How to Weld Plastic – Plastic Welding Basic

Plastics welding is the process of joining two pieces of Thermoplastics at heated state and under a pressure as a result of cross-linking of their polymermolecules. The work pieces are fused together with or without filler material. The joint forms when the parts are cooled below the Glass Transition Temperature (foramorphous polymers) or below the melting temperature (for crystalline polymers). Thermosets (thermosetting resines) in cured condition cannot be welded, since cross-linking of their molecules has completed.

Plastics welding processes:

• Hot Gas Welding
• Hot Plate Welding
• Ultrasonic Welding
• Spin Welding
• Vibration Welding

How to Plastic Welding Using Hot Gas Welding Method 

Hot Gas Welding is a plastics welding process, utilizing heat of hot gas stream. The gas (usually air) is heated by electric heating elements mounted within the welding gun. The torch (welding gun) directs the heated gas toward the work piece surfaces and a rod of filler material. The edges of the joined parts and the filler rod material are fused together and pressed. The polymer molecules are cross-linked when the work pieces cool down, forming a strong joint. Hot Gas Welding is manually operated process requiring high level of the operator skill. Some polymers (e.g. Low Density Polyethylene (LDPE), High Density Polyethylene (HDPE)) oxidize at incresed temperature therefore they are welded by hot Nitrogen.

Applications of Hot Gas Welding:

• Containers;
• Tanks for storage chemicals;
• Ventilation ducting;
• Tubes;
• Repair works.

Process parameter for hot gas welding

Process parameters Description
· Temperature- Temperature of hot gas
·Gas – Composition of hot gas(air, carbon dioxide, hydrogen, oxygen or nitrogen)
· Angle – Include angle between weldment and rod, angle between gas nozzle and weldment.
· Travel Speed – Rate at which weld is being deposited
· Weld force  – Amount of force applied to the filler rod
· Filler rod  – Composition of filler rod
· Gap distance –  Distance between gas nozzle and workpiece
· Weld joint  – butt joint and double strap fillet joint.

Welding Procedure in Hot Gas Technique

The hot air gun is used for hot air welding. The gun consists of a main body which contain heating element. This is anon contact soldering for high requirements. The air mass flow and the temperature can be adjusted in a prescribed range; the basic element of hot air gun is shown in the figure
2. The nozzles can be easily replaced so that each component is soldered by using the suitable nozzle, air and temperature setting.

As a manual process, weld quality is dependent on skill. There is no recognised non-destructive technique, that conclusively shows the presence of defects in plastic welds that could lead to weld failure. Therefore, it is recommended that good quality welder training is received and that welder certification is adopted.

Ultrasonic Welding
Ultrasonic Welding is a plastics welding process, in which two work pieces are bonded as a result of a pressure exerted to the welded parts combined with application of high frequency acoustic vibration (ultrasonic). Ultrasonic vibration transmitted by a metal tool (horn, sonotrode) causes oscillating flexing of the material and friction between the parts, which results in a closer contact between the two surfaces with simultaneous local heating of the contact area. The plastic melts in the contact area, the polymer molecules are cross-linked, forming a strong joint. Ultrasonic Welding cycle takes about 1 sec. The frequency of acoustic vibrations is in the range 20 to 70 kHz (commonly 20-40 kHz). The ampltude of the acoustic vibrations is about 0.002” (0.05 mm). Thickness of the welded parts is limited by the power of the ultrasonic generator. Ultrasonic Welding is used mainly for processing amorphous polymers (Polysterene (PS), Acrylonitrile-Butadiene-Styrene (ABS))

Applications of Ultrasonic Welding:

• Medical equipment (filters, face mask, valves, cardiometry reservoir);
• Automotive components (glove boxes doors, filters, valves, airflow sensors);
• Appliance (vacuum cleaner, steam iron, dishwasher components);
• Electrical equipment (switches, terminal blocks,connectors);
• Electronic and computer components;
• Toys.

Advantages of Ultrasonic Welding:

• Short welding cycle;
• Easily automated and controllable;
• Small amount of flash forms;
• Low energy consumption;

Disadvantages of Ultrasonic Welding:

• Only small and thin parts may be welded;
• Tool design is required.

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