Step 1: Before release

First, the purpose of moving forward

1, which can better control the bulk density of molded products in a lower range.

2. It can reduce the density gradient in the molding process, and can be expanded and molded uniformly.

Second, the pre-launch mechanism

In the preforming process, the beads containing foaming agent are slowly heated to above 80℃, and then they begin to soften (the temperature is lower during secondary foaming). The foaming agent in the beads is heated and vaporized to generate pressure, which makes the beads expand to form disconnected bubbles (closed cells). After preform, the bubble diameter is about 80um, and there are about 5500 small bubbles per cubic centimeter, and the bubble wall thickness is only 65438±0um.

At the same time, steam also permeates into the expanded battery, increasing the total pressure in the battery. With the continuous penetration of steam, the pressure increases and the volume of beads increases. This process has been going on. In the process of pre-injection, with the continuous penetration of steam, it is very important to increase the total pressure in the battery. Only by evaporating foaming agent (such as n-pentane), even if there is no loss at all, the beads can only increase by about 25 times at most. The reason why the foaming rate can be as high as 100 times is that the total pressure in the expanded cell is increased by the penetration of steam, and the volume of the cell is further expanded.

It should be pointed out that the permeability of water vapor to EPS bead film is several times faster than that of n-pentane gas or air, and the main factor affecting its permeability is temperature. When heated, the foaming agent will also ooze from the beads. Therefore, in the pre-development process, the rate of vapor permeation entering the cell should exceed the rate of foaming agent oozing out of the cell, so that most of the foaming gas will remain in the cell, thus increasing the total pressure in the cell, and the foaming agent will not overflow in the cell, so that the polymer will be stretched in a rubber state with sufficient strength to balance the internal pressure, so that the beads can be pre-developed, the polymer can be stretched and the beads can be pre-expanded.

Third, the propulsion method.

Pre-dryer can be divided into continuous type and intermittent type. No matter what type of pre-dryer, EPS raw materials are added to the machine and foamed by steam heating. Obviously, the foaming ratio and foaming speed of different raw materials are inconsistent. However, the foaming ratio and foaming speed of different raw materials are inconsistent. Moreover, all kinds of pre-machines can control the foaming ratio by adjusting various parameters.

Taking the continuous preforming machine as an example, we can control the pre-forming packing density of EPS by adjusting the feeding speed and the height of the discharge port under the condition of constant body volume and steam flow.

Intermittent pre-blower means that after the machine throws EPS material once, it pre-delivers the material to a certain volume, and then discharges the material in the machine and blows it clean. Then feed, pre-send, release ... and so on. Because the dosage of each batch of materials is certain, the pre-development volume of each batch of materials is also certain, thus ensuring the volume density after pre-development, and the volume density of materials after pre-development changes little, generally staying below 3%.

If the content of foaming agent is too high, it will lead to uneven pre-foaming, frosting of foam, product deformation and cracking, and it is also easy to lead to the decrease of foaming ratio, long heating time, poor adhesion between EPS beads during molding, product shrinkage and particle shedding. The optimum content of foaming agent should be 4.8%-7%, and because foaming agent is volatile, the temperature of raw material warehouse should not be too high (below 20℃) and the storage time should not be too long.

Step 2: curing

1. Definition: the process of drying, cooling and battery pressure stabilization of pre-developed EPS particles.

Second, the curing reason:

Beads that have just been pre-developed are all wet. A certain amount of gaseous foaming agent and water vapor left in the beads during pre-development suddenly meet with cold when they leave the machine for cooling, so that the gas in the honeycomb cell condenses and negative pressure (local vacuum) is formed in the cell. At this time, the foamed beads are soft, easy to deform and inelastic, and cannot be directly molded.

Third, the curing mechanism:

Put the newly developed EPS beads in the air for a period of time, on the one hand, dry them, on the other hand, make the air penetrate into the cells through the cell membrane, so that the pressure in the cells is balanced with the external pressure, and the beads are elastic.

Fourth, the advantages of curing:

By improving the further expansion, the melting between beads and the elasticity of beads in the foaming process, the quality of EPS products can be improved.

Verb (abbreviation of verb) Note:

1. In the curing process, the foaming agent also diffuses outward, so the storage period of pre-developed beads should not be too long, and it is generally appropriate to use 8- 12h.

2. There is also an optimal curing temperature. According to the experimental measurement, the curing environment is 18℃-22℃, the outside air permeates the battery, and the pentane gas in the battery basically does not diffuse outward. When the temperature is higher than 22℃, the air seepage velocity is accelerated, and the pentane in the cell diffuses outward and is lost. When the temperature is lower than 18℃, the speed of condensed pentane being sucked into EPS again slows down and the air permeability decreases. Therefore, the optimum ambient temperature for curing pre-expanded polystyrene beads is 65438 08℃ ~ 22℃. The ideal curing warehouse should be equipped with a hot air (temperature 18℃-22℃) conveying system to shorten the curing cycle.

Step 3: Forming

1. Definition: The solidified expandable polystyrene beads are filled in a closed cavity, and hot steam is directly introduced into the cavity through the pores of the mold wall in a short time, so that the beads soften and expand after being heated. Due to the limitation of the cavity, the expanded beads can fill all the gaps and completely bond into a whole. After cooling and solidification, the foamed plastic product is released.

Second, the classification:

1. packaging molding: foam products with specified shapes are directly produced by using packaging molding machine and corresponding molds, which are widely used in shock-absorbing packaging of electrical appliances and light industrial products, fresh-keeping transportation of fruits and seafood, lost foam casting and other fields.

2. Sheet forming: a sheet forming machine is used to produce large-volume rectangular foam products, which are cut into sheets and used as building insulation and decoration materials.

At present, the annual output of the above two kinds of foamed molded products in China is basically the same. Molded products are widely used in the developed areas in the south and coastal areas, and the plates for building insulation are mainly produced in the north.

Although there are differences in density and use between foam products and foam boards molded by molds, their molding mechanism is the same and the molding process is similar, so the molding process of EPS foam is described by compression molding.

Third, the molding mechanism

Molding includes two stages, steam heating and cooling.

The pre-cooked beads are heated by steam, and the air can't escape in about 20s-60s. After being heated and expanded, pressure is generated, and the sum of the pressures is greater than the steam pressure heated outside the beads. At this time, the polymer softens, the pressure in the cell after the foaming agent vaporizes is greater than the external pressure, the beads expand again, and the beads fill the gaps between the beads to form an integral block, forming a foamed plastic product with the same shape as the mold. After cooling with water, they can be shaped and polymerized.