WO1998019848A1

WO1998019848A1 - Process at an injection moulding of parts with hollow profiles

Info

Publication number: WO1998019848A1
Application number: PCT/SE1997/001742
Authority: WO
Inventors: Anders ÅBERG
Original assignee: Perstorp Ab
Priority date: 1996-11-04
Filing date: 1997-10-17
Publication date: 1998-05-14
Also published as: SE9604010D0; SE507950C2; AU4971997A; SE9604010L

Abstract

Process where a part made of thermoplastic resin is manufactured. The mould (1) is provided with at least one material inlet (4), at least one fluid channel (10) and at least one moveable mould member (2). The mould (1) is further provided with at least one pressure sensor (5). The injection moulding process is initiated by reducing the volume of the mould cavity, whereby an amount of molten thermoplastic resin is injected. A skin is allowed to form in the resin. The pressurised fluid is introduced into the molten resin, forming a fluid cavity (11). The fluid exerts a pressure towards the walls of the mould cavity (3). This pressure is detected by the pressure sensor (5), which guides the movement of the moveable mould member (2). The volume of the mould cavity (3) increases while the fluid is allowed to flow into the growing fluid cavity (11). The resin is allowed to solidify further while the fluid pressure is maintained.

Description

Process at an injection moulding of parts with hollow profiles.

The present invention relates to a process at an injection moulding of parts with hollow profiles in a mould with an adjustable mould cavity volume.

Transport packages manufactured through injection moulding of thermoplastic resin are quite common nowadays. These are normally reinforced by means of ribs and profiles so that they will meet the demands on mechanical stability. A disadvantage with such ribs and profiles are that the weight of the package is increased and that dirt collecting pockets, which are difficult to clean, are formed. It can also be desired to design parts with thicker profiles for other reasons. This will, however, in common injection moulding processes cause problems since the material thickness should not differ to much within the part. Differences in material thickness will inevitably cause problems with warping of the part.

This problem can however be solved by injecting for example a gas through separate inlets or together with the molten thermoplastic resin. The weight of the part can hereby be decreased and the ribs and profiles be made thicker, as seen from the outside. It is also known that a closed hollow profile is stronger than an open profile, as for example a traditional rib, with the same amount of material used. Hereby the number of ribs and profiles can be reduced whereby the product will be easier to clean.

One disadvantage with gas channels achieved through injection of a gas together with the thermoplastic material is that the thickness of the material in the part will be very difficult to guide since the thermoplastic resin is very soft in a molten or a semi-molten state. This might even cause a so called blow-out where the fluid cavity within the thermoplastic resin ruptures the bubble of molten thermoplastic resin. This problem can be avoided by allowing the thermoplastic resin to solidify closest to the surface of the mould cavity before the introduction of the gas. The mould cavity will then have to be completely filled with thermoplastic resin whereby the surplus material will have to be transferred into a secondary cavity when the gas is introduced. A disadvantage with this method is that the resin in the secondary cavity is redundant, which means that the material saving is lost anyhow. The material can of course be reused through recycling, but a polymer chain break-down which adds through the process is the inevitable result of such reuse. The mould can also be provided with moveable mould members so that the volume of the mould cavity can be increased in connection to the introduction of the gas. Several parameters will, however, affect the final result, whereby the products received will not always fill the different demands such as mechanical stability and even material thickness.

The above mentioned problems has through the present invention been solved whereby a part, made of thermoplastic resin, with well defined hollow profiles have been achieved. The invention relates to a process at an injection moulding procedure where a part made of thermoplastic resin is manufactured. The part includes a hollow profile formed by means of a pressurised fluid and a mould. The mould is provided with at least one material inlet for molten thermoplastic resin and at least one fluid channel through which the pressurised fluid is injected and evacuated. The mould is also provided with means for increasing and decreasing the volume of the mould cavity of the mould by means of at least one moveable mould member. The moveable mould member is operated by means of hydraulic cylinders, pneumatic cylinders, stepping motors or the like. The invention is characterised in that the mould is provided with at least one pressure sensor placed mainly as far as possible from the material inlet, preferably on the part of the mould cavity that will be filled with molten thermoplastic resin last.

In a mould provided with only one fluid channel the channel will be used for injecting and evacuating the fluid. If the mould is provided with more than one fluid channels, a first cannel or a first group of channels can be used for injection of the fluid while a second channel or a second group of channels can be used for the evacuation of the fluid.

