Method and inspection device used for the cyclic production of injection molded parts

The invention relates to a manufacturing inspection method for the cyclic production of injection-molded parts in multi-cavity casting molds involving a row-by-row monitoring of the injection-molded parts after they are removed from the casting molds, said monitoring being effected by sensors, particularly, photoelectric barriers. The invention also relates to an inspection device used during the cyclic production of injection-molded parts in multi-cavity casting molds involving a row-by-row monitoring of the injection-molded parts after they are removed from the casting molds, said monitoring being effected by sensors, particularly, photoelectric barriers.

TECHNICAL AREA

The invention relates to a manufacturing inspection method for the cyclic production of injection-molded parts in multi-cavity casting molds involving a row-by-row monitoring of the injection-molded parts after they are removed from the casting molds, said monitoring being effected by sensors, particularly, photoelectric barriers. The invention also relates to an inspection device used during the cyclic production of injection-molded parts in multi-cavity casting molds involving a row-by-row monitoring of the injection-molded parts after they are removed from the casting molds, said monitoring being effected by sensors, particularly, photoelectric barriers.

THE CURRENT ART

A distinction is made in injection molding between two different areas of application for multiple molding. A first application is the production of relatively thin-walled parts. Such parts are cooled in the mold after completion of the actual injection-molding process until they have been brought into a stable condition and can be ejected and packaged after the halves of the mold are separated.

A second application can be illustrated with the production of PET (polyethylene terephthalate) objects. PET objects are thick-walled injection-molded parts. The wall thickness must be thick enough so that there is sufficient wall material for a subsequent blow process with associated significant increase in volume. A PET bottle must retain sufficient wall thickness after the blow process. The thick walls of the injection-molded parts have a major disadvantage for the molding process; the cooling time is two to four times greater than the molding time itself. If the preform must remain in the mold until it has cooled sufficiently, then the throughput of the entire injection-molding machine is correspondingly reduced. In practice the productivity of the machine is kept at high efficiency by having only the first portion of the cooling of the injection-molded part take place within the mold in such a way that the injection-molded part can be removed from the mold without damage and allowed to cool subsequently. Injection molds are designed for the production of 50 to 200 preforms per cycle. The corresponding number of injection-molded parts are removed by robotic equipment while still hot and these are placed directly into a post-cooler or transferred by means of a special removal device from the mold to a post-cooler. In the interests of efficient production, each step in the process is carried out in the shortest possible time. Nevertheless, the removal and transfer of the individual parts is not entirely without its problems. For various reasons an individual part can remain caught in the cooling cavity of the transfer device or in the post-cooler. If an injection-molded part is stuck in the mold half, this can damage the mold when it closes. The same is true for both the removal device and the post-cooler. For the sake of efficiency both the mold cavities and the cooling positions are arranged in rows.

In the current art at least the removal of the injection-molded parts from the molds is monitored by means of photoelectric sensors. A beam of light is passed across an entire row of preforms to determine whether any injection-molded parts are still within a mold-half or whether all preforms have actually been removed from the mold. If even a single preform remains on the positive mold-half, the light beam does not reach the detector. This immediately causes an error signal and the closure of the mold is stopped. Generally the individual preforms are removed by hand and the injection process is started up again. It is understandable that with the increasing number of mold cavities in the tooling and the corresponding number of injection-molded parts per molding cycle, the requirements for process control are increasing. The concept of post-cooling and the extra handling steps involved multiplies the possibilities for error.

The object of the invention is to develop a means of monitoring as well as a monitoring device for protection of the tooling that recognizes, in real time if possible, errors in the process of transferring injection-molded parts in association with the molding process and thereby permits these errors to be corrected without consequent damages.

PRESENTATION OF THE INVENTION

The procedure according to the invention is characterized in that for each row a sensor is fixedly disposed in such a way that the injection-molded parts in are presented to that sensor, particularly in that they are individually moved past that sensor.

The device according to the invention is characterized in that for each row at least one fixedly disposed sensor is disposed in such a way that the injection-molded parts obligatorily are presented to the sensor, particularly in that they move past the sensor for a check of a full and/or empty situation at each position of the row.

