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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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| The Moulding Process |
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Heating curing is the next step of our process. This not only accelerates the curing process of the freshly molded blocks, but also assure that the material is dimensionally stable and provides a completely, dry material for best fabrication results.
In case of an efficient cooling supported by vacuum, the residual moisture contained in EPS mouldings is maximally 5 to 6% of the mass (compared to 15 to 20 % when cooling without vacuum support ). Usually, the mouldings need not be dried anymore. According to latest technology, EPP mouldings require a subsequent treatment in a heat Chamber which keeps a temperature of up to 60°C for 4 to 24 hours depending on the raw material and the moulding density. This so- called "curing" is necessary to remove the residual moisture from the spaces between the beads, to balance the shrinkage that has developed during demoulding and to guarantee the dimensional accuracy of me moulmngs.
Consumption of process energy
Air consunaption The air required for demoulding depends on the volume of the steam chamber in which the air cushions for transfer and ejection of the moulding are generated. The steam chamber volume results from the steam chamber dimensions width x height x depth in m3. Generally, an air cushion of pe = 0.5 bar is sufficient. Considering the losses due to escaping air and after a completed transfer and demoulding, the air consumption can be roughly calculated as follows:
Vair≈Vsteam chamber(in m3) 1.5 bar loss factor 3 to 5 (in m3/cycle).
It is more complicated to determine the air quantity required for filling. Depending on the number of fill injectors, on the filling time and the filling method, the result is very different air quantities. It is, of course, not enough only to consider the most favourable air consumption when choosing the filling method. When filling from a pressurized storage tank, the filling time can be reduced by approx. 40% compared to the filling time of storage tanks without pressure. This process requires a relatively high air consumption when filling EPP according to the compression method.
Steam consumption. The following explanations refer to conventional moulds made of light metals with perforated mould walls. The steam consumption is predominantly determined by the mould weight Mmould and the temperature difference Tmould between heating and cooling. The heat amount Q closed in kJ supplied with the steam results from Qclosed=Qmould+Qfoam+Qloss*
If the mould weight is known, the heat amount for the mould can generally be determined by Qmould=Mmould Cpmould △ Tmould However, this is hardly the case in practise but the mould makers or machine producers should be able to provide experience values. The required heat quantity for the moulding material M foam results from Qfoam = Mfoam Cp foam Δ Tfoam* With a skeleton substance of 2 to 10% (referred to the volume), the voluminous particle foam materials have a very low mass and this is why Q foam can be neglected. The specific heat capacity cp of EPS is, for example, 1.3 kJ/kg K and 1.7 kJ/kg K of PP. In the case of EPS, this results in a steam quantity of 0.10 kg per 1 kg particle foam at a temperature rise to 150°C. The heat quantity Q loss consists of the steam losses during parallel and cross steaming, of the steam volume in the steam chamber after moulding, as well as of the radiation lost to the surrounding of the steam chamber. Q loss can hardly be cal culated. Nevertheless, to achieve any useful data, the weight of moulds made of aluminium alloys were determined over a certain period of time and the steam consumption of shape moulding machines of different sizes were measured. The values resulted from this are the basis for the following estimated formula: Msteam steam quantity in kg/cycle Mmould Δ Tmould Msteam 2300 Mmould mould weight in kg, Δ Tmould temperature difference between heating and cooling in K; standard value for A T mould at an EPS mould mass of approx. 40 to 50 K, at an EPP mould mass of approx. 80 to 90 K, nl variable factor depending on the steam pressure in the steam chamber; nl ≈ 4 at EPS, n l≈ 6 at EPP, V steam chamber volume of the steam chamber in m3. This estimated formula can be used when the mould weight and steam chamber volume are known. The latter can easily be determined. In case the mould weight is not known, it can be estimated and the steam consumption can be roughly determined. Cooling water consumption In regard to the cooling water consumption, it can be assumed that, after moulding, the heat quantity Q mould plus the heat quantity contained in the steam volume of the steam chamber must be removed again. The predominantly used water cooling with vacuum support requires a separation of the cooling water quantity into a smaller amount which is used to cool the mould and into a larger amount which is used to cool the condenser usually in stalled between steam chamber outlet and vacuum connection. The water quantity required for cooling the vacuum- supported mould can be estimated as follows:
MwaterI Mmould ΔTmould /2300+ n2 Vsteam chamber/ 70- TwaterI
Description of the symbols used in the equation:
Mwater water quantity for cooling the mould in dm3/cycie.
n2 variable factor, depending on the steam pressure in the steam chamber
n2≈650 for EPS n9 ≈ 1600 for EPP,
Twater I℃ temperature of the water intake, usually 30 to 50°C The steam removed from the steam chamber into the condenser by vacuum must be co densed by means of a further cooling water quantity M waterII. The water quantity in dm3/cycle required for this can be estimated by Mwater II Mmoula ΔTmould 0.22 + n2 Vsteam chamber/ Δ Twater II Δ Twater II is the temperature difference between inlet and outlet of the condenser. It must be considered that, at an absolute pressure of 0.2 bar (manometer indication -0.8 bar), the temperature in the condenser must be kept under the boiling temperature of 60°C. This means that a water outlet temperature of maximum 55°C must be kept. Consequently, Δ Twater II- 55°C -Twater II is in front of the condenser. In practise,ΔTwater II should at least be 25 K. This estimation can also be used to determine the cooling water quantity of mould cooling without vacuum system, which means Mwater I can be determined like Mwater II. Experience has shown that the "mixed" outlet temperature of the steam chamber can amotmt to 60 to 75°C. Special methods to produce EPS mouldings
The conventional moulding production mentioned above refers to moulds which are cooled by water or which walls are perforated by numerous nozzles for venting and steam supply. Below, special methods are described which deviate from above mentioned features in one or more aspects.
