CA2274850A1

CA2274850A1 - Process for manufacturing injection mouldings

Info

Publication number: CA2274850A1
Application number: CA002274850A
Authority: CA
Inventors: Karl Hehl
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-12-13
Filing date: 1997-12-03
Publication date: 1998-06-18
Also published as: DE19651879A1; EP0944466A1; ATE202740T1; JP2001505833A; WO1998025751A1; EP0944466B1; DE19651879C2; DE59703974D1

Abstract

In a process for manufacturing injection mouldings in an injection moulding machine for processing plasticized masses, a regulated subsequent pressure is applied by the injection moulding unit (13). At least in the areas of the moulding away from the sprue, the subsequent pressure is generated by a regulated volume alteration of the mould cavity. The subsequent pressure can thus be reliably distributed during the subsequent pressure phase even in the case of complex mouldings.

Description

Method of producing moldings Specification Reference to related applications The present application claims the priority of the German patent application 879.2, filed on the 13.12.1996, whose disclosed content is hereby expressly also made the subject matter of the present application.
Technical Field The invention relates to a method of producing moldings in accordance with the preamble of claim 1.
Under the term "cross-section of flow" is understood the cross-section actually flowed through by the plasticisable substance during the injection process, per section of the mold cavity. The cavity of the mold includes all the cavities of the mold through which the plasticisable material flows as it is injected, i.e. if necessary also the spree.
Prior art Underlying the preamble is a method which, among experts, may be equated with injection-stamping. This comparison is apt if, before the injection of the plasticisable substance, the mold is closed apart from a gap, the plasticisable substance is completely injected into this gap and the gap is then closed, injection-stamping the molding. Basically there is also the possibility, during the injection process, of opening the already closed mold again or reducing the force already applied for keeping the mold closed. This method is used in order to manufacture e.g. thin walled parts such as CDs, since there are here occasionally difficulties in pressing by a normal injection process the plasticisable substance into the last corners of the mold cavity.
After the injection of the plasticisable substance, the latter is cooled in order to shape the molding. In this cooling phase, there are inevitably contractions of the substance, such as e.g. a liquid plastic, which are normally compensated for by a holding pressure phase with the aid of the plasticising screw. To this end, the plasticising screw is held under pressure during the holding pressure phase to correspond with a pre-determined profile. Typically this pressure profile is such that after the change from the injection phase into the holding pressure phase, the highest holding pressure is set in order then to become smaller and smaller towards the end of the holding pressure phase. The course of the pressure profile is derived from the material-dependent course of the volume contraction as a result of pressure and temperature according to the typical pVT diagram for each material during the cooling phase.
2 0 These processes for the shaping of standard moldings are sufficiently well-known;
however problems arise if the mold part has regions with a different cross-section of flow, such that notches or sprues seal in the cooling phase so early that the holding pressure cannot be maintained in a controlled n~anaer in this region.

i i , , . ~. DL
Altered page - 2a From DE-As 1 I 27 579 is known a way of so altering the holding pressure of the plasticising cylinder initially that, although the substance is injected into the open mold, the latter is then closed during the injection process, this being intended to ensure that constantly so much material continues to be injected fi~om the plasticising cylinder that the nozzle does not close. It is essential here that the mold is completely closed towards the end of the injection process. Furthermore, I adjustable holding pressure is to be applied from the nozzle in known fishion;
however, control of the holding pressure from the closing side of the mold does not come about, such that there is no overlaying of a controlled holding pressure inside the injection mold.
From the Japanese disclosure document 58 008622 is known a way of injecting material into a mold cavity via an injection molding unit. The injection itself comes about with the mold closed. A portion of the mold wall is movable, however, such that, after holding pressure has been applied, the size of the mold cavity can still be influenced. The holding pressure is applied, however, by the injection molding unit.
There is no mention of control of the holding pressure both on the injection molding 2 0 side and by the movable part within the injection mold.
From EP 348 129 A2 is known a way of generating a vacuum in the injection mold before the injection process. The material is then injected with a sudden pressure increase, such that there is no premature hardening even on the surface. The nozzle -.
is then closed and adjustment of the holding pressure only occurs on the mold closing side. The mold cavity is not affected by holding pressure from both sides.

