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Machine Output 2There are probably as many ways to analyze machine production cycles as there are molds. Comparing machine styles for overall performance requires a careful look at many factors including, product type, mix and variety of molds, demolding times, initial capital requirements, floor space and labor available. In a similar manner to the analysis in Machine Output 1 which looks at the effect of cycle imbalance on the relative performance of turret and independent carousels, this analysis takes a look at how shuttle machines perform compared to fixed and independent carousel machines for situations where the demolding cycle is extended for some reason on one arm. This might be caused, for example, by complex mold construction, many inserts or a lot of graphics. Two styles of shuttle are considered:
1-Arm Shuttle
2-Arm Shuttle (Single Oven) There are other versions of shuttle available such as 3-Arm Shuttles, Double Arm with Double Ovens (effectively two 1-Arm machines in-line) and in-line shuttles with multiple stations but these are not considered here. Also, a 1-Arm shuttle can be considered to act in the same fashion as a clamshell (single station) machine. The overall productivity and ability of these machine styles to handle cycle imbalances are compared against a 3-Arm Turret, a 4-Arm Turret and a 3-Arm 5-Station Independent machine as shown below:
3-Arm Turret
4-Arm Turret (2 Cooling Stations)
3-Arm 5-Station (1 Main Cooling Station) The comparisons use a base cycle setting of 15 minutes oven, 15 minutes cool and 15 minutes demolding starting from a cold start with no mold in the oven. Assumptions and Data: The effect of an increasing demolding cycle on one arm is used to examine the effect of a cycle imbalance on production rates and station delays. RotoCycle Software was used to analyze the output over 24 hours as the demolding cycle for one arm becomes progressively longer by 1 to 40 minutes (i.e. the demolding / servicing time increases from 15 minutes to 55 minutes). Note: For a turret or independent machine, the station at which the imbalance occurs does not necessarily change the overall productivity of the machine - all arms are affected in series and only the pattern of delays differs. However, for a 2-Arm shuttle machine there is a major difference if the imbalance occurs at the oven station or the unload/cool station. If the imbalance is in the oven, the second arm must wait until it is free before progressing; if the imbalance is in the unload/cool station, then the other arm can use the oven station and continue to cycle, effectively bypassing the other arm. This can be important when prototyping or using molds that have very complex servicing requirements. The number of complete cycles produced in 24 hours for each style of machine are shown in the table below as the imbalance (increase in demolding time on one arm) grows. The data is also plotted in the graph below.
The results of the comparisons are summarized in the following sections: 1. Overall Machine Output On a per machine basis, the 3-Arm Turret, 4-Arm Turret and 3-Arm Independent machines are all very similar when the cycles are balanced and produce more arms per day than the shuttle style machines. The 4-Arm Turret produces slightly more arms as the imbalance or delay on one arm increases. The 2-Arm shuttle and 1-Arm shuttle obviously produce less overall production with fewer arms in service. Note the fact that the output from a 2-Arm shuttle levels off as the cycle imbalance becomes excessively long - see '3. Bypass' below. Although output per machine is lower, single station machines can also be very flexible platforms for manufacturing operations that are linked to other assembly processes - parts can be made to order for a production line. Also, single station machines will typically require more floor space and can therefore enable easier set up of complex secondary operations in line with the machine. Multiple products coming from a turret style machine can make secondary areas cluttered and difficult to manage. 2. Output per Arm However, on a per arm basis, overall productivity for a 1-Arm shuttle machine is equivalent to a 2-Arm shuttle, a 3-Arm turret and a 3-Arm, 5-Station independent machine. Productivity on a per arm basis is lower on a 4-Arm turret although the total production output for the machine is roughly equivalent to the other carousels. This per-arm output figure is dictated by the longest segment of the molding cycle (in this case all equal at 15 minutes). Three 1-Arm shuttles combined can produce the same output as a 3-Arm turret or a 3-Arm, 5-Station Independent. However, there is an increased initial capital cost for this format, more maintenance of three separate systems and three separate demolding stations to manage. However, it does allow the option to build molding capacity gradually as demand allows. If demolding time can be reduced, overall output can increase in a shuttle style machine as this saved time is automatically transferred to the next molding cycle - this typically will not affect production output on a carousel machine unless all the arms can be improved in the same manner. 3. Bypassing Arms One great advantage of shuttle machines is the ability to effectively 'bypass' arms. If cycle times are dramatically different or there is work that needs to be done on one side of the machine or to a mold, it is possible to continue working on the other side of the machine without interruption. This can clearly be seen for the 2-Arm shuttle machine in the above table when the cycle imbalance raises the demolding time for the delayed arm to beyond twice the normal demolding time of 15 minutes - the overall output of the machine remains almost steady as the arm with the shorter cycle is able to take advantage of the downtime for the arm being delayed. The delayed arm continues to produce at the same rate as the other machines on a per arm basis. 4. Two 2-Arm Shuttles vs. A 4-Arm Turret An effective combination of machines consists of two 2-Arm shuttles positioned opposite each other across a common operator aisle-way. This requires two operator stations manning one demolding station from each machine. The overall output of these two machines is some 25% greater than an equivalent 4-Arm turret when compared for small cycle imbalances (up to 10 minutes). When the imbalance is great, and the ability of the shuttle machine to bypass becomes evident, this produces an even greater improvement. There are some factors to consider in this set-up:
The choice of using two 2-Arm shuttles together vs. a turret or independent machine will depend upon the mix of products needed to be molded. Parts with the same cycles and similar demolding times which are less than the heating and cooling portions of the cycle will probably be most effectively molded on a turret style machine. Parts with dramatically different cycle times or which require a lot of servicing will probably benefit from the flexibility of a shuttle machine. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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