Transfer technology
In the mid 1970s, transfer machines were developed which work with two separate moulds and without water cooling. The essence' here is that the EPS moulding is filled and foamed in one mould and then transferred to a second, cold mould to stabilize it.
Fig. F1- 29 shows the principle of transfer technology. Filling and moulding with steam takes place like in the conventional moulding production. The cold mould does not have to be especially cooled. The low steam consumption is an advantage as a water cooling is not necessary. However, the great hopes that were initially put in this technology did not come true in practise. The surface structure and the mechanic stability of the mouldings can not be compared to conventionally produced mouldings. The size of the moulding is also limited as the moulding, which has been de- moulded and not yet stabilized, must be transferred to the cold mould in a very short period of time. Large and fast moving mould masses place high requirements on machines and technology. In addition, there are higher mould costs for two moulds which cannot always be recovered by saving energy.
Moulding method without nozzles
Thin - walled EPS mouldings, especially drinking food, are produced according to thin walled technology without mould perforation. Furthermore, high- quality mouldings with low residual moisture and smooth, often slightly shining surfaces are produced in moulds without the usual perforation of the mould walls, e.g. lost- foam models for full mould casting with gasifiable particle foam models. One characteristic feature of the moulds is that the steam for heating and the cooling water have no direct access to the mould cavity. The low amount of steam to fuse beads is supplied separately from the heating steam. It is very complicated to vent the mould cavity during filling and heating The basic idea of separating the heating steam and fusion steam dates back to 1963 and was published in a patent application in 1969. Further patent applications about processes and devices to separate heating steam and fusion steam were published in the following years. Basic examples of different ways to separate the steam supply are shown in Fig. F1 - 30 and Fig. F1 - 31. Fig. F1 – 30 shows the supply of fusion steam via a separate steam distribution chamber. Fig. F1- 31 shows an example of the schematic structure of a mould for lost- foam parts. The openings for the steam supply are very small and thus they do not leave any imprints on the
moulding.
The shape moulding machines are especially constructed for moulds with separate steam supply. Except from steaming, the moulding production hardly differs from the process mentioned
above. During filling and steaming, the mould cavity is vented via a small crack between the separating surfaces of the mould. Cooling supported by vacuum is generally not applied here.
LTH Technology
Meanwhile, the so- called LTH technology (Low Temperature Horizontal) has been developed. On the surface, a shape moulding machine for LTH technology hardly differs from conventional shape moulding machines. However, the mould differs greatly from conventional moulds. The most important thing here is the inner construction of the mould, not the kind of mould connection (quick mould change).So far, approx. 95 % of the process energy has been used for heating and cooling the
mould and for the accompanying losses. The LTH technology has been developed to improve upon this by keeping the mould temperature constant during expansion and by only feeding foam with process energy. This takes place when all mould surfaces, which have contact with steam, are sealed - off from all media. This guarantees that, as mentioned above, when the operation temperature of approx. 85℃ has been reached, the foam is only supplied with the temperature necessary for fusion. It is not necessary to cool the mould because the steam condenses on the insulated mould surfaces and on the bead surfaces after the energy has been supplied. By applying sufficient vacuum this condensate evaporates again and can be discharged. During this process, the evaporated condensate is extracted from the foam and thus helps to cool and stabilise the moulding. Considerably shorter cycle
times result from this as the period of time for heating and cooling the mould becomes unnecessary. An important advantage of this process is an energy saving of approx. 60% of the total energy. Apart from steam, the compressed air quantity can also be reduced by decreasing the steam chamber volumes.
Since its development, LTH technology has proven a success predominantly in mass-production and most of all in the packaging of household appliances, which are partly used in- house directly at the appliance manufacturer. A disadvantage here are the high mould costs. As already mentioned above, because all energy- carrying parts are sealed off from against the media, the construction of the mould is very expensive.
Recently, a modified version of the pure LTH process has been presented. This is a so-called ECO- LTH process in which only the mould surfaces that directly contact the EPS moulding material are sealed. As always, however, the energy distribution areas are cooled by water in the short run. In this way the same cycle time can be achieved. The largely constant contour at the mould surface, however, requires a higher energy consumption during steaming and a slightly higher air consumption. An important advantage of this method is the saving of mould costs so that it is economically attractive to use this energy- saving technology for considerably lower piece numbers |
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