WO1988008785A1

WO1988008785A1 - Method of joining plastic material

Info

Publication number: WO1988008785A1
Application number: PCT/SE1988/000223
Authority: WO
Inventors: Ingemar Persson
Original assignee: Exploweld Ab
Priority date: 1987-05-05
Filing date: 1988-05-02
Publication date: 1988-11-17
Also published as: SE8701846D0; SE467729B; SE8701846L

Abstract

A method of joining materials, which consist of or contain as component macromolecular organic compounds, where at least a first one (1) of the material parts to be joined with a second material part (2) is accelerated to high speed by means of one or several detonated explosive charges (16; 19) and is allowed to collide with the second material part (2) in a joint plane. The invention is characterized in that the relative speed (vv) in a direction perpendicular to the joint plane, with which speed the material parts (1, 2) collide with each other, is caused to be such, that the pressure arising between the material parts (1, 2) at the collision is higher than the respective yield point of the materials, but twenty times lower than the yield point, at the same time as the collision successively in a direction in parallel with the joint plane is caused to run with a speed between about 1000 m/s and 2000 m/s, preferably between 1400 m/s and 1700 m/s.

Description

Method of joining plastic materia l

This invention relates to a method of joining plastic material.

It is known to join plastic material by glueing, thermal fusion, ultrasonic welding, friction welding etc. Plastic materials constitute a very large group of materials with widely different properties and application fields. In certain cases the plastic materials form the matrix in composite materials. This implies, that it is not possible to apply only a single one of the aforementioned methods to all plastic materials and application fields.

In the field of metallic materials it is known to join most of the metals and metal combinations by means of explosion welding, which is a rapid and reliable method and can be used without requiring large investments.

The present invention proposes a technique by means of which plastic material can be explosion welded.

The present invention, thus, relates to a method of joining material, which consists of or comprises as component macromolecular organic compounds, where at least a first one of the material parts to be joined with a second material part is accelerated to high speed by means of one or several detonating explosive charges and is permitted to collide with the second material part in a joint plane, and is characterized in that the relative speed v_v in a direction perpendicular to the joint plane, with which speed the material parts collide with each other, is caused to be such that the pressure arising between the material parts at the collision is higher than the yield point of the respective material, but is lower than twenty times the yield point, and simultaneously the collision successively in a direction in parallel with the joint plane is caused to proceed with a speed D between about 1000 m/s and 2000 m/s, preferably between 1400 m/s and 1700 m/s. The present invention is particularly suitable for use when pipes of HDP-plastic (High Density Polyethylene) are rapidly and simply to be welded together to form pipelines for gas etc. Another special application field is the joining of carbon-fibre reinforced plastic materials for the aviation industry.

It is obvious, however, that the present invention can be utilized for most of the macromolecular materials. The present invention is explained and described in the following with reference to the accompanying drawings, in which

Fig. 1 is a schematic lay-out of an explosion welding process. Fig. 2 is a speed diagram.

Fig. 3 is another speed diagram.

Fig. 4 illustrates different zones in an explosion welding process.

Figs. 5-10 show different basic arrangements of two material parts to be explosion welded one to the other.

Figs. 11a-11e show different exemplifying arrangements for joining plates or pipes.

Fig. 1 shows in a schematic manner the material flows occurring at an explosion welding process. A first material part 1 is accelerated against a second material part 2. The collision zone is designated by a. In reality, the collision front a moves from the right to the left in the Figure.

It can be imagined theoretically, without changing the relative motion pattern of the parts, that the collision zone is stationary and the material parts 1,2 move from the left to the right in the Figure.

The continuity conditions of the process yield the speed D to be constant everywhere in the longitudinal direction of the material parts. The vector diagram in Fig. 2 shows that the speed v_p, with which the first material part 1 is slung against the second material part 2, has the argument ø =ß/2 relative to the normal to the surface, which moves downward and during the process forms an angle ß with the upper surface of the second material part 2.

In Fig. 2 ø + δ + 90 = 180° and ß + 2 δ = 180°, and therefore Ⴔ = ß/2.

In Fig. 3 the speeds are illustrated with reference to an explosion welding process.

The speed vector v has the composants v_v perpendicular to the collision surface of the second material part 2 and v_h in parallel therewith.

The composant v_v is braked momentarily at the collision and gives rise to a pressure wave, which partly moves upward in the material part 1 and partly downward in the material part 2.

When the amount of the composant v_v is sufficiently great, the pressure created near the collision surface will be sufficiently high for fluidizing the material. The viscosity of the fluidized material can be influenced within certain limits. The fluidizing depth further is affected on both sides of the collision surface by the amount of v_v. The composant v_h is the difference in speed between the colliding surfaces of the material parts 1,2 and affects the formation of the joint in such a way that, when its amount reaches a certain value, the flow between the fluidized layers of the material parts 1,2 transforms from laminar to turbulent character. The said value is coherent in known manner with the. viscosity.

When the pressure in the joint surface ceases, the fluidized layer closest to the collision surface solidifies, and the turbulent wave pattern "freezes", as shown in Fig. 4. The pressure wave produced at the collision is reflected against the surfaces of the material parts 1,2 which are opposed to the collision surfaces. After the reflection, the pressure wave returns to the joint surface as a draw wave, thereby relieving the joint area.

In Fig. 4 the flowing zone of material, which arises, is indicated by a screened area 3. The propagation and, respectively, direction of the pressure wave produced at the collision between the material parts is illustrated by the lines 4 and arrows 5 , respectively. The lines 6 and, respectively, arrows 7 illustrate the propagation and direction, respectively, of the pressure wave reflected against the surfaces 8,9.

The reflection points are indicated by the numerals 10 and 11.

As illustrated in Fig. 4, the flowing zone solidifies due to the draw wave running inward from the surfaces 8,9, when it arrives at the joint area whereby said wave pattern 12 freezes. The propagation of the solidified joint area is illustrated by the lines 13,14.

It is known at explosion welding of metals, that the amplitude and wave length of the "frozen" wave pattern increase with increasing time until the draw wave relieves the joint area. This implies, that the turbulence or wave image initiated at the front of the collision increases in amplitude and wave length during the time when the material layers closest to the Joint are flowing. Increased material thickness of the material parts implies, that the draw wave arrives at the joint area at a later time relative to the collision time and, therefore, the amplitude and wave length of the wave pattern increase. The said time, of course, depends also to a high degree on the shock wave speeds of the material. The shock wave speed for a material is equal to the sonic speed for the material at shock waves with lower amplitude and a function of the sonic speed at higher shock wave amplitude. When the time between the time of the collision of the material parts and the time of the freezing of the wave pattern is long, for example due to low shock wave speed, the turbulence tends to increase so much, that it gives rise to strong whirl formations, which in their turn by inner friction cause a temperature so high that the material melts thermally. This results in cavities in the completed joint and reduces the strength thereof.

The aforesaid with regard to the process of explosion welding also applies to explosion welding of metals. The shock wave speed for macromolecular organic materials normally is considerably lower than for metals. This implies, that said time is long until the joint area is relieved. According to what is explained above, this gives rise to great turbulence and whirl formations, resulting in the formation of cavities. As, moreover, the fusion temperature for macromolecular materials is very low, compared with that for metallic materials, a substantial thermal melting of the material occurs readily in the joint area. Explosion welding of macromolecular materials, such as plastics, therefore cannot be carried out by means of the parameter relations applied at explosion welding of metals.

The present invention proposes a method of joining materials, which consist of or contain as components macromolecular organic compounds, where at bast a first one of the material parts to be joined is accelerated to high speed by means of one or several detonating explosive charges and is allowed to collide with a second material part in a joint plane, i.e. a method of explosion welding macromolecular materials.

It was found by surprise, that an explosion joint of high quality is obtained when argument and amount for the said vector v is adjusted in the way described below to the yield point, shock wave speed and fusion temperature of the macromolecular materials.

