Omar Khalid
Plastic pipe welding is a technique that joins two units without the need for bulky or costly fittings. It is used in the construction of patient monitoring devices, diagnostic instruments, and machinery used in life-saving operations. Welding plastic gas pipes can be a tricky process that requires precise temperature control and special equipment. Various welding techniques are available, such as extruded bead welding, hot gas welding, hot plate welding, and induction welding. Each technique involves heating and softening the plastic before pressing the pieces together to form a strong and durable bond.
Characteristics and Values Table for Welding Plastic Gas Pipes
| Characteristics | Values |
|---|---|
| Techniques | Pipe welding, butt fusion welding, hot plate welding, extruded bead welding, hot gas welding, induction welding, high-frequency/RF welding |
| Use Cases | Construction of patient monitoring devices, diagnostic instruments, machinery used in life-saving operations, irrigation, plumbing fittings, water tanks, industrial parts |
| Benefits | Stronger end product, time and cost-efficient, ability to withstand harsh chemicals |
| Materials | Polyvinyl chloride (PVC), Polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE/FPA) |
| Tools | Soldering gun, heat gun, propane torch, hot air welders, conductive metal piece, high-frequency generator |
| Precautions | Precise temperature control, practice on scrap pieces, wear a mask to avoid inhaling smoke, avoid overheating to prevent plastic decomposition |
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What You'll Learn
Hot plate welding
The hot plate welding process can be divided into four phases: matching, heating, change-over, and welding/forging. In the matching phase, the geometry of the weld surfaces is matched to the theoretical welding plane. The weld surfaces are heated through conduction by physical contact with the hot plate. The hot plate temperature should be 86 to 212 °F (30 to 100 °C) above the melting temperature of the material being welded, and a constant pressure between 0.2 and 0.5 MPa is applied against the hot plate.
During the heating phase, the temperature of the plate must be high enough, and the time of this phase should be long enough to achieve a sufficient depth of melt. A brittle weld will form if the melt depth is too thin, and there will be excess flash and unfavourable orientation of molecules at the joint interface if the melt depth is too thick.
Once the weld surfaces have been sufficiently heated and melted, the hot plate is removed, and the molten plastic of the two parts is pressed together to complete the weld. The pressure during this phase must be enough to form a strong molecular bond and squeeze out any air to create an airtight seal. If the pressure is too high, too much molten plastic will be squeezed out, resulting in a weaker bond and deformed parts.
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Soldering gun use
Soldering guns are used for welding plastic pipes, especially those made from high-density polyethylene (HDPE) and polypropylene (PP). These types of plastics do not take glue well, so welding is the best method to join them.
A powerful soldering gun (also called a soldering iron) with a wattage of 50W to 75W should be used. It is important to wear protective gear and ensure proper ventilation when using a soldering gun. The tip of the soldering gun should be flat-faced, if possible. Before welding, the pipe should be pre-warmed using a heat gun on a low heat setting. This can be practised on a scrap piece of the same material as the pipe.
To weld the pipe, the soldering gun is used to melt the plastic along the joint. Consistent pressure should be applied to ensure a strong bond. Hot staples can be used to reinforce the weld, providing additional support and durability. It is important not to overheat the plastic as this can weaken the weld. Once the weld is complete, it should be allowed to cool naturally. Force-cooling the weld can cause stress and weaken it.
When welding plastic, it is important to stagger the start and stop points. The heat should be kept low enough to melt the bonding area without changing its colour. The weld will not necessarily flatten and will still have a rod/roll appearance.
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Induction welding
The welding apparatus used in induction welding contains an induction coil that is energised by a radio-frequency electric current, creating a high-frequency electromagnetic field. This field acts on the electrically conductive workpiece, generating resistive heating through induced currents known as eddy currents. The heat generated in the metal is then transferred to the surrounding plastic by conduction, softening it and allowing it to be welded.
To ensure a strong weld, it is important that the metal piece is sufficiently heated to soften the plastic. Once the plastic is softened, the two pieces can be pressed together and allowed to cool, forming a permanent bond. This process can be highly automated and is often used for long production runs, especially for welding the seams of pipes.
It is important to note that the depth of heating penetration is influenced by the frequency of the electromagnetic field, with higher frequencies resulting in shallower penetration. The composition and temperature of the metals being welded also play a role in determining the penetration depth. By controlling these variables and utilising a reliable power source, induction welding can be a highly effective method for joining plastic gas pipes.
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Extruded bead welding
To weld plastic gas pipes using extruded bead welding, follow these steps:
First, cut some strips of the same material as the pipe. These strips will be used as filler material. It is important to use a material that is compatible with the plastic pipe and that has a similar melting point.
Next, heat the pipe and the filler material. This can be done using a soldering gun, a propane torch, or a hot air welder. It is important to control the temperature precisely to ensure that the plastic does not decompose or become brittle.
Once the pipe and filler material are hot enough, use a welding rod and a handheld plastic extruder to apply a bead of very hot plastic between the two sections of the pipe. The plastic extruder forces the molten plastic out against the parts being joined, creating a strong bond.
After the bead of plastic has been applied, hold the two sections of the pipe together until the plastic cools and hardens. This will ensure a strong weld.
Finally, check for leaks by turning on the gas and inspecting the repaired area. If there are any leaks, they can be sealed using plumber's tape. It is important to follow safety precautions when welding plastic gas pipes, including wearing protective gear and ensuring proper ventilation.
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Hot gas welding
The temperature of the hot gas is carefully regulated to ensure that it is sufficient to melt the plastic without causing degradation or burning. This precise control over the heat and welding parameters enables accurate and controlled melting of the plastic surfaces. The choice of gas depends on factors such as the type of plastic being welded, the desired welding temperature, the need for an inert atmosphere, and the equipment available. For example, nitrogen, an inert gas, is often used as it does not react with the plastic material, helping to prevent oxidation or degradation during the process. Other specialised gases such as argon or carbon dioxide may also be used depending on the specific requirements.
It is important to note that hot gas welding is only applicable to thermoplastic materials that can be heated and melted repeatedly. Thermosets, the other group of plastic materials, cannot be welded using this technique. Additionally, mastering the art of plastics welding, including hot gas welding, requires significant education and continued on-the-job training to ensure that the welds are of high quality and meet the required specifications.
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Frequently asked questions
Plastic pipe welding is a technique that joins two units of plastic pipe, as opposed to syncing them with bulky or expensive fittings.
There are several types of plastic pipe welding, including:
- Extruded bead welding
- Hot gas welding
- Hot plate welding
- Induction welding
- High-frequency or RF welding
The tools and equipment needed for plastic gas pipe welding may include a soldering gun, a powerful soldering iron, a heat gun, a propane torch, and a high-frequency generator.
