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Feedthroat Temperature Differences…

I received this question the other day from an experienced molder…

Cindy
We have 2 different brands of machines. On one brand, the feedthroat temperature is 85-90ºF, yet the other brand has a temperature of 150-190ºF. Is this difference OK…? and could you comment on what effects the higher temperature will have on the process?
My Response
Regarding the Machine Differences – In reality, there is no standard convention for feedthroat temperature measurement. Some machines place the transducer near the cooling lines, some place them closer to the rear heater bands. Additionally, some machines will measure the temperature of the water passing through the feedthroat. In your case, the best way to compare the effectiveness of the feedthroat cooling is to use a surface temperature probe to measure the water temperature entering and leaving the feedthroat on both brands of machines.
Regarding the Effects of Temperature – The feedthroat temperature should low enough to prevent the bridging of material. Always ensure the feedthroat does not get cold enough to induce condensation within the feedthroat.
Additional Thoughts
When such a discrepancy exists between two different brands, it is typically caused by differences in design.
-Andy

Screw Flight Diameter Along the Screw

I received this question as a follow-up question regarding a different blog, and decided it warranted it’s own entry.

MJ
Does the screw have the same diameter in the front, middle and back screw flight diameter?

My Response
The typical injection molding screw has the same outer diameter along the entire length of the screw. When measuring for wear, the highest amount of shear, and wear, typically occurs in the transition and metering zone.
I have seen some cases where a company may use variying clearances in the metering zone for mixing, or adjust the clearance in the feed zone to accomodate specific feed issues.
Some manufacturers, especially those for micro-molding, will occasionally use a tapered barrel to facilitate material conveyance in the feed zone.
Additional Thoughts
Keep in mind… if you are using a barrier screw, you will have two melt channels in the transition zone with one flight having more clearance than the other. 
-Andy

Packing With High Pressure Molding

During a recent consultation, I was asked this question…

Engineer
Could you explain how to pack-out a thin-walled part which requires a large amount of pressure to fill?
My Response
In such a case, the pressure to fill the part is typically very high… therefor, the pressure at transfer is also high. In such a case, you may have to complete the part filling at a pressure approximately the same as the pressure at transfer.
In some cases, this pressure is actually higher than the pressure at transfer… depending on the amount of pressure that is necessary to fill the remainder of the mold cavity. The benefit to this is you are using just enough pressure to fill the mold cavity, which will help reduce the amount of flash which is produced.
Additional Thoughts
Some processors have a tendency to fill the mold completely during first stage because they are not used to using a high second stage packing pressure.
-Andy

Conditions For The Dynamic Check Ring Repeatability Test

I received this questions from one of our advanced blog readers…

Milan
Before I perform the dynamic check ring repeatability test, must I optimize the screw recovery, decompression, back pressure, and feed zone temperature.
note: The Dynamic Check Ring Repeatability Test monitors the ability of the check ring to mold consistent short shots by weighing and comparing the part weight of 10 cycles. For more read: 

Dynamic Check Ring Repeatability Test


My Response
It is always nice to have everything optimized…. though these will typically not cause enough variability to fail a check ring repeatability test. If your variability seems high as a result of such a test, I would suggest purging the barrel and optimizing these parameters before I went through the effort of pulling the screw or making any equipment changes.
Additional Thoughts
Also, for those readers interested in learning more about optimizing you melt, I recommend the following blog entry: Optimizing Screw Recovery…
-Andy

Machine Variation vs. Process Variation

I received the following question in an email yesterday.Since it was lengthy, I have done my best to reduce it to a more concise inquiry…

Nathan
We have an LCP process where the tolerances for cushion, charge time, fill time, and pressure are very narrow. We are maintaining a consistent cushion with only 0.2mm deviation, yet we are noticing short shots consistently. The detection capabilities of the machine do not seem to be adequate for detecting short shots, is there anything you suggest?
My Response
The biggest problem I see is that you have set you tolerances for both the inputs and outputs which relate directly to each other. Although LCPs are highly crystalline and have a very sharp melting point, their ability to flow into the mold is highly dependent on the rate of injection. As a result, any shift in molecular weight, molecular weight distribution, or additives during lot changes will result in a non-compliance. Eventually, you need to re-evaluate your tolerances.
Ultimately, it appears your issues are not related to variations in your machine, but a lack of flexibility in your process resulting in variation.
Additional Thoughts
Such a process often requires a short shot with a high packing pressure to complete mold filling.  This will ensure that there is always enough pressure available to fill the part completely.
I have a few questions to address this week, but I intend to post a blog about high-pressure molding next week. This should help you better understand how to establish such a process.
-Andy

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