Blow molding is the process of forming a molten tube (referred to as the parison or preform) of thermoplastic material (polymer or resin) and placing the parison or preform within a mold cavity and inflating the tube with compressed air, to take the shape of the cavity and cool the part before removing from the mold.
Any hollow thermoplastic part can be blow molded.
Parts are not just limited to bottles, where there is one opening and it is usually smaller in diameter or size than the overall body dimensions. These are some of the most common shapes used in consumer packaging, however there are other typical types of blow molded parts, including, but not limited to:
- Industrial bulk containers
- Lawn, garden and household items
- Medical supplies and parts, toys
- Building industry products
- Automotive-under the hood parts
- Appliance components
Blow Molding Manufacturing Processes
There are three main types of blow molding:
- Extrusion blow molding
- Injection blow molding
- Injection stretch blow molding
The main differences among them are the method of forming the parison; either by extrusion or injection molding, the size of the parison and the method of movement between the parison and blow molds; either stationary, shuttling, linear or rotary.
In Extrusion Blow Molding-(EBM) the polymer is melted and the solid extruded melt is extruded through a die to form a hollow tube or parison. Two halves of a cooled mold are then closed around the parison, pressurized air is introduced through a pin or needle, inflating it into the shape of mold, thus producing a hollow part. After the hot plastic has cooled sufficiently, the mold is opened and the part is removed.
In EBM there are two basic methods of extrusion, Continuous and Intermittent. In continuous, the parison is extruded continuously and the mold moves to and away from the parison. In Intermittent, plastic is accumulated by the extruder in a chamber, then forces through the die to form the parison. The molds are typically stationary under or around the extruder.
Examples of the Continuous Process are Continuous Extrusion Shuttle machines and Rotary Wheel machines. Intermittent extrusion machines can be Reciprocating Screw or Accumulator Head. Various factors are considered when selecting between the processes and the size or models available.
Examples of parts made by the EBM process include many hollow products, such as bottles, industrial parts, toys, automotive, appliance components and industrial packaging.
With respect to the Injection Blow Systems – (IBS) process, the polymer is injection molded onto a core within a cavity to form a hollow tube called a preform. The preforms rotate on the core rod to the blow mold or molds at the blowing station to be inflated and cooled. This process is typically used to make small bottles, usually 16oz/500ml or less at very high outputs. The process is divided into three steps: injection, blowing and ejection, all done in an integrated machine. Parts come out with accurate finished dimensions and capable of holding tight tolerances—with no extra material in the formation it is highly efficient.
Examples of IBS parts are pharmaceutical bottles, medical parts, and cosmetic and other consumer product packages.
Injection Stretch Blow Molding- (ISBM) the Injection Stretch Blow Molding- (ISBM) process is similar to the IBS process described above, in that the preform is injection molded. The molded preform is then presented to the blow mold in a conditioned state, but before final blowing of the shape, the preform is stretched in length as well as radially. The typical polymers used are PET and PP, that have physical characteristics that are enhanced by the stretching part of the process. This stretching gives the final part improved strength and barrier properties at much lighter weights and better wall thicknesses than IBS or EBM—but, not without some limits such as handled containers, etc.. ISBM can be divided into the One Step and Two Step process.
In the One Step process both preform manufacture and bottle blowing are performed in the same machine. This can be done in 3 or 4 station machines, (Injection, Conditioning, Blowing and Ejection). This process and related equipment can handle small to high volumes of various shape and size bottles.
In the Two Step process the plastic is first molded into the preform using an injection molding machine separate from the blow molder. These are produced with the necks of the bottles, including threads on the open end of the closed end hollow preform. These preforms are cooled, stored, and fed later into a re-heat stretch blow molding machine. In the Two Step Reheat Blow process, the preforms are heated (typically using infrared heaters) above their glass transition temperature, then stretched and blown using high-pressure air in the blow molds.
The Two Step process is more suited to very high volumes of containers, 1 liter and under, with very conservative use of resin providing great strength, gas barrier and other features.