The injection moulding process is initiated by reducing the volume of the mould cavity to a minimum by sliding the moveable mould member to its innermost position. An amount of molten thermoplastic resin is thereafter injected through the material inlet into the mould cavity so that the molten thermoplastic resin exerts a pressure towards the walls of the mould cavity. The pressure is detected by the pressure sensor. The molten thermoplastic resin is allowed to solidify somewhat so that a skin of solid thermoplastic resin is formed closest to the walls of the mould cavity. The pressurised fluid is thereafter introduced through the fluid channel, into the molten thermoplastic resin wherein a fluid cavity is formed in the same. The fluid in the fluid cavity exerts a pressure towards the inner walls of the mould cavity via the molten thermoplastic resin and the skin of solidified thermoplastic resin. This pressure is detected by the pressure sensor which guides the movement of the moveable mould member so that this is slided outwards. The sliding motion of the moveable mould member is suitably started at a predetermined pressure. The velocity of the mould member is suitably higher at a higher pressure. The volume of the mould cavity will hereby increase while the fluid is allowed to flow into the growing fluid cavity in the molten thermoplastic resin until the moveable mould member has reached its outermost position. The molten thermoplastic resin is then allowed to solidify further while the pressure in the fluid cavity is maintained. The pressure in the fluid cavity is thereafter decreased to ambient pressure by evacuating the fluid so that the mould can be opened and the part be removed. The process can then be repeated. The fluid used is suitably a gas, preferably an inert gas. A second pressure sensor is suitably placed in direct contact with the fluid. This pressure sensor is suitably placed in connection to the fluid channel, between the cavity and the valve. The results of the measurement from the first and the second pressure sensors are compared, whereby the degree of solidification of the thermoplastic resin can be determined. The pressure on the walls of the mould cavity will decrease when the thickness of the solidified thermoplastic skin, closest to the inner walls of the mould cavity, increases. This effect is especially pronounced in connection to corners in the mould cavity. The difference between the pressure exerted on the walls of the mould cavity and the pressure within the fluid cavity will hereby increase as the thermoplastic material solidifies through cooling. The pressure sensors are suitably pressure sensors of piezo-electric type.

At least one temperature sensor is suitably placed in connection to the inner walls of the mould cavity whereby at least the degree of solidification in the outer skin of the molten thermoplastic resin can be detected. The solidification time, before the introduction of pressurised fluid or the initiation of the increase in the volume of the mould cavity, can hereby be guided through signals from the temperature sensor.

The pressure of the fluid in the cavity is preferably guided by the pressure sensor placed in connection to the mould cavity. The pressure sensor opens and closes a fluid valve so that a desired fluid pressure is maintained within the cavity whereby the moveable mould member can be slided out with a constant velocity.

The invention is explained further together with enclosed drawings showing an embodiment of the invention wherein, -figure 1 shows schematically and in cross-section a part of a mould with a moveable mould member in its innermost position and a mould cavity that just has been filled with molten thermoplastic resin.

-figure 2 shows schematically and in cross-section a part of a mould with a moveable mould member in its outermost position and a mould cavity where a gas has been injected so that a gas cavity is formed in the molten thermoplastic resin.

Figures 1 and 2 show different steps in the same embodiment of the process according to the invention.

Figure 1 shows schematically and in cross-section a mould 1 with a moveable mould member 2 and a mould cavity 3 which has just been filled with thermoplastic resin through a material inlet 4. The volume of the mould cavity 3 was reduced to a minimum prior to the injection by sliding the moveable mould member 2 to its innermost position. The mould 1 is further provided with a first pressure sensor 5 separated from the material inlet 4. The molten thermoplastic resin exerts a pressure on the walls of the mould cavity, which pressure is detected by the first pressure sensor 5. A guiding signal from the first pressure sensor 5 will, via a control unit 6, close a resin injection valve 7 when the muold cavity 3 is completely filled with thermoplastic resin. The thermoplastic resin is then allowed to solidify somewhat so that a skin of solid resin is formed closest to the walls of the mould cavity 3. A temperature sensor 8, connected to the control unit 6, measures the temperature of the resin. The control unit 6 will open a fluid valve 9 when the temperature reaches a predetermined value. A gas under pressure will thereby be introduced through a fluid channel 10 into the molten thermoplastic resin so that a gas cavity 1 1 (fig. 2) is formed in the same. The gas in the gas cavity 1 1 exerts a pressure towards the walls of the mould cavity 3 via the molten thermoplastic resin and the solidified skin of the same. This pressure is detected by the first pressure sensor 5. The first pressure sensor 5 will, via the control unit 6, guide the manoeuvring of the moveable mould member 2 so that this is slided outward at a predetermined pressure. The volume of the mould cavity 3 will hereby increase while the gas is allowed to flow into the growing gas cavity in the molten thermoplastic resin until the moveable mould member 2 has reached its outermost position. The thermoplastic resin is allowed to solidify further while the pressure in the gas cavity is maintained. The degree of solidification of the thermoplastic resin is controlled by combining temperature measurement with a differential pressure measurement between the first pressure sensor 5 and a second pressure sensor 12, placed in the fluid channel 10. The thermoplastic resin will shrink as it solidifies. The difference between the pressure exerted on the pressure sensors 5 and 1 1 will therefore increase. This effect is especially pronounced if the first pressure sensor is placed close to a corner. The process will be concluded by decreasing the pressure in the gas cavity 1 1 to ambient pressure by evacuating the gas through the fluid channels 10. The mould 1 can thereafter be opened and the part removed whereupon the process can be repeated.

