Ultrasonic Plastic Welding

Plastic Welding Basics: How to Weld Plastic

Plastic welding is a type of welding that is applied to semi-finished plastic materials. It is a process of joining softened surfaces of materials, usually using heat (excluding solvent welding). The heat is used to melt the plastic and fuse the two pieces of plastic. Generally, plastic welding can be achieved in three stages: surface preparation, heat and pressure application, and cooling. This process has a wide range of applications in industry and everyday life. It is frequently used in manufacturing to join plastic parts, like automobile parts, pipelines, and electronic components. It may also be used to repair plastic products like toys, household items, and car bumpers.

What is Plastic Welding?

Plastic welding is a process that makes use of heat to join softened thermoplastic surfaces. When compatible thermoplastics are heated, they establish a molecular bond, which leads to plastic formation. During manufacture, the surfaces must be pressed, subjected to heat and pressure, and cooled.

Many welding methods have been devised to join semi-finished plastic materials. Thermoplastic welding methods are categorized as external or internal heating methods according to the heat generation mechanism at the welding interface. It should be noted that developing a high-quality weld depends not only on welding methods but also on the weldability of the base materials. Plastic welding is utilized in various industries, including infrastructure construction, automobile repairs, plumbing, water tanks, and heat exchanger manufacturing.

In general, plastic welding can be employed 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

How to Weld Plastic?

Welding serves as the optimal solution to join plastic parts or repair cracks. Achieving a flawlessly blended plastic can be a difficult task. Nevertheless, this process can be effortlessly executed by following the steps described below.

Step 1: Cleaning and Setting up the Plastic

1. Set up a Workspace in a Ventilated Area

Firstly, you should set up your workspace in a well-ventilated location to avoid dust and fumes. Utilize fans and open windows and doors close to air out the space. When preparing the plastic, you can be extra safe by wearing a dust mask and protective polycarbonate glasses. Additionally, put on long-sleeved clothing and heat-resistant gloves for protection.

2. Remove Debris from the Plastic with Soap and Warm Water

You can remove debris by cleaning the plastic with soap and warm water. Mild dish soap or detergent also works. This process can eliminate all impurities that weaken the weld. When finished, dry the plastic with a clean, lint-free cloth. To remove stubborn stains, consider utilizing a liquid solvent known as methyl ethyl ketone (MEK).  

3. Choose a Welding Rod That Matches Type of Plastic 

Selecting a matched welding rod based on the letter identification on the plastic is necessary. Letter identifiers are often printed on many plastic items. For example, you can check for the initials PE (polyethylene), PP (polypropylene), or PVC (polyvinyl chloride) on the plastics. Hence, remember to select a rod that matches the type of plastic you intend to weld. Although you can frequently repair a crack in a single piece by melting plastic around it, keep a welding rod handy to fill any gaps.

 

plastic welding rods

4. Use a Welding Rod Testing Kit

Utilize a welding rod testing kit if you are uncertain about the type of plastic material. A rod testing kit includes a variety of diverse plastic welding rods. To conduct the test, attempt to select a rod that bears the closest resemblance to the plastic material. Apply heat to the end of the rod, as is often done in welding, to attach it to a clean spot of the plastic. Next, try to pull the rod off the plastic with a pair of pliers. If it remains connected, it is the same substance as the plastic.

5. Remove Paint on the Plastic with 80-grit Sandpaper

If the area you intend to weld has been painted, you can remove it with rough sandpaper, such as 80-grit sandpaper. Lightly but consistently apply pressure as you rub the sandpaper back and forth across the surface. In addition, you may use a drill-attached abrasive disc or sanding wheel. Scraping away paint with a paint scraper is also a good solution.  

80 grit Sandpaper

6. Clamp and Tape Plastic Pieces Together

Plastic parts are clamped and taped together to secure the joint. Establish the joint before turning on the torch. Push the plastic parts on the bench as close together as possible. Then, secure the pieces to the table with C-clamps. As necessary, wrap the parts with foil tape to keep them pressed close together, but avoid covering the area where you intend to weld.

