- The main reasons for bubbles
Raw material problems
Water or low molecular volatiles (such as solvents and plasticizers) are mixed into the rubber material.
Uneven mixing, air or impurities inside the rubber material.
Improper storage of the rubber material, damp or expired.
Improper vulcanization process parameters
Vulcanization temperature is too high or too low: Too high temperature causes premature vulcanization of the rubber surface, and the internal volatiles cannot be discharged; too low temperature causes insufficient vulcanization and gas residue.
Insufficient vulcanization pressure: The gas in the rubber material cannot be discharged or the residual air is compressed.
Insufficient vulcanization time: The rubber material is not completely cross-linked, and the gas is not fully escaped.
Mold design or use problems
Improper mold exhaust structure (such as insufficient number of exhaust grooves or blockage).
The mold cavity design is complex, the rubber flow path is long, and the gas cannot be discharged in time.
Mold surface contamination (oil stains, mold release agent residues) causes the rubber flow to be blocked.
Operation problems
Improper loading method: The rubber material does not fully fill the cavity, forming air pockets.
The shape or size of the preformed rubber is unreasonable, resulting in air being trapped during mold closing.
The pressure of the vulcanizer is unstable or the holding time is insufficient. - Rubber silicone molding solution from CHINA
- Raw material control
Drying treatment: The rubber is preheated and dried before use (such as 60~80℃ oven treatment for 1~2 hours) to remove moisture and volatiles.
Optimize the formula: reduce the amount of volatile additives, or choose plasticizers/softeners with low volatility.
Mixing improvement: ensure uniform mixing, extend the thin pass time to exhaust the air inside the rubber; use a vacuum mixer. - Vulcanization process optimization
Adjust the vulcanization temperature: select the appropriate temperature according to the type of rubber (such as NR is usually 140~160℃) to avoid surface scorching caused by excessive temperature.
Increase the vulcanization pressure: increase the mold closing pressure (generally 10~20MPa) to ensure that the rubber is compacted and the gas is discharged.
Extend the vulcanization time: ensure that the rubber is fully cross-linked, and pressurize in stages if necessary (such as pre-pressing and exhausting before full-pressure vulcanization). - Mold improvement
Add venting grooves: open venting grooves (depth 0.01~0.03mm) on the mold parting surface or at the end of the rubber flow, and clean the blockage regularly.
Optimize the flow channel design: shorten the rubber flow path to avoid gas retention; use multiple gates for rubber injection.
Mold surface treatment: keep the mold clean, polish regularly, and reduce the amount of release agent (use internal release agent instead). - Operation specifications
Rubber preforming: cut the rubber into a shape that matches the mold cavity to reduce the amount of air wrapped when closing the mold.
Sectional mold closing: first close the mold at low pressure to exhaust the gas, and then vulcanize at high pressure.
Vacuuming assistance: use a vacuum vulcanizer to remove the gas inside the rubber before closing the mold. - Equipment maintenance
Regularly check the vulcanizer pressure system to ensure stable pressure.
Calibrate the temperature control system to avoid local temperature deviation.
III. Preventive measures
Process monitoring: Regularly check the moisture content of the rubber (such as using a moisture meter).
First article inspection: observe the position of bubbles during mold testing and optimize the process or mold in a targeted manner.
Environmental control: keep the humidity in the production workshop below 50% to prevent the rubber from absorbing moisture.
The bubble problem can be effectively reduced or eliminated by systematically checking the raw materials, processes, molds and operation links. If the problem persists, it is recommended to optimize the parameters by DOE (experimental design) in combination with specific cases.
:Rubber Auto Products
The generation of bubbles in rubber products is a common problem in the production process. Bubbles on the surface of the product not only affect the appearance, but also the quality. There are many reasons for the generation of bubbles in molded products, including vulcanization parameters, mold structure, operation process, water or gas in the rubber, thickness of the product, etc., so how should it be solved in actual production? Let’s share the common reasons and solutions with you. - Analysis of the causes of bubbles in molded products:
The main reasons are: - Material problems. There is moisture in the rubber material during mixing, storage and use. The moisture is not removed, resulting in bubbles, or dampness due to weather changes. Most of the reasons may be that there is a problem with the raw materials. Try another batch of materials; add degassing agent or moisture absorbent (calcium oxide) to the formula.
- There is gas wrapped in the rubber, and the gas is not easy to remove, and bubbles are generated during vulcanization.
- Improper setting or blockage of the mold vent hole will also produce bubbles, and the exhaust is not properly discharged (improve the rubber vent groove, and simplify the mold deconstruction as much as possible without leaving dead corners).
During the rubber refining, the air is wrapped inside the material, resulting in the material and air entering the mold together during the processing. If the mold does not have a vent groove or the exhaust process is not set during the processing, the air is easily trapped in the mold, causing bubbles or pores in the product. - In the production of rubber products, there are bubbles after vulcanization. It may be a problem in the rubber formula. The materials should be reasonably matched and materials that help exhaust (recycled rubber, rubber powder, degassing agent, calcium oxide, etc.) should be appropriately added.
- Poor bonding between rubber and metal will cause a large amount of gas to remain in the bonding area.
- Reasons for bubbles during vulcanization of rubber products
- Uneven rubber mixing and non-standard operation. According to the reasonable order of adding materials, the number of thin passes and the back pressure process after parking and aging.
- The rubber film is not parked in a standardized manner, and the environment is unsanitary. Management is not standardized. Foreign impurities.
- The material has moisture (add some calcium oxide during mixing)
- Insufficient vulcanization, it looks like bubbles when it is not mature.
- Insufficient vulcanization pressure.
- There are many impurities in the vulcanizer, and small molecular impurities are decomposed in advance, and bubbles remain in the product
- The exhaust design of the mold itself is unreasonable, and the gas cannot be discharged in time when the rubber material is punched!
- The product is too thick, the rubber material is too little, the rubber heat transfer is slow, and after the surface is vulcanized, the rubber fluidity decreases, causing material shortage, so bubbles may be generated. Adjust the vulcanization rate, not too fast.
- The exhaust is not discharged well during the vulcanization process. Improve the number of exhausts and the exhaust time interval.
- Formula problem, the vulcanization system needs to be improved. It is recommended to choose low temperature and slow speed (relatively).
Solution: Improve the vulcanization pressure and time
a. Extend the vulcanization time or increase the vulcanization speed. Reasonable vulcanization temperature range.
b. Thin it several times before vulcanization. Park it for aging and then pressurize it.
c. Exhaust more times during vulcanization. Arrange the exhaust time interval reasonably.
d. Reduce the Mooney of the mixed rubber and choose a rubber with low air tightness (raw rubber or formula combination reduces air tightness). - Poor rubber-metal bonding will cause a large amount of gas to remain in the bonding area, and the gas will shrink when pressurized.
