SE434241B - PROCEDURE FOR IMPROVING A HEAT WELDING STRUCTURE, IMAGE BETWEEN TWO THERMOPLASTIC PIECES - Google Patents

Description

UT 10 2C 25 30 35 RO 7813035-8 grinding in an area around the joint between the bonded edges or by cutting the strand. This was followed by a polishing step.

In many applications, however, the integrity, reliability and durability of the weld or connection are essential. As an example, when thermoplastic pipes are bonded to each other with a hot weld, it is very important that the weld exhibits the required strength and durability for purposes including transporting fluids under varying temperatures and pressures in an environment which may be exposed to significant vibrations. As a second example, it can be mentioned that some battery glass is produced by hot welding technology.

These battery glasses contain a liquid electrolyte and support a series of heavy electrodes. Once placed in place, the battery lenses are subjected to vibrations and sometimes to shock impulse forces and consequently the welds must have a significant strength and service life in order to remain usable for a long period of time.

To test the integrity and reliability of these welds, a number of methods have been developed. One method means that a very strong electromagnetic field is established over the weld, whereby it is determined whether dielectric disturbances occur. If there are small pores and / or cracks in the weld, the dielectric strength of the weld will be reduced and a spark will be generated when the electromagnetic field is established over the weld.

Another method for testing the integrity and reliability of welds means that a mechanical impulse force is generated against the weld in order to determine its breaking resistance.

In the battery glass industry this is achieved by allowing a weight in the form of a spear to fall from a predetermined distance on the weld so that a very high point pressure differential is formed. Of course, other percussion methods can be used. depending on the design requirements for the finished product. Methods for measuring the reliability and strength of welds have been found to be useful for many purposes as the integrity of a weld is critical.

Using these and other known test methods, it has been found that the formation of hot welds by simple heating of the edges of thermoplastic material to be bonded and subsequent pressing of the edges against each other so that welds i \ | ° \ I) TJWR * j \ l 13 p; \ Jl 20 BO \ 1 ~ | UI H0 auAuTv '7813035-8 late formed results in a decreasing tensile strength of the material at the weld; ie the tensile strength of the material at the weld can be 55% of the tensile strength of the starting material or lower.

In addition, the extent to which the dielectric test fails as determined by the generation of a strong electromagnetic field across the weld can be increased by as much as 100-fold compared to the extent to which the dielectric test fails for the starting material. Furthermore, the impact strength of such welds, when the materials are tested by allowing a spear weight to fall on the weld, has been found to be significantly reduced compared to the starting material and the impact strength also varied considerably at various points along the welds and from one weld to another. deteriorate. Furthermore, the bending strength is reduced, especially the bending strength during bending, for the weld around the welding shaft significantly.

Accordingly, the invention relates to an improved process for bonding thermoplastic materials to each other thereby improving the bond strength and reliability of the bond.

Furthermore, the invention relates to an improved apparatus for bonding thermoplastic materials to each other.

Accordingly, the invention relates to an improved method and apparatus for bonding thermoplastic materials to each other. The process comprises steps in which the edges of the thermoplastic materials are heated to at least the melting temperatures.

The heated edges are then pressed against each other, whereby a strand is formed as a result of the pressure of the two thermoplastic materials which abut against each other, whereby a weld joint is formed. the string is formed along the weld. Thereafter (1) the weld is heated to at least about its melting temperature, possibly at elevated pressure, and then (2) it is cooled rapidly so that the physical properties of impact strength, dielectric strength and depression flexural strength of the plastic material in the vicinity of the weld are improved. known equipment for heating and chipping the edges of the thermoplastic materials so that a weld is formed. The improved apparatus according to the invention comprises a strip of material which can be heated and cooled relatively quickly.

The strip which preferably has the shape of the weld is supported by an insulating material which has a network of depressions over the entire surface thereof which supports the strip. The strip can be heated, _ _ .l -._._.._.__........_....._...___...__-.___...... _ .. 10 15 20 25 30 35 H0 * It heated through that ambient air is sucked over the joint area and the strip through 7813035-8 for example by an electric current and is cooled by sucking air from the area surrounding the strip through the network of depressions a_and.out therefrom through a vacuum pump. During operation, the strip of material is pressed after the weld has been formed against the weld formed during the welding step and heated to around the melting temperature of the plastic material. the weld joint area and the strip are then quickly cooled by the network of depressions. When the plastic material has cooled sufficiently, the strip is removed from the plastic material.

