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Review of Joining Methods for Plastics

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Welding-plastics refers to practical methods of fabricating objects by joining permanently plastic materials or polymers.

Two types of plastics are known.

Those made of thermosetting resins undergo irreversible cross linking reactions by curing, in the process of being manufactured in useful shapes.

Welding-plastics is not applicable to them because they are destroyed by heat, without reaching again plastic state.

Adhesive bonding or mechanical fastening are the methods used to join thermoset resins.

Welding-plastics is applicable only to the thermoplastic types, those that gain plasticity through heat and then return to solid state upon cooling.

The materials are known by the names polyethylene (PE), polypropylene (PP), polyvinyl-chloride (PVC) and many others that are modifications or derivations therefrom.

The names of the classes of thermoplastics are useful to identify properties that meet specific requirements.

Although a specialized subject and quite different from welding metals, Welding-plastics is not difficult to learn by practicing the methods.

It might complement the activity of any shop serving certain industries and open business to new opportunities.

Thermoplastics can be manufactured economically in complex shapes, are relatively strong with low weight, are chemically inert.

They can outlast metals for corrosion and abrasion resistance or containment of chemicals.

Their intrinsic limitations however are service temperature and pressure, which must be within material capabilities.

Welding-plastics is ideal for thermoplastics and can be performed by different methods which deliver energy to the joints.

Heat can be supplied from an external source or internally by friction, or by electromagnetic effects with proper inserts.

Welding-plastics means heating to thermoplastic state, applying some pressure for the polymer chains to intermingle, and letting the material cool under pressure.

It is obtained by fusion as soon as the transition temperature is exceeded.

Such temperature is called the glass transition temperature for amorphous polymers, and melting temperature for crystalline ones.

Once the plastic state is achieved, the polymer chains acquire sufficient energy, under moderate pressure, to interdiffuse between the elements being welded together.

A similar effect can be partially obtained with amorphous polymers of identical or similar solubility, like polycarbonate, acrylic and polystyrene resins.

Joining is achieved by using a proper solvent that plasticizes the surface to promote chain diffusion and joining.


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Methods of Welding-plastics

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The Thermal methods of Welding-plastics include the five described hereafter.

1 - Hot Tool welding starts with bringing a heated tool in contact with both surfaces and applying a moderate pressure to provide good contact. As soon as the required thickness of heated material is obtained, the tool is removed and the surfaces to be joined are brought together under pressure and allowed to cool down.

Key parameters can be easily optimized and controlled, and systems can be automated for mass production as in the automotive manufacturing industry.

An Article on Welding Thermoplastics was published (11) in Issue 160 of Practical Welding Letter for December 2016.
Click on PWL#160.

2 - Hot Gas Welding-plastics uses a stream of heated air or gas directed by a torch toward the joint surfaces and a filler of the same material, that is pushed to fill the gap. The operation is mostly manual and skill dependent. As the gas temperature is much higher (although suitable to each material) than what required for the fusion of thermoplastics, the process is not energy efficient. It is used for continuous welding of tanks, containers and tubes, and for field repair.

3 - Extrusion welding, a variation of the above method, uses a plasticized filler supplied by a heated extruder, as the joining material to be pushed into the gap by a welding Shoe. It is used when a larger size of filler material is needed than what available as standard welding rod sizes. The process can be automated for long seams in thick section components of large bodies.

4 - Infrared (IR) welding is performed by heating the joint with focused, elliptical reflectors surrounding a tubular IR lamp. The thin, focused beam is made to heat up, by a robotic fixture, alternatively both the surfaces to be joined. No filler is added. When the surfaces are hot, the lamp is shut off and the joint sides are pushed together under pressure. The process is automated.

5 - Laser welding is being applied for Welding-plastics at low power with Carbon Dioxide lasers, without the use of filler materials. Satisfactory penetration and small heat affected zone have been reported.

Three processes depend on Friction at the bond surface for heat generation. As such they are more energy efficient than the external heating types described above.

1 - Vibration welding is obtained by providing relative oscillatory motion to surfaces in contact under variable pressure. In linear machines the movement is parallel to the straight bond line. In general there is a lower fixed platen to which one of the elements to be joined is fastened. The other element is fastened to a movable platen, vibrated at 100 to 500 Hz (cycles per seconds).

Pressure, Amplitude, frequency and duration of applied vibration influence the heat produced and the outcome of welding. The process can be easily controlled and automated. Mostly for producing butt joints.

2 - Spin welding is similar in principle to friction welding as used for metals. This process is limited to joints exhibiting circular symmetry. One element is static. The other one is rotated and pressed to rub the static surface. Once the amount of heat developed is sufficient, motion is stopped while axial pressure is maintained until the welded part cools down.

The machines can include sensors and control devices to increase reliability of automatic operation.

3 - Ultrasonic welding is considered the most widely used Welding-plastics process. It is a process whereby mechanical energy, developed through the transformation of high frequency alternating current into mechanical vibration (by piezoelectric ceramic transducers) is transmitted through a metal tool called a horn (or sonotrode) to the interface of two overlapping surfaces to be joined, supported by a static suitable anvil, producing intimate contact and a welded joint.

