How To Weld Plastic: The Easy Way

Welding plastic? Most people didn’t know it was even possible.

When plastic cracks or breaks, sometimes the cheapest method is to weld the pieces together. It is a simple process and one that every welder should learn to master.

We show you how to weld plastic the easy way.

What Types of Plastics Can Be Welded?

You must identify what type of plastic you are working with. Luckily, most plastics are stamped with an identity label to let you know. This is especially true in the automotive industry.

You can weld thermoplastics using a nitrogen welder. The most common types of plastic, like polyethylene, ABS, nylon and polycarbonate, can all be fused. These plastics are typically used on applications like overflows, washer bottles and other practical uses.

If you are unsure about what type of plastic you have, you can buy welding test kits that contain several plastic rods. All you need to do is find a clean surface on the plastic you are welding and choose a rod with similar qualities. Melt the rod until a bead forms, and then allow it to cool. If you cannot lift it off with a pair of pliers, it is the right blend for the job.

10 Methods To Weld Plastic

There are several ways you can weld plastic. Some are better than others, depending on the plastic you are fusing. Here are the 10 most common methods.

Extruded-Bead Sealing

Placing a heated bead of the same material you are welding between the joints is often enough to seal the gap. Press the two pieces together immediately once you have laid the bead, and the heat should cause it to seal tight.

Friction Welding

Friction welding is commonly used with thermoplastics. The welder applies fast, angular movements to lay down the heat. It is a variation of the spin welding process, for when the two parts are not symmetrical.

High-Frequency Welding

This method involves plastic making contact with an electrode powered by a high-frequency welding machine. This heats the surface, ready to be joined.

Hot Gas Welding

This method is more akin to welding metal. Welding guns contain a gas or electrically heated chamber where nitrogen, or dry air, is fed towards the weld line. The heated gas melts the joint, while a rod of the same thermoplastic material is applied to the joint to seal it.

Hot Plate Welding

Press the two plastic surfaces against a hot metal plate coated in polytetrafluoroethylene (PTFE) to prevent it from sticking. Once the plastic starts to melt, hold them together using firm but light pressure until it cools and the joint hardens.

Induction Welding

A conductive metal insert melts the two parts of the plastic. The insert is heated thanks to a high-frequency welding machine, and the plastic starts to melt. At this point, the pieces are pushed together until a seal forms and it fuses. As the joint cools, the plastic hardens.

Laser Welding

A laser passes along the weld line as the two plastic pieces are pushed together. The heat from the laser fuses the plastic, generating sufficient heat to make the weld possible.

Solvent Welding

A solvent is applied to soften the surface of the plastic as they push together. Adhesion comes when the solvent evaporates and absorbs into the plastic material, or a polymerization effect occurs.

Spin Welding

Spin welding involves the circular cross-sections of the material spinning at a rate that causes friction, which melts the interface of the plastic. When the rotating stops, the friction generated is enough to bond the pieces together with firm pressure.

Ultrasonic Welding

The thermoplastics heating and sealing is achieved through mechanical vibration pressure of ultrasonic frequencies of 20 to 40 kc (kilocycles per second). Electrical energy transfers to ultrasonic vibrations by a transducer, targeted at the area of the plastic to be welded. The vibration creates localized friction, which then creates heat, and the plastic fuses.

How To Weld Plastic

We show you the step-by-step guide of how to weld plastic, including hints and tips.

1. Identify the Plastic

Knowing what plastic you are welding is crucial to getting the best results. Typically, you should find an ID label that tells you, especially if you are working on bumpers and car trims.

2. Clean and Prepare

Clean the plastic with warm soapy water to remove any loose debris. If you need something with a bit more power, try a detergent to scrub away stubborn marks. Grease and stains gathered over the years on the plastic’s surface weaken the weld and cause contamination.

Now dry off the plastic with a lint-free cloth. If the plastic has any paint on the surface, remove that too using 80-grit sandpaper.

3. Prepare the Workspace

Make sure you have sufficient ventilation or that you are wearing a suitable N95 face mask. Fumes from plastics can be harmful if inhaled. Open nearby windows and doors to create maximum airflow, and keep other people away from the area.

Also, make sure you have a sturdy welding table, set on flat and even ground.

4. Put on Personal Protective Equipment

Safety is paramount. Full stop! Always use a safety visor (a welding helmet is not needed because it is the arc that causes damage to your eyes) and a pair of leather welding gloves to protect your hands. Wear long-sleeves or a welding jacket to keep your arms safe, cotton pants and work boots.

5. Select the Welding Rod

Use the letter identification on the plastic to match the welding rod. Look for the letters PE (Polyethylene), PP (Polypropylene), and PVC (Polyvinyl Chloride). Choose a rod that matches, so, a polyethylene rod for welding polyethylene.

You can use a rod testing kit, which contains a variety of rods. Simply melt the rod that best resembles the plastic, placing a bead on a clean part of the surface as a test. When it cools, see if you can pull the bead off the plastic using pliers. If it sticks, you have the right rod.

6. Clamp Your Workpiece

Set both pieces of plastic on the table’s flat surface, pushing both ends together, making the neatest joint you can. Clamp them in place using “C” clamps. Wrap the pieces in foil tape, but keep the area to be welded clear.

