What Is Blow Molding?

Plastic Products of the South is a South Carolina based molded plastics company providing products and services to multiple industries. Below are terms we use in the industry. Please see our FAQ page for common questions regarding custom blow molds.

  • Parting line: The point at which the two halves of the mold come together. The parting line typically leaves a corresponding thin line on the molded part, but through careful mold design, build and maintenance it can be minimized.
  • Flash: Excess plastic that squeezes out of the mold during molding. Flash can be found at the parting line, mold inserts or fasteners. Flash is typically undesired and can be minimized or eliminated with careful tool maintenance.
  • Cycle Time (Takt time): The time it takes to mold one part. For example, if it takes 15 seconds to extrude the plastic tube, 15 seconds to mold the part, and 5 seconds to remove the part from the mold, the cycle time is 35 seconds. Notice, however, that secondary operations such as dimensional cooling or trimming do not affect machine cycle time, but may affect overall output, especially if a single operator is responsible for molding and trimming.
  • Setup: In most cases the molding machine must be adjusted and purged prior to each production run. If a color change or plastic change is involved, a few hundred pounds of plastic may have to be run through the system in order to purge the old material. The parts which have been run during a purge typically have color swirls or flecks and are cosmetically inferior.
  • Blow Pipe: The mold feature that injects air into the part during molding. As the mold closes, the blow pipe penetrates the extruded plastic tube, and begins to blow air into the plastic until it fully conforms to the interior of the mold. The blow tube always leaves a corresponding hole in the side of the molded part. In most cases this hole is not objectionable, but if it is, it can be plugged after molding.
  • Mold Maintenance: The regular repair and upkeep of the mold as it begins to show signs of wear. All molds will wear as they are used, and the amount of wear depends on the mold material – steel or aluminum. Steel is more costly, but lasts much longer. Mold maintenance usually includes repairing the parting line, removing any dirt, debris, oils and fingerprints, and touching up any dings, scratches and textured areas. The molder should establish a reliable process and schedule for mold maintenance.
  • Extrusion Die: An extrusion blow molding system will first extrude a continuous tube of plastic, which is then lowered to the mold tooling for molding. The extrusion die determines the diameter and thickness of the resulting plastic tube, and therefore the part’s weight and thickness.
  • Mold Inserts: Tooling in the mold that contain certain molded part features. Static inserts are used for textured areas, logos, or date stamps that may need to be changed or updated, or may be more susceptible to damage. Having multiple inserts on-hand can greatly reduce downtime. Dynamic inserts are used to mold certain features that cannot be produced with the two halves of mold themselves. These inserts are usually tied to hydraulically or pneumatically operated cylinders that move the inserts into position after the two halves of the mold have closed.

Contact us if you have any questions about custom blow molding.