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| How to find us ? |
| TEL: |
+86-21-68453855 |
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-86-21-52303505/6/7 |
| FAX: |
+86-21-50453855 |
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+86-21-52303508 |
| URL: |
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| Email: |
info@foameps.com |
| epsmachinery@foameps.com |
| ADD: |
201607.NO.368 Xindan Road.Shanghai.China |
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| Pre-expansion |
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The Pre-expansion phase of manufacturing is simply the swelling of the small bead to almost 50 times its original size through heating and rapid release of the gas from the bead during its glass transition phase.
Pre- expanders
Dr. Fritz Stastny did not only invent EPS in 1949, he is also known as the father of pre-expanders. His original constructions of continuous and discontinuous pre- expanders, which he described in a device patent, were already designed in the form of standing and
insulated agitator tanks. The design of the agitator in cooperation with the breaker rods guarantee an evenly loose bead bulk without hardly any agglomerations. In order to achieve that the raw material is evenly supplied with heat energy, steam flows through the tank from bottom to top.
Continuous Pre- expanders
Continuous pre- expanders are operated without pressure and are thus equipped with an open- top agitator tank in which raw material is continuously fed from the bottom by an adjustable screw conveyor.. Steam is "also fed continuously into the pre- expanding room through openings that are positioned shortly above the tank bottom. An agitator and fixed breaker rods make it more difficult for bead clusters to develop; they keep the beads in motion and make sure that al ready pre-expanded beads of lower density move to the top of the bead bulk and fall into a shaft through a height- adjustable discharge opening.
Pre- expanders for the so- called "second pass expansion" of already expanded materials, are equipped with an especially large feed screw in order to transport the already expanded material to be steamed again. All pre- expanders have in common that the completely expanded beads trickle into a so- called fluid bed dryer where they are dried and
stabilised before they are transported into the storage silos.
Continuous pre- expanding
First of all, a screw conveyor or a bucket conveyor transports the material as carefully aspossible into the store tank. A conveying screw continuously feeds the material into the pre- expander where steam flows through it. The continuous pre- expansion process, can be achieved after a second or third pass.
The capacity of a continuous pre- expander ranges from 200 kg to 3000 kg per hour.Here, the throughput capacity depends mainly on the type and quality of the raw material and the volume of the pre- expander.
Continuous pre- expanders show some disadvantages compared to discontinuous pre- ex-panders: first, in regard to the minimum densities that can be achieved, second, when it comes to the processing of fine- beaded materials and third, in the processing of lowpentane material. The major advantage of continuous pre- expanders, however, is their low investment. Moreover, they are simple to install and to operate. In the field of second pass expansion, however, discontinuous pre- expanders are superior.
Discontinuous Pre- expanders
These kinds of pre- expanders differ in some details to the continuously operating machines.
Pressurized pre- expanders are often equipped with tanks that are approved for a maximum operating pressure of 0.49 bar. Some series allow pressures up to 1 bar while special pre- expanders allow pressures up to 5 bar.
Different from continuous pre- expanders, the raw material is dosed in batches. It is important to measure the raw material exactly in order to achieve consistent densities. The best results are generally achieved by using gravimetric dosing systems. The beads are fed into the expansion tank via an opening in the lid of the pressure tank.
As usual, the steam flows through the expansion tank from bottom to top. A homogeneous distribution over the total floor space is important. This is why high- quality machines are equiped with wedge wires over the total floor space. The control of the steam pressure is especially important because it adjusts the temperature in the expansion tank. If EPS of a high density is to be pre- expanded, air is added to the steam flow. In this way, the temperature of the steam- air mixture is decreased and the heat transmission is drastically pressure) Numerous mechanical and physical characteristics of EPS expanded plastics are, among others, influenced by the expansion pressure. This makes the expansion pressure an important process parameter and thus it is used as a measured variable for the process control (e. g. generally in the case of block moulds). When standard material brands are moulded, steam pressures of pe = 0.5 to 1.3 bar are used. These pressures are applied to the mould cavity and the moulding material for a few seconds. When material brands remain stable under heat, are moulded, higher steam pressures are necessary depending on the type and density of the mould, which also requires corresponding machines, moulds and process conditions.
