WO2009061200A2

WO2009061200A2 - Injection molding method and injection molding apparatus

Info

Publication number: WO2009061200A2
Application number: PCT/NL2008/050714
Authority: WO
Inventors: Jan Piet Ooms
Original assignee: Green Investments Bvba
Priority date: 2007-11-08
Filing date: 2008-11-10
Publication date: 2009-05-14
Also published as: NL1034658C2; WO2009061200A3

Abstract

An injection molding method, wherein a mold, included in a press, is brought from an opened position to a first closed position, whereby at least one mold cavity is formed in the mold, having a first volume, after which an at least partially liquid mass is introduced into the mold cavity, after which the mold is brought further into a second closed position, wherein the volume of the mold cavity is smaller than the first volume and the mold cavity takes a shape which is product-forming.

Description

Title: Injection molding method and injection molding apparatus

The invention relates to an injection molding method. The invention relates in particular to an injection molding method wherein use is made of displacement of material to be injection molded in a mold cavity.

From WO2004/024416, a method for injection molding products is known wherein in a mold, use is made of a movable wall part which is held in a retracted position during injection of, for instance, plastic into a mold cavity - and after that is moved forward with speed, displacing the plastic. With such a method, injection molding can take place in an advantageous manner because particularly low closing forces can be utilized and because especially thin- walled products can be manufactured, in all sorts of two-dimensional and three-dimensional shapes. Furthermore, in the products, little or no stress is formed and little or no loss of material properties occurs.

However, the injection molding apparatuses to be used for this known method are relatively complex in structure and use. The object of the invention is to provide an injection molding method which can utilize a relatively simple apparatus or at least an apparatus which deviates at least in part from the known apparatus.

In a first aspect, an injection molding method according to the invention is characterized in that a mold is used which is included in a press. The mold is brought from an opened position in a first closed position, whereby at least one mold cavity is formed in the mold, having a first volume. Then, an at least partially liquid mass is introduced into the mold cavity. After that, the mold is brought further into a second closed condition, wherein the volume of the mold cavity is smaller than the first volume and the mold cavity takes a form which is product-forming. Incidentally, herein, "after that" is understood to mean at least but not exclusively after the start of the introduction of the mass.

The invention further relates to an injection molding apparatus. In a first aspect, an injection molding apparatus according to the invention is characterized in that a press and a mold are provided, wherein the mold is provided with at least one buffer. With the buffer, when the buffer is in a first position, complete closure of the mold is prevented, while with the buffer in a second position, complete closure of the mold is possible.

In a second aspect, an injection molding apparatus according to the invention is characterized in that a press and a mold are provided, wherein the press is provided with at least one buffer. With the buffer, when the buffer is in a first position, complete closure of the mold is prevented, while with the buffer in a second position, complete closure of the mold is possible.

The invention furthermore relates to a control device for an injection molding apparatus or injection molding method.

In a first aspect, a control device according to the method is characterized in that at least one control valve, an injector and at least one buffer are provided. The control device is set for bringing and/or holding the at least one buffer under high pressure for a first period of time and controlling the injector for dispensing a mass during at least a part of this first period of time. Furthermore, the control device is designed to stop dispensing mass by the end of the first period of time by controlling the injector and reducing the pressure in the at least one buffer in a second period of time contiguous to the first period of time.

The invention will be elucidated further on the basis of the drawing. In the drawing:

Fig. 1 schematically shows, in cross-sectional side view, an injection molding apparatus in a first position;

Fig. 2 schematically shows, in cross-sectional side view, an injection molding apparatus according to Fig. 1 in a second position;

Fig. 3 schematically shows, in cross-sectional side view, an injection molding apparatus according to Fig. 1 in a third position; Fig. 4 schematically shows, in cross-sectional side view, an injection molding apparatus according to Fig. 1 in a fourth position, and

Figs. 5A and 5B show an alternative embodiment of an apparatus, schematically represented in a first and second closed position. In this specification, identical or corresponding parts have identical or corresponding reference numerals. In this specification, injection molding is understood to mean at least but not exclusively introducing under pressure into a mold cavity an at least substantially liquid or viscous mass, which is allowed to cool in the mold cavity such that an at least partial product is obtained that can be taken from, can fall or can be ejected from the mold cavity. Injection molding press is understood to mean at least any apparatus with which an injection molding cavity can be closed and/or can be held closed. In the shown and described embodiments, in each case, a mold with one mold cavity will be used. However, it will be clear that also molds with more than one mold cavity can be used, such as for instance multiple molds, stack molds and the like. The invention will be described on the basis of plastic injection molding, but other materials too can be processed, such as metals and, for instance, biopolymers.

