Injection Molding Machine Having Mold Locking Unit Equipped With Toggle Drive and Hydraulic Locking Mechanism

The present invention relates, in general, to injection molding machines and, in particular, to a new and useful mold locking arrangement having a thermal force control feature.

In injection molding machines, for example, for light metals or plastics, special, mostly complicated constructual provisions are necessary to keep the magnitude and direction of the forces exerted through a tie rod supporting the mold parts constant, for example, or to control them in a desired way. For example, in injection molding machines for plastics equipped with a toggle mold-locking mechanism in which the locking force must be taken up by a plurality of tie rods, these rods are submitted to strain or bending by which the locking force acting on the mold parts is varied. In addition, unequal variations in length, or bending, of the different tied rods may cause an unsymmetrical distribution of the locking force over the parting surfaces of the mold. There are mechanically or hydraulically operated centralized adjusting devices known by means of which the locking force of such machines can be adjusted, or controlled, automatically. Such devices are relatively inaccurate in operation and make no allowance for inevitable wear. Also, the prior art constructions for controlling the locking force are particularly expensive.

The present invention is based on a concept of eliminating these drawbacks by controlling the force thermally, not mechanically, or hydraulically.

An object of the present invention is to provide an injection mold with locking device comprising, a fixed backing plate, a fixed mold part plate, one or preferably more thermally expandible and contractable tie rods connecting the fixed mold part plate and the fixed backing plate, a movable mold part plate, drive and locking means connected between the fixed backing plate and the movable mold part plate for moving the movable mold part plate into a molding position with the fixed mold part plate and locking the movable mold part plate at this position, and thermal force control means associated with the one or more tie rods for controlling the temperature thereof and thereby altering the force between the fixed and movable mold part plates.

Since the length and, thereby, the force exerted by the tie rods is variable by varying the temperature of the rods, which may be obtained in a simple and accurate way by heating or cooling, no mechanical adjusting means are necessary. The force control through temperature variations can be effected with simple means. The construction is inexpensive and existing installed machines can be equipped with such a thermal control in a simple manner.

Another object of the invention is to provide a method of thermally controlling the force exerted by the movable mold part plate on the fixed mold part plate in a molding position.

A still further object of the invention is to provide an injection mold with locking device which is simple in design, rugged in construction and economical to manufacture.

In the following, the invention is explained, by way of example, while considering a mold locking mechanism of an injection molding machine for plastics in which the force exerted through the tie rods is thermally controlled.

For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings.

The drawings consist of a single FIGURE which diagrammatically and schematically show the workings of the invention.

Referring to the drawing, in particular, the invention includes a mold locking device comprising, as known per se, a back-up plate 3 for supporting a hydraulic locking cylinder or means 1, on one side, and a toggle mechanism 2 forming the drive, on the other side. Plate 3 is secured to a machine frame (not shown). An opposite backing element is formed by a mold supporting plate 4 which is also fixed and secured to the machine frame. Plate 4 supports a mold part 4a. A movable mold-carrying plate 5, on which the movable mold part 5a is supported and against which both the toggle mechanism 2 and auxiliary hydraulic drives 6 apply, is guided, in a manner known per se, on tie rods 7 (four in the present example) extending between and secured to the two opposite fixed plates 3 and 4. Tie rods 7 are usually made of metals as is known. An electrical heating sleeve 8 is engaged over each of tie rods 7 (only two are shown). It will be understood that heating sleeves with a liquid heat carrier or bores may also be provided. In such a case, tie rods 7 may not only be heated, but also cooled. Further, each tie rod 7 is equipped with a strain pickup 9 (only one is shown) which may be an inductive transmitter, for example. Also associated with each tie rod 7 is a temperature sensor 10.

The strain pickups or sensors 9 are connected to a force measuring device 11, while the heating sleeves are connected to a temperature controller 12.

Since any force acting on tie rods 7 results in some small elongation thereof, the detected instantaneous elongation is a measure of the magnitude of this force. This force may be read, for example, on the force measuring device 11. On the other hand, every temperature variation in the tie rod corresponds to a definite variation in length of the rod due to thermal expansion and contraction. For each machine, exact relations between the tie rod temperature and the locking force can easily be determined and put into tables, for example. This enables the operator, as indicated at H in the FIGURE, to adjust, on controller 12, the temperature change required for changing the respective measured locking force. The force may also be controlled automatically. In such a case, the actual values of force and temperature are supplied to a force controller 13 by which the required temperature variation in tie rods 7 is effected in accordance with the desired value of the locking force and through temperature controller 12.

In the foregoing, it has been assumed that the temperature, and thus the locking force, of all four tie rods 7 is controlled in the same way. It is easily possible, however, to control the force effective in each of the tie rods separately, not only by producing a completely assymmetric locking force, but also, by increasing the force through the two upper tie rods relative to the lower ones. This may make allowance for an asymmetric geometry of the mold cavities, for example. But also inaccuracy in construction (for example, unequal length of the tie rods, oblique position of one of the mold supporting plates, etc.) may thus be compensated for.

It will be understood that temperature variations in the tie rods require time, in practice, some minutes. This time, however, is satisfactory for all practical purposes. Also, experience has shown that considerable variations in force are obtainable with relatively small temperature variations. For example, in an injection molding machine for plastics having a locking force of 500 KN (with a tie rod length of 1,000 mm and tie rod diameter of 53 mm), force variations per rod of 5.45 KN for each degree centigrade of temperature variation may be obtained, which corresponds to about 4.4% of the rated force of the machine.

Known circuits and off-the shelf devices exist to make up the block diagram shown in the FIGURE which includes the temperature control apparatus 12, the strain measuring apparatus 11 and the force controller 13.

As may be learned from the foregoing, the technological and constructional measures to be taken to control the locking force thermally are very simple. The necessary heating or cooling devices, pickups, measuring and control devices, can easily be mounted on already set up machines. In addition, as compared to mechanical adjusting devices, they have the advantage of not being exposed to wear and being hardly susceptible to disturbances.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.