1. Introduction to vulcanization method
The rubber molecules are cross-linked to form a network structure, thereby improving the performance of the rubber. This change is called “vulcanization”. Natural rubber was originally vulcanized with sulfur, where the term “vulcanization” comes from. According to different vulcanization methods, it can be divided into hot vulcanization, cold vulcanization, high-frequency vulcanization, high-energy ray radiation vulcanization, etc.
(1) Hot vulcanization
Thermal vulcanization is the vulcanization of rubber by heating it with electricity or steam. Used to vulcanize various model products, various tapes, rubber sheets, tires, tapes, etc.
(2) Cold vulcanization
Cold vulcanization is the vulcanization of rubber at room temperature and is mostly used for products bonded with natural vulcanization glue. The glue is equipped with a highly active super accelerator. Products related to this glue can be vulcanized by being left at room temperature for a period of time. Especially large-scale tape products, such as rubber water pipes, capsules, etc., often use this method. Rapid natural vulcanization glue can also be produced to achieve a very high bonding strength in a short time.
Another cold vulcanization method is impregnating sulfur-free products with a dilute solution of 2%-5% sulfur monochloride for vulcanization. Because it can only be used in film products and sulfur monochloride is highly toxic, it is rarely used in the aviation industry.
(3) High frequency vulcanization
Under the action of a uniform high-frequency alternating electric field, the rubber molecules move with the changes in the electric field. Due to the movement, mutual friction, and displacement between the molecules, the rubber compound generates heat, thereby achieving the purpose of vulcanization.
(4) High energy ray radiation vulcanization
Using Co 60 to emit gamma rays as the energy source at room temperature, the radiation energy can detach the hydrogen atoms of the rubber molecules, generate free radicals on the rubber chain, and produce cross-linking effects, causing the linear molecules of the rubber to form a network structure, completing the vulcanization process. High-energy ray radiation can vulcanize rubber without vulcanizing agents and accelerators and obtain excellent physical properties. Due to its high cost, it is only used for special products. But it can simplify the process and speed up production, so it has development prospects.
2. Introduction to the molding vulcanization process
(1) Hot refining
The rubber should be heated before vulcanization to destroy the gel produced when it is parked and improve the plasticity and uniformity of the rubber. The method is to cut the rubber material into small pieces and use a rag soaked in aviation gasoline or benzene to wash away the dirt, dust, and related marks on the rubber material. Let dry at room temperature for 10-15 minutes. Then heat it on the rubber mixing machine for 3-5 minutes and thin it several times to make the surface of the rubber material soft and smooth. Then, cut into slices according to the specified size. The heat-refined rubber should be used within 12 hours. Outdated rubber materials can be blended into rubber materials of the same brand, batched at an amount of 30%, and then reheated before use. The heat-refined rubber should be soft and smooth, without large wrinkles, pits, bubbles, and other defects.
(2) Blanking
Use a punch, scissors, or performing machine to make the rubber material into a blank of a particular shape and size. Its shape should be similar to the mold cavity, and its weight should be accurate ( generally 5%-10% heavier than the net weight of the part).
(3) Preparation of metal parts
Blowing sand
In order to increase the adhesion between rubber and metal, the surface of metal parts used for molding must be free of rust, oil, and oxide layers and must be hand-blown. After sand-blowing treatment, the surface of the parts should be rough, uniform, and without metallic luster, and they should be immersed in clean gasoline for later use. The best use period is within 48 hours. When sand blowing is not allowed on non-machined surfaces, special fixtures should be used for protection. When sand blowing, the size of quartz sand particles and the pressure of compressed air should depend on the specifications of the parts and the surface condition of the metal parts. The quartz sand used must be screened dried, and must not contain oil. The sand blower must be equipped with an oil trap to remove the oil in the compressed air (regular inspection and analysis).
Brass plated
Brass plating is used to increase the bonding strength between rubber and metal parts. When plating brass, the composition, pH value, and temperature of the plating solution must be strictly controlled. Brass-plated metal parts should not be stored for too long, otherwise, the combination with rubber will be affected, so it is required to be used immediately after brass plating. The brass layer should be free of bubbles, spots, and oil stains. The brass-plated parts should be immediately placed in aviation gasoline and stored for later use (the storage time should comply with the process regulations)
(4) Vulcanization
The rubber molding process includes loading, mold closing, vulcanization, molding and mold cleaning. As mentioned before, the rubber material should be heated before vulcanization, and the blank should be cut into appropriate shapes and sizes and then put into a mold preheated to the specified temperature for vulcanization. The blank assumes a plastic state under heat and pressure, fills the mold cavity, and becomes a vulcanized product after being maintained for a certain period of time. When molding rubber products, in order to eliminate the air in the mold cavity and volatile matter in the rubber material to avoid defects such as stuffiness in the product, pre-pressure can be performed 2-3 times continuously. This operation is called “deflation”. Whether deflation is required depends on the rubber material used, the shape of the product, and the structure of the mold. Affecting vulcanization quality are vulcanization time, temperature, and pressure.
Vulcanization time
When raw rubber containing sulfur is heated and vulcanized, a chemical reaction will occur between the sulfur and the raw rubber to produce structural changes. The amount of bound sulfur is related to the vulcanization time. When the temperature is constant, free sulfur decreases as the reaction time increases; that is, the amount of bound sulfur gradually increases. If the time is not enough, the product will be under sulfur; if the time is too long, the product will be over sulfur. Both of these are detrimental to the finished product and should be avoided.
