HK1110546A - Apparatus and method for manufacturing plastic products - Google Patents

Description

Apparatus and method for manufacturing plastic products

Technical Field

The present invention relates to an apparatus for manufacturing plastic products.

Background

For the production of plastic products, injection molding devices are generally used, which have a plasticizing unit and a mold with at least one mold cavity. In the plasticizing unit, the plastic is plasticized by rotating a plasticizing screw in an idle manner while heating a certain amount of plastic. Subsequently, with the aid of a plasticizing unit, plasticized plastic is introduced into the mold cavity of the mold under high pressure and allowed to solidify. Thereafter, the formed product can be removed and in the next injection molding cycle a new product can be formed.

Such devices have the disadvantage of being particularly heavy and complex. In particular, a heavy press is used to keep each mold cavity closed during filling. Otherwise, the mold will be pressed open by the filling pressure, which will result in an unacceptable product. A further disadvantage is the need for relatively large and rapid movements of the plasticizing unit components to obtain the desired plasticization and pressure build-up.

Disclosure of Invention

It is an object of the present invention to provide an apparatus for manufacturing plastic products in a mold, whereby at least a number of the disadvantages of the known injection molding techniques and apparatuses can be avoided.

In particular, it is an object of the invention to provide such a device which is simple in construction and use.

It is a further object of the invention to provide such a device which is relatively advantageous in use and which is particularly advantageous in use.

It is a further object of the present invention to provide an apparatus for manufacturing plastic products, whereby a complete filling of at least one mold cavity can be obtained at a relatively low pressure.

It is a further object of the invention to provide a method for manufacturing a plastic product, which method can in particular use relatively low pressures.

It is a further object of the invention to provide such a method in which a relatively simple device can be used.

At least a number of these and other objects are achieved with the apparatus and method according to the invention.

With the device according to the invention, a buffer chamber is utilized in which a quantity of fully or partially plasticized plastic can be stored before being fully or partially introduced into the mold cavity. In order to introduce the plastic into the mold cavity, an ejection element, for example a plunger, is then used, which introduces the desired amount of plastic into the mold cavity. The plasticizing unit is thus separated from the means for pressing the plastic into the mold cavity and a relatively light and simple device can be obtained.

A further advantage of the apparatus is that, in practice, the or each buffer chamber can be built into the mould so that it can be placed or removed together with the mould. A pressing device which can comprise, for example, a simple plasticizing screw or the like can thus be used universally. It can furthermore be of simple design.

The or each buffer chamber is preferably designed such that the plastic introduced therein is kept in a plasticized state, for example by providing heating means.

Preferably, the or each buffer chamber has at least one relatively rapidly movable wall portion at a speed at least such that the plasticized plastic is maintained in a plasticized state until the mold cavity is completely filled, or conversely at a speed such that a slightly solidified plastic is remolded to improve the melt, at least further liquefying the plasticized plastic. As a result of the use of such a mould, the or each movable wall part is thus brought into the retracted position during the introduction of the plastic into the mould cavity and into the advanced position only at or after the end of the filling of the mould cavity, so that the final volume of the mould cavity for forming the desired product is reached, whereupon the advantage is obtained: a particularly low filling pressure can be used so that the mould and the associated further devices can be designed to be simple.

Preferably, the mould is then arranged so that, as a result of this movement towards the advanced position, the pressure exerted by the or each movable wall portion is substantially equal to the pressure exerted by the mould halves, so that the mould will simply remain closed as a result of its own weight. Surprisingly, it has been shown that, as a result of the speed of movement, which preferably produces adiabatic heating of the plastic in the mold cavity, the pressure prevailing in the mold cavity is relatively low with respect to the pressure at which a comparable product is produced in a conventional injection molding apparatus, for example of the order of 1/3 conventional pressure, more particularly 1/4. The mold can thus be kept closed in a particularly simple manner. Furthermore, the auxiliary devices to be used, such as the press, can be of lighter design or even redundant.

In the device according to the invention, it is advantageously possible to provide more than one buffer chamber, which can be connected to the same or different mold cavities. The buffer chambers can be supplied from the same pressure device.

With the device according to the invention, each time during the solidification of the plastic in the mold cavity, the or each buffer chamber can be filled with new fully or partially plasticized plastic so that the pressing device can plasticize the plastic and feed it through the supply channel, preferably in a continuous manner. The screw (if any) in the press device then need not be moved linearly, at least not so fast that acceleration or deceleration occurs, e.g. 1G or more. This advantage can be further enhanced when at least two buffer chambers are used for the same mold cavity.

