GB2030280A - Heat gun - Google Patents

Abstract

A portable hot air gun has a handle assembly 1 on which are separately mounted a venturi tube 2 and a gas nozzle 9 directed into one end of the venturi tube which has at its other end a burner provided with an igniter operable from the handle assembly 1. The tube 2 leads via a diffuser 55 having radial vanes 60 into a fishtail-shaped combustion chamber 21 having a slot outlet 22, the burner being surrounded by a rectangular cage 20. The handle comprises a pistol grip 4 with a trigger 5 to open a gas valve (in housing 48) and to operate a piezo electric igniter unit 34. A second trigger (81), Figs. 4, 5 (not shown), may be provided to hold the first trigger in its depressed position. <IMAGE>

Description

SPECIFICATION Shrinkmaster gun This invention relates to an air gun or is more specifically concerned with a gun for producing hot air in order to heat shrink a polyethylene film onto articles in order to package them in a taut, air tight, envelope capable of being easily removed when the articles are to be used.

Currently available hot air guns are designed to be used manually and provide a stream of hot gas which is directed onto the film to be heat shrunk.

The gas stream must be diffuse so that a large area of the film is heated to shrinking temperature, and must be free of hot spots which might melt a hole in the film. The gas which is combusted is commonly propane and the appropriate combustion conditions are obtained by passing a jet of the propane gas through a venturi which is open at a gas inlet end so that air is sucked through the venturi tube with the gas jet. An impeller at the outlet end of the venturi tube ensures thorough mixing of the gas and air which are then passed into a mask in which combustion takes place in the presence of further air. The mixture of air and combustion gases discharged from the mask are directed onto the film to be heat shrunk.

Conventionally, the gun is provided with a pistol grip having a trigger for controlling ignition of the gas-air mixture in the mask. Such ignition is achieved by a piezo electric generated spark. A valve, which may be controlled by a second trigger on the handle, ensures that the propane gas is discharged into the venturi tube immediately prior to ignition. With such an arrangement, release of the operator's grip on the handle causes extinction of the burner automatically. This "dead man" facility has advantages from the point of view of safety.

Currently available hot air guns are relatively expensive. The triggers and valves on the handle result in a complex mechanism having several moving and interacting parts, and the assembiage of gas jet, venturi tube, impeller and mask is mounted so as to be balanced on the handle for the comfort of the user. Flexible gas tubing which carries propane gas to the gun is normally connected to the bottom of the pistol grip of the handle and special connections conduct the gas from the handle ducts which extend along the venturi tube from the handle to the gas jet. To simplify the assembly and ensure rigidity, the venturi tube is normally made as a casting and has the ducts passing lengthwise through it.

An object of this invention is the provision of a relatively simple, and therefore cheap, construction of hot air gun.

In accordance with the present invention a hot air gun has a handle assembly on which are separately supported a venturi tube and a gas nozzle directed into one end of the venturi tube which has at its other end a burner provided with an igniter operated from the handle assembly.

An advantage of the invention is that by having the nozzle mounted on the handle assembly instead of on the venturi tube at one end, the construction of the venturi tube can be greatly simplified so that cheap fabrication techniques can be used for its construction instead of more expensive casting techniques. The gun is therefore cheaper to manufacture and lighter in weight.

Preferably the handle assembly includes a pistol grip having a trigger which, when depressed, sequentially opens a valve allowing gas to jet from the nozzle and thereafter operates the igniter to start the burner. Suitably this result is achieved by using an arrangement in which operation of the trigger displaces a carriage along a slideway to depress a plunger of piezo electric spark generating unit, and simultaneously to move a camming device past a gas valve operating element so that the gas valve is opened prior to the spark generating unit operating.

Preferably the pistol grip is provided with a catch for holding the first mentioned trigger in the depressed position and a second trigger which when depressed when the first trigger is depressed retains the catch in its holding position and when released allows the catch to be released so that the first trigger can return to its nondepressed position.

