METHOD FOR PRODUCING HOLLOW BODIES MADE OF PLASTICS

Abstract

A method for producing hollow bodies made of plastics by blow molding or deep drawing using a deep-drawing tool or a blow-molding tool, comprising the extrusion of preforms from thermoplastics, the method comprising a wall thickness control of the preforms, wherein a measurement of the wall thickness of the preforms takes place within the tool on the mold cavity side on at least one reference point of a preform, the measured value obtained is compared as an actual value with a predetermined target value and the wall thickness control is triggered to change the wall thickness of the preform during extrusion or between extrusion cycles depending on the deviation between the actual value and the target value.

Claims

1-11. (canceled)

12. A method for producing hollow bodies made of plastics by blow molding using a blow-molding tool, comprising the extrusion of preforms from thermoplastics, the method comprising a wall thickness control of the preforms, wherein the preforms are placed as sheet-like plasticized preforms in an open blow-molding tool and are first formed into shell-like intermediate products within the blow-molding tool using the heat of plasticization from the extrusion, wherein a measurement of the wall thickness of the intermediate products is carried out within the open or closed blow-molding tool on the mold cavity side on at least one reference point of a preform, the measured value obtained is compared as an actual value with a predetermined target value and the wall thickness control is triggered to change the wall thickness of the preform during extrusion or between extrusion cycles depending on the deviation between the actual value and the target value in the sense of regulating between extrusion cycles.

13. The method according to claim 12, wherein the intermediate products are joined together to a closed hollow body in a further method step, wherein the measurement is carried out following the molding of the intermediate products.

14. The method according to claim 12, wherein at least one built-in part is joined to at least one shell-like intermediate product before the shell-like intermediate products are joined to form the hollow body, wherein the measurement of the wall thickness takes place before the mounting of the built-in part and preferably in the area of a joint surface provided for the built-in part and wherein the step of joining the built-in part is only carried out if the deviation between the actual value and the target value does not exceed a predetermined variable.

15. The method according to claim 12, wherein the shell-like intermediate product is fixed within the blow-molding tool during the measurement.

16. The method according to claim 12, wherein the measurement is carried out by at least one measuring device which is selected from a group of measuring devices comprising tactile sensors, ultrasonic sensors, capacitive sensors or optical sensors, in particular laser sensors.

17. The method according to claim 12, wherein the measurement is carried out by at least one measuring device which is installed on a tool divider, a joining frame or an intermediate frame of the blow-molding tool at at least one measuring point provided for this purpose.

18. The method according to claim 17, wherein the measurement is carried out as an optical measurement, in particular as a laser measurement.

19. The method according to claim 12, wherein the wall thickness control causes a nozzle gap adjustment on an extrusion head.

20. The method according to claim 12, wherein an initial and possibly repeated zero position setting or calibration of the measuring device is provided.

21. The method according to claim 12, wherein the shell-like intermediate products are discharged from the process by a given amount in the event of a discrepancy between the actual value and the target value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention is explained below with reference to an embodiment shown in the drawings:

[0035] FIG. 1 is a schematic representation of the cavity of a blow-molding tool for producing hollow bodies according to the so-called half-shell method, which illustrates the calibration of the measuring device used in the method;

[0036] FIG. 2 is a schematic representation corresponding to that in FIG. 1, which illustrates the measurement process; and

[0037] FIG. 3 is a schematic representation of a laser sensor during the measurement process, which illustrates the operation of the laser sensor.

DETAILED DESCRIPTION

[0038] The method according to the invention is explained below using the so-called half-shell method. This is an extrusion blow molding process, in which two preforms are placed as sheet-like plasticized preforms made of thermoplastics in an open blow-molding tool and are first formed into shell-like intermediate products within the blow-molding tool using the heat of plasticization from the extrusion, wherein the intermediate products are joined together in a further method step to form a closed hollow body. Either a tubular preform is extruded, which is cut lengthwise on opposite sides, or two sheet-like preforms are extruded. The preforms are preferably extruded in the direction of gravity, it being possible for one or more extrusion heads to be arranged directly above the blow-molding tool. As already mentioned at the beginning, the process according to the invention can also be carried out as a conventional extrusion blow molding process or as a deep-drawing process.

[0039] In the drawings, the cavity 1 of the blow-molding tool is shown schematically. Parts of the blow-molding tool have been omitted for the sake of simplicity. The blow-molding tool comprises two partial cavities, which are each formed in so-called blow mold halves 3. Each blow mold half 3 is arranged on a mold clamping platen of a blow-molding machine.

[0040] The mold clamping platens are in turn arranged on a locking frame and can be moved towards and away from one another via the locking frame. The blow-molding tool further comprises a center tool as an intermediate frame 2 and optionally a joining frame, which can be arranged on a common machine frame.

[0041] In FIG. 1, only the empty cavity of the blow-molding tool is shown, the blow mold halves denoted by 3 being closed against the intermediate frame 2. Two laser sensors 4 are attached to the intermediate frame 2, specifically at predetermined positions of the intermediate frame 2 provided for this purpose. FIG. 1 illustrates the process of calibrating the laser sensors 4. The distance from the relevant point of the intermediate frame 2 to the inside of the partial cavities is measured via the laser sensors 4. The process of calibrating or zero position setting of the sensor system, which is illustrated in FIG. 1, is initially carried out at the start of production and then from time to time at irregular intervals. This calibration can be necessary every now and then, for example, to compensate for temperature changes over the course of the day. During the calibration, for example, an offset can also be set which takes into account, for example, the shrinkage delay of the hollow body to be manufactured.

[0042] FIG. 2 illustrates the measurement process during the production method of the hollow body. The measurement process is also carried out by means of the laser sensors 4, whereby, as schematically illustrated in FIG. 2, shell-like intermediate products 5 are already formed in the cavity 1 of the blow-molding tool. The shell-like intermediate products 5 are still in the hot-plastic state in the method step shown in FIG. 2 and are optionally fixed in the partial cavities using negative pressure. While the shell-like intermediate products 5 are in the state shown in FIG. 2 and the blow mold halves are closed against the intermediate frame, the measurement process is carried out. The distance is measured in each case from the relevant laser sensor 4 of the intermediate frame to a reference point on the inside, i.e. the side pointing into the cavity, of the shell-like intermediate product 5. The difference between a measurement with an empty tool and a measurement immediately after the shell-like intermediate products 5 have been formed results in the wall thickness of the shell-like intermediate product 5.

[0043] The measured value is supplied to a regulating and control device, depending on the deviation from a given target value, the wall thickness control is triggered correspondingly in order to realize an action in the control or an action in a wall thickness program.

[0044] Reference points are selected as reference points in the sense of the invention, which are critical for the wall thickness of the finished hollow body or for the installation of built-in parts or add-on parts.

[0045] FIG. 3 illustrates the laser measurement method using a laser sensor 4. The method is carried out as a triangulation process. A laser beam emitted from a laser light source 6 strikes an object at an angle, in the present case the shell-like intermediate product 5 formed in the partial cavity, is reflected by the latter and detected by an electronic image converter 7. The connection between the light rays from and to the object forms a triangle, which enables the geometric determination of the distance to the object.

REFERENCE NUMERALS

[0046] 1 cavity [0047] 2 intermediate frame [0048] 3 blow mold halves [0049] 4 laser sensors [0050] 5 intermediate products [0051] 6 laser light source [0052] 7 electronic image converter