一,Cell structure issues
1.Uneven cell structure (inconsistent in size, co-bubble)
The cut surface of the product shows obvious large cell areas and dense small cell areas, with uneven distribution, which affects the hand feel and compression rebound.
It caused by many reasons：
1)Uneven mixing: This is the most fundamental reason. The foaming agent and vulcanizing agent were not evenly dispersed in the rubber compound, resulting in an excessively high concentration of the foaming agent in some areas and the formation of large bubbles.
2)Improper plasticity of the rubber compound: The rubber compound is too hard (with low plasticity), has poor fluidity, and the resistance to bubble growth and expansion is large, making it easy to form cells of different sizes. If the rubber compound is too soft, the strength of the bubble walls will be insufficient and they are prone to merge.
3)Mismatch between vulcanization and foaming speeds: When the foaming speed is greater than the vulcanization speed, the bubbles have sufficient time to grow and merge, resulting in unevenness. When the vulcanization speed is greater than the foaming speed, the rubber compound sets prematurely, and the bubbles cannot grow fully, resulting in insufficient foaming and high density.
Improper loading method: The rubber compound is placed in an unreasonable position in the mold cavity, resulting in an overly long flow path and gas accumulation during the flow.
2. Insufficient or excessive foaming rate (density out of control)
phenomenon: The product density deviates from the design value, either too heavy or too light and too soft, and the strength does not meet the standard.
It caused by many reasons：
1)Inaccurate dosage of foaming agent: Incorrect weighing or the foaming agent itself has become ineffective.
2)Improper process parameters: Temperature: The mold temperature is too low, and the decomposition of the foaming agent is insufficient; If the mold temperature is too high, the foaming agent will decompose violently in an instant, which may cause gas to escape or break through the rubber compound.
3) Pressure: The forming pressure is too high, which inhibits the growth of bubbles.
4)Time: Insufficient vulcanization time and demolding of the rubber compound before it is fully set may lead to bubble rupture or product shrinkage. Expired rubber compound storage: The activity of the foaming agent or vulcanizing agent decreases during storage.
3. The ratio of open holes to closed holes is unbalanced
Phenomenon:
1)Excessive closed cells: The product has good rebound but feels relatively hard, and has a large permanent compression set.
2)Too many holes: The product is very soft but has poor resilience, and is prone to compression and deformation, like a sponge.
It caused by many reasons：
Rubber compound strength: The cohesion of the rubber compound itself (gel strength) is the key. High strength, the bubble wall is not easy to break, and it is easy to form closed cells. Low strength and prone to forming open holes.
Foaming and vulcanization balance: As above, the vulcanization speed is slightly faster than the foaming speed, which helps stabilize the bubble wall and form closed cells.