"3R" unscrewing mold technology.
Although plastic closures are produced in thousands of different styles, they can be classified according to three main molding methods: stripped, collapsible core, and unscrewed. For closures with internal threads, the manufacturing method is often dictated by thread definition and resin used.
This article briefly describes the first two options for molding closures with internal threads - the stripped and collapsible core methods. It then examines in depth the unscrewing options, and presents a new approach, the patented "3R" (Rotating Ratchet Ring) design.
Normally, stripped closures are produced on straightforward molds that simply force the molded product over the core. This is the simplest and most efficient method. However, parts may develop dimensional inconsistencies, distorted threads (when they are ejected too early), and deformed walls from thread displacement. As a result, the parts can be unacceptable for both cosmetic and performance reasons, and downstream capping efficiencies may be reduced.
Part design is also subject to limitations. Parts must be made of a nonbrittle material, such as polyethylene or polypropylene. They must also have a shallow thread with a rounded profile, and usually not more than one turn of thread.
To produce a deeper and more defined thread requires a different approach. Similar in operation to stripped thread molds, the collapsible core mold incorporates a stripper ring that ejects the part over the core. The core, however, is made up of three or more segments. The center segment acts as a wedge. In its forward position, it keeps the outer segments in place to produce the desired molding surface. As the center section is retracted from the part, the outer segments close in to release the thread, permitting the stripper ring to eject the part.
In some designs, the center segment of the core has no undercuts or threads. This permits the center segment to be withdrawn from the part. As a result, the closures have an interrupted thread, which is not always acceptable. Movement of the outer segments does not permit the molding of a sealing feature on the top face of the core, a fact that limits this approach to lined closures.
Many different methods for molding parts that require unscrewing have evolved over the years. They provide various degrees of performance and reliability, and can be classified according to distinct methods of part removal. Either the core is unscrewed from the part, or the part is unscrewed from the core.
Rotary core molds represent the most widely used method of molding threaded closures. Of the many variations that exist, the so-called "rack and pinion" mold is the most common. Figure 1 represents a typical cross section of a rack and pinion mold. The sequence of ejection is as follows:
1) The rack moves in a linear direction and drives a gear that is attached to, or is part of, the core.
2) The resulting rotational movement unscrews the core from the part. A ratchet ring prevents the part from rotating with the core, by means of small teeth embedded into the skirt of the closure.
3) As the unscrewing takes place, the ratchet ring is indexed forward by cams to maintain engagement with the part.
Although many successful variations on this theme exist, some limitations are inherent to most of the designs. The limitations are explored in depth for purposes of comparison with the new 3R unscrewing design:
Core cooling. To minimize the rack stroke required on a rack and pinion mold, the drive gear on the core must be kept as small as possible. This can restrict cooling channel size for the larger-diameter closures.
Rotary seals. The core cooling fluid must be contained with rotary seals. Although seal designs are constantly improving, they can be a frequent source of water leaks, even after relatively short periods of use (see explanation below).
Core rotation. The core rotation can lead to wear between the core and ratchet ring tapered shutoff. The wear will be seen as vertical flash on the closure (see explanation below).
The presence of contaminants in the cooling fluid is a major contributor to leaks when rotary seals are used on a rotary core mold. Particles may be present in the form of sand or silt, or mineral, sulfate, silica, and phosphate buildups. They may also be the result of corrosion - mainly microbiological-influenced corrosion (MIC), which also produces scale deposits as a byproduct. Chemical additives that are used to control corrosion can also attack the rotary seals.
For all practical purposes, contaminants cannot be totally eliminated. Particles can get between the seals and sealing diameter, and eventually cause wear grooves - even on a highly polished hard-chrome surface.
Even without the presence of sediment, the rotary seals are subjected to wear. Back-up seals are often provided on rotary core molds. However, the back-up seals are generally not designed to run dry and can fail before the primary seal. Constant monitoring is required. If the cooling fluid does leak past the seals, it can lead to extensive corrosion damage to the bearings, which are normally positioned beside the seals.
