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THE BASICS: Plastic Welding

Plastic welding is a huge domain and certainly too large for the scope of this article. Therefore, I will limit my discussion to the types of plastic welds that are generally available to plastic fabricators. This article will give you the basics on plastic welding. It will assist you in your choices about the welding needs within your company.

Some people think plastic welding is an easy operation. This might be so, but there are some things to consider if you want to make a quality weld. First, not all types of plastic are weldable. Thermoset plastics (which do not melt when heat is applied) are not weldable. Thermoplastics (which melt when heated) can be welded numerous ways. Second, only plastics of the same type are weldable. Each plastic type has it is own molecular structure and weld temperature. Therefore, you can not weld together different types of plastic (such as polyethylene and polypropylene).

Before you start welding with most welding systems, surface preparation of the plastic is essential. This time-consuming work is necessary for a quality weld. Surface preparation reduces surface oxidation and other contaminants such as grease, dust or damage. Some people forget this important step, but with most systems, a quality weld needs surface preparation. To prepare the surface for welding, take a scraper and remove the first layer of the material. Keep this area clean and dust-free at all times and prepare only the area where you are working on.

Numerous welding systems are on the marketplace. There is no "be all that ends all" welding system. Each fabricator has to establish the type of weld that his facility needs to make. The following list identifies and explains some of the available welder types:

Electric socket: Use electric sockets to weld plastic (such as polyethylene pipes) together. Electric power heats up a copper wire inside the socket. The plastic melts and expands. The pressure of the expansion joins the pieces.

Ultra sonic: Ultrasonic welding uses an acoustic tool to transfer vibration energy through the plastic parts and into the weld area. The friction of the vibrating molecules generates heat, which melts and welds the plastic. When the plastic reaches a molten state, vibration stops. Apply pressure while the molten plastic solidifies. Use this system is to weld smaller manufactured parts together.

High frequency: High frequency welding is similar to ultra sonic welding except that HF welds are continuous and sonic welding is like a stamping process. The typical use for these kinds of welders is welding thin materials. (Think of a plastic sewing machine without a needle).

Hot plate: With Hot plate welding, bring the plastics together against the plate and hold them for a certain length of time and pressure. Plastic parts melt on both sides of the plate. Separate the parts, and remove the heat plate. Bring both plastic parts together again with a certain amount of pressure until they are cool. With welds of this type, it is important that both parts have the same shape (like pipes with the same ID and OD). They must also fit together very well for a quality weld.

Spin: Spin welding joins round plastic parts by bringing the parts together, with a circular, spinning motion. Hold one part still and rotate the other. The friction between the two parts generates heat that causes melting and fusing (when cool).

The above systems have some application in the plastics fabrication industry but more common in the fabrication industry are the following group of welders:

Hot air/Hot gas welding: This type of welding uses hot air (or gas) to pre-heat the plastic surface and melt the welding rod. The most common types of welders are stick and extrusion welders. Oxidization on plastic can make welds weaker. If oxidization is a concern, hot gas (such as nitrogen) may replace hot air because the moisture in the air may increase oxidation. With respect to temperature consistency, hot gas welders have the advantage over hot air systems.

Extrusion welding: An extrusion welder has a screw (auger) mechanism, which conveys rod or pellets (depending on welder size), through a barrel. Inside the barrel the plastic mixes, compresses, and melts. The auger pushes the melted plastic out of the welder through a 'shoe' and creates the weld bead. The extrusion welder pre-heats the weld area with hot air or hot gas.

Injection welding: This unique welding system uses a combination of heated tip and injection pressure to form its welds. The hot (interchangeable) tip melts the surface of the plastic and creates a weld zone into which molten plastic is injected. There is an actual physical mixing of the weld bead and the plastic. When cool, the weld is the strongest in this group of welding systems. Because the tip cracks the surface of the plastic, there is no need to prepare the surface unless heavy contamination exists. This saves considerable time for the fabricator. Drader Injectiweld manufactures an injection-welding gun. It is excellent for fabricating, prototyping, and repairing a wide range of thermoplastics. The interchangeable tip gives the Injectiweld versatility. Weld bead sizes vary between 1/16" and 5/8". Drader is introducing its latest hand-held welder at the NPE in Chicago.

Please remember that the above explanations are very basic. Contact the author if you wish to have further assistance with a particular welding process.

There are many welding processes in the marketplace and with each one, there are three parameters to consider.

1. Heat

2. Pressure

3. Time

Heat: Each plastic melts within a certain temperature range. Stay between the minimum and maximum weld temperatures. When you drift outside this zone, the weld quality diminishes.

Pressure: Pressure allows the plastic molecules of the materials to mix. With plastic welding, the best bonding occurs when there is an actual physical mixing of the plastics. Pressure, when too high or too low, reduces the quality of the plastic weld.

Time: Plastic needs a certain time to melt and a certain time to cool down. Give the material this time. You should not speed up the cooling time. Do not cool down the weld with water or cold air unless you want a large amount of stress applied to the weld bead.

Some think plastic welding is easy, but there are many basic considerations to make in creating a quality plastic weld.

Written by Andy Bramer, Plastics Technologist for Drader Injectiweld, manufacturers of plastic welding systems.

For more information, contact the author at 780-440-2231, 800-661-4122 ext. 236, E-mail: abramer@drader.com, Web: www.drader.com.

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