Blow molding process is an alternative to produce hollow products out of thermoplastic materials. Blow molding evolved from the ancient art of glass blowing today it is a leading plastic manufacturing process because of the wide range of resins that can be blow molded and because of the many methods of blow molding. Blow molded parts range in size from the very small to extremely large parts bottle and containers are the most common blow molded products but other products such as highway barrels automotive components double walled cases, toys medical items and structural panels are produced.
Blow molded product are formed from thermos plastic resins such as HDPE, MDPE, LDPE, PET, PP and PVC, TPE, PS, PC PTFE and Nylon. Most thermoplastic blow molded products are produced via extrusion blow molding using high density polyethylene (HDPE). But there are many other additional blow molding processes which include injection blow molding, biaxial stretch blow molding and co extrusion blow molding. These blow molding processes all use elements of extrusion or injection molding or a combination of the two. In addition these blow molding processes all share four common stages.
- Plasticizing or melting the resin.
- Parison production when referring to most blow molding operations or preform when referring to biaxial stretch blow molding.
- Parison or preform inflation and cooling in the blow mold
- Ejection from the blow mold.
- In extrusion there is a 5th stage for finishing and trimming the product.
Blow molding processes all use the same blowing technique whether it be through a blow pin, a needle, and a stuffer or core rod. It is in the production of the parison or the preform where these processes differ.
The process of extrusion blow molding involves applying heat and pressure to thermoplastic to produce what is called the melt. This melt is then forced through a die to produce a parison which is dropped, trapped or conveyed to an open blow mold for subsequent blow molding. Extrusion blow molding using an intermittent or continuous method for them of the molten parison. Intermittent produces a parison only when the blow mold is ready. These parisons are produced by using either a reciprocating screw or an accumulator and ram style machine.
Injection blow molding incorporates elements of convention thermoplastic injection molding with blow molding and is generally more economical than extrusion blow molding for containers under a ¼ liter in size and large production runs. Injection blow molding machines typically contain three stations:
- The injection station
- The blow station
- And the strip or eject station
The injection station is basically an injection molding machine, a ram usually a reciprocating screw forces the melted thermoplastic resin under pressure onto a metal core rod held within a closed split parison cavity injection mold. After the outside skin of the parison has set the parison mold opens and the core rod carrying the parison rotates to the blow station. One the core rod is in position the blow mold closes a trigger bar opens the core rod air passage the air lifts the parison from the core rod except where it held at the neck and expands it to the shape defined by the blow mold cavity. One the part has cooled sufficiently the blow mold opens and the part is indexed to the next station for stripping or ejection.