Another factor that can contribute to water leaks is the rotation of the core. Rotation is affected by numerous factors, including internal clearances in the bearings; clearances in the bearing installation; placement of the bearings in relation to the rack, and the distance from each other; the overhang beyond the bearings; misalignment between the bearing plates; and wear. All of these factors permit the core to oscillate during rotation, which constantly compresses and decompresses the seals. The faster the core rotates, the faster the seal has to react to the constantly changing clearances. By slowing down the rack speed, one can reduce the risk of leaks, but at the cost of an increased cycle time.
Any oscillation of the core as it rotates can lead to wear at the core and ratchet ring tapered shutoff. Alignment bushings between the core and ratchet ring are often provided, but the bushings wear and require monitoring.
Water pressure, which acts against the inside of the core, exerts constant pressure to force the core forward. Any clearances in the bearings or washers, used for axial alignment, will permit the core to move forward as it rotates. This maintains engagement with the ratchet ring taper during the initial rotation, and can accelerate taper wear.
Because of the cam mechanism design that is normally used on most rack and pinion molds, any wear on the cam, cam follower, or wear plates can lead to a delayed separation of the tapers during unscrewing, and premature mating of the tapers during reset. The result is that a portion of the core rotation is done with the tapers engaged. Again, this can accelerate taper wear.
An alternative to the rotary core mold is to unscrew the part from the core. This can be accomplished in two ways: by building the unscrewing function into the mold, or by using a machine that performs the unscrewing function outside of the mold - a so-called unscrewing system.
The unscrewing system is based on a mold with two core halves, and a three-tie-bar machine equipped with an unscrewing mechanism. The core halves are mounted to a swing plate that rotates around one of the tie bars. The cavity half is mounted to the stationary platen. Following is the sequence of ejection:
1) Parts are molded on the first set of cores.
2) The plates then swing 180 [degrees] around the tie bar.
3) Parts are then molded on the second set of cores while the first set is engaged by unscrewing chucks.
4) The chucks grip each part externally and unscrew it from the core.
5) The chucks then index back and eject the parts.
This method removes all the moving and rotating components from the mold. The mold is simplified and maintenance requirements are reduced.
Removal of rotating shutoffs eliminates vertical flash, and rotary seals are replaced by static O-rings. Grease is not required because the rack, gear, and bearings have been removed, and optimum water channels can be used to maximize cooling.
Because the molds are considerably simpler and require much less maintenance, and because the molding cycles are faster, the method has proven to be an economical approach when compared to conventional unscrewing molds. Economical, that is, provided that the product lends itself to this method and the production requirements are sufficiently high to justify the high initial costs.
3R (Rotating Ratchet Ring)
A new method of unscrewing parts from stationary cores, by means of an in-mold unscrewing mechanism, is the patented 3R approach. Figure 2 represents a typical cross section of a 3R mold. Following is the molding sequence:
1) As the mold opens, the air pistons move the bearing plates forward, taking up a small space between the cam and cam follower. This provides an initial clearance between the tapers of the cores and the rotating ratchet rings.
2) A hydraulic cylinder pulls the cams and racks, which, in turn, drive a series of pinions.
3) Each pinion drives a set of four rotating ratchet rings, which unscrew the closures by means of small teeth embedded into the skirt of the closure.
4) The forward movement of the bearing plates maintains this grip as the part is unscrewed. The rate at which the bearing plate moves is controlled by the cam and cam follower arrangement.
5) At the end of the bearing plate movement, compressed air helps eject the parts.
6) Prior to mold close, the bearing plates are returned by the air pistons before the rack/cam mechanism is repositioned.
7) The unidirectional clutch in the drive gear permits the rack and cam structure to be reset after the mold is closed.
