Plastic Welding Experiences In the past I have advocated plastic solvent welding with ABS solvent to rejoin ABS plastic parts without repainting. Solvent is inexpensive but it does takes considerable time for a solvent weld to become strong. Plastic welding is well known to be the strongest ABS repair returning the material to original strength but it is usually taken for granted that repainting will be necessary. These are experiments that have been done with plastic welding to show that it can be done without repainting. Plastic welders come in two general types. Hot air welders and soldering iron type welders. I tried both but a soldering iron or soldering type welder is by far the easier to use if repainting is not desired. This appears to be true for several reasons. Below is a photo of a Weller 60 watt soldering iron that has been used to plastic weld a fair number of ABS panels without repainting. The low power of the iron helps to prevent overheating. Figure 1 Figure 2 shows a hot air welder which uses an 800 watt adjustable heating element and a compressor to set the air flow. These two adjustments require a thermometer to set the actual welding temperature for the type of plastic being welded. One advantage of this type of welder is that 800 watts can create a high enough temperature to weld nylon at about 752 degrees F.
Figure 2 The disadvantage is that the heat is spread over a larger area which is good for production work requiring speed but is a disadvantage when the opposite surface is coated with paint that is not to be damaged. Experiments with this type of welder revealed that when attempting to heat a crack in plastic, the area heated is sufficiently large that distortion of the plastic can take place due to the cracked plastic having acquired internal stresses that suddenly cause the heated plastic to to move unpredictably. Here is an example of a fairing that was heated with hot air from the back side but the front side suddenly puckered causing the cracks to open up even though the paint was not damaged by the heat. The entire area around the cracks became soft like putty and quite pliable so that sections could be moved with fingers. Plastic is a poor heat conductor so it retains heat internally for some time allowing the plastic to be manipulated to some extent in and out of the picture but it is not easy to move the sections in the plane of the photo to close the gaps that opened up. Figure 3
The plastic welder shown in figure 4 is made by the same manufacturer. It comes with stainless steel mesh which will be very useful. Based on the specifications of the manufacturer, this soldering iron type of welder should not work on ABS plastics due the specified maximum tip temperature of 525 degrees F. which is only 274 degrees C. The typical melting temperature of ABS plastic is 350 degrees C. Figure 4 A test with the welding rods shown in figure 6 revealed that this welder did not have any problem with melting the ABS rods. A subsequent measurement of the tip temperature of this welder showed that its actual tip temperature was 383 degrees C. or 721 degrees F. which is nearly 200 degrees F higher than specified. This is very fortunate since the shape of this welder's tip lends itself to reshaping cylindrical welding rods into shapes such as required for rebuilding broken mounting tabs as will be demonstrated later.
Figure 5 Figure 6 The price of the welder is normally about $20 but it can be found on sale for as low as $12. The disadvantage of this welder is that the tip is not replaceable. The tip is also made of soft metal that will easily bend if it is used to apply any significant pressure to the work. The reviews on this welder show that many users have broken the tip only to find no replacement part available. In the description of the experiments below it will become apparent that the broad triangular foot of the plastic welder gives it advantages over the simple soldering iron that will speed work.
The Experiments Trench with back fill Using the tip of the soldering iron push it into the material to create a trench as shown in figure 7. This tacks the two pieces together. Then back fill the the trench by moving the material that was forced out of the trench in the first step back into the trench. I found making circular motions while over the area worked well to refill the trench. Keeping a finger on the weld seam on the opposite side is a good way to determine if the plastic is getting too hot. If you cannot keep your finger on the seam it is too hot. This procedure does not take any welding rods at all and can provide sufficient strength for parts that are normally not stressed. It is not as strong as the original material because the trench is typically from 30 to 50% of the depth of the material to avoid damage to any paint on the opposite side. Figure 7 Trench with welding rod back fill This technique is similar to the above but instead of back fill with the material forced out of the trench, a welding rod is heated and pushed down into the trench. This technique can approach the strength of the original material even welding from only the unpainted side since new material is being added on the back side making the overall thickness as thick or thicker than the original plastic. With a soldering iron, circular motions around the end of the plastic welding rod will melt enough to fill the trench. A true soldering iron type plastic welder like the one in figure 4 makes this go much faster. Instead of swirling motions to melt off the end of the rod into the trench, the plastic welder's broad triangular foot can be used to melt a good section of welding rod and push it down into the trench.