Step 2: Joining the Plastic

1. Preheat the Welding Gun

Ensure the welding gun is preheated to the desired temperature for at least 2 minutes. Since various types of plastic melt at different temperatures, setting your welding gun up properly is critical. The required temperature falls within the range of 200 to 300 °C (392 to 572 °F). Beyond that temperature range, the plastic either catches burning or fails to melt sufficiently.

Plastic Welding Gun

2. Secure Plastic Together by Tack Welding the Ends

Pin loose bits of plastic together by melting the ends of the join before beginning the actual weld. Gradually apply heat after attaching a tack welding nozzle to your welding gun. Allow the plastic to start melting, causing the pieces of plastic to join together. This will prevent the plastic from moving as you weld.

3. Trim the End of the Welding Rod with Angle Cutting Pliers

Holding the pliers diagonally toward the rod’s end would be best. Then, cut it and sharpen the rod to a point. In the absence of pliers, employ a trimming knife to scrape the rod meticulously until it assumes a point. Providing the rod with a pointed end can enhance the likelihood of obtaining a smooth, steady weld without a large bubble of plastic where you begin. Allow the welding gun to cool before replacing the nozzles and inserting the welding rod. However, remember to reheat the gun before beginning the weld.

4. Insert Welding Rod into Speed Nozzle on Welding Gun

A speed nozzle features an opening that allows you to hold the welding rod securely while melting it onto the joint. Once the nozzle has been attached to the plastic welding heat gun, insert the rod into the second opening on top. Insert the cut end first to use it to begin the weld. 

You may alternatively hold the rod horizontally above the joint and melt it with pendulum welding, which involves sweeping the gun or torch back and forth. It takes a little longer but works well in tight spots.

5. Move the Tip of the Welding Gun over the Plastic

Gradually move the welding gun’s tip across the plastic surface to perform a speed weld. Begin at the top of the crack or area to be joined. Keep the gun at a 45-degree angle, with the nozzle’s edge contacting the plastic. Next, heat the plastic until it begins to melt. While moving the welding torch along the joint, feed the welding rod into it with your free hand. The plastic and welding rod will readily melt without burning if this is done. Consider speeding up the torch’s pace if you see a plastic burning or color change.

6. Swing the Welding Gun back and forth in Pendulum Welding

If you carry out a pendulum weld, swing the welding gun back and forth. Position the nozzle approximately 2.54 cm (1.00 in) above the crack while angling the gun at a 45-degree inclination. On the opposite side, place the welding rod at a 45-degree angle. Sweep the nozzle back and forth 3 or 4 times while holding the rod in position to melt it. Continue repeating this as you proceed down the plastic to finish the weld.

You can use pendulum welding if you don’t have a plastic heating gun with a speed nozzle. It’s simple to perform using a propane torch. This is also an excellent approach to filling a tight joint that a speed nozzle cannot readily reach.

Step 3: Finishing the Weld

1. Wait at least 5 minutes for the Plastic to Cool

Restore the plastic to room temperature before proceeding with further operation. A maximum of five minutes should pass before the plastic grows to cool. Observe until the welded plastic becomes solid. You may proceed with working on it if no heat comes from it.

It is optimal to repair a weld before its cooling process. A good weld seems to be generally smooth and uniform. Add more welding rods or use your gun to smooth out the melted plastic as required.

2. Sand the Welded Joint down with 120-grit Sandpaper

Utilize 120-grit sandpaper to sand the welded joint. Smooth the rough ridges on the weld to provide a uniform appearance that matches the surrounding plastic. It would be best if you rubbed the sandpaper back and forth across the weld. Make the weld appear level with the surrounding area, but avoid damaging the plastic. Consider using a sanding wheel on a rotary tool to finish this quickly. However, be cautious since plastic is quite soft and easily scratched.

3. Finish the Plastic with 180 and 320-grit Sandpaper

To smooth out the weld, use sandpaper with a finer grit, like 180 and 320-grit Sandpaper. Although higher-grit sandpaper is less abrasive and finer, it can scratch plastic if handled carelessly. Similar to how you sanded the weld with 120-grit sandpaper, rub it until it is uniform in appearance and feels smooth to the contact. 

