Thermosets and thermoplastics are the two most common types of plastic used in blow molding. The main difference between them is the way they process heat: under high temperatures, thermoplastics will melt, whereas thermosets will solidify.
Characteristics of Plastics
This is because under heat, a thermosetting polymer cannot be weakened; its three-dimensional chain will cross-link and strengthen irreversibly. On the other hand, thermoplastic polymers hold the shape of a one-dimensional string of monomers whose bonds weaken rapidly under heat, thus causing the plastic to melt and allowing it to be re-shaped. When cooled, the thermoplastic bonds will strengthen and the plastic will re-solidify.
Needless to say, many molding processes require the use of thermoplastics or thermosets, including extrusion blow molding.
Extrusion Blow Molding
Extrusion molding is an industrial process that melts raw thermoplastics in the form of granules. The plastic is fed into a machine through a revolving chamber, which will melt it and force it through something called a “die,” which is similar to a mold in that it provides shape, but it does so by cutting materials as they are forced through. The finished products are often long and rough shapes such as rods, sheet metal, or pipes.
The Process of Molding Plastic
Blow molding is the process of molding plastic into a hollow shape. It can be done through either an Extrusion method. In the Extrusion method, the melted plastic is forced through a die to be shaped like a tube. The product’s mold then entraps the tube. From the top, a blow pin is inserted through the tube and air pressure inflates the plastic into the hollow shape of the mold it is trapped in. In the Injection method, the process is very similar except a die is not used to form the tube; the tube is formed when melted plastic is directly injected into a mold made specifically for it.