Q&A: Electromagnetic Welding of Plastics
A resource sponsored by SPE’s Decorating & Assembly Division
by Steven M. Chookazian
Emabond Solutions, LLC
What is electromagnetic welding?
Electromagnetic welding, also more commonly known as the Emabond Process, is a design and assembly method that provides a simple, rapid and reliable assembly technique to produce structural, hermetic or high-pressure welds 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 bonded using a specially formulated thermoplastic resin, commonly referred to as the EM resin material, which is 100 percent solids, environmentally friendly and highly reliable. The process is so versatile it can bond almost any thermoplastic, filled or unfilled, to itself plus certain dissimilar thermoplastics. The electromagnetic welding process can easily join such difficult-to-join materials as polyolefins and elastomers, as well as engineering thermoplastics.
The electromagnetic welding process offers designers an alternative enabling technology, assembly method and design tool that picks up where traditional plastic welding methods leave off.
How does Emabond work?
The image below illustrates the “before” – “during” – “after” phases of a successful weld created within a typical tongue and groove joint design.
Before joining – The EM resin preform is deposited in the joint. The mating parts are brought together and placed within a fixture containing a work coil, which conforms to the weld line geometry. This phase easily is automated or operator-initiated.
During joining – The activated coil heats the EM resin, causing the adjoining surfaces to melt. Energy is consumed only during the actual heating cycle, which typically is 1 to 30 seconds. Low clamping force is applied via the specially designed fixture to allow efficient transfer of melt temperature to the substrate.
After joining – The EM resin has filled the gap. The process has fused the mating parts, resulting in a polymer-to-polymer permanent weld. Note the compact joint cross section when compared to frictional methods of assembly, which typically require broader land area and or flash traps.
Why should I consider electromagnetic welding?
There are many advantages when compared to alternative methods of assembly. On a broader scale, the advantages can be segmented into design, aesthetic and manufacturing.
- Superior joining of polyolefins – PP & PE (all densities)
- Highly filled polymers, such as glass-, talc- and mineral-filled
- Ability to provide a shear joint design with gap-filling properties
- Meets leak-proof, high-pressure and strength specifications
- Meets code requirements – e.g. NSF for potable water and certain FDA applications
- Clean process with no particulate generated
- Flash-free weld line
- Compact joint design
- Clean, smooth, distortion-free bond line
- No surface pretreatment required
- Precise heat delivery at the weld line
- Environmentally safe – completely solvent- and VOC-free
- Quiet operation
- Reduce scrap: Zero waste capability
- Ability to reverse the process to reclaim internal components or reweld assembly
- Reduce overall operating costs through waste reduction
The process often has been considered as the ideal method for critical high-performance applications where the cost of weld failure is of great concern. It is a non-contact, nonviolent method of assembly that is gentle on parts.
Do I have to incorporate a specific joint design interface?
Proper joint design is essential to the ultimate success of the weld, regardless of the welding method. Commonly used welding methods (for example, ultrasonic, vibration, spin, hotplate and laser welding) require specific joint designs to provide optimum performance. Electromagnetic welding in this regard is no different. Leak-proof and pressure-tight joints generally require a tongue and groove type of design.
Since the EM resin material located within 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 EM resin material can be compared to filling a cavity in injection molding. The following formula is generally used to determine the amount of material required to fill the joint:
where Ae = cross-sectional area of the EM resin material
Av = cross-sectional area of the void in the joint
k = constant, ranging from 1.02 to 1.07 depending upon the amount of joint interface pressure desired and the material being welded.
Many approaches exist when designing a proper joint. An influencing factor 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 of the final assembly. If a leak-proof weld is required, it is best to use a tongue and groove design. If high-pressure is needed, then a step joint or a tall tongue and groove joint designed to put the EM resin material in shear would be most desirable. If all that is required is a static flow airtight seal or a structural weld, a flat-to-flat flange may be sufficient. Typically, a tongue and groove joint that places EM resin material in shear is employed when leak-proof, pressure-tight welds are required.
What are the commonly used EM resins forms?
EM resins are 100 percent solid and available in a wide variety of forms. The composition and final form is fully dependent on the materials being joined, performance requirements and – ultimately – the joint design configuration. The material composition used for the welding process consists of two major components: the susceptor material (typically either fine iron or stainless steel particles) and the thermoplastic resin matrix, which is compatible with the materials being joined.
The most common form would be an extruded profile – typically round, although it can be rectangular. Other forms include sheet, which can be die stamped into special shapes, or ribbon for flat-to-flat joining. EM resins also are offered as injection-molded gaskets and can be co-injected directly into one half of the part being assembled.
What kind of equipment is required?
A standard welding machine consists of five key components: 1) high-frequency, solid-state generator operating at 13.56 Mhz; 2) pneumatic press; 3) controls and 4) water cooling for the high-frequency components and application-specific work coils. All four of these components are typically integrated into a seamless unitized machine. The fifth component is the application-specific tooling or welding fixture, which consists of a water-cooled copper work coil and holding fixtures matching the part geometry. The five key components must be optimally designed to achieve the desired results.
What are some typical end-use applications?
The process has been used effectively on a wide range of demanding applications, including automotive, filtration, plumbing, medical devices, industrial, office furniture and consumer appliances. Historically, greater than 80 percent of the applications have been leak-proof and pressure-tight, often with demanding specifications where the cost of failure is high.