How to fix bubbles in the middle of an extruded rubber hose

Analysis of the reasons for bubbles on the surface of rubber products: 1. Presence of moisture (add calcium oxide during mixing). 2. Incomplete vulcanization; the product appears to have bubbles due to insufficient curing. 3. There are many sulfiding agent impurities; small-molecule impurities decompose prematurely, leaving bubbles in the product. 4. The mold’s exhaust design is inadequate, preventing air from being discharged promptly when the rubber material is extruded An effective rubber additive. Article source: China Rubber and Plastic Pipe Industry Network. A challenge arises during the production of HNBH rubber compounds: due to the different origins of the raw materials used, their quality varies. Even though materials that are prone to absorbing moisture, such as fillers and activators, are dried, it is still difficult to control the quality of the raw materials; if not careful, the rubber compounds can contain moisture. When products are vulcanized using this rubber compound, water evaporates rapidly into vapor at high temperatures. Rubber has extremely poor air permeability; gases get trapped inside the product, forming bubbles, which results in a waste rate as high as 57.1%. Due to the high cost of this rubber compound, it results in huge economic losses. Various attempts were made to achieve this by reprocessing the rubber compound at higher heating temperatures to evaporate the moisture, but with little success. I thought again of CaO, which is the main component of lime; it can absorb CO2 to form CaCO3, and it can absorb moisture to form Ca(OH)2. However, ordinary CaO and Ca(OH)2 are difficult to disperse evenly in the rubber compound, resulting in white spots always appearing in the rubber compound or vulcanized product, which affects the quality of the product and is not acceptable. Ultimately, the following experiments were conducted, yielding good results. 1 Tests 1.1 Main raw materials: Hydrogenated nitrile rubber (HNBR), Therban C3446 with an AN content of 33.5%, produced by Bayer Germany; kaolin (clay, pottery clay, white clay) with a particle size of 1250 mesh, produced in Beijing; Rhenogran CaO-80, in granular form, containing 80% CaO and having a density of 2.2 g/cm3, produced by Rheinchem (Qingdao) Co., Ltd.; all other materials are commonly used in the rubber industry. 1.2 Sample preparation and testing: 10 parts of Rhenogran CaO-80 were added to the mixed, moisture-containing rubber compound, and samples were prepared and tested using conventional methods; the results are shown in Table 1. 2 Results and Discussion Since Rhenogran CaO 80 is a pre-dispersed rubber chemical, it is a composite formed by CaO combined with thermoplastic elastomers and special dispersants, the latter two of which are highly compatible with rubber. In this way, CaO can also be easily dispersed uniformly within the rubber compound along with TPE and dispersants. The reason why CaO and Ca(OH)2 can absorb moisture in the rubber compound so effectively is not simply due to physical adsorption, because in that case the moisture in the compound would only be transferred to CaO or Ca(OH)2; it would still remain as moisture, and bubbling would occur during the vulcanization of the compound. The key here is that CaO can absorb water and react with it to form Ca(OH)2; as long as sufficient CaO is added to the rubber compound, all the moisture present in it can react with CaO, and no bubbles will be formed during vulcanization. The resulting Ca(OH)2 is also an extremely fine solid powder that contains no moisture; it is non-toxic and tasteless, with a density of 2.24 g/cm3. It can be well dispersed in rubber compounds, and due to its strong alkalinity and hygroscopicity, it serves as an inorganic accelerator, activator, and humectant for rubber. During the vulcanization of rubber compounds, it absorbs additional moisture, thereby preventing the formation of bubbles. In fluororubber and other halogen-containing rubber compounds, Ca(OH)2 is sometimes intentionally added as an acid absorber to increase the cross-linking rate and the degree of vulcanization, reduce the value of compressive set, and extend the service life of the products, especially sealing products. Based on this, adding 10 parts of Rhenogran CaO80 to the above-mentioned rubber compound, which forms severe bubbles during vulcanization due to moisture, yielded exceptionally significant results. Through comparison, the product qualification rate increased from 42.9% in the past to 95.9%. It has no adverse effect on product performance and has achieved good economic results. During production, the additives such as fillers and activators, which are used in relatively large quantities, are prone to absorbing moisture and do not deteriorate even when heated at 80°C for several hours, are dried first. Then, appropriate amounts of desiccants are added to prevent moisture from entering the rubber compound, which thus helps to completely avoid the formation of bubbles during the vulcanization process. The addition of CaO improves the cross-linking rate of the rubber compound, the degree of vulcanization, and the properties of the vulcanized rubber, without any adverse effects; generally, 3 parts are sufficient. 3 Conclusion Rhenogran CaO-80 is a rubber additive that requires only a small amount to achieve significant effects; adding a modest quantity (such as 3 parts) to the compound used in the production of high-precision rubber products can effectively eliminate bubbles formed during vulcanization, thereby significantly improving the quality rate of the products and economic efficiency. I don’t know how to attach files; Table 1 wasn’t sent. Last edited by QXZ-1966 on 2008-7-30 14:41]

Is it because the exhaust design of the mold itself is unreasonable?

It is recommended to use vacuum pumping during extrusion

The original poster explained it too simply, just mentioning extruding the rubber tube. What kind of rubber do you need? Silicone rubber, or EPDM, or CPE, or something like that. How do you carry out vulcanization – with hot air or continuous vulcanization? Otherwise, how can one guess what the reason is!

The second floor is talking about surface bubbles, while the original poster is referring to bubbles inside; they’re not the same! Ask the original poster whether it’s the double-two-four hot-air vulcanized silicone rubber that’s having the problem

The rubber hose extrusion process does not require vacuum processing; blistering in the rubber, whether on the outside or inside, generally occurs after vulcanization. It is very likely related to your rubber mixing method – either the dispersion is not proper or the additives have become damp. Additionally, if there are issues related to the formulation, it is necessary to consider the type of rubber compound being used, and make appropriate adjustments to the choice of additives in order to resolve the problem.

Agree with the person above. As a supplementary note, the original poster can try using a sieve with larger mesh size, which helps to remove air mixed in the rubber compound.