The invention is explained in more detail with reference to preferred embodiments thereof shown in the accompanying drawings, in which Fig. 1 shows in simplified form a weld joint having a rounded strand formed on each side of the weld; Fig. 2 shows a simplified perspective view of an embodiment of the apparatus used to achieve improved hot plate welding; Fig. 3 shows a perspective view of a preferred embodiment of apparatus for obtaining an improved hot plate weld; Fig. N shows an enlarged side cross-sectional view of the apparatus of Fig. 3; Fig. 5 shows an enlarged sectional view of a weld made according to the present method; Fig. 6 shows a cross-sectional view of a simplified apparatus using the embodiment of Fig. 3; Fig. 7 shows two end sections which can be welded together to form a battery glass; Fig. 8 shows a side elevational view of the battery glass formed by welding the two end sections shown in Fig. 7; and Fig. 9 shows a plan view of the battery glass of Fig. 8.

Fig. 1 shows a cross-sectional view of a weld joint formed by heating the respective edges 10 and 12 of two pieces of thermoplastic material. After the respective edges have been heated to the melting temperature therefore or until they have become plastic, the edges are pressed against each other so that a weld joint is formed. The pressure on the molten plastic edges of the thermoplastic material which arises as a result of the edges being forced against each other gives rise to a rounded strand 11 on each side of the weld. and 15 'near each edge 10 and 12 illuminate in a simplified manner the part of the thermoplastic material which has been reheated during the dashed lines 13 the welding process. Welding defects determined by the above-mentioned spear impact test procedure often occur between and along the respective boundary lines 15 between the reheated portions 13 and the unheated portions 17 of the thermoplastic material. ln 20 R) UI BO Å0 r-í? g, _ uuALn-Y va1so3s-a Welding defects determined by the spear impact test procedure indicate that the thermoplastic material is more brittle and less stretchable than the starting material. Although reduced tensile strength of the material has been noted in the weld, fragility and reduced extensibility of the material at the weld in comparison with the starting material have more serious side effects with the welding process. In addition, the material at the joint is characterized by a much higher degree of failed dielectric tests with regard to dielectric requirements in industry than the starting materials do.

Various reasons have been proposed for the variations in weld joint impact strength at the welded joints, namely (1) that the molecular weight distribution in the thermoplastic material may affect the weld joint impact strength and may give rise to the variations; (2) that the material has been oxidized during welding and has consequently become more brittle; and (3) that the crystalline structure of the material in and near the weld line has become coarser due to the welding suspension. However, it has not been possible to determine that any or a combination of these effects would lead to a reduction in the impact strength of the thermoplastic materials at the weld.

According to the invention it has been found that the impact strength of the material at the weld can be increased and that the error rate in the dielectric test of the material at the weld can be substantially eliminated, by (l) heating the material at the weld to a temperature at least about the melt temperature of the thermoplastic , optionally at elevated pressure, and by (2) quenching the heated joint to a temperature at least lower than the melting temperature. When two pieces of thermoplastic material are welded, as pointed out above, a strand appears along at least one side of the welded joint. According to the present method, the strand can be removed before the steps including heating and quenching, but preferably it is not removed.

The exact heating temperature depends on exactly which thermoplastic materials have been welded and can, for example, be as low as 150 ° C for branched polyethylene and can be up to ü80 ° C when the thermoplastic is a high density polyethylene thermoplastic.

This means that the exact heating temperature depends on the melting temperature of the thermoplastic materials, ie the temperature at which it turns into molten form. As a practical guide, the exact heating temperature can be determined for a specific terbole by selecting the temperature at which sufficient g occurs within a time period of up to about 25 seconds.

Pressure is applied to the weld during the heating step or after the heating step. The pressure must be high enough for the material at the weld to be completely flat. In practice, the pressure can vary greatly depending on the apparatus and the temperatures used and can vary from 0.14 to 0.62 MPa. If the string formed at the weld is not removed, the pressure must be sufficient to crush the string against the weld until it is substantially flat. According to a preferred embodiment, pressure is preferably applied during the heating step and the combined effect of the heat and pressure conditions is sufficient to crush the strand until it is substantially flat.

