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| How to find us ? |
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+86-21-68453855 |
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-86-21-52303505/6/7 |
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+86-21-50453855 |
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+86-21-52303508 |
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info@foameps.com |
| epsmachinery@foameps.com |
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201607.NO.368 Xindan Road.Shanghai.China |
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| The Moulding Process |
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According to the author, so far shape moulding machines have been described best by Jo hann Friedrich Jegelka during the talk he gave on the occasion of a VDI - K conference in Munich, Germany. Quote:
Shape moulding machines can be defined as machines to discontinuously produce mouldings of particle foam materials. Vertical and horizontal operations are determined by the closing and opening movement of the mould halves. Except for special cases, shape moulding machines are predominantly produced in horizontal construction. Their main components are:
- closing unit with basic steam chambers
- filling unit
- pipes and valves for air, steam and cooling water
- ejector mechanism and
- programme control
Furthermore, modem shape moulding machines are equipped with devices to remove and stack mouldings. The general controls enable inputs of pmgrmmne parameters, process pressures and movements via a screen. It is possible to save mould specific data as well as to connect it to a central recording system. The machine sizes are determined by the mould surface in m2 or by the inner dimensions (height and width) of the steam chamber. The standard sizes of the most machine producers are between 0.4 m2 and 2 m2. In special cases, mould surfaces of over6 m2 are also produced. Apart fromspecific equipment for single processsteps, the stiffness of the steam chamber construction and the closing force of the moulds are decisive factors for the suitability of shape moulding machines to process various particle foam materials. Here, the corresponding processing temperature of the particle foam materials plays a major role. According to VDI steam tables, the processing temperature is connected to a steam pressure in the state of saturation which is to be applied to the steam chamber. The steam pressure is effective on the clamping surface, within the sealing surrounding the entire mould surface on the point of separation of the two steam chamber halves. Thus, the closing force of the closing unit must always be higher than the lifting force generated by the steam pressure. An exception is the production of mouldings of EPP according to the compression method. In this case, the lifting force generated by the compressed air is decisive.
The closing unit consists of a frxed and a movable steam chamber half, of the bars which guide the movable side, of devices for opening and closing as well as of devices that keep the steam chamber closed. Depending on the design of the machine, the steam chamber halves are separately fixed on solid steel frames or the frames are made of thick- walled piping in the form of a chamber. Here, the structural piping is for the supply and discharge of process media. The steam chamber back side or back wall in which the fill injectors and ejectors are anmlged is typical for all shape moulding machines but it is also problematic in regard to the load caused by the chamber pressures. On the one hand, the surface of the back wall must be able to freely adjust the fill injectors and ejectors and on the other hand, it must safely support the entire surface.
Depending on the design of the machine, the back wall equipped with the required devices is either on the fixed or on the movable side. When installed at the fixed side, an external mechanism outside or in front of the machine is necessary to operate the ejectors.
The advantage of an installation at the movable side is that the ejection is included in the opening stroke. The devices for opening, closing and keeping the steam chamber closed consist mainly of hydraulic drives. Hydraulic cylinders transfer the motion and the closing force. In EPP processing when large mould surfaces and high closing forces are required, it is more efficient to separate the motion and the development of the closing force. Hydraulic cylinders that are too big and directly pressurized either require huge amounts of oil and, for their purposes, oversized hydraulic ~gates or their motion periods must be unnecessarily prolonged. The closing force that develops independently of the motion can be generated by locking units arranged around the edge of the steam chamber.
A specific model of a closing unit for high closing forces is equipped with hydraulic plunger cylinders which are arranged parallel to the guiding cylinders. The hydraulic plunger cylinders which are filled with hydraulic fluid are only pressurized when the closing force is generated. During motion, the hydraulic fluid moves without pressure between the plunger hydraulic cylinders and a hydraulic tank. This enables a separation of motion and development of high closing forces which in turn is and advantage to short motion periods and to the dimensioning of the hydraulic aggregate. Besides a favourable supply of closing power, this arrangement has the advan tage that the complete back side of the movable steam chamber half remains free and can be used as filling and e jection side. The portal- shaped frame on the movable side guarantees free ac cess to the inside of the machine and makes it possible to install blocked moulds even from the back of the mov able steam chamber half.
Moulds
In the non- cutting shaping process, the halves of a two- piece mould are generally divided in female and male mould. The ex pressions male side and female side, defining female side as the hollow part, were intro duced in the terminology of moulds for parti cle foam mouldings. In most cases, female and male side are divided in two mould halves and the fill injectors and ejectors are arranged at the female side. It is also possible to produce moulds with male and female side in the so- called one side construction. This, however, may cause some disadvantages during the process. The basic differences are demonstrated in Fig. F1 -18 and F1- 19. Fig. F1- 17 shows the one- side construction "male and female"and Fig. F1- 18 shows the divided construction. Except from low production costs, the advantages of the one- side construction are demoulding without ejectors and a shorter mould opening stroke. When opening the mould, air presses the moulding to the opposite surface and ejects the moulding from there. On the other hand, there are the following disadvantages: the venting during the filling of the mould cavity is not optimal when a venting crack between the mould surfaces is required: an un favourable cross flow of the steam during cross steaming may cause poor fusion in the region X and the transfer of the moulding to a removing device may be problematic. The mould walls are perforated by pressed- in hole type nozzles or slotted nozzles which are arranged on even surfaces at a regular distance of 30 to 40 mm and with a diameter of 10 to 12 mm. In narrow cavities often diameters of only 4 to 8 mm are possible so that the distance be tween the nozzles should be as small as possible. The nozzles in the mould halves opposite to each other should be staggered. The outsides of the mould, especially the top surfaces, must not show any indents where water could accumulate. Perhaps, drainage ducts leading to the outside are to be worked in. A PTFE coating of the mould sides is recommended as this is an advantage to demoulding, filling and to corrosion protection.