With the device according to the invention, preferably a plunger or similar expelling element is used in the buffer chamber, said plunger being under a back pressure such that when the buffer chamber is filled, the plunger is pushed away against the back pressure. Thus, the plastic fed into the buffer chamber is continuously kept under pressure so that a homogeneous mixture can be obtained and/or maintained and gas inclusions can be prevented or possibly eliminated.

With the device according to the invention, the pressure used can be kept relatively low, for example less than 100 bar for products formed at injection pressures of typically 180 bar. For example, plasticized plastic can be introduced and held in the buffer chamber at a pressure of more than 30 bar, the back pressure being about the same or a few bar higher. The mold, in which the plastic pressure in the mold halves is approximately equal to or only slightly greater than the pressure that can be generated on the projecting surface in the mold cavity, is then held closed by its own weight. Thus, the closure device can be of a particularly light-weight design or can even be omitted.

With the device according to the invention, the ageing of the plastic during the forming process is effectively prevented, at least also reduced, as a result of the temperatures and pressures occurring and the short cycle times. Moreover, as a result of the lower pressure in the die, its properties are not adversely affected, even in relatively thin products under long flow paths. The fact is that shear or other stress factors of the molecules in the plastic are reduced to a minimum.

With the device according to the invention, the mold cavity can be filled in a simple, volume-controlled manner. Here, the or each buffer chamber preferably has a volume comparable to the final volume of the mould cavity (i.e. the product forming volume), or an integer multiple thereof. In other words, the volume of the or each buffer chamber is equal to N x V, where N is an integer and V is the product forming volume of the mould cavity. If the volume of the buffer chamber exceeds one time the product forming volume, the discharge means are preferably arranged to introduce into the mould chamber plasticized plastic having a volume equal to the product forming volume of the mould chamber.

The invention also relates to a method of forming a plastic product, wherein a buffer chamber is utilized. Thus, the product can be formed in a simpler manner than using a conventional injection molding apparatus.

Preferably, with the method according to the invention, during or immediately after the desired amount of fully or partially plasticized plastic is fed into the mold cavity, the at least one movable wall part is moved against or in the plastic, so that an improvement in the melt occurs, at least an increase in the flowability of the plastic. Here, adiabatic heating phenomena preferably occur in the plastic.

Surprisingly, it has been shown that there is a direct connection between the front surface of the or each movable wall part of the mould cavity and the pressure prevailing therein. Without wishing to be bound by any theory, this is shown as a result of the improved melt in the mould and the speed of movement of the or each wall portion, so that pulses comparable to pulses in water are formed, instead of more static pressure variations as in conventional compression moulding.

The kinetic energy introduced into the movable wall part is converted virtually completely (e.g. about 90%) into heat in the plastic. For PP, for example, an injection temperature of 220 ℃ may be used. The temperature in the plastic will be reduced to, for example, 207 c before the or each wall portion is displaced. Then, due to the movement of the wall, so much energy is supplied to the plastic that the temperature therein rises to, for example, 252 ℃. The melt index (melt) then rises, for example from about 38 to 45 ℃.

These values are merely examples and should not be construed as limiting in any way. Further preferred embodiments are given in the dependent claims.

Drawings

In order to clarify the invention, embodiments of the device and the method according to the invention have been described and illustrated with reference to the accompanying drawings. In the drawings:

fig. 1A-C schematically show, in cross-sectional side views, an apparatus according to the invention in three different steps of a method according to the invention;

FIG. 2 schematically shows two buffer chambers with plungers and supply lines according to the invention; and

fig. 3 schematically shows a part of a device according to the invention in an alternative embodiment.

Detailed Description

The embodiments shown and described should not be construed as limiting in any way. They are used for illustrative purposes only. In different embodiments, identical or corresponding parts are marked with identical or corresponding reference numerals.

In the description, a pressing device is to be understood as meaning at least a device by means of which the at least partially plasticized plastic can be moved continuously or discontinuously. Preferably, the plastic can be plasticized in a pressing device, for which purpose the device can be designed as a simple plasticizing device. Incidentally, the pressing device in the apparatus according to the invention can also be designed as a conventional plasticizing unit for an injection molding apparatus.

In the description, the product forming volume is understood to mean the volume of the mould cavity, which is such that the mould cavity has the shape of the product eventually formed in the cavity. The product may also be an intermediate product which is provided in successive production passes, for example with sprayed or coated further product parts in the same or different mould cavities. If the same mold cavity is used, for example the movable wall part can be retracted to a position beyond the next product forming volume, the next quantity of plastic can be introduced again and each respective movable mold part can be brought into the next applied or intended product forming volume.

Fig. 1 schematically shows an apparatus 1 according to the invention, showing three successive steps.

