DE29904994U1 - Device for cutting hard foam - Google Patents

Description

Scaritec AG
CH-4333 Münchwilen

Device for cutting Rigid foam

DE-OS 40 10 927 describes a high-temperature fiber cutting device which comprises a blade with a cutting edge. The blade contains a resistance heating element which is embedded in silicon nitride. The device is supplied with a voltage of 220 V.

A hand-held cutting device for rigid foam boards is also known from EP-A-862 974, which is declared to be an integral part of the present application. This hand-held cutting device comprises a handle to which a holder is detachably attached by means of a bayonet lock. A bracket is attached to the holder, over which a heating wire is stretched.

The present invention is based on the object of specifying a device with which rigid foam can be cut in a simple manner for universal use. This object is achieved by the combination of features of the claims.

An embodiment of the invention is explained below with reference to the drawing. In it:

Figure 1 shows a longitudinal section through an inventive

raaßse device,

Figure 2 shows an enlarged section of the

position according to Figure 1, and

Figures 3 and 4 show two variants of the corresponding control device.

The device 1 according to Figures 1 and 2 comprises a tubular, metal holder 13 which is designed for insertion in a hand-held cutting device according to EP-A-862 974. For this purpose, the holder 13 has two cylindrical shaft sections 2, 3, a flange 4 and two protruding pins 5 for the bayonet lock in the handle. A heating rod 6 is inserted into the holder 13 at the front end. The heating rod 6 is secured in the holder 13 with two threaded pins 7. At the rear end, an insulating sleeve 8 is inserted into the holder 13, into which a contact pin 14 is pressed. The contact pin 14 is connected to the heating rod 6 with a cable 9. The heating rod 6 consists of a thin-walled tube 18 with a wall thickness of 0.1 mm and an external diameter of 3 mm. The tube 18 consists of an electrical resistance material, such as that used for heating wires. The tube 18 encloses a conductor wire 11 with an intermediate layer of electrical insulation 10, e.g. a ceramic tube. One end of the wire 11 is connected to the cable 9. The other end is hard-soldered to the pressed end of the tube 18. The heating current with a voltage of e.g. 40 V flows via the holder 13, the tube 18, the wire 11 and the cable 9 to the contact pin 14.

Figure 3 shows a control device 22 which is particularly suitable for operating the device 1 according to Figures 1 and 2. The tube 18, together with a reference piece 19 made of the same material, e.g. a shorter piece of the tube 18, which is connected electrically in series, forms a half bridge of a full bridge circuit 28 which is completed by two resistors 29, 30. The supply voltage is applied to the end 16 of the reference piece 19 and the holder 13. The temperature measurement of the heating tube 18 is tapped at the contact pin 14 and at the fourth bridge point 31 between the resistors 29, 30. The tapped alternating voltage is fed via

a rectifier 32 and amplified by an amplifier 33, which actuates a relay 34. The alternating voltage present on the supply cable 26 is applied to the primary coil 37 of a transformer 38. Its secondary coil 39 has a center tap of eg 10 V and an end tap of eg 40 V. The relay 34 switches the voltage applied to the holder 13 between these two values.

The device described works as follows: In idle mode, the bridge 28 is tuned and the output of the rectifier 32 is almost zero. The lower voltage is applied to the holder 13. If a Styrofoam plate is now cut with the device 1, the tube 18 cools down and its resistance drops. This detunes the bridge 28 and the rectifier 32 supplies an output voltage which switches the relay 34 via the amplifier 33. The higher voltage is then applied to the holder 13 so that the heating power increases. The cutting speed is increased accordingly. As soon as the cut is finished, the tube 18 is heated again to the same temperature as the reference piece 19 so that the bridge 28 is calibrated again. The relay 34 drops out and the heating power is switched back to idle mode.

Because the pipe 18 has a relatively low temperature of e.g. 400 ^° C when idling, this prevents the cut material from being scorched at the start of the cut, which would result in an unclean cut. Despite this relatively low temperature, a high cutting speed is achieved by automatically switching the heating power.

In order to ensure that the pipe 18 is not heated too much in the area where it is not in contact with the material to be cut, a current limiter 40 known per se can be installed in one of the lines 23, 25.

