WEB EXCLUSIVE : Getting the most out of robots and automation

Formerly used mostly for limited part-removal functions, robots are increasingly used as components of highly integrated automation systems.

For some injection molders, automation may be as simple as using a single sprue picker to separate runner from parts, while for others it may be a stand-alone automated workcell or even an entire factory full of machines completely integrated with robots and downstream automation.

Automation provides molders the ability to improve cycle performance and product quality through consistent and repeatable operation. In addition, automation is often necessary to perform functions that could not otherwise be achieved.

Sprue Pickers and Traverse Robots
One of the simplest forms of automation is the sprue picker. Sprue pickers are typically used on machines of less than 150 tons where the parts are subgated and there is a need to separate the runner from the parts during takeout. Sprue pickers are typically pneumatically driven and are limited to performing simple in/out operations. Most injection molding machines can achieve greater returns from flexible robots and automation that perform other functions during the molding cycle.

The next level of automation is typically performed by linear or traverse robots that provide in/out operations in addition to traverse motion along a beam, followed by up/down motions to place parts. These robots also offer various levels of programmability and flexibility. Pneumatic, electric, and a-c servo drives can be used. Additional axes of rotation offer great flexibility to position parts for complex or unique applications. Any extra cycle time beyond what is required to perform the simple in/out part removal, traverse, part placement, and return to the start position is available for the robot to perform other operations.

For example, if the total molding cycle time is 30 seconds but it takes only 2 seconds in/out time to remove the part from the molding area and an additional 8 seconds to traverse and place the part, there are still 20 seconds left for the robot to perform other functions. The additional time is available to conduct secondary operations such as stacking parts on a conveyor or directly stacking parts into boxes automatically fed along an indexing conveyor. A molder might simply want to place parts at a height other than the machine centerline for downstream operations.

Sometimes the robot is required to take parts out based on the machine centerline, with little consideration given to part placement. On a 500-ton machine with a centerline of 70 inches off the floor, robot sizing for only parts removal would place parts more than 60 inches off the floor. This is not ideal for secondaries such as a conveyor belt at a height of 36 to 40 inches.

A robot equipped with an electric vertical arm allows parts to be positioned below the press centerline and could even stack parts. Equipped with an electric traverse, it could perform other operations such as feeding a degating station mounted on the traverse beam. It is, however, very important to ensure that the robot is capable of completing all operations within the given cycle time. Automation should not extend the cycle time.

Using Robots for Quality Control
Quality control (QC) functions are often overlooked when specifying a robot. A robot can sample each batch and place those samples apart from the product flow. QC may require a subroutine, a unique sequence, or even the opportunity for the operator to push a button at any time to sample parts without loss of count, if it is important to production counting or filling a box.

The robot can run subroutines or sound an alarm based on information from the injection molding machine's statistical process control (SPC) via SPI or E12 interface, machine-vision cameras, a weigh scale, or other quality control input. Rejected parts can go to QC chutes or drawers or directly to a granulator to close the loop back to the hopper.

Managing an Automation Project
For either custom or proprietary molders, new project needs during the life of a robot may dictate a need for unique sequences. Programming flexibility and payload capacity determine the robot's suitability for new applications. Although the term "programmable" is widely used, many robots offer only "selectable" functions such as C-axis parts rotation, vacuum on and off, or sprue sensing and gripping. It is important to know whether the robot is freely programmable or limited to specific sequences. Many molders want complete flexibility to perform unique sequences and the ability to assign peripheral equipment inputs and outputs.

Also, determine whether in-house engineers can perform the programming. If not, it may be necessary for the vendor or even a third party supplier to perform the changes.

Icon-based operator interfaces, common today, allow almost any operator to program easy pick-and-place setups with minimal training. More complex operations and integration of secondary equipment requires a programmer to generate unique sequences.

Ensuring that proper project management is available is critical to the success of any automation project. Often, project management is shared between the customer and robot supplier. The project management starts with a discussion about the workcell objectives, part design, mold design, cycle analysis, automation requirements, footprint, work flow, systems integration requirements, safeties, and so on. The project manager then prepares a layout drawing as the blueprint of the workcell. Project management extends to all aspects of the project such as arranging installation, rigging, and start-up of the workcell.

Some molders do not involve the automation vendor at the start of a project, for whatever reasons. This can result in cycle time penalties because parts cannot be easily removed from the mold with basic tooling, or additional motions are required to orient the part within the workcell.

The automation partner should be involved from the onset of the project to lend expertise in how best to handle the parts. An automation applications engineer can ask questions to help optimize the part and mold design. For example, ask "Can the part be produced in a hot runner mold to save the cost of a degator? Can the parts be ejected off the other side of the mold to eliminate the need for a flip-over station in the downstream?" These simple questions often are overlooked.

The Second Life of a Robot
It is unlikely a robot will be used for its entire life in the molding program for which it is originally specified, even if that is in a long-term, dedicated molding cell. Well-maintained robots can last eight to 10 years or more, through millions of cycles. Rebuilt robots effectively have a second life, and last even longer. It is, therefore, important to get a robot that can meet future needs. No one wants a robot to be obsolete in a few years when its first automation project is finished.

A common mistake is to specify the robot with a payload (parts plus end-of-arm-tooling weight) capacity only sufficient to meet the current application. Even if a robot can handle the greatest part weight that a given machine can produce, it may not be able to handle heavier end-of-arm-tooling that might be called for if the machine is switched to insert molding, for example.