The apparatus 1 according to fig. 1 comprises a mould 2 having a first part 3 and a second part 4. The first part 3 rests on a base (not shown) and comprises two slides 5, which slides 5 are movable from two sides opposite each other, which is achieved with means 6, for example hydraulic, pneumatic or electric means. In the embodiment shown, these are piston-cylinder assemblies. The sliders 5 each have an inclined surface 7 facing upward in fig. 1. A movable wall part 8 with a complementary sloping underside 9 rests on the sloping surface 7. The part 10 of the wall portion 8 facing away from the underside 9 extends in a mould cavity 11, which mould cavity 11 in turn is bordered by the stationary parts of the mould 2, at least with the mould closed as shown in fig. 1. The second mould part 4 rests on the first part 3 and is held thereon, for example by gravity and fitting guides (not shown). The mould cavity 11 is partly located in the second mould part 4. Lifting means such as piston-cylinder assemblies 12 or the like are provided to separate the first mold part from the second mold part in order to remove the product.

A buffer chamber 13 located in the supply channel 14 is provided in the second mould part 4. The supply channel 14 comprises a first channel portion 14a between the outlet opening 15 in the bottom 16 of the buffer chamber 13 and the mould cavity 11, and a second channel portion 14b extending from an inlet opening 17 in the bottom 16 of the buffer chamber to a pressure device 18, which is schematically shown on top of the second mould part 4. The press device comprises a plasticizing screw 19 (shown schematically in fig. 1A) which can be rotated about a longitudinal axis 20 to plasticize the plastic. Such plasticizing devices, like the pressing devices, are very well known in practice and are not described further here.

A discharge element in the form of a piston or plunger 21 carried by a piston rod 22 of an energizing cylinder 23 is provided in the buffer chamber 13. This discharge element 21 is movable in a direction substantially at right angles to the bottom 16 of the buffer chamber 13.

The device 1 according to fig. 1 can be used as follows.

In the position shown in fig. 1A, the ejector element 21 has moved to the greatest distance from the bottom 16 and introduces fully or partially plasticized plastic 24 from the pressure device 18 into the buffer chamber via the second channel portion 14 b. The movable wall part 8 has been moved maximally away from the opposite wall 25 of the mold cavity 11 by moving the slides 5 away from each other. The volume of the mold cavity is then relatively large, larger than the product forming volume VP as shown in fig. 1C. Preferably, the volume VB of the buffer chamber 13 in which the plastic 24 is buffered is equal to the product forming volume VP.

From the position shown in fig. 1A, the ejector member 21 is moved in the direction of the bottom 16, so that the plastic 24 is pressed into the mold cavity 11 through the first channel portion 14a, as shown in fig. 1B. It is clear that the mold cavity is only partially filled with plastic 24.

The slides 5 are then moved rapidly towards each other so that the movable wall part 8 is pressed in the direction of the wall 25, preferably at a relatively high speed. In particular, the slides 5 are moved with the result that the plastic 24 is moved in the mold cavity 11 as a result of the movement of the movable wall part 8, adiabatic heat development processes occur in the plastic, so that at least partial plasticization occurs in the plastic. The flow properties of the plastic are then improved as a result of the melt change and the plastic can be moved into all parts of the mold cavity under substantially no pressure, at least with minimal resistance and no stress. Fig. 1C shows the end position of the movable wall part, in which the product forming volume VP of the mold cavity 11 has been reached.

In the position shown in fig. 1C, the product formed in the mould 2 can be hardened by solidification of the plastic, after which the mould is opened to remove the product. Furthermore, the completely or partially plasticized plastic 24 is again forced from the pressure device 18 into the buffer chamber 13, so that the discharge element 21 is pressed away from the bottom 16 of the buffer chamber 13. The ejector element 21 then stops to a certain extent as a result of, for example, the back pressure exerted thereon by the actuating cylinder 23, which prevents air from being introduced into the plastic 24. As a result of the back pressure, the plastic will become or at least remain homogeneous and any gaseous inclusions will be squeezed out. Simultaneously with or after the removal of the product, the movable wall part 8 is moved back to the position shown in fig. 1A, so that the device 1 is ready again for the next production cycle.

In the device according to the invention, the supply of plastic 24 to the mold cavity can be volume-controlled so that a very suitable filling is always obtained. This allows the post-pressurization to be omitted. In addition, in this way, the plastic can be prevented from being pressed in the mold cavity, so that an optimally weighted product can be obtained. This is advantageous in particular with regard to the toughness of the product. However, the movable wall part can also be moved such that the product forming volume is smaller than the introduced plastic, resulting in a certain degree of compression of the plastic. So that the density will increase.