After the pressure treatment, the heated weld is rapidly cooled immediately. Rapid cooling involves rapid cooling of the netted weld to a temperature which is at least lower than the melting point of the thermoplastic material, and preferably to a temperature at which the thermoplastic does not exhibit adhesive properties. The rapid cooling step is performed so that the material at the welding sweep will solidify again.

The rapid cooling must be immediately followed by pressure treatment or hot pressure treatment of the weld. This means that rapid cooling according to the invention does not include that the pressure-treated or hot-pressure-treated weld may be allowed to cool at ambient conditions. Namely, it has surprisingly been found that rapid cooling after the steps involving melting the two thermoplastic edges and joining these edges under pressure, i.e. immediately after the formation of the weld, i.pm fi dikaivendigennedfïrzæt the weld has improved properties with respect to dielectric properties, impact strength and depression flexural strength around the weld shaft. Rapid cooling can be carried out, for example, by immersing the treated joint in water.

To eliminate the problem of reduced tensile strength, low impact strength and high relative error rate in the dielectric test and the problems due to a rounded strand extending along the longitudinal length of the weld, an apparatus has been developed which is shown in its simplest form in fig. Fig. 2 shows an insulating strip 19, which according to a preferred embodiment consists of a ceramic material or a high-temperature plastic, R (11AUTy g 'pineapple-s 7 such as Torlon, which forms a polyamide. A strip 21, preferably of titanium alloy 6AlüB having a thickness of 0.30 mm and a width of 25 mm is arranged over the insulating strip 19. As can be seen from the figure, the insulating strip 19 has the double function of an electrically insulating material and heat insulating material. and as a quick cooling mechanism of, inter alia, the strip 21.

In the annealed state, the strip 21 has an electrical resistivity of about 160 uncm, excellent corrosion resistance and a tensile LW breaking limit of 90 MPa at room temperature. The high strength of the alloy is suitable for it to withstand the local compressive forces generated when it first comes into contact with the rounded weld bead. The high strength is also suitable due to forces which arise on the titanium strip when it is exposed to high temperatures. For example, when the titanium strip is heated to 230-26000, its length increases due to thermal expansion. On the other hand, the plastic material outside the welding zone, ie the area 13, can be prevent the area of the strand at the heated strip 21 from expanding or contracting along the weld during the 2G 2 process including crushing the strand and cooling the formed smoothed strand.

Consequently, the longitudinal expansion and contraction of the heated strip 21 can give rise to a shear stress between the strip 21 and the weld strand material. This could give rise to cracks in the surface of the plastic material and possible throwing of the plastic object formed by the welding step. Consequently, the titanium alloy strip 21 is subjected to a longitudinal tensile stress at room temperature which slightly exceeds the maximum thermal stress occurring below heating and cooling.The tension is kept constant during the heating of the strip 21 with prior art, for example this tension can be produced by screws 55 and 56 as shown in Fig. 6. In this way each point along the alloy strip 21 remains in substantially the same position with respect to the strand during heating and cooling, and consequently the length of the heated section of the strip 21 remains substantially constant, since a voltage at room temperature of about 2N MPa is necessary to provide the necessary tension on the strip, which voltage significantly reduced at high temperature, the strength of the strip 21 must be quite high.

H0 It is obvious that a tensile strength of 90 MPa is more than till- \) | \ I \ ,, ..._..._._.... - _ _. . ... VH -___ H _..- VV _.._...___...._.....___._._._..... P? 10 15 20 25 S0 55 H0 78-130353-8 8 sufficient for the voltage values induced in the strip 21.

As shown in Fig. 1, the heated strip 21 together with its insulating carrier 19 is pressed against the string 11 so that it is caused to melt and becomes plastic. The string is pressed and flattened against the welding area. During this operation, the molten thermoplastic material will adhere to this strip 21. After the strip has cooled to below the melting point of the thermoplastic material, the adhesion of the strip to the thermoplastic material ceases and the strip can be removed from the material.

