Emabond process for thermoplastics assembly with emphasis on polyolefins and TPE's.

Electromagnetic welding of thermoplastics provides a simple, rapid and reliable assembly technique to produce structural, hermetic or high-pressure seals on most thermostructural, hermetic or high-pressure seals on most thermoplastic materials and TPEs. It employs the basic principles of induction heating by developing fusion temperature at the abutting interface of parts to be welded using a thermoplastic electromagnetic interlayer.

The process is so versatile it can weld almost any thermoplastic, filled or unfilled, to itself plus certain dissimilar thermoplastics and non-thermoplastic materials, such as paper to thermoplastics. The electromagnetic welding process can easily join engineering, high performance resins and such difficult-to-weld materials as polyolefins and nylons.

Understanding the design criteria for electromagnetic welding is essential to the product designer. It involves an understanding of the process itself, the work coil designs and the joint design. The purpose of this article is to relate some of these fundamentals and apply them to specific applications.

Process description

Welding one thermoplastic part to another is achieved by inductively heating a ferromagnetically-filled thermoplastic medium layer at the joint interface to the fusion temperature of the abutting substrate. A chemical bond is obtained across this interface. The electromagnetic EMA medium layer consists of a thermoplastic matrix, which is either the same as, or compatible to, the abutting substrate to be welded. This thermoplastic matrix incorporates a dispersion of fine micron-sized ferromagnetic powders such as iron or stainless steel. Heat losses develop from the filler concentration within the thermoplastic matrix of the medium perform via eddy currents and hysteresis losses when the entire composite is subjected to a high frequency alternating current source. The heat developed in the filler melts the thermoplastic matrix, which in turn locally melts the abutting substrate's surface to be welded. Thus, the EMA material heats via induction and fusion is achieved by conduction.

Key benefits

The ability to provide reliable leakproof, high pressure and/or structural welds on a variety of applications is discussed in the following examples. Applications can range in size from very small, fractional inch diameters to large part assembly requiring 20 more feet of bondline. Continuous welding is also available.

Leakproof

Many applications demand reliable leakproof performance often times consistent with very high cosmetic features. Electromagnetic welding has repeatably been selected over hot plate fusion welding due to consistent performance. These key benefits are very common in the small consumer appliance field for applications like water kettles, coffee makers, insulated beverage containers and other items.

High-pressure

Applications present very demanding requirements since they typically require welding of highly filled, typically glass, polyolefins or high temperature engineering materials. High burst pressures and long-term cycling are required in the filtration and water tank applications. Often the designer has the ability to optimize the part design to obtain superior performance and low part cost.

Structural

Applications can cover a wide range of physical sizes and performance requirements. And excellent example is joining of a large, long, multiple weld line in a station wagon loadfloor which demands high static and torsional loading. It is made using a 40% glass matte reinforced polypropylene. Electromagnetic welding is extensively specified for joining of large compression molded thermoplastic composites.

Turnkey system components

The welding system is generally provided as a complete turnkey single station semi-automatic system designed with tooling for a specific application(s). Four basic components comprise the process, including an induction generator that converts 50-60 Hz electrical to 3-8 mHz output frequency with output power from 2-5 kW; work coils which are water-cooled copper inductors which develop the desired magnetic field intensity through the EMA material; fixturing, designed into a press generally used to hold parts in place in the magnetic field; and electromagnetic materials that are essentially the key to the whole process and are typically supplied in the form of molded gaskets, extruded profile preforms, sheet or an integral part of the molded product via co-injection or insert molding.

General design considerations

When considering the electromagnetic process for an application, the following list of items is important to consider for design.

* Material to be bonded - determines the thermoplastic matrix material to be utilized for the Emaweld preform.

* The environment for the end-use product - influences the filler type - either iron or stainless steel non-oxidizing.

* Physical properties required, e.g., leakproof to 60 psi. Will suggest a preferred contained joint design.

* Geometry of the bond line - determines the work coil design and fixturing requirements to hold the part as required.

* Coupling distance of the coil to the joint - influences the filler particle size, power output and frequency, and resulting operating efficiency and cycle time.

* Volume of units - determines parts handling method, manual, semi-automatic or fully automated.

* Size of part and/or joint length - determines the power output necessary.

* Production rate is influenced by filler type, filler particle size, coil type, cross-sectional dimension of Emaweld material, power output, frequency output, part size and joint configuration.

The two most important aspects which will determine if a part can be fused via electromagnetic welding are the part geometry as it relates to work coil design and joint design. All the other variables such as power output, frequency, filler type/size and polymer type are controllable and are tailored for each specific application.

A thorough understanding of coil design options and the proper joint design is critical when designing for the electromagnetic welding process.

Work coil design

Primary consideration for a successful electromagnetic bonding application is the proper design of the work coil.

Work coils used in electromagnetic bonding are somewhat similar to those used in standard induction heating for annealing or heat treating of metals between 200-500 kHz. In electromagnetic bonding of thermoplastics, higher frequencies, typically between 3-8 mHz, are used. Because of higher frequencies, coil design becomes important in order to reduce any tendencies for overloading and arcing.

Work coils can be manufactured in a wide variety of types and styles, depending upon the shape and size of the object to be welded and the bond area. The work coil design must follow certain principles if maximum efficiency of the high frequency generator is to be obtained.