'i From DE-U 88 06 805 is known an injection mold in which an additional movement of the tool is derived from the closing movement. This movement comes about shortly before cooling and thus before the molding has completely or partially sealed. Here a gap in the injection mold is reduced in size and the material is pressed from the middle into two mold cavities at the edge. In addition, this i movement can also come about through a separate pressure means. This is thus able to be equated with injection-stamping, without the holding pressure being controlled by this means.
Summary of the invention Proceeding from this state of the art, the purpose underlying the invention is to make possible, in the holding pressure phase, reliable distn'bution of the holding pressure even on complicated parts.
This object is fulfilled by a method with the features of claim I .
In this method, the contraction of the plasticisable material is realized even in the thick-walled region of the mold cavity, which can no longer be reached after the sealing of the regions having a low cross-section of flow, in that the contraction of the molded part is compensated for at least in these regions by a controlled volume adjustment of the mold cavity by means of the mold closing unit. In the other regions, which may be reached. by the holding pressure built up b~ the injection 2 0 molding unit; the contraction of the molded part is compensated for by the injection molding unit, if necessary in conjunction with the mold closing unit or the pressure means. By this means, individual regions of the molding can be targeted specifically in respect of the holding pressure, the possibility also existing in principle of letting the holding pressure take a different course in different regions and by this means, with a gradual reduction of the holding pressure, inner tensions occurring in the molded part through material displacement in too cold a melting are avoided.
The only pre-requisite for this is that both the injection molding unit may be adjusted in respect of the holding pressure and the volume alteration of the mold cavity may be adjusted in respect of the force that is necessary to do this.
The units effecting the holding pressure can generate, independently of one another, a common pressure or force profile. The holding pressure is thus controlled by co-operation between the injection molding unit and the mold closing unit and if necessary a separate pressure means according to claim 2. In an extreme case, it is also possible actively to relieve the stress on the mold cavity e.g. by withdrawing the screw. If the molded part has regions close to the spree, which lie between the spree and the regions remote from the spree, it does not cause any damage if the region close to the spree seals before the holding pressure has been correspondingly built up and reduced in the regions remote from the spree. In the regions remote from the spree, the holding pressure can be controlled by the mold closing unit.
Short description of the figures Fig. 1 A diagrammatic view of an injection molding machine with associated 2 0 control possibilities, ' Fig. 2 a flow chart for screw-dependent stamping of the molding, Fig. 3 a flow chart for filling the mold cavity in dependence on the mold path, as well as Fig. 4 a schematic view of a molding acted on according to the method, Fig. 5 a view according to Fig. 4 with additional injection-stamping.
Detailed description of preferred embodiments The invention is now explained in greater detail by way of example with reference to the attached drawings. However, the embodiments are just examples which are not intended to limit the inventive concept to a specific physical arrangement.
The method is used for producing moldings which are manufactured on an injection molding machine for processing plasticisable substances. As plasticisable substances can 6e considered plastics, powdery _substances or even ceramic substances. The method can be carried out basically on any injection molding machine; however reference is made below to the structure of a plastics injection molding machine shown in the drawings.
An injection molding machine of this type has, according to Fig. 1, a mold closing unit 11, which normally receives a mold 12 between its mold carriers. The mold consists, as per the embodiment given by way of example, of two parts 12b, which .
enclose between them, in the closed state, a mold cavity 10. The mold cavity is accessible from outside via a spree 14, through which the plasticisable substance, 2 0 such as e.g. a plasticised thermo-plastic plastic, can reach the mold cavity. The plasticisable substance is plasticised in a plasticising cylinder 18 of an injection molding unit 13, preferably by a feed screw 17 configured as the feeding means, and led to the sprue channel in a correspondingly metered amount.
At the beginning of the shaping operation, the mold closing unit 11 transfers parts 12b of the mold 12 into a position in which the complete clamping force, necessary for the production of the molding, is not yet applied, indeed if necessary no such force is yet applied. The plasticised substance is injected into the mold thus closed or almost closed, by the injection molding unit, into the mold cavity 10. Once the plasticised substance has been injected, the necessary locking force to produce the molding is applied. Thus the mold is first of all filled and now the plasticised substance has to be cooled so that the molding 12 can be formed.
In order to counteract any contraction of the molding during cooling, controlled holding pressure is applied by the injection molding unit 13. To this end, the injection unit 19 of the injection molding unit 13 is triggered via a control valve 1 S.
The control valve I S is col~neeted t4 the two cylinder cavities of the injection unit 19 via two lines, whilst the two further lines are connected on the one hand with a pressure source (connection P) and on the other hand with a tank (connection T) to recirculate a hydraulic medium. In the two lines 20, which lead to the injection molding unit 13, pressure sensors 2I, 22 are provided which record the actual 2 0 pressure values P~~~ P~,,B in the two lines 20. The pressure difference of the actual pressure values thus determined corresponds to a pressure or a force which the feed screw exerts on the mold cavity I0. The actual pressure values are notified to a regulator 23, which adjusts the pressure according to a graph of desired pressure 28 to pre-determined desired pressure values PS. Naturally, with an electrically driven injection molding unit, similar values are produced in a different manner, as is common knowledge to the expert.
At the same time, however, the holding pressure is also influenced in a controlled manner by an alteration in the volume of the mold cavity 10, e. g. on the closing side of the mold by the mold closing unit 11. This acts particularly but not exclusively on regions l0a of the mold cavity, remote from the sprue _ and having a large cross-section of flow, and the corresponding regions of the molding which would otherwise be cut off from the holding pressure applied by the injection molding unit by regions lOc of the mold cavity which have a small cross-section of flow sealing premaxurely. To this end, the mold closing unit is provided with hydraulic medium from a control valve 16 via Iines 24. This control valve is also connected on the one hand with a pressure source (connection P) and on the other hand with the tank (connection T). Via a pressure sensor 25, preferably a mold inner pressure sensor, the actual pressure value P~t~mold, which bears on the mold is measured which corresponds to the force for holding the mold closed. The actual value is notified to a controller 26, which tracks from the actual value a desired force value PF
in accordance with the graph of desired force or desired pressure 27. Instead of applying the volume change only via the mold closing unit, this can come about together with some other pressure means such as a stamping die 30, which may be 2 0 adjusted separately and independently from the mold closing unit.
Through the controlable injection molding unit in conjunction with the mold closing unit which can be adjusted in respect of the clamping force, from both sides controlled holding pressure can be exerted in a defberate manner also on different regions of the mold cavity 10. The actual holding pressure values on the molding can of course be measured also by mold inner pressure sensors, as indicated optionally in Fig. 1 by the reference used with P/P~~ ~~;~,~. In these cases, adjustment is again made at the control valves I5, I6.
If the holding pressure has been lowered in a controlled manner at least in regions l0a remote from the spree and having a large cross-section of flow, through the application of a controlled clamping force by means of the form closing unit 11, the molding can be removed tom the mold. In Fig. 4 is shown a mold cavity IO which can be influenced by a corresponding two-sided control of the holding pressure.
The molding produced in this mold cavity is e.g. a compact disc, the region IOc representing the CD. Proceeding from the central spree 14, first of all a region l Ob, close to the spree and having a large cross-section of flow, is provided which merges via. a region IOc, close to the spree and having a low cross-section of flow, into the region 10a, remote from the spree and again having a large cross-section of flow. If such a part is produced in the known method according to prior art, the region lOc seals so early that holding pressure cannot be applied correctly any more in the region l0a remote from the spree. The plasticisable substance led into the mold cavity 10 via the spree 14 is therefore influenced as far as the region lOc, close to the spree, by the injection molding unit 13 with a controlled holding pressure. Thus the thick-walled region l Ob, close to the sprue, is influenced by the 2 0 injection molding unit 13 in respect of the holding pressure, the holding pressure PS
being set. In the region I Oa, remote from the sprue, on the other hand, the holding pressure applied by the injection molding unit no longer has any influence.
For this reason, the holding pressure is applied there by adjusting the clamping force via the mold closing unit 11 in the form of holding pressure PF.

