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Machine Output 1During a number of recent visits to molders, I have been asked about the suitability of particular styles of machine for an application. There are obviously many factors to consider in a given situation and there are as many permutations as there are products. However, one basic comparison commonly occurs: 3-Arm Turret vs. 3-Arm 5-Station Independent vs. 4-Arm 5-Station Independent
3-Arm Turret
3-Arm 5-Station (1 Main Cooling Station)
4-Arm 5-Station (2 Main Cooling Stations) These are three of the most common styles of machine in use and are often being used incorrectly in terms of cycle balance. As a result, molding performance and the 'rhythm' at the machine is less than desirable. To study the relative performance, I used the following basic cycle settings starting from a cold start with no mold in the oven:
Assumptions: Variation in cycles can occur in any station - increasing in likelihood from the heating cycle to the cooling cycle to the demolding cycle. This study examines the effect of the demolding cycle of one arm becoming increasingly out of balance with the other arms. The results will be similar for out of balance cooling or heating cycles. RotoCycle software was used to analyze the output over 24 hours as the demolding cycle for one arm becomes progressively longer by 1 to 10 minutes (i.e. the demolding time increases from 15 minutes to 25 minutes). Overall production falls for all the machine styles but each exhibits specific performance characteristics. The specific data calculated can be seen in tabular form at Raw Data. The results are summarized in the following sections. 1. Effect of Increasing Cycle Imbalance on Overall Machine Output The first stage of assessment looks at the total number of arms produced over a 24 hour period as the demolding time increased for one arm. This could also apply to an increased oven cycle or cooling cycle. Changes to the cycle for only one arm is used for a basic comparison - the analysis obviously becomes more complex when the cycles change for all the arms.
The 3-Arm 5-Station machine with cooling conditions (a) and (b) is affected more by delays on demolding than the 3-Arm Turret machine. The 4-Arm 5-Station machine is affected least in terms of overall number of arms cycled through. Therefore, on the face of it, a 4-Arm 5-Station machine is a good choice for total productivity, right? Not quite - let's take a look at some of the internal details in the data. 2. Effect of Increasing Cycle Imbalance on Oven Delays
When there is a delay on a turret machine, all arms have to wait in their current station. Typically in the case of the oven, this means that two things are occurring: (1) the oven rear doors are open and (2) the parts are being cured for longer than programmed. The figure above shows that when a delay or longer demolding cycle occurs on one arm, an arm is delayed in the oven station up to 17.5% of the total oven operating time (for a 10 minute imbalance). For a 3-Arm 5-Station machine, there are no arms waiting in the oven at any time due to the two levels of redundancy available. This is true for both sets of cooling conditions. In the case of a 4-Arm 5-Station machine, the fact that there is only one free station (one level of redundancy) means that arms are delayed in the oven station up to 12% of the time at a 10 minute imbalance. A 3-Arm 5-Station machine therefore frees the machine from oven delays that dump heat into the factory and produce overcured parts. 3. Effect of Increasing Cycle Imbalance on Cooler Delays
Delays in the 1st cooler are affected by machine style and the way in which the cooling time is divided (in this case condition (a) or (b)). A 3-Arm Turret machine is delayed the same amount of time in the cooler and the oven. A 3-Arm 5-Station machine is delayed a similar amount when the 1st cooler is programmed for 0 minutes (condition (a)) - i.e. arms will be idle in the wait station a proportion of the time. A 3-Arm 5-Station machine has no delays in the wait station when the cooling cycle is evenly divided between the two stations (condition (b)). A 4-Arm 5-Station machine has arms delayed in this station a very large proportion of the time for both conditions.
The results are somewhat different for the 2nd or main cooling station. The 3-Arm Turret and 3-Arm 5-Station (a) machine perform in a similar fashion but when the cooling cycle is less than the demolding time (3-Arm 5-Station machine condition (b)), there is an arm delayed in this station up to 30% of the time (for a 10 minute imbalance). A 4-Arm 5-Station machine actually has the lowest delay in this station for cooling condition (a) but the highest when cooling condition (b) is used. Note that the combined level of delays for the 4-Arm machine is highest. Arms sitting idle in a cooling station affect the way in which a part cools and therefore will affect the impact performance and size of the part. If the arm cannot proceed through a cooling cycle uninterrupted, the ability to control the cooling cycle is reduced. 4. Effect of Increasing Cycle Imbalance on Pre-Oven Delays
A 3-Arm Turret does not have a pre-oven (2nd load/unload) station and is not included in this graph. Arms held up in the pre-oven reduce as the level of imbalance increases. A 4-Arm 5-Station machine has an arm waiting ahead of the oven almost 95% of the time when the cycles are balanced (i.e. 15 minute demold). A 3-Arm 5-Station machine has a much lower level of delay. 5. Effect of Increasing Cycle Imbalance on Overall Station Delays
An overall measure of station delays is taken by combining the times that arms are idle in a particular station and dividing by the number of stations. The graph above shows the amount of time that arms sit waiting as cycle imbalance increases. Given that a turret machine format is the base level, a 3-Arm 5-Station machine exhibits lower levels of arm delays and a 4-Arm 5-Station machine exhibits considerably higher levels of delays. Note that even when the cycles are balanced, arms will be delayed on station for a 4-Arm machine (for the cooling conditions selected using a fixed total cooling time) for over 20% of the time. Delays on-station will affect cycle repeatability and therefore both product quality and overall production rates will be reduced. Minimizing delays and improving flow at the machine is critical for consistent repeatable production. Conclusions:
Therefore:
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