injection blow molding design

injection blow molding design and construction

The design of tooling for injection blow molding as discussed herein is intended for equipment utilizing the horizontal rotary index method of core rod transfer. This principle has been the most successful to date and is used by over 90% of the machines now in service. Figure 1 shows the mold and machine.

injection blow molding design

Figure 1

Principle of Operations

As seen in a simplified tooling layout before two sets of molds are required, along with three sets of core rods. To insure alignment and speed mold change-overs, each set of molds is mounted on its own mold base or die shoe. In the first or parison mold, melted resin is injected over the core rods, filling the cavities and fixing the weight of the container. At the same time the neck finish is molded completely. This set of core rods carrying the preformed parisons is rotated to the second or blow station where they are enclosed within the blow molds and blown to the desired shape. The blown containers are then rotated to the stripping station for removal, oriented if desired. Since there are three sets of core rods, all three operations are simultaneously conducted by sequential rotation of the core rods.

Mold Design

The constraints of container length,diameter, and number of cavities are imposed by the physical dimensions of the machine at hand. Parison molds are normally mounted so as to place their centroids of projected area directly under the centerline of clamp pressure. The distance, then, from the face of the rotating head to the center of pressure will determine the maximum container length. Machines are available to cover the range of V2 to 12 in, length. The maximum container diameter is about 1/4 in. Iess than the mold opening, to allow a minimum of l/s in. top and bottom clearance for the blown container to rotate from the blow station to the stripper station. Present machines use from 4 to 6 in. mold opening, depending on the size.

The maximum number of cavities is normally limited by the clamp tonnage of the parison mold and the material being molded. The following clamping requirements are suggested as minimums:

The maximum number of cavities

the nature of injection blow process places some limitations on the be made. Injection-blow molding is well suited to wider mouth bottles and conventional shapes. Because the core rod is a cantilever beam, its L/D ratio should be 12/1 maximum to prevent deflection during injection. A 32 oz. cylinder round style container requires a core rod about 9 in. long. Specified with a 24 mm finish, the L/D ratio is an unworkable 14/1. Moving up to a 28 mm or 33 mm would reduce the L/D to a manageable ratio.

Swing weight becomes a consideration with containers over 32 oz. capacities that have wide necks. The core rod for a 48 oz. capacity container with a 110 mm finish weighs about 9 lb. Mounted at the extremes of the swing radius, they impose severe inertial loadings on the transfer mechanism, as indexing takes place in about 1 sec.

One advantage of injection blow is the diametrical and longitudinal programming of the parison by shaping the parison mold, the core rod,or both. This two-dimensional programming becomes especially advantageous in the production of oval containers. Up to an ovality (container width/container thickness) ratio of 1.5/1 quite satisfactory containers can be blown from circular cross-section parisons. Up to ovality ratios of 2.2/1 can be handled with oval cross-section parisons. Generally the ovalization is done to the parison mold; the core rod remains round. Extensive parison ovalizing, perhaps above 35%, can lead to selective fill during injection and result in visible knit lines in the finished bottle. Under these circumstances it will be necessary to ovalize both the core rod and parison cavity to obtain the de-sired distribution in the finished container. Some provisions to prevent the core rod from rotating must then be incorporated. Ovality ratios above 3/1 are not suggested. The preferred blow ratio is between 2 and 3/1,measured as the average Parison OD against the bottle OD. This normally will yield an ideal parison thickness from . 120 to .180 in. with predictable expansion characterizes Increasing the blow ratio by reducing the core rod diameter, reduces e Projected area, often enough to allow another cavity to be fitted. Unavoidably, the parison thickness must be increased, to maintain a constant weight,and experience has shown parison thicknesses above .225 in, to be unpredictable in their blowing characteristics.

Injection Blow Process

Injection-Blow Process

Injection Blow Process

The injection-blow process combines the function of blowing with the precision of injection molding; it requires a parison-forming mold with integral neck ring, multiple core rods (blow pins) and a blow mold. A core rod is moved into the parison mold and material is injected into the mold to form a Precision parison with a fully formed neck. Within certain limits, the parison can be shaped internally by the core rod,and externally by the parison to control the uniformity of the wall thickness of the blown product, lie the body of the parison is still hot and plastic, the core rod with the Parison on it is lifted from the parison mold and then transferred to the blow mold. Air is introduced through the core rod to blow the parison to the final form of the product. After cooling, the product is removed from the core rod as a completely finished article without the need for secondary trimming operations.

The injection-blow process is used to manufacture products requiring close control of dimensions, weight, capacity, and material distribution. There are three main types of injection-blow machines commercially available. They are discussed below.

Three and Four-Station Rotary Machines. The parison mold and blow mold are mounted in a fixed radial position with respect to each other. The core rods are mounted in a head which indexes intermittently to transfer the core rods from the parison station to the blow station and then to an ejection station. The four-station machine has an extra station, after the ejection station, for conditioning (heating or cooling) the core rod before it is transferred into the parison mold.

The rotary machines are the most widely used of the injection-blow machines because of their versatility in producing precision containers in a size range from a fractional ounce to 64 ounces.

Two-Station Machine. In comparison with the four-station rotary machine, the ejection and conditioning stations have been eliminated. Ejection of the finished product occurs as the blow mold is opened. Conditioning in a two- station machine is normally accomplished by the circulation of temperature-controlled fluid through the core rods.The two-station machines are used primarily for producing specialized containers for paint or food products.

Adaptive Tooling. Various kinds of systems have been designed and built to adapt standard injection molding machines for blow molding. Such systems generally find application in injection molding companies that want the flexibility to occasionally use their standard machines as blow molders.

Injection Blow Process


Formed Neck Process

In this process, the neck of the container is precision-formed by pressing the neck ring integral blow pin against the extrusion die and extruding the plastic material into the interior space. After the neck is formed, the neck ring and blow pin move away from the extrusion die while additional material is extruded at a controlled speed for the section of the parison which form the body and base of the container. In one variation of the process, a short blow pin is used and the parison is pinched off by the mold halves which close on it. In another variation, a long blow Pin is used which supports the entire length of the parison; when the desired parison length has been extruded, it is cut off and the parison is rotated 180° by the blow pin to the blow mold. Commercially available machines are not yet in wide use, but proprietary machines employing the first variation of the process have been used for many years.