WO1996025285A1

WO1996025285A1 - Stretch blow forming method and blow forming press

Info

Publication number: WO1996025285A1
Application number: PCT/CH1996/000054
Authority: WO
Inventors: Ronald Siegrist; Bruno Stillhard
Original assignee: Procontrol Ag
Priority date: 1995-02-17
Filing date: 1996-02-19
Publication date: 1996-08-22
Also published as: DE19680085D2; EP0765213A1

Abstract

The invention proposes reaching the upper stage of the compressed blowing air adiabatically instead of isothermically, as previously, so enabling the entire blowing process to be carried out with the smallest possible amount of energy, and the greatest possible amount of energy to be recovered via a piston pump (14), for example, after blow forming. Preferably, for a first phase, low-pressure blowing air (N) of between 8 and 12 bars is generated isothermically in one or two stages and then adiabatically generated high-pressure blowing air (H) of between 30 and 40 bars reaches the actual pressing stage. Energy can also be recovered electrically by the same means as are used for driving the piston pump (14). A highly significant portion of energy is saved as a result of the upper stage of the compressed air stage being reached adiabatically and provided with a compressed air cushion at the rear of the piston and an additional high-pressure reservoir. Optimum use is made of the dynamic cycle of advance and return movements of the piston. All the sequences are co-ordinated by a central control system (ST).

Description

Stretch blow molding and blow molding press processes

Technical field

The invention relates to a method for blowing, in particular for controlled stretch blow molding of a thin-walled bottle body from a preform, a first blowing phase being carried out with compressed air at low pressure and a second blowing phase with high pressure blowing air. The invention further relates to a blow molding press.

State of the art

Stretch blow molding is a rational method to produce extremely thin-walled hollow bodies. The best known are bottles of 1 to 2 liters for mineral water. A thermoplastic material such as e.g. Pet is used, from which a preform is produced in a previous step. The preform is heated to 150 to 180 ° C and introduced into a blow mold while hot. The compressed air is usually blown into two stages, first at a low pressure of about 10 bar and then at about 40 bar into the finished mold. After a short solidification phase, the compressed air is released from the bottle, the mold is opened and the finished bottle is removed. An entire manufacturing cycle takes two to three seconds. Attempts have already been made to reduce at least part of the compression energy by taking back the blown air into a container system. However, the considerable effort and relatively small profit prevented this type of pressure energy recovery from being used more widely. It was not recognized that dynamic processes were not mastered to the extent required. Presentation of the invention

The invention was based on the object of optimizing the energy balance for stretch blow molding in relation to the provision of the blowing air with the least possible investment in equipment.

The method according to the invention is characterized in that the compression and decompression of the blown air takes place adiabatically in the phase with higher pressure to optimize the energy balance and energy conversion. The pressure generator for the higher pressure preferably works alternately as a compressor gas pressure motor. The invention further relates to a blow molding press for blowing, in particular for controlled stretch blow molding of hollow bodies made of plastic, in particular PET, from a preform or hose by means of compressed air, the compressed air system consisting of a preferably isothermally operating low-pressure air system and a high-pressure system, characterized in that the compressed air system for the high pressure blowing air is designed as an adiabatically operating pressure generator and for energy recovery, preferably as a compressor gas pressure motor.

According to the invention, it has been recognized that the greatest profit is possible if energy recovery is already taken into account in the generation of compressed air by the fact that in the highest pressure stage the compressed air is no longer cooled, but is compressed adiabatically and expanded again by backflow into the compressed air generator. The compressed air of the lower pressure level, e.g. pre-compressed to 8 to 12 bar and cooled with the practice known per se, and the heat of compression is removed. For the high-pressure phase, an enclosed high-pressure air cushion is moved back and forth and compressed or expanded. In some cases it is possible to recover the heat generated here in a known manner. According to the invention, however, the upper pressure stage, for example. from about 10 to 40 bar performed adiabatically without air cooling. The air compression is particularly preferably carried out by a single piston movement for the compression and vice versa with the corresponding backward movement. The The high-pressure part takes place in the manner of a spring or air pressure spring, but with precisely controlled movements. The temperature increases during compression by about 80 to 140 ° C. The air thus approximates the temperature of the preheated molding. The first blow molding of the preform is advantageously carried out with the isothermally generated compressed air. Compressed air is blown into the preform at approximately 8 to 12 bar. The adiabatically generated compressed air can then be fed continuously without pressure jump via corresponding valve controls and the pressure increased accordingly to the aforementioned 40 bar. A reversing control for the compressor piston then expands the compressed air again by means of a backward movement or return movement of the piston. The compression heat is recovered during the backward movement because the temperature increase during the adiabatic compression and expansion is brought back to the initial temperature. There is only little heat loss. Since a full cycle time only lasts two to three seconds, the heat loss during the short period of high pressure remains low. Any heating over several cycles of the recovered air is essentially compensated for by mixing with the cooler air of the lower pressure level. Due to the excellent dynamics or control dynamics of the servo motors, the actual high-pressure blowing phase can now be controlled very precisely. So it is possible to regulate any pressure curve, e.g. between a complete pre-compression of the high pressure blowing air or any pressure increase during the high pressure blowing process.

