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Complications With 1st – 2nd Stage Transfer…

A technician encountered this interesting problem yesterday…

Kevin
We have been working to achieve 95-97% fill during first stage fill with zero hold time and pressure. As soon as I apply hold pressure and time, the screw continues at speed after transfer. My machine has a hold speed minimum setting, but no option to turn this setting off. I am actually observing a pressure spike from 15 bar to 25 bar at transfer. To get an acceptable part, I need to profile a drop in injection speed before transfer as well as increase the transfer position in 5mm increments until the pressure spike at transfer was eliminated.

Can you offer any thoughts as to why this is happening? Is there something I missed?

My Response
Many machines have a bad habit of allowing the screw to overshoot when 2nd stage pressure is turned on, but stop immediately when 2nd stage is turned off. On most of the newer electric and counter-pressure molding machines you can actually watch the screw stop instantly when the second stage is turned off. Unfortunately, a few of these fancy machines are factory set to overshoot the moment the second stage is turned on… this means that the same screw which stopped on a dime will now overshoot and flash just by turning on the second stage time (even when the pressure setting is at zero).
Basically, this is a silly attempt to mimic older hydraulic machine behavior. In such a case, you should turn on the second stage time to a normal value, then set the lowest value possible for pressure and minimum speed. You should now see the screw overshoot on your particular machine. This will allow you to establish a more representative 1st stage short shot.
On any machine, you must establish a 1st stage short shot which represents the machine’s behavior during production… this will allow you to fill and pack out the part more consistently.
note: Whenever possible, you should turn off the ability for these newer machines to overshoot. The more accurately the machine performs the desired actions, the more consistent the process. 
Additional Thoughts
Although each machine is different, many newer machines have the option to turn on and off the injection overshoot such as a choice between sharp or gradual transfer. Other machines may have this option buried in the maintenance portion of the controls. You may have to contact the manufacturer to find out where this option is located.
-Andy

Balancing Family Molds…

A common blog reader asked this question regarding family molds…

Jim
What is the best way of balancing a family mold?
My Response
When balancing family molds, it is best to approach it in two steps.
Step 1 – Balance the runner system – When possible, try to balance the runner system so that each gate starts filling the mold cavities at the same time. Keep in mind, different rates of fill will affect this balancing.
Step 2 – Balance the gates – After you have determined that all the mold cavities have begun filling at the same time, then adjust the gate dimensions to balance the filling of the mold cavities.
In the best case scenario, you could… 1) short shot the runner system and see all the mold cavities begin to fill at the same time, and 2) end filling with a short shot on each mold cavity, so that all the cavities can complete filling and begin packing under pressure control.
Additional Thoughts
Most people only adjust the thickness of a gate to restrict or promote flow, yet this has a significant impact on the shear thinning of the polymer. Adjustments to the length, width or number of gates can often promote or restrict flow, yet have less impact on the shear heating and thinning of the polymer.
-Andy

Same Supplier… Same Grade… Bigger Dimensions…

I received a follow-up question regarding a blog entry last week…

ED
We switched our product line over to 4710 PE 100 extruder grade PE about 2.5 years ago. The material was produced in Belgium. We sized all of our new tooling to this material.In February they started making the material in Texas. The supplier says the material is the same. All of my processes were established with Decoupled II process using cavity pressures sensors.

Machines, mold, melt, and barrel temperatures are the same.

When I match the templets from before the dimensions of the parts are too big. I think they have changed the carbon black. I have suspected that a Nucleating Agent has changed. If it has changed the crystalinaty it will affect the shrinkage. Can this be in the Carbon Black? Or can the Nuculating Agent be some where else?
My Response
Although a change in nucleating agent may have occurred, it is more likely that there is a change in the polymer. During polymerization, high density polyethylene can grow, branch, and propagate in many ways. Different material manufacturing conditions can result in a change in Average Molecular Weight, Molecular Weight Distribution, as well as the amount of branching even though the melt flow index is the same.
First, I would request some rheological data from the material suppliers from both facilities. If you can compare some detailed shear rate and viscosity data, it can help you better determine the differences between the grades.
You may wish to investigate the difference in rheological characteristics between both the old and new. A Differential Scanning Calorimeter, or DSC, can be used to determine the relative degree of crystallinity between the samples from your past and current production lines. This may also give you some data regarding any differences in additives.
Additional Thoughts
If you really want to delve into the material, you can use a capillary rheometer or parallel plate rheometer to measure the viscosity characteristics across multiple decades of shear using different temperatures. These types of tests will provide very accurate information regarding the behavior and characteristics of the material.
-Andy

The Purpose Of Feedthroat Cooling…

This question arrived via email yesterday…

John
Why is there water running through the barrel of an injection molding machine?

My Response
The basic purpose of the water is to cool the feedthroat to avoid bridging. The feedthroat is the area of the barrel where the material passes through from the hopper the screw.
If this area gets too hot, the material may clump or stick together, creating a blockage known as bridging. If the area is cooled too much, condensation may occur resulting in visual defects or a change in mechanical properties. Condensation in the feedthroat most often occurs during a production delay, such as a mold or material change.
Additional Thoughts
For a related entry on feedthroat cooling, please review: Feedthroat Temperature Differences…
-Andy

How Colorant Affects Shrinkage

I received a detailed question via email the other day, I will do my best to convey the nature of the question in this blog…

Kevin
I am running as part in acetal and am having difficulty in maintaining the same dimensions as natural when running colorant:

When molded with natural: overall length = 49.33

Natural + yellow 130C: overall length = 49.32

Natural + green 326C: overall length = 49.20

To increase the overall length of the green part…
Mold temp was increased from 60 to 70ºC
Hold pressure was increased 65 to 110 bar
Hold time was increased to 6.0 to 7.5 seconds

As a result, the part was the correct length, but the weight increased from 23.96 to 24.29 grams.

My question is, is there any technical information regarding colors, pigments, etc. and their affects on materials?
My Response
There is much information available online, but there are a few things that you can also investigate in-house and with your supplier.
1. Any additive will change the melting characteristics of the polymer. You should always perform a tact temperature study as well as check and document the temperature of your melt with each polymer/additive combination. Some additives may cause the polymer to stick to the screw rather than the barrel, requiring an unnecessarily high screw speed to recover the shot. Adversely, one colorant may cause the polymer to melt very smoothly, resulting in a lower melt temperature. To better explain this, one of my customers (who processes only one base resin) showed me a screw with different colored stains along the transition zone. Each stain indicates the melting characteristic of that material/colorant combination. Some colors caused softening near the feed zone, while others softened closer to the metering zone with a rainbow of colors in between.
2. In additional to documenting the process outputs such as fill weight, peak pressure at fill, back pressure, etc. you should also consider taking a picture of the short so that you can visually match the appearance of the colored first stage short shots to the natural first stage short shot. Although most additives act as plasticizers, some additives will actually increase the viscosity of the polymer melt… especially if it reduces the melt temperature. This consequence can be improved by optimizing the screw recovery for each formulation you use.
3. Contact your material supplier and request any Technical Service Bulletins regarding acetals and/or the colorants you use. Also request drying specifications and annealing suggestions. In many cases, materials like acetals will provide more consistent dimensions when annealed.
Additional Thoughts
In semi-crystalline polymers, such as nylon, colorants and additives can also affect semi-crystalline site nucleation and growth. 
For, more information about tact temperature studies, please read: Optimizing Screw Recovery…
For free multimedia presentations on process documentation, scientific molding, and in-mold rheology, please visit:  http://www.traininteractive.com/free/webinar/player/
-Andy

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