It is usual to provide hot air guns with a metal impeller. Preferably such an impeller comprises a stack of identical spoked coaxial plates, each plate being rotated relative to the adjacent plate about the axis of the stack. The plates are preferably rivetted together, and may be provided with a roughened surface to impede any relative movement between the plates after rivetting.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a side view of a hot air gun; Fig. 2 shows the gun in section; Fig. 3 is an exploded view of the flame gun; and Figs. 4 and 5 show a modified pistol grip of the hot air gun, in the released and operative positions respectively.

The flame gun shown in Fig. 1 comprises a handle assembly 1, a ventud tube assembly 2, and a mask 3.

The handle assembly 1 has a pistol grip 4 equipped with a trigger 5 hinged at its lower end to the grip 4 and carrying at its upper end a trunnion 6 connected to an arm 7 capable of movement along a slot formed in the underside of a support 8. A flexible gas supply tube 10 extends to a connection at the rear end of the support 8 opposite the position of a gas jet nozzle 9. The front portion of the support 8 has volted to it the venturi tube assembly 2.

The venturi tube assembly 2 comprises a length of metal tube having a flared mouth 11 at its end adjacent the nozzle 9, which constitutes the gas inlet end of the venturi, and a uniform internal diameter. At its other end 14 it is welded into an annular end ring 1 5. Extending along the underside of the tube is a conduit 1 6 through which extends an insulated wire 1 7 for conveying a high tension voltage to an ignition electrode 18.

The mask 3 comprises a slotted cage 20 of metal which is of rectangular cross-section, the longer side of the rectangle extending vertically and being symmetrically arranged around a divergent wedge-shaped combustion chamber 21 of a burner having a slot outlet 22. The chamber 21 tapers towards the outlet 22 from a cylindrical tube section 23 which is fitted to the end ring 1 5 and held in place by screws 24. The inside surface of the end ring 15 is outwardly flared at25 as clearly shown in Fig. 2.

Fig. 2 the internal construction of the hot air gun in more detail. The trigger 5 is hinged at 30 to the pistol grip 4 and is provided with a slot 31 at its upper end to receive the trunnion 6 which projects down from a sliding carriage 32 mounted in the support 8. The support is of channel section so that its floor provides a guiding slideway for the carriage 32. A piezo-electric high voltage spark generating unit 34 is fixed in the forward end of the support 8 by a bracket 35, and is equipped with a plunger 36 which, when pressed sufficiently into the unit 34, triggers operation of a spark generator. The voltage produced by the spark generator is conveyed by the wire 1 7 to the ignition electrode 18.

The plunger 36 is biassed by a spring inside the unit 34, to bear on a piston flange 37 formed as part of the carriage 32. A second, parallel, flange 38 is provided at the rear end of the carriage 32 and locates one end of a coiled compression spring 40. The other end of the spring engages a fixed leg 41 of a cover strip 42 which closes the top of the support 8 beneath the nozzle 9.

A strengthening web 43 extends between the two flanges 37, 38 and, like them and the trunnion 6, is integrally formed with the carriage from a hard-wearing plastics material, such as nylon. The cover strip 42, pistol grip 4 and trigger 5 are likewise formed of insulative plastics materials whereas the support 8, the venturi assembly 2 and the mask 20 are made of metal.

The upper surface of the web 43 is formed towards its rear surface with a ramp surface 44 providing a cam and extending between two flats 45 and 46. A spring-loaded ball 47 rides on the ramp surface 44 and controls opening and closing of a gas valve (not shown) located in a housing 48 into which the nozzle 9 and an end-connector 50 of the tubing 10 are screwed.

The forward upper end of the support 8 has bolted within it a block 51 welded to the underside of the ventun assembly 2. A gap 52 between the under forward end of the block 51 and the top of the pistol grip 4 allows the wire to be led through from the conduit 1 6 to the sparkgenerating unit 34.

