Processor Tips


Know Your Mold-Building Terminology

By: Robert Beard, P.E., Honored Fellow SPE 12. August 2013 21:53

For many years I held a seminar called Purchasing & Quoting of Plastic Parts aimed at OEM purchasing and molding personnel.  Attendance has waned during the recession, but the need for knowledge is still there, especially where conformal-cooled molds are concerned. 

I have seen many purchase orders over the years worded only  “Build a 4-cavity mold to produce ABC,” and nothing else. So it continues with conformal-cooled molds:  “Build a 4 cavity conformal cooled mold to produce ABC”.  People who purchase conformal-cooled molds need to understand the technology so that they know how to specify a conformal-mold in a P.O.

So what questions should OEMS or injection molders be asking their moldmaker? My list:

1. Who is designing the mold?  

2. What analyses will be done?  (You’re paying for these; put it in the P.O.)

3. How much experience do they have with each of the software packages?

4. Who is building the conformal inserts and what are they responsible for?

5. Who has the responsibility for the whole mold?

You have to have an understanding of the technology in order to write a good P.O., whether you are an OEM or a molder.

About the Author

Robert A. Beard is president of Robert A. Beard & Associates, Inc. which was formed in 1984. He has more than 40 years of experience in plastics. He presents seminars nationally and internationally entitled: Purchasing & Quoting of Plastic Parts, and Virtual Workshop On Trouble Shooting The Injection Molding Process. He has been elected to the prestigious grade of Fellow and is a Honored Service Member in the Society of Plastics Engineers, and has served as the Chairman of the Fellows Selection Committee. Contact: (262) 658-1778; email: rabeard@plastic-solvers.com; website: plastic-solvers.com

Busting the Conformal Cooling Myths

By: Robert Beard, P.E., Honored Fellow SPE 8. August 2013 16:43

Conformal cooling is opening up new ways of doing things with new tools to solve problems. As cooling lines get smaller and closer to the core and cavity wall, and take a torturous path through the mold, the hydraulic resistance is increasing for each channel. There is a myth, at the floor level, that if the main water inlet to the mold is connected to say a 12-port manifold, that the manifold splits the water into 12 equal parts. This is not true. Hydraulic resistance determines that. The higher the hydraulic resistance is in each channel, the less water that goes into the channel.

In a conformal mold, it is important to measure the flow rate of each cooling line and calculate the Reynolds number to see that it is above 5000 for turbulent flow. This can be done by installing a flow meter with a metering valve on each cooling line on the return manifold so that each cooling line can be manually balanced.

If we continue to do what we have always done, we deserve to get what we’ve always gotten.

For more on the myths of conformal cooling, check out an upcoming new FastTrack training program on September 4th and 5th, near Toledo, OH, sponsored by Plastic Technologies, Inc. (PTI), which will feature two modules— Conformal Cooling for Injection Molding (September 4th) and Medical Plastics Design and Processing (September 5th).

Conformal Cooling Seminar Outline


1.) Understanding heat management

2.) How resin selection affects heat management.
How resins can be modified to cycle faster.

3.) Choosing the right mold metal.

4.) Understanding how Fluid Dynamics impacts Dynamic Heat Transfer.

5.) Alternative cooling technologies to be used with conformal cooling.

6.) Conforming Cooling Technologies, including a European technology
presentation not seen in North America.

7.) Examples why Moldflow and Computational Fluid Dynamics (CFD) are important,
if not necessary, tools for designing conformal cooling channels

 

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About the Author

 

Robert A. Beard is president of Robert A. Beard & Associates, Inc. which was formed in 1984.  He has more than 40 years of experience in plastics. He has been president of the Chicago and Philadelphia sections of the Society of Plastics Engineers, and has served as National Councilman for the Chicago Section.  He presents seminars nationally and internationally entitled: Purchasing & Quoting of Plastic Parts, and Virtual Workshop On Trouble Shooting The Injection Molding Process.  He has been elected to the prestigious grade of Fellow and is a Honored Service Member in the Society of Plastics Engineers, and has served as the Chairman of the Fellows Selection Committee. Contact: (262) 658-1778; email: rabeard@plastic-solvers.com; website: plastic-solvers.com

Calibrate Those Instruments

By: Timothy Womer 28. June 2013 09:37

I was recently asked to visit a sheet processor to determine the cause of a major screw design problem. So, as always, I started at the beginning to gather all of the technical information to determine the root cause. This facility had 5 large extrusion sheet lines, and they were issues with all 5 extruders.

With the extruder at room temperature, I set up three dial indicators on the discharge flange of the barrel in the X,Y and Z axis.  Then I turned on the barrel heaters to the standard zone setting to make sure that the barrel thermally expanded in the Z-axis direct as much as it should theoretically, and that the X and Y indicators move minimally.

 

The simple equation to determine the amount of expansion that a barrel should grow is:

 

ΔL=0.00000633 X ΔT X L

where:

                                             ΔL = The change in length

                                             ΔT = The change in temperature, in this case from room

                                                       temperature to the barrel zone setting

                                                     

                                               L =  The heated length of the barrel

 

Amazingly the barrel grew within about  0.030-in. of the theoretical change in length, which in this case was approximately 0.750 in.

 

Then I measured the flight OD on several of the screws for various designs to determine if there was a consistent wear pattern. There was, so that was noted.

 

Then I gathered all of the process data.  This is a very important part of doing a “CSI” on screws.  This is where you collect the given throughput rate at a given screw speed against the headpressure during that timed rate check, motor load and melt temperature.

 

The motor load reading is taken from ammeter on the control panel; the screw speed is taken from the tachometer.  If at all possible, it is best to have the customer’s plant manager to check the motor load with a hand held meter to verify that the ammeter on the control panel is reading correctly.  As for checking the screw speed, this typically can be done by using a stop watch and counting the rotation of the drive quill at the back of the gearbox.