Fig. 2 shows a part of an alternative embodiment of a device according to the invention. In this embodiment, two buffer chambers 13A, 13B are provided, both of which terminate in the same mold cavity 11. A valve 26 is arranged between the buffer chambers 13, by means of which valve the plastic flow through the second channel portion 14B can be alternately directed to the first 13A and the second buffer chamber 13B. Thus, in addition to a shorter cycle time, it is easier to provide the required amount of plasticized plastic 24 in one buffer chamber 13.

Fig. 3 schematically shows a part of a further alternative embodiment of the device 1 according to the invention, in which also two buffer chambers 13A, 13B are provided, which are both connected with the mould cavity via a channel portion 14 a. However, in this embodiment, each buffer chamber 13A, 13B is connected to a separate pressure unit 18A, 18B. With such a device, two different plastics can be introduced simultaneously and/or successively into the same mold cavity. If a mould cavity with one or more movable wall parts is used, the first product part can first be formed in the manner described above, after which the movable wall parts 8 are retracted, thereby emptying a part of the mould cavity, which is slightly larger than the second product forming volume. Then, plastic from the second buffer chamber 13B can be introduced into the chamber. Subsequently, the movable wall part 8 is moved forward again, so that the final 2K product is obtained. It is clear that in the manner shown in fig. 2 and 3, also more than two buffer chambers 13 can be used, optionally in combination with the embodiment shown in fig. 1 and 2. Furthermore, a plurality of buffer chambers can be used simultaneously in order to fill the mold cavity by means of different channel portions 14 a.

In the pressure device 18, a heating device 27 is arranged in the usual manner. Furthermore, around each buffer chamber 13, a second heating device 28 is arranged, whereby the plastic 24 in the buffer chamber can be kept in a plastic state and possibly also be increased. In the buffer chamber, the plastic 24 is kept under pressure at all times.

By way of example, examples of pressures and temperatures in the method according to the invention are given, compared to conventional injection moulding methods for forming similar products.

At a temperature of about 220 ℃, the plastic PP is introduced into the buffer chamber 13 and then into the mould chamber. The plastic is then cooled in the mold cavity 11 to an average temperature of about 207 c, at least close to the walls of the mold cavity. Thereafter the movable wall part 8 is moved forward at such a speed that the energy introduced into the plastic 24 is converted to a large extent into heat, resulting in a temperature increase to about 252 c. This results in an increase in melt index, and thus from about 38 to about 45, and a corresponding decrease in viscosity. In addition, a more favorable melt flow rate is obtained. The plastic 24 is introduced, for example, at a pressure of about 30 bar, while the back pressure during filling of the buffer chamber is a few bar higher. In contrast, in conventional injection molding, PP is introduced into the mold cavity at the same temperature, requiring the application of a much higher pressure (e.g. 180 bar or higher) to cause the plastic to be completely dispersed within the mold cavity. The plasticizing device then requires a much heavier design to produce the desired pressure. This involves the plasticizing unit being subjected to particularly high accelerations and decelerations, resulting in particularly heavy loads. In addition, heavy presses are used in order to compensate the pressure arising from the injection pressure in the closing pressure in order to keep the mold closed. In the process according to the invention, preferably the lowest possible pressure is used, in particular less than 180 bar, more in particular less than 100 bar, and still more in particular less than 60 bar. It has been found that the injection pressure can be less than 50 bar, for example about 30 bar, whereas conventional injection moulding must use pressures of 180 bar or more to form comparable products. Furthermore, with the method according to the invention, a lower shot size is sufficient to obtain the same volume of product.

In the device according to the invention, it is possible, by the way, to use conventional plasticizing units and presses, for example if a mold cavity without movable wall parts is used.

In the method according to the invention, the volume of each buffer chamber 13 may be equal to the product forming volume of the mould cavity, but may also be a multiple thereof, in particular an integer multiple. The ejector member 21 can be controlled such that a desired volume of plastic 24 is introduced into the mold cavity at all times, in particular in correspondence with the product forming volume VP.

The invention is not limited in any way to the exemplary embodiments shown in the description and the drawings. Many variations are possible within the scope of the invention as outlined by the claims.

Thus, other means may be used to evacuate and/or fill the buffer chamber, and furthermore the back pressure may be provided in a different manner, e.g. by means of hydraulic or pneumatic damping, electric means, etc. A mold cavity with other and/or more movable wall parts may be used. Further, for example, a movable core, an insert, or the like can be used in such a mold. Also, a plurality of molds may be used in an apparatus or method according to the present invention. In particular, all combinations of parts of the different embodiments are understood to be included and disclosed in this specification.