The invention also allows a number of particularly advantageous configurations, the energy being recovered to the maximum. According to preferred refinements, the high-pressure blowing air for a single blowing phase is generated by only one feed movement of a compressor piston and is supported by a compressed air cushion on the piston side. This has the great advantage that, in addition to the pure conversion of the piston thrust, the dynamic portion of the energy is fully usable or convertible. The compressor piston will driven by an electric motor, in the phase of decompression of the high-pressure blowing air the piston-side compressed air cushion is compressed as an energy store, and during the high-pressure blowing phase the drive of the compressor piston takes place both electromotively and from the energy store. This means that two energy potentials can be used in both directions of movement of the compressor piston. The piston-side compressed-air cushion can also be connected to an additional high-pressure accumulator, so that the compressed-air cushion is decompressed or compressed in counter-stroke with the compression / decompression of the blown air.

The expansion of the high-pressure blowing air is used until the low pressure of the first phase is reached to support the backward-controlled drive of the compressor pistons. The compressor piston can thus be driven in a controlled manner in both directions in the manner of a dynamic spring. When blowing, the energy is alternately used via the electromotive drive of the pressure generator and the piston-side compressed air cushion and when the blown air flows back, the expansion pressure of the return flow air and the electromotive compressor drive. It is also possible to additionally provide a mechanical spring at least on the piston rear side. The blowing pressure for the lower pressure phase is usually generated isothermally in several stages. The ratio of the adiabatically and the isothermally generated compressed air can be determined by the choice of the respective maximum pressure, in particular on the basis of the desired final temperature of the blown compressed air. The system is preferably designed such that the pressure at the end of the backflow corresponds to, or at least approximates, the pressure of the isothermal pressure stage. After closing a valve between the compressor piston and the finished mold, the residual compressed air from the finished bottle can be blown outside. The effective piston volume and possibly the blow-off pressure e.g. through the piston travel is selectable. It is possible to increase the initial pressure for the adiabatic pressure stage with the compressed air of the isothermal pressure stage. The compressed air system for the high-pressure blowing air preferably has a compressor piston which is electrically controllable in both directions, the compressor piston being designed as a spring, in particular as a gas pressure spring, in cooperation with the electromotive drive. The pressure generator for the high pressure is preferably driven by an AC servo motor or a vector-controlled motor. However, any other motor can also be used, provided that it has sufficient control behavior. Very short, rapid and large changes to the rules are particularly required.

The compressor piston is preferably driven by a servo motor, the overdriving from the servo motor to the piston rod being effected by a rack and pinion gear or a ball roller or planetary roller spindle.

It is also possible that a container collecting device is provided for taking over the low-pressure blow-off air from the finished mold, after the phase of energy recovery of the adiabatic pressure stage.

Brief description of the invention

The invention will now be explained in more detail using an example. Show it:

FIG. 1 shows a preform and a finished PET bottle, which is known per se; FIG. 2 shows schematically the basic structure of an entire blow press device according to the invention; FIG. 3 schematically shows a solution according to the invention with a concept of the course of forces and pressure for the high pressure phase; 4 shows the course of the cycle in relation to the shape movement and the course of pressure.

Ways of Carrying Out the Invention A preform 1 has a given filling volume vol. 1 according to an inner diameter d and its length lv. The finished bottle 2 has a filling volume vol. 2 according to the inner diameter D and the length LF. At the end of the stretch blow molding process there is bottle 2 Compressed air according to Vol. 2 and e.g. 40 bar, which are blown into the open in the prior art.

