How to Weld Plastics Together

Plastic Welding or Hot Gas Welding is a process of applying hot gas using a welding gun on a broken plastic surface, making it soft and applying pressure or weld rod to form a common joint. The result is a stronger product improving its anti-tampering degree and eliminates the need for replacement. Welding, in general, saves a lot of money and has application in almost all streams of Manufacturing industry raking economic benefits to distributors and clients.

Plastic welding is not a complicated process unlike other welding operations nor requires heavy electrical devices, can be performed by non-professional welders. There are some important precautionary steps that must be kept in mind when handling the hot weld gun and fragile materials. 

As we move to the actual process, ensure the workplace is safe and restrict others as a part of initial safety measure. Plastic weld is relatively less risk and simple in operation. Follow Critical safety precautions to make the process more fun working with . Keep in mind to protect firstly, your body [wear protective gloves, glasses, and gear] and second, wielding gun and peripherals [Read the safety manual and instructions from the tool].

Take care not to damage the floor or table you are working on. Always keep the weld tools in a safe position during both operation/idle mode, the hot air from the nozzle must not come into contact with body parts, inflammable objects, electric wires, and devices.

 

STAGE 1

Initial Set up and Inspection of Plastic

1.  Select an Optimum Space with adequate aeration and ventilation. 

 Plastic welding involves micro risks to be cautioned of, indoor place of operation is not advised. Select an outdoor location and in an unavoidable situation like indoor, make sure the place is sufficiently ventilated. Make room for ventilation by opening the windows and doors. After the workplace set up, put on the protective gear, gloves, and glasses. Using microfiber mask, safety goggles made of Trivex or Polycarbonate can help check hazardous situations. Working indoors in summer or humid seasons with hot weld gun may take a toll on body temperature and concentration. Taking regular breaks, hydration can help lessen the effect.

  • Request other people to leave the place of operation by politely explaining the situation.

2. Use experts advised Industrial protective apron and anti-heat hand gloves.

Covering your body with protective clothing ensures safe working conditions, using industrial leather hand gloves gives a good anti-thermal layer to your hands when working. In case of unconventional welding operations mind wearing a helmet visor to protect from accidents or splashing of heat waves on face or hair.

  • Custom made visor for plastic weld can be preferred, conventional huge helmets may not be useful in this operation. 

3. Prepare the surface of the plastic by cleaning it with soapy water to remove dust and residues.

Preparation of the weld surface is the most important pre-requisite of the Plastic welding process, any negligence will lead to weld failures. Plastic surfaces are prone to accumulation of dust, soil and surface corrosion as they become old. They may also contain layers of oil, grease, paints or chemical which must be thoroughly removed. The surface erosion should be leveled using sandpaper on thin plastic material and sanding disc (abrasive) on thick plastics. 

The next step is to clear the debris off the surface by rinsing the layer with water and low concentric detergent mixture, it must be noted that industrial quality chemical or detergent agent may introduce chemically reactive agent on the surface affecting the welding process. After washing the surface with water, use a piece of cloth or towel to wipe the surface thoroughly making no room for moisture on the surface.

  • The sanding disc must be used delicately without affecting the structural bond of the plastic,  air drills or electric drills can also prove handy in such a situation. The use of abrasive should be carried under the supervision of experts as it may damage the structure of the plastic.
  • Using solvents for surface preparation is not advisable, it may remove the soil and dust off but tends to react with plastic.
  • In the case of strong stains, using a liquid solvent like methyl ethyl ketone in the right amount to get rid of such stains.

4. Select the matching welding rod with the help of a letter identification marker on the plastic. 

Plastic products generally come with letter identifiers imprinted on their surface, every plastic product differs in their mechanical, thermal and chemical properties. It should be taken care of to identify the correct plastic-type and select the compatible welding rod. Plastic weld is similar to a conventional welding process wherein the chemical reaction is only on surface-level using thermal energy in bringing the plastic to its melting point.

Letters like PP (polypropylene), PE (polyethylene), and PVC (polyvinylchloride) signify the type of plastic. Select the appropriate welding rod matching the plastic-type.  The primary function of a welding rod is to connect the broken surface by forming a homogeneous bond between the two. 

