are a significant workhorse in the extrusion blow molding
of hollow plastic
articles such as bottles for food storage
Shuttle machines are either single-sided or dual-sided machines, and can be manufactured to produce one- to six-layer containers - although the number of suppliers who produce 4-6 layer machines is limited. In a single sided machine, the mold "shuttles" under the flowhead, closes to capture the parisons, then moves away from the head. Blow pins are then forced downward into the molds, helping to "calibrate" the necks while air is forced into the cavity to blow the container. The shuttle motion allows the bottles to be blown and cooled to the side, without interfering with the parisons, which are continually extruding from the flowhead. In a double sided shuttle machine, there is a mold on each side of the flowhead, one shuttling to the right, and one to the left, which generally doubles the output of a single-sided machine.
Shuttle machines may extrude single or multiple parisons, and are characterized by the number of parisons and the horizontal spacing between the parisons. For example, a "4x100" shuttle extrudes four parisons, spaced 100 mm between the centers. This would require a platen (for attaching the molds) greater in size than 400 mm, to accommodate the required mold width. The horizontal or angled shuttling distance is thus greater than 400 mm for a 4x100 shuttle machine. In general, shuttle machines up to 2x100 mm spacing are considered small machines; shuttles up to 6x100 mm spacing are considered mid-sized machines; shuttles larger than this are typically referred to as "long-stroke" machines.
Shuttle machinery is used widely in the production of personal care bottles, medical bottles, and some small industrial containers.
Sequence of Operation
The steps required for a shuttle machine to blow mold a hollow plastic object can be described by the following sequence of operations:
- As the dropping parisons approach the length of the object to be blown, the mold, in open position, shuttles sideways to a point directly under the head of the machine.
- The molds close to capture the parison.
- A knife cuts the parisons directly above the molds. This may be either a cold knife (cutting with a sharp edge) or a hot knife (burning through the parison).
- The molds shuttle away from the head until they are directly under the blow pin stations. If the mold movement is horizontal, the extruder head is made to bob up vertically, so that the continuously extruding parisons do not drag against the mold as it moves sideways. In some shuttle machinery, the molds shuttle down at an angle, eliminating the need for the head and extruders to bob upwards.
- The blow pins are forced down into the still-open necks of the containers, calibrating the necks of the containers. In most cases, the blow pins punch down onto striker plates, which form the top edge of the neck to a precise flat dimension.
- Air pressure is applied to blow the containers. In many cases, the blow air is turned on before the blow pins enter the open neck of the parison, to force the plastic outward and ensure a good neck formation.
- After the containers have cooled, the molds open, and again shuttle under the head of the machine. As the molds close on the molten parisons, masking stations that are attached to the sides of the mold close over the outside of the previously blown containers, which are still held in place by the blow pins.
- The blow pins retract, leaving the containers held only by the masks.
- As the molds again shuttle sideways, the masks transfer the formed containers sideways to a punching station. Punches come forward to remove the tails, top moil, and any handle (grip) slugs away from the bottles.
- The bottles are then conveyed out of the machine. This may be done by transferring the bottles onto conveyor belts, by takeout devices, or by simply dropping the bottles into a chute or onto a takeaway conveyor.
Advantages and Disadvantages
Shuttle blow molding equipment offers the following advantages:
- Relative low cost compared to other extrusion blow molding machinery
- Capable of producing high-quality "calibrated neck" finish with blow pins
- In-machine trimming - finished bottles exit the machine
- Capable of producing bottles of all shapes - including handleware
- Coextrusion capability, with up to six layers of plastic
Shuttle machines have some limitations:
- Not cost effective for extremely high volumes
- Reduced bottle weight consistency compared to rotary wheel machinery, due to inevitable variations among the number of unique parisons that must be extruded in shuttle equipment
- Cycle time disadvantage when compared to reciprocating screw machines and rotary wheel machines, particularly when producing lightweight containers
- Complexity of the hydraulic and control systems
There are many suppliers of shuttle blow molding machinery. Some of the major global suppliers include: