5.6 PLASTIC WELDING

 

• Well suited to joining of thermoplastics.

 

• Types of plastics used in welding are,

 

 

• Plastics to be joined should be compatible. A common method is based on trial and error testing.

 

• To determine plastic types burning small samples with a low flame gives the following observations,

 

 

• Welding of thermoplastics involves heating, contact, cooling and bonding.

 

• Joints may be weakened by incomplete fusion, oxidation or thermal degradation of the plastic.

 

• Melting may be done by,

- gas or electric gun

- heated tool

- induction heating

- friction

- spinning

 

• Sheet welding,

- heat and pressure are applied at an overlapped joint between thin sheets.

- rollers join the sheets (one roller is often motor driven and heated while the other just applies pressure).

- typical variables are,

- roller temperatures

- feed rate

- pressure

-underheating leads to a loose seam.

- overheating leads to a hole formation.

- parametric setting is very sensitive.

- welds can be done on tables with a hand roller and a heat gun.

- advantages

- simple tools

- disadvantages

- hard to set parameters

- preparation of welded sheets.

 

- if welded plastics to repair cracks, drill holes at the ends to stop crack propagation.

 

• Hot air/gas welding,

- Used successfully with molded parts in,

- PVC

- polyvinylidene chloride

- polyethylene

- acrylic

- polychlorotriflourethylene

- Operation steps,

1. pieces positioned but a gap of 1/16” left

2. a suitable welding (often same material) rod is pushed into the gap

3. a hot blast often 400-600°F is directed at the tip of the welding rod and surfaces to be welded. * if a torch to focus distance of 1-2” is used, a drop in temperature of 200°F will occur, the resulting temperature should be the melting temperature of the plastic.

-The final strength ranges from 50% maximum for high density materials, to near 100% for low densities.

- The heating guns are similar to common hair dryers with heaters and fans, and vents to control air flow rates.

- The heat calls for safety measures.

- Nozzles - a variety of nozzles and tools are available.

- Advantages,

- simple tools

- Disadvantages,

- welding angles hard to set

 

• Tack welding,

- parts are put in position.

- the gun temperature is allowed to heat up (a tack welding nozzle is used).

- the gun is put at an angle of 30-40°F to the weld and held in place until melting begins.

- the gun is slowly drawn along the seam.

- since the tack weld is weak (used for positioning) subsequent welding is required.

 

• General welding,

- operation

1. the gun is held 90° to the weld and a rod is inserted.

2. once the rod starts to melt, the gun is turned to a 45° angle and moved steadily along the weld.

3. The gun is moved in an elliptical path over the weld with an amplitude of about 1”.

4. The rod is forced into the groove with a pressure of about 3-6 lbs. This pressure prevents air from entering the weld. An angle of 45° to 90° is used for the rod.

5. When ending a weld, the heat is turned off, and after cooling the rod is twisted off, or for continuous welds there should be an overlap of 1/2”.

6. If required a weld can be restarted by cutting the previous weld at an angle, and starting from that point.

 

• Speed Welding

- the rod and gas are fed side by side.

- the rod is heated in the gun, and is “wiped” out as it leaves the gun.

- when starting pressure is applied to the rod and a sharpened tip is forced into the work.

- as the rod starts to melt, the gun is lowered to 45° and drawn along. The welding rod is pulled in itself.

- moving the tip too fast will result in beading and too slow will result in charring.

- the weld is stopped by standing the gun at 90° to the surface and pulling the gun off. The rod is then cut off.

 

• Tractor Welding (Machine Welding),

- a hot air gun and rollers are driven over a surface by motors.

- a tape can be dispensed that will join the sheets or the two sheets can be overlapped.

- advantages,

- fully automated

- easy to set parameters

- disadvantages,

- special equipment required

 

• Ultra Sonic Welding,

- basically a high frequency vibration is directed through a plastic joint. The vibration causes friction, and then heat, often causing a solid bond in less than a second.

- frequencies above 20 KHz.

- the distance the vibration travels has a great deal to do with determining the classification.

- very well suited to rigid thermo plastic parts.

- good designs make direct application of the vibrations possible.

 

- a smaller contact area increases the energy concentration. As a result V-notches, tongues, pins, and other special joints are commonly used.

 

- if remote sealing is necessary, thicker walls should be incorporated into the part design

- epoxy molds can be used to reinforce weaker parts when doing this operation.

- advantages,

- fast

- clean

- no extra materials needed

- disadvantages,

- tool design required

- simple design rules not always available

 

• Linear Vibration Welding,

- similar to Ultrasonic Welding, except that frequencies are about one hundred Hz and amplitude are mm.

- this is best used with high coefficient of friction, low viscosity plastics.

 

• Spin/Friction Welding,

- two parts are spun and the contact area builds up heat through friction and pressure. The pressure forces a good bond between parts and drives out bobbles.

- flashing may occur with this method.

- advantages,

- produces a good weld

- air does not enter during welding

- inexpensive machines, such as drill presses may be used

- disadvantages,

- circular weld joints are required

 

• Testing Plastic Welds,

- a handheld gun can be used to generate arcs. The sparks are generated with voltages up to 55 KV at 200 KHz.

- operation,

1. The gun is calibrated to spark at distances just over the weld thickness (to a ground plate).

2. the ground plate is placed behind the weld.

3. as the probe is moved over the weld, sparks will jump when a gap in the weld moves between the probe and the ground plate.

 

 

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