There is a power supply with a converter that elevates the frequency of the electrical current from the grid, then the transducer that transforms electrical into mechanical energy, then a booster that modifies the shape and magnitude of vibrations an finally the horn (or sonotrode) that vibrates the material to be welded, while it is clamped unto the stationary anvil. Converter, booster and horn must resonate at the operating frequency.

For Welding-plastics the vibrations under pressure are in the direction normal (perpendicular, 90 degrees) to the interface, the frequency is mostly 20 to 40 kHz (kilohertz or thousand cycles per second) and the amplitude of oscillations is in the range of 20 to 80 microns (thousandth of mm).

The direction of vibration is at variance from that of Vibration welding described above where the oscillations are parallel to the joint. Vibrations generates frictional heat at the interface, melting the materials together.

Medical items are among the most demanding applications. Other uses are for Appliance, Automotive, Consumer, Electrical, Packaging and Toys.

Four processes depend on Electromagnetic effects for Welding-plastics as described in the following.

1 - Resistance (or Implant) welding is done by inserting a conducting wire or mesh at the interface of the parts to be welded together. The insert and the surrounding material are heated up by the resistive effect to the passage of electric current. Upon melting the two surfaces are welded together. The insert remains in place with no further use.

This method is also called electrofusion and is used to join together lengths of a tube by inserting the ends into a collar were electrical resistance wires are embedded. Electrical current through the wires provides welded junctions between each end of the tube and the common collar, realizing the junction.

Cleanliness is an important requirement, to avoid inclusion of impurities. The method is useful and portable especially for joining of polyethylene pipes.

The new Issue No. 30 of our Mid Month Bulletin for October 2008 reports on useful Resources on Electro Fusion Welding as several links to valuable Online Sources.
Click on PWL#062B to see it.

2 - Induction welding is performed by preplacing at the interface an insert layer or a thin sheet of thermoplastic resins including conductive and ferromagnetic particles. Heating is obtained by eddy current induction from a water cooled copper coil energized by a high frequency (2 to 10 MHz) power supply. The heated layer or tape melts the surrounding material and completes the joint.

This method can handle joints of complex form, but needs the recurrent procurement of suitable filled tapes and, for each configuration, the production of proper induction coils of correct design and performance.

3 - Dielectric (or Radio Frequency) welding is produced by submitting the surfaces to be welded to much higher radio frequency (15 to 100 MHz), through a brass electrode that applies energy directly to the plastic held in a press that clamps the sections together. The primary application if for packaging.

4 - Microwave welding is obtained by heat produced while irradiating the parts to be joined (held under slight pressure) with microwave frequency radiation. Although most thermoplastics do not heat up when irradiated, by simply inserting suitable implants (including metal, carbon or conducting polymers), local heating takes place.

The main advantage is that by strategically placing suitable implants one can submit to microwave radiation a whole structure that will only be heated where the implants are located. The technique is considered to be still in development stages, but promising for mass production.

Forming Thermoplastics

One should note that raw material required is available as plates of the needed thickness. To form the material to the various design shapes one may need sheet benders or rotationally molding machines, that are provided with means to heat the plates in order to form them without breaking. Heat and time setting are material specific.

NDE

Of the Non Destructive Evaluation methods applicable to Welding-plastics, visual inspection and leak tests are the most frequently used. Liquid penetrant testing has limitation in that solvents used must not attack the materials. Radiographic methods are applicable only with low kilovolt techniques, because of low absorption coefficient. Ultrasonic inspection with compression waves have been used with some success.

Specifications

Specifications for Welding-plastics equipment, welding and testing procedures, welder education, design practice, and certification are established in Europe.

In the USA the American Welding Society developed and approved similar requirement documentation for the improvement and progress of this technology.

ANSI/AWS G1.1M/G1.1:2006
Guide to Ultrasonic Assembly of Thermoplastics
Edition: 1st
American Welding Society / 06-Jul-2006 / 96 pages

ANSI/AWS G1.2M/G1.2:1999 (R2010)
Specification for Standardized Ultrasonic Welding Test Specimen for Thermoplastics
American Welding Society / 31-Aug-1999 / 29 pages

ANSI/AWS G1.6:2006
Specification for the Qualification of Plastics Welding Inspectors for Hot Gas, Hot Gas Extrusion, and Heated Tool Butt Thermoplastic Welds
American Welding Society / 06-Jul-2006 / 24 pages

ANSI/AWS G1.10M:2001
Guide for the Evaluation of Hot Gas, Hot Gas Extrusion, and Heated Tool Butt Thermoplastics Welds
American Welding Society / 21-Feb-2001 / 46 pages

AWS B2.4:2012
Standard Welding Procedure Specification for Welding Procedure and Performance Qualification for Thermoplastics
American Welding Society / 10-Apr-2012 / 56 pages

Watch the following Video on

Vinyl Welding Process for Vinyl Windows

http://www.youtube.com/watch?v=derXrNMTN0M

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