The essential parts of the compressed air system according to the new invention are shown schematically in FIG. A two-stage isothermal compressed air generator 3 has two air coolers 4. The low pressure air VN is in a container 5 with, for example. pressed up to 12 bar. In a mostly two-part blow mold 6, a bottle 2 is shown fully pressed. Compressed air of the lower pressure level is fed via a compressed air line 7 and jointly pressure line 8 into the opening 9 of the bottle 2. The common pressure line 8 can either be opened or closed in the main line by means of valve 10 or in an outflow or blow-off line 12 by means of valve 11. With the reference numeral 13, a compressed air recovery is indicated, from which the low pressure air e.g. again an inlet valve de compressed air generator 3 can be supplied. With a compressor piston 14 is de high pressure of, for example, a piston or a piston rod 15 de 30 to 40 bar generated. Arrow 16 indicates the blowing pressure generation for the high pressure and arrow 17 indicates the expansion of the high pressure blowing air. The compressor piston 14 is driven via an AC servo motor 18 via a double-rack gearbox 19. Electrical energy can be generated during the backward movement (arrow 17) and can be done in some way via a rectifier and an electrical memory 20 or directly back into the network . The system is controlled by a control ST, by means of which all process parameters such as start, stop, valves etc. can be set and optimized during stretch blow molding.

The low pressure system consists essentially of the low pressure air generator 3 and the pressure vessel 5. The high pressure system has a, designed as a compressor gas pressure motor 25 High pressure generator with the compressor piston 14. The high-pressure blowing air is generated in the pressure chamber 21 on the front side of the compressor piston 14, for which purpose the piston rod 15 is moved in the direction of the arrow 16 at the given time. The high pressure builds up in the bottle interior 22, following the blowing phase of the low pressure. At the back of the piston there is a compressed air cushion 23 which is directly connected to an additional high-pressure accumulator 24. The pressure ratios can be selected depending on special requirements. In particular, the values determined in practice as optimal (low pressure e.g. 8 to 12 bar, high pressure e.g. 8 to 40 bar) can be set. It is important that the design of all volumes is correct. The principle also allows values outside the specified values. An important point is the size of the additional high-pressure accumulator 24, which, for example. can have a size from 10 lt. to 100 lt. During the high pressure phase, the piston 14 is clamped, as it were, between the pressure in the pressure chamber 21 and the pressure in the compressed air cushion 23. At the same time, the exact movement of the compressor piston is controlled via the servo motor 18 or the corresponding control signals from the control ST. Two important sections are the two end positions, at the end of the injection of high pressure air when the piston is far left, and after the blowing process is completed when the piston is far right. In normal operation, the electromotive drive must also be used to ensure that the piston is held in position in the end positions or that it maintains the respective pressure and that the piston pushes when necessary.

Two complete working cycles are shown in FIG. 4. The form movement FBew is vertical, the time Zt horizontally. The movement of the mold halves is shown in the upper half of the figure. The most important sections are: closing the mold (30), holding the closed mold (31), opening the mold 32 and opening the mold 33. Prg means press closed. In the same time, the pressure curve of the blowing air is recorded directly below. Vertical the pressure D. The pressure curve is a theoretical curve, whereby with a lower limit line 35 the upper value of the low pressure air, with an upper limit line 36 the upper pressure of the high pressure air is indicated. The first blowing phase 37 takes place via a low pressure system. The second blowing phase 38 from a high pressure system. At the end of the blowing process, the high static pressure must be maintained for a short time (pressure maintenance phase 39). Then, by reversing the compressor piston, the high-pressure air from the now finished PET bottle is lowered (40) until the cut with boundary line 35. After the lower boundary line 35 has been reached, the high-pressure system is separated by closing a corresponding valve. The low-pressure air remaining in the PET bottle can be discharged outside (curve 41). The time between two blowing phases is required to take the finished bottle out of the mold and insert a new preform. A complete cycle is shown with Zyi, Zy2.