The forward end of the cylindrical tube section 23 has located within it a combined diffuser and impeller device 55. This comprises an impeller unit 56 formed as a nylon moulding and having a central boss 57 formed with a forwardly facing well 58 and surrounded by a ring of outwardly projecting impeller blades 60 whose radially outer edges terminate on the locus of a cylinder. At their rear (i.e. upstream) ends, the blades 60 are surrounded by an integrally formed collar 61. A nylon sleeve 62 surrounds the outer edges of the blades and abuts the collar 61, the outer diameter of the blade and the collar being the same and sized to fit tightly into the tube section 23.The axial length of the collar 61 is such that its forward end protrudes beyond the blades 60 and serves to space a flat stainless steel mesh 63, providing a diffuser, forwardly from the mouth of the well 58 by about 3 mm. The mesh 63 forms the back of the combustion chamber 21 and the rim of the mesh is located by being trapped between the end of the wall of the combustion chamber and the collar 61.

Fig. 3 shows how the parts are made and assembled. In practice it will be found that for each gas pressure and nozzle size, there is an optimum length of venturi tube and its spacing from the nozzle 9 influences the performance.

When the gun is to be used the operator squeezes the pistol grip 4 to depress the trigger 5.

This is resiliently opposed by the spring 40 and the bias built into the spark unit 34. As the trigger is moved, the trunnion 6 slides the carriage 32 forward so that the plunger 36 is depressed into the unit 34, the spring 40 is compressed between the flange 38 and the leg 41, and the ball 47 rides up the ramp 44 to open the gas valve. The nozzle 9 discharges a stream of gas into the venturi tube assembly 2 and air is entrained in the stream by virtue of the flared mouth 11. The mixture of gas and air is whirled by passage through the impeller and discharged through the diffuser mesh 63 into the combustion chamber. By the time the gas enters the combustion chamber the grip of the operator's hand on the pistol grip has increased sufficiently for the carriage displacement to trigger operation of the spark generating unit.The spark produced thereby at the electrode 1 8, ignites the gas in the combustion chamber and a stream of hot gas is discharged from the forward end of the mask 20.

The presence of the well 58 in the boss of the impeller is important. Without it, combustion at the centre of the mesh 63 occurs with a yellow flame which is undesirable. It seems that the well 58 together with the spacing between the mesh and the boss, alters the gas/air mixture flow pattern behind the mesh in such a way that combustion occurs downstream of the mesh with a blue flame which is advantageous. The mesh 63 also serves to prevent blow-back of the flame from the combustion chamber and acts as a heat shield to prevent the nylon impeller temperature rising to a softening temperature.

As soon as the grip of the operator's hand on the pistol grip eases, the springs drive the carriage 32 back to the position shown in Fig. 2 at which the gas suppiy valve is closed.

Figures 4 and 5 illustrate an improved pistol grip for the shrink gun already described.

Components corresponding to those of Figures 1 to 3 are shown by reference numbers increased by 100. The pistol grip 104 is provided with a main trigger 105 acting by means of slot 131 on trunnion 106 to operate the gas valve and igniter.

In these figures a spring 71 acts between the gun frame and a lug 72 above the trunnion to return the components to their rest position in place of the spring 40 of Figures 1 to 3.

In this embodiment, the main trigger 105 has a pin 73 on which a catch 74 is pivoted. The catch has a depending lug 75 which on depression of the main trigger 105 engages the far side of a pillar 76 joining the two sides of the pistol grip 104. Without assistance the lug 75 would not retain the main trigger 105 in its depressed position, and this assistance is provided by a second trigger 81 on the opposite side of the pistol grip 104 to the main trigger 105. The thumb operates the main trigger 105 while the fingers, particularly the index finger, operate the second trigger 81.

The second trigger 81 is formed with a slot 82 surrounding a pivot pin 83. A spring 84 urges the trigger downwards relative to the pin 83 so that the pin 83 tends to be located at the top of the slot. A tension spring 85 urges the bottom of the second trigger 81 towards a pin 86 on the grip 1 04. so that the top end 87 tends to protrude from the grip until a stop 88 engages the wall of the grip 104. The trigger 81 is formed with a recess 89 facing the catch 74, the upper surface of the recess being tapered inwards from the mouth to an intermediate point 91 after which it tapers outwards again.