'i The injection itself comes about in such a way that either a gap S (Fig. 1 ) is injected into, which is closed after or during the injection process. This can e.g.
come about in that the mold is not completely closed or is only closed as far as a certain force region e.g. below the maximum clamping force. Or there is the possibility of opening the already closed mold again through the injection pressure as the plasticised material is injected, the parts 12b of the mold 12 moving away firom one another again. If no opening comes about in this way, during the injection process at least the clamping force which is not yet completely applied can be reduced. If then the clamping force needed to produce the molding 10 is applied, the deformation here occurring leads to a process which is comparable with injection-stamping. A corresponding method sequence is shown in Fig. 2.
In step S 1, the mold is closed as far as a coining gap ds. Thereafter, in step S2, the injection of the plasticised substance is started. If the feed screw 17 reaches a certain injection stroke mark according to the enquiry in step S3, the mold is closed in step S4. At this point, the two possibilities for adjusting the holding pressure come into play. On the side of the injection molding unit, in step S5, the course of the holding pressure is adjusted in controlled fashion. On the closing side of the mold, a program of the clamping force is started as per step S6. If both programs have finally led to a lowering of the holding pressure, after cooling has occurred, 2 0 the mold is opened in step S7 and the part can be removed from the mold in step S8.
Alternatively, however, there is also the possibility that once, during the closing movement, the moveable half of the mold 12 has covered a certain pre-determined path, the injection process is started. Fig. 3 represents a corresponding method sequence, in which steps comparable with the preceding method sequence as per Fig. 2 are provided with the same reference numbers. The method sequence starts with the process " mold close " as per step S9. I~ during the closing of the mold, a specific clamping unit stroke mark is reached shortly before the mold closes or with a Iow clamping force as per step S 10, the injection process as per step S 1 I
is started. At this point, the two holding pressure closed loop controls in the form of the holding pressure on the side of the injection molding as per step SS and in the form of the program of the clamping force on the side of the mold closing as per step S6 come into play again. After the adjustment of the holding pressure has ended, the mold can then be opened again in step S7 and the part can be removed from the mold in step S8.
The advantage of this variant is that the time needed for positioning the mold closure can be saved.
Preferably the necessary volume of the plasticised substance for filling the mold cavity is not injected completely during the injection process and a pad of the substance can be provided in front of the screw for later adjustment. The lowering of the holding pressure comes about on the side of the mold closing by reducing the maximum clamping force.
When the volume of the mold cavity 10 is changed, the connection between the 2 0 regions 10a, having a large crass-section of flow, and the regions 1 Oc, having a small cross-section of flow, can be interrupted at flue same time as a result of the path ds (Fig. 5) of the mold necessary for this, which is the equivalent of a stamping of the notches or sprees.