7. Power On

Plugin the welder and switch it on. Allow the welding gun to preheat for 2 minutes. Each plastic has a different melting point, so your welding gun set-up is crucial. You’ll likely need a temperature between 200 and 300 degrees Celsius (392 and 572 degrees Fahrenheit).

Anything hotter and the plastic will overheat and melt or burn.

8. Tack Weld the Plastic

Secure the plastic in place by placing a tack welding nozzle, attached to the end of your welding gun, on the ends of the joint. Apply small amounts of heat and wait while the plastic melts. Now push the pieces together.

9. Trim the Welding Rod

Using angle-cut pliers, trim the end of your welding rod diagonally to hone a point. You should now have a flat surface on the underside. This gives you a better chance of a smooth weld, avoiding large bubbles of molten plastic.

You will need to wait for the welding gun to cool down before inserting the rod. Similarly, once inserted, you will need to let the gun heat up again.

10. Insert the Welding Rod

Speed nozzles have openings that hold the rod in place as you work along the weld line. If you didn’t have one supplied with your particular welding gun, you can buy them separately. Fit the nozzle to your torch and then feed the cut end of the electrode into the second opening, making sure that the clipped end will make contact with the surface of the plastic.

11. Start To Weld

Move the rod from the top of the crack, working towards you. Only start this process once the gun has heated, and you can see the rod melt as it touches the plastic surface. Hold the gun at 45 degrees, feeding the welding rod with your free hand as you move.

The watchword to a successful outcome is consistency. Don’t rush it, and keep a steady pace.

12. Try Different Techniques

Hold the nozzles an inch above the joint, with the gun at a 45-degree angle. Hold the rod at the same angle from the opposite side and move the nozzle back and forth as it melts. Keep doing this 3 or 4 times as you move the plastic as it melts to complete the task.

This is a trickier method than using a speed nozzle because you have to master the control of the gun and the rod separately. The secret is to move the rod in a swinging motion, gently brushing the plastic’s surface as it heats and melts.

13. Finish the Weld

Wait 5 minutes for the weld to cool. Let it come to room temperature before you start to neaten the joint. Remember to put your welding gun in a safe spot as it cools to avoid injury or fire.

If you can see the molten plastic turning solid and there is no heat coming from the weld line, it is safe to work on.

14. Sand the Weld

Use 120-grit sandpaper to file back any raised prominent edges. Only apply light pressure, working in consistent sweeping motions. When you are satisfied the weld line is flush and neat, wipe away the dust with a cloth.

15. Finish Up

To achieve finer detail, use 180 or 320-grit sandpaper. Higher-grade paper provides a better finish and is less abrasive. Still, avoid overworking the plastic as this risks scratching it.

FAQs

Is Plastic Welding Worth It?

Plastic welding is worth it in certain circumstances, but you need to understand more about it to know when to use it and avoid it. Plastic welding is used in various industries, most notably the automobile sector.

To give you an idea of how long-lasting plastic welding can be in the right conditions, consider the process used to make plastic car bumpers. These bars must have the high tensile strength to comply with relevant safety regulations.

You should know that a lot of factors affect the durability of plastic welding, starting with your experience as a welder. A plastic weld is as good as the welding rod used, but temperature and material quality also have a say.

What Temperature Do You Weld Plastic?

For most plastics, you need a temperature between 464°F and 518°F. Some plastics require higher or lower temperatures, but the interval mentioned earlier is the most common.

If you try to weld at a too-high temperature, you will notice the plastic starts to burn and release smoke. When you expose plastic to high temperatures and burn it, its surface turns brown.

Should the welding temperature be too low, the weld will easily break, and the rod will not penetrate into the base material.

When you use the correct welding temperature, PVC will have a shiny finish, and PP & PE will have a dull finish.

What is the Easiest Plastic to Weld?

Thermoplastics (and specifically polypropylene) are the easiest ones to weld. This type of plastic has several uses, and it’s great to work with if you are a welder.

That’s because PP has high tensile strength and superior chemical resistance. To weld PP, you typically need a temperature of 572°F.

Polyethylene comes in a close second place because it can resist abrasion and doesn’t absorb water. This plastic is used to make cutting boards and bins. To weld it, you typically need temperatures of around 518°F.

What Plastic Can’t Be Welded?

You can’t weld thermoset plastics because once formed, they are no longer affected by heat enough to become “moldable” once again. This includes plastics such as epoxy and nylon.

Can You Weld Plastic with a Lighter?

You can weld plastic with a lighter, depending on what type of plastic you want to work with. As a general safety rule, don’t use the lighter on the highest flame setting because you might burn your fingers.

Note that you can only weld thermoplastics using a lighter, and the pieces would have to be relatively small for the weld to be successful. Never attempt to use a lighter to weld plastics for heavy-duty applications.

What Is the Best Plastic Weld Glue?

The best plastic to weld glue is epoxy because of its adhesive bond strength. Epoxy adhesives provide chemical resistance and increase the plastic’s structural strength.

However, other types of glue might be better for certain plastics. What you plan to use the end product for will also determine the better glue. You can even use hot glue guns if you want to fix a small plastic weld around the house.

Plastic Fantastic

Welding plastic is satisfying and relatively easy to master. It could save you a heap of cash and get the job done without extra hassle. As with all welding tasks, it comes down to preparation, a little bit of knowledge, and taking a steady approach.

If you follow these steps, you should achieve a satisfactory result the next time you find yourself welding plastic.