how to weld plastic Using Hot Gas Welding and Ultrasonic Welding

The plastic welding is a process of joining softened or malleable surfaces of materials with the help of heat. There are numerous techniques available to weld plastic. However, in this blog, I will try to cover the basic things for your understanding.

In the upcoming section, I explain a little about the major types of plastic and how you can use different methods to join or weld them.

Types of Plastics

Broadly the plastics can be classified as thermosets plastics and thermoplastics plastics.

Thermoset Plastics or Thermoset Composites

These are synthetic materials which take a uniform structure when heated. However, remember that these cannot be reheated or remolded after the first heat formation. In layman language thermosets are like cakes, they are baked once but if you again bake or heat them, they may burn.

Further, after initial heating, these materials become resistant to high temperatures, oils and specific chemicals and also to corrosion as well. The common examples are epoxy, phenolic and so on.

Now you can guess what is the best way to join or weld such materials which cannot bee reheated. Yes! Adhesive joining.

Thermoplastics or Thermosoftening Plastics

These are basically plastic polymer materials that are easy to mold after reaching a certain temperature and it hardens upon cooling. In layman language, a thermoplastic is like ice-cream, you can heat it and it will again take any shape which is not possible in case of cakes or thermoset plastics.

So, thermoplastic welding is a process which joins the pieces of thermoplastic materials using pressure, cooling and heating. In this method, firstly the surface of thermoplastic is heated till its melting point, also called thermoplastic state which generally ranges between 480°F (250°C) to 1022°F (500°C).

The common examples of thermoplastic materials are PVC, polyethylene, polypropylene etc. There are several methods to weld these materials but for now, I will discuss hot gas welding and ultrasonic welding.

Methods to Weld Plastic

Hot Gas Welding of Thermoplastics

Hot gas welding or hot-air welding is a welding process of joining to thermoplastics with the aide of heat. A heat gun also known as hot air welder generates a stream of the hot air which softens the materials which are to be joined and also the plastic filler rod. Note that all these materials should have plastic of similar composition otherwise it may ruin the quality of the weld.

This method can be used for all thermoplastics except PVC. since this is a popular fabrication method so it is largely used on manufacturing smaller arrangements like water or chemical tanks, plumbing fittings, heat exchangers and so on.

In short, it is a swift welding technique that can be performed continuously. You just have to join two plastic sheets with the help of hot gas and then rolled together.

Ultrasonic Welding for Thermoplastics

This is an industrial technique in which high-frequency (15 kHz to 40 kHz) ultrasonic low amplitude vibrations are applied to the two thermoplastic materials to be joined together. These materials are held together under adequate pressure to make a solid-state weld.

Besides, it maintains the temperature below the melting points of both the materials to be weld to avoid any unwanted damage. This technique is completely free from any use of nails, bolts, soldering materials, adhesives or glues. The noteworthy point is that this process can be effectively used to join even dissimilar materials. 

In short, it is one of the fastest heat sealing processes available. Even, it produces strong welds with high durability. 

Steps to Weld the Plastic

Here, I will quickly try to tell the steps to follow before and during welding your plastic materials.

1. Cleaning and Arrangement

  • It is recommended to set up your workspace preferably in a ventilated or airy area. This arrangement will guard your material against fumes, dirt and dust.
  • Wear a decent pair of welding gloves (maybe leather), fire-resistant fully covered clothing and relevant workboots. 
  • Don’t forget to cover your face with a welding helmet or a mask. For welding of plastic, you don’t have to wear a welding helmet as this process doesn’t involve intense arc and spark.

 2. Thorough Cleaning of your Material

  • Soak a sponge in warm water and try to scrub off as much debris as you can. Also, you may use a mild detergent or dish wash, in case the oil, grease, dust or dirt is stubborn.
  • This step is an important process as it will decide the quality of your weld. If it is not cleaned properly, you may get weak welds.
  • After cleaning dry the plastic with a dryer or a lint-free cloth.
  • Note that the industrial-strength detergents may leave soapy films behind so avoid using such detergents. Rather you can try methyl ethyl ketone to remove stubborn stains.
  • If there is any paint on the surface, you may use an 80 grit sandpaper to scrub-it off.

3. Match the Welding Rod Same as Your Plastic Material

  • The plastic material generally has a letter identifiers such as PP (polypropylene), or PVC (polyvinyl chloride). Select the welding rod the same as your plastic which is to be weld.
  • You may also take the help of welding rod testing kit, in case you are not sure of the plastic you have.

4. Carefully Arrange the Plastic Pieces

  • Place the plastic pieces in the position you want them to weld and push them close. You can use C-clamps to avoid any movement and use foil tape to keep them close together. However, do not cover the area or part you want to weld.
  • The pieces should be tightly joined and in the exact position so that you don’t have to adjust them once the welding torch is on.

5. Joining or Welding of Plastic

  • Check the melting point of your plastic pieces as every material has its own melting point. Set the temperature of the welding gun below the melting point of your material so that they don’t burn or melt.
  • Secure the plastic with a tack welding at the ends to avoid any movement in between. Now slowly move the tip of welding gun over your plastic for a quick weld. Hold the gun a little downward at around 45 degrees angle and slightly touch the nozzle edge to the plastic.
  • If you see that the plastic is changing colour or burning, move your welding torch at a faster speed. Once you are done, let the plastic cool for at least 5 minutes.
  • Check the welded plastic if it turned solid. Now for finishing, sand the welded joint with 120 grit or 320 grit sandpaper. 
  • You may start with lower grit sandpaper. Since it is coarse, it will wear away more plastic welds. Higher grit sandpapers are suitable for finishing purposes.

Summing it Up

I have tried my best to cover as much information I could. There are a number of methods and techniques available to weld plastic materials. However, I have covered the most common methods. 

I will try to cover more techniques may be in my upcoming blogs. You can write to me to share your personal experiences you had during your welding career. Till then take care and stay safe!!

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