After the heat energy for the fusion of the beads has been reached through steaming, the moulded part cannot be directly removed from the mould cavity (demoulding) because the particle foam would continue to expand uncon- trollably during demoulding and could tear and stick to the mould cavity. Because, in contrast to pre- expanding, the internal pressure (ex- pansion pressure) in the connected or fused beads is still present after steaming and is only reduced slowly. The required minimal period of time for the reduction of the expansion pres sure depends primarily on the EPS material brand, the density, the maximum thickness of the particle foam, the steaming degree and the expansion pressure directly after steaming. The period of time required for the reduction of the expansion pressure is an important value for moulding productivity and is thus an important factor for processing efficiency.Contrary to pre- expanding, cooling by means of the surrounding air is not sufficient when used as the main and "short" cooling process when moulding. The internal pressure has to be reduced over several bead layers (foam thickness) as opposed to the very low cell wall thickness of a single bead. Thus, other and more efficient ways to reduce the foam pressure and to stabilise (cool) the particle foam must be used. The latest developments in technology are the principles of water cooling and vacuum used either individually (e. g. vacuum cooling at block moulds) or in combination (e. g. Water and vacuum cooling in shape moulding machines).
In the case of pure water cooling, the mould cavity is solely cooled by water. During this phase of moulding, water serves as a heat release for the particle foam. With this, in the direct contact region between particle foam and mould, the cell substance that is still soft can be stabilised and the pressure can be reduced quickly. The stabilised skin makes the complete foam particle dimensionally stable, especially in the period directly after demoulding. At the same time, the contact temperature between mould and foam is only reduced to the necessary demoulding temperature, but no lower.
Even today, some older plants still work with a controlled water cooling only. The disadvantages here are that the parts are demoulded with a relatively high content of residual moisture and that, inevitably, the mould temperature sharply decreases. This causes, on one hand, the foam to be dried more intensively afterwards and on the other hand that more heat is necessary for demoulding because the mass in the mould is cooled and heated in cycl6s. Simultaneously, water can accumulate in the mould cavity which causes problems in the moulding process (e. g. during filling).
Almost all of these disadvantages can be avoided when the design of the mould does not allow water to get in touch with the foam, e.g. in the case of dry expansion or in combination with vacuum cooling. A vacuum works in two ways When stabilising EPS particle foam: on one hand by creating a higher pressure gradient between the foam and its surroundings and, on the other hand, by the evaporation of the left over condensate in the particle foam and the mould cavity; moisture and heat is also removed. In this way, the time period for declining the expansion pressure can be considerably reduced. Moreover,the residual moisture of the particle foam is reduced after moulding.
The vacuum cooling process was adopted by the EPS processing industry in the early 1980s. First, it was applied in the production of mouldings but it was constantly improved by optimising and adapting the EPS material brands to the machines. The acquired advantages
of vacuum technology were gradually applied to block production. In the field of EPS processing, vacuum cooling can be seen as the most significant technological development of the past 15 years. The EPS material brands offered on the market today allow a fast reduction of the expansion pressure, high demoulding temperatures and low energy consumption at high productivity.
The above mentioned process steps for moulding pre- expanded EPS beads are put into action by the specific design of the mould cavity, the construction of the processing plant and the moulding parameters. The mouldings are produced on shape moulding machines, predominantly when comparably high numbers of mouldings are to be produced. The shaped mould (mould nest) is attached to the steam chamber of a shape moulding machine and can be exchanged depending on the required items and cavities. For the production of large rectangular semi- finished products, block moulds (block mould plants) are used which work with fixed geometric forms for the mould cavity. |
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