Plastics Training & Technology Blog: September 2010 Archives

How Much Re-Training...

By Andy Routsis on September 29, 2010 10:10 AM 0 Vote 0 Votes

Yesterday, I received a question I get asked all the time...

Jason

Should I do any re-training of my employees?

My Response

We recommend regular retraining of your employees for two reasons...

1) Each time someone takes the training, they will pick up, learn, or understand something new.

2) Retraining also ensures they do not lose track of the fundamentals, which can often happen over time.

Basically, your employees should always be learning and reviewing,,, especially in the complex and diverse field of plastics processing.

Additional Thoughts

Any good plastics professional should be should always be learning and honing their skills. What I know, and the approach I take to processing adjusts, and improves as every year passes. Likewise, when I refer back to educational and reference materials I have used in the past, I always find good information that I forgot or have not used in a while.

-Andy

Open-Loop and Closed-Loop Process Sheets...

By Andy Routsis on September 27, 2010 2:29 PM 0 Vote 0 Votes

I was recently at a customer with both open-loop and closed-loop molding machines...

BD

Can a standard, machine-independent, process sheet be used for all our molding machines?

My Response

In this case, the best way to process an open-loop machine is much different than a closed-loop molding machine. An open-loop machine should be full at the end of 1st stage... while a closed-loop process should be short-shot at the end of 1st stage.

As a result, molders who process both types of machines should have one machine independent process sheet specifically for open-loop machines, and one process sheet for closed-loop molding machines.

Additional Thoughts

It is always preferable to process with a closed-loop molding machine... and it is almost always cost beneficial to replace your open-loop molding machines when possible.

-Andy

Volumetric or Gravimetric Feeder?

By Andy Routsis on September 20, 2010 8:05 AM 0 0 Vote 0 Votes

I recently received this question...

JC

How do I decide whether to choose a volumetric or gravimetric feeder?

Note: A gravimetric feeder measures the flow's weight and adjusts the feeder output to maintain the desired set point. On the other hand, a volumetric feeders, by definition, do not measure the weight of the flow.

My Response

Having said this, volumetric feeders are inexpensive and simple but are open loop devices. They will not detect or make adjustments for changes in material density. Because they are open loop devices, volumetric feeders are also unable to measure the flow for data acquisition. A volumetric feeder may be good for your application if your material has little variation in density and pellet uniformity.

Most materials do have variations in density great enough to warrant the use of a gravimetric feeder.

Most feeder manufacturers have the resources to determine whether a given material can be fed volumetrically at a required accuracy or if a gravimetric feeder is required.

You must also consider what you will be feeding based on cost and concentration. If you need to feed a material in at 1% by weight a gravimetric feeder is almost always a must. Also, for expensive additives, the material saved by having a gravimetric feeder may pay for itself.

-Andy

Avoiding Check Ring Leakage...

By Andy Routsis on September 17, 2010 3:32 PM 0 0 Vote 0 Votes

I was recently asked this question...

Joe

How do I avoid check ring leakage?

My Response

Many heated discussions have arisen as a result of check ring leakage, but the simple fact is: All check rings allow some plastic back-flow (leakage). There are specialized products on the market which try to eliminate check ring leakage, but they eventually cause excessive barrel wear... resulting in inconsistent leakage.

In most important factor with check ring performance is the consistency of check ring leakage. Inconsistent leakage can result in defects such as Flash, Short Shots, and Sinks. For this reason, it is important to routinely perform a dynamic check ring repeatability test to ensure proper performance.

Additional Thoughts

For more about this test, please feel free to read:

Conditions For The Dynamic Check Ring Repeatability Test

Dynamic Check Ring Repeatability Test

-Andy

Selecting a Part Weighing Scale...

By Andy Routsis on September 17, 2010 3:30 PM 0 Vote 0 Votes

I was recently asked this question about weighing parts...

TC

If I am weighing and documenting part weight for process documentation, what scale should I use?

My Response

The accuracy of a scale must be +/- tenth of a percent of the total part weight or better. For example, if you are weighing a 100+ gram part, the accuracy of the scale should be at least +/- 0.1 gram. If the accuracy of the scale is inadequate, the test results cannot be trusted.

Additional Thoughts

The scale’s accuracy can be usually found in the owner’s manual or on the scale itself.

-Andy

Controlling Warpage

By Andy Routsis on September 16, 2010 11:15 AM 0 Vote 0 Votes

I received this inquiry the last week...

Karl

I have a single cavity mold with a part varying in wall thickness from 3 to 5 mm. The part is warping near the transition of the thin to thick section. What can I do to overcome this problem?

My Response

Like all warpage yours is caused by differential shrinkage. In this case it sounds like you have an uneven temperature gradient throughout the wall thickness of the part.

You should start by checking your coolant temperatures. If the cooling lines enter on the thin side of the part and exit on the thick... the water temperature will already be higher on the exit side of the mold. I suggest hooking up your cooling lines on the thick side of the part first.