Finally, the shrinkage that must be added to the set dimensions of the mouldings has to be considered. The shrinkage of EPS is between 0.6 and 0.7 % and the shrinkage of EPS is between 1.5 and 2.8% depending on the type. In any case, this should be checked with the producers of the moulding materials. A foam pressure metering device and a temperature probe control can monitor the process. The foam pressure metering device is a pressure receiver to determine the internal mould pressure when moulding. The point of demoulding can be determined by monitoring the foam pressure reduction (end of moulding stabilisation).
The temperature probe and the transducer connected to it determine the mould temperature during the moulding and the subsequent cooling. With the help of temperature measurement,the required cooling time and cooling water quantity can be determined. The construction shown in Fig. F1 -20 shows that first only the mould and the receiving frame are installed in the steam chamber of the shape moulding machine and the accessories re quired for the operation are installed afterwards.
According to latest developments in technology, the moulds are installed completely with the required accessories in order to save the time involved in this and to avoid machine down times.
With these quick change moulds the male side and the back plate are fixed by spacer pieces and the cooling lead as well as the accessories required for operation is mounted before they are installed. the mould halves prepared like this can be moved into the open steam chamber from outside and inside and can be fixed by means of clamps or hydraulic quick clamp cylinders.
Due to higher pressures, the EPP version is equipped with a thicker back plate and a
more stable support. A calculation of the plate dimensions and the required support has to be included in the mould construction. By means of a movable electric hoist installed on top of the machine and with the help of centerings, the heavy mould blocks can be intalled and adjusted to a central position, on the portal- shaped stand or on the fixed steam chamber half according to their installation direction. Then the movable steam chamber half can be moved slowly over the mould block.
Removal of the mouldings
The collection of the mouldings of various sizes is often a problem that has not yet been satisfactorily solved and that is mainly caused by the shape moulding machines. If mouldings are ejected and collected like bulk material it takes much time afterwards to sort,bundle or pack them. The transport of the mouldings can be simplified by a controlled removal and magazining or stacking. A pre-
condition is that the shape moulding machines are equipped with all- purpose devices. Basically, the usual construction of shape moulding machines provided two possibilities. The mouldings can both be removed towards the top or the bottom and transported to a magazine or to a stacking device. Many devices that are used remove the mouldings to the bottom are specific to customer needs.
A precondition to remove the mouldings on the top and front side of the machine is that the machine construction allows this transport direction as well as a sufficient room height. For this purpose, the foil unit and the ejectors must be installed on the movable
mould half. Fig. F1 - 21 shows the principle of removing to the top with subsequent stacking. This kind of removal has the following advantages:
- The removing device can be easily retooled in a mould change because only the plate with the suction or gripping arrangement that is suitableto the mould must be changed.It is not necessary to change or adjust the stacking device. The ascending stroke of the stacking table is automatically adjusted to the moulding
height by means of a firmly positioned light barrier.
- The mouldings are safely transported to the removing plate because the moulding itself is hot moved during the transfer.
- Regular stacks can be created even with multiple tools with an irregular geometric arrangement of the mouldings.
- The space below the closing unit remains free which enables a free access to the inside of the machine, especially during the mould change.
- No production interruption due to faults caused by mouldings that were not transferred safely or fell down during a removal to the bottom.
Economic Aspects of Moulding Handling
Erich Streichsbier describes the economic conditions of using removing and stacking systems in EPS moulding production as follows:
When we look at the economic aspects of parts handling, the following points have to be examined:
- batch sizes
- shape of the mouldings
- creating moulding sets
- mould cavities
- personnel costs
The shares of the handling costs were determined with the help of examples from Germany, Great Britain and Norway, see table F1- 1. This is not to asses the efficiency of the different operating structures but to demonstrate the relation between personnel and the use of removing systems.
Case A, table F1- 1, shows a mixture of products which can only partly be removed and stacked in a regular manner by modern removing systems. Case B shows a plant for the production of fish boxes which configuration enable an automatic removing, stacking and bundling of the mouldings. Personnel costs greatly influence whether handling is economic with or without handling devices. The personnel costs are mainly influenced by the location of the processing company and differ greatly between Europe, the Far East and the USA.
