Extrusion Blow Molding is one of several blow molding methods to produce plastic bottles, jars, and jugs. It is a common blow molding technology used to process many different plastics, including HDPE, PVC, PC, PP, and PETG - and very common with bottles requiring multi-layer technologies (bottles with special oxygen / moisture barrier requirements to improve product compatibility & stability).
Steps of EBM process
Extrusion Blow Molding is the simplest type of blow molding. A hot tube of plastic material, called a parison, is dropped from an extruder and captured in a water cooled mold. Once the molds are closed, air is injected through the top or the neck of the container; just as if one were blowing up a balloon. When the hot plastic material is blown up and touches the walls of the mold the material "freezes" and the container now maintains its rigid shape.
For colored bottles, colorants are often fed into the extruder at a controlled rate and mixed with the resin as they are being melted. If barrier and adhesive layers are required, they are fed separately and combined in a co-extrusion head
When a bottle is produced using this blow molding method, excess material is created when the mold closes around the parison. This 'flash’ must be removed to complete a finished bottle or jar. Flash at the bottom (called the tail), the top (the moil), and for handled ware - a “handle slug” needs to be trimmed. Typically, the flash is removed upon mold release (trimmed in mold). For bottle with handles (handleware), offset neck and special circumstances, the flash is extracted via a secondary operation downstream.
The short video below depicts an EBM process for a wide mouth bottle being produced with it's flash trimmed downstream.
In addition to removing the flash, secondary (downstream) operations include flame treatment, leak detection, and post mold decorating. Flame treatment is sometime used prior to bottle decorating to improve ink adhesion. Leak detection, on the other hand, is always required to ensure the integrity of the bottle before it leaves the production line. If a bottle fails the leak detection test, it is rejected and sent back for regrind.
Advantages of EBM
There are several advantages of EBM, one of which is lower mold costs compared to ISBM or IBM and the capability of forming handles. Another advantage of EBM is multi layer capability. In some instances, EBM bottles can have up to 7 layers of materials that include virgin plastic resin layers that encase a regrind (PCR) layer, barrier layer, and adhesive layers to bond the other layers together.
Although specific capabilities differ from one manufacturer to the next - other capabilities unique to EBM includes IML (in mold labeling), window stripe on bottles, angled and offset necks, handleware, and multiple necks.
In terms of logistics and cost benefits, EBM mold fabrication time is often shorter and relatively inexpensive compared to other blow molding methods, and many EBM manufacturers have interchangeable mold parts which allow the same bottle to be molded with different neck finishes, body, or embossment inserts. Since the inserts are interchangeable, the job change-over time are often relatively short.
Identifying EBM Bottles
EBM bottles are easily identifiable by their pinch off line across the base of the bottle. This 'line' is created as the mold cavity closes on the parison and the tail is trimmed off.