by Jason Holbrook, Krauss Maffei Corporation
When adding robotic automation to production lines, project owners should consider involving all parties, like the manufacturers of the machine, robot, automation, end-of-arm tool and guarding. There are many considerations that can impact the time, money and labor saved when automation is brought into a facility, and a robust team provides a better opportunity to capitalize on those savings. Here are five factors that should be considered.
1. Space Requirements
As industries pursue the most efficient means of production, space requirements have become an increasingly important feature of production cell design. Simply put, facilities are trying to shoehorn more production into tighter spaces, while also maintaining a safe and ergonomically correct working environment for the laborer. Historically, a fully automated injection molding machine cell has consumed approximately three times (3x) the machine width to include the machine, guarded production cell, supporting equipment and work-in-process (WIP). More recently, this is being reduced to two times (2x) the width and, in some cases, one times (1x) the width by utilizing a more efficient linear cell design.
To achieve a cell two times (2x) the width, keep the automation on the non-operator side of the molding machine, integrating the guarding to the top of the conveyor or elevating the conveyor itself above 2,200mm from the floor. By restricting robot strokes from exceeding this area, a very tight production cell can be accomplished. Both options have advantages and disadvantages, but most disadvantages easily can be overcome with creative thinking.
Achieving a cell one time (1x) the width most commonly is done utilizing a longitudinally mounted (L-Mounted) robot, which traverses to deposit the parts at the clamp end of the machine. The additional traversing stroke required adds a little cost to the robot, but minimizes the guarding and conveyor cost, while also providing more real estate – which adds its own value. The idea of this design is to place more machines next to one another with all of the parts deposited at the clamp end, so the machine operator simply can walk along an aisle to tend a lot of machines. Two-platen machine designs contribute to this solution because their clamp is two-thirds the length of the old traditional ram or toggle machines, and there is no hydraulic tank or breather bags to contend with above the clamp.
2. Safety Issues
Cell layout is critical to the efficiency of a production cell. Too often, this is overlooked in the initial stages of project, resulting in perimeter guarding encompassing an entire cell, consuming too much real estate and hindering movement and interaction for the operators managing the cell.
Guarding often is an afterthought, which often leads – at the eleventh hour prior to production readiness – it to be inadequately erected. ANSI establishes clear descriptions that OSHA adopts for oversight. These requirements have changed in recent years to improve the safety of the operator. OSHA inspections have become more critical in this area, making it important for cell designers to educate themselves on these latest revisions. One good example is the overhead solid panel guarding in any operator aisle where a moving piece of the automation operates overhead – this aisle must be guarded with a solid panel. Another mistake that often is seen is the use of metal mesh or expanded metal perimeter guarding in areas where polycarbonate panels would be the proper choice. The distance the perimeter guarding of metal mesh needs to be from any potential moving part of the automation is directly related to the mesh hole size being used. Typically, this distance can be from 24″ to 36″ around the perimeter, resulting in guarding three times as large. If this is done with polycarbonate panels, the perimeter guarding can be as close to the automation as it needs to be. So, although polycarbonate panels are more expensive than metal mesh, when following ANSI/OSHA guidelines, the polycarbonate solution may require less square footage, meaning it will be less expensive.
The use of light curtains often is suggested in place of solid panels, and there are areas where this can be very beneficial, but keep in mind that inertia of the human body has to be taken into account. A solid panel will stop a human body from moving through a barrier; a light curtain will not. The distance of the light curtain to any moving part of the automation, congruent to the time it takes for that automation to come to a complete halt, determines the distance the light curtain needs to be positioned from the automation. Most often, this too results in a larger guarded area, where the solid panel is the better choice. That said, light curtains have their place in automation when applied correctly.
3. Wrist Choices
Injection molding applications have, more often than not, been automated with (3-axis) linear robots compared to (6-axis) articulated robots. Although the later have become more popular in recent years, they bring with them their own restrictions not inherent to linear robots.
To compensate for the flexibility that a typical 6-axis articulated robot has over a 3-axis linear robot, manufactures of linear robots now offer 5-axis and 6-axis servo robots where an A-B-C-Servo wrist is added to the bottom of the vertical arm on a linear robot. This provides the best of both worlds – faster mold harvesting of the linear robot, with articulated human-like movements for post-molding applications, while still being mounted to the top of the stationary platen and not consuming real estate beside the press, as is typical of a floor-mounted 6-axis robot.
