Welding Processes for Plastics Explained

Welding Processes for Plastics Explained

Plastic welding is a fabrication process that joins thermoplastic materials by using heat. It has become an essential tool in manufacturing and engineering as it overcomes some problems related to traditional welding methods.

The term welding is generally associated with joining metal parts, so using the term with plastics might sound a bit foreign. But by definition, welding is the process of fusing two or more parts by heat, whether the parts are from metal or plastic, does not play a role here.

What Is Plastic Welding

Plastic welding is used to join thermoplastics by heating plastic pieces until they are malleable and then joined into a unified structure. It is used in a wide range of sectors from infrastructure building, automotive repairs, plumbing, and manufacturing of water tanks and heat exchangers.

Plastic welding can be used on the following materials:

  • Acrylonitrile-butadiene-styrene (ABS)

  • Polycarbonate (PC)

  • Polyethylene (PE)

  • Polyethylene terephthalate (PETE or PET)

  • Acrylic or polymethyl methacrylate (PMMA)

  • Polypropylene (PP)

  • Polyvinyl chloride (PVC)

  • Other materials

As with other welding methods, surfaces must be clean to achieve the highest weld quality. Plastics are joined at their melting point. Once cooled, they are fused completely.

Plastic Welding Advantages

Plastic welding is becoming increasingly popular among manufacturers in various industries since it is highly efficient and covers a wide array of materials. Unlike some other joining methods, such as fasteners and adhesive bonding, in most cases, it eliminates the need to purchase additional components and materials. Plastic welds are also lightweight and cost-effective while still achieving quality results.

Additionally, plastic welding offers versatility and compatibility with different joint shapes. The welds are permanent and the process is safer when compared to some other traditional welding methods since it produces minimal fumes.

Plastic Welding Methods

There are many methods for plastic welding, each with its advantages and disadvantages. The methods are classified as internal and external

The chosen method should depend on the type of welded materials and their shape as well as the required weld strength. We’ll highlight some of the most common ones below:

Hot Gas Welding

Hot Gas Welding HDPE
Hot Gas Welding HDPE

Hot gas welding uses a specially designed heat gun with additional plastic welding rods or sheets in order to create malleable and easy-to-join pieces of plastic. The equipment doesn’t require electricity. It is portable and easy to use. The downside of hot gas welding is the slow heating rate compared to other methods. Hot gas welding isn’t suitable for thicker plastics such as PVCs and acrylics.

  • Free-hand welding (fan welding)

    A stream of hot air is applied to the weld joint and the plastic weld rod. The melted welding rod is fused into the joint as it reaches working temperature.

  • Speed welding (speed tip welding)

    Speed tip welding uses a plastic welder with a working principle similar to a soldering iron. It softens up the plastic and the molten weld rod continuously exits a feed tube, fusing with the workpiece.

Laser Beam Welding (LBW)

Laser Beam Welding of Plastics
Laser Beam Welding of Plastics

Laser welding is a fast and accurate plastic welding process that uses a concentrated heat source while subjecting the workpiece to pressure. Laser welding can be automated through robots, creating clean welds that require little to no post-weld processing. LBW equipment is initially expensive and the process isn’t suitable for plastics with thicknesses above 0.5 inches (12.7 mm). Laser beam welding can sometimes lead to porosity and brittle results.

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Ultrasonic Welding

Ultrasonic Plastic Welding
Ultrasonic Plastic Welding

Ultrasonic welding uses high-frequency, low-amplitude mechanical vibrations (15kHz – 40kHz) to generate heat. The vibrations create friction, which allows the plastic polymers to melt, creating a bond between two plastic surfaces. Ultrasonic welding is fast and doesn’t use direct heat application. This makes it an excellent choice for joining plastics such as polyvinyl chloride that are prone to generating fumes when used with other techniques. However, it can only be used to create overlapping joints on thin strips of low-moisture plastics. So the field of application is rather limited for this process.

Spin Welding

Spin Welding Plastic in Slow Motion
Spin Welding Plastic in Slow Motion

Spin welding is a rotational friction welding process that uses pressure and friction between a stationary and moving plastic (lathe, milling machine, drill press). Heat and pressure melt and bond the two pieces of plastic as it cools down. This process is quick and straightforward, which makes it compatible with most plastics. Surface preparation is required to achieve symmetrical surfaces before joining the plastic parts.

Vibration Welding

Vibration Welding of Plastics - Linear Welding
Vibration Welding of Plastics - Linear Welding

Vibration welding joins plastic materials by heat generated through pressure and friction between rubbing parts. It has two classifications:

  • Linear

    One plastic part is stationary while the other material moves back and forth, rubbing the inert plastic at a set frequency and amplitude.

  • Orbital

    The upper plastic material moves through a circular motion, creating a welded joint to the stationary plastic material. The process is similar to spin welding but uses a more complex machine, in which the moving part rotates in a small circle. Significantly smaller than the size of the whole joint.

Vibration welding doesn’t need any consumables or surface preparation, making it suitable for creating welded joints between irregularly shaped plastics. The downside is that it requires expensive vibration welding equipment and can only weld specific plastics.

Hot Plate Welding

Hot Plate Welding Plastic
Hot Plate Welding Plastic

Hot plate welding is a plastic welding technique that uses a hot plate to melt the surfaces of the plastics. The pieces combine to create strong, welded plastic as it cools down. This method is preferred when welding large parts of plastic. It is also faster than other welding techniques. As a drawback, it cannot weld plastics below 0.1 inches (2.5 mm). Hot plate plastic welder also requires frequent maintenance.

High-Frequency Welding

Radio-Frequency aka High-Frequency Welding Explained
Radio-Frequency aka High-Frequency Welding Explained

High-frequency or radio-frequency welding uses an electromagnetic field (13MHz-100MHz) to melt and join plastic polymers. As the high-frequency generator creates heat and melts plastic by causing the molecules to oscillate at high speeds, pressure is simultaneously applied to create a strong weld joint. This method is incredibly efficient as it can reach weld speeds at around 100 to 120 m/min. Welding operators must wear protective gear and take precautions, as the generator can radiate a large amount of heat.

Solvent Welding

Solvent Welding Acrylics
Solvent Welding Acrylics

A solvent or a solvent blend (commonly methyl ethyl ketone) is applied between plastic parts to create a strong weld joint. Unlike other methods that use a plastic filler rod or softened plastic to join plastics, solvent welding uses the chemical bond of the solvent. This technique is simple and inexpensive, perfect for plastic pipes, scale models, and general fabrication. Improper surface preparation or solvent application may cause stress cracking and weak joint integrity.

Plastic Welding Safety

Although plastic welding is generally deemed safer and simpler than known welding methods such as flux-cored welding, for example, it is still best to understand the safety precautions. Protection from fumes and burns can be practiced by having proper ventilation and wearing protective clothing.

Wrapping It Up

Plastic welding covers a wide range of applications such that many industries take advantage of creating strong material bonds. Destructive and non-destructive testing methods are essential in making sure that the plastic welding process is kept at a high quality. While these testing methods minimise risk and improve welding safety, a software-operated welding process further improves the precision of welding plastic.

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