Heavy Diverting in Packaging

A major dairy packaging company operating multiple plants in Texas and surrounding states had a need to improve the inspection and reject station for their half-gallon gable top carton filling line.  The improvement in the inspection system’s speed and accuracy meant the station would no longer be the critical path for improving overall line speed and productivity provided the reject portion of the station could match the inspection system’s performance.  They chose to use a SMAC moving coil actuator to solve this problem.

“I don’t think the designer of the inspection system fully understood the importance of choosing the right reject mechanism until they got done evaluating their own inspection solution” observed Steve Curtiss, SMAC’s Eastern Territory Sales Manager.  “It was then that they realized that for the total solution to be a success and for the customer to leverage all the improvements they were providing with their inspection device, they needed a reject device that preformed comparably.”

The previous reject system used an air cylinder that was unsatisfactory due to its slower speed, low response time, poor reliability and short life span.  In addition, the pneumatic reject system needed to generate so much velocity at the end of its stroke to reject the large carton that it had a subsequently high velocity at point of product impact that it was causing jams, spillage and damage to the carton.

“Calculating the necessary speed of the station has to include the variability in this speed and it’s response, and not just the published new, out-of the-box speed of the device” added Steve Curtis.  The variability of pneumatic devices is based, in part, by the stiction that occurs with the air seal wearing and getting clogged, especially when on in use for a period of time. It also is impacted by variations in air pressure and volume when it needs to operate.  So while the published speed of the previous pneumatic cylinder may have appeared to be sufficient on paper, the fact that that this speed had to be reduce by the variance made it insufficient. 

Response time, or the time it takes the device to start moving once a signal is sent from the controller, also plays a factor in designing the speed of a reject station.  Pneumatic devices typically have response times of +/- 200 msec.  This response time must also be added to the overall cycle time of the device when calculating the fasted dependable cycle time of the station.  SMAC actuators have a response time of +/1 msec.

Increasing productivity involves reducing downtime as well.  “Customers are often so focused on solving their immediate issues that they often forget to consider the overall impact of their decisions on what type of diverter to use” explains Steve Rowland, P.E.  “Choosing SMAC’s actuator with a designed life expectancy of >200 Million cycles, compared to 10-20 million cycles for air cylinders, will save even more money in the long run due to not having to take the line down for repairs and replacement as often.”  This does not include the reduced downtime because the SMAC has push-button changeover and there is virtually no debug time after changes.

Rejecting a large package like a half-gallon gable top carton requires a lot of energy.  With the energy needed being represented by the equation E =mv², it can be seen that the energy increases with the mass (m) of the container and/or actuator rod but, more importantly, the square of the velocity (v).  For a pneumatic cylinder to achieve the necessary reject energy for a larger carton the velocity at the end of the stroke must be high.  It also results in a lot of energy having to be absorbed by the unit at the end of the stroke to stop it.  This energy causes loud noises.  It also imparts significantly higher damage to the unit to stop the cylinder at the end of the stroke which equates to further reduction in life expectancy and more frequent replacement.  

Higher required reject energy/velocity by an air cylinder results in a subsequently higher velocity at the time of product impact because the velocity of the air cylinder can’t easily be controlled throughout the stroke.  Higher impact energy means greater chance of carton damage, product spillage and jams.  Product spillage equates to lower productivity due to clean up costs and associated potential downtime to do so.  

The velocity, stroke and force of the SMAC actuators can control velocity, stroke and force throughout the motion profile.  This allows the velocity/impact energy to be reduced at the time of carton impact while having the acceleration necessary to still achieve the reject velocity at the end of the stroke.  This control means that the actuator can also be slowed down at the end of the stroke to reduce noise and wear on the actuator.  The actuator’s controller allows for multiple motion profiles to be stored and/or sent to the actuator to allow push button changeovers during and between product runs. 

SMAC actuators have built in feedback of velocity, stoke and force. This feedback means that not additional sensors are necessary to validate the actuator’s moves.

“I had never seen an electric actuator in operation before,” said the industrial engineer at the facility.  “The decrease in spill cleanup and lack of product damage alone made it (SMAC actuator) justifiable to me.  But now that I see how fast and efficient it is, and how easy it is to set up, we are looking to eventually change over all our reject mechanisms to electric cylinders.”