 

In this case the control panel ammeter was reading correctly, but the screw speed was not.  The customer’s setup sheet showed that their standard setup was to have the extruder operating at 70 rpm, but when I counted the revolutions of the drive quill, I was getting 92 rpm.  This is an error of 24%! 

 

I then checked the tachometer on the line next to the one that I was gathering the process data from and the tachometer on it read 86 rpm but when I did the count, it was only rotating at 70 rpm. This meter was mis-calibrated by 23%!!!

 

So, the moral of the story is, the only thing worse than no data is BAD data.  In this case, the customer immediately had their maintenance people re-calibrate all of their control instruments.

 

NOTE: Sometimes the screw rotation is faster than what a person is able to visually observe. In these cases, I take the advice given to me when I was a kid by an old mechanic mentor of mine (who only had a 4th grade education)...I  “count the clicks.” I had no idea what he meant until he showed me.

Howard took this machinist scale (a pencil or pen will work) and turned on the chuck of the engine lathe in his shop, then took the scale and let it rub against the chuck. On an extruder it can be a small bolt in the back of the rotating drive quill or the drive key on the shank of the screw.  Then with your stopwatch in one hand the “clicker” in the other, you can count the number of times that bolt or key hits the end of the scale, pencil or pen...or the number of clicks.  “Count the clicks.”  Very simple but very effective.

 

Just make sure that your instruments are calibrated on a regular bases and also do a check and balance when gathering data.  Never trust what you think  you see the first time.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com
 

Overhung Loads

By: Timothy Womer 10. June 2013 11:06

Recently a processor installed a new screw into its 6-in., 32:1 L/D extruder. Within a few weeks the hard facing that had been welded on the flight OD started to pop off. The flight failure was in an isolated area, so it was then assumed it was due to a poor weld bond. The screw was ultrasonically inspected and the remaining flights showed good bore, but the entire screw had to be rebuilt.
 
Within about 4 weeks after it was returned, the customer called and said that the flights had failed again in the same area, which was located about in the middle of the screw and within about a 12-in. length. The screw was returned to the manufacturer and repaired a second time, sent back to the customer and reinstalled.
 
Once again, in about another 4-5 weeks the customer called again to say the screw had been pulled and yet again the flights had failed in the same exact location. This was unbelievable! 
 
After much research and review of the screw design, it was considered that there was something wrong with the extruder—and not the screw—and most likely the problem  was due to thermal expansion.
 
It was time to make a plant visit.  The first thing I noticed was that the 900-lb, 6-in. screen changer—located approximately 36 in. in front of the front barrel support— had no support under it, causing a cantilevered overhung load.
 
So to determine if the barrel was expanding properly, the die and adapter were disconnected from the extruder. Then, three dial indicators were mounted so that they contacted with the screen changer. The indicators were mounted independent to the extruder at 12 o’clock, 3 o’clock, and also on the face of the extruder, so that the movement could be measured in the X, Y and Z axis. The die and cart had been pushed up close to the screen changer so that it could be used as the independent support for the indicators.
 
Also, a dial indicator was mounted independently near the middle of the barrel between the front barrel support and the face of the feed throat housing, at the 12 o’clock position. This was done to observe if the barrel would bow upwards, which be an indication that the barrel was not thermally expanding forward properly.
 
Calculations were made to determine the theoretical amount of thermal expansion that should be expected when the barrel zones were set at the processing temperatures of the extruder. The expected expansion was to be approximately 0.434 in. at an average barrel temperature of 410°F. Once all of the indicators had been properly “zeroed” on the “cold” extruder, the barrel zones were turned on and allowed to heat up.
 
Within an hour, the barrel only expanded forward approximately 0.400-in., but the screen changer had dropped 0.045-in. and the middle of the barrel had lifted 0.032-in. for a total deflection of 0.077 in. Also, it was evident and measured that the barrel had only moved forward at the front barrel support a distance of 0.253 in.
 
From all this it was concluded that the overhung load from the screen changer was causing a bind in the area of the front barrel support and not allowing for smooth and uniform expansion of the barrel in the axial direction. 
 
A support for the screen changer was fabricated and installed to eliminate the overhung load. Also, it confirmed by a major screen changer manufacturer that they recommend a screen changer cart for all screen changers 6-in. diameter and larger. and even for smaller extruders also.
 
Lesson learned: Excessive overhung loads and non-uniform thermal expansion will cause premature screw and barrel wear.
 
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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

Screw Speed vs. Recovery Time

By: Timothy Womer 29. April 2013 14:24

Many times I have visited molders to help them with processing issues. In reviewing their setup parameters, I find many times that the screw speed is not setup to operate at its most effective rate.
 
There have been times, for example, when a customer was processing Xenoy— Sabic’s blend of semi-crystalline polyester (typically polybutylene terephthalate, or PET, and polycarbonate)—and the molder was complaining that the screw was degrading the resin. The screw was turning at a fairly moderate speed and was recovering in 5 seconds, then sitting in the back position before injecting for an additional 8 seconds. Then during the injection stroke, degassing and smoke was evident at the nozzle.
 
After witnessing several cycles of operation in the above manner, I suggested that the screw rotation be reduced to the point where the screw got back just in time so that a count of “3” could be obtained. Then the screw was stroked forward to fill the mold. The screw speed was practically reduced by half, and within about three shots the degraded material was flushed out of the barrel, and the smoking stopped.
 
This simple change in the processing parameter reduced the shear on the resin and allowed the process to be improved. Not only was the degrading eliminated, but most of all the overall melt temperature was reduced which in turn reduced the cooling time and, therefore, reduced the overall cycle time.
 
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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com




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