When retooling a robot to suit a new mold or adding downstream components to a workcell, a molder could re-engineer and build the new workcell in-house, go back to the automation vendor, or bring in a specialized integrator or custom engineering house. Each approach has merit, provided the original equipment is sufficiently flexible.

Proper training is essential to maximizing robot utilization, uptime, and cycle time. Many robot vendors offer specialized training to ensure that molders get the most out of their automation investment. The cost of training is readily offset by the long-term benefits of having operators who fully understand the programming and maintenance requirements of the system.

Advanced Applications Superseding Simple Pick-and-Place Functions
Many applications demonstrate the usefulness of today's automated work cells. Insert loading with robots and shuttle tables or vibratory devices is becoming increasingly popular as molders try to increase profits with high precision, stabilized cycle times, and reduced scrap. It is now almost standard to use robots to insert items like nuts, bolts, pins, appliques, films, foils, sensors, clips, and fasteners.

In one example of specialized insert molding, an automotive fuel rail required five inserts to be placed in each of two cavities at various angles. The uniqueness of the orientation required programming for special robot motion. It was worth the effort, however, as the automation eliminated cycle inconsistencies and scrap resulting from the previous manual operation. (An operator formerly opened the safety gate every cycle, hand-placed the inserts, closed the gate, let the machine mold the part, opened the gate, removed the part, and went through the processes repeatedly.) A standard traverse robot was able to perform all of the needed functions, and within very limited time constraints.

Another example is the loading of thin, flexible films for overmolding. A static bar is used to attract and collect the insert and then place it in the molding area. Vacuum cups can distort the film.

Overmolding is another common insert molding application requiring specialized end-of-arm-tooling to provide an unblemished part finish. The robot uses one end-of-arm tool to collect the finished parts, while a second tool collects, transfers, and inserts the substrate to the mold for overmolding. This was an alternative to using rotating molds.

Another common task for molders is part degating, which can be done either by hand or with a dedicated fixture. With articulating robot motion control of up to six axes of freedom and advanced control systems, a traditional linear top-entry traversing robot can now manipulate the part into unique orientations and present the part-edge gate to a moving nipper system. Degating with flexible automation can be accomplished using the robot, eliminating the need and cost of dedicated hardware that would require additional floor space in the workcell.

A servo axis motion of the nipper arm and servo a-c wrist can get at most cutting surfaces to make excellent cuts. The degating attachment mounted on the robot beam or floor can now perform the function with simple programming of the robot to position the part as required in front of the degator nipper. It is very easy to change the program for the next mold to perform degating rather than setting up dedicated hardware each time a different part is molded. Robots offering flexible automation with degators have been shown to achieve excellent gate vestige, typically +.15 to -.05 inch. The robot supplier can recommend mold gating designs to simplify degating.

Downstream Operation Trends
Robots are also quite capable of performing other downstream operations to further use any remaining time within the overall cycle. For example, automatic box-filling workcells are becoming commonplace. Empty boxes are fed along an indexing conveyor to a fill station, where they are stopped. The "fill box" is moved into place by a push cylinder to frame up the box and to make certain the x-y-z reference is set to allow filling with a robot. The robot picks each shot and places it into the box. If the shot is too large or cavity spacing too great, either collapsible tooling or pick-and-place routines are sequenced to allow for the packing layout.

In some instances, an automatic tool changer may be used to allow for two EOATs. First the removal EOAT picks and places the shot before the robot switches over to the pack-out EOAT to pick and place parts into box. The robot may also be used to place interlayer sheets between layers of parts. Cycle time permitting, the primary (and only) robot can use one set of vacuum grippers to pick parts from the mold and a second set of outboard cups to pick-up cardboard interlayer sheets and place them into the box.

Demands on the robot and workcell will continue to increase. Functions such as labeling and inspection will use robots more extensively. In most cases, traverse or linear robots are used for their speed in and out of the mold. They require minimal mold-open time and little floor space. The ability to easily program robot motions and the increased ability to achieve articulating motions with standard robots will provide many new opportunities to perform more downstream operations than in the past. In all cases, until the part removal robot's actions have used all the available cycle time, there is more the robot can do than simply pick and place parts.

Other trends will benefit molders in the future. Web-based and plant-linking information systems for robots and automation are increasingly important. This allows users to monitor the operation of work cells and be notified remotely of any problems. Increasingly, custom downstream and EOAT components are becoming more flexible through programming options rather than fixed hardware. Increased integration of the robot, automation, and the injection molding machine for saving and transferring mold setup recipes will lead to faster and more efficient start-ups. The use of robots also will be supported by continued advances in "fully teachable and user friendly" robot controls. Automation suppliers will also continue to increase their role in developing specialized solutions for niche applications by being involved through partnering with moldmakers at the start of molding projects.

Editor's note: David Preusse, vp of sales and marketing for Wittmann Automation Systems, has extensive experience in working with robots and automation specifically for plastics processing.

Contact Information

Wittmann
Torrington, CT; David Preusse
(860) 496-9603
www.wittmann-ct.com

Yushin
Cranston, RI
(401) 463-1800
www.yushin.com

Elwood Corp./Mark 2 Automation Group
Oak Creek, WI
(800) 541-8813
www.elwoodcorp.com

Conair
Pittsburgh, PA
(412) 312-6000
www.conairnet.com

Ranger Automation Systems
Shrewsbury, MA
(508) 842-6500
www.rangerautomation.com

Star Automation
Menomonee Falls, WI
(262) 253-3550
www.starautomation.com