How to weld Plastic

The word plastic is derived from the Greek word plastikos, which means “fit for molding.” Plastics are made up of small units of molecules bonded together into long chains called polymers. A greater tonnage of plastics is produced annually than of all nonferrous metals combined.

Plastic in engineering has two types:

1. Thermoplastic Plastic

2. Thermosetting Plastic

How to weld Plastic

Click here to Get Guide on How to Weld various materials

Weldability of Plastics

Choosing the right plastic for the welded construction, for example, a pipeline depends on their operating conditions and the material properties such as strength, chemical and heat resistance, required security, and economics. Here it must be considered that by targeted changing of certain characteristics, i.e. by modification, the most varied plastic types can be produced.

Important welding parameters are:

  • Temperature: The joint surfaces are taken into the thermoplastic state by heat energy (hot-air, heating elements, friction). Open flames are not permitted for heating since plastics, due to their reduced heat conductivity, will burn on the surface before heat has penetrated deep enough. Forced cooling by water or compressed air is eventually
    dangerous, because it leads to the freezing of stresses (residual welding stresses), which put excessive load on the component and the material.
  • Pressure: The interweaving of the molecule threads must be activated by exerting pressure force, because the plastic melts are semi-liquid.
  • Time: Due to the very low heat conductivity of plastics, the duration of the heat input and cooling must be observed with much care. In case of holding too long the welding temperature application, there is a risk of thermal damaging; one must also consider the elongation or shrinking of plastics during heating or cooling, as they are much larger in comparison with metals.

Various types of Plastic, their full form & welding temperature

CodePlasticWelding temperature
ABSAcrylonitrile Butadiene Styrene662°F (350°C)
PC/ABSPolycarbonate/Acrylonitrile Butadiene Styrene662°F (350°C)
PAPolyamide (Nylon)752°F (400°C)
PBTPolybutylen Terephtalate662°F (350°C)
PCPolycarbonate662°F (350°C)
PEPolyethylene572°F (300°C)(hard)
518°F (270°C)(soft)
PPPolypropylene572°F (300°C)
PP/EPIDMRubber-modified Polypropylene572°F (300°C)
PURPolyurethane572°F/662°F (300°C/350°C)
PVCPolyvinyl Chloride572°F (300°C)(hard)
662°F (350°C)(soft)
GRP/SMCGlass Fiber Reinforced PlasticsN/A

Note: Only thermoplastics are weldable. Thermosetting plastic in cured condition cant be welded. Not all polyurethane (PUR) is weldable. Glass fiber reinforced plastics are not weldable.

Plastic welding principle

Plastics welding is achieved in three phases, surface preparation, application of heat and pressure, and cooling.

The plastics welding process involves the joining of two pieces of Thermoplastics at a heated state and under a pressure as a result of cross-linking of their polymer molecules. The plastic pieces are fused together with or without filler material. The weld joint forms when the fused parts are cooled below the Glass Transition Temperature (in case of amorphous polymers) or below the melting temperature (in case of crystalline polymers). Thermosets (thermosetting resins) in cured condition cannot be welded, since cross-linking of their molecules has completed already.

Plastic welding Processes:

  1. Laser Welding or Laser Beam Welding (LBW)
  2. Ultrasonic Welding
  3. Hot Gas Welding
  4. Spin Welding
  5. Vibration Welding
  6. Hot Plate Welding
  7. Friction Welding
  8. Frequency Welding.

Hot gas plastic welding process & its steps

Phase I: Pre-Cleaning the Plastic

For our first stage, we’ll define basic steps so that you understand how to set-up your weld and clean your plastic material. We’ll cover each step in more detail below.

Step 1: Create Your Workspace

  • Ensure you are going to weld in a well-ventilated area, having safety heat-resistant gloves and clear safety glasses. You are not required dark glasses as there is no welding arc.

Step 2: Clean the Plastic

  • Clean the plastic using some soap and warm water. Scrub away all the debris that you can with a sponge. If that isn’t enough, you can also wash the plastic with dish soap. Get rid of any dirt, grease, or other particles that might weaken the weld as you clean your plastic. After your plastic is clean, get a lint-free cloth and dry off the material to ensure there is no moisture.

Step 3: Select a Matching Welding Rod

  • Find out the type of plastic you are welding. Some plastics have letters on them that help identify what they are, like PE for Polyethylene or PVC for polyvinyl chloride. Select a welding rod that matches the plastic you are going to welding.

Step 4:  Remove Paint/ any coating with Sandpaper/Emery paper

  • Using some 80-grit sandpaper, remove the paint you find on the plastic. If you see that the area of plastic you want to weld still has color on it, remove the color by using sandpaper.

Step 5:  Clamp the Plastic Pieces Together

  • Clamp the plastic parts which are to be joined using a clamp or tape to hold the parts in position. Heavy parts can behold manually to have them in position before joining. Ensure the right fitup before tacking.

Phase 2: Combining Your Plastic Pieces Together

After we have done cleaning and preparing the workspace, proceed for joining the plastic pieces. Follow these steps:

Step 1: Preheat Your Welding Gun

  • During preheating the welding gun, we need to leave it on for at least two minutes to reach a hot temperature throughout the nozzle. However, depending on the type of plastic you are welding, you’ll need to set your temperature appropriately. Most plastics melt at temperatures between 392°F and 572°F. If you aren’t sure what temperature to set your welding gun at, refer above in this post for the welding temperature for various plastic grades.
  • Remember, don’t linger too long in one particular spot or you’ll apply too much heat, and possibly burn part of the plastic.