Fig. 1 schematically shows, in cross-sectional side view, a portion of an injection molding apparatus 1. For the sake of simplification, drive means, base and supply means for the plastic mass, and for instance ejectors and the like have been omitted from the drawing. It will be directly clear to the skilled person how these can be built-in, insofar as desired, and how conventional modifications known per se on the basis of existing injection molding apparatuses can be utilized within the invention.

Fig. 1 shows a first table 2 and a second table 3, part of an injection molding press 100, which tables 2, 3 can move relative to each other in the direction F and can jointly form, for instance, a mold 101. On the first table 2, on the side proximal to the second table, a first mold part 4 is fitted. On the second table 3, on the side proximal to the first table, a complementary second mold part 5 is fitted. By moving at least one of the tables 2, 3, the mold parts 4, 5 can be pressed one on the other, thereby forming a mold cavity 6 in a manner to be discussed later. On the first table 2, as a part of the first mold part and/or part of the press, an injector 7 is provided, for injecting the plastic mass into the mold cavity.

In the embodiment shown, the first mold part 2 comprises a base part 8 with a raised middle part θ, and a closing plate 10 having a central opening 11 which fits around the middle part 9. An end face 12 of the middle part 9 has a shape which defines at least a part of the mold cavity 6. The injector 7 extends substantially through the middle part 9 and terminates with an opening 13 in the end face 12. In the opening 13, for instance a shutoff, such as a needle shutoff, can be provided. The second mold part 3 comprises, on the side proximal to the first mold part 2, a cavity 14 which defines a second part of the mold cavity 6, surrounded by a closing surface 15 which can cooperate with a closing surface 16 of the closing plate 10. The opening 11 preferably fits as sliding fit around an outer wall 17 of the middle part 9, so that a seal is obtained and plastic from the mold cavity 6 is prevented from flowing therebetween or being pressed therebetween.

On the side of the base part 6, the closing plate 10 is provided with two piston rods 18 which are each coupled to a piston 20 included in a chamber 19 in the closing plate 10. The piston rods 18 each project through an opening in the closing plate 10 and project by an end 21 in a complementary cavity 22 in the base part 8. On two opposite sides of the piston 20, a first 23 and a second channel 24, respectively, are in fluid communication with the chamber 19. A control device 25 is connected to the channels 23, 24 for controlling supply and discharge of a fluid, such as a non-compressible or little compressible liquid, for instance hydraulic oil, towards the chamber 19, on both sides of the piston 20. The pistons with the chambers form buffers 26. The control device 25 can further comprise, for instance, two collecting basins 27A, B connected to the channels 23, 24. The control device can further comprise the injector 7 or at least be connected thereto in order to gear buffering and injection of mass to each other. The buffers 26 can also form part of the control device 25.

In the control device 25, per buffer 26, at least one pump 28 can be provided, for pumping the fluid into the chamber 19, via the first channel 23, on a side of the piston 20 remote from the first table 2. Optionally, also a pump can be provided in the other channel 25. A central controller 30 can be provided for controlling for instance the pump, the injector 7 and the press 100. The press 100 can for instance be controlled in a manner known per se, such as hydraulically, pneumatically or electrically. The base part 8 can be fitted on the first table 2 via an intermediate plate 29.

In the embodiment shown in Figs. 1 - 4, the mold is provided with one mold cavity 6, in the form of a plate or dish part or the like. This is shown merely by way of illustration. Other forms of products to be formed are possible, by adapting the form of the mold cavity both substantially two- dimensionally, such as the plate or dish part shown, and substantially three- dimensionally, such as for instance trays, containers and the like. With a three-dimensional product, a part of the mold cavity can extend along the wall 17 of the middle part 9, for instance between the middle part 9 and the plate 10. The mold cavity could also extend deeper or less deep into the second mold part 5. Further, several mold cavities 6 can be provided, next to each other and/or one above the other, which can for instance be filled with the mass via different injectors 7 and/or via connecting channels.

An injection molding apparatus according Figs. 1 — 4 can for instance be used as follows, wherein as fluid, oil is selected and as mass plastic.