Generally speaking, rapid and reliable heating is dependent upon the ability to construct a work coil to suit the application and the capacity of power output of the generator. In other words, the generator should be of sufficient power and frequency to heat the EMA material rapidly. Once the correctly powered generator is selected, if the shape of the part allows itself to be surrounded by a work coil with uniform coupling distance, then the application is a logical candidate for the electromagnetic bonding process. Most applications utilize a power output rating of 2-5 kW. Multiple work coils and generators can be employed on a single application to increase speed or accommodate multiple welds simultaneously.

The induction coil quickly elevates the temperature of the EMA material located at the bond interface by passing high frequency current through the coil and is inductively absorbed into the EMA material. Heat develops in the EMA material via hysteresis and eddy current loss. The coil becomes the transformer's primary with the EMA functioning as the secondary. The EMA material is in no way a part of the closed electrical circuit, so the generation of heat is solely through induction. The EMA material is typically nonconductive.

Heat is generated via energy losses within EMA material, which results in a rapid temperature rise within the EMA material. In ferrous powders which have magnetic properties, these energy losses are a result of hysteresis and eddy currents up to the Curie point (where magnetic properties cease to exist), at which point continued heating is created through eddy currents losses only.

Since electromagnetic fields become exponentially more concentrated as the coupling distance to the coil is decreased, there is a distinct advantage in locating the bond line as close to the coil as possible to maximize the heat transfer propagated into the EMA. The strength of the magnetic field varies inversely to the square of the distance between the EMA and the coil. This means that the coupling distance will have a direct relation to the amount of heat generated in the EMA material in a given length of time.

Joint design

There are many approaches to the proper joint, depending upon the application. Some of the influencing factors would be whether the part is injection molded, blow molded, extruded profile or thermoformed. The most critical factor for determining the proper joint design is the performance requirements. If a hermetic seal is required, it is best to use a tongue and groove design. If a high-pressure hermetic seal is needed, then a step joint or a tongue and groove joint designed to put the EMA material in shear would be most desirable. If all that is required is a static flow air tight seal or a structural weld, a flat to flat flange would be sufficient. Assuming there is sufficient room within the package limits, we would typically recommend a tongue and groove that places EMA material in shear. This type of joint provides the best sealing reliability and strength. If EMA material needs to be dispensed automatically via extruder, then a flat to groove is most appropriate. Typically, these types of applications have been for high volume structural welds.

A proper joint design is essential to the ultimate success of the weld. Since the EMA material located at the joint interface becomes molten when activated, it flows under pressure into the voids and irregular surfaces to produce reliable welds with near zero reject rates. Ideally, the molten flow should be contained and subjected to an internal pressure against the abutting weld surface. The flow of the EMA material can be compared to filling a cavity in injection molding.

The following formula is generally used to determine the amount of material required for filling the joint:

Ae = kAv, where Ae = cross-sectional area of the EMA material; Av = cross-sectional area of the void in the joint; and k = constant, ranging from 1.02 to 1.07, depending upon the amount of joint interface pressure desired and the material being welded.

Design freedom

The electromagnetic welding process offers unique advantages over many conventional methods of assembly. The electromagnetic process is able to successfully weld highly filled thermoplastic materials, very large parts, three-dimensional joint planes and difficult-to-weld materials such as polyolefins.

The fillers and fibers in filled materials have replaced part of the thermoplastic matrix, which results in resinpoor areas that present a problem for most conventional welding techniques. When welding these filled resins, the thermoplastic matrix of the EMA material fulfills a dual purpose: It not only transports the fusion heat, but also provides the necessary additional resin to ensure polymer-to-polymer linkage.

The EMA process is able to weld very large bond lines up to 20 feet in length in one shot; something other conventional methods have a great deal of difficulty accomplishing. With the EMA process, we are able to do continuous area web sealing of large thermoplastic sheet materials like woven and non-woven polyolefin materials. Essentially, the sheets are overlapped with the EMA material at the interface and fed past a small coil welding the two sheets together as it passes through the electromagnetic field.

With joints in more than one plane, the generation of heat by mechanical friction techniques becomes very difficult or impossible. Irregular welds and contoured welds in different planes are readily welded with the EMA process.

One of the most interesting side benefits of the electromagnetic process is the ability to reactivate the bond line and separate the weld if desired.

Not all applications fit the electromagnetic welding process profile, however. For those that present unique problems, that demand reliable leakproof, high pressure or structural welds, consider electromagnetic welding.

Conclusions

The electromagnetic welding process is a fusion welding technique used to assemble thermoplastic parts that require leakproof, pressure-tight or structural welds. The process offers a number of advantages, including welding of similar as well as dissimilar thermoplastic materials; cost reduction through improved design and production yield; welds crystalline or amorphous materials effectively; tolerant of high filler loading; accommodates wide tolerances - material will melt, flow and fill voids between parts; weld lines can be continuous or three-dimensional, allowing part designers flexibility; and long and multiple bond lines can be achieved in one single cycle.

References

[1.] B.E. Wagner, "The assembly of an Azdel load platform with an electromagnetic welding system," SPE Conference NA TEC 1983.

[2.] Emabond Systems, Ashland Chemical Co., Technical Literature, Bulletin 1688, entitled "Emaweld."

[3.] M. Chookazian, "Design criteria for electromagnetic welding of thermoplastics," Technical Paper, 1982.

[4.] M. Topping, "Electromagnetic welding of thermoplastics and specific design criteria."

Steven M. Choolazian,
Ashland Chemical