Figure 8 Trench with welding rod back fill and with fiberglass mesh Fiberglass is added to improve the strength of the first and second technique. The tip of the welder is used to push the fiberglass cloth into the ABS material to give it greater strength. This technique can be very useful if the total thickness of the welded part cannot be thicker than the original part since the fiberglass can be embedded into the original ABS plastic by pushing it below the surface on the back side. The following four photographs show the fiberglass used and how a small piece is slowly pushed with the soldering iron tip below the surface of the ABS plastic. Figure 12 shows the depth the fiberglass was embedded and what happens when the joint is tested to failure. While strong it is not as strong as the original ABS plastic since the failure happened at the same joint. Figure 10 Figure 9
Figure 11 Figure 12 Trench with welding rod back fill and with aluminum screen mesh This technique is similar to the above with the substitution of aluminum mesh available from most hardware stores for window screen. The aluminum works well because as the tip of the welder heats the aluminum, aluminum conducts heat well acting as an extension of the welder tip making it very easy to embed. Its strength is similar to fiberglass except that it does not work as well in applications where the plastic part might be constantly being flexed since the aluminum fibers will work harden and crack. If the plastic is stressed so that it cracks with aluminum mesh embedded in it, the aluminum fibers will stick out of the break as shown in figure 15 which can be painful if it is accidentally grabbed. Figure 14 Figure 13
Figure 16 Figure 15 Trench with welding rod back fill and with steel chicken wire screen Chicken wire is very coarse screen but is steel making it stronger than fiberglass or aluminum. Due to its coarseness, the plastic can crack but the steel fibers hold the pieces together. In the test to failure shown in figure 19, the chicken wire was actually cut with diagonal pliers for the photo because some strands of chicken wire did not break when failure occurred. Figure 18: Figure 17:
Figure 19: Trench with welding rod back fill and with stainless steel mesh Stainless steel is about five times stronger than aluminum so this technique provides a very strong weld. It has the same disadvantages as aluminum mesh in that it can work harden if the part is subject to a lot of bending. This material came with the welder in figure 4 but is not easy to find in a store. Online prices for a similar size piece of material that came with the kit cost almost as much as the entire kit. Forming ABS rods into shapes for rebuilding mounting tabs on fairings This is possible due to this welders broad triangular shaped foot. Welding rod can be cut into short lengths as shown in figure 21. The ends are tacked together to hold them in position. The foot of the welder can then be placed over the rods to press them flat into any desired thickness to create flat pieces suitable for rebuilding mounting tabs. Figure 20 below shows a broken headlight mounting tab. The formed ABS flat piece is altered with the tip of the welder to fit the space to be bridged. Just heating the sides of the replacement section and the sides of the opening then placing the replacement section into the opening is sufficient to tack the piece in place for it to be welded.
