Blow Molding of Plastic Containers

The most desired shape for a rigid plastic container for many products is a bottle or jar. However, bottles and jars, even those with large necks, cannot effectively be produced by injection molding because of the difficulty of getting the solid core out of the molded object. As glass blowers learned centuries ago, an effective way to produce such containers is used to use air pressure to shape the inside of the object. In blow molding air pressure to shape the inside of the object, while a mold shapes the outside. Blow molding is the only practical way to make plastic bottles and jars, and this method is also used for large plastic containers such as drums. There are two major types of blow molding, extrusion and injection blow molding.

Injection Blow Molding

Injection blow molding combines injection molding of a precisely formed parison with blow molding of the finished container. Therefore, it is able to give fairly accurate control over container dimensions, especially in the critical area of bottles. Injection blow molding is more expensive than injection molding alone, since it requires two sets of molds and two molding processes. However, it is capable of producing shapes that cannot be produced by injection molding. It is also produces very little scarps. Injection blow molding can be used with resins without sufficient melt strength to be handled by extrusion blow molding. Injection blow molding is used for most PET bottles, as well as most of bottles used for pharmaceutical products. It generally cannot economically produce bottles with handles.

    Injection blow molded bottles (IBM) are always made in a two-step process. First, a preform, or parison, is produced by injection molding. Next, this perform is placed in a second mold and blown to produce the final container shape. The injection molding of the preform is identical to the injection molding process, except that the mold has three parts, the two halves of the cavity and a core rod. Hot runner molding is used almost universally, so no trimming is required. The bottle finish is formed in the injection molding step, and the parison typically has a test tube-like shape. Usually the preform is cooled only enough maintain its shape with the support of the core rod, before being transferred, still on the core rod, to the blow mold. In this step, air is introduced at high pressure through the core rod, to blow the bottle. Blowing pressure are higher than for extrusion blow molding because of the cooler parison temperature, and hence the higher viscosity of the parison. As in extrusion blow molding, the mold is cooled, usually with water. When container is cool enough to maintain its shape without distortion, it is ejected from the mold. Bottles produced by this process can be identified by mark left by the gate on the bottom of the bottle at the initial injection molding stage.

    Injection blow molded bottles are generally blown on the same machine as the one making the performs. It is a multi-station machine, where first station does the injection molding, the second station blows the bottles, and the third station ejects the finished bottle from the machine. The process is often arranged on a horizontal table. However, the cavitation is limited by the sizes of the rotary table.

Extrusion Blow Molding

Basic Extrusion Blow Molding Process – Extrusion blow molding is the oldest, and still the most common, process for producing plastic bottles. The first step is extrusion of a hollow plastic tube, just as it done for blown film, except that the extrusion is usually in a downward direction for making bottles, and usually in an upward direction for film. The two halves of the mold next close on the tube, capturing it as it is cut off from the extruder. A blow pin or a needle is inserted and air is blown into the mold, expanding the tube, or parison. In some cases the blown pin cooled by water, assists in forming the finish by compressing the finish section into the mold, rather than simply blowing it in. This results in a smooth interior in the finish region. In the needle blow case, the needles is inserted into a part of the molded object that is trimmed off forming the final container shape, and the inside of the finish is formed only by air. The mold is cooled, usually with water, to solidify the plastic. When the container is cool enough to maintain its shape, it is ejected from the mold. The flash is trimmed from the container neck and bottom, as well as from other areas that are pinched off, for instance to form handles or offset neck.

    The mark left from the removal of the flash serve as an easy means for identification of extrusion blow-molded containers. Usually, this is easiest to see on the bottom of the container. It typically appears as a rough area along the mold parting line, centered in the middle of the bottom and running half or so of the distance to the heel of the bottle. It is also possible, on careful examination, to identify the roughness at the top of the finish, or on other areas where flash was trimmed.

    The flash, after being trimmed, usually is immediately up and fed back into the extruder at a controlled rate, mixed with the virgin resin. The use of regrind can be problematic for heat-sensitive resins like PVC, especially if the proportion of the flash is high.

    The process of the parison extrusion can be continuous or intermittent. For intermittent extrusion, the melt from the continuously rotating extruder may be fed into an accumulator, from which it is periodically ejected, or a reciprocating extruder like those used for injection molding may be used. Continuous extrusion is preferred for most packaging applications. It provides higher productivity and reduces thermal degradation, since the melt is not help up. Intermittent extrusion is commonly used for the production of very large blown containers, where a large parison must be produced in a very short time.