Plastic Welding – Repairing Cracked Or Broken Plastic Items

Most people have had an experience with a broken plastic part such as a child’s toy, automobile parts, lawn furniture, industrial equipment, etc. Plastic as commonly known has a limited lifetime. How long a product made of plastic might last depends on the environmental conditions the parts are used in. Cold weather, improper protection from UV rays, and chemical contamination all have an effect on the life span of any plastic part. Such circumstances are not easy to change or control. Often damage can occur that is mechanical in nature. Plastics can break due to a hard impact, for example an accident can break your car’s bumper or your water tank can get blown off from its pedestal and hit the ground. Sometimes it can cost less to fix the broken parts than replacing with new ones or a damaged piece would be difficult to replace because it is ‘built-in’ to a complex system. An alternative could be to repair it.

Plastic Repair

When attempting to repair any broken plastic item the key factor is to know the material. The material will affect the repair process and method you choose. Thermoplastic and thermoset materials are entirely different from each other, thermosets cannot be welded, but most types of thermoplastics are. This leads to different types of repair. Since the basis of this article is about plastic welding it will discuss thermoplastics more than thermosets.

You can repair plastic made of Thermosets by using Phenolics or Epoxies, and use either fibreglass or carbon fibres as reinforcement to the repaired area.

Plastic Repair on Thermoplastics

Now, let’s take a closer look at how you can make a quality repair on any items made of thermoplastics. As always is the case with plastic repair you should first determine the type of plastic that needs repair. This is extremely important because if your welding rod is of a different material than the broken plastic material, the two will not bond together. Certain thermoplastic groups called “Polyolefines” have a very low surface energy making it very hard to bond them with an adhesive or glue. Simply said, nothing will stick to them. Some suppliers offer polyolefine adhesives that will produce a chemical reaction and increase the surface energy. This will last only a very limited period of time and therefore is not recommended. When it comes to plastic repair a proper weld on these materials would always be the best however, you can give it a go with the adhesive as well, to see what works best for you. There are other types of thermoplastic materials, which are perfect for using adhesives on.

Choosing the Plastic Welding Rod

If welding is your chosen solution for plastic repair, the welding rod needs to be of the same material as the piece that is damaged. Remember your welding rod can be of the same colour as the item that needs repair. Colour though is not an indication of the type of plastic material, the material itself needs to match. Occasionally you might find a stamp located somewhere on the part identifying the type of plastic.

Types of Plastic Repair

There are two different kinds of plastic repair commonly practiced. One type of plastic repair is a crack repair and the other type of plastic repair is to fix a broken part that is missing.

Let’s start with the crack repair first. Stress causes fractures inside the material. Stress can occur for numerous reasons from the manufacturing process to the storage, handling or even the use of the piece. You can prevent the crack from advancing any further by drilling a slightly larger hole (app 2 mm or 0.08 in diameter) than the fracture itself at the ends of all cracks. The round shape of the hole will distribute the stress more evenly and will help to stop further cracking. If the fracture extends up the entire length of the plastic part, a bigger gap appears. This shows another sign of stress development inside the plastic. When trying to do this type of plastic repair, new stress might emerge, so handle these areas with caution.

"V" grooving the fracture ensures superior weld strength since an increased welding zone allows the filler material (welding rod) to fill the gap much more effectively. The best tool to do this kind of preparation is a right angle stick scraper. The most suitable angle for an injection welder is of 80º to 90º and for hot air units it is 60º to 70º. After you have the tool, tack or clamp the parts into the appropriate position. Before starting any plastic repair, both the welding rod and the surface needs to be clean, oxidation, dust, grease and other contamination reduces the weld strength substantially. You can do this with a blade scraper.

Plastic Repair by Plastic Welding

Another crucial part in plastic repair is the weld temperature. Some plastic welders can say they can weld plastic much faster by cranking up the temperature. This makes it more efficient’. This might be correct, but when doing this they don’t think or are not aware of what is taking place inside the plastic. It is important to know that thermoplastics have a certain weld temperature range. Too hot or too cold can result in a poor quality weld. Temperatures above the maximum weld temperature will harm the molecular structure of the plastic. A mixing of molecules will not be possible anymore because the material has been destroyed. With a temperature too low, the welding rod and the parent material cannot mix properly. Even though it might stick, it cannot be called "a weld" because the material and its properties is a major concern. This commonly applies to plastic repairs as well as to all other plastic work-ups.

How much pressure is applied on the welding rod is very crucial when filling the prepared crack. The mixing of molecules will not happen if the pressure is too high or too low; hence some practice is essential. After you are finished with the plastic welding, smoothing down the weld bead to the surface to avoid new cracking areas. When possible, weld the crack from either side. This takes a bit longer, but will enormously increase the quality of the repair.

The same plastic repair process can be used for the repair of a plastic item where some pieces are missing. Areas with missing pieces normally show irregular shapes. Often it is easier if you cut out the entire damaged area in a shape that is round, square or rectangular. Square and rectangular shapes should have rounded corners to make it easier to weld. A good fit of the replacement piece makes the repair process simpler, quicker and stronger. A material of the same thickness will avoid weak spots in the repair as well. Irrespective of whether the replacements were cut out of sheets, pipes or other damaged parts it is very important that the material, the thickness and the shape fit the missing area.

Involving the right plastic welding equipment can make a big difference in the quality and appearance of the section repaired. Most widely used plastic welding equipment for plastic repairs are the hot air plastic welder or the injection plastic welder. Hot air plastic welding units are relatively cheaper, however their use requires quite some skill to make a strong weld.

Plastic Welding On Thermoformed Parts

When doing plastic welding on thermoformed (vacuum formed, press formed) parts, troubles might occur when the heated air hits the surface and starts spreading in all directions. It is almost impossible to control this airflow. Thermoformed pieces pack a high amount of stress in the formed areas. When the hot air reaches these spots, it might burn a hole in the section or will cause it to deform. This is where the advantage of an injection welder comes into play. An injection plastic welder uses a heated aluminum tip instead of hot air to melt the parent material. The heat spread is localized only to where the tip touches the weld area hence deforming or holes will not occur as easily. The quality of the weld will show a higher strength.