In Fig. 1 is shown an opened position of the mold 101, wherein the first 4 and second mold part 5 are moved away from each other by the press 100. Via the channels 23, on the side remote from the first table 2, oil has been introduced into the chamber 19, below the piston 20. Herein, below is only understood to refer to the position in the Figures. As a result, the piston 20 is forced virtually completely against the side of the chamber 19 proximal to the first table, so that the piston rod 18 pushes off against the base part 8, and the closing plate 10 is pushed away from the base part 8 over a distance S. By thereupon switching off the pump 28 and/or closing valves 31, 32 in the channels 23, 24, the pistons 20 are held in the respective position. As shown in Fig. 1, the closing surface 16 of the closing plate 10 can then be approximately flush with the highest part 33 of the end face 12. The relative position of the closing surface 16 can, for that matter, also be different with respect to the end face 12.

From the position shown in Fig. 1, the mold 101 can be closed in a first closed position, as shown in Fig. 2. In this position, the closing surface 16 of the closing plate 10 sealingly abuts against the closing surface 15 of the second mold part 5. The closing plate 10 is held at a distance S from the base part 8 by the buffers 25. The oil can withstand the closing pressure of the press, applied to the mold 101. In this condition, the mold cavity 6 has an internal volume that is relatively large. The end face 12 of the middle part 9 is held at a distance (S+d) from the opposite surface 34 of the mold cavity 6, wherein d is the desired local wall thickness of the product 102 to be formed. The controller 30 controls the injector 7 and plastic is injected into the mold cavity. As a result of the relatively large volume and especially the relatively large distance (S+d) between the injection opening 13 and the opposite surface 34, the plastic can be introduced rapidly and with relatively little pressure into the mold cavity. During injection, the flow path the plastic travels is relatively short and wide. As a result, the properties of the plastic are preserved well. Furthermore, the heat of the plastic can be maintained relatively well in that there is a small contact surface between the plastic and the mold cavity. As a result, degeneration of the plastic can be prevented at least for the greater part and furthermore, injection is possible at lower temperature than when a comparable product is injection molded with a mold having a fixed volume mold cavity, so that the cooling time can be relatively brief. Furthermore, thinner and more complex products can be manufactured than with a fixed volume mold cavity, and moreover, more viscous plastics can be utilized. As, among others, the injection pressure can be relatively low, the closing pressure of the press can be kept relatively low in comparison with injection molding with a fixed volume mold cavity. During injecting and cooling down of the plastic, this closing pressure will be maintained at least to a large extent.

After at least a part and preferably all plastic has been introduced into the mold cavity, the pressure in the chamber 19 on the side of the pistons 20 proximal to the table 3 is at least partly removed, so that the first table 2 can be moved further with respect to the second table 3 in the direction F into the second closed position shown in Fig. 3. Here, oil from the respective part of the chambers 19 is displaced to the respective collecting basins 27A, reducing the respective volume, while oil can be introduced from the collecting basin 27B into the chamber 19 on the opposite side of the piston 20. As a result, the middle part 9 with the end face 12 can be brought closer to the opposite surface 34, so that the mold cavity 6 has a second volume, smaller than the first volume. In the second closed position, the mold cavity has a form which is product-forming. This is understood to mean at least but not exclusively that the mold cavity has a form of a product 102 such as it can be withdrawn from the moid cavity, as shown in Fig. 4, after the plastic has sufficiently cooled and solidified and the mold 101 has been opened. In this condition, the distance between the end face 12 and the opposite surface 34 is approximately equal to the desired thickness d of the product 102. This thickness d for that matter needs not be the same over the entire product.

By bringing oil under pressure to the side of the piston 20 proximal to the first table 2, the closing plate 20 can be pressed with additional pressure against the closing surface 15 of the second mold part 5. As a result, the closing force can be increased. During movement of the middle part 9 with respect to the second mold part 5, from the first closed position in Fig. 2 to the second closed position in Fig. 3, the plastic in the mold cavity 6 is forced to spread itself through the entire mold cavity. As a result, the desired filling is obtained. Here, the plastic will exhibit a good flowing pattern. Furthermore, through control of the outflow of the oil on the one side and/or supply of oil on the other side of the respective pistons 20 in the chambers 19, the speed of movement and the pattern of movement of the middle part 9 with respect to the second mold part 5 will be controlled so that flowing can be optimized. Through a suitable movement pattern, for instance, the temperature of the plastic can be kept relatively high, so that the viscosity remains relatively low and flowing remains possible even in relatively narrow parts of the flow paths. Optionally, the temperature can even be increased with respect to the injection temperature, by generating sufficient friction in the plastic and/or between the plastic and the wall of the mold cavity 6. After the plastic mass has spread in the mold cavity 6, it is allowed to cool and solidify at least partially. Then, the mold 101 is opened, as shown in Fig.4, and the product 102 is ejected or taken out and moreover, the pistons 20 are returned in the direction of the side of the chamber 19 proximal to the first table 2, by supplying oil and/or discharging oil at the opposite side, to the side of the pistons 20 proximal to the first table. As a result, the injection molding apparatus 1 is once again prepared for a following injection molding cycle.