Also, compare the entering and exiting water temps to ensure the difference is not to great. If your exiting water temperature is fairly greater than your entrance temperature you may want to consider getting a more powerful chiller.

A chiller with a stronger pump and higher volumetric throughput will have better cooling uniformity and water temperature throughout the mold.

You may also want to try increasing the packing pressure to reduce the amount of material shrinkage, increase the pack time until the part weight no longer increases, or profile your packing pressure to obtain a more uniform shrinkage across the part.

-Andy

Dryer Capacity Calculation

By Andy Routsis on September 13, 2010 12:35 PM 0 0 Vote 0 Votes

I was just asked...

JW

What factors should I take into account when calculating the capacity of a new dryer system?

My Response

When calculating the capacity of a dryer system you need to determine your total dried material consumption. Your dried material consumption can be calculated by multiplying the number of machines by their respective output in pounds. If you have 10 machines consuming 4 ounces of plastic a minute you are consuming 150 lb/hr.

Once you determine your material usage you must take into account the drying time of your material. Most plastics will vary in the type, extent, and duration of drying needed for the application. This must be determined before the dryer capacity can be calculated. Ask the dryer manufacturer typical drying times for the material you are processing. Multiply your hourly material consumption by the drying time and your result is the minimum dryer capacity. For instance, in the example above if the material drying time was 2 hours the dryer capacity would have to be at least 300 pounds.

Additional Comments

The example above was for the minimum capacity needed. If the dryer manufacturer offers a 400 pound capacity machine you may want to choose that instead, in case you decide to expand your production... or if the material takes slightly longer to dry.

-Andy

Venting Problem

By Andy Routsis on September 10, 2010 9:50 AM 0 0 Vote 0 Votes

I was recently asked...

George

We just received new material and started getting burn marks on our parts near the end of fill. What do you think is causing this and how can it be fixed?

My Response

Burn marks usually appear when high temperature gas combusts in front of the melt during mold filling. In your case the new material may have a slightly higher flow rate and the mold is not venting quick enough with your current injection velocity. You should check and make sure your vents are clean and able to vent properly.

Many polymers gas-off in the molten state releasing particles that can clog the venting system. You may also want to check the melt flow rate and compare it with data from previous batches of material. If possible you may want to increase your fill time a bit to allow for longer venting.

-Andy

Sizing up Ejector Pin Diameters

By Andy Routsis on September 7, 2010 1:20 PM 2 Vote 0 Votes

I was asked the question...

Dan

"How one will decide the ejector pin diameter to eject the part safely

without any stress mark observed on the part?"

My Comments

To avoid permanent distortion of the plastic moldings, the number, location, and design of the ejector components must be developed to apply a low and uniform state of stress across the moldings. If the ejector force is uniformly distributed across many points in the mold cavity, then the molding will be uniformly ejected from the mold without any permanent distortion.

My Response

You must first have an estimate of the ejection force required to remove a molding from the core. The ejection force required to remove a molding from a mold core is a function of the normal force between the surface of the molding and the surface of the mold, together with the associated draft angle, the coefficient of friction, and the vacuum force. The coefficient of friction will vary from approximately .5 for highly polished surfaces to more than 1.0 for rough or textured surfaces.

The Ejection Force will then be:

Cos(Draft Angle) x Coefficient of Friction x Force(normal) + Vacuum force

There is a minimum push area that is required to avoid excessive shear stress on the plastic molding. More ejector pins will increase the pushing area and reduce the shear stress at each ejector pin location. The mold should be designed such that the perimeter around all the ejector pins provides a shear stress about one half the yield stress of the plastic material. The following equation relates the total perimeter of the ejector pins with the ejection force, plastic yield stress, and wall thickness.

Sum(Perimeter of Ejector Pins) > [2 x Ejection Force]

Plastic Yield Stress x Wall Thickness

If you cannot add additional ejector pins you will have to increase the ejector pin diameters until the sum of their perimeters satisfies the equation above. If more pins were added the diameters could be smaller as long as the equation above was satisfied.

-Andy

Screw Recovery Alarm

By Andy Routsis on September 2, 2010 9:59 AM 0 0 Vote 0 Votes

I was just asked…

Jim

"What is the value of having a minimum screw recovery alarm? I can see the maximum happening but the minimum I'm not so sure. I guess if someone puts the screw speed up to purge and does not put it back it may be useful. Is there anything else I might be missing?"

My Response

I believe you are correct Jim. Sometimes companies put alarms and options on the machine just because it's easy to program.

Additional Thoughts
The only benefit to such an alarm may be to monitor tampering of screw RPM but this is extremely rare.

-Andy

September 2010 Archives

How Much Re-Training...

Open-Loop and Closed-Loop Process Sheets...

Volumetric or Gravimetric Feeder?

Avoiding Check Ring Leakage...

Conditions For The Dynamic Check Ring Repeatability Test

Dynamic Check Ring Repeatability Test

Selecting a Part Weighing Scale...

Controlling Warpage

Dryer Capacity Calculation

Venting Problem

Sizing up Ejector Pin Diameters

Screw Recovery Alarm

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