In Figs. 2 and 3 are given the possibilities, shown in boxes in broken lines there, of opening or closing the mold according to the program as an alternative on the mold closing side to the program for the locking force as per step S6. This can come about e.g. in that the parts 12b of the mold 12 are moved away from one another during the injection process as a result of the injection pressure and thereafter the coining gap 8 occurring in Fig. 1 or ds in Fig. 5 is closed again under the influence of the mold closing unit I I. -Fig. 5 makes clear finally that the parts can be driven together as far as a stamping gap ds; the movement which then occurs during injection stamping can also be used to interrupt simultaneously the connection between the regions 10a, having a large cross-section of flow, and in the region l Oc with a low cross-section of flow.
Likewise, different regions of a mold cavity can also form different moldings, which are subjected to different holding pressure profiles on the injection molding side and on the mold closing side.
It goes without saying that this specification can be subjected to the most different modifications, alterations and adaptations which are within the range of equivalents to the pending claims.

Claims

page 11

1. Method of producing moldings on an injection molding machine for processing plasticisable substances, using a mold (12), which is accommodatable in a mold closing unit (11) and has at least one mold cavity (10), which has a sprue (14), and the plasticisable substance is injected into said cavity by means of an injection molding unit (13), the mold cavity (10) having regions (10a) with a large cross-section of flow and regions (10c) with a small cross-section of flow, by comparison, the method having the following steps:
- transferring the parts (12b) of the mold (12) into a position, in which the full clamping force required to produce the molding is not yet applied, - injecting the plasticised substance into the mold cavity (10) of the mold (12) through the spree (14), - applying the required clamping force, - applying a controlled holding pressure through the injection molding unit (13), and - removing the molding (10) from the mold, characterized in that the controlled holding pressure, on the one hand, is generated into regions (10c), close to the sprue and having a small cross-section of flow, by means of the injection molding unit, and, on the other hand, is generated both in the regions close to the sprue, insofar as they are overlaid with the holding pressure applied by the injection molding unit (13), and in the regions (10a), remote from the spree and having a large cross-section of flow, by means of the mold closing unit (11), the mold (12) being moved according to a clamping force so as to change the volume of the mold cavity in a controlled manner.

2. Method according to claim 1, characterized in that an additional change in volume of the mold cavity (10) is effected by means of a separately controllable pressure means (30).

3. Method according to claim 1, characterized in that the parts (12b) of the mold (12) are moved away from each other during the injection process as a consequence of the injection pressure.

4. Method according to claim 1, characterized in that, during the injection process the clamping force, which has not yet been fully applied, is changed according to a profile for such force.

5. Method according to claim 1, characterized in that, during the application of the required clamping force, the molding (10) is at least partially injection-stamped.

6. Method according to claim 1, characterized in that, as soon as the movable part of the mold (12) has covered a certain prescribed ,distance during the closing operation, the injection according to a prescribed profile is started.

7. Method according to claim 1, characterized in that, as soon as the movable part of the mold (12) has covered a certain prescribed distance during the closing operation, the application of the holding pressure according to a prescribed profile is started.

8. Method according to claim 1, characterized in that the volume of the plasticisable substance required to fill the mold cavity is not fully injected during the injection process, and in that the holding pressure is controlled via the injection molding unit so as to supply the residual volume.

9. Method according to claim 1, characterized in that the holding pressure, at least in regions (10a) remote from the spree and having a large cross-section of flow, is controlled by means of the force for keeping the mold closed in dependence on an internal meld pressure determined in each case by an internal mold pressure sensor.

10. Method according to claim 1, characterized in that, in the event of a change in volume of the mold cavity (10), the connection between the regions (10a) having a large cross-section of flow and the regions (10c) having a small cross-section of flow is interrupted at the same time.