It has been found that the welded joint which has been heated according to the invention must be cooled rapidly in order for the benefits including improved tensile strength, improved strength and improved dielectric properties to be achieved for the material at the weld joint. Accordingly, a plurality of holes 23 are received in the strip 21, each of the holes communicating with the other holes through a channel 25 formed in the insulating carrier 19. According to one embodiment, cool air is blown through the channel 25 and out through the holes 23. and around the heated thermoplastic material as illustrated by the arrows in Fig. 2. Thereby the thermoplastic material and the strip are rapidly cooled whereby the desired crystalline structure is achieved, i.e. the flattened thermoplastic material has a smooth, closed surface having a very low dielectric relative error rate.

The reshaped welding strand forms a flat addition layer of material which is laminated to the starting material. Consequently, the possibility of a small crack in the weld that would give rise to undesirable leakage is significantly reduced.

Figs. 3 and U show a preferred embodiment of the apparatus according to the invention. 29, for example made of ceramic or a high temperature resistant plastic, a groove 35 formed through the center thereof having a plurality of transverse grooves 33 of relatively small size formed along the length of the insulating strip 29. above the insulating strip 29 is a heating strip or a heating strip 31 placed which is preferably made of the titanium alloy EALÄV having a thickness of 0.30 mm and a width of 25 mm.

As shown therein, an insulating strip exhibits the description of the embodiment shown in Fig. 2, initially at room temperature for a voltage at a value greater than the maximum voltage due to heating to the strip shall be held in place and in the same position with respect to the thermoplastic strand | ... | UI 20 30 ÄO 7813035-8 one during the heat equalization operation.

In the embodiment shown in Fig. 3, ambient air is sucked in through the grooves 33 by means of a vacuum pump (not shown) which establishes a reduced air pressure of 0.2 atm. By sucking in cool ambient air through the grooves 33, a more homogeneous distribution of air around the strip 31 and the flattened string is achieved and consequently a more homogeneous cooling of the strip 31 and the flattened thermoplastic material.

The groove 35 should have a relatively smaller width to form a carrier for the strip 31, and consequently the groove must be deep for the intake air from each of the grooves 33 to be led to the vacuum pump. In addition, the grooves 33 should be wide enough to provide a large cooling surface for the strip 31 but should not be so wide that the strip 31 does not receive adequate support.

According to a preferred embodiment, the grooves 33 are 1.7 mm wide and only 0.17 mm deep, each groove being separated by a 1.3 mm wide portion. This groove structure is designed to keep the bending stress in the strip 31l low and at the same time to provide a relatively large area between the strip and the cooling air. During the heating cycle, the grooves can simultaneously act as insulators which prevent a large heat transfer to the insulators 29. The central channel 25 is deep and narrow so that it has a very small area relative to the strip 31 which could induce transverse bending stresses while at the same time has a sufficiently large cross-sectional area for the air from the grooves 33 to be led to the vacuum pump.

According to the embodiment shown in Figs. 2 and 3 and H, when electricity is passed through the strips 21 and 31, they become hot enough for the strand 11 shown in Fig. 1 to be transferred to around or above the melting temperature thereof. The carrier 20 for the insulators 19 or 29 and the strip 21 or 31 press the heated strip against the string so that the string is flattened against the welding area until it is substantially flat. The reheated welding material is then bonded to the plastic in the welding area as illustrated in Fig. 5 so that an improved weld is formed. The joint in Fig. 5 is not shown to scale in order to make it clear how the smoothed strand forms a thin extra layer of bonded plastic material at the welds.

Fig. 6 shows a simplified cross-sectional view of an apparatus for manufacturing battery glass using the embodiment in Fig. 3; The strip 31 is arranged over the insulating material 29 which in turn is supported on a steel frame 60 which defines a closed space S. The space S communicates with a vacuum pump BH and opens into the channel 35 which in turn communicates with the grooves. 33.

A power source 36 is connected to the strip 31 through a wire 38 for connecting electrical current thereto. When the vacuum pump is in operation, it sucks air under reduced pressure over the strip 31 and the weld joint area and both of these are cooled. A copper coating 57 (which for simplicity is shown on an enlarged scale) is provided on the strip 31 in such an area of the strip 31 which does not come into contact with the plastic material to prevent overheating of the strip in such areas.

