be scrapped after a few minutes of use. Therefore, the mold design must ensure product size stability.
的 The main factor affecting the dimensional accuracy of the product is the shrinkage of the rubber. In the case of small product size tolerances, the mold design must first consider the shrinkage of the rubber. The reason for the shrinkage of the rubber compound is that the thermal expansion coefficient of the rubber compound is greater than that of the metal mold material.
NBR products generally have a dimensional tolerance of 0.5% to 1.0%. The allowable tolerance of the mold size of American molded rubber products depends on the product. The tolerance of the mold size of general products is 0.127 mm, and the tolerance of the mold size of special precision products is 0.005 mm. And shape changes, solid product processing size tolerance is 0. 254 to 0. 635 mm, sponge product processing size tolerance is 0. 254 to 1. 520 mm. Pre-heating the rubber compound before vulcanization can effectively reduce the internal deformation of the rubber compound; after cooling, the shrinkage of the rubber compound exceeds the cavity, and the degree of shrinkage varies with the rubber composition (especially the type and amount of filler). The change is related to the internal stress before the rubber is molded. The effect of filler on the shrinkage of NBR molded products is shown in Table 1.
The cause of overflow can be attributed to insufficient or uneven vulcanization pressure, too small escape holes and too large cavity area. According to the design principles, the volume of the skin-escape hole should be 15-20 times the volume of the cavity to be sufficient to accommodate the overflow. If the overflow is required to be small, the stress between the cavity and the escape hole is small. In a mold with overlapping cavities, the distribution and thickness of the overflow edge vary greatly depending on the curing pressure. Generally, the thermal tear strength of NBR is lower than that of other synthetic rubbers. Therefore, the vulcanization pressure of NBR products should be appropriate, and care should be taken not to form a reversed cut when the finished product is ejected from the mold. In addition, the glue flow groove at the edge should be as close to 0.4 ~ 3.2 mm as possible near the end of the cavity.
2.3 Vulcanization conditions
2.3.1 Vulcanization temperature
BRThe compression curing temperature of NBR rubber is usually 140 ~ 200 ℃, the specific curing temperature depends on the physical properties, size and compression method of the product. Low temperature vulcanization can reduce product quality problems caused by scorch and mold contamination, etc., and can also make the product obtain better physical properties. Large-scale product parts need a relatively flat vulcanization process. At different vulcanization temperatures, the physical properties of the product can be optimized by adjusting the vulcanization time.
NBR compounds can be preheated effectively by a dielectric method. This pre-heating method can evenly heat up the rubber compound before molding, accelerate the vulcanization speed, and overcome the problems of uneven vulcanization and skew of thick products. Product skew is caused by the central part of the product surface being continuously vulcanized by heat after vulcanization.
BRNBR compound is vulcanized at high temperature, and each operation step needs to be performed quickly. For example, the compound can stay in the cavity of 153 ℃ for 2 minutes and then pressurize and vulcanize. In addition, with high temperature vulcanization, mold scale will also increase, thereby increasing the number of mold cleaning. Mold fouling is generally caused by the accumulation of crumbs and release agents.
2.3.2 Vulcanization pressure
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 kept stable. NBR rubber vulcanization pressure is generally 1.4 ~ 3.4 MPa.
The overfill of the finished product leads to waste products. This problem is often attributed to the mold design, which is actually caused by insufficient vulcanization pressure. Insufficient pressure during the molding process will often produce pores, external bubbles and deformation; excessive pressure may cause distortion.
NBR products can be segmented vulcanized, that is, the NBR rubber is first vulcanized and set in a flat vulcanizing machine, and then vulcanized in two stages.
2.3.3 Vulcanization time
Vulcanization time is the time that the rubber compound is vulcanized in the mold to obtain the best performance under a certain temperature and pressure. If the vulcanization time is too long, cracks will occur when the product is demolded, which can be solved by vulcanization at low temperature for a long time. If the vulcanization time is too short, the product is under sulfur, and it is easy to tear at the parting line. It can be solved by increasing the vulcanization temperature.
3 · Conclusion
的 The factors affecting the quality of NBR molded products are mutually restricted. When quality problems occur in NBR molded products, it is necessary not only to adjust the formulation factors, but also to find the reasons from the aspects of compounding rubber preparation, mold design and curing conditions, and propose improvement measures to ensure product quality and output.