Let's discuss why silicone parts get moldy from a structural engineer's perspective.

As a product structural engineer, I'm often asked questions like: "This silicone sealing ring has only been used for six months, how come it's already moldy? Is it a problem with the material?" My usual answer is: the material is only part of the reason; the bigger problem lies in the structural design. Today, from a structural engineer's perspective, let's discuss the engineering logic behind silicone mold. I. Understanding Silicone Pure silicone (polydimethylsiloxane) is an inorganic polymer material with a Si-O bond in its main molecular chain, exhibiting extremely high chemical stability. Mold's enzyme system simply cannot break down this structure. What are the molecular structural characteristics of silicone rubber? What are its main uses? _obsh foaming agent|environmental accelerator Therefore, strictly speaking, silicone itself doesn't mold. Mold is caused by deposits on the silicone surface. But as structural engineers, we can't stop at concluding that the material itself is fine. What users see is mold; what we need to solve is-why does the dirt adhere so firmly to the silicone and cannot be washed off? II. Analysis of the Causes of Mold Growth in Silicone Components 1. Surface Microstructure From a structural design perspective, any contact surface is a mating surface. During use, silicone components repeatedly rub against hard objects, causing micro-scratches to form on the surface. These scratches are typically between a few micrometers and tens of micrometers deep-invisible to the naked eye, but sufficient to accommodate mold spores and organic residue. Once moisture enters these micro-grooves, they form miniature reservoirs where mold multiplies rapidly. Engineering Implications: If silicone components need to be in long-term contact with food or the human body, the design target for surface finish (Ra value) should be as low as possible. However, in reality, the surface treatment process for silicone components in many products is not included in the design review, which is a deficiency in the design process. 2. Structural Gaps This is where structural engineers should reflect most. Many silicone components are designed with deep grooves, narrow gaps, and dead corners for ease of assembly or aesthetic purposes. For example: The assembly gap between the silicone sealing ring and the metal/plastic shell The mating gap between the silicone button and the panel The inner fold of the silicone folding structure These locations share a common characteristic: moisture can get in, but cannot evaporate. Furthermore, cleaning tools cannot reach them at all. From a DFM perspective, these structures may look fine in 3D modeling, but in actual use, they become major mold-prone areas. Engineering Implications: During design reviews, in addition to considering assembly tolerances and appearance, an additional dimension should be added-cleanability assessment. For every deep groove and crevic, ask yourself: Can the user clean it? Can the moisture dry completely? 3. Material Selection The hardness (Shore A) of silicone directly affects its surface density. Generally speaking: Many products choose low-hardness silicone to achieve a soft touch. However, softness means larger molecular gaps, which means it is easier to absorb organic matter and moisture. Engineering Implications: Material selection cannot be based solely on feel. A balance needs to be struck between user experience and durability. If soft silicone must be used, subsequent surface treatment processes (such as applying an anti-fouling coating) must be appropriate. 4. Draft Angle and Demolding Design This is something many non-structural engineers might not consider. Silicone parts are typically molded, requiring a draft angle in the mold design. Insufficient draft angle will cause the silicone part surface to be scratched during demolding, resulting in microscopic tears or burrs. These damages become entry points for dirt and grime during subsequent use. Furthermore, an improperly designed venting channel in the mold can lead to tiny pores on the silicone part surface, which are also breeding grounds for mold. Engineering Implications: During mold review, in addition to focusing on dimensional tolerances and appearance, the impact of demolding on surface integrity should also be considered. A well-designed mold should produce silicone parts with a surface free of pores and scratches. III. Structural Engineer Design Recommendations ✅ Design Phase Reduce deep grooves and dead angles: Use open structures whenever possible instead of closed structures, and use large rounded corners instead of right-angled grooves. Controlling Surface Roughness: For silicone parts that come into contact with food/human bodies, the recommended surface finish is Ra ≤ 0.8μm. Consider Removable Design: If silicone parts must be exposed to humid environments, design them with a removable structure for easy periodic deep cleaning or replacement. ✅ Material Stage Prioritize Food-Grade Liquid Silicone (LSR): LSR has significantly higher purity and density than solid silicone, resulting in better stain resistance. Appropriate Hardness Selection: Avoid ultra-soft silicone (<30A) unless there is a strong justification for optimal tactile feedback. ✅ Mold Stage Draft Angle ≥ 3°: Reduces demolding damage. Properly Arrange Venting Channels: Avoids surface porosity. Mold Surface Polishing: Silicone parts produced from mirror-polished molds (SPI A1/A2 grade) have a denser surface and are less prone to contamination. In Conclusion While silicone mold may appear to be due to uncleanliness, it is a result of a combination of factors related to materials, structure, and manufacturing processes. As structural engineers, our responsibility goes beyond simply making products functional and aesthetically pleasing; it's also about ensuring their durability in real-world usage scenarios. While mold may not affect functionality, it directly impacts user experience and brand reputation.