User:Benatar.1/WE7406 Article 6

= Implant induction Welding =

Implant induction Welding is a joining method used in plastic manufacturing. An induction coil is used to excite and heat electromagnetically susceptible material at a joint interface and melt a thermoplastic for joining. The susceptible material can be contained in a gasket placed between the welding surface, or the actual components of a composite material. Its usage is common for large, unusually shaped, or delicate parts that would be difficult to weld by other methods.

Physical Mechanisms
An induction coil will excite certain materials when exposed to its electromagnetic field. In non-magnetic electrical conductors like aluminum, nickel, or copper, an alternating electromagnetic field will induce Eddy currents in the material. These currents generate thermal energy through Joule heating. Ferromagnetic materials like iron and carbon steels will see heating from both Eddy current formation and from Hysteresis losses.

Material Considerations
Induction heating is an efficient method of transmitting thermal energy to electrically conductive or magnetic materials. Unfortunately most thermoplastics are non-magnetic and excellent insulators. To take advantage of induction heating for thermoplastic welding purposes, a susceptible implant must be used as an intermediary. Nearly any electrical conductor or ferromagnetic material may be used as an implant. Carbon fiber is of particular interest due to its widespread use in composite materials. Provided there are closed loops of carbon within the composite structure, eddy currents can be induced in the material. Unidirectional carbon fiber composites can have poor susceptibility when fiber to fiber contact is limited. Implant styles include meshes, fibers, and fine powders. The most common gasket design is a thermoplastic composite with suspended susceptible fibers. This composite gasket can be formed into any shape required for the welding application. The gasket matrix is typically made of the same thermoplastic being welded. In situations where two dissimilar materials are to be welded, the gasket material is usually a blend of the two thermoplastics.

Equipment
An induction generator is used to produce high frequency current. Frequencies used for induction welding are in the 2-10 Mhz range. An induction coil converts the high frequency current into the necessary alternating magnetic field. High currents used in induction welding produce large amounts of heat in the induction coil. The heat is removed using liquid cooling and a heat exchange system. Fixtures are used to hold the parts in position during welding, and a press applies and maintains pressure during heating and cooling.

Welding Steps
An implant rich gasket is placed at the surface to be welded. In some cases the implant is already contained in a composite material, or it has been included at the joint by some other means. Pressure is applied to the joint to force out air cavities and ensure good bonding. An electromagnetic field is applied by the induction coil which heats the implants. Heat conducts into the surrounding thermoplastic, melting the gasket and creating a melt layer at the joint surfaces. The applied pressure flows the molten thermoplastic and fills the joint. When sufficient bonding has been achieved, the induction coil is turned off and the joint is cooled under pressure. For large items with long joints, the coil can be scanned along the length of the interface. This is used frequently during the manufacture of plastic pressure tanks.

Power
Typical induction generators provide a power output of 1-5 kWs. High power output is necessary for longer and larger joints. Power output must also be increased as coil distance from the joint increases, due to decay of electromagnetic fields with distance.

Pressure
Even distribution of the molten polymer in the joint is imperative for strong bonding. Weld pressure must be sufficient to flow the molten gasket and fill the joint.

Weld Time and Cooling Time
Weld time will vary based on the joint size, the volume of susceptible implant material, and the power and frequency. Weld times can be very fast since no preheating is needed, and heat generation happens exclusively at the weld joint. This also benefits the cooling time. With so little heat wasted on the bulk of the part, cooling times are brief. Under 1 second for some applications.

Advantages

 * Implant induction welding does not require physical contact with a heat source, so it is useful for joining components with unusual dimensions or delicate surfaces.


 * The induction coil can be scanned along the surface of long joints. Extremely large parts to be effectively welded using this method.


 * Heat generation is limited to the exact area where it is required for joining, so thermal stress generated by welding is low.


 * The joint can be reopened using induction for repairs or recycling.


 * The heating and joining steps are simultaneous, so cycle times are short.

Disadvantages

 * There are additional costs due to the implant and gasket material. For small and simple parts, this can make the method cost prohibitive.


 * Heating is limited by the penetration depth of the electromagnetic field. Care must be taken to avoid uneven heating in complex joint designs.


 * Implant material at the joint can have an impact on strength.


 * The electromagnetic field may affect metallic or electronic components of the part.

Food Packaging
Implant induction welding is heavily used in the production of Tetra Pak containers for products like juice boxes. The use of induction heating shortens the sealing time versus other joining methods that use external heat, and avoids damage to the paperboard layer from direct contact with hot tooling. An aluminum foil layer is used to block oxygen diffusion into the packaging, so no additional implant material is needed.

Automotive Manufacturing
The automotive industry makes large scale use of implant induction welding for the manufacture of large plastic items such as bumpers, plastic body panels, and fuel tanks. Manufacturing costs of components with complex geometries are brought down by manufacturing the parts in separate pieces, to be assembled later using induction welding.

Tamper Proof Packaging
Polyethylene coated aluminum foil is induction welded to the top of many food, supplement, and drug containers. The seal helps retain product quality and provides evidence if the product has been tampered with.