Step 6: Simulate a Pendulum Weld

  • Now you’ll need to move your welding gun from side to side, much like you would if you were performing a pendulum weld. When you do this, keep the nozzle about a full inch above the crack area you are welding. As you do that, tilt your welding gun using a 45-degree angle on the opposite side. Keeping the rod in place, now move the nozzle across it a few times to melt it. Then, move down the rest of the area to finish the plastic weld.  If you don’t own a plastic gun that also has a speed nozzle, then pendulum welding can help. That’s because pendulum welding can be done just using a propane torch. You can also use pendulum welding anytime you want to fill a problematic joint your speed nozzle can’t seem to handle or reach.

Phase 3: Finishing the Weld

  • When you are ready to finish your mold, you’ll want to start by allowing the plastic you’ve been working with to cool first. You need to wait for five minutes, possibly more, for this to happen.

Step 1: Fix the Plastic if Necessary, then Cool the Plastic

  • Wait for the plastic to return to room temperature before you finish your weld. You’ll notice right away that welded plastic cools much more quickly than welded melded. Still, you can wait as long as you feel it is necessary. Make sure you check to ensure that the plastic is turned solid. Also, wave your hands around it to make sure there is no further heat coming off of the plastic before you begin.  Also, if you need to fix a part of your weld, you’ll want to do that before the plastic cools off.
  • Check to make sure your welds look good and smooth. You will need to add more of the welding rod or use your gun to smooth the plastic out before you let it cool down.
  • At this point, you are ready to put your welding gun away. Make sure you store it in a safe place until it has cooled down.

Step 2: Sand Your Welded Joint

  • Start sanding down your welded joint using 120-grit sandpaper. With the sandpaper, smooth out any rough areas of the weld so that everything has a smooth, consistent look. You’ll need to use slight pressure as you do this and rub the sandpaper up and down the plastic piece. Do your best to make the weld look as level as the other parts of the plastic surrounding it. However, be careful not to scratch the plastic as you are doing this. You can also use a sanding wheel on a rotary if you have one.

Step 3: Finish the Product

  • Now you’ll need to get some 180 and 320-grit sandpaper as well. Start using the finer sandpaper options to make the weld look as smooth as possible.

Heated Element Welding

Usually, an electrically heated heating element serves as the heat carrier, which is furnished with an anti-adhesive layer made of PTFE, in order to avoid the adhesion of the thermoplastic material.
With direct-contact hot tool welding, the heating element is between the joint surfaces, whereas with indirect hot welding the heat is transferred through a joining part to the joint surface. Due to the poor thermal conductivity of plastics, the indirect hot welding method is only suitable for welding thin foils.

Heated tool butt welding

Heated tool butt welding can be carried out manually and by machine, whereby pressure pipes can be welded only by machine. The crucial points here are that the previously cleaned and planed weld contact areas of both parts to be welded are pressed against an electrically heated hot tool for adjustment.
Thereby the eventually existing uneven parts and scratches are melted. Subsequently, the joint surfaces are, with reduced pressure, placed against the heating element and heated for an exactly defined time interval.
Then the heating element is quickly removed from the weld surfaces and the pipe endings are quickly joined under a pressure of 0.15 N/mm2 weld area. The welded parts should remain under pressure in this joint restraint, up to complete cooling.

Hot gas welding

Whereas heating element welding is executed without filler material, hot gas welding uses a plasticized welding rod (for hard thermoplastics) or a welding cord (for soft thermoplastics) made of the same base material and is pressed into the welding seam. This is done, with a few exceptions, by hand.

Testing of Plastic Welds Joints

For the testing of plastic welds, it is specially referred to the leaflets DVS 2203 “Testing of welded joints of thermoplastic materials” and DVS 2206 “Testing of components and constructions from thermoplastic materials”. These leaflets contain detailed summaries of the respective standards and regulations.
These non-destructive test methods enable, however, only quantitative comparison statements (good/bad), but no quality statements for the long-term behavior of a weld. Plastics can, however, deteriorate over time. By defined long-term tests, it is possible for example to test the minimum service life of plastics under certain operation conditions for at least 50 years although the actual testing period is much shorter. For foil welded joints, special test methods are applicable, e.g. cleavage test by weight, cleavage test by spring, bending change test, a manual test of peel strength, etc.

The destructive test methods mentioned previously require a specimen from the component. This can be realized in practice by extending the weld beyond the required dimension. The excessive piece serves then as the test specimen. However, it is often the case that no test specimen can be taken from pipe joints. For a rough estimate, there are several testing possibilities that provide information on the quality of the weld by an appropriate experience of the tester. Suitable reference points result first from the welding protocol that must be completed by the plastics welder who is subjected to regular monitoring.
Additional information regarding the quality of the welded joint results from the compression tests and the visual checks.

Causes for defects in Plastic Welding

Plastic welding is also prone to welding defects similar to usual fusion welding processes. In plastic welding joints, the following are the factors that can cause the welding defects:

  • Inhomogenities: Material, moulded components (pores, cavities)
  • Joint preparation: Aligning of the pipe axis (joint surface misalignment)
    Parallelism of joint surfaces
    Pipe plug-in unit
    Clamping tools
  • Foreign material: Oil, grease
    Dust, sand
  • Welding parameters: Temperature
  • Joining conditions: Mechanical stress during the welding process

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