IR welding

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IR welding is a welding technique that uses a non-contact heating method to melt and fuse thermoplastic parts together using the energy from infrared radiation. [1] The process was first developed in the late 1900s, but due to the high capital cost of IR equipment the process was not commonly applied in industry until prices dropped in the 1990s. [1] [2] IR welding typically uses a range of wavelengths from 800 to 11,000 nm on the electromagnetic spectrum to heat, melt, and fuse the interface between two plastic parts through the absorption and conversion of the IR energy into heat. [1] Laser welding is a similar joining process that applies IR radiation at a single wavelength. [1]

Contents

There are many different welding techniques that use IR heating, with the three major modes being surface heating, through transmission IR welding (TTIr), and IR staking. [1] A variety of heating configurations have been applied to these techniques such as scanning, continuous illumination, and mask welding. [1] Advantages such as faster and controllable non-contact heating applicable for a wide range of simple or complex part geometries sets IR welding apart from other forms of plastic welding. [1] [3] CO detectors, IV bags, and brake transmission lines are just a few of the many products that utilize IR welds. [1]

History

IR welding is categorized as a form thermal plastic welding alongside hot gas welding, hot tool welding, and extrusion welding. [2] Although infrared radiation was first discovered in the 1800s, IR was not applied as a source of heat until the beginning of WWII when it was found to be more effective than the fuel convection ovens of that time. [4] IR radiation was first tested for the welding of thermoplastic polymers in the late 1900s, but the process was relatively new and not fully understood. [2] IR welding systems offered faster heating times than the other forms of thermal welding, but the high capital costs limited its development. With a decrease in the price of equipment in the 1990s, IR welding has become more popular in the industry. [1]

Physics of IR welding

Electromagnetic spectrum Electromagnetic spectrum -eng.svg
Electromagnetic spectrum
Electromagnetic wave interactions Wave reflection medium.png
Electromagnetic wave interactions

IR welding typically uses wavelengths from 800 to 11,000 nm on the electromagnetic spectrum. Plastics interact with IR radiation through reflection, transmission, and absorption. Incident IR radiation can either be reflected off the surface of the plastic, transmitted through the plastic, or absorbed into the plastic as other forms of energy including thermal energy. The ratio of these three interactions depends on the wavelength of the IR radiation and the receiving plastic's properties. Amorphous plastics are generally optically clear and can transmit almost all incident IR radiation. For this reason they are commonly used in TTIr. Semi-crystalline plastics can diffuse incident IR radiation between the amorphous and crystalline boundaries, reducing the transmittance and increasing the absorbance of the material. The higher absorptivity results in more heat generation for a given IR source. Additives such as clarifying agents can be used increase a plastic's transmittance while dies and pigments can be used increase the absorbance of a material. Increasing amounts of these additives can decrease the strength of both the material and the welded joint. [1]

The closer the IR radiation source, the higher incidence efficiency on the material. IR radiation is most effective when directing radiation normal to the part. Radiation energy always affects the surface of a part while the depth of penetration that the energy can reach is dependent on the plastic's crystallinity. [3]

Equipment

IR Sources

Potential IR welding sources include quartz lamps and ceramic heaters which can generate a wide range of IR wavelengths. Laser welding employs IR sources that operate at a single wavelength such as CO2 lasers, Nd:YAG lasers, laser diodes. The equipment selected for each welding process stems from the type of radiation produced. Quartz lamps produce wavelengths of around 1,000 to 5,000 nm and ceramic heaters produce wavelengths of around 5,000 to 10,000 nm. [1]

Attachments

P-wave technology utilizes an IR lamp and a pre-placed focusing device such as an IR transducer or film that can filter and focus IR radiation at a desired wavelength and increased intensity within a selected area to improve weld penetration with minimal surface damage. This method allows improved IR welding of polymers with higher melting temperatures such as most fluoropolymers and polyketones. [5]

IR welding techniques

The three major welding techniques used in the industry today include surface heating, through transmission IR welding, and IR staking. All IR welding techniques contain the following six basic steps in some form: [1]

Basic steps of the IR surface heating mode Basic steps of the IR surface heating mode.png
Basic steps of the IR surface heating mode
TTIr Welding Diagram Transmission welding diagram.png
TTIr Welding Diagram
  1. Loading of parts into the welding system that will hold the parts in place
  2. Insertion of IR source in front of the face of each part that will welded together
  3. Application of IR radiation to melt a thin layer of plastic on the surface of each part
  4. Change-over in which the IR source is removed from the face of each part
  5. Clamping of the parts to join the melted surfaces together under pressure as they cool and solidify
  6. Unloading of the parts after the weld has been made