Table F1-1.Costs shares lof the production turnover in %. Companies in Europe
Product unit A: mixed moulding production B: always the same mouldings,
e.g. boxes
use of material 30 to 36 41 personnel 28 to 30 14 process costs ( oil, gas, cur rent, water, maintenance) 5to8 7 logistics ( packaging material, freight, ear pool) 9 to 10 14 other expenses, like own tools, development, administration,etc. 28 to 16 24 In Europe and the USA, removing systems would be absolutely necessary if they could be used universally. However, this has failed so far because the particle foam processing industry did not have enough money at their disposal to invest in intelligent handling systems which meet their requirements. An alternative between the production with manual handling and the production with regular removing and stacking can be found in Asia, Japan and in the USA where many companies carry out the production and the packaging on various floors in their production facilities.
Moulding prduction
In practice, the progrmmne control consists of many individual steps and a number of variants for the various particle foam materials. Despite this, the moulding production is identical in the main process steps which can be described as follows.
Filling
The filling unit consists predominantly of a storage tank and of the fill injectors operated by compressed air which suck in the foam beads and transport them to the mould cavity. Fig. F1 - 22 shows the structure and function of a fill injector. When the closing piston is open, a vacuum is created by the compressed air escaping from the air nozzles. In this way the foam beads are sucked in, seized by the escaping compressed air in the region of the air nozzles and blown into the mould cavity. In most cases, the storage tank is a pressure tank and is additionally subjected to a reduced compressed air. This increases the pressure discrepancy from the tank to the fill injectors and shortens the filling time. When the mould cavity is completely filled, back blowing starts automatically. Due to the back pressure that develops at the filling outlet, the compressed air is diverted and now flows Table F1 - 1. Costs shares of the production turnover in %. Companies in Europe towards the storage tank. In this way, the surplus foam beads are removed from the flu injector and the supply hose and are returned into the storage tank. To achieve an ideal filling, an optimal venting of the mould cavity must be guaranteed. When the compression method is applied in order to fill compressible foam particles without blow ing agents, the venting of the closed steam chamber must be controlled by pressure in order to maintain the required compression pressure.
Moulding
After the mould has been filled, the foam particles are moulded and fused by the heat energy of the steam. In order to achieve a good heat transfer, the air in the steam chamber and between the foam beads must be removed. Air cushions cause lower temperatures and lead to a poor fusion and to spaces between the beads. The air is completely removed when the steam is spread over the entire width of the steam chamber and enters it from top to bottom so that the developed condensate can also flow out of the chamber. Heating the mould and foaming generally takes place in three to four steps.
During purging, shown in Fig. F1 - 25, the steam inlet valves and outlet valves are open.
Steam flows through both steam chamber halves from top to bottom so that air and condensate are discharged.
During the following cross steaming step, Fig. F1 -26, a cross flow of the steam through the mould cavity is generated according to the displayed valve position, removing the air between the foam beads. At the same time, the mould starts to heat and beads start to fuse. The selected direction of the cross steaming depends on the design of the moulding. In some cases, especially with complicated mouldings of EPS and with mouldings of EPP, a cross steaming in alternating directions is neces sary. Cross steaming is followed by autoclave steaming, as seen in Fig. F1- 27. Here, both inlet valves with controlled steam supply are open until the selected steam pressure and an even fusion of the particle foam is achieved.
Cooling and stabilising
After expansion, the mould and moulding have a temperature of 120°C with EPS and 150°C with EPS, depending on the type of particle foam. The moulding must now be cooled to remain dimensionally stable. EPS particle foams require to be cool ed toapprox. 80°C, EPP particle foams require less cooling. As soon as the internal pressure of the moulding has been created after cooling, demoulding can start. Cooling takes place by spraying water on the moulding. EPS particle foam materials are almost always cooled with the help of a vacuum. First, the mould is cooled to a temperature of approx. 95° by means of a defined amount of water. Then or at the same time, a vacuum up to 200 mbar is created in the steam chamber by a central vacuum installation or by a water ring vacuum pump in the machine in order to decrease the boiling temperature of the sprayed- on water. The heat created though steaming is removed from the mould by the re- evaporation of the water. This leads to a temperature reduction until the required demoulding temperature is reached. During re- evaporation, the condensate in the moulding is largely evaporated again. After cooling supported by vacuum, the moulds and mouldings are almost dry. In order to prevent the steam that has developed during the re- evaporation from entering the vacuum pump, a further cooling and condensation in the condenser is required. Cooling supported by vacuum is only useful when the possible de moulding temperatures are above 60°C. In lower temperature regions, the vacuum is hardly effective as the re- evaporation required for the heat abstraction and drying can not be achieved. The mouldings remain wet so that there is hardly any difference to directly cooling the mould to the demoulding temperature.
Demoulding
The mouldings are de- moulded by compressed air which is generally supported by ejectors. Before opening the mould and during opening of a slow motion speed, one mould half is subjected to transfer air. The compressed air cushion transfers the moulding to the ejection side. The moulding is then ejected from the open mould by means of ejection air and supported by ejectors. Afterwards, the moulding is transferred to a removing device, if present. |
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