The inventors are aware that, particularly in cases of protection of tooling, motion of the injection-molded parts relative to the sensors offers multiple advantages. Primarily it is the reliability of the monitoring that is increased. It is easier to obtain an unambiguous signal if presence or non-presence can be determined with a motion. Row-by-row monitoring of injection-molded parts is known in the current state of the art. The associated sensors are fastened to the molds. Each sensor monitors an entire row of positive molds and ensures that the molds cannot be closed when one or more molded parts remain on the positive mold after removal.

The new solution suggests that in cases of tooling protection, each injection-molded part move past the sensor individually. This has the major advantage that if the process control is computerized, the injection-molded parts can be counted. For each injection-molding cycle, the predicted quantity of injection-molded parts can be given and the actual number can then be determined for each removal and the determination made of empty and full conditions at the removal means. This opens entirely new possibilities. Monitoring of process steps following removal of the injection-molded parts can be provided with means of displaying all of the injection-molded positions being monitored so that empty molds or the retention of one or more injection-molded parts can be determined, advantageously with the indication of position. In the processing of PET molds it is common practice to continue with production even though problems exist with one or more mold cavities. The supply of melt to the affected cavities is stopped. The new solution has the enormous advantage that even this operation is still positively monitored. The applicable data can be stored in the memory of the computer and continually modified. Safety in production is here again maintained completely. Preferably the sensors are disposed in proximity to the injection-molding molds to ensure the quickest possible accomplishment of a safety intervention in the process control.

The new solution permits monitoring of the injection-molded parts in either the device for removal from the injection molds and/or in the post-cooler. In the event that the injection-molded parts are preforms, then the process control is carried out in proximity to the post-cooler. Each preform is pressed into the cavity of the removal device and/or post-cooler far enough so that essentially only the threaded portion protrudes from the cavity. The threaded portions are monitored by the sensors and because of the significant size of the threaded portions, this permits a high degree of reliability of signal capture. Because the plastic materials of the preforms are more or less transparent, there exist some preferred sensor techniques.

The new solution further permits a large number of particularly advantageous structural designs. Each sensor is preferably designed as a split optical switch such that each injection-molded part must pass through the gap of a sensor. In the case of PET molding, the threaded portion passes through the gap in the split optical switch. Preferably all sensors are disposed in a single plane on a common carrier bar, which is disposed perpendicular to the direction of motion of the injection-molded parts and in proximity to the injection molds. The common carrier bar is fixedly disposed with reference to the removal device. Row-by-row monitoring is sufficient for monitoring the post-cooler. There the post-cooler moves relative to the sensors in such a way that the rows that have been cleared are the rows that are monitored.

The monitoring device can include visualization means for the display of contingent errors relative to an empty condition or to a discrepancy between the predicted full condition and the actual condition. In the event of repetitive errors, this permits exact localization and rapid goal-oriented correctional measures to be taken with regard to either restricted operation or mechanical correction of the error.

APPROACHES AND IMPLEMENTATION OF THE INVENTION

In the following, reference will be made toFIG. 1.FIG. 1shows schematically a primary preferred design for a production unit for PET moldings.FIG. 1shows an entire injection-molding machine with a machine bed1, upon which a fixed mold mounting plate2and an injection unit3are attached. An end backup plate4and a movable mold mounting plate5are supported and are axially displaceable on the machine bed1. The two fixed plates2and4are connected by means of four rods6which extend through the movable mold mounting plate5. Between the backup plate4and the movable mold mounting plate5is a drive unit7to provide the closing force. The fixed mold mounting plate2and the movable mold mounting plate5each carry a mold half8and9in which a plurality of mold cavities are formed in order to provide a suitable number of hollow injection-molded parts. The part molds8′ are illustrated as rods, to which the hollow injection-molded parts adhere immediately after the opening of the mold halves8and9and are thereafter ejected. At this time the injection-molded parts10are already in a semi-hardened condition and are indicated by dotted lines. The same injection-molded parts10in a completely cooled condition are shown at the upper left ofFIG. 1, where they are just being ejected from a post-cooling device19. The upper rod6is illustrated as broken in order to better show the details between the opened halves of the mold. The four most significant phases in the handling of the injection-molded parts after completion of the injection process in accordance with the invention ofFIG. 1are:

“A” is the removal of the injection-molded parts or preforms10from the two halves of the mold. The semi-hardened parts are picked up by a removal device11that is lowered into the space between the two opened halves of the mold (Position “A”) and lifted by this device into Position “B” (pickup device11′ inFIG. 1).