Start with sandpaper of a lower grit at all times. Because it is coarser, it wears away the plastic more. Doing so will save the higher grit sandpaper for the finishing task. 

Different Techniques of Plastic Welding 

Many plastic welding techniques are used in the plastic industry. Here are some frequently employed techniques to weld plastic.

Hot Gas Welding

Hot gas welding, commonly known as hot air welding, is a plastic welding process that applies heat. A hot air welder jets hot air that softens the parts to be joined and a plastic filler rod. Note that these rods must be of the same or extremely similar plastic. Smaller products such as chemical tanks, water tanks, heat exchangers, and plumbing fittings are commonly fabricated using hot air/gas welding.

Free-hand Welding (Fan Welding)

A stream of hot air is directed toward the plastic weld rod and the weld joint. As the melted welding rod reaches working temperature, it is fused into the joint.

Speed Welding (Speed Tip Welding)

Speed tip welding employs a plastic welder that functions in a working principle similar to a soldering iron. The process involves the plastic being softened and the molten weld rod consistently flowing out of a feed tube, resulting in the fusion with the workpiece.

Laser Welding

This approach necessitates one part to possess transmissivity to a laser beam, and the other part must either possess absorptivity or have a coating at the interface that exhibits absorptivity to the beam. Both parts are subjected to pressure as the laser beam advances along the joining line. The beam moves through the first part and gets absorbed by the other part, or the coating, producing sufficient heat to soften the interface and establish a permanent weld.

Plastic welding commonly uses semiconductor diode lasers. In the joining of plastic materials, diode laser systems provide the following advantages:

  • Superior to adhesive bonding in terms of cleanliness.
  • No micro-nozzles can get blocked.
  • No liquids or fumes to impact the surface finish.
  • There are no consumables.
  • Increased throughput.
  • Can reach workpieces with difficult geometry.
  • Process control at a very high level.

Laser plastic welding

Ultrasonic Welding

High frequency (15 kHz to 40 kHz) low amplitude vibration is utilized in ultrasonic welding to generate heat through friction between the materials to be joined. The interface between the two parts is carefully engineered to focus energy for optimal weld strength. Almost every plastic substance may be treated with ultrasonic. This technique is the most rapid heat sealing method currently accessible.

Spin Welding

Spin welding is an application of friction and pressure between a stationary and moving plastic (lathe, milling machine, drill press) to create a rotational friction weld. The two pieces of plastic are melted and fused with the use of heat and pressure, which then solidifies when it cools. The process is rapid and simple, making it suitable for most plastics. Before joining the plastic parts, surface preparation is necessary to obtain symmetrical surfaces.

Vibration Welding

During vibration welding, the plastic part is rubbed at a certain frequency and amplitude. This causes frictional heat to be generated, which melts the joint piece and forms the welded joint. Vibration welding is classified into two types:

Linear Vibration Welding

It melts the plastic parts’ joints using frictional heat created by moving one plastic part relative to the other at a specified displacement. After the vibration stops, the process proceeds at a constant pressure to the point where the welded joint cools.

Orbital Vibration Welding

The process entails vibrating the upper portion of the plastic part in a continuous circular motion in every direction. This produces heat, which melts the plastic part. The vibration ceases when the welded joint reaches its melting point, allowing it to solidify. 

Hot Plate Welding

Hot plate welding is a plastic welding process that melts the surfaces of plastics using a hot plate. As the pieces cool, they unite to form a strong, welded plastic. When welding big plastic pieces, this approach is ideal. It is also more efficient than other welding processes. However, it cannot weld plastics less than 0.1 inch (2.5 mm). A hot plate plastic welder must also be serviced regularly.

High-Frequency Welding

High-frequency or radio-frequency welding melts and joins plastic polymers using an electromagnetic field (13MHz-100MHz). Pressure is applied simultaneously to form a strong weld joint while the high-frequency generator produces heat and melts plastic by forcing the molecules to oscillate quickly. This process is extremely efficient, with weld rates ranging from 100 to 120 m/min. Welding personnel must wear safety gear and take measures since the generator emits a lot of heat.

