Our first step was to ask if we could get a sample of the part and the process sheet.  After looking at the part and reviewing the process sheets we noticed the following:   First, key set points like the dryer settings were not included in the process sheets.  We saw this as a potential red flag.  With polycarbonate it is very important that the material be dried correctly with the proper equipment.  Polycarbonate requires a dryer setting around 240 degrees for four hours (following the material recommendations of course, some may vary around 250 degrees for four hours) but doing this requires a high-heat dryer.   It is always good to verify that the moisture is 0.020% or less prior to molding.

Further looking into the setup sheets we noticed that the injection pressures were all on the high side of the recommended range.  This can be a sign that the gate size or nozzle orifice may be a potential suspect.  Running the incorrect gate size or nozzle size can induce molded-in stress.

We also noticed a lack of process monitoring; the set limits would allow the press to continue to run outside the manufacturer’s recommendations.  If uncontrolled, incorrect barrel temps, pressures or screw cushion can all be reasons for in-molded stress.

In looking at the part, the molded stresses were obvious, particularly when looking through a polarized lens under strong lighting.  The stresses create a rainbow effect in the translucent material.  Our next step was to measure the gate size and we found it to be much smaller than what we would recommend for PC.

So we had plenty to consider from the start, and these are just a few possible reasons for PC cracking.  We’ve also been told by the material manufacturer that some mold release sprays can attack polycarbonate.  They even had a story about an operator whose hand lotion was found to be the culprit for cracking parts.  This is one reason we do not allow silicone mold release in the plant and insure our operators use gloves on polycarbonate jobs.

After ruling out all of the above possibilities, it’s possible that some part designs may require annealing for stress relief.  Annealing of the plastic part is the process of heating the post molded part to just below its softening point, then keeping it at the high temperature for a period of time before cooling it slowly back to room temperature.  This can relieve some molded-in stresses but isn’t a desirable solution in most cases.

Processing polycarbonate at the manufacturer’s recommendations is the key to stress-free and crack-resistant parts.  If, for any reason, you are unable to follow the recommendations you should ask yourself why and correct the problem at its roots.

Written By:

Pat Collins
Molding Operations Manager

I remember when I first started working with robotics on molding presses. Back then, they had to be adjusted by climbing all over the robot, and the programs were only capable of the basic “L” and “U” movements. In many cases the drop zone location on the up and down movement had to be set the same as the pick location on the up and down movement. The presses would have a mechanical stop to hold the mold in the open position and an alignment pin above the locating ring on the front half to verify that the mold was perfectly aligned each and every time it was set in the press. Even with this it was a challenge to keep everything aligned. It was also very important for the oil temp to be correct at startup. Old hydraulic presses would not open to the same distance or eject the parts correctly at the wrong oil temperature. You would need to re-adjust after running for a few hours or even days. It was an ongoing battle keeping everything lined up. The robots had pneumatic up and down movement with a servo drive existing only on the traversing and kick movements. For something designed to make a processor’s life easier they certainly brought their fair share of pain!

But like all newer technologies, issues were addressed one by one and improvements came out consistently. We now have servo movements on all three axises, with options for rotation and flip servos as well. We are able to tie the robotics directly into the process monitor on the press and automatically divert parts at startup and any time the process parameters move out of tolerance.

We continue to install alignment pins on the front half of the molds but the newer presses hold the open position / ejection forward position much better (especially newer electric presses). Now robots have evolved from the painful era of trial-and-error setup to a nearly scientific setup and operation.

I have worked with many different models and brands over the years and have been lucky to have worked with some of the best built and best supported robots on the market. Recently I attended a Flex Teach class for Yushin robots. The Flex Teach system allows the user to create motion programs for the robot using a personal computer. The same programs can also be modified using the touch panel controller. What I like best about the Flex system is that it utilizes the PC as a training tool for the robot when it is offline. This can save countless hours of down-time and allow operators that would not feel comfortable practicing on a live press to start learning the Flex Teach system. Just knowing that they won’t have to worry about damaging expensive molds or end of arm tools (or more importantly, themselves and others) opens the doors for every operator to catch up to speed.

Even robotic systems from just a few years ago were no comparison. They, too, were fully programmable and also had servos with CNC type controllers, but these models required hundreds of command lines and an extensive knowledge of the programming language to run. The program itself consisted of several parts: a run program, reference program, and home program for every job. Making adjustments to a program became a trial-and-error nightmare. More importantly, valuable press time was lost in the mix. Considering today’s shortened deliveries and 24/7 production jobs, fiddling with the programs is something most molders can live without. I wish I could have done some of the work offline with a program tool like the Flex Teach system. We now have the ability to take our time (with minimal pressure) and do most of the programming offline while the press is still running.

With the old system, programming mistakes would have to be caught during the standard process of verification referred to as “stepping through the program” and tweaked accordingly. The Flex system allows us to run the program or changes through a simulator and verify that it looks good on the computer screen before being transferred to the press via a SD memory card and loaded onto the robot. For good measure we continue to step through the program to verify a second time, but there is no doubt this saves time in the process.

All of these new features have made robots perform more consistently and adds to their versatility, performing tasks like sorting, de-gating, counting, boxing, and stacking. Robots can even place small inserts and verify their placement these days.

So molders, learn to love your robots. They work tirelessly, exactly, and without a complaint or absent day. They can be a molder’s best friend (though you can still keep the dog). Yes, robots surely have come a long way!

Written By:

Pat Collins
Molding Operations Mgr.