Molders' Guide to Do-It-Yourself Robot Tooling
An injection molding robot is no better than its end-of-arm tooling (EOAT). All the potential benefits of robots--increased productivity, quality, and safety, as well as reduced scrap--are influenced by the effectiveness with which the EOAT does its job. End-of-arm tooling may perform tasks as simple as sprue picking and demolding or as advanced as degating, insert loading, parts reorientation, and assembly.
If a molder can build and/or repair molds, then EOAT design and engineering are within its technical capabilities. While many molders depend on either robot suppliers or specialty tooling houses for their EOAT, molders that design and build their own often achieve substantial cost savings, reduced lead times, and greatly increased flexibility to meet job changes in a timely manner.
At a minimum, molders should make their own simple and mid-level EOAT. Difficult tools, with secondary functions or movements, can be outsourced until in-house personnel have gained sufficient skills. On the other hand, if your operation involves mainly dedicated robot applications that will run many years without changes, it may be more economical to continue sourcing the occasional new EOAT from outside vendors.
Surprisingly, in-house EOAT design after a new mold is delivered often produces results quicker and more efficiently than so-called "concurrent" engineering done off-site by a tooling supplier. A huge amount of information must be collected prior to designing the EOAT, and the chances for error in accumulating, transmitting, or interpreting that data are substantial. All changes to parts or molds must also be resubmitted to the EOAT vendor. On the other hand, those who design EOAT on-site with ready access to the molding machine, molds, and parts can readily confirm details and thus reduce the number of iterations and the level of risk in the process.
Information you'll need
While not every item listed below will apply to every application, you may have to obtain the following in order to design your own EOAT:
Robot data: EOAT mounting dimensions, robot dimensions, maximum strokes, speeds, and payload.
Molding machine data: Dimensions, strokes, distance between tiebars, mold-open distance, clamp repeatability.
Mold data: Mold prints (ejector-side and cross-section views) with dimensions on all critical components, including ejector strokes.
Part data: Samples, drawings, material, demolding temperature, and non-standard conditions (e.g., flash).
Molding cycle time: Mold-open time and overall cycle.
Plant data: Location and dimensions of associated downstream equipment (like conveyors) with which the EOAT must interact.
By far the best source of molding-machine data is direct measurements taken on the production floor. Manufacturer's specifications and plant layout prints will also provide essential data. Videotape is extremely useful in helping the EOAT designer visualize demolding conditions clearly. The video should include at least six complete cycles or one minute per view, showing the following:
Top views, side views (i.e., from the molding machine's front and back) of the mold-opening and ejection cycle, and views of cavity faces on both mold halves.
Long views showing surrounding activities and equipment on all sides of the molding press.
EOAT and mold interface
A few more factors bear special mention, as they affect part removal and thus influence equipment selection and EOAT engineering. The key to successful part removal is to never lose control of the parts between the mold and the EOAT. The EOAT must secure the parts at a point during ejection that maintains them within the position tolerance required by downstream operations.
The ejection stroke must be set to the minimum needed to eject the parts with minimal force. If the parts fall during or after ejection, the EOAT must be interlocked with the ejectors so it can grip or guide the parts to maintain the desired orientation and position tolerances. If the parts remain on the mold within the desired downstream tolerance, then the robot will not have to interlock with the press ejection system.
When mounting a mold in a machine, the sprue bushing on the stationary platen determines the horizontal and vertical mold positioning. However, some horizontal, vertical, and angular variation in mounting is still possible. Thus the EOAT must be engineered to tolerate this range of variation.
You will need to know the mold-opening position tolerance of the press with the mold mounted and running at the desired cycle time. On older or larger presses, mold-open position may vary up to ±0.25 in. These presses should be upgraded to provide a narrower repeatability range, if possible. In any case, the EOAT or robot will have to accommodate normal clamp-position variance of at least ±0.032 in. One approach is to design the EOAT with spring-loaded suction cups to absorb any positional tolerances. EOATs that require precise gripping tolerances may require the grippers or EOAT plate to be compliant in order to self-adjust to clamp-position variances. Even highly precise servo-driven robots sometimes are equipped to adapt to varying clamp positions.
