Several questions about lost foam casting——foam molding of EPS foam

Expandable polystyrene resin (ExpandablepolyStyrene) is a liquid foaming agent with a low boiling point added to polystyrene beads. Under the condition of heating and pressure, it penetrates into the polystyrene beads to make them swell. Into expandable polystyrene resin (beads).

Molding process

1. The bubble core formation stage of the foam: when the synthetic resin is added with chemical blowing agent or gas, when the temperature is increased or the pressure is reduced, gas will be generated to form a foam, and the gas will be formed when the gas exceeds its saturation limit in the melt or solution. When the solution is saturated, the gas will escape from the melt and form bubbles. At a certain temperature and pressure, the decrease of the solubility coefficient will cause the dissolved gas concentration to decrease, and the released excess gas will form bubbles.   

2. Bubble core growth of foam: In the foaming process, the cell growth rate is determined by the growth rate of the internal pressure of the cell and the deformation ability of the cell rate. After the bubble is formed, since the pressure of the gas in the bubble is inversely proportional to the radius, the smaller the bubble, the higher the internal pressure, and increase the number of bubbles through nucleation, and the expansion of the bubble expands the growth of the bubble. The main factors that promote foam growth are the increase of dissolved gas, the increase of temperature, the expansion of gas and the merging of bubbles.

3. Stable solidification of the foam: If the cell growth process is not interrupted at a certain stage, some cells can grow to a very large size, so that the material forming the cell wall reaches the material rupture limit, and finally all the cells will be connected in series. , The entire foam structure collapses, or all the gas slowly diffuses from the cells to the atmosphere, the pressure of the gas in the foam gradually attenuates, then the cells will gradually become smaller and disappear.

Therefore, it is important to control the growth rate and stability of pores in foam formation. This can be accomplished by causing the polymer matrix to suddenly solidify or gradually reducing the deformability of the matrix. Many methods to stabilize the foam can reduce its surface tension, reduce gas diffusion, and stabilize the foam. For example, in the foaming process, the viscosity of the plastic liquid can be increased by cooling the material or cross-linking the resin to achieve the purpose of stabilizing the foam.

According to the heating method or heating medium, the foam molding of foam can be divided into: high frequency heating molding, hot water molding, steam cylinder foam molding and press foam molding. Due to the high cost of high frequency heating, it has not been widely used in production. Hot water molding also has many shortcomings, such as the inability to produce foam patterns with a wall thickness greater than 50mm, the core is prone to defects such as poor adhesion of beads, and high energy consumption. It has also been phased out.

Compressor air chamber foaming molding is to fill pre-expanded beads into a foaming mold with air chambers. The molds are installed on the upper and lower or left and right pressure plates of the press with a mechanical clamping device, one of which is Movable, the superheated steam enters the mold through the air chamber through the pores on the mold, so that the beads are foamed; then the cooling water is passed through the same channel to cool the mold and the foam, and the desired foam pattern can be obtained .

Forming mechanism

1. The formation of bubble core

The so-called bubble core refers to the original bubble, which is the place where the gas molecules initially gather. The initial stage of the plastic foaming process is to form a large number of bubble nuclei in the plastic melt or liquid, and then expand the bubble nucleus into a foam. There is a free space with zero pressure in the molecular structure of polymers, and different polymers have free spaces of different sizes. Some polymers have a larger free space, which can accommodate the infiltration of certain blowing agents. Generally speaking, the above two conditions must be met at the same time to form a bubble core.

RN Hacoard et al. provided the basis for the molecular framework theory. They took polystyrene as the object and studied its molecular structure. It is inferred from the compressibility of polystyrene that there is free space in its molecular framework, and its internal pressure is zero. When the temperature is lower than Tg, the free space accounts for about 13%. The maximum amount of pentane entering these spaces is 6.5%~8.5%. A Ringram and WrightHA support the above argument. They used experiments to prove that the saturation capacity of pentane in EPS is 8%~8.25%, which is very close to the above inference.

According to the molecular framework theory, the following conditions must be met to form the bubble nucleus.

A. As the polymer of the foam plastic matrix, a sufficient amount of free space should be used in its molecular frame to gather a sufficient amount of foaming agent to form a bubble core.

B. Blowing agents generally use low-boiling organic liquids, which can penetrate into the free space of the polymer molecular framework under certain conditions, and are subject to greater force to make them difficult to volatilize. In addition, the boiling point of the blowing agent must be lower than the softening point of the polymer. Therefore, although there are many organic liquids with low boiling points, not many are really suitable as blowing agents.

C. The low-boiling point foaming agent gathered in the polymer molecular framework, and its molecules are continuously diffusing. Therefore, polymers containing low-boiling foaming agents should not be left in the atmosphere for a long time.

Bubble expansion process

Approximate calculation formula for bubble growth:        

R(t)=kDCoTn 

Where R(t)––the bubble radius that changes with time t;      

T––Time;    

D—-The diffusion coefficient of gas

Co––– initial solution of gas;     

K–––correction factor (related to polymer viscosity and elasticity)

Stabilization and curing process of bubbles

Any system with gas-solid or gas-liquid coexistence is mostly unstable. The bubbles that have been formed can continue to expand, or they may merge, collapse or burst. The realization of these possibilities depends mainly on the conditions at the bubbles.

  (1) Curing method 

  In order to prevent the bubble from rupturing, on the one hand, we can start by improving the viscoelasticity of the melt, so that the bubble wall has sufficient strength and is not easy to rupture. On the other hand, the expansion speed is controlled to take into account the time required for the stress relaxation of the bubble wall.

  (2) Factors affecting curing

  The curing of thermoplastic foam is mainly carried out by cooling, and cooling is also the main factor affecting the curing speed.

In order to make the heat of the bubble diffuse into the surrounding air or cooling medium through various heat transfer paths. More often, air or cooling medium is used to directly or indirectly cool the surface of the bubble. However, because the foam is a poor conductor of heat, it often appears that the foam on the surface has been cooled and solidified and the core temperature is still very high during cooling. At this time, if the cooling and shaping are not enough, although the skin layer has been solidified and shaped, a large amount of heat from the core will continue to be transferred, which will make the temperature of the skin layer rise again. Coupled with the expansion force of the core foam, it may be shaped, but the core A large amount of heat will continue to be transferred, and the temperature of the skin layer will rise, and the expansion force of the core foam may deform or destroy the shape of the foam. Therefore, the cooling and solidification of foamed products need to have sufficient cooling and setting time and cooling efficiency to ensure. However, the cooling rate should not be too fast, especially for polymer foams with larger shrinkage.