The pistons 20 can have a relatively small surface, as the closing force the press 100 needs to apply required for closing the mold 101, can remain relatively low. If the injector 7 is provided in the middle part 9, it can remain stationary with respect to the first table 3, which offers technical and economic advantages.

In a variant (not shown) the, a or each mold cavity 6 can extend further, for instance into or between the closing surfaces 15, 16 and/or below one or more chambers 19. Also, both mold parts 4, 5 can have a design comparable to the first mold part 4 in Figs. 1 - 4. The parts of the mold cavity outside the end face 12 can for instance have a fixed form and size and be filled only through displacement of the plastic. They can also be provided with an individual runner, for instance with multipoint injection. Injection of plastic can also take place only via such a part or via an edge injection, instead of via the injector mentioned.

In the first closed position, the mold is closed at a distance from maximal, in the second closed position it is closed further and in particular is closed maximally. Herein, closed maximally is understood to mean but is not limited to a position in which the tables 2, 3 of the press are brought at a minimal distance from each other, wherein each distance is defined, for instance, by the total thickness of the mold 101, as shown in Fig. 3. At a distance from maximally closed means that movement in the direction of maximally closed is still possible. In Fig. 5A and B, schematically, a part of an injection molding apparatus 1 is shown, in a further alternative embodiment. Here, the first mold part 4 is clamped on the first table 2 and the second mold part 5 on the second table 3. The first mold part 4 is provided with or connected to the injector 7 and bears a part of the mold cavity 6, comparable to the end face 12 in Fig. 1 - 4. The second mold part 5 comprises a base part 8, an annular first movable part 40 and, movable therein, a second part 41. The second part has an end face 42 with a form comparable to the surface 34 in Figs. 1 - 4. End face 12 and end face 42 form substantially a mold cavity 6, comparable to that according to Figs. 2 and 3. The second part 41 is borne by one or more pistons 43 in a matching number of first chambers 44. The first part 40 is borne by one or more pistons 45 in a corresponding number of second chambers 46. The or each first chamber 44 is in communication via for instance a control valve 31 with at least one second chamber 46. The first part 40 has a closing surface 15 proximal to the closing surface 16 of the first mold part 4. When closing the mold 101, the closing surfaces 15, 16 press against each other so that a first closed position is obtained. Oil or another pressure medium is brought below the first part 40, so that this projects over a distance Si with respect to a top face 47 of the base part 8. Here, the second part 41 is moved in the direction of the second table 3, so that the face 42 lies at a distance S2 below the face 47. In this condition, represented in Fig. 5A, the mold cavity has a relatively large volume. When at least a part and preferably all plastic has been introduced into the mold cavity 6, the or each valve 31 is opened and, as a result of the pressure applied by the press 100 to the mold 101, a part of the oil is forced from the second chambers 46 to the first chamber 41, so that the first part 40 is displaced in the direction of the second table 3 and simultaneously the second part in opposite direction. As a result, the volume of the mold cavity is reduced until the mold is in the second closed position as represented in Fig. 5B. Here, the plastic is spread through the mold cavity. After the product 102 in the mold cavity has solidified at least to a large extent, the mold is opened, the product 102 is taken out and the first and second parts 40, 41 are returned to the initial position shown in Fig. 5a. To that end, a pump can for instance be provided (not shown) for pumping the oil back from the second to the first chamber. Depending on the difference in size of the chambers 44, 46, the distance over which the first and second part 40, 41 are moved with respect to the base part 8 is defined.

With an apparatus 1 and method according to the invention, in mold labelling can be effected in a simple manner, in particular because the pressure is relatively low during injection. Furthermore, injection to an inside of a product can be effected in a simple manner, or to a side opposite a label, in case labelling takes place.