With all the above application-related information in hand, it is possible to begin a preliminary design concept for the EOAT, using the hardware options described below.
Getting a grip
Three methods are commonly used for gripping parts: vacuum, mechanical grippers, and air cylinders. Across all industries (including plastics), vacuum accounts for 65% of EOAT gripping applications, according the Robotic Industries Association. This is due mainly to the low cost of vacuum grippers. They are also lightweight and easy to design, set up, and reuse. Additionally, a wide range of standard components is available, and there is a built-in system to verify part acquisition.
Where a part has no sufficiently flat area to accept standard suction cups, custom-designed gasket suction devices may be used. These tend to be expensive and less durable, so they are used only when other gripping methods won't work. For further discussion of vacuum options.
Whatever vacuum arrangement is chosen, it is important to first determine the gripping force required. Select and test the size and number of suction cups that best maintain control over the part during ejection from the machine and during the robot's motions.
The second option, grippers, are used for parts that can't be secured adequately by standard vacuum methods. It is not uncommon to combine vacuum cups and grippers in one EOAT. The grippers can stabilize and accurately position a part while vacuum cups control and verify the part during ejection.
Selecting appropriate grippers means balancing a wide range of criteria, including gripping force, jaw-opening stroke, weight, and size. Grippers must be large enough to grab the part and small enough to be integrated within limited available space. Other considerations are gripper accuracy or repeatability, and whether a verification system is built in.
A rule of thumb calls for a 10:1 ratio between the force of each gripper finger and the part weight. For example, a gripper with 30 lb of force can handle a 3-lb part. Because the robot payload limits include the weight of EOAT, lighter grippers allow greater design flexibility for the rest of the EOAT. While lower payloads also tend to improve robot speed and positioning accuracy, heavier grippers are often more reliable and crash tolerant.
Grippers are available in a wide range of configurations. They may have two or three fingers, pneumatic or electric drives, and drive action in one or two directions (single or double action). Grippers are available with either angular (scissors-like) motion or parallel motion. Single-action grippers use pneumatic force in one direction while a spring performs the opposing stroke. The spring action is usually available in either direction of stroke, so that the gripper may be "sprung" either open or closed. Double-action grippers, which include some pneumatic and all electric ones, are powered in both directions.
The third main gripping method, pneumatic cylinders, are usually arranged in opposing pairs and act much like oversized grippers. Cylinders are typically used where vacuum is unattainable, where the configuration of the part offers no convenient gripping points for grippers, or where the application requires greater gripping force and/or longer stroke or openings than grippers provide.
If the part must be supported during the ejection stroke, guides can be added to the EOAT. Opposing cylinders may be actuated sequentially, so as to shift the position of undercut parts to aid removal. Cylinders may have a single piston, dual pistons acting in opposite directions, or even four pistons at 90¡ to each other. Cylinders may also have either single or double action. As is the case with grippers, single-action cylinders are generally less expensive but also less accurate in the spring position than the double-action type.
Additional options can be used where standard methods do not have sufficient grip to pull parts off the mold or where only Class A surfaces are exposed. One gripping mechanism for such cases consists of a hook-shaped finger configured to pivot behind the part.
A collet chuck may also be used to distribute gripping force evenly on the ID or OD of cylindrical parts. A pneumatic cylinder typically actuates the chuck.
Be aware of opportunities to pull parts from the mold by grabbing the sprue or runner rather than the parts themselves. Almost any of the methods described above can be used for sprue/runner gripping.
In order to avoid closing the mold on unejected parts, verification is an essential corollary to part removal. As mentioned above, vacuum gripping systems have a built-in verification system in the form of a transducer that attaches to the vacuum generator. If any cup fails to achieve proper suction on the part, the transducer does not send a signal to the robot controller, which is programmed to "pause" the press cycle until part removal is manually confirmed or the cycle reset.