“B” is the transfer position of the removal device11with the preforms10to a transfer carrier12.

“C” is the transfer of the preforms10from the transfer carrier12to a post-cooling device19.

“D” is the row-by-row ejection of the cooled and stabilized preforms from the post-cooling device19.

FIG. 1shows a snapshot, so to speak, of the four main steps in handling. In Position “B” the vertically arrayed injection-molded parts10are taken by the transfer carrier12and12′ and swung by the transfer device in the direction of the arrow P into an upright position as shown in phase “C”. Transfer carrier12consists of a supporting arm14that swings about an axis13and that carries a holder plate15to which a carrier plate16for centering rods8″ is disposed in parallel with some separation. Carrier plate16is adjustably parallel to holder plate15so that the hollow injection-molded parts10can be extracted from the removal device11and then swung into position “C” so that they can be inserted into the post-cooling device19which is positioned above them. This transfer takes place by enlarging the distance between holder plate15and carrier plate16. The still semi-solid injection-molded parts10are completely cooled down in the post-cooling device19and after the post-cooling device19has been moved into Position “D” they are ejected and dropped onto a conveyor belt20. The label30is the water cooling with associated inlet and outlet tubing, which are indicated by arrows for simplicity and are assumed to be known. The label31/32indicates the air side, where31represents the air inlet for inflation and compressed air and32represents the vacuum and exhaust line.

InFIG. 1the horizontal plane is designated EH and the vertical plane EV. The horizontal plane EH is defined by the two coordinates X and Y and the vertical plane by the coordinates Y and Z. The Z-coordinate is vertical and the X-axis is perpendicular to it. The motion of the individual devices and the automation is only schematically indicated by arrows.FIG. 1represents a possible basic arrangement, one that can be implemented in a great number of variations in accordance with the new invention: the transfer carrier12makes a swinging motion as well as a linear motion in the X-coordinate. The transfer carrier12can also be designed to make a controlled motion in the Y-coordinate. Because the transfer carrier already has a controlled motion in the X-coordinate, an exact positioning of the preforms present on the holder rods on the transfer carrier can be made in the X-direction by means of closed or open-loop control of the motion. For the transfer of the preforms to the post-cooler19, in this example the post-cooler19is moved into a fixed position in the X-direction and the transfer carrier is moved in the Y-direction by means of closed or open-loop control into the desired position. In the preferred embodiment, the means of motion for the post-cooler19in the two coordinates X and Y for exact positioning for the ejection and for the transfer of the preforms are under closed or open-loop control. Thus the post-cooler19and the transport carrier12are each moved into a defined position.

FIG. 1shows two locations for a final inspection:

the area in which the preforms are removed from the injection molds with split-switch units50to provide protection for the tooling.

The post-cooling area with a photoelectric barrier50′ for ejection monitoring.

In both cases the process is carried out by preprogrammed or controlled mechanical means. The beginning of each working cycle must be clearly defined.

Before the mold halves are closed, it must be assured that there are no individual injection-molded forms10stuck in the tooling from the previous cycle. From the point of view of tooling protection, the most important safeguard is that, in the event of an irregularity, either the process is halted or at least the next closing of the tooling is prevented. This can involve either not initiating the closing process or, if necessary, that the closing be halted during the closing process.

Before the removal device11is returned to the space between the open mold halves8and9, it must be determined that the space is completely empty.

Before loading the post-cooler19following ejection of the completed parts, it must be ensured that no individual preforms10are stuck in any of the positions.

Considering the importance of monitoring the finished product, the most important tasks are tooling protection and monitoring the multi-cavity molds8and9. A monitoring device50is permanently installed in proximity to the two mold halves8and9and connected by a support53to the removal device11, as indicated by the auxiliary line52. A second possibility for applying the new invention is in the area of the post-cooler19. The associated monitoring device is designated as50′. Means of visualization are indicated at54and the memory and computer are shown as55,56.