If the two triggers are depressed together, the catch 74 engages the tapered top surface of the recess 89 and the pillar 76. The reaction on the second trigger lifts it so that the pivot pin moves to the bottom of the slot against the compression of the spring 84 so that the lug 75 can pass between the tapered surface of the recess 89 and the pillar 76. The catch and second trigger then move downwardly until the undersurface 92 of the main portion of the catch engages the pillar. The intermediate point 91 of the second trigger now engages the catch 74. While the second trigger 81 is depressed, the first trigger 1 05 cannot escape from its depressed position.The effort to keep the second trigger depressed is much less than that required to depress the main trigger, so that once the gas valve has been opened and the igniter operated, the operator is only required to exert a small pressure on the grip to keep the gun operating. In the embodiment of Figures 1 to 3, the full pressure must be continuously exerted on the trigger 4 to keep the gun operating. The gun still has a fail-safe facility, since as soon as the second trigger 81 is released, the spring 85 will rotate it anticlockwise and the catch 74 will move over the tapered portion of the recess 89 and lift the second trigger to release the catch from the pillar 76.

The catch 74 and the second trigger 81 are made of 25% glass filled nylon.

In place of the diffuser and impeller device 55, a metal device may be provided formed by a stack of identical spoked coaxial plates, each plate being rotated relative to the adjacent plate about the axis of the stack, and the plates being rivetted together. The rivet head is preferably formed with a small well corresponding to the well 58. Each plate has eight narrow spokes and the angle between adjacentplates is such that the stack is twisted through 450 between the front and back plates. This is achieved by a relative angle between adjacent plates of just under 30 in a stack of eighteen eight-spoked plates. The front and rear surfaces of the plates are stamped with a regular dot pattern to roughen these surfaces to impede relative rotation between the plates when rivetted together.

The above described air gun is simple to use, easy to assemble, and cheap tc manufacture.

Also, as the gas suppiy control components are independent of the venturi tube assembly, the latter can be of simple construction which makes expensive casting unnecessary.

Although nylon has been mentioned as the plastics material for the impeller and other parts of the gun, it is to be understood that other plastics material, such as resin based fibre reinforced material, may be used equally well.

Claims (10)

1. A portable hot air gun having a handle assembly on which are separately supported a venturi tube and a gas nozzle directed into one end of the venturi tube which has at its other end a burner provided with an igniter operated from the handle assembly.

2. A gun as claimed in Claim 1, in which the handle includes a pistol grip having a trigger which, when depressed, sequentially opens a valve allowing gas to jet from the nozzle and thereafter operates the igniter to start the bumer.

3. A gun as claimed in Claim 2, comprising a carriage displaceable along a slideway by operation of the trigger to depress a plunger of piezo electric spark generating unit, and simultaneously to move a camming device past a gas valve operating element so that the gas valve is opened prior to the spark generating unit operating.

4. A gun as claimed in any one of the preceding claims, in which a diffuser and impeller device is located at the opposite end of the venturi tube to the nozzle and the diffuser comprises a metal mesh through which gas enters the combustion zone of the burner.

5. A gun as claimed in Claim 4, in which the impeller comprises a central boss having radiating blades curved to produce whiriing motion of the gas air mixture passing them, and the diffuser is spaced from the blades and boss which is formed with a well facing the mesh.

6. A gun as claimed in Claim 5 wherein the impeller comprises a stack of identical spoked coaxial plates, each plate being rotated relative to the adjacent plate about the axis of the stack.

7. A gun as claimed in Claim 2 or any claim dependent thereon, wherein the pistol grip comprises a second trigger which when depressed can hold the first mentioned trigger in its depressed position.

8. A gun as claimed in Claim 7 wherein said second trigger is adapted for radial and pivotal movement about a pivot axis.

9. A gun as claimed in Claim 7 or Claim 8 wherein said first and second triggers are mounted on offset sides of said pistol grip.

10. A portable hot air gun substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 3 alone or as modified by Figures 4 and 5 of the accompanying drawings.