Now, there are plenty of times where the 6-axis articulated outperforms the 3-5-6-axis linear robot, and those are typically where orbital movement of the post-molded part through an automated cell is most efficiently done, where a linear robot simply doesn’t have that degree of freedom or tatk time to complete the operation. Herein lies the importance of 6-axis robots, and they are often more properly used in conjunction with a linear part harvesting robot in post-molding applications.
4. EOAT Design
End-of-Arm Tool (EOAT) design, like guarding, often is an afterthought not considered until the mold has arrived, and the molder is under pressure to produce production volume parts. It is very common then for a process engineer to fabricate an EOAT on-site from an erector set of components on hand, without the art of form and function being utilized to its fullest. More often than not, this then is the same EOAT running the application months into production, causing headaches at set-up and contributing to inconsistent position tolerance.
An EOAT should be considered part of a system that operates with the mold. From the part’s point of view, the mold is its initial “fixture,” holding its position in very tight tolerance. Once it’s released from the mold, tolerances are added to that position. The accuracy of the EOAT picking the part from the mold will determine the accuracy to which it is being handed off. If positioning tolerance is critical to part positioning post-molding, then the stack-up of tolerances needs to be considered from the mold to the EOAT to the automation. In these cases, consider the EOAT as one of three parts to a system:
The stack-up of tolerances from this three-point system determines the position accuracy of the part; the robot and molding machine are simply carriers for the first two pieces of the system. Operators often find that the stack-up of tolerances from the system exceeds far beyond the allowances of the part assembly print. What results is the manufacturer chasing one end of the allowance to the next – a self-defeating exercise.
An EOAT should have nearly as much forethought given to it as the mold design, with the idea being to hold the stack-up of tolerances as tightly as possible. The EOAT should be built in such a robust manner that it doesn’t beat itself to death through daily use. Keeping features tightly positioned to main mounting position and nesting the parts in Delrin pockets is very common practice to a successful EOAT design. Part sensing and independent vacuum and gripper circuits also contribute to proper EOAT function, so make sure to consider these facts when outfitting a robot.
5. Vibration Isolation
Lack of vibration isolation in an automated cell contributes to more problems than most consider:
- premature component wear
- position inaccuracies
- extended cycle time
Vibration, as with most energy, travels in a sine curve. Less resistance results in lower frequency. The more mass, the lower the resistance and the lower the frequency becomes. As mass is reduced throughout the movement of vibration, the higher the frequency becomes. In the case of sine curves from the forces of the molding process, once it terminates, it will become noise, heat or vibration – vibration being the path of least resistance. A molding machine is designed to have the forces travel through the stationary platen, down through the frame of the machine to the vibration isolation pads and, finally, to the concrete floor. When a robot is positioned on top of the moving platen, its like setting a tuning fork on top of the platen. The mass of the stationary platen is very large compared to the riser of the robot. The mass of the riser of the robot is larger than that of the traversing stroke – and, so on. By the time vibration gets all the way through the kick strokes, vertical arm and wrist flip to terminate at the EOAT, the mass of material for that sine curve to travel through has been reduced exponentially, resulting in uncontrollable vibration. The further the strokes are extended, the higher the frequency of this vibration, and the worse it becomes.
The solutions to such problems are rather simple:
- Ground the vibration.
- Reduce the length of travel needed to execute the operation.
To ground the system, consider adding an outrigger vertical support leg to the traversing beam of the robot. The more mass this leg has, the easier it will be for the vibration to take that path of least resistance to a vibration isolation pad at the bottom of the beam to the floor. This prevents the vibration from working its way out through the kick stroke, reaching the EOAT. To minimize strokes, consider positioning insert pick-up or degating stations as close to the traversing beam and robot riser as possible, rather than mounting them on the floor where the robot must extend itself to max stroke. This will tighten the capability of position tolerances and minimize the impact of vibration. This also prevents premature wear and tear on linear bearing surfaces.
In conclusion, there are many areas of improved performance to be gained when early forethought of the production cell is realized.
Jason Holbrook is sales manager for Krauss Maffei Corporation. In Krauss Maffei’s injection molding machine division, Holbrook brings with him a strong background in plastics injection machine technology and automation in several key end markets, including automotive, appliance, packaging, medical and consumer. For more information, email [email protected] or visit www.kraussmaffeigroup.us.