DE29809587U1 - Device for molding plastic parts - Google Patents

Description

Device for molding plastic parts

The invention relates to an apparatus for molding plastic parts with an extruder and a molding area to which molten thermoplastic is supplied from the extruder and which has a mold wall with an inner molding surface. In particular, the invention relates to an apparatus for molding plastic pipes with an extruder and a molding tunnel to which molten thermoplastic is supplied from the extruder for molding the pipe, which has a downstream open end for ejecting the pipe from the molding tunnel and which is formed by first and second mold jaws which have inner molding surfaces defining the shape of the pipe.

Typically, the plastic pipe is manufactured by applying a vacuum to its outer surface or by applying internal pressure using an injection molding process. After the pipe has been molded, it is usually cooled by cooling the mold jaws. As the pipe continues to travel with the mold tunnel, the cooling becomes less and less effective because the pipe shrinks as it cools, creating an air gap between the mold jaws and the pipe. The pipe is thus insulated from the mold jaws and can hardly transfer any heat to the mold jaws as it travels with the mold tunnel.

The invention is based on the object of improving the cooling effect of the mold area on the plastic part located in the mold area.

The object is achieved according to the invention in that in a device for molding a plastic part of the type mentioned at the beginning, a gas inlet is provided through the mold wall, through which gas is supplied to the mold area on the plastic part, whereby both cooling of the plastic part and cooling of the mold wall can be achieved, and furthermore a gas outlet is provided at a point remote from the gas inlet, through which the gas can be removed from the mold area.

In particular, in a device for forming plastic pipes of the type mentioned at the outset, first wall openings are provided through the molding surfaces of the first mold jaws, furthermore, a device for supplying a gas into the molding tunnel through the first wall openings of the first mold jaws onto the plastic pipe for cooling both the plastic pipe and the inner molding surfaces of the mold jaws and a gas outlet arranged at a distance from the first wall openings for discharging the gas from the molding tunnel.

Due to the device according to the invention, after the plastic part has been formed, gas is released through the inlet on the mold wall
onto the molded plastic part, which cools both the outer surface of the plastic part and the mold wall. The gas is then removed from the molding area of the device through the outlet located away from the inlet.
30

In a device for forming plastic pipes in a forming tunnel with a circular cross-section, the cooling gas is introduced into the forming tunnel through the forming jaws arranged on one side of the forming tunnel and is guided around the pipe over its circumference.

The invention can be used in combination with an internal cooling system which increases the cooling effect on the plastic part

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improved even further.

Preferred embodiments of the invention are described in more detail below with reference to the drawing. The drawing shows:

Figure 1

a side view of a device for forming a plastic pipe,

Figure 2

a perspective top view of a pair of mold jaws of the device shown in Figure 1,

Figure 2A is a plan view of one of the mold jaws shown in Figure 2,

Figure 3 is a section through the forming tunnel of the device shown in Figure 1, showing a vacuum-formed tube,

0 Figure 3A is a section through the mold tunnel of the

1 with a tube formed by internal pressure,

Figure 4

a section through the forming tunnel of the device shown in Figure 1, showing the initial phase of cooling the pipe,

Figure 5

a section through the forming tunnel of the device shown in Figure 1, showing the final phase of cooling the pipe and

Figure 6

a section through the mold tunnel of another preferred embodiment.

In Figure 1, the device for forming a plastic pipe is generally designated 1. In this device, molten thermoplastic material is fed from an extruder into a forming tunnel arranged downstream of the extruder.

At the downstream end of the forming tunnel, the pipe P is led out of the device 1.

The mold tunnel is formed by an upper track of mold jaws 3 and a lower track of mold jaws 4. The mold jaws meet in the area of the mold tunnel, generally designated 5.

Figure 2 shows a pair of mold jaws 4, which are connected to one another in the longitudinal direction, from the lower track of the mold jaws. As can be seen from this figure, these mold jaws have an inner mold surface, generally designated 7, which defines the outer shape of the pipe P. In this exemplary embodiment, the inner mold surfaces of the mold jaws have alternating elevations and depressions to produce a rib-shaped pipe.