Surface Heating

Surface heating includes heating and melting of the interface between plastic parts with IR radiation and forcing the parts together into a molten joint that solidifies as one part. This process can be split into 3 phases as shown in the figure to the right: A) Loading of parts, insertion of the IR source, and IR application. B) Change-over with the removal of IR source and clamping of the parts to join them. C) Unloading of the parts after the weld was made. [1]

Through Transmission IR Welding (TTIr)

TTIr welding is the joining of an IR transparent part to a second part such that the IR radiation travels through the transparent part and heat the surface of the second part as shown in the figure to the right. IR wavelengths are generally within 800 to 1050 nm. To make a transparent part absorbent to IR radiation, the addition of dies or colorants such as carbon black can be used. Highly absorbent thermoplastic films can be placed at the joint to receive the IR radiation and melt the interface during welding. Using these methods, TTIr welds can be completed between parts of both the same or different materials. [1]

IR Staking IR Stacking.png
IR Staking

IR Staking

IR staking includes the localized welding of a thermoplastic stud or stake from one part into the cavity of a non-weldable part to form a mechanical fastener. As shown in the figure to the right, the polymer part and non-weldable part are first placed together (A), then the projecting polymer is melted and formed around the non-weldable part to fasten the two together (B). The stud can be heated through directed TTIr when pre-placed within the cavity of an IR transparent part, then melted to deform it into a button shape required to fill the cavity before solidifying. Surface IR radiation can also be used to soften a plastic stud which is then pressed into a button-shaped die to form a head before cooling and solidifying. [1]

Heating Configurations

IR systems generally rely on one of three surface heating methods: scanning, continuous illumination, and mask welding. [1]

Scanning

Scanning involves the movement of an IR beam across the surface of a part using either an automated motion system or galvanic mirrors. Equipment is limited by the speed of movements across the part's surface to maintain uniform temperatures on the surface. In TTIr welding, scanning allows the un-melted portion of the part to act as a mechanical stop in order to maintain the joint gap between the two parts. [1]

Continuous illumination

Continuous illumination uses more than one IR radiation source to heat the entire joint interface at the same time. Part tolerances or fit is not as crucial with this method as the entire surface will be melted before welding. This method is useful when welding parts with complex geometries, employing the multiple IR sources to evenly heat all forms of joint interfaces. [1]

Mask welding

Similar to continuous illumination, mask welding utilizes multiple IR sources to completely illuminate a joint interface while placing an IR radiation mask over the parts to control which regions will form a melt layer. [1]

Materials

Below is a list of materials well known for their IR weldability:

Advantages / Disadvantages

Advantages

Disadvantages

Applications

New joining technologies using IR welding are critical for fabricating complex parts and assemblies at high speeds and low costs. [3] Although IR plastic welding has many advantages over other types of plastic welding, limitations such as equipment costs and susceptible materials properties reduce the amount of industrial applications of the method. [1] A few examples of current industrial applications are shown below:

CO detector Carbon monoxide alarm.jpg
CO detector
IV bag Iv1-07 014.jpg
IV bag
PE pipe Fixing the PE pipes (8073330485).jpg
PE pipe

Related Research Articles

Infrared Form of electromagnetic radiation

Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around 1 millimeter (300 GHz) to the nominal red edge of the visible spectrum, around 700 nanometers (430 THz). Longer IR wavelengths are sometimes included as part of the terahertz radiation range. Almost all black-body radiation from objects near room temperature is at infrared wavelengths. As a form of electromagnetic radiation, IR propagates energy and momentum, with properties corresponding to both those of a wave and of a particle, the photon.

Welding Fabrication or sculptural process for joining materials

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as brazing and soldering, which do not melt the base metal.

Thermoplastic Plastic that becomes soft when heated and hard when cooled

A thermoplastic, or thermosoft plastic, is a plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.

Thermosetting polymer Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst. Heat is not necessarily applied externally, but is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Ultrasonic welding Welding process

Ultrasonic welding is an industrial process whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together. When used to join metals, the temperature stays well below the melting point of the involved materials, preventing any unwanted properties which may arise from high temperature exposure of the metal.

Plastic welding Welding of semi-finished plastic materials

Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.

Polyoxymethylene Engineering thermoplastic

Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic polymers, it is produced by different chemical firms with slightly different formulas and sold variously by such names as Delrin, Kocetal, Ultraform, Celcon, Ramtal, Duracon, Kepital, Polypenco, Tenac and Hostaform.

Heat sealer

A heat sealer is a machine used to seal products, packaging, and other thermoplastic materials using heat. This can be with uniform thermoplastic monolayers or with materials having several layers, at least one being thermoplastic. Heat sealing can join two similar materials together or can join dissimilar materials, one of which has a thermoplastic layer.

