Introduction: Broken Plastic Box? Weld It Back Together!

Typically what happens when a plastic case shatters is that it gets thrown away; a tape patch really doesn't cut it. However, if you come down to your local TechShop you can preform a sturdy repair to last through plenty of use until a replacement is found! The "welder" is basically a concentrated heat gun that is used to melt the plastic just enough to join pieces together again. All tools to perform this repair are available through a TechShop membership, specifically this was done at TechShop San Jose.

For this Project you will need:
  1. A plastic Welder
  2. A Dremel Tool w/sanding head
  3. Adjustable crescent wrench
  4. Some sort of plastic filler material (preferably same color as broken plastic)
  5. Gloves
  6. Safety Glasses
One note on filler material: It is preferable to get some sort of stock plastic that you know is safe to use from a distributor. (i.e. TAP plastics). In a pinch however it can be almost anything, just be sure to do this in a well ventilated area. For example, the filler material I used for this repair was a small disposable plastic measuring cup, roughly an oz. or 2 in size. 

Before repairing the box itself, I highly recommend practicing on some scrap plastic to lock in the techniques explained in the next few steps. If you mess up on the box itself it will makes things more difficult in the long run, rather than making it easier.

Step 1: Getting Started

The initial set up is fairly simple.

Included with the plastic welder is the instruction manual, and in that manual it gives a chart of temperatures in accordance with the different nozzle types. Select which nozzle is best for your repair, the included types are featured in the first photo. When you install the head, make sure you tighten it down a quarter turn with the adjustable wrench; I learned after welding a bit that it will shake itself loose over time. On the bottom of the plastic welder you will see the knob that adjusts the temperature needed for welding. For this project I was around level 2-3, adjusting accordingly for when the gun heated up and cooled down. W Before you get started, you need to place the welder in a clear open space away from any other objects. It is key to turn the welder on and let it run for roughly 3-5 minutes to get up to temperature, otherwise it will just be blowing cold air. The welder featured does not need an air compressor so it's a bit more accessible, but takes longer to warm up.

It is important to mention safety here as well. When working with the warmed up welder, it is crucial to wear gloves through this process. At first it may seem safe to do without, seeing how it's not nearly as high temperature as a MIG or TIG welder. However, the temperature needed will be around 300°F so you can easily burn a finger.

Step 2: Tack Welding

The first step in welding something back together does not involve filler material. Essentially you will be tack welding the box in several places to hold it together while the rest of the work is done. It's really just holding the box together and placing the blast of air over the seam just long enough to partially melt each side. You want the plastic to begin to turn transparent but not begin to warp. (This is important seeing how if the box warps, it may further lower the quality. This is also true if not more important while welding with filler material.)

One note: If welding without filler material doesn't work, use a small amount just to make sure.

Step 3: Welding Along a Seam

Once the box has been tack welding together enough so it won't easily come off it is time to start using filler material. Feel free to experiment with different methods and techniques while preforming this step seeing how it was largely experimentation on my part. What I discovered that worked, was that I first heated up the filler material slightly until it was transparent. My reasoning with this is that the warping of the filler material doesn't matter, while the structure of the box does. I then placed the near-melting plastic on the seam of the crack while applying the heat to both the box and the filler. The air from the welder will force the plastic against the seam which will force the material in to the crack while fusing with the box's plastic. Once this process is started, slowly drag the filler material and the welder along the seam, depositing plastic as you move along.

Remember! Do this in a safe and clear environment to avoid all hazards and maximize safety.

Keep in mind once you have welded the entire seam, it won't look pretty. Since the box pictured here is clear, there is little finish work available compared to something that is painted. However, there is something that can be done in the next step!

Step 4: Sanding Down the Weld

Rotary Tool Safety: It is very important to not wear gloves and to wear safety glasses during this step. A golden rule with rotary tools is to avoid gloves, the fabric can get pulled in and twisted around even though this is a small hand tool. With safety glasses, there is a high chance that plastic will be thrown in to your eyes while doing this and so it is important to be safe about it. It may be a simple project but that doesn't mean to ignore precaution.

To get rid of the gnarly plastic along the crack it has come time to break out the Dremel tool! For this you will want to keep the RPM down on the tool to be sure to not melt the plastic. I started at the lowest setting and moved up from there, gauging what was needed based on effectiveness and speed. A bit too fast and you will risk melting the plastic as well as grinding too deep, and too slow it will take an unnecessary amount of time to sand down the weld.

If you have a painted or otherwise coated box, there will be an extra step of refinishing over the sanded plastic. It's really on a case to case basis with what

After you are satisfied with the sanding, you are done! This fix will make the box fully functional once again and will save you some money before upgrading the container to something new. All of these tools and more are included with a TechShop Membership so come on down!