Step 1: Understanding Plastics
Hot melt adhesive or simply hot glue is really a plastic. It is very sticky when molten and is called an adhesive for that reason. It is specially formulated for adhesion. Nevertheless, it is still just another kind of plastic. It is softer and more flexible than ABS plastic. It is a very useful plastic on its own and can be used to make all kinds of things. Don't be afraid to experiment.
In this instructable, I will demonstrate how to not only repair a broken piece of plastic, but even to do some rudimentary molding to fabricate plastic parts. The piece I will be repairing is a large cover for an air handler in a automobile.
Step 2: Gather Your Materials
- Hot glue gun and glue sticks
- Duct tape
- Backing material
- Mold release agent
- Shaping utensil
1) Since we are going to be using a lot of plastic in this repair, it really pays to get the long glue sticks. You can lay a lot of plastic without running into the dreaded end of stick problem.
2) You can use a lot of different kinds of tapes. You mainly just need to be able to mount the backing material to make a mold. Masking tape should also work.
3) For a backing material, I used aluminum foil and thin cardboard. As will see later on, the foil stuck to the plastic despite the mold release agent. I have found that "saran" type plastic wrap tends to not adhere to hot melt glue and therefore may be a better backing material.
4) In order for the hot glue to not stick to the mold, you will need what is called a "mold release agent". Fortunately for us, ordinary petroleum jelly or "Vaseline" works great for such an agent.
5) For a shaping utensil, I just used an old spoon. Hot melt glue comes off most metals easily when it is cool, so this makes a great tool.
Step 3: Make Your Mold
Follow the captions on each pic so follow how each step brings the mold into the perfect shape. Here is a summary:
- I loosely tape the foil to the piece, covering the hole.
- Next, I completely cover the backside of the foil with duct tape, making sure the tap is bonded to the foil, making a two layer backing.
- Since the foil-tape layer does not have much of a shape, I then tape a thin layer of cardboard to the backside.
- Next, I press the foil-tape against the cardboard to make a nice flat shape conforming to the contour of the piece. For cardboard, I used a tissue box. You can also use a cereal box. They both use thin but stiff cardboard.
- Apply the mold release agent. Don't forget this important step!
Step 4: Apply Hot Glue
Start by applying glue along the edges. Next, apply glue across the face. It will self-level pretty well at this stage. I filmed a short video of the process here.
For this repair, I needed a fairly thick (1/4") layer of plastic, so I then made a second pass. This layer was more lumpy than the first. I also made sure I overlapped the glue across the original plastic as well. You can apply petroleum jelly to your spoon and smooth the glue as well.
Another technique for smoothing the glue/plastic is to use a hot air gun. I didn't do so in this case since most people don't have such a tool, and I didn't need a perfect result. There is also some risk involved with using hot air as it can also melt your original piece. If you decide to use one - proceed with caution!
In the second photo, you can see how perfectly the mold side came out.
I ended up with a problem on removing the mold. The aluminum foil stuck to the hot glue, despite the mold release agent. I am not sure why this happened. In future projects, I think I will try using "Saran" type plastic wrap instead of foil. I have found it to come off the cooled glue/plastic even without mold release agent, so it ought to work very well.
Step 5: All Done
Hot melt adhesive is a superior plastic in almost every way. The only time this plastic can't be used is where rigidity is needed. For these cases, shape the part you need from wood and hot glue it into place. The wood part will also be superior to the plastic it replaces. I have replaced broken shelf brackets in a refrigerator using this technique. These brackets must hold a lot of weight, and therefore must be very strong. The combination of wood and hot glue fit the bill perfectly.
Feel free to experiment. Don't be intimidated. You have nothing to lose. If you fail, the worst case is you will have to throw the broken part away. If you succeed, it is like having a brand new part. The possibilities are endless.
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Next time you have a broken piece of plastic, don't throw it away - hot glue it!