FIGS. 2a,2band2cshow schematically three different installations for the removal device11. InFIG. 2a, for instance, only the top four preforms have actually been picked up by the removal device11. The removal device11passes through the split switch unit50with the protruding preforms10. After the removal device11has passed entirely through the split switch unit50, it determines that the preforms from position5and position6are missing in that particular row and consequently are still stuck in the mold half. An immediate stop order is practically instantly triggered for the machine via computing means55,56. The two mold halves8and9cannot close again. An alarm is sounded so that the operator of the machine can remove the two preforms at position5and position6by hand and the process can start again. There is a second possibility that the two mold cavities were not actually active. The two preforms at position5and position6were not injected. There is the possibility that this was programmed into the recipe memory56. During processing in this case, it would now be established that the two preforms in position5and position6were simply not produced. Production could then continue with the least possible amount of time lost.

FIG. 2bshows the situation for a transfer from the removal device11to the post-cooler19via a transfer carrier12(FIG. 1). Here again there is the possibility of an error. One or even more preforms could become stuck in their cavities in the removal device11. This would mean that at the next pickup of hot preforms from the mold halves8,9, two preforms would collide, with possibly major resultant damage to the tooling.

FIG. 2cshows the empty-check. The removal device11traverses through the photoelectric barrier from top to bottom with some preforms10possibly stuck inside. Any stuck preforms would be detected and the process immediately interrupted until any stuck preforms have been removed by hand.

FIG. 3shows a post-cooler concept in a compact design. The invention as shown inFIG. 1still applies to the injection-molding machine. A post-cooler40with a plurality of cooling cavities10occupies a vertical transfer plane, that is, a plane within the Y and Z coordinates. In the position shown, the 22 mold halves8and9are in the open position, so that the post-cooler40can be introduced into and removed from the empty space42between the mold halves. The post-cooler40has a total of three degrees of freedom: a horizontal axis of motion in the Y coordinate, a vertical axis of motion in the Z coordinates and an axis of rotation43. The axis of rotation43serves only for the ejection of the cooled preforms onto a conveyor belt20. This will not be discussed further. The axis of rotation43is attached by bearings to support48and exhibits a vertical drive45. Vertical drive45is slidably disposed on a carrier plate46associated with a horizontal drive47.

Also inFIG. 3is the monitoring device50″ as well as sensors in proximity to the injection molds. The injection mold cavities are disposed in equally spaced vertical rows V1, V2, V3, V4, etc. Monitoring device50″ is fixedly attached to the machine frame. Instead of using a removal device, the preforms are here inserted directly into a post-cooler40. In the example shown, the cooling positions are disposed in offset rows, in such a way that the post-cooler can accommodate three or four times the quantity produced in an injection cycle. In computer/memory means49, an image of the injection and the cooling positions is established so that in the event of an error the exact position of the problem can be determined.

The post-cooler equipment shown inFIG. 4exhibits several parallel rows {circle around (1)}, {circle around (2)}, {circle around (3)}, {circle around (4)}, In the example shown there are 12 cooling cavities70in each row, each of which accepts one preform10. The cooling cavities70can be disposed much more closely together than the cavities in the injection molds. For this reason there are not only more parallel rows but the rows are also advantageously offset as expressed in the their X and Y dimensions. This means that for the first injection cycle the holes are labeled {circle around (1)}, for a second injection cycle the holes with number {circle around (2)}, etc. If in the example with four parallel rows, all the rows with number {circle around (4)} are filled, the rows with number {circle around (1)}, as described, are being prepared for ejection onto the conveyor belt20. The remainder proceed in this manner during the entire production run. In the example shown the total post-cooling time is on the order of three or four times the injection time. The cooling Channel21for the water cooling must be optimally designed so that the water cooling takes place as uniformly and as efficiently as possible for all cooling tubes. In addition, the compressed air and vacuum provisions in the post-cooling device must be controllable row-by-row so that at some particular time all rows {circle around (1)} or {circle around (2)}, etc. can be activated simultaneously.