Figure 20: Three pieces of ABS welding rod are cut to form the missing triangular section as shown below. The base plate is the sheet metal plastic welder holder that came with the plastic welding kit shown in figure 4. Figure 21:
The three pieces are tack welded on the left side to hold them together. Figure 22: Then the triangular foot of the welder is placed over the top and used to flatten the rods to the proper thickness. Figure 23:
This is done on both sides as shown below. Figure 24: Next the angled edge is formed to conform to the matching edge on the headlight tab. Figure 25:
Then the two edges are heated and the formed part attached temporarily as shown below. Figure 26: A pair diagonal pliers is used to cut off the excess material. Figure 27:
A groove is dragged in the area where the two pieces are joined and any voids filled with welding rod. Figure 28: It looks homely but it is strong. Figure 29:
The discoloration comes from ABS material left on the welder tip that is burnt black. A steel wire brush comes with the welder and does a good job of cleaning off this residue. Figure 30
Next three pieces of ABS rod are formed to create the face for the mounting tab. Figure 31 The front piece is formed and tacked into position as before. Figure 32
Then welded as the prior triangular piece. Figure 33: Again, the piece is trimmed to size with diagonal pliers. Figure 34
Figure 35 shows the final result. It is not pretty but it is strong even without stainless steel mesh reinforcement. The spring clip was put in place to locate the hole that needed to be drilled. The replacement section is a bit thicker than the original material so the the spring clip fit very tight. In spite of that, the spring clip could be pulled on and off with locking pliers with no ill effect. Figure 35 This headlight mount included a threaded brass ferrule that is serrated on the outside with a solid bottom which was probably pressed into the ABS originally. They had not pulled all the way out but were loose so that the ferrule rotated in the plastic as the screw turned. They were removed by placing a screw in the ferrule, then heating the screw and ferrule until the hot metal parts melted the ABS and released the ferrule. Originally, I tried just heating the ferrule until the plastic softened to see if the plastic would flow around the ferrule and lock it in place without removal but it did not work. What did work was removal of the ferrule, then filling the opening with small amount of ABS welding rod but not enough to close off the hole, then using the foot of the welder to heat the ferrule until it melted the ABS enough to slowly sink down into the post on its own.
Figure 36 Restoring ABS screw holes This is a problem that can occur when a self tapping screw is directly screwed into ABS. Sometimes the post cracks or the threaded part of the post has given way from over tightening or having the screw pulled out by force. Below is an example of this type of hole. Figure 37 Three methods were tested in restoring this type of hole. The first was to externally weld any visible cracks in the post, then use the welder tip to try to force as much welding rod as possible into the hole.
Figure 38 This worked only to a limited extent. The welding rod is larger in diameter than the screw hole so these tests indicated that ABS plastic could only be forced about 1/4 into the hole. This limited the restored strength to 1/4 of the opening. While it worked, the screw could not be tightened as much as with an undamaged hole. Next, more ABS welding rod was forced into the hole using a drill bit of smaller diameter than the hole by heating the drill bit shank until it became hot enough to melt and push the plastic down further into the hole. Figure 39 This worked better but was very tedious.
What worked the best was simply to drill out the damaged hole to the same diameter as the welding rod and then heat the rod while inserting it into the hole. Figure 40 Next a new hole is drilled in the new plastic. Figure 41 Notice that the new hole is not centered. This was how the hole was located from the factory which may be the reason the hole failed since it was thin on the lower left side. To prevent that from recurring, the plastic welder was used to beef up the outside so that the hole became centered.
Figure 42 This particular fairing has two headlights with one ferrule and two mounting tabs each shown at the locations circled in green. All six were broken and repaired with plastic welding. Figure 43
Using polypropylene welding rod to restore mounting tabs on headlight assembly Figure 44 shows an intact headlight mounting tab from the Kawasaki EX500. I never had any success with any glue on the headlight assembly so it was a pleasant surprise to find that polypropylene welding rod works very well. Figure 44 Figure 45 shows a broken tab. Figure 45
Figure 46 shows how a welding rod is melted into the damaged area. The green color indicates it is a polypropylene rod. Figure 46 Figure 47 show this process continued until the entire missing tab has been reproduced. Figure 47 The final step is to drill the mounting hole and paint as shown in figures 48 and 49 below. Both the left side upper and lower mounting tabs were broken on this headlight assembly.
Figure 48 Figure 49 Krylon Semi-Flat spray paint is almost a perfect match for the black of the EX500 headlight assembly. It is not really necessary to paint since the mounting tabs are not visible when assembled into the front fairing. Three light coats worked fine. Figure 49 is after the second coat. A little green can still be seen in the upper left tab. August 31, 2013