In the foam molding process or foamed polymer materials, a honeycomb or porous structure is formed through the addition and reaction of physical foaming agents or chemical foaming agents. The basic steps of foam molding are the formation of bubble cores, the growth or expansion of bubble cores, and the stabilization of bubble cores. Under given temperature and pressure conditions, the solubility of the gas decreases until it reaches a saturated state, allowing excess gas to be eliminated and bubbles formed, thereby achieving nucleation.
Introduction
Foaming
A general term for the foam molding method. According to the foam manufacturing method, it can be divided into two categories: the method of mixing the plastic raw materials for foaming and then producing the foam in one process is called the one-step foaming method, also known as the direct method, and polyurethane foam is its typical representative.; The method of producing foam plastics in two processes is called the two-step foaming method, also known as the intermittent method. In two-step foaming, the previous process is called pre-foaming or pre-foaming. At this time, the foam or beads have not fully expanded and the density is high. The beads produced in this way are expandable beads. The latter process is called post-foaming or secondary foaming and produces a fully expanded, low-density final foam product. Polystyrene, polyethylene foam, etc. are made using this method. Industrially, foam molding methods are classified as follows: (1) Continuous foam molding by calendaring, extrusion or conveyor belt methods; (2) Production of final-shaped foams one by one by injection molding, which is called intermittent molding. foam molding. (3) Inject the liquid or particulate raw material complex into the mold according to the usage requirements to foam it, which is called on-site foaming molding.
Technical introduction
Foaming is the process of creating a microcellular structure in plastics. Almost all thermosetting and thermoplastic plastics can be made into foam plastics. Commonly used resins include: polystyrene resin, polyurethane resin, polyvinyl chloride resin, polyethylene resin, urea-formaldehyde resin, phenolic resin, etc.
According to the cell structure, foam plastics can be divided into two categories: if most of the pores are connected to each other, it is called open-cell foam plastic; if most of the pores are separated from each other, it is called closed-cell foam plastic. Open-cell or closed-cell foam structure is determined by the manufacturing method.
1. Chemical foaming is a gas produced by the thermal decomposition of a specially added chemical foaming agent or a chemical reaction between raw material components, causing the plastic melt to fill the cells.
The gases released by chemical foaming agents when heated include carbon dioxide, nitrogen, ammonia, etc.
Chemical foaming is commonly used in the production of polyurethane foam.
2. Physical foaming Physical foaming is a method of dissolving gas or liquid into plastic, and then causing it to expand or vaporize and foam. Physical foaming is suitable for many types of plastics.
3. Mechanical foaming is a foaming method that uses mechanical stirring to mix gas into the liquid mixture, and then forms cells through the shaping process. This method is often used for urea-formaldehyde resin, but it is also suitable for others such as polyvinyl formal, polyvinyl acetate, and polyvinyl chloride sol.
Foam molding principle
The formation process of bubbles is to first dissolve gas in liquid polymer or heat the polymer to a molten state, while generating gas and forming a saturated solution, and then forming countless tiny bubble nuclei through nucleation. Therefore, the molding and shaping of foam plastics are generally divided into three stages: the formation of bubble cores, the growth of bubble cores, and the stabilization of bubble cores. Then it is expanded into a foam with the required foam structure. Finally, the structure of the foam is fixed through assimilation and shaping to obtain a foam plastic product. The forming factors of these three stages are different. The forming mechanisms and influencing factors of these three stages are introduced below.
The process of formation of bubble nuclei
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 nuclei into foam. The so-called bubble core refers to the original microbubble, which is where the gas molecules initially gathered. A gas phase is added to the polymer liquid phase, and the gas is distributed in the solution to produce foam. If very fine solid particles or tiny bubble nuclei are added at the same time, a second dispersed phase as a gas will appear, which is beneficial to the formation of foam. The added substances that facilitate foam formation are called nucleating agents. If nucleating agents are not added, large-pore foam will easily form.
The formation stage of bubble cores plays a key role in the quality of molded foam. If a large number of uniformly distributed bubble nuclei can appear in the melt at the same time, it will be beneficial to obtain a bubble body with fine and uniform cells; if only a small number of bubble nuclei are added to the melt, the final foam formed will be small and uneven., the foam is denser and of poorer quality – so it is very important to control the formation stage of bubble nuclei during the foaming process.
Conditions for the formation of bubble nuclei
A chemical foaming agent (or gas) is added to the molten plastic or liquid mixture, and the gas (or added gas) produced by the chemical reaction will produce a gas-liquid solution. As the amount of generated gas increases, the solution becomes saturated. At this time, the gas will escape from the solution to form bubble nuclei. At this time, a gas-liquid two-phase is formed in the solution – the process of forming bubble nuclei in a gas-liquid solution is called nucleation., nucleation can be divided into homogeneous nucleation and heterogeneous nucleation. In actual production, nucleating agents are often added to facilitate nucleation to occur at lower gas concentrations. Nucleating agents are usually fine solid particles or tiny pores. If nucleating agents are not added, coarse pores may form.
Bubble growth process
Increasing the amount of dissolved gas, raising the temperature, causing gas expansion and bubble merging are beneficial to promote foam growth. After gas forms bubbles from small bubbles, the gas pressure inside the bubble is inversely proportional to its radius. The smaller the bubble, the higher the internal pressure – when the two When bubbles of different sizes come close to each other, the gas diffuses from the small bubbles to the large bubbles, causing the bubbles to merge. At the same time, the number of bubbles is greatly increased through the action of the nucleating agent, and the expansion of the bubbles expands the pore size of the bubbles, which causes the foam to continue to expand. Therefore, after the bubbles are formed, the gas expands due to heat and the bubbles merge, which promotes the continuous growth of the bubbles.
