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Surface Preparation - Plastic Welding

Surface Preparation and Tacking - plastics repair welding
Techspan's Tech Tips will help guide you in achieveing a good understanding of the Plastic Welding process.  Each month we will provide you with new tips and techniques on how to get your Plastic Repairs right first time. Today we are looking at "Surface Preparation" and "Tacking".

Surface Preparation:
As with all forms of welding, cleaning of plastic components prior to welding is as important as any other welding preparation in achieveing strong, structural welds. Poorly prepared plastic components will lead to weld failures. Plastics surface preparation involves ensuring all soils are removed. Any paints, oils, greases and even UV degredation must first be removed prior to welding. With large parts, eg. Bumper bars, washing the part is usually a good start. This will ensure we are removing any atmospheric soils, eg. Mud, Traffic films etc . Use a soft sponge with warm water - don't use any industrial detergents as these can also introduce film coatings that may also effect the welding results. Any painted surfaces should also be cleaned right back to the parent material.

Surface Preparation - What tools do I use ?:
Paints and other remaining soils should be removed easily using a standard abrasive wheel (sanding disc) with any high speed air drill or electric drill will do the trick. A grinding disc will make quick work of this process, but be warned - plastics yeild easily to abrasives, so take care you dont end up damaging other sections of the part where the wheel can 'catch' and 'drag' across the component.

 
VIDEO - Preparing the plastic surface

This process can create a fair bit of dust, so we recommend the use of a face mask when performing this task. Other methods for preparing plastics include - paint scrapper, sharp blade, clean wire brush and even a solvent. These other tools can also be employed for assisting in your plastics preparation - Note: the least used method is that involving solvents. Solvents can assist you in removing greases & oils, however different plastics will also react differently to a range of solvents - so finding a suitable solvent for the specific plastic you are working with may be more trouble than its worth. Imagine attempting to clean a high gloss plastic part with a solvent that reacts with the plastic making it sticky and dissolving the paint - so my advice is not to use solvents. Not only are they harzardous to store, but can be harzardous to use and even more harzardous to your plastic parts. Once the part has been properly cleaned are prepared, we are now ready for the "Tacking" step.

Tacking:
Welding of plastics components relies on a number of factors -
- Surface Preparation
- Good joint interfaces
- Plastics Identification / Correct welding rod selection
- "V" Grooving, using the rotary burr
- Tacking
- Correct welding temperatures
- Welding technique
 
A Rotary Burr is used to provide a "V" groove along the weld line.

 
VIDEO - 'V'ing out of the welding line

Tacking of plastics components prior to welding, helps the welder to achieve two things -
1. Good joint interfaces and
2. Alignment of the plastic components

The "Tacking Nozzle" can be used to ensure close joint interfaces and alignment of broken parts.  Unlike metal welding, where tacking is carried out at the ends of the welding area, Plastic Welding requires "Tacking" to be carried out where ever you intend to place welding rod.

 
VIDEO - Tacking of the weld area

So if you have a crack of 100mm in length, you must "Tack" along the entire length (100mm) of the weld area. The "Tacking Nozzle" is an easy 'push-fit' onto the Techspan welding tool. It heats up to the corresponding temperature set on the welding tool, then simply run the tip of the tacking nozzle along the welding area. You will notice this will provide a slight "swagging" together of the two materials - closing up any gaps in the weld area.  If you notice any "un-tacked" areas, simply run the tacking nozzle over the weld line again, until there are no visible gaps between the mating surfaces. 

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