Claims (22)

1. An apparatus for manufacturing plastic products, comprising a mold in which at least one mold cavity is provided, at least one runner ending in the mold cavity, wherein a pressure device is further provided for operatively supplying a quantity of at least partially plasticized plastic into the runner, and wherein at least one buffer chamber for buffering plasticized plastic supplied from the pressure device is provided between the pressure device and the at least one mold cavity, and wherein at least one discharge element is provided for pressing plasticized plastic from the at least one buffer chamber into the at least one mold cavity.

2. Device according to claim 1, wherein the at least one mould cavity has a first volume in the first product forming, closed position and wherein the at least one buffer cavity has a buffer volume at least equal to the first volume, more particularly equal to a discrete numerical multiple of the first volume and preferably equal to the first volume.

3. Apparatus according to claim 1 or 2, wherein said at least one mould cavity comprises at least one movable wall portion for defining said first volume, said wall portion being retractable to a position in which the respective mould cavity has a second volume larger than said first volume.

4. An apparatus according to any one of the preceding claims, wherein the pressure device comprises the discharge element.

5. An apparatus according to any preceding claim, wherein the pressure device comprises a plasticising screw.

6. Apparatus according to any preceding claim, wherein the or each buffer chamber includes heating means.

7. An apparatus according to any preceding claim, wherein the or each discharge element comprises a back pressure device for causing a plunger of the discharge element to squeeze plastic flowing into the respective buffer chamber during filling of the buffer chamber.

8. An arrangement according to any one of the preceding claims, characterized in that at least two buffer chambers are provided, and that at least one valve is provided between the pressure device and the at least two buffer chambers, with which valve one buffer chamber and the other buffer chamber can be placed alternately in communication with the pressure device.

9. An apparatus according to any one of the preceding claims, wherein at least two buffer chambers are provided, and wherein the inlet channel portion extends from each buffer chamber into one mould cavity.

10. An apparatus according to any preceding claim, wherein the pressure device is arranged to introduce plasticised plastic into the or each buffer chamber at a pressure of less than 180 bar, more particularly less than 100 bar and preferably less than 60 bar.

11. Apparatus according to claim 10, wherein the pressure device is arranged to introduce plasticized plastic into the or each buffer chamber at a pressure of less than 50 bar, in particular about 30 bar.

12. Apparatus according to any preceding claim, wherein the or each mould cavity comprises a movable wall part which is movable within the mould cavity at a speed such that the quantity of plastic provided in the mould cavity is at least partially re-plasticised.

13. Apparatus according to any one of the preceding claims, wherein each mould cavity comprises a movable wall part which is movable in the mould cavity at such a speed that a quantity of plastic provided in the mould cavity is at least partly compressed.

14. Device according to claims 12 and 13, characterized in that the movable wall part is arranged to move rapidly so that the plastic is compressed during adiabatic heating.

15. A method for manufacturing a plastic product in a mold cavity, wherein a quantity of fully or partially plasticized plastic is introduced into the mold cavity, said plastic is fully or partially plasticized in a press device and then introduced into at least one buffer chamber, from where a quantity, in particular said quantity, of plastic is introduced into the mold cavity by means of a discharge element, such as a plunger.

16. A method according to claim 15, characterized in that the quantity of plastic is introduced into the buffer chamber under a back pressure to prevent or to discharge gaseous inclusions in the plastic.

17. Method according to claim 16, characterized in that the plastic in the buffer chamber is brought into and/or maintained in a homogeneous condition.

18. Method according to any one of claims 15-17, characterized in that the plastic is introduced into the at least one mold cavity in a volume-controlled manner.

19. Method according to any one of claims 15-18, characterized in that plastic is introduced into the mold cavity of the closed mold, at least a part of at least one wall portion of the mold cavity being formed by a movable wall portion, which movable wall portion has been brought into a retracted position during the introduction of at least a part of the plastic into the mold cavity, so that the respective mold cavity has a relatively large volume, and is subsequently brought into a forward-moving position for determining the final product-determining volume, while at the same time at least a part of the plastic is ejected from the mold cavity.

20. Method according to claim 19, characterized in that the volume of plastic introduced into the mold cavity is equal to the product-determining volume.

21. Mould for use in an apparatus according to any of the claims 1-14 or a method according to any of the claims 15-20, characterized in that a supply channel is provided in the mould, which comprises at least one buffer chamber for buffering fully or partly plasticized plastic.

22. A mould according to claim 21, wherein at least one mould cavity has at least one movable wall part, whereby the volume of the mould cavity can be reduced to the product forming volume when the mould is closed.