Factors affecting bubble growth
There are many factors that affect the expansion of gas in liquids, which can be summarized into two categories: one is raw materials, including the type and amount of raw materials, such as the type of foaming agent, solubility and diffusion coefficient, etc.; the other is molding and processing conditions., including the molding process, process conditions and equipment structural parameters, such as molding temperature, pressure, shear speed and die geometric parameters, etc. These parameters have a greater impact on the expansion of bubbles. During the bubble expansion process, the surface tension of the polymer and the viscosity of the solution are the main factors that hinder the growth of bubbles. The degree of action of these two factors must be appropriate. However, during the entire foaming process, the melt viscosity of the plastic decreases due to the increase in temperature. At this time, due to overheating of local areas (commonly known as hot spots ), or due to the action of defoaming agents, the surface of local areas of the melt becomes The reduction in tension will cause the cell wall membrane to become thinner and even cause the foam to collapse.
To control the expansion process of bubbles, it is necessary to understand the power and resistance of bubble expansion and the relationship between various influencing factors. There are many factors that affect bubble expansion, such as the rheological properties of the polymer, the type and amount of foaming agent and nucleating agent, molding process and equipment structural parameters, etc. In the process of bubble growth, the surface tension and viscosity of the solution are important factors that hinder the growth of bubbles.
In order to obtain high-quality foam plastics with uniform, dense and light cells, a large number of uniformly distributed bubble nuclei and supersaturated gas should first be formed in the melt at the same time during foam molding. The ratio of the total amount of supersaturated gas in the melt to the number of bubble nuclei determines the size of the bubbles. The greater the ratio of the sum of the bubble surface areas to the melt’s outer surface area, the greater the amount of supersaturated solution that diffuses from the melt to the bubble surface and enters the bubbles. This can reduce the amount of gas lost from the outer surface of the melt and improve gas utilization. If the number of bubble nuclei is too small, more gas will be lost from the outer surface of the melt to the atmosphere. As a result, the amount of bubbles obtained by each bubble nucleus may be more, but the gas utilization rate is low. The foam thus obtained has large cells, large quantity, and high quality, and has poor economic benefits. Therefore, to produce high-quality foam, a large number of bubble nuclei and supersaturated gas must exist in the melt at the same time.
Bubble stability
Most systems in which gas and liquid phases coexist are unstable. During the foam formation process, due to the continuous generation and expansion of bubbles, countless bubbles are formed, which increases the volume and surface area of the foam system and thins the bubble wall thickness, making the foam system unstable; already formed bubbles can continue to expand, Either bubbles merge, or bubbles collapse or rupture. The occurrence of these phenomena mainly depends on the conditions in which the bubbles are located. During the foam molding process, to control the increase of pores and stabilize them, the following measures can be taken:
- Use appropriate polymers, foaming agents and other compounding agents.
- Control the surface tension, viscosity and elastic modulus of the material by controlling the temperature of the process and the time in each temperature range. When the pores increase to a certain extent, timely cooling will make the viscosity and elastic modulus of the foamed material higher, and the fluidity will be poorer, making it difficult for the pores to move, merge and be stabilized.
- For rubber and thermosetting plastics, the cross-linking speed can be controlled. When the pores in the material increase to a certain extent, the cross-linking degree will be reached high enough in time, thereby greatly increasing the viscosity, reducing the fluidity, and stabilizing the pores.
- For some thermoplastic plastics, appropriately add some surfactants (such as silicone oil ) to reduce the interfacial tension between the resin and the pores, which is also beneficial to stabilizing the pores.
Foam molding equipment
There are two types of foam molding equipment: molding machines and steam cylinders. For mass production, medium and large foam shapes are mostly formed by molding machines; for small and medium production batches and small shapes, steam cylinders can be used.
1. Molding machines are divided into vertical molding machines and horizontal molding machines according to their mold opening direction, as shown in Figure 1.
The mold opening method of the vertical molding machine is horizontal splitting, and the mold is divided into an upper mold (moving mold) and a lower mold (fixed mold). Its characteristics are:
- The mold is easy to disassemble and install;
- It is convenient to place loose blocks and inserts in the mold;
- Easy to take the mold manually;
- Small footprint. Vertical molding machines are available in simple and automatic control types.
The mold opening method of the horizontal molding machine is vertical splitting, and the mold is divided into a left mold (fixed mold) and a right mold (moving mold). Its characteristics are:
- The upper and lower space at the front and rear of the mold is wide, and a pneumatic core-pulling mechanism can be installed to facilitate the production of complex foam patterns with multiple core-pulling structures;
- The water and air in the mold are discharged smoothly, which is conducive to dehydration and drying of the foam pattern;
- High production efficiency and easy to implement fully automatic computer control;
- The structure is more complex and the price is higher.
2. The manual steam cylinder forming device has two types: separate and horizontal. It has a simple structure, low investment, and can be self-made. The molding process is controlled by workers, but the labor intensity of molding is relatively high.
There are also two types of mechanical steamers: vertical and horizontal. Vertical mechanical steam cylinder can be transformed into a vertical forming machine. It is to place several molds on the workbench, then close the steam cylinder; start the control program to complete heating, water spray cooling and other processes; open the steam cylinder, manually remove the molds, and take out the pattern. Compared with molding by a molding machine, steam heats the mold from outside to inside, making it difficult to form a steam flow that penetrates the foam pattern. Condensation water is easily generated in the center of the thick section, which affects the fusion of beads. Therefore, the foaming time is much longer than that of a molding machine with an air chamber, and it is only used for the production of small patterns and sprues.