According to the invention, the relative speed v_v in a direction perpendicular to the surface of said second material part facing to the first material part, i.e. perpendicular to the joint plane, at which speed the material parts collide with each other, is to be caused to be such that the pressure arising between the material parts 1,2 at the collision between the same is higher than the yield point of the materials, but twenty times lower than the yield point, and at the same time the collision successively is caused to run in a direction in parallel with said surface of said second material part, i.e. in parallel with the joint plane, with a speed D between about 1000 m/s and 2000 m/s, preferably between 1400 m/s and 1700 m/s.

According to a preferred embodiment, said collision pressure is between 5 and 20 times the yield point. As a comparison can be stated, that at explosion welding of metals the collision pressure is more than 50 times higher than the yield point for the metallic materials.

It was found that a higher speed than about 2000 m/s gives rise to shock waves of much too high strength, which after said reflection destroy the joint area and/or the material in the material parts.

The explosives to be used at the present invention, thus, are explosives with low detonation speed. Pulverous explosives, for example, diluted with an inert material can be used. The number of imaginable explosives is very great. Nitroglycerine explosives, for example, are suitable. One such explosive is marketed by Nitro Nobel, Sweden, under the trade name GURIT. Another suitable explosive is Nitroguanidine (CH₄N₄O₂). The expert easily can provide an explosive with a detonation speed suitable for the application at present in question.

When the thickness and shock wave speeds of the material parts are equal, which is usually the case at the joining of plastic materials, according to the aforesaid a collision of the two reflected pressure waves is obtained just in the newly formed and not yet solidified joint layer, which collision tends to wear apart the joint. This is to a high degree a problem when explosion welding is to be carried out of macromolecular materials.

This problem is avoided according to several preferred embodiments of the invention.

According to a first embodiment where both material parts 1,2 have substantially or exactly the same thickness perpendicular to the joint plane, at the free surface of one material part a buffer material is located, the shock wave impedance, i.e. the density times the sonic speed, of which is substantially the same as the shock wave impedance for the material part, against which the buffer material is located.

In Figs. 5 and 6 the numerals 1 and 2 designate the material parts, the numeral 15 designates the buffer material, and the numeral 16 designates the explosive or the impulse generating member.

According to another embodiment, the material parts 1,2 used have different thickness perpendicular to the joint plane, as illustrated in Figs. 7 and 8, where the explosive charge is designated by the numeral 16. Hereby a strong draw wave in the joint area is prevented.

Fig. 7 illustrates a usual setting before the detonation of the explosive, where the two material parts 1,2 are in parallel with each other. At such a setting, thus, the speed D of the collision front must be controlled only by selection of the explosive.

According to a preferred embodiment illustrated in Fig. 8, the surfaces of the two material parts 1,2 to be joined have been arranged, prior to the joining, so as to form an angle with each other. According to this embodiment, the speed D of the collision front can be controlled by adjusting the angle between the material parts 1,2.

For macromolecular materials, which require the speed of the collision front to be located in the lower one of the speed interval stated above, it is advantageous to select an explosive with low detonation speed and to arrange the material parts 1,2 with an angle to each other as illustrated in Fig. 8. At the arrangements shown in Figs. 6 and 7, the explosive charge is caused to act directly on the material part 1 to be accelerated.

Especially for materials where the speed of the collision front must be low, an alternative according to a further embodiment is that the explosive charge is caused to indirectly affect the material part 1 to be accelerated via a material inert to the detonation, as illustrated in Fig. 9.

In Fig. 9 the numeral 17 designates the inert material. This material or pressure transfer medium can consist, for example, of plastic, rubber, metal, gypsum, paraffin or a liquid, after suitable choice and order, in order to reduce the speed D of the collision front to a desired value. According to another embodiment illustrated in Fig. 10, said inert material is a projectile body 18, which is caused to accelerate by means of the explosive charge l6 against the first material part 1 and to collide with the same in order thereby to accelerate the first material part 1 against the second material part 2. Hereby the speed of the collision front is reduced.