Like the rotary core and collapsible core methods, the 3R design has the advantage of running in a standard machine. It does, however, have some additional benefits over those approaches:
* The thread on a closure is usually the thickest wall section, and tends to regulate the cycle. If a closure is unscrewed too early, the frictional forces experienced at the thread could overcome the internal strength of the part. This causes the thread to swell and distort, which leads to capping problems. With the 3R approach and elimination of any restrictions to core cooling, the size of the cooling channel can be increased and brought closer to the molding surface. Thus, the part can be ejected sooner.
* As the core is no longer rotated and physical demands on the core material have therefore diminished, it is also possible to place more emphasis on thermal conductivity properties in the selection of the core material (e.g., beryllium copper). This can bring significant additional improvements in heat removal and further reductions in cycle times.
* As on the unscrewing system described above, the 3R method's stationary cores permit replacement of the rotary seals with static O-rings. This greatly reduces maintenance requirements and risk of water leaks.
* If humidity levels are high, condensation can also occur on the cooled cores. If the alignment bearings are in contact with the core, corrosion of the bearings can occur. The 3R design places the alignment bearings over the ratchet rings, away from any potential moisture buildup.
* The 3R design reduces the risk of wear between the core and ratchet ring tapers, and the subsequent vertical flash, by means of the following:
1) Providing an initial clearance between the core and ratchet ring tapers before rotation (see ejection sequence).
2) Use of a unidirectional-clutch, which allows the ratchet ring to remain stationary during reset of the cams and racks.
3) Reversing the cam mechanism to allow the bearing plates to follow, rather than be driven by, the cams. This ensures no delay in forward movement of the bearing plate, even after cam wear and cam follower wear.
4) Improved alignment, as needle bearings at the ends of the ratchet ring eliminate any overhang.
* The implementation of the unidirectional clutch and the reversal of the cam mechanism allow the racks and cams to be reset after the mold has been closed. This eliminates any delay in the cycle (rack reset) and permits the flexibility of a double ejection stroke, if required.
* Another benefit of using the unidirectional clutch is that, in the unlikely event of a seizure (or if the torque design criteria are exceeded), the clutch will slip and no longer transmit torque before any damage occurs to the gear teeth.
* The stationary core also permits accurate rotational alignment of the thread start, to the cavity.
* The gear ratio, drive gear to ratchet ring, results in a reduced rack stroke for most closures.
* Compared with the rotary core design, the number of racks required has been halved, which allows the hydraulic cylinder to be positioned between the racks. As the racks and cams are positioned behind the core, the hydraulic cylinder can also be sunk into the mold shoe - behind the air and water services required for the core. The benefits of the repositioned hydraulic cylinder include reduced overall height of the mold, making it easier to handle; reduced risk of part contamination in the event of a hydraulic leak; and shorter racks and cams (also the result of the reduced rack stroke required).
* Another major benefit is the improved accessibility of modular components. This permits fast product changeovers and replacement of inserts without removal of the mold from the molding machine. The latching over of the front bearing plate exposes most of the major components, including ratchet rings, drive gears, clutch mechanisms, pinions, wear plates, alignment bearings, thrust washers, air pistons, and seals.
Rotating core and 3R molds, producing identical 38-mm closures, were compared. Both molds and hot runners were manufactured by the same supplier and were tested in the same Husky LX300 machine, with a reciprocating screw injection unit. All of the process setup parameters were identical.
Part weight: 5.0 grams
Part material: HDPE
Manufacturer: AutoChem; Number: 2110 MN50
Color: White; % Concentrate: 3%
Mold process temperature: 10 [degrees] C
Results: The 3R mold was able to produce acceptable parts at a cycle time 1.5 sec (18.3%) faster than the rotary core mold. Most of the cycle improvement was a result of reduced hold and cooling time (0.9 sec). The remainder of the improvement resulted from a faster ejection and mold close time (0.6 seconds faster). See Fig. 3.
P. Weick, "Threaded Closure Molding: An Alternative Approach," SPE ANTEC Tech. Papers, 34, 264 (1989).
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|Author:||McCready, Derek R.|
|Date:||Oct 1, 1996|
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