Assembling plastic model is easy, right? Of course it is, but there is some science to the process that will help give you a better understanding of how plastic works and why you should or shouldn’t use the glue you are using.
To start with, most plastic models are going to come still attached to the sprue. This is what a sprue may look like after the thermoplastic is formed and released from the injection molding process.
Thermoplastic is a term that defines a polymer that becomes pliable or moldable above a specific temperature, and returns to a solid state upon cooling. Most game manufacturers use polystyrene to make their plastic miniatures. It is important to know what the material your model is made of to help you correctly choose the proper glue.
“What? Proper glue? I just use Super Glue for everything.” And Super Glue will work, but it has some disadvantages when assembling plastic models. Super Glue (cyanoacrylate) will create a mechanical bond when glueing plastic to itself. However, when we choose a proper glue, like plastic cement (made of Xylene, Ethyl Benzene, and n Butyl Acetate or methyl ethyl ketone) then we get a chemical bond. Plastic cement actually dissolves a layer of the plastic on each half of the joint before chemically welding them together.
Why do you care? Maybe you don’t, but a properly welded plastic joint will not ever come apart. Like a previously broken and mended human bone you will break the model somewhere else before a cemented joint comes apart.
How to use plastic cement:
Step 1: Get a good quality plastic cement with a fine point applicator. My favorites are Testors Plastic Cement and Model Master Plastic Cement. They come with a nice metal fine point applicator.
Step 2: Clean and dry the surface of your model (I like to use a small amount of liquid dish soap and a tooth-brush). While this step is not always necessary, having no-contaminates on the surface of the glue joint will ensure that you get the best possible bond.
Step 3: Fine a place that is well ventilated and get yourself a respirator. Mos glues are toxic and the fumes can cause some unwanted side effects.
Step 4: Apply a thin layer of plastic cement to both sides of the joint and let the pieces sit for 20 seconds (this allows the solvent to begin to soften the surface of the model).
Step 5: Place the two pieces in contact and hold the joint together until the solvent has evaporated. If you’re welding an arm onto a Space Marine you can hold to the count of 5 and then set the model aside. If you’re welding something larger with more weight and leverage you might want to use a small clamp. Ideally you want the joint to be undisturbed for at least a few hours. I believe that 80% strength will be achieved within about 12-24 hours and full strength within a few days to a week. Basically, give it until the next day before you prime the model if you can wait.