The mold packs 4 are designed in particular for vacuum forming pipes by providing them with small vacuum slots 9 in each recess in the mold surface. To vacuum form the pipe, negative pressure is applied through these vacuum slots to the mold surfaces of the mold jaws, which presses the molten thermoplastic material onto the mold surfaces of the mold jaws. The vacuum slots can be formed in sections or continuously along the semi-circular mold surface of each mold jaw, as shown in Figure 2A. Figure 2 shows two vacuum slots in each recess of the mold jaws. However, only one slot or more than two slots can be provided in each of the recesses.

The mold jaws 3 in the upper track of the mold jaws have a construction identical to the mold jaws 4.

Figure 3 shows the vacuum forming of the pipe as it takes place in an upstream area of the forming tunnel 5. As can be seen from this figure, the two forming jaws 3 and 4, both of which are attached to the forming

jaw supports 2, tightly closed in the mold tunnel. A vacuum source 11, which, as shown in Figure 3, is arranged outside the mold jaw, is in direct communication with the small slots 9 in the upper mold block at separate locations 16, 17 and 18, respectively, through channels 13, 14 and 15. The channels 13 , 14 and 15 lead directly through the wall of the upper mold jaw, as shown in Figure 3. A similar arrangement is located in the lower mold jaw 4, wherein a vacuum source 19, which is again arranged outside the mold tunnel, is in communication with the vacuum slots 9 opening onto the mold surface of the lower mold jaw via channels 21, 22 and 23. The channels 21, 22 and 23 in turn reach the vacuum slots 9 at separate locations 25, 26 and 27, respectively.

As shown in Figure 3, when vacuum is applied by the upper and lower vacuum sources, the plastic is drawn onto the inner mold surfaces of the two cooperating mold jaws to form the tube P.

As shown in Figure 3A, the plastic pipe can also be formed using the same mold jaws 3 and 4 by air pressure acting on the pipe from the inside, which is indicated by arrows in Figure 3A. The air pressure presses the molten thermoplastic outwards against the mold surfaces of the mold jaws.

Regardless of the molding process chosen, i.e. the application of vacuum to the pipe from the outside or the application of pressure to the pipe from the inside in the injection molding process, the pipe is cooled by using the openings in the mold surfaces of the mold jaws, in this case by using the vacuum slots leading through the mold jaws 9. After the pipe has been formed and while it continues to travel with the mold tunnel, whereby the thermoplastic solidifies and maintains the shape of the pipe, a cooling gas is introduced into the mold tunnel through one of the mold jaws, where it hits the pipe. The cooling gas is cooled by a

The gas is then removed from the mold tunnel via an outlet located far away from the point where it is introduced into the mold tunnel.

As shown particularly by the arrows in Figure 4, the cooling gas is fed to the upper mold jaw 3. In the case that the tube has been formed by external vacuum application, the vacuum, while still existing in the upstream region of the mold tunnel, is replaced in the more downstream region of the mold tunnel by a flow of cooling gas into the mold tunnel after the tube has been formed and has solidified sufficiently to retain its shape. The channel 11 now serves to feed the cooling gas. The cooling gas, which may be, for example, ambient air or air cooled by a cooling device or which may be otherwise suitably generated, is fed under pressure into the channels 13, 14 and 15 and in the present case through the vacuum slots 9 of the upper mold jaw. The vacuum slots form an inlet into the mold tunnel so that the cooling gas is directed onto the outer surface of the tube P.

As shown in Figure 5, after an initial cooling, the tube shrinks away from the inner mold surface of the mold jaws to a smaller outer diameter, creating a gap G between the tube surface and the inner mold surface of the mold tunnel. This allows the cooling gas to spread circumferentially around the tube, as shown by the arrows in Figure 5, to the vacuum slots of the lower mold jaw, where the cooling gas is then withdrawn from the mold tunnel. The slots in the lower mold jaw, which continue to be subjected to vacuum, now provide outlets for the cooling gas from the mold tunnel.

However, to produce an initial shrinkage of the tube away from the mold tunnel wall, the introduction of a cooling gas into the mold tunnel is not necessary. The shrinkage occurs naturally as a result of the normal cooling of the tube. Therefore, the cooling gas can also be used after the natural shrinkage.

natural pipe shrinkage, which creates a gap G between the pipe and the mold wall, so that the cooling gas can be fed through one of the two mold jaws into the mold tunnel and fed around the pipe to the other mold jaw.

The cooling gas provides two advantages. On the one hand, it causes the pipe to cool and solidify more quickly. On the other hand, it reduces the temperature of the mold jaws so that they can absorb more heat from the pipe, which in turn leads to the pipe cooling and solidifying more quickly than is the case with conventional methods.

