The rotational molding products are formed by the particles which lead into the rotational molding products step by step. So what are the factors and the whole process that harm the quality of products in the whole process? Today, I will talk about it for you
1. The typical bubble shape and temperature in the whole process of forming bubble nucleation.
When the mold shell is sent into the heat treatment furnace heated to 300 ~ 400 ° C, the mixed powder in the mold shell is heated slowly. When the temperature rises to the polymer melting temperature, the polymer powder just begins to melt. The polymer melt flows with the rotation of the mold shell, encapsulates the foaming agent and produces foaming mother, in which the foaming agent is dispersed. It must be noted that if the polymer melt stops fluidity and does not encapsulate the foaming agent very well, bubbles will be generated between the foaming agent and the polymer melt, which will adversely affect the bubble shape. There is no cutting exercise based on the whole process of rotational foaming. Therefore, the zero shear viscosity of polymer melt is a key rheological parameter in the whole process of polymer melting. When the temperature of the polymer pregnant mother is higher than the dissolving temperature of the foaming agent, the foaming agent just begins to dissolve, causing vapor in the polymer pregnant mother. That creates bubble cores.
2. Bubble growth when the foaming agent dissolves and produces bubble core, the bubble core grows again and becomes bubble.
It should be noted that the bubble growth principle of organic chemical foaming agent is completely different from that of physical foaming agent. Physical foaming involves introducing a volatile liquid into a polymer melt under a certain pressure, reducing the vaporization of the liquid in the polymer melt and producing foam. The pressure difference is the key driving force for bubble growth. The high pressure vapor in the core causes the bubble to swell. However, due to the existence of pressure difference, part of the vapor will melt in the polymer melt. The bubble propagation rate is affected by the vapor generation rate, the vapor melting rate and the compressive strength of the polymer melt. -Generally speaking, the key factors of bubble growth are bubble growth time, temperature, vapor volume, liquid negative pressure or polymer melt pressure and polymer melt viscosity and ductility.
3. Bubble coalescence is accompanied by bubble growth, and neighboring bubbles will converge.
With the increase of the gap composition, the bubble wall becomes soft, and the relative density of bubbles begins to decrease. This is due to the weakening of the compressive strength of the bubble wall, and the initial instability of the bubble, a large number of bubbles agglomerate together. According to the law of thermodynamics, the area of bubbles decreases and the neighboring bubbles tend to coalesce. The foam plastics with smaller bubble size and more bubbles have stronger structural mechanics and thermal properties. Therefore, bubble coalescence should be prevented in the whole process of rotational molding to obtain the ideal bubble core density. The key reason for bubble coalescence is the insufficient compressive strength of polymer melt. The compressive strength of the melt can be defined as the ability of the polymer melt to resist the expansion of bubbles during the whole process of gas volume expansion and bubble wall softening. Therefore, the reliability of bubble wall increases with the increase of melt compressive strength. The compressive strength of melt increases with the decrease of temperature, so it is necessary to maintain ultra-low temperature to avoid bubble coalescence during the whole process of rotational molding.
4. Bubble passivation is another factor to reduce the relative density of bubbles in the whole process of bubble growth.
The size of the two adjacent bubbles is not the same. Due to the existence of pressure difference, the vapor will flow from the smaller bubble to the larger bubble. As a result, smaller bubbles become smaller and smaller. On the other hand, larger bubbles become larger and larger, and two bubbles become one big bubble. In addition, bubble coalescence will result in the decrease of bubble relative density and the decrease of physical properties. At present, a lot of scientific research work has been done in the whole process of rotational molding, including the harm of bubble coalescence and bubble passivation on the physical properties of foamed plastics.
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