The strip 31 must be kept under tension as indicated above with reference to Fig. 2. Screws 55 and 56 schematically illuminate a device for achieving this tension; but obviously there are many equivalents that can be used instead.

When the screw 56 is tightened, one end of the strip 31 is wound up, whereby the necessary tension on the strip 31 is provided. As can be seen from Fig. 6, the insulating material 29 on which the strip 31 is supported is arranged on a flat surface. However, the surface supporting the insulating material need not be flat but may have a surface which corresponds to the surface of the thermoplastic workpiece at the weld.

Thus, if two thermoplastic pipes are welded together, the surface can be annular or cylindrically shaped. A device comprising a piston and a cylinder acts on the frame 60 so that contact is made with the weld joint Ål. ag A second apparatus 58 is schematically illuminated on the opposite side of the weld joint U1 by the plastic material lü and serves to heat and level the formed strand on the other side of the weld joint H1.i Example Using the apparatus shown in FIG. 6, the embodiment illustrated in Figs. 3 and H was applied to make a battery glass of a propylene copolymer blend of the type shown in Figs. 7-9.

The elongate battery glass of Figs. 8 and 9 comprises two end portions 37 and 39, as shown in Fig. 7, each of which is open at the top and which has an elongate open side and three elongate closed sides. The distance from the surface defining the elongate open side of each end portion to its opposite closed side is at least several times less than the distance from the top to the bottom thereof. In general, the distance between the open side and the opposite side is about l / H to l / 13 of the distance from the top to the bottom of the portions 37 and 39. their end portions 37 and 39 have a wall thickness which is substantially the same from the top to the bottom thereof; that is, no taper or surface reduction occurs from top to bottom and each of the end portions 37 and 39 forms a mirror image of the other portion. stretched end portions so that the battery glass illustrated in Fig. 8 and Fig. 9 is formed. The process for manufacturing the battery end portions 57 and 59 is set forth in USP U 118 265 to which reference is hereby made.

The main requirements for a battery glass are that it is resistant to battery acid, does not show leakage, essentially shows exact dimensions, is resistant to shrinkage when the battery overheats, has a high impact resistance to be exposed to accidents during battery manufacturing and use thereof, has a homogeneous width and length from top to bottom, ie no surface reduction occurs, has straight sides that do not bulge out or in and has a capacity to be bent and / or deformed during handling to hot rupture of the battery glass end ends 37 and 39. at the respective length shall be prevented.

As previously pointed out, when the respective edges of the end portions 37 and 39, illustrated in Fig. 7, are heated to the melting temperature and then bonded together to form a weld, the molten plastic material and the outside of the glass are formed from that of rig 1 and Fig. 8. and 9 illuminated type. After the weld, including the strands, has been cooled, the insulator 29 and the belt 21 are arranged as shown in rig 3 along both the inside and the outside of the weld area against the strands formed during the initial welding step using an apparatus of the type illustrated in Fig. 6.

The titanium alloy strip 21 is then heated for a time interval ranging from 2.5 seconds to more than 20 seconds while being pressed against the strings. Thereby the strands melt and are flattened towards the heated welding area 13 as illustrated in Fig. 1 whereby a flat weld is formed as illustrated in Fig. 5. The strip 21 and the flattened strand are then cooled by sucking air of room temperature through the grooves and through ka After the flattened string has cooled sufficiently to no longer adhere to the strip 21, the strip and the insulating carrier are removed, whereby the finished welded battery glass is obtained. 10 15 20 25 30 35 7813035-8 12 It has been found that when longer heating and cooling times are used, the point impact strength of the welds increases. However, it has also been found that if longer heating cycle times are used, the battery glass falls, especially at the upper end next to the open end of the glass, ie the glass curves inwards or outwards to an unacceptable extent. The casting that occurs at long heating times obviously depends on the shrinkage that occurs in the plastic material after it has been heated to the melting point. The material in the area above which the strand is flattened is thus brought to a temperature at or near the melting point thereof and consequently shrinks during cooling, when on the other hand the surrounding material which has not been reheated does not shrink.