Hot plate welding, also called heated tool welding, is a thermal welding technique for joining thermoplastics. A heated tool is placed against or near the two surfaces to be joined in order to melt them. Then, the heat source is removed, and the surfaces are brought together under pressure. Hot plate welding has relatively long cycle times, ranging from 10 seconds to minutes, compared to vibration or ultrasonic welding. However, its simplicity and ability to produce strong joints in almost all thermoplastics make it widely used in mass production and for large structures, like large-diameter plastic pipes. Different inspection techniques are implemented in order to identify various discontinuities or cracks.

Rheological Weldability (RW) of thermoplastics considers the materials flow characteristics in determining the weldability of the given material. The process of welding thermal plastics requires three general steps, first is surface preparation. The second step is the application of heat and pressure to create intimate contact between the components being joined and initiate inter-molecular diffusion across the joint and the third step is cooling. RW can be used to determine the effectiveness of the second step of the process for given materials.

Active thermography is an advanced nondestructive testing procedure, which uses a thermography measurement of a tested material thermal response after its external excitation. This principle can be used also for non-contact infrared non-destructive testing (IRNDT) of materials.

Vibration welding refers to a process in which two workpieces are brought in contact under pressure, and a reciprocating motion (vibration) is applied along the common interface in order to generate heat. The resulting heat melts the workpieces, and they become welded when the vibration stops and the interface cools. Most machinery operates at 120 Hz, although equipment is available that runs between 100–240 Hz. Vibration can be achieved either through linear vibration welding, which uses a one dimensional back and forth motion, or orbital vibration welding which moves the pieces in small orbits relative to each other. Linear vibration welding is more common due to simpler and relatively cheaper machinery required.

Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.

Laser welding of polymers is a set of methods used to join polymeric components through the use of a laser. It can be performed using CO2 lasers, Nd:YAG lasers, Diode lasers and Fiber lasers.

Spin welding is a form of friction welding used to join thermoplastic parts. The parts to be welded must be round, and in plane with each other. Like all other welding methods this process utilizes heat, time, and pressure to create a weld joint. Heat is generated via internal friction generated between the two parts when rotating and subjected to a load normal to the weld joint. This frictional heat causes the plastic to melt and a bond to be created.

Extrusion welding is one of the processes used to weld thermoplastics and composites, developed in the 1960s as an evolution of hot gas welding. It can be a manual or automated process.

Advanced thermoplastic composites (ACM) have a high strength fibres held together by a thermoplastic matrix. Advanced thermoplastic composites are becoming more widely used in the aerospace, marine, automotive and energy industry. This is due to the decreasing cost and superior strength to weight ratios, over metallic parts. Advance thermoplastic composite have excellent damage tolerance, corrosion resistant, high fracture toughness, high impact resistance, good fatigue resistance, low storage cost, and infinite shelf life. Thermoplastic composites also have the ability to be formed and reformed, repaired and fusion welded.

Radio-frequency welding, also known as dielectric welding and high-frequency welding, is a plastic welding process that utilizes high-frequency electric fields to induce heating and melting of thermoplastic base materials. The electric field is applied by a pair of electrodes after the parts being joined are clamped together. The clamping force is maintained until the joint solidifies. Advantages of this process are fast cycle times, automation, repeatability, and good weld appearance. Only plastics which have dipoles can be heated using radio waves and therefore not all plastics are able to be welded using this process. Also, this process is not well suited for thick or overly complex joints. The most common use of this process is lap joints or seals on thin plastic sheets or parts.

Implant resistance welding is a method used in welding to join thermoplastics and thermoplastic composites.

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

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 Grenwell, David A., Benatar, Avraham, Park, Joon Bu (2003). Plastic and Composites Welding Handbook. Cincinnati: Hanser. pp. 271–309. ISBN   978-1-56990-313-1.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. 1 2 3 Stokes, Vijay (1989). "Joining Methods for Plastics and Plastic Composites: An Overview". Polymer Engineering and Science. 29 (19): 1310–1324. doi:10.1002/pen.760291903.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 Chen, Yang Shiau (1995). "Infrared heating and welding of thermoplastics and composites". ProQuest Dissertations Publishing. ProQuest   304207573.
  4. ↑ Arnquist, W (1959). "Survey of Early Infrared Developments". Proceedings of the IRE. 47 (9): 1420–430. doi:10.1109/JRPROC.1959.287029. S2CID   51631730.
  5. ↑ "New Approach to IR Welding Bonds More Engineering Plastics". April 2004.
  6. ↑ No, Donghun (2005). A study of the combined socket and butt welding of plastic pipes using through transmission infrared welding (Thesis) – via OhioLINK.
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