Process factors affecting the quality of NBR moulded products
🕙 2023-02-14 👆
Nitrile butadiene rubber (NBR) is mainly used to prepare rubber products. The thermoplasticity of NBR increases with the increase of temperature and acrylonitrile content. Good thermoplastic rubber molding fluidity, suitable for the preparation of molded products. Moulded rubber products are mixed with certain shape and quality into the mold, vulcanized on the plate vulcanizing machine, and then demoulding, trimming and made. The quality of moulded rubber products is generally determined by the compound formula and processing technology. This paper mainly discusses the technological factors affecting the quality of NBR moulded products.
●Common quality defects of NBR moulded products
The purpose of molding is to make rubber products have a certain shape and physical properties, and avoid surface defects. The common quality defects of NBR moulded products are as follows: tear at the parting line, surface flow marks, not smooth, spray, warped or rolled up, heavy skin cracks, internal bubbles, excessive size shrinkage, tear during demoulding.
●Factors influencing the quality of NBR moulded products
Poor dispersion of the compound will affect the physical properties of NBR moulded products. For example, sulfur in NBR solubility is small, not easy to disperse, often lead to rubber products stripping edge peeling or surface white, when vulcanized rubber stretching can also see sulfur particles. The uneven distribution of the plasticizer will cause the declamation of the compound before molding, produce bubbles, and reduce the vulcanization property of the peroxide compound. Impurities in the compound will lead to loose rubber and appearance defects of precision products. Insufficiently mixed compound should be remixed in time. Coke burning compound is difficult to flow in the process of molding vulcanization, and coke burning will lead to defects such as uncompaction, delamination, shrinkage and distortion of the product.
The fluidity of the compound in the mold cavity depends on the viscosity of the compound. The viscosity of the NBR compound is usually determined by the plasticizer and the filler. The use of plasticizer or extension of plasticizing time is difficult to reduce the viscosity of the rubber, for the molding method of NBR adhesive Mooney viscosity [ML (1+4) 100 ℃] is 35 ~ 130. If the viscose viscosity is small, curing pressure is not enough, curing products will produce bubbles; If the viscosity of the rubber material is large, it is not easy to fill the mold when vulcanized, and the products are easy to cause the lack of rubber or marks.
In order to prevent adhesion, NBR rubber under the film surface can be coated with isolator, but can not use stearic acid powder, because stearic acid is insoluble in NBR, will play the role of excessive lubrication, affecting the adhesive in the molding. When the NBR adhesive is combined with the metal core, the powder separator can not be used, and the viscosity of the adhesive and the metal contact surface must be ensured.
The size of the rubber material can not be exactly the same as the size of the corresponding cavity. The quality of the rubber material should be larger than that of the product. The overflow of rubber (or the overflow edge) during the molding will play the dual role of removing the air in the cavity and maintaining the vulcanization pressure. 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, can not be full of mold cavity, will cause a lack of glue; If the amount of glue is too much, it will hinder the mold closure, resulting in substandard products. For the general plate vulcanizing machine, the rubber material of the separate mold should be loaded 15% ~ 20% more, the rubber material of the pressed separate mold should be loaded 5% ~ 10% more, and the rubber material of the pressed mold should be loaded about 2% more.
●Mold design
NBR is widely used in the manufacture of O-rings, U-rings, seals and washers. When the product size tolerance exceeds the specified tolerance range, its service life will be significantly shortened, even a few minutes of use will be scrapped. So mold design must ensure the stability of product size.
The main factor affecting the dimensional accuracy of the product is the shrinkage of the rubber material. In the case of small dimensional tolerance of the product, the mold design should first consider the shrinkage of the rubber material. The reason for the shrinkage of rubber is that the thermal expansion coefficient of rubber is greater than that of metal die material.
The dimensional tolerance of NBR products is generally 0. 5% to 1. 0%. The allowable tolerance for die size for moulded rubber products in the United States varies with the product, with a general product die size tolerance of 0. 127 mm, special precision products mold size tolerance is 0. 005 mm; The machining dimensional tolerance of rubber products varies with the change of rubber type, formula, size and shape, and the machining dimensional tolerance of solid products is 0. 254 ~ 0. 635 mm, sponge product machining size tolerance is 0. 254 ~ 1. 520 mm. The high frequency preheating before vulcanization can effectively reduce the internal deformation of the rubber; After cooling, the shrinkage rate of the rubber exceeds the mold cavity, and the shrinkage degree varies with the composition of the rubber (especially the variety and dosage of the filler), and is related to the size of the internal stress of the rubber before molding. The effect of filler on the shrinkage of NBR moulded products is shown in Table 1.
●Conclusion
The factors influencing the quality of NBR moulded products are mutually restricted. When NBR moulded products have quality problems, we should not only adjust the formula factors, but also find out the reasons from the aspects of compound preparation, mold design and vulcanization conditions, and put forward improvement measures to ensure product quality and yield.
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