Another advantage of using vacuum slots for both the supply and discharge of the cooling gas, preferably cooling air, is that whether they are continuous or sectioned, they form cooling air channels that extend completely through the main body of the mold jaws. The cooling air therefore not only flows around or alongside the inner mold surfaces of the mold jaws, but also penetrates the mold jaws. This greatly improves the cooling of the mold jaws. This increases the ability of the mold jaws to cool the pipe, particularly when they return to the upstream end of the mold tunnel after one revolution on their path at a much lower temperature than normal cooling, coming into direct contact with the plastic and exerting the greatest cooling effect on the plastic extruded into the mold tunnel.

Figure 6 shows a slight modification of the mold tunnel, which consists of pairs of mold jaws 21 and 25 with channels 23 and 27 respectively. These channels initially form vacuum channels during the formation of the pipe P and later serve as cooling gas inlet and outlet. They lead into the interior of the mold tunnel along the parting surfaces of the mold jaws, with which they lie against one another at the point marked 29 in Figure 6, and form an alternative to the channels directly in the wall.

the holes formed in the mold jaws of the embodiments described above.

If an even greater acceleration of the tube cooling in the mold tunnel is desired, a cooling device, such as a cooling mandrel or a cooling gas, can be used inside the tube in combination with the external cooling of the tube described.

In another embodiment, a cooling gas is supplied to the inner mold surface of the mold tunnel, for example through slots in the mold jaws, along the parting surfaces of the mold jaws, or through other inlets provided in the mold surfaces of the mold jaws. After both the molded plastic pipe and the mold jaws have cooled, the gas is removed again at the downstream end of the mold tunnel, which is opened for the ejection of the pipe from the mold tunnel, as described above with reference to Figure 1.

Claims (9)

Lippert, Stachow, Schmidt & Partner 26- May 1998 Patent Attorneys Frankenforster Straße 135-137 D-51427 Berglsch Gladbach Manfred A. A. Lupke Concord, Ontario Claims 1. Apparatus for molding a plastic part, with an extruder and a molding area to which molten thermoplastic is fed from the extruder and which has a mold wall with an inner mold surface that forms the plastic part, characterized by a gas inlet that extends through the mold wall and through which a gas is fed to the molding area and directed onto the plastic part so that the gas cools both the plastic part and the mold wall, and a gas outlet arranged at a distance from the gas inlet through which the gas can be removed from the molding area. 2. Device (1) for forming a plastic pipe (P), with an extruder (2) and a molding tunnel (5) to which molten thermoplastic is fed from the extruder (2) for forming the pipe (P) and which has a downstream open end for ejecting the pipe (P) from the molding tunnel (5), the molding tunnel (5) being formed by first and second molding jaws (3 and 4, respectively) which have inner molding surfaces (7) defining the outer shape of the pipe, characterized in that the first molding jaws (3) have first wall openings through their inner molding surfaces (7) and a device is provided for feeding a gas into the molding tunnel (5) through &iacgr;&ogr; the first wall openings of the first mold jaws (3) onto the surface of the pipe (P) so that both the pipe (P) and the inner mold surfaces (7) of the mold jaws (3, 4) are cooled, and for discharging the gas from the mold tunnel (5) through gas outlets arranged remotely from the first wall openings. 3. Device according to claim 2, characterized in that the second mold jaws (4) second wall openings which extend through the mould jaws (4) and form the gas outlets. 4. Device according to claim 2, characterized in that the downstream end of the mold tunnel (5) forms the gas outlet. 5. Device according to claim 3, characterized in that all wall openings are connected to a vacuum source (11, 19) for the purpose of vacuum forming the 0 tube (P) in an upstream region of the forming tunnel (5) before initiating cooling of the tube. 6. Device according to claim 5, characterized in that the inner mold surfaces (7) of the mold jaws (3, 4) have alternating elevations and depressions, the wall openings having small slots (9) in the depressions. 7. Device according to claim 6, characterized in that the slots (9) in the recesses are continuous. 8. Device according to one of claims 2 to 7, characterized by an air cooling device with which the gas can be cooled before entering the mold tunnel (5). 9. Device according to one of claims 2 to 8, characterized by a pressure source with which the pipe can be subjected to excess pressure from the inside for the purpose of injection molding.