Plastic bottles can be molded in a variety of ways. The manufacturing method is typically determined by the bottle shape, size, material, location, and desired cost. The basics of each manufacturing process are very similar: plastic pellets (resin) are heated, then the plastic is formed to the shape of a bottle when it is extruded, injected, or blown into metal tools and cools. COMPAX can help you manufacture your plastic bottles through Extrusion Blow Molding (EMB), Injection Blow Molding (IBM), Injection Stretch Blow Molding (ISBM), Co-Extrusion (Co-Ex) or Injection Molding (IM).
Extrusion Blow Molding (EBM)
The extrusion blow molding process forms a continuous, hollow, round tube of plastic. This extruded tube of plastic is referred to as a parison.
The two halves of the mold clamp into a closed position around the parison, and pinch one end shut, like biting on the end of a straw.
Air is blown into the parison through the open end, expanding the plastic tube to fit each side of the cold mold cavities.
When the bottles are formed and cooled, the mold opens, and the bottle is removed. There may be extra plastic around the edges of the bottle, this extra plastic is called flash. Flash is a part of the EBM process, occurring when the mold clamps closed around the parison. You can tell a bottle is extrusion blow molded by looking for a seam along the bottom or sides. Flash around the neck block is called a moil, where flash around the base of the bottle is called a tail. Typically, moils and tails are reprocessed throughout the production run of the bottle. This is called using regrind.
Typical materials used to form EBM bottles are PE, PP and PET. When looking at EBM bottles, it’s important to keep in mind that blow molding is a more variable process, that requires consistent in-line quality inspections to ensure bottle weight, wall thickness, and neck dimensions say within required specification. Through the EBM process, it is easier to make a trigger sprayer or handleware bottle, which are more difficult to form in injection blow molding.
Injection Blow Molding (IBM)
The process of injection blow molding is capable of holding tighter neck dimensions than extrusion blow molding, along with a more precise gram weight. In IBM, plastic is injected into injection mold cavities to form what looks like a test tube, but with a defined neck– these are called preforms. The preform is then transferred into the blow mold cavities, where it is blown into bottles. When cooled, the bottles are ejected.
This process takes place in one machine. The preform parisons are injected, then rotated to the blow mold, final part is blown, cooled, and ejected.
You can tell a bottle is injection blow molded by looking for a gate “bullseye,” typically found on the bottom of the bottle. There may be parting lines along the side of the bottle, but they won’t be raised seams in the same way of an extrusion blow molded bottle.
Injection Stretch Blow Molding (ISBM) or Stretch Blow Molding
The stretch blow process is more typically used for PET. There are both one-step and two-step processes. The main difference between stretch blow and injection blow is that in ISBM, the preform parison can be blown both laterally and length-wise. One-step is typically used for specialty applications, requiring unusual neck or preform shapes, or handleware. One-step ISBM, similar to IBM, takes place within one machine. Two-step is seen frequently used for the beverage bottle market, as the bi-axial stretching allows for the container to be light-weighted, reducing the costs of extremely high volume runs. In two-step, the parison and neck of the bottle are preformed, ejected, and cooled. These are referred to as “preforms.” Then, the preform is inserted into the blow mold, and is blown to the final shape of the bottle.
Co-extrusion blow molding produces multi layered bottles. Extruded together in the parison, the layers are bonded into the final bottle. The Co-Ex process can be used to achieve cosmetic effects, like adding soft touch matte without the secondary process of spraying, or keeping PCR on the outside layer of the bottle with virgin material on the inner to protect your formula. This process can also be used for bottles in the same way it is for tubes, to extend the barrier properties of the bottle.
Injection Molding (IM)
It is rare to see injection molded bottles, but the process may be used to mold a specialty application where extremely tight tolerances are required. In injection molding, plastic is injected into a closed mold between the cavity and core inserts. Pressure forces the plastic into the cavity, and the part cools. The mold opens, and the finished part ejects. For a narrow mouthed container, like a bottle, too many efforts and expenses would have to be taken with ejecting the part from the injection mold. This is typically why you will not see injection molded bottles, and they are more frequently blown. You may see an injection molded “bottle” if it is straight walled, or for an application like an inner airless bottle that requires threading on both the inside and outside of the neck.