Plastic is a word to describe a wide range of materials. In our modern world we interact with plastic every day. We drink from plastic water bottles, type on plastic keyboards, drive cars with plastic parts and eat food served in plastic containers. Most plastics we use can be broken down into 6 categories. If you ever notices on something made of plastic is a triangle with a number in it. Sometimes that triangle has letter below it and sometimes that triangle is in the shape of recycle symbol. The number in the middle is whats most important. It tells what kind of plastic it is or more correctly, what kind of resin the plastic was made from. The numbers start with one and goes to 7. One to 6 are specific kinds of plastic and number 7 is a catch all for anything that that does not fit into numbers 1 to 6.
Making plastic is a lot like cooking or better said, baking. In baking you add eggs, water, oil, flour and sugar to begin your cake or pie. These foods are made up of elements like hydrogen, carbon, oxygen and nitrogen. These elements combine to form chemicals. Depending on how much egg you add or how much flour you add you cake or pie will taste very different. Plastics are like food, depending on how you make and mix your resins you get different kinds of plastic.
Most plastics we use were melted down and formed into products we use. When we are done with them, most can be remelted and reused. It would be nice if any number 4 plastic resin could be melted with any other number 4 plastic resin and reused. But different manufacturing process requires slightly different formulas of resins.
What are the different kinds of plastic?
The different kinds of resins that make our plastics each do well at somethings and not so well at others. Some of the common plastics we use are (warning: here comes the big words) High Density Polyethylene, Polyethylene Terephthalate, Polyvinyl Chloride and Polystyrene. Each of these have abbreviations. HDPE, PET, PVC and PS. And you guessed it they each have a number. If you a geek and want to remember High Density Polyethylene, fantastic, or you can be kinda techie and say HDPE or keep it simple and just call it plastic number 2. How do you know it plastic #2, well it stamped usually on the bottom of the item. Flip over almost any plastic thing and there is the number. We have been using the number system since the last 1980 and it was created by the Society of the Plastics Industry (SPI).
Three of the most common plastics are:
- # 1 plastic is Polyethylene Terephthalate (PET) and is what water bottle are made of.
- #2 plastic is High Density Polyethylene (HDPE) and is what milk jugs are made of.
- #5 plastic is Polystyrene (PS) and is what Styrofoam cups are made of.
Click here is you want to go deep into the plastic resins and their codes.
3D PRINTING PLASTICS
The 2 most common plastics for 3D printing are: ABS and PLA. They fall into the #7 plastic category of “Other”. There is experimentation with using HDPE with 3D printing but it is not commonly used yet.