Most grippers incorporate either a reed switch or Hall-effect sensor to detect open, closed, or half-open positions. Cylinders often have sensors as well. In instances where grippers or cylinders do not have a means of verification, an external sensor or one in the fingers should be added to the EOAT or elsewhere in the robot system. Options include photoelectric, inductive, and capacitive sensors. A universal verification approach consist of a programmable video imaging system that looks at the mold. These video systems can detect mold damage and insert placement, as well.
Manipulating the part
Once the EOAT has grabbed the part, it is often desirable to manipulate it before releasing it. Especially in the case of undercut parts, it may be necessary to shift or rotate the part in order to fully release it from the mold. Other useful motions include the following:
- Reorienting parts for placement on conveyors or in trays or boxes, or for hand-off to downstream equipment.
- Flexing living hinges or closing caps.
EOAT may incorporate a number of additional components to perform these tasks. In theory, almost any kind of motion device can be incorporated into the EOAT, within the limits of available size, payload, and program flexibility. In practice such devices generally consist of these:
- Wrists for rotations up to 360¡.
- Linear slides.
- Rotary actuators.
- Air nippers.
- A second set of tooling for downstream functions.
- Quick-change adapters, manifolds, and connectors.
Nippers may be added to EOAT where space and payload allow. Of course, nippers may also be mounted elsewhere on or beside the robot and need not be part of the EOAT.
Tying it all together
The EOAT's functional components must be mounted on a base plate or modular frame. For larger EOAT (used on presses of over 500 tons), modular tubing results in a lighter assembly and less engineering and fabrication cost.
The part configuration and mold and machine dimensions influence base-plate designs and the components required. Structural integrity must be balanced against the need to minimize size and weight. Aluminum is the most common material, although plastic or composites may be used for weight savings.
EOAT built up from standard extrusions or modular kits is offered by a few manufacturers. It offers many benefits, including ease and speed of assembly and reconfiguration for use on other molds. On the downside, modular frames tend to be somewhat bulky for small injection machines and slightly less rigid than solid plates.
For jobs requiring quick mold changes, the EOAT should have dowel-pin bushings to locate precisely on the robot wrist. For applications like insert molding that require precise positioning either inside or outside the mold, guide pins and bushings should be used on the EOAT, mold, and/or downstream equipment.
Put it to the test
After assembly, the EOAT should be bench tested. Vacuum systems should be inspected for leaks. The operation of all switches and verification systems should be confirmed. All components on a single circuit should be checked for simultaneous operation.
Perform a "shake test" as an initial check of the EOAT's ability to maintain control over the part without damaging it in any way. Confirm that hoses and wires are properly routed and restrained so they do not interfere with EOAT functions.
Next, mount the EOAT on the robot, check for interference, and then actuate all valves and switches to ensure proper functioning. If the design includes a wrist, flip it back and forth to check clearances. With the injection machine in manual mode, open the press, bring the robot arm into position, and check the alignment of the EOAT to the mold. Check the routing for all hoses and wiring again. Then teach the robot its positions for part removal and for integration with downstream equipment.
Run through one complete molding cycle manually and readjust the robot, EOAT, or program as needed. Then run the injection machine in automatic mode at slow speed and gradually increase speed as EOAT functionality is confirmed. Keep increasing the speed until an error occurs or until the target cycle time is reached with no errors. Record or save all set-up data, including speeds, positions, and times.
Learning is incremental
The first in-house EOAT design project should be a simple one. With so many variables, it is inevitable that designers will go down a few blind alleys, even in basic part-removal applications. But they will learn much in the process. By gradually increasing the complexity of subsequent projects, it will be possible to develop a high level of in-house expertise to handle virtually any EOAT requirement.
While still on the learning curve, however, it is useful to work with the robot supplier when tackling a new challenge. Chances are good that the robot maker has encountered similar challenges before and may have good advice on how to grasp or manipulate an unusual part more reliably, delicately, or economically. A few robot vendors even offer training in EOAT design. This training is worthwhile for any molder contemplating in-house EOAT development.
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