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R&B Plastics integrates
motion control functionality into an off-the-shelf PLC to optimize blow
molding
PLC handles control
functionality with built-in integration of parison control, synchronized
camming of blow pins with carriages, linear positioning of shuttles and
30 zones of heat control.
by Jim McMahon
Like most blow molding
original equipment manufacturers, R&B Plastics Machinery, LLC (R&B) has
to coordinate different machine motions very closely. Its machines had
been using closed-loop hydraulic motions with dedicated motion control
cards, which are not only expensive, but also somewhat difficult to
coordinate with programming.
However, recently the
company decided it would be more efficient to control these hydraulic
motions directly from its PLCs rather than having to use dedicated
motion control cards. In conjunction with Siemens Energy & Automation,
Inc., R&B has successfully integrated the motion control functionality
of its blow molding machines, including parison control, into
off-the-shelf PLCs, and in the process increased productivity, reduced
machine cycle times and cut costs by reducing the amount of components
required.
Specialists in Plastics Machinery
Founded in 1980 and based in Saline, Michigan, R&B provides solutions
for plastics machinery, molds, tooling, and trimming systems. It offers
programmable controllers, touch screen operator interfaces, independent
parison controls, inclined or horizontal machines, free standing platens
with mold face compensation, in-machine trimming, single screw
extruders, and trimmers and deflashers. The company also designs and
builds custom equipment for various processing requirements and provides
precision built blow mold and trim tooling products.
Additionally, it offers
machinery audits, machinery installation/removal, reconditioning, and
metal cutting machinery services.
PLC and Motion Controller in One Unit
One particular R&B blow molding machine has nine axes of
servo-hydraulics - three are for parison control and six are for
motion control. It has two shuttles, two molds and two blow pins.
Clamp motions open and close the molds, blow pins move up and down,
and carriages, which hold the clamps, move underneath the parison
head, and then out to the blow station.
“R&B is using a standard
Siemens S-7 PLC to control these positioning motions,” says David Chin,
a technician with NDC Technologies, a Siemens strategic partner in the
controls upgrade. “We provided a set of function blocks that gave R&B
the position control with a PLC. Other machines will control motion with
a motion controller, a dedicated controller for servo-controls and
positioning. We were able to customize the design of the function block
to achieve the same type of functionality and performance compared to a
stand-alone controller.”
“A PLC is not usually
associated with high precision for servo-control, that is why a
dedicated motion controller is normally used,” explains Chin. “But here,
the motion control function that is handling the positioning of various
components, which are all hydraulically actuated, is controlled by a
standard Siemens S-7 PLC.”
The PLC is also controlling
30-plus zones of heat control from the hopper to the extruders and
through to the parison head. Twelve of these zones are for heat-cool,
and 18 are for heat only. The heat control system, starting with cold
plastic in the hopper to 415 degrees F at the parison head, is critical
to maintaining a temperature tolerance of +/- one degree throughout the
machine.
The off-the-shelf Siemens
S-7 PLC uses function blocks written in standard IEC-61131 languages to
control all servo hydraulic axes. It does not require a different motion
control environment. By integrating motion control capabilities into the
PLC, like the S-7, faster performance and much more efficient processing
capability is provided, as opposed to the use of dedicated PLC and
motion control hardware.
Integrated Parison Control
The first of the motions that R&B was trying to manage through the PLC
on its blow molding machines was the parison. Normally, blow molding
machines are controlled in either one of two ways: by a machine
controller, and a completely separate and independent parison
programming unit with a dedicated motion control card; or by the use of
PC-based automation using specialized and often proprietary code, to
handle the motion control requirements. R&B wanted to integrate the
parison programming right into the main machine controller. Programming
speed with Siemens S-7 PLCs has become fast enough to make this
possible.
The parison control data
comes directly from the position transducer as an analog input. All of
the processing is done as regular PLC code, and then an analog output is
used to control the hydraulic valve. This is done without using any
specialized hardware.
Integrated Hydraulic Axis Control
R&B then integrated the rest of the servo hydraulic axes into the PLC.
The position transducer data for these axes come into the PLC via SSI
modules. If required, the position data coming from the SSI modules can
be synchronized to the PLC program scan using Siemens’ Profibus
Isochrone mode. Again, standard analog output modules are used to
control the hydraulic valves.
Synchronized Camming of Blow Pins with Carriages
Another function that R&B has integrated into the PLC is camming of the
blow pins with the carriages. As the carriage brings the mold toward the
blowpin position, the blowpin traverses downward following a programmed
“cam” profile. Since the position of the two axes is coordinated, this
can be accomplished with minimal margin for error.
Before integrating this
function into the PLC, R&B was unable to do this synchronized camming
because the blow pins and carriages were not able to communicate. The
protocol programmed into the S-7 PLC makes this functionality possible
for the first time with blow molding.
Reduction in Dry-Cycle Time
In blow molding, the time it takes to actually blow the bottle or cool
the bottle is typically similar on all machines. But there is a
difference between machines from a cycle-time standpoint, specifically
with dry-cycle time, when the machine is not actually physically blowing
the bottle. This is measured in how long it takes the machine to get
back to the point where it is blowing the next bottle or set of bottles.
The motion control functions
within the S-7 PLCs provide a superior level of coordination of the
dry-cycle motions, reducing time intervals between specific actions. It
enables multiple motions to be coordinated together - position-based as
opposed to time-based. This allows significant time, as much as 20
percent, to be taken out of the dry-cycle, giving R&B a cycle-time
advantage.
PLC-Based Automation for Blow Molding, a Better Solution
The advantages to using a proven, off-the-shelf PLC-based automation
solution for blow molding compared to dedicated hardware for motion and
parison control are significant. Since all servo-functions are
controlled from one function block, this means better coordinated and
faster machine cycle times, more simplified and faster programming and
less equipment. Additionally, dedicated hardware is manufactured in
small production runs bringing into play quality control factors,
long-term support issues, and the need to re-engineer and fix problems
with the product, all of which require additional financial
justification.
Compared to PC-based
controls, proven PLC-based automation solutions also provide a more
attractive controls package for the blow molder, including a more rugged
unit, use of solid state memory, no dependency on third-party operating
systems, longer product life cycles and more long-term support. One of
the key benefits of PLC versus PC control is the ease with which PLCs
retain data when they are shut off (such as with a power loss).
“We now have one controller
that is controlling our PLC functions and motion control functions,”
says Jake Losee with R&B Plastics. “We do not have the communication
delays that we had before using a stand-alone card. Also, we have recipe
storage of all of our axes. Before, we had separate software packages
that could only be used by a technician who understood a particular
motion card. Now, we are able to put it all on our HMI (Human Machine
Interface).”
“One of our goals was to
eliminate equipment,” explains Losee. “Adding additional equipment means
more costs, more maintenance and more software, which requires more
training for our maintenance personnel. The Siemens S-7 PLCs, with its
integrated motion and parison controls, has delivered a highly optimized
controls solution on our blow molding machines.”
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