Blister defects in rubber products are a frequent problem in factories and cost many companies millions of dollars each year.
1. Curing Pressure
Always make sure that the rubber is vulcanized under sufficient pressure, otherwise air bubbles are likely to appear.
2. Bubbling Out
During the vulcanization of rubber products, it is important to increase the vulcanization pressure to ensure that the bubbles are in a compressed state.Sufficient pressure will keep the bubbles at a very small size, which will reduce the appearance of bubble holes.Bubble out point is the shortest vulcanization time point of the rubber product without the appearance of bubble holes.If the bubbles appear before the center part of the rubber product has reached 30% of the optimum vulcanization time, then the size of such bubbles is almost invisible to the naked eye.Of course, 30% is a very rough estimate and varies with the type of rubber, e.g. some types of rubber may be 25%, while some types of rubber with a very low modulus of vulcanization may be as high as 98%.During the vulcanization process, the modulus will continue to increase and the bubbles will become less likely to grow.Sometimes bubbles will appear after vulcanization, but they are so small that they are not visible to the naked eye.
3. Stepwise Decompression Vulcanization of High-Pressure Tanks
For rubber products vulcanized in high-pressure tanks, in order to avoid the appearance of blisters, the practice of gradually reducing the vulcanizing pressure in the later stages of vulcanization is called "gradual decompression vulcanization".During this gradual pressure reduction, the encapsulated gases gradually seep out of the rubber.
4. Peroxide Sulphation
For peroxide vulcanized rubber compounds, due to the high vulcanization temperature and rapid vulcanization speed, the gas cannot be discharged in time, and the surface of the product is prone to bubble defects. A feasible solution is to reduce the vulcanization temperature.
5. The Influence of Dispersion on Cell Structure
To ensure that the adhesive components, especially filler dispersion is uniform, poor dispersion will cause vesicle problems, this is because the undispersed particles can be used as a vesicle formation of the bubble nucleus.
6. Poor Dispersion and Overplasticization
Excessive plasticizing of the raw rubber can cause blistering problems, on the other hand, uneven dispersion can also lead to blistering problems.
7. Avoiding Moisture in Raw Materials
The following materials should be protected from moisture as much as possible:
1)Raw rubber.
2)Carbon black.
3) Silica.
4) Clay.
5) Nylon fiber.
6) Fibrin fiber.
7) Stearic acid.
8) Antioxidant.
The raw rubber and the various compounding agents should be kept in a dry place and sometimes even dried before use.
8. Avoidance of Volatile Components in Raw Materials
Avoid the use of process oils, filler oils and plasticizers with high volatility.
9. Dehumidification of Compounded Materials
The moisture should be removed from the placed compounds before subsequent processing.Here are some suggestions :
1 ) For dip board adhesive, allow enough time for dehumidification.
2 ) Place the compound in a dry place.
3 ) Do not leave the mixing rubber for too long, e.g. more than 2-3 days.
4 ) If necessary, put the compound back into the kneader to remove the moisture.
5) Consider adding calcium oxide as a desiccant, but also consider that calcium oxide may reduce some of the physical properties of the vulcanized rubber.
10. Process
Consider the following options to minimize the appearance of blisters :
1 ) Reduce the thickness of the calendered film to avoid inclusion of gas, and then stack the thin film in multiple layers to achieve the required thickness.
2 ) Prepare thick film directly with a roller die head instead of stacking multiple layers in a calender.
3 ) Avoid high preform pressures when molding.
4 ) When molding, avoid oversized molds (compared to vulcanized plates).
5 ) When molding, avoid too long curing time.
6 ) Extrusion or injection molding, to avoid the viscosity of the rubber material is too low, so that it is easy to pack into the air
7 ) extrusion or injection molding, to avoid the rubber material too strong shear thinning behavior.
8 ) When injection molding, avoid too small injection volume.
9 )When injection molding, avoid too short residence time in the mold.
10)Choose the extruder with vacuum exhaust pipe.
11) The screw speed in the extruder should be slow.
12)Pressurizing the outside of the steam high-pressure vulcanizing tank can reduce the chance of air bubbles in the rubber.
13) Vulcanization with hot air or steam can provide a certain pressure, so that the rubber is vulcanized at a lower temperature and reduce the bubble generation.
14) When vulcanizing in high pressure tanks, the outer sheath can maintain a certain pressure to reduce the generation of air bubbles.
15) In continuous vulcanization at atmospheric pressure, to avoid high vulcanization temperature.
16) In continuous vulcanization, the use of "pressurized liquid intermediary" can be considered.
17) Continuous vulcanization can make the rubber material to form a "cross-linked surface layer", this surface layer can reduce the production of air bubbles.
This layer reduces the formation of air bubbles, and the faster the vulcanization rate, the better the cross-linked layer.
11. Control the Extrusion Temperature
When using an extruder with a vacuum vent, higher temperatures may be considered to discharge volatile components as quickly as possible.However, extruders without vacuum vents should be operated at lower temperatures to ensure a higher viscosity of the compound and to effectively avoid air bubbles.
12. Talcum Powder
It is reported that the use of talcum powder in rubber compounds can effectively reduce the gas trapped. However, if talcum powder is not well dispersed, it may instead cause the formation of bubbles.
13. Sulfurized Rapeseed Oil
Lue's research found that using vulcanized rapeseed oil as a compounding component of the rubber reduced blistering.
14. Vulcanizer Bleeding
Parts tend to pack in gas during preform, so opening the vulcanizer to bleed off the gas can reduce the generation of air bubbles to some extent.This is done by slightly opening the vulcanizer to bleed the air and then closing it again.
15. Molding under Vacuum
When molding, a vacuum device is applied so that the encapsulated gas can be vented.
16. a Ventilation Shaft
For some rubber parts with complex shapes, when molding, it is generally necessary to consider setting exhaust holes on the equipment.