  • For instance, plastic weld on polyethylene (PE) supports polyethylene welding rod. While it is common to weld a minor crack or fissure by melting regions around it and conjoining the surface without using a welding rod. This requires expertise but it is advisable to carry a welding rod for the purpose.
  • In the case of the non-visibility of the plastic identifier marker, it is advisable to seek the assistance of experts.

5. Find the type of Plastic using a Welding rod testing kit when uncertain.

Welding rod testing kit comes handy with possible welding rods of different plastics. It can also be selected by closely examining the rod based on their resemblance and common mechanical properties. To begin with the test, heat one end of the rod and try to fuse it with the plastic surface. If both stick to each other, it implies made of the same material and plastic weld can proceed. If the materials are not the same, the surface will not fuse or join with the welding rod. This trial-error method requires patience and carefully pulling off the welding rod from the plastic surface is important to avoid further widening of the crack.

  • Testing and finding weld rods can be time consuming as in some surfaces only one rod has a chance for compatibility. It is better to find the type of plastic with the help of a letter identifier. The properties and type of welding rod are detailed in the test kit manual which can be referred.
  • Welding rods and testing kits are readily available online and in retail stores. 

6. Scrubbing the Plastic Surface to remove Paint layers and residues using Sandpaper.

The weld area is required to be clean and chemically inert, it’s important to remove the paint layers and any residuals which may affect the quality of the plastic weld. Scrubbing can be done using 80-grit sandpaper by gently rubbing back and forth across the region. Care should be taken not to expose the layer underneath the plastic.

  • In the case of bigger and tougher materials, abrasive disc instead of sandpaper or even tools like mild chippers or a paintbrush can be used. The abrasion should be mild without disturbing the crack or the material.

7. Fix the Plastic Material or Pieces using Clamps and Tapes on a Platform. 

The material or pieces must be joined with each other before the weld. Fix the plastic pieces on a table or mount without leaving any distance between the two. The pieces then must be pinned to the platform using C-clamps. Further, foil tape can be wrapped covering the material or pieces in such a way that they are close. Do not cover the area that needs to be welded.     

  • The plastic pieces must be maintained in the right position and secured as close as possible for a good weld. The static position of the pieces or materials gives less room for a bad weld.

STAGE 2

Welding the Plastic

1. Preparing the Welding Gun and Pre-heating. 

The welding gun should be checked for any blockage in the air vent, monitor the peripherals before switching on. The unit must be connected to a properly earthed switch unit. The ideal temperature is between 200-300° C (392 and 572° F), any deviation from this range will either burn the plastic or melt the plastic incompletely dampening the weld quality. The gun should be preheated in the given temperature range for at least 2 minutes. There are different temperature settings for different plastics, it is important to garner the respective temperatures.

  • For instance, the ideal temperature for propylene and polyurethane is 300°C (572° F). 
  • The heat setting for PVC plastics should be set at about 275°C (527° F). 
  • The temperature settings for polyethylene should be around 265° C (509° F).

2. Weld the ends of the material using the Tack Welding method. 

Tack welding is done on the ends to ensure pieces stick together throughout the welding process, it can be done by melting the loose plastics at the end of the joint. Using the tack welding nozzle, apply light heat on the piece. The piece will start melting after a few seconds, enabling elasticity which is the right time to join both the pieces.  A good tack will maintain rigidity throughout the welding process.

  • Identifying and installing tack welding nozzle is easy, it is differentiated by a tube-like structure with the fin at the end. 
  • Tack weld joints differ for each material, smaller pieces can be joined by a tack at the ends whereas larger work-pieces should be tacked repeatedly at appropriate distances for better stability and rigidity. 

3. Condition the welding rod by trimming its end. 

Trimming of welding rods is crucial for accurate weld, more importantly in smaller and thinner materials. The rods can be trimmed using pliers by giving a diagonal cut at the end. The process should be repeated until the point gets honed properly. Knives or hardened blades can also be used to scrape the welding rod.

  • The pointed end of the welding rod gives a better chance for smooth and steady weld, eliminating unwanted residual accumulation.
  • It is important to take precautionary steps when removing the nozzles as it retains heat, the nozzle should be removed once the welding gun cools off. 
  • Make sure the gun is preheated again before the actual weld process begins.