The projectile body 18 can be made of plastic, rubber, metal, gypsum, wood fibre etc. and can either be plane- -parallel as shown in Fig. 10 or have a varying cross--section.

The projectile body 18 also can be such that it is welded on the first material part 1.

At the embodiment shown in Figs. 9 and 10 the inert material also has the effect, that said pressure arising between the two material parts 1,2 at their collision is reduced compared with the case when the explosive charge acts directly against the first material part 1.

In Figs, 11a-11e different exemplifying settings for explosion welding of two material parts 1,2 of macromolecular material are shown schematically, where the materials have the shape of plane plates or are tubular. When tubular materials are referred to, Figs, 11a-11e are sections along a radius. The numeral 19 designates explosive charges.

According to one embodiment, the said material parts 1,2 consist of a homogenous material.

According to another embodiment, however, one or both material parts can consist of a composite material or a laminate .

Experiments carried out in practice have shown that a surprisingly good joint is obtained when two material parts of plastic are joined according to the present invention. The present invention, thus, proposes a method of explosion welding macromolecular materials to each other.

It is, of course, necessary to adapt the choice of explosive and the arrangement of the material parts prior to joining, the use of buffer material, a material inert to the detonation or a projectile body, and the angle between the material parts prior to the joining to the macromolecular material or materials in question, in order thereby to bring about the conditions defined in the attached claims. Such adaptation causes no problems for the expert.

The present invention must not be regarded restricted to the embodiments described above or to the embodiments exemplified in the Figures, but can be varied within the scope of the attached claims.

Claims

1. A method of joining materials, which consist of or contain as component macromolecular organic compounds, where at least a first one (1) of the material parts to be joined with a second material part (2) is accelerated to high speed by means of one or several detonated explosive charges (16;19) and is allowed to collide with said second material part (2) in a joint plane, c h a r a c t e r i z e d i n that the relative speed (v_v) in a direction perpendicular to the joint plane, with which speed the material parts (1,2) collide with each other, is caused to be such, that the pressure arising between the material parts (1,2) at the collision is higher than the respective yield point of the materials, but is twenty times lower than the yield point, at the same time as the collision successively in a direction in parallel with the joint plane is caused to run with a speed (D) between about 1000 m/s and 2000 m/s, preferably between 1400 m/s and 1700 m/s.

2. A method as defined in claim 1, c h a r a c t e r i z e d i n that said collision pressure is between five and twenty times the yield point.

3. A method as defined in claim 1 or 2, where both material parts (1,2) perpendicular to the joint plane have substantially or exactly the same thickness, c h a r a c t e r i z e d i n that one material part (1;2) at its free surface abuts a buffer material (15), the shock wave impedance of which substantially is the same as the shock wave impedance for the material part (1;2), to which the buffer material (15) abuts.

4. A method as defined in claim 1 or 2, c h a r a c t e r i z e d i n that the material parts perpendicular to the joint plane have different thickness.

5. A method as defined in claim 1,2,3 or 4, c ha r a c t e r i z e d in that the material parts (1,2) prior to the joining have the surfaces to be joined so arranged, that they form an angle with each other.

6. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the explosive charge (16;19) is caused to act directly on the material part (1;2) to be accelerated.

7. A method as defined in any one of the claims 1-5, c h a r a c t e r i z e d i n that the explosive charge (16) is caused to indirectly act on the material part (1) to be accelerated via a material (17ll8) inert to the detonation.

8. A method as defined in claim 7, c h a r a c t e r i z e d in that said inert material is a body (18), which by the explosive charge (16) is caused to accelerate against the first material part (1) and to collide with the same, in order thereby to accelerate the first material part (1) against the second material part (2).

9. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that each of said material parts (1,2) consists of homogenous material.

10. A method as defined in any one of the claims 1-8, c h a r a c t e r i z e d i n that one or both material parts (1;2) consist of a composite material or a laminate.