Abstract: We take the shock-absorbing bushing product as an example. This paper systematically analyzes the necessary conditions to avoid degumming quality problems when the rubber and skeleton are bonded using adhesives and proposes methods and measures to prevent degumming issues.
Shock-absorbing products that combine rubber and metal are widely used to ensure the comfort of car rides. These include shock-absorbing structural products that combine natural rubber and metal with adhesives that bond rubber and metal. After the product is vulcanized, if the bonding is poor, it is easy to cause early wear during the use of the product, produce abnormal noise, and transmit vibration, causing the shock absorption function to fail and unable to bring the intended comfort to the driver and passengers.
1. Analysis of the causes of sleeve degumming
Structural form of shock-absorbing bushing
Shock-absorbing bushing products are mainly composed of an outer frame, natural rubber, and an inner frame. The production process generally includes skeleton pre-treatment, adhesive application, vulcanization, and diameter reduction detection. In the case of good vulcanization and good bonding, the use requirements of the shock absorber bushing can generally be guaranteed, and the shock absorber bushing does not need to be replaced when the car travels 30,000 to 60,000 kilometers. However, in the case of poor bonding, early wear and tear failure is likely to occur within 20,000 kilometers of driving.
Analysis of the causes of degumming
The bonding conditions between natural rubber and metal skeleton are very strict. Poor bonding will occur in the following aspects, causing quality problems such as degumming of the shaft sleeve.
The surface cleanliness of the skeleton pre-treatment does not meet the requirements.
During the production process, a large amount of grease is often attached to the surface of the metal skeleton during storage, and the structure itself is also rusted. If these two types of pollutants are not thoroughly cleaned, a clean metal surface will be formed on the surface of the metal skeleton, and the adhesive will not be able to adhere to the skeleton’s surface. Developing an effective bond between metal skeleton adhesives is difficult without sufficient combination, diffusion, and infiltration.
The inner and outer frames are degreased and shot blasted before use to remove surface debris and oil. But sometimes, after shot blasting, it is found that there are still oil stains on the skeleton’s surface. During further investigation, it was found that the pellets had been used for a long time to process a large number of oily products, and the pellets were already full of oil. After the shot blasting, the skeleton’s surface was contaminated by secondary oil pollution.
Effect of surface roughness of skeleton pre-treatment on bonding
The material of the inner frame is mostly 45# steel or 20# steel that has been tempered and carburized. In order to increase the bonding area and strength, the surface of the skeleton is generally shot blasted to increase the outer area of the structure. However, when inspecting some degummed products, it was found that the 20# steel inner frame that had been tempered, carburized, and shot blasted with chromium alloy sand did not meet the surface roughness requirements of Ra45~60 and was only Ra20, with a smooth surface.
The honeycomb surface is not formed, resulting in insufficient contact area between the metal skeleton and the adhesive, resulting in low bonding strength of the product and degumming problems. When analyzing this problem, it was found that after the tempering and carburizing treatment, the hardness of the skeleton can reach 45HRC. The hardness is too high, and the high-speed pellets cannot hit dents on the surface. And the more they hit, the smoother it becomes, and it cannot form a practical honeycomb shape. The surface cannot meet the bonding requirements.
diameter reduction on bonding after surface treatment of exoskeleton
After the product was made, when the bushing was pressed and degummed, it was found that the primer adhesive was partially peeled off from the surface of the outer frame, and the rubber itself was not in an ideal state. When investigating the reasons, it was found that in order to improve the anti-rust performance of the exoskeleton, some exoskeletons need to be phosphate on their surfaces before applying adhesive so that a dense phosphating film is formed on the metal surface. When the outer frame of the shaft sleeve is reduced in diameter, there will be a change in the outer diameter of 2 to 5%. However, when the outer diameter is squeezed, the phosphating film will be damaged with the extrusion deformation, reducing the relationship between the frame and the phosphating film. Adhesion, thereby reducing product bonding strength.
The impact of improper painting methods on bonding
The edge film thickness of the outer frame does not meet the painting requirements, resulting in low bonding strength between the adhesive and the frame, resulting in degumming problems in the product.
2. Take preventive measures and countermeasures
After shot blasting, degreasing and drying are performed. The degreasing treatment can be placed after the shot blasting treatment, and the skeleton can be degreased and dried after shot blasting, effectively avoiding secondary pollution of the skeleton.
Before the heat treatment of the frame, the frame is first shot blasted to meet the surface roughness requirements and then tempered and carburized to improve the stiffness of the inner frame. Shot blasting is performed later to remove oxide scale or debris on the surface.
Arrange the phosphating treatment after the diameter reduction process. Generally, the purpose of phosphating the skeleton surface is to improve the corrosion resistance of the skeleton surface. This process can be arranged after the product is reduced in diameter. While meeting the anti-rust requirements, the adhesive is directly bonded to the metal surface of the frame to achieve good bonding performance requirements. The new process technology of phosphating protective treatment after vulcanization of metal-rubber shock-absorbing products has no adverse effects on the physical properties of NR vulcanized rubber and steel-NR bonding properties and has no significant impact on the appearance and fatigue performance of the product, which is completely satisfactory. Production technology requirements.
The operator must change the painting method when painting the edge or even change the painting method to the cutter twice to meet the standard film thickness requirements and increase the film thickness detection at the edge. After improvement, the film thickness requirements are met.
3. Conclusion
(1). The bonding strength between the metal skeleton and rubber is the main reason that affects the degumming of the shaft sleeve. The surface cleanliness, roughness, surface treatment, and adhesive application methods of the metal skeleton can all improve the bonding strength and prevent degumming problems.
(2). We analyzed the factors in the product manufacturing process and came up with methods to prevent degumming, which provided a basis for the process design of the product. The product process requires continuous improvement and innovation to ensure stable batch quality of the product. At the same time, attention must be paid to the details of the process, and standardization must be formed to solidify the improvement results.
