Expanded foam manufacturing process
- Granules preliminary foaming.
- Conditioning of pre-foamed granules.
- Polystyrene blocks forming.
- Conditioning of polystyrene blocks.
- Cutting polystyrene blocks on plates.
- The use of polystyrene waste.
1.1. Brief description of raw materials
Foamed self-extinguishing polystyrene containing 5-6% of the pentane and isopentane mixture is a foaming factor and is used as a raw material. This mixture is contained in polystyrene granules in dissolved form.
The raw material has the form of granules obtained by suspension polymerization of styrene. It contains a substance that reduces flammability-fire retardant.
After heating to a temperature of 90-100°C, under volatilizing pentane, granules increase their volume (foaming process) at about 30-65 times. In industrial practice, water vapor is used to foam polystyrene, which also penetrates into the granules and increases the impact of pentane.
International designation of polystyrene being foamed: EPS self-extinguishing FS.
To store only in factory, tightly closed package or containers arranged in ventilated areas or under a canopy, away from sources of heat and fire. It is highly recommended to store raw materials at a temperature not exceeding 20°C.
The product stored at the recommended temperature should be used no later than in 3-6 months from the date of the product examination specified in the quality certificate. The product from partially emptied or damaged containers should be used immediately.
In production facilities, raw materials can be stored in amount not exceeding its average daily consumption.
1.2. Processing of foamed polystyrene.
The final density of the finished product is determined at the stage of pre-foaming.
An important indicator is the pressure control during the foaming process, for continuous prefoaming device 0.015-0.03 MPa, for cyclic one 0.015-0.02 MPa.
There are two methods of product apparent density change:
- by changing the amount of feedstock;
- by changing the level of foaming material in the working chamber;
The first and the second method affect the time spent foaming the material in the working chamber. The third method affects the temperature in the chamber.
The effect of the time spent in the foaming device on the apparent density of the product is shown in Fig.1.2.
If the residence time of the raw material in the foaming device is too long, the granules begin to shrink and the density increases; if the temperature is too high, the foamed granules can form lumps. Both of these phenomena can occur simultaneously. And have a direct impact on the quality of the final product.
Duration of pre-foaming
Fig. 1.2. The dependence between the apparent density and the duration of foaming
In order to obtain a low density (< 12 kg/m3), two-stage foaming is used. Two-stage foaming is carried out using the same equipment that is used for single-stage foaming, with the supply of pre-foamed raw materials through a secondary foaming system.
In order to achieve optimal results of foaming granules should be saturated with air (air conditioning process) before foaming at the second stage.
Pre-foamed granules enter the dryer with a boiling layer in which the warm air (temp. approximately 30-40°C) passes through the perforated bottom of the dryer, dries and pushes the granules in the direction of the discharge fan.
The air stream should be distributed so that the granules drying and moving process flows evenly along the entire length of the dryer (adjustment is carried out by means of dampers in the dryer air chambers).
One of the extremely important factors affecting the foaming of polystyrene is the duration of the raw materials storage. The older the raw material is, the longer the foaming will be and the harder it is to achieve the required apparent density of foamed granules. Therefore, the shelf life of raw materials in a sealed package is limited to six months.
1.3. Technical equipment of pre-foaming unit
2.1. The basic process of granules conditioning
In the course of conditioning, air penetrates into the foamed granules due to the vacuum formed in them, and moisture in the form of steam and pentane is released from the foamed granules into the atmosphere, the remains of the raw materials polymerization process which were not exposed to the reaction. This gas exchange is possible due to the gas permeability of polystyrene shells.
Fig. 1. Granules of foamed polystyrene in the process of conditioning
The rate of air diffusion into the granules is mainly due to the apparent density, ambient temperature and the size of the granules. The purpose of removing moisture from the granules' surface in the dryer with a boiling layer is to obtain 100% of the apparent surface through which the gas exchange is carried out.
The evaporation rate of pentane also depends on density, ambient temperature and granule size. Pentane evaporates slowlier from the large granules than from those of small diameter, due to the ratio between the surface of the granule and its mass.
2.2. Technical equipment of the granule conditioning unit
Silos used for conditioning of foamed granules are manufactured in the form of a light metal structure of the rack type with containers made of air-permeable fabric.
When moving the foamed granules with a jet of air strong electrostatic charges accumulate on the granules surface. Therefore, it is extremely important to thoroughly ground all metal elements of silos, transport pipelines and other equipment.
2.3. The parameters of the granules conditioning
The ambient temperature in granules conditioning work should not be below 15°C, at a lower temperature, the duration of conditioning increases. In summer, at temperatures above 20°C, the conditioning time is reduced, and at lower temperatures — extended.
When transporting fresh granules to silos, their apparent density increases as a result of collisions with the pipeline walls. Therefore, when setting the foaming parameters, it is necessary to take into account the increase in density during transportation.
3.1. Characteristics of the forming process
When producing blocks, foamed granules are placed in the forming chamber up to its complete filling. Then the saturated dry water vapor at a pressure of 0.2-0.4 MPa enters the mould, which leads to a further increase in the volume of granules. Due to the fact that the granules are in a closed chamber, the free space between them is first filled, and then the granules get interlocked.
1) filling 2) blowing 3) steaming 4) cooling 5) disbanding
4) steaming - growth
5) steaming — setting time
6) delivery end
7) vacuum cooling
An important factor in the block steaming is the supply of the appropriate amount of steam to the chamber in the free space between the granules in the shortest time. This requires adequate ventilation (blowing), the purpose of which is to remove the air before the start of the steaming process. Insufficient duration of blowing leads to inhomogeneous density and poor sintering of the block.
It is also important to maintain a constant high temperature of the mould, otherwise the steam consumption increases significantly (Fig.3.1.3) and the steam becomes wet, which reduces the quality of the granules adhesion.
The pressure that the block exerts on the inner walls of the molding chamber is approximately 0.08 MPa. In order to remove the block from the mould without damaging it, this pressure must be reduced to a value of about 0.01 MPa. The time required to reduce the unit pressure, i.e. the cooling time, depends on the foam grade.
In the blowing and cooling phase the vacuum is applied with the purpose of intensification of the steaming process and acceleration of the cooling process.
f) steam boiler
4.1. Brief description of process blocks conditioning
After the end of the forming process, the blocks are conditioned. Air conditioning is carried out in order to reduce humidity and eliminate internal strains arising during forming. In addition, the processes of gas diffusion and pressure equalization inside the granules with atmospheric pressure, similar to the processes occurring in the conditioning of pre-foamed granules.
In the process of blocks conditioning the sequence of their use, corresponding to the sequence of forming is highly important, that is, during the selection of blocks for cutting it is necessary to start with the "old" ones.
Blocks cutting is carried out by means of a rheostat wire heated to the appropriate temperature.
All waste enter the shredder, where from it is pneumatically transported to the secondary use.
The colouring of EPS plates should be the same as the colouring of pre-foamed polystyrene granules.
It is necessary to carry out random check of plates-at least 2 pieces on each block length.
If the plates meet the requirements, then after stacking they are sent to the packaging.
If the deviation from the required dimensions exceeds the permissible value, you should once again check one plate on the entire length of the block, determine the cause, make the appropriate adjustment of the gaps between the segments of the rheostat wire.
It's necessary to check in the same way the dimensions of the plates obtained by cutting the next block.
Plates that do not meet the requirements are sent for secondary use.
Waste is reused in the production of blocks.
6.1. Unit technical equipment
The device is designed for grinding polystyrene waste, resulting in crumbs used as an additive to polystyrene granules in the production of polystyrene blocks. The size of crumbs obtained in this way is up to 15 mm.
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