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May 12, 1999

6 Min Read
Purchasing Basics:Robots for press-side handling

Molders, custom and proprietary, are increasing their demand for automation to get the parts out of the mold and get them on their way through the plant. There are many reasons to consider automation, and it's important to focus on your own priorities among these reasons. Here are some of the benefits automation projects focus on achieving. Which are your priorities?

  • Part quality. Automation, properly applied, brings a consistent cycle to the molding process, reducing scrap through a repeatable process. Automation also prevents free fall of parts out of the mold.

    Productivity. In addition to consistent cycle time that helps keep parts on an even keel, robots work continuously, 24/7. Many applications see cycle time improvements, as well as better uniformity. Extra ejection strokes are eliminated, for example.Labor utilization. Direct labor associated with part production can be greatly reduced, freeing manual labor for better, value-added jobs.Worker safety. Automation can work within areas too hazardous for operators, or handle repetitive tasks that might lead to repetitive stress injury. Profitability improvements. Molders assume that there will be a bottom line payoff of the initial investment. Does this become the overriding factor?

Robot payloads

When comparing payload specifications of robots from different manufacturers, be aware that testing and rating methods may not be directly comparable, Payload ratings should apply to robots running at their maximum speed for the design life of the robot.

Press tonnage

50-100                        

100-150

150-300

300-500

500-700

700-1000

1000-2000

2000-3000

Once the objectives have been communicated to the potential vendors, you will find that many of them have detailed information requests that will help you through the process, probably in more detail than nearly any other purchase. If you are automating an existing molding operation, one very good place to start is with a video of your process as it presently exists. Even if it's not a new part, start with part information.

The Part and Mold
What is the part or parts? What does it weigh, how many are in a cycle, and how delicate is it? Is the sprue molten on ejection, formed, or is there no sprue? How many cavities are in the mold? Is the mold hot or cold runner? When the mold opens, how is the part oriented in the mold? If the mold is not already built, is there opportunity to influence the orientation because of the robotic handling? How is the part ejected from the mold? Can the ejector stroke be set so that parts are not dropped but are held? Where is the part gated? Is it hot when it comes out of the mold?

What specific functions have been identified as desirable? What sequences do you need the robot to repeat?

Rules of thumb

  • Molding machines running at cycles more than 15 seconds long offer the opportunity to have the robot do more than part removal and orientation. Consider degating, parts separation, weighing, hot stamping, stacking, palletizing, box loading, tray filling, QC staging, insert loading, and ultrasonic welding, among others.

  • When calculating payload, remember that the end-of-arm tooling for a robot that loads inserts is about twice as much as that for part removal only.

  • Robot speed should be specified 15 percent quicker than the shortest current overall cycle, to allow for future process improvements.

  • The more frequent the mold change, the more likely it is you will need a CNC robot.

  • Replacing an operator with a robot typically increases output by 20 percent.

  • After five years, the resale value of a pneumatic robot is 20 percent its original cost; for a servo robot, 55 percent of the value is retained.

If it is to be used for many different parts, how often are molds changed? By what means? Does your process require traceability, so that parts from different cavities need to be kept separate?

The Machine
Specify the machine by manufacturer and age. What tonnage is it, and what is the control method and control voltage? Is it presently attended by an operator? Is it an insert application, requiring loading of inserts as well as removal of parts? Do you want the robot to load the inserts? Or could the inserts be loaded into a tray by the robot, and the tray loaded into the machine by an operator? An operator-loaded fixture reduces the overall cost of automation, although the operator is not eliminated. However, the consistent press cycle benefit still applies.

Get out your measuring tape. Most robot vendors have a whole series of measurements around your molding machine they need to have. Ask for a blank sheet and fill in the dimensions, or communicate in writing all the details on the machine's dimensions, such as distances for clearance-overhead obstructions, top of the platen to top of the safety gate, mold centerline to floor, platen edge to outside of safety gate, thicknesses of platen halves and mold halves, tiebar diameter, guide pin length on both sides, and so forth.

All these measurements will help determine the envelope in which the robot can operate. Is there enough room beside the press for a side takeout robot? It's faster, but also restricts access at the rear of the press, has a more limited work envelope, and requires a larger, more awkward main arm.

Selecting a robot drive based on application

Application need

Drive technology

Sprue or runner removal
for 100 percent part
and runner separation

Pneumatic sprue picker

Simple part removal with
limited flexibility, such as
removing part and
placing it on a conveyor

Three-axis pneumatic
(least expensive beam type)

Place parts in rows
on a conveyor, with
multiple mold changes

Two-axis pneumatic
Single-axis electric, horizontal
(increased flexibility)

Palletizing parts at the press

Two-axis electric
Single-axis servomotor
for long cycle or three-axis
CNC/servo for short cycle
(increased speed and flexibility)

Boxing parts at the press
(multiple positions/
multiple layers)

Three-axis CNC
(easily programmable)

Automation cell where secondary
equipment can be simplified
or eliminated with use of
programmable robot

Three-axis CNC

Insert molding applications

Three-axis CNC/servo
Two-axis CNC
Single-axis asynchronous

Multiple insert or automation
applications with secondary
equipment

Three-axis CNC

Programming
Will this be a robot in a permanent application that is programmed at the robot manufacturer's factory on a hardware basis? Pneumatic robots are usually programmed that way. Will you need to change programming frequently so that a software approach (servo robots) is better? Will the operator need to learn how to interface with the robot's controls, or will a programming specialist be available? Is offline programming or programming via a teach pendant more important to you?

Cycle Time

The overall estimated cycle time is important, but the mold open time is also critical. Only a small part of that overall cycle can be devoted to mold extract. If the robot is expected to use only 21/2 seconds out of a 20-second cycle for part takeout, for example, the robot manufacturer needs to know this. If this is a new project, you can work from reasonable cycle time estimates based on other projects. Most of all, be sure that your candidate vendors understand what your objectives are as they prepare proposals for your evaluation.


With our thanks . . .
. . . to a number of companies who provided material for this article. They include CBW Automation,Conair,Fanuc Robotics,Remak North America,Wittmann Robots, and Yushin America.


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