The actual dosage of sulfur depends on the characteristics of the product. If you use too much, it will become hard glue; if you use too little, it will not have enough hardness and strength. For soft glue, the dosage is preferably less than 5%. In fact, part of the added sulfur will always remain in the vulcanized rubber. The residual sulfur should not be too much. Otherwise, if it exceeds a certain solubility, it will crystallize or penetrate into the surface of the vulcanized rubber and cause sulfur spray (when the sulfur dosage is appropriate and the vulcanization conditions are If not used properly, sulfur spraying may also occur ).
Spraying sulfur will directly affect product quality. Sometimes blooming occurs. The occurrence of bloom will reduce the bonding force between the rubber material and the parts, causing inconvenience to the molding of multi-layer products. When a bloom occurs, the solvent can be used to wash away the bloom. If blooming is severe, rubber solvent can be used to moisten the rubber surface before processing. However, in some cases, blooming is beneficial. The film formed by blooming can protect the rubber surface from erosion by sunlight and air. The blooming caused by adding excessive antioxidants is more resistant to aging.
Generally speaking, the problem of products blooming is not a big problem, but the problem of products spraying sulfur should arouse vigilance, and the cause should be found out (whether there is too much sulfur in the rubber compound or the vulcanization conditions are not strictly controlled), and it can be eliminated quickly.
Vulcanization temperature
Temperature changes have a great impact on the vulcanization reaction speed. If the vulcanization temperature increases by 10 °C on average, the vulcanization speed can be doubled; that is, the vulcanization time can be shortened by about half. As the vulcanization temperature increases and the vulcanization speed accelerates, the production capacity of the equipment will increase. However, the vulcanization temperature cannot be too high because, during high-temperature vulcanization, oxygen dissolved in the rubber can easily cause an oxidation reaction of the rubber, which will reduce the aging resistance of the product. If there are fibrous materials (cotton yarn, rayon, etc.) in rubber products, they will be destroyed after being exposed to high temperatures, greatly reducing the strength of the product. At the same time, due to the poor thermal conductivity of rubber, it is also very difficult to make products reach high temperatures evenly.
Vulcanization pressure
During the vulcanization process, the reaction between the vulcanizing agent and the raw rubber molecules is a non-gas phase reaction, and the pressure has little effect on the reaction speed. However, the rubber often contains moisture, adsorbed air, dissolved gases, etc. During the vulcanization process, the water evaporates due to the heating of the rubber, the decomposition of some compounding agents, and the hydrogen sulfide and other gases released by the vulcanization side reaction, which will cause vulcanized products to produce Porosity or degumming defects. Vulcanization under pressure can eliminate these defects. At the same time, the density of the product is increased due to pressure, and the wear resistance and aging resistance are also improved. Of course, the pressure should not be too high. If it is too high, the performance of the vulcanized rubber will not improve much, but more equipment power will be consumed. It should be noted that if the vulcanization pressure fluctuates too much, it will affect the dimensional consistency of the product. Generally, the specific pressure required for the vulcanization of rubber products is 15MPa-25MPa for thick products and 5MPa-15MPa for thin products.
Treatment of problems found during the vulcanization process:
a) After vulcanization starts, if the temperature can rise to the specified minimum vulcanization temperature within 15% of the vulcanization time of the part, the vulcanization time of the mold part does not need to be increased.
b) After vulcanization starts, if the temperature cannot rise to the specified minimum vulcanization temperature within 15% of the vulcanization time of the part, the vulcanization time of the mold part should be increased (increase the actual heating time by one-half).
c) After vulcanization starts, the mold part will be scrapped if the temperature cannot rise to the specified minimum temperature within 50% of the part’s vulcanized time.
3. Rubber product inspection standards
(1) Appearance inspection
In the vulcanization process, the first mold parts of each work shift undergo visual inspection, while for finished product acceptance, 100% of the parts undergo visual inspection. The method of appearance inspection generally adopts visual inspection. When checking the finish of the working surface of the part, the finish specified on the mold drawing is usually used as the standard.
(2) Dimensional inspection
The vulcanization process is subject to first mold inspection; finished product acceptance is generally subject to random inspection. However, for parts with strict size requirements or parts that require mechanical processing and parts that need to be stably processed in the working medium, in principle, 100% of the main dimensions should be inspected.
Have been vulcanized and deburred and need to be parked for 6 hours before dimensional inspection. Rubber parts have large elastic deformations. If the measurement method or measuring tool is improperly selected, the accuracy of the measurement will be affected. General measuring tools and instruments should be used for general trial mold identification and trial production, while special measuring tools should be used for batch production.
(3) Performance test
a) In the Sauer Type A hardness test, the thickness of the sample should not be less than 6mm, the width should not be less than 15mm, and the length should not be less than 40mm. If the thickness does not meet the requirements, the same samples can be overlapped for measurement, but no more than four layers are allowed, and the top and bottom must be parallel.
b) Oil swelling resistance test, immerse the sample in oil, acid, alkali, or other liquid media, and measure the changes in size, volume, or weight at a specified temperature after a certain period of time.
c) Test of rubber-metal bonding strength, cross-section method. Generally, a ø3 steel needle is used to pick the rubber on the metal part. If there are uniform rubber bonding marks on the surface of the metal part after the rubber is picked and broken, the bonding strength is considered qualified. For each working class, 1-3 parts are generally selected for bonding strength inspection during the first mold inspection and finished product acceptance.
