Factors Affecting the Foaming Process in Rotational Molding Products
Release time:
2021-07-20
Rotational molding products are formed step by step by rotating the particles imported into the rotational molding mold into rotational molding products. So what factors and processes affect the quality of the products in this process?
Rotational molding products are formed step by step from the particles introduced into the rotational molding mold. So, what factors and processes affect the quality of the products during this process?
1. Bubble Nucleation Typical bubble morphology and temperature during the processing and molding of rotational molding products. When the mold is sent into a heating furnace preheated to 300~400℃, the mixed powder in the mold begins to be gradually heated. When the temperature reaches the melting temperature of the polymer, the polymer powder begins to melt. The polymer melt flows with the rotation of the mold, wrapping around the blowing agent, forming a foaming matrix, in which the blowing agent that has not yet begun to decompose is dispersed. It is important to note that if the polymer melt stops flowing and does not wrap the blowing agent well, bubbles will form between the blowing agent and the polymer melt, which will negatively affect the bubble morphology. The foaming process of rotational molding products basically has no shear motion, so the zero-shear viscosity of the polymer melt is an important rheological parameter during the polymer melting process. When the temperature of the polymer matrix is higher than the decomposition temperature of the blowing agent, the blowing agent begins to decompose, generating gas in the polymer matrix. This forms the bubble nuclei.
2. Bubble Growth After the blowing agent decomposes to form bubble nuclei, these bubble nuclei continue to grow into bubbles. It is worth noting that the bubble growth mechanism of the chemical blowing agents used in rotational molding products is completely different from that of physical blowing agents. Physical foaming involves injecting a volatile liquid into the polymer melt under a certain pressure; as the pressure drops, the liquid in the polymer melt vaporizes, forming foam. The pressure difference is the main driving force for bubble growth. The high-pressure gas inside the nuclei causes the bubbles to expand, but due to the pressure difference, some gas will dissolve in the polymer melt. The bubble growth rate is influenced by the gas generation rate, gas solubility, and the strength of the polymer melt. Generally speaking, bubble growth is mainly affected by bubble growth time, temperature, gas volume, fluid static pressure or polymer melt pressure, and the viscoelastic properties of the polymer melt.
3. Bubble Coalescence As the bubbles grow, adjacent bubbles will coalesce. These neighboring bubbles initially just connect, sharing a common bubble wall. As the void component increases, the bubble wall becomes thinner, and the density of the bubbles begins to decrease. This is because as the strength of the bubble wall weakens, the bubbles become unstable, leading to more bubbles coalescing together. According to thermodynamic laws, the surface area of the bubbles decreases, and adjacent bubbles tend to coalesce. Foaming materials with smaller bubble sizes and narrower bubble size distributions have better mechanical and thermal properties. Therefore, in the rotational foaming process, bubble coalescence should be prevented to achieve an ideal bubble nucleus density. The main reason for bubble coalescence is the insufficient strength of the polymer melt. Melt strength can be defined as the ability of the polymer melt to resist bubble expansion during the gas volume expansion and the thinning of the bubble wall. Therefore, the stability of the bubble wall increases with the increase in melt strength. Melt strength increases as the temperature decreases, so it is essential to maintain a low temperature during the foaming process of rotational molding products to minimize bubble coalescence.
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