Blow Molding
Manufacturing process using air pressure to form hollow plastic containers from a heated tube of plastic.
Blow molding is a manufacturing process used to create hollow plastic parts like bottles, drums, and 5-gallon pails. The process works by heating plastic resin (typically HDPE pellets) until molten, forming it into a tube called a parison, clamping the parison inside a two-piece mold, and blowing compressed air (60-100 PSI) to expand the plastic against the mold walls. There are three main types of blow molding: extrusion blow molding (EBM) — the most common for large containers like 5-gallon pails; injection blow molding (IBM) — used for smaller, precision bottles; and stretch blow molding (SBM) — used for PET bottles. EBM is the primary process for industrial HDPE containers because it can produce thick-walled, durable parts with excellent chemical resistance. Key process parameters that affect container quality include: melt temperature (typically 350-400°F for HDPE), parison programming (varying wall thickness for even distribution), blow pressure (60-100 PSI), mold cooling time, and flash removal. Modern blow molding machines can produce a 5-gallon pail every 60-90 seconds. For UN-certified containers, blow molding parameters must be tightly controlled and documented. Each mold cavity produces containers that are independently tested, and the batch must pass drop testing, stack testing, and hydraulic pressure tests before receiving UN certification markings.