FIG. 4shows an offset arrangement of the rows, corresponding toFIG. 3. It is important in any case to consider the motion of the post-cooler with the preforms10relative to the sensors50, which are advantageously fixed in position. With a corresponding image of the cooling positions as well as the mold cavity positions, any error can quickly be associated with a position by means of the memory/computer means, and the required control commands to can be issued.

FIG. 5shows a preferred design with a plurality of split-switch barriers60,60′,60″, etc. for tooling protection which are fastened to a common carrier bar63. The threaded portion61extends out of the cooling cavities62and moves perpendicularly to the plane of the drawing inFIG. 5. Each sensor or each photoelectric barrier is connected to the computer/memory54,55,56by means of a signal conductor64.FIG. 5is the application at the removal of the injection-molded parts from the mold cavities, in which it is desired to count the parts produced. Each photoelectric barrier has a transmitting component65as well as a receiving component66. If a threaded portion passes through the gap of the split-switch barrier, a suitable signal is produced.

FIG. 6shows the application of a photoelectric barrier80,80′ at a stand-alone post-cooler19. Here the object is to monitor with a light beam81the complete ejection of all injection-molded parts after completion of the post-cooling. Counting is not necessary here. The post-cooler19moves perpendicularly to the plane of the drawing into either the pickup or the ejection position as desired. The photoelectric barriers are fixedly attached to a longitudinally movable frame82. In the solution in accordance withFIG. 6, the preforms move through the light beam row by row. In a further embodiment, the transfer carrier may be subjected to photoelectric-barrier monitoring in the form of a split-switch unit or with transmitted light or a light beam81for an entire row. [Legends onFIG. 6: Longitudinal frame movement together with post-cooler; Post-cooler with transverse motion]

FIG. 7shows a preferred embodiment of a cooling cavity62. Cooling cavity62consists of a double-shell cooler70,71and an inner cooling cavity72. The inner cooling cavity72has an internal contour that closely fits the exterior shape of the injection-molded part. Particularly interesting in this respect is the rod-shaped component73. The preform lies snugly within the curved shape of the interior of the cooling cavity. The cooling cavity72exhibits a relatively thin constant wall thickness up into the dome region. In this way the cooling conditions, particularly the heat flux through the total area of the preform which is enveloped by the cooling cavity, are optimized. At the peak of the dome is an air connection74through which either compressed air (for ejection) or vacuum (to suck the preform up into contact with the dome) can be provided. It can further be seen fromFIG. 7that the entire threaded portion G extends out from the cooling cavity and is usable for sensor monitoring.FIG. 7schematically shows the application of a photoelectric sensor with a reflector74. The reflector sends the light beam back to the transmitter. By means of a suitable mirror device in the sensor device76, the signal is processed and conducted through a signal conductor64to the monitoring device54,55,56. [Legends onFIG. 7: Vacuum/compressed air; Water cooling; Transmitter/receiver; Reflector]

FIG. 8shows a further embodiment of the sensor that is designed as an image sensor. In this way laser, microwave or infrared radiation can be used.FIG. 8shows a laser sensor based on optical edge-recognition. The edges of the injection-molded part are most easily recognized when they are passed through the laser beam. A laser diode with focusing optics (L) produces a highly visible tiny point of red light on the forward surface of the sensor device28. The laser beam (X) strikes the surface obliquely. In accordance with light scattering theory, the major portion of the light is reflected in the direction (Xx). Detector D1then receives more light (XD1) than detector2(XD2). If the laser beam encounters an edge, precisely the opposite is the case. D2receives more light than D1. The compact device, thanks to its well-focused laser beam, detects even the smallest edge. A built-in microcontroller suppresses undesired multiple pulses by switching the device into an inactive state during a dead-time following each detected upper edge. Various programs guarantee an optimal adaptation to all counting problems:

Fixed dead-time: settable in milliseconds

Dynamic dead-time: the microcontroller continually monitors the pulse train and eliminates multiple pulses even with a varying conveyor speed since the dead-time is dynamically adapted to the pulse train.

Synchronization with machine rate: synchronization with the machine rate by means of a synchronizing input (from a tachometer, for example). In this way the dead zone corresponds to a defined distance, completely independent of conveyor speed.