The control device 22 can also be operated with direct current. In this case, the rectifier 32 is omitted or is connected downstream of the transformer 38. The relay 34 can also be replaced by an electronic switch. The response threshold of the amplifier 33 can be adjustable, for example, by a potentiometer 41.

The tube 18 should have a reasonably constant temperature resistance coefficient of preferably more than 0.01%/°K over the temperature range used, in any case one which does not change sign in the temperature range used.

Figure 4 shows another variant of the control device 22. In this variant, the secondary coil 39 has only one tap. The amplifier 33 is connected to a switching unit 56 which, when the output of the amplifier 33 is low, only allows 17 full waves per second of the secondary alternating current to pass through, for example, and when the output is high, all 50. This type of circuit has the advantage over the known dimmer circuit that interference with other devices is avoided. The switching unit 56 can, for example, contain a ring counter with three counting stages which counts the increasing zero crossings of the alternating current and, in idle mode, activates an electronic blocking switch in the second and third counting stages, which interrupts the power supply to the holder 13.

With the device described, cuts in hard foam boards can be made particularly easily because the heating rod 6 can be moved in any direction. The low supply voltage compared to conventional heating rods enables the direct heating of the outer tube 18 by current flowing through it, without endangering the user with electric shocks. This makes it possible to achieve a small outer diameter of the heating rod 6.

Because the pipe 18 is heated directly by the current flowing through it, it reaches operating temperature within seconds. This design also makes it possible to use the control device 22 described, which automatically increases the heating value from an idle value to a higher value at the start of cutting and automatically switches back after the end of the cutting process. This makes it possible to achieve a high cutting speed without the risk of scorching the rigid foam at the start of cutting.

Deviating from the embodiment described, it is also possible to install the reference piece 19 directly in the holder 13. However, this requires three contact points for electrical connection to the handle. This variant has the advantage above all that several devices with heating rods β of different lengths and already coordinated reference pieces 19 can be provided for the same handle without the full bridge 28 having to be coordinated each time. This variant is also particularly useful if the holder 13 is directly and firmly connected to a handle that has corresponding openings for the air supply to cool the reference piece 19. In this variant, the same control unit 22 can be used for different devices 1 with, for example, different lengths of the heating rods 6 or differently shaped heating rods 6.

The heating rod 6 can also be designed in an arc shape instead of a straight line. This opens up additional areas of application for the device 1, e.g. in the design of sculptures made of hard foam.

Claims (7)

1. Device for cutting rigid foam, comprising a holder ( 13 ) to which an electrically heatable heating element ( 6 ) is fastened, characterized in that the heating element is designed as a tube ( 18 ) whose first end is fastened to the holder ( 13 ), that an electrical conductor ( 11 ) is arranged in the tube ( 18 ), which is separated from the tube ( 18 ) by an insulation ( 10 ), and that the second end of the tube ( 18 ) is connected to the free end of the conductor ( 11 ). 2. Device according to claim 1, wherein the wall thickness of the tube ( 18 ) is less than 0.2 mm. 3. Device according to claim 1 or 2, wherein the second tube end is brazed to the conductor ( 11 ). 4. Device according to one of claims 1 to 3, wherein the insulation ( 10 ) consists of ceramic. 5. Device according to one of claims 1 to 4, wherein the holder ( 13 ) is designed as a plug-in unit for insertion into a handle, wherein the first tube end is electrically conductively connected to a first contact element and the second end of the conductor ( 12 ) is connected to a second contact element ( 14 ). 6. Device according to one of claims 1 to 5, wherein it additionally comprises a control device ( 22 ) which switches the heating power of the tube ( 18 ) between two values deviating from zero, namely from an idle value to a higher value and back. 7. Device according to claim 6, wherein the tube ( 18 ) is electrically connected in series with a reference element ( 19 ) of the same nature, with which together it forms a half-bridge of a full-bridge circuit ( 28 ), and wherein the one diagonal voltage of the full-bridge circuit ( 28 ) is used as a measured value for switching from the open-circuit value to the higher value and vice versa.