Plastics Welding

There are multiple plastic welding processes that are in use in industry that for a direct weld between the two parts being joined.  This “weld” is not in the same sense as a fusion weld in metals but is more like a solid-state bond.  Described below are commonly used welding processes.

Hot Plate Welding

Hot plate welding works by heating the two surfaces to be welded by pressing them against a hot plate with a non-stick PTFE surface.  Once the surfaces are hot enough, the hot-plate is removed and the surfaces are pushed together and allowed to cool.  Hot plate welding is popular because of low capital cost and wide process window.  The joint strength are very high and can approach parent material strengths.  However, cycle times are longer, 10-30 seconds, as compared to other techniques such as ultrasonic welding.  The PTFE coating can be used for plastics that can be heated below 250C; for temperatures greater than that one has to use either a plate without any coating or a heat the surface with a radiant non-contact heating.  Another approach would be to heat the surfaces with infra-red energy source.

Conventional hot plate welding is at temperatures in the range of 30 to 100C above the melting point of the thermoplastics.  In non-contact hot-plate welding, temperatures are 300-400C above melting point of the plastic.  Other factors include pressure/time/displacement during heating, joining pressure, and cooling time.  Applications include plastic barrels, air intake manifolds, fuel tank assembly and ports, and transport pallets.

TTIR (Through Transmission Infra-Red) Welding

As the name indicates, this welding process depends on transmission of IR energy to the weld interface through one of the parts being welded.  Process is ideally suited for two thermoplastics components that can be brought into intimate contact at the weld locations and where at least one of the components is transparent to IR energy; plastics that are transparent to visible light are also transparent to IR energy.  Simple configurations include a sandwich of two flat sheets of plastic or two pieces of plastic butting against each other.

The IR energy, once it is transmitted through the transparent component, has to be absorbed at the interface on the other component.  Hence it helps if the other component is not transparent; any colored plastic works but black colored plastic would be ideal.  If both plastics are transparent, one can add a thin layer of black plastic sheet as an "braze" between the two.  The absorbed energy melts the plastic surface and then by conduction also melts the transparent plastic which has to be held in intimate contact during the whole process.

IR energy can be delivered over an area with halogen lamps or can be focused to a spot with a laser fiber.  Laser energy sources can include YAG or Diode lasers.  Depending on the part geometry, a direct diode source can also be used.  For seam welding applications, such as enclosures or other containers, the laser energy can be delivered at a spot and the spot is then moved along the required trajectory to form a seam weld.  Other sources of energy such as CO2 laser light (10.64 microns) does not transmit through any plastic thus making it difficult to heat the interface.  However, such light can be used if the interface is directly exposed to the light or if the parts being welded are very thin sheets and absorption on the exposed surface is sufficient to generate the welding heat.

Ultrasonic Welding of Plastics

Ultrasonic welding is perhaps the most common welding technique for plastics.  Possible reasons include speed, relatively simple fixture design, modest capital costs, and applicability to a wide range of thermoplastics.  In UW, the weld energy is provided in terms of a horn that vibrates at high frequencies ranging from 15-35 kHz; the vibrations are transmitted through the parts being welded by suitable part and horn design.  When the energy travels across the weld interface, it produces localized softening of the plastics which then bond under the action of the applied force.  Other applications of UW apparatus to create a bond include Ultrasonic Swaging and Ultrasonic Staking.

Once the horn and fixture are properly aligned, the process has few variables to control including energy, power, pressure, and/or time.  Proper design of the horn is critical and becomes more important as the weld size increases.  Frequent aligning checks and tuning checks should be conducted to ensure proper process control.  One good aspect of the welding process is that with experience, one can hear a good or bad weld based on how the long the weld is on and what kind of sound it makes.

Common applications include assembling enclosures such as transformer housings, telephone handsets, and battery pack casings.  UW is also used for sealing food cartons and also finds applications in "sewing" garments that have greater than 50% synthetic content such as lab gowns.