一,Cell Structure Issues (Core Issues)
1.Uneven cell structure (varying in size)
Phenomenon: The cross-section of the product shows a distinct mixture of large and small cells, with an irregular distribution, which affects the hand feel and mechanical properties of the product.
Cause:
1)Uneven mixing: The A/B components (especially the foaming agent) were not mixed thoroughly, resulting in an excessively high concentration of the foaming agent in some areas and the formation of large bubbles.
2)Improper stirring speed and time: Before injecting into the mold, if mechanical stirring is required to introduce physical foaming such as nitrogen, stirring too fast or too slow, or too long or too short, will all lead to uneven bubble sizes.
3) Viscosity mismatch of the rubber compound: If the viscosity of the rubber compound is too high, the resistance to the movement and merging of bubbles in the rubber compound is large, and it is easy to form bubbles of different sizes. If the viscosity is too low, the bubbles tend to float up and merge into large bubbles.
4)The mismatch between vulcanization and foaming speeds: This is the key reason. If the foaming speed is much faster than the vulcanization speed, the bubbles have sufficient time to merge and grow, resulting in unevenness. Conversely, if the vulcanization is too fast, the bubbles will be fixed before they can grow, which may result in insufficient foaming.
2. The closed-cell rate is either too high or too low
Phenomenon: High closed-cell rate: The product has good resilience but poor permanent compression set, and feels relatively hard. High opening rate: The product is soft and breathable, but has poor resilience and is prone to compression and deformation.
Reason:
1)Rubber compound formula: The structural strength (modulus) of the silicone itself is the key. The high-strength rubber compound film wall can better encapsulate gases and form closed cells. Low-strength rubber compounds are prone to rupture when the bubbles expand, resulting in open holes.
2)The type and dosage of foaming agent: Different chemical foaming agents have different decomposition gas rates and solubilities of gases in the rubber compound, which directly affect the stability of the pore wall.
3)Vulcanization speed: A faster vulcanization speed helps to stabilize the cell structure quickly and form more closed cells. Slow vulcanization gives the bubbles more time to break and connect, forming open pores.
3.The aperture is either too large or too small
Phenomenon: The average size of the cells deviates from the design target.
Reason:
1)Dosage of foaming agent: The more foaming agent is added, the more gas is produced and the larger the pore size is usually.
2)Nucleating agent: Nucleating agents that are not added or added in insufficient amounts (such as fine particles like zinc oxide) cannot provide sufficient bubble nucleating points, resulting in a small number of bubbles but large individual bubbles.
3)Pressure control: Mold pressure and injection pressure affect the nucleation and growth of bubbles. Releasing pressure too quickly will cause the bubbles to expand rapidly.
二,Surface quality issues
1.Rough surface, orange peel, and bubbles
Phenomenon: The product's appearance is not smooth, with a texture similar to orange peel, and even bubbles with surface cracks can be seen.
Reason:
1)Excessively high mold temperature: When the rubber compound comes into contact with the surface of the mold cavity, the surface foaming agent decomposes violently in an instant, and the gas breaks through the unvulcanized surface, causing roughness and bubbles.
2) Adhesion of the rubber compound to the mold: Before the rubber compound expands, it fails to adhere well to the surface of the mold cavity, and gas accumulates at the interface to form surface bubbles.
3)The surface finish of the mold: If the mold itself is not smooth enough, it will aggravate surface problems.
2. The surface skin is either too thick or too thin
Phenomenon: Foamed products usually have a dense skin layer. Too thick leather affects its softness, while too thin leather affects its strength and appearance.
Reason:
1)Mold temperature: Mold temperature is the key to forming the skin. High mold temperature causes the surface rubber compound to vulcanize rapidly, preventing foaming and forming a thin skin. The mold temperature is low, the surface vulcanization is slow, and the gas generated inside may lift the surface layer, forming a porous thick skin.
2) Thermal conductivity of the rubber compound: Poor thermal conductivity of the rubber compound can lead to a large temperature difference between the inside and outside, accelerating the formation of the skin layer.
三,Issues regarding Dimensions and Physical Properties
1.Large density deviation
Phenomenon: Unstable product density, or uneven density in different parts of the same product.
Reason:
1)Inaccurate injection volume: This is the most direct cause. The injection volume for each mold cavity must be precise and consistent.
2)Unstable foaming process: All the above-mentioned cell problems (unevenness, different sizes) will cause density fluctuations.
3)Insufficient vulcanization: Products with insufficient vulcanization have poor support and may be overly compressed after demolding, resulting in higher density measurement values.
2. Large shrinkage rate or deformation
phenomenon: The product shrinks severely after demolding, or warps and twists occur.
Reason:
1)Cell structure: For products with an open-cell structure, after demolding, the internal gas escapes, which can cause significant dimensional shrinkage.
2)Insufficient vulcanization: The crosslinking density is not high enough, the polymer chain segments cannot be effectively fixed, and the shrinkage rate increases.
3) Uneven cooling: After the product is taken out of the mold, if the cooling rates of different parts are not consistent, internal stress will be generated, leading to deformation.
4)Mold design: The ejector pin design is unreasonable, causing the product to deform during the ejection process.
3. Poor resilience and compression set
phenomenon: The product cannot return to its original shape after being compressed, resulting in permanent indentations.
Reason:
1)Cell structure: An excessively high open-cell ratio is the main cause of poor permanent compression set.
2)Degree of vulcanization: Severe under-vulcanization can lead to insufficient strength of the silicone rubber molecular network, making it unable to provide adequate resilience.
3)The inherent properties of the rubber compound: The selected base silicone raw rubber has poor resilience.