In an alternative method, the mold cavity 6 can be enlarged after a first amount of plastic has been introduced into the mold cavity for instance by reducing the closing pressure of the press somewhat and/or returning the closing plate 10 with the aid of the buffers, in particular the pistons 20, in the direction of, or towards the first closed position as represented in Fig. 2, after which a second amount of plastic can be introduced. This may be a different plastic, so that for instance a 2C injection molded product can be obtained. A second injector can be provided, for instance in the second mold part 5.

The invention is not limited in any manner to the embodiments represented in the description and the drawing. Many variations thereon are possible within the framework of the invention as outlined by the claims.

For instance, buffers can be designed differently, for instance in that motor driven elements are used which can control stepwise closing of the mold via the first closed position to the second closed position. Also, a mechanical blocking can be built in, instead of or in addition to the buffers, which can hold the mold in the first closed position as shown in Fig. 2 and, upon removal of the blocking, releases it for closing in the second position, as shown in Fig. 3. In Figs. 1 - 4, the first table 2 is of movable design. However, naturally, also the second table can be of movable design, or both tables. The closing direction can be arranged in any desired position, depending on, for instance, the press to be used. The mold can have a different construction, as long as it can take at least the desired first and second closed position.

Claims

1. An injection molding method, wherein a mold, included in a press, is brought from an opened position into a first closed position, whereby at least one mold cavity is formed in the mold, having a first volume, after which an at least partially liquid mass is introduced into the mold cavity, after which the mold is brought further into a second closed position, in which the volume of the mold cavity is smaller than the first volume and the mold cavity takes a form which is product-forming.

2. An injection molding method according to claim 1, wherein the mold in the press is brought into the first closed position by closing the press as far as some distance from maximally, and is brought into the second closed position by closing the press virtually maximally.

3. An injection molding method according to claim 1 or 2, wherein in the mold at least one volume of fluid is brought under pressure by and/or when bringing the mold into the first closed position, and wherein upon pressing the press further to the second closed position of the mold, said at least one volume is reduced and/or lowered in pressure.

4. An injection molding method according to any one of the preceding claims, wherein in the first closed position a movable wall part of at least one mold cavity is held in a rearwardly drawn position and upon bringing the mold in the second closed position is moved in the direction of an opposite wall part of the mold cavity.

5. An injection molding method according to any one of the preceding claims, wherein during closure of the press, a buffer allows closure of the mold into in the first closed position, wherein through removal of pressure in the buffer, the press can close the mold into the second closed position.

6. An injection molding method according to any one of the preceding claims, wherein the mold is brought from the first closed position into the second closed position when at least virtually the entire mass has been introduced into the mold cavity.

7. An injection molding apparatus, provided with a press and a mold, wherein the mold is provided with at least one buffer with which, in a first position of the buffer, complete closure of the mold is prevented and, in a second position of the buffer, complete closure of the mold is possible.

8. An injection molding apparatus, provided with a press and a mold, wherein the press is provided with at least one buffer with which, in a first position of the buffer, complete closure of the mold is prevented and, in a second position of the buffer, complete closure of the buffer is possible.

9. An injection molding apparatus according to claim 7 or 8, wherein the at least one buffer is included between clamping plates of the press.

10. An injection molding apparatus according to any one of claims 7 — 9, wherein the at least one buffer is provided with a control device with which in the buffer a high pressure and a low pressure can be built up.

11. An injection molding apparatus according to any one of claims 7 - 10, wherein in the at least one buffer or the joint buffers a pressure can be built up which is such that the buffer(s), at least in the first position, can withstand the closing pressure of the press.

12. An injection molding apparatus according to any one of claims

7 — 11, wherein the or each buffer is a hydraulic or pneumatic buffer, provided with an overflow valve.

13 An injection molding apparatus according to any one of claims

7 - 12, wherein at least two buffers are provided.

14. An injection molding apparatus according to any one of claims

7 — 13, wherein an injector is provided which terminates in a space of a mold cavity or runner channel which, with the at least one buffer in the first position, has a first volume and, with the at least one buffer in the second position, has a second volume, wherein the second volume is smaller than the first volume.

15. An injection molding apparatus according to any one of claims 7 — 14, wherein the press is a hydraulic or electric press.

16. A control device for an injection molding press, provided with at least one control valve, an injector and at least one buffer, wherein the control device is set for bringing and/or holding the at least one buffer under high pressure for a first period of time, controlling the injector for dispensing a mass during at least a part of said first period of time, and, by the end of the first period of time, stopping the dispensing of mass by controlling the injector and reducing the pressure in the at least one buffer in a second period of time contiguous to the first period of time.