A method for eliminating the throwing problem means that the welded battery glasses are preheated to 80-90 ° C before the equalization process. This causes the whole glass to shrink slightly upon cooling and consequently significantly reduces the difference in shrinkage between the material adjacent the weld and the remainder of the battery glass. However, this method is undesirable in a production line because the extended cooling cycle required with preheated glass significantly increases overall manufacturing. the time of the battery glasses. It has therefore been discovered that by using a very short heating time of 3-Ä seconds and heating the titanium alloy strips to a higher temperature, an improved weld exhibiting high point impact strength without any significant throw can be achieved. The degree of casting is further reduced by using a method comprising sucking in relatively cool ambient air under the strip 31, whereby the battery glass material adjacent to the strips is cooled. This means that a narrower zone of heated plastic material is subjected to shrinkage which in turn reduces the distortion of the walls of the battery glass due to shrinkage. Consequently, by using a relatively short heating time cycle and sucking in air from the area surrounding the heated plastic material, the total time cycle for treating the weld can be kept lower than 30 seconds.

The invention has been described above with respect to preferred embodiments thereof. However, it will be apparent to those skilled in the art that many variations and modifications of the invention are possible. _ - _ - _ -------- _--------

Claims (7)

7813035-8 \ '$ Patent Claim A method for improving a hot weld joint (10, 12) formed between two thermoplastic pieces, which pieces have been joined so that a weld is formed, a string being formed along at least one side of the length of the weld, characterized by: (a ) that the weld joint (10, 12) and the thermoplastic material in the vicinity thereof are subjected to an elevated temperature and an elevated pressure which is sufficient for the temperature of the thermoplastic material to be raised to at least about the melting temperature thereof and which is sufficient for the thermoplastic material strand to be pressed plant against the weld; and (b) rapidly reducing the temperature of the heated thermoplastic material to at least below the melting temperature of the thermoplastic material by sucking ambient air around the extruded thermoplastic material. A method according to claim 1, characterized in that the heating step comprises providing an electrically conductive strip (31) of relatively high resistivity and that a current is passed therethrough so that sufficient heat is generated for the thermoplastic material to melt, and that the heating strip shielded against the string under a pressure such that the combined effect of heat and pressure compresses the string until it is substantially flat; wherein the temperature reduction step comprises rapid cooling of the area defined by the substantially flattened string, by suction of ambient air under reduced pressure over the area. Apparatus for carrying out the method according to claim 1 for improving a hot weld joint (10, 12) formed between two pieces of thermoplastic material, which two pieces have been joined so as to form a weld, a string being formed along at least one side of the length of the weld , characterized by: a device (31) for heating the weld joint (10, 12) of thermoplastic material to at least about the melting temperature of the thermoplastic; a device (33, 35) for rapidly reducing the temperature of the thermoplastic material in and in the vicinity of the weld to at least below its melting temperature. H. Apparatus according to claim 3, characterized in that the temperature reducing device comprises a device (29) for supporting the heating device (31), which supporting device has an opening (35) in the center thereof over which heating furnaces are arranged; and a device (3Ä) for suction - hfl r - VL. '* ï -a>'. 'J ~ 1 ...- - i. ,. '^' .... «. 1¿, i ~; nä. to ambient air over the weld, past the heater (31) and through the opening (35). Apparatus according to claim H, characterized in that the heating device comprises an electrically conductive strip (31) of titanium-containing material and a device (38) for connecting a source of electricity (36) to the strip. Apparatus according to claim 4, characterized in that the heating device consists of a strip of electrically conductive material (31) of high electrical resistivity; a device (38) for connecting electrical energy to the strip; The reduction device comprises a device for supporting the strip having at least one opening (35) in the center thereof, which strip is arranged over the opening; a frame (60) defining a closed space and having an opening (35) in a surface thereof for connection between the outside of the frame and the enclosed space, which strip (31) and which carrier device (29) are supported by the frame (60) so that the opening in the carrier communicates with the opening in the surface; and a device (3U) communicating with the enclosed space, for sucking ambient air from the area around the strip through the opening in the carrier (33) and through the opening in the surface (35) of the frame into the enclosed space. _ Apparatus according to claim 6, characterized in that it comprises a device (55, 56) whereby the strip (31) can be subjected to a bias so that the strip when electricity is passed therethrough is not displaced with respect to the thermoplastic material.