FIG. 3 schematically shows a blow molding press according to the invention with an associated displacement pressure diagram (center of the picture). The given values show a calculation example. Pl is the pressure curve in the pressure chamber 21, P2 is the pressure curve in the compressed air cushion 23. The (+) and (-) designate the energy gradient which is greatest in the respective end positions. Only in the sense of a pictorial model below the path pressure diagram. a spring mass oscillator is shown. Since the compressed air is locked in on both piston sides, resp. the pressure and the corresponding force drop depending on the position. The area designated by Ek represents the kinematic energy which is temporarily stored in the moving mass M. Since, as schematically indicated with Ek, part of the potential energy is temporarily stored as kinetic energy, less electrical energy is generated for intermediate storage. Figure 3 shows a two-cavity shape. It can simultaneously stucco 2 1 - liter _bottles, a servoeletrischen Blasstation- be made per module. In the new process, the high-pressure air is "generated" at the time of blowing by a single-acting piston pump driven by a servo motor and then recovered again. In the example considered, the supply takes place via an 8 bar low-pressure supply. In the servo cylinder with e.g. 120 mm diameter and 800 mm stroke, 9 liters of air under 8 bar is compressed to 40 bar or 3 liters volume without heat loss. After blowing, the same air is decompressed and largely recovered. In order to ensure that the cycle time is at least not extended, but rather becomes shorter, you can count on 6 liters of recovered air and 2 liters of air to be replaced per blowing cycle and module.

With the new system, around 90% of the energy can be saved in the area of high pressure. So far, the energy requirement for high pressure was 2 to 3 times greater than the energy requirement for low pressure.

Claims

claims

1. A method for blowing, in particular for controlled stretch blow molding, of a thin-walled bottle body from a preform, a first blowing phase being carried out with compressed air at low pressure and a second blowing phase with high pressure blowing air, characterized in that the phase with higher pressure is used for Optimization of the energy balance and energy conversion, the compression and decompression of the blown air takes place approximately adiabatically.

2. The method according to claim 1, characterized in that the high-pressure blowing air for a blowing phase is generated by a feed movement of a compressor piston and the decompression takes place by the backward movement of the compressor piston and the expansion of the high-pressure blowing air until reaching the low pressure of the first phase at backward-controlled Drive the compressor piston is used.

3. The method of claim 1 or 2, so that the pressure generator for the higher pressure is alternately coupled as a compressor gas pressure motor and works with an electric motor / generator.

4. The method according to any one of claims 1 to 3, characterized in that the compressor piston is driven by an electric motor, wherein in the phase of decompression of the high-pressure blowing air, a compressed air cushion is compressed on the piston side as an energy store, and during the high-pressure blowing phase the drive of the compressor piston is driven by an electric motor and from the Energy storage takes place.

5. The method of claim 4, d a d u r c h g e k e n n z e i c h n e t that the piston back compressed air cushion is connected to an additional high pressure accumulator, such that the compressed air cushion is decompressed or compressed in push-pull with the compression / decompression of the blown air.

6. The method according to any one of claims 1 to 5, characterized in that the compressor piston is driven in both directions in the manner of a dynamic spring, alternately when blowing the energy via the electromotive drive of the pressure generator and the piston-side compressed air cushion and when the blown air flows back the expansion pressure of the return air and the electromotive compressor drive are used.

7. The method according to any one of claims 1 to 6, characterized in that the high pressure phase is adiabatic and the compressed air is generated isothermally for the first low pressure phase, the ratio of the adiabatically and the isothermally generated compressed air or the ratio of high pressure and low pressure by the Choice of the respective maximum pressure or the pressure of the low-pressure air is determined in particular on the basis of the desired final temperature of the compressed air.

8. Blow molding press for blowing, in particular for controlled stretch blow molding of hollow bodies made of plastic, in particular of PET, from a preform by means of compressed air, the compressed air system consisting of a preferably isothermally operating low-pressure air system and a high-pressure system, characterized in that the compressed air system for the high-pressure blowing air is preferably adiabatic working pressure generator and is designed for energy conversion as a compressor gas pressure motor.

9. Blow molding press according to claim 8, so that the compressed air system for the high-pressure blowing air has a compression piston which is electrically controllable in both directions, the compression piston being designed as a spring, in particular as a gas pressure spring, interacting with the electromotive drive.

10. blow molding press according to claim 9, that the drive of the pressure generator for the high pressure has an AC servo motor or a vector controlled motor.

11. Blow molding press according to claim 10, that the compressor piston can be driven via an AC servo otor, and the overdriving from the servo motor to the piston rod takes place by means of a rack and pinion gear or a ball roller or planetary roller spindle.

12. The blow molding press according to claim 11, which also means that it has a high-pressure auxiliary accumulator which is connected to the compressed air space on the piston rear side.