4. Install the welding rod on the gun. 

The selected welding rod after its end gets trimmed should be inserted into the gun through a speed nozzle opening. The opening holds the rod along with the weld nozzle so that the heat fuses both the surface of the plastic and the rod.  The nozzle unit can be bought separately if not included with the welding gun. The nozzle comes with two-hole, dedicated for the gun and weld rod separately. The rod with its clipped end should be installed into the second hole.

  • Care should be taken after the tack is applied on the surface as it can be hot, let the nozzle cool or change the nozzle with the help of thermal proof gloves or pliers.
  • Alternatively, the weld can also be done without in inserting the welding rod into the nozzle opening by a process called “Pendulum Welding”, it needs extra expertise but proves effective for tight spots or welding smaller region.

5. Hover the heat gun gradually over the surface. 

Begin the process of slowly applying the heat from the top reaching downwards or the direction that seems comfortable to you. The gun should be held at approximately 45-50 degree angles for better visibility of the weld region and stability. When the hot gun air comes in contact with the plastic, observe the region to see if it starts to melt. As the surface starts to melt, gradually hover the weld gun along the crack or joint while feeding the rod using the other hand. 

  • The best practice for achieving consistent welding is to move the gun keeping a constant pace. The temperature exchange should be done only to melt the plastic and the rod, not high and intense enough to burn them.
  • In situations like overheat or over melting of pieces, immediately hover the gun through the weld region at a faster pace. Care should be taken that the heat doesn’t pass to other parts of the plastic that can result in deformation or discoloration.
  • Avoiding air bubbles along the weld path ensures better weld quality and a stable surface level. It is better to avoid the accumulation of a weld rod feed in one spot.

6. Pendulum Weld and Important steps to be followed.  

 If you are following the pendulum method, it is important to keep few things in mind, the weld gun should be tilted at an angle of 45 degrees at a distance of 2.56 cm from the crack or weld region. Simultaneously, place the welding rod in the opposite direction at an angle of 45 degrees over the crack, meeting at the point of the gun nozzle. When you are ready to weld, wave the nozzle back and forth repeating until the spot reaches the melting point. Repeat the same process as you move down. 

  • Pendulum weld is an effective technique when dealing with smaller pieces or tight weld spots, also helpful in case if your weld gun doesn’t support or has speed nozzle. It can be performed using a simple propane torch.
  • This technique needs the synchronization of both your hands.
  • The hot wave should be hovered at a constant speed to prevent burns or hyper melting of plastics. You should be able to swing your weld gun back and forth to focus mild heat on the weld spot. 

STAGE 3

Finishing and Post Plastic Weld Process

1. Allow the plastic to cool for a few minutes

Let the weld piece be brought to the room temperature before any post-weld process. The plastic will cool down within minutes depending on the room temperature and ventilation. Welded plastic turning solid is an indication to proceed further works. Thoroughly check the region to ensure a stable and solid-state of the plastic.

  • Perfect weld has a consistent surface build with a smooth texture. This state can also be achieved using additional weld rod feed and tampering with the help of a welding gun.  
  • The next step is to securely rest aside the welding gun after all the work. It still carries heat and stationing it on a heat resistance holster is a safe practice. Let it come to room temperature.  

2. Scrub the welded joint using Sandpaper.

The weld region contains an uneven concentration of plastic and weld rod feed, it also contains air bubbles leading to the formation of rough ridges. These ridges can be smoothened out using 120-grit sandpaper. The sandpaper should be applied to the weld with pressure good enough to remove the ridges and look level with the surrounding area. This should be done without disturbing the areas adjoining the weld.

  • A faster solution is to use a rotary tool fitted sanding wheel that can do the trick on hard and rigid plastic surfaces, this is not advisable for thinner plastic materials. 

3. Fine-tune the weld surface using 320 and 180 grit sandpaper.

After removing the ridges, the sandpaper of finer grits should be scrubbed on the surface to make the spot smooth and shiny. Less abrasive and higher grit sandpaper is an ideal tool for this process. The surface should be rubbed with sandpapers until the chips and small ridges on the weld spots disappear to give a smooth texture.

  • Selection of appropriate sandpaper is important for the post-weld process, lower grit sandpaper is good for wearing out ridges right after the weld, while higher-grit sandpaper is best for surface finishing.
  • Scrubbing should be only limited to weld regions, enough care must be taken not to wear other regions.