Solvent Welding

We often use a solvent or solvent blend (typically methyl ethyl ketone) between plastic parts to forge a robust weld joint. Unlike other techniques of joining plastics that require a plastic filler rod or softened plastic, solvent welding relies on the chemical bond of the solvent. This process is easy and low-cost, making it ideal for plastic tubing, scale models, and general manufacturing. However, stress cracking and poor joint integrity can be caused by improper surface preparation or solvent application.

welding plastic

Advantages of Welding Plastic

Plastic welding for manufacturing has several advantages, which is why it is used in various sectors and for rapid prototyping. Here are a few advantages:

  • Plastic welds are suitable for providing leak-tight or airtight seals.
  • No need for extra consumables like fasteners, solvents, or glues. This reduces complications, risks, and costs accrued due to extra consumables.  
  • Compatible with any Joint Shape. You can weld parts with curved or irregular shapes easily.
  • It’s a cost-effective process and requires a low need for consumables and a low cycle time.
  • Because this process does not need many consumables, it reduces the weight of the final welded products.
  • The welded joint is permanent when compared to other plastic joining mechanisms.
  • Unlike many other welding processes, plastic welding usually requires minimal surface preparation.

Disadvantages of Welding Plastic

Let’s see some disadvantages of Plastic welding:

  • It is greatly dependent on the compatibility of the materials. The plastics being joined should be of the same or very similar type for the strongest welds.
  • Many welding techniques need certain joint designs, which can be challenging to mold.
  • The cost of welding is higher than other available techniques in the market.  
  • Plastic welds tend to have localized internal stresses which act as crack initiation points and areas of weakness. 

plastic welding applications

Applications of Plastic Welded Parts

Plastic welding refers to various processes used to join plastic parts together. Some common applications that utilize plastic welded parts include:

Aeronautics – Plastics used in aeronautics necessitate precise permanent welding that does not compromise their properties. As a result, the method is viable for fabricating plastic welded parts, including trays, interior panels, and holding tanks.

Automotive Industry – Hot plates or laser welding combines many interior and exterior plastic car components. This includes bumpers, instrument panels, door panels, armrests, etc. Plastic welding allows for strong, lightweight, and cost-effective joints.

Medical Devices – Equipment like IV bags, blood and solution bags, tubing, and disposable medical tools often involve plastic welded seams and joints. The seals need to be very strong to avoid leakage. Spin welding and ultrasonic welding are commonly used.

Plumbing Systems – Plastic pipes, tubes, tanks, valves, and fittings are frequently joined via socket welding, electrofusion welding, or butt welding processes. This creates watertight joints for water supply systems.

Food and Beverage Packaging – Various types of plastic packaging are sealed using heat sealing, ultrasonic, or hot plate welding. Heat-sealed plastic pouches, trays, bags, and other containers help preserve food freshness.

Electronics – Many components in devices like computers, printers, and appliances are plastic. Methods like vibration, laser, and infrared welding combine plastic casings, parts, and assemblies in electronics.

Toys and Sporting Goods – Toy building blocks, model kits, inflatable play items, and protective sports gear often utilize welded plastic joints. Spin, hot gas, and stake welding provide safe, durable connections.

Summary

Plastic welding refers to joining plastic pieces together by applying heat or pressure. It’s often assisted by a filler rod to bond the two damaged plastic pieces together. Plastic welding is an indispensable technique across countless industries and applications. It creates strong, long-lasting plastic components and products more efficiently and affordably than other joining methods.

References

Plastic welding – From Wikipedia

How to Weld Plastic – From wikiHow 

FAQs

Plastic welds are robust molecular bonds. The weld is hermetically sealed depending on the welding technology employed.

Plastic welding is impossible with thermoset materials because they cannot be re-melted. The materials that will be welded are thermoplastics since they can be re-melted and re-shaped more than once.

Major industries include aerospace, appliances, automotive, chemical plants, construction, consumer goods, electronics, manufacturing, marine, and medical devices.

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