What are the rubber molding solutions -1?

I. Common Types of Bubbles Before addressing the problem, quickly locating the "source" of the bubbles is crucial. 

Based on international experience, bubbles typically present three typical forms, each indicating a different cause: 

1. Single large bubble (often located in the final filling area): This usually indicates poor mold venting or improper pre-forming shape 

of the rubber compound, causing air to be physically "locked" inside the cavity. 2. Shrinkage cavities in the center of thick-walled areas: 

These are often not actual gases, but rather vulcanization shrinkage defects. This is because the rubber compound shrinks after vulcanization,

 and the already vulcanized outer skin cannot compensate, resulting in a vacuum cavity in the center. 

This is related to a high shrinkage rate in the formulation and rapid initial vulcanization (surface scorching first). 

3. Randomly distributed microbubbles: These usually originate from moisture or volatiles in the raw materials.

 Moisture vaporizes at high temperatures, or certain low-molecular-weight additives decompose,

 producing a large amount of gas that cannot be released in time.

II. Material and Formulation Factors: The rubber compound itself is both the main body of the finished product and a major source of gas. 

Improper control of the formulation and raw materials is tantamount to planting "gas landmines" at the source.

 1. Moisture and Volatile Matter Moisture absorption by raw materials during storage, 

or the presence of low-volatile components in plasticizers and resins in the formulation, 

are the most common causes of microbubbles. When the mold temperature rises (usually above 100℃),

 moisture vaporizes, causing a rapid expansion in volume. If the rubber compound has already begun cross-linking at this time,

 the gas cannot escape, thus forming bubbles. Solutions: Physical drying: Preheat and dry the rubber compound before

 placing it into the mold, for example, by treating it in an oven at 60~80℃ for 1~2 hours, which can effectively remove free moisture. 

Chemical desiccant: Add calcium oxide (a desiccant) to the formulation. 

Calcium oxide reacts with trace amounts of water to produce calcium hydroxide, chemically consuming moisture, 

and the reaction product is a solid that does not produce gas. Raw material optimization: 

Select plasticizers and resins with low volatility to reduce the source of gas. 2. Poor dispersion of compounding agents and impurities Uneven mixing,

 especially the clumping of small components such as sulfur and accelerators, not only causes localized weak physical properties, but the interfaces of these

 "small particles" are often a breeding ground for air adsorption. In addition, impurities or scorched rubber particles cannot fuse with 

the main body during vulcanization, forming air-like voids. Solutions: Strictly adhere to the feeding sequence to ensure adequate dispersion 

of compounding agents. Perform a thin pass through the compound to break up agglomerates and expel trapped air from 

the compound using the shear force of the roller gap. After mixing, the compound needs to be left to mature 

(e.g., NBR recommends a maturation period of at least 16 hours) to relax stress and allow internal gases to continue escaping.

 3. Compound Viscosity and Mooney Viscosity

Compound viscosity (Mouney viscosity) has a dual impact on venting. If the viscosity is too low,

 the flowability under vulcanization pressure is too good, easily blocking the venting channels and preventing air from escaping;

 if the viscosity is too high, flow is difficult, trapping air and forming bubbles. Solutions: 

Adjust the formulation appropriately to control the Mooney viscosity of the compound within a suitable range. 

If excessive viscosity leads to trapped air, consider appropriately reducing the viscosity or extending the low-pressure venting time