The velocity of the moveable mould member 2 can also be guided so that it depends on the pressure in the gas cavity 1 1 . The outwards directed motion will hereby be initiated at a certain threshold value and the velocity will increase with increasing pressure or decrease with decreasing pressure. The pressure is measured either by the first pressure sensor 5 or the second pressure sensor 1 1 or by combining the two.

The invention is not limited to the embodiments shown since these can be varied in different ways within the scope of the invention. The differential pressure measuring can, for example, as described in connection to figure 1 and 2, be replaced by measuring the temperature utilising the temperature sensor 8 only. The second pressure sensor 12 will hereby be excluded.

In cases where the desired placement of the fluid valve 9 is in direct connection to the mould cavity 3, a second pressure sensor 12 can be arranged in connection to the middle of a straight surface of a part of the mould cavity 3 that forms the outer surface of the hollow profile. The shrinkage of the thermoplastic resin caused by the solidification will hereby not affect the measured result from the second pressure sensor 12 as much as from the first pressure sensor 5, placed in connection to a corner. The cooling phase, when the resin solidifies, can alternatively be guided by time only. The temperature sensor 8 and second pressure sensor 12 can hereby be excluded.

Claims

1 . Process in an injection moulding procedure where a part made of thermoplastic resin is manufactured, which part includes a hollow profile formed by means of a pressurised fluid and a mould (1 ), which mould ( 1 ) is provided with at least one material inlet (4) for molten thermoplastic resin, at least one fluid channel ( 10) through which the pressurised fluid is injected and evacuated and means for increasing and decreasing the volume of a mould cavity (3) of the mould ( 1 ) by means of at least one moveable mould member (2) which is operated by means of hydraulic cylinders, pneumatic cylinders, stepping motors or the like, c h a r a c t e r i s e d in that the mould ( 1) is provided with at least one pressure sensor (5) placed mainly as far as possible from the material inlet (4), preferably on the part of the mould cavity (3) that will be filled with molten thermoplastic resin last, whereby the injection moulding process is initiated by reducing the volume of the mould cavity to a minimum by sliding the moveable mould member (2) to its innermost position whereupon an amount of molten thermoplastic resin is injected through the material inlet (4) into the mould cavity (3) so that the molten thermoplastic resin exerts a pressure towards the walls of the mould cavity (3) which pressure is detected by the pressure sensor (5), that the molten thermoplastic resin is allowed to solidify somewhat so that a skin of solid thermoplastic resin is formed closest to the walls of the mould cavity (3), that the pressurised fluid is introduced through the fluid channel (10), into the molten thermoplastic resin wherein a fluid cavity (1 1 ) is formed in the same, which fluid in the fluid cavity (1 1 ) exerts a pressure towards the inner walls of the mould cavity (3) via the molten thermoplastic resin and the skin of solidified thermoplastic resin, which pressure is detected by the pressure sensor (5), that the pressure sensor (5) guides the movement of the moveable mould member (2) so that this is slided outwards, and that its velocity is suitably higher at a higher pressure, wherein the volume of the mould cavity (3) increases while the fluid is allowed to flow into the growing fluid cavity (1 1 ) in the molten thermoplastic resin until the moveable mould member (2) has reached its outermost position, that the molten thermoplastic resin is allowed to solidify further while the pressure in the fluid cavity (1 1) is maintained, whereupon the pressure in the fluid cavity ( 1 1 ) is decreased to ambient pressure so that the mould (1 ) can be opened and the part be removed after which the process can be repeated.

2. Process according to claim 1 c h a r a c t e r i s e d in that the fluid used is a gas, preferably an inert gas.

3. Process according to claim 1 or 2 c h a r a c t e r i s e d in that a second pressure sensor (12) is placed in direct contact with the fluid, that the results of the measurement from the first and the second pressure sensors (5 and 12 respectively) are compared, whereby the degree of solidification of the thermoplastic resin can be determined.

4. Process according to any of the claims 1 - 3 c haract e ri s e d in that the pressure sensors (5 and 12 respectively) are pressure sensors of piezo-electric type.

5. Process according to any of the claims 1 - 4 c h ar a c t e r i s e d in that at least one temperature sensor (8) is placed in connection to the inner walls of the mould cavity (3), whereby at least the degree of solidification in the outer skin of the molten thermoplastic resin can be detected, and the solidification time before the introduction of pressurised fluid or the initiation of the increase in the volume of the mould cavity can be guided through signals from the temperature sensor (8).

6. Process according to any of the claims 1 - 5 c haracteri s e d in that the pressure of the fluid in the cavity is guided by the pressure sensor (5) placed in connection to the mould cavity (3) which opens and closes a fluid valve (9) so that a desired fluid pressure is maintained within the cavity whereby the moveable mould member (2) can be slided out with a constant velocity.