Andy Routsis: April 2010 Archives

I received this question today from a blogger...

Blogger
How should I proceed, when I want to adjust the process on an older pressure-controlled machine (not velocity controlled)?

My Response
Although each of these older machines have their own specific limitations... there is a basic strategy to use with open-loop pressure-limited injection molding machines:

1) Fill and pack using first stage pressure - The mold should be completely full with small sinks near the gate area

2) Transfer using time - Using second stage hold to eliminate any sinks at the gate area.

Additional Thoughts
Many of these molding machines have really accurate limit switches on the injection unit. Unfortunately, the variability in mold filling as a result of viscosity changes can often cause a process to bounce between shorts and flash.

-Andy
I was asked this question the other day...

Milan
Does it make any sense to perform a load sensitivity test on a electric machine?. I did few tests, but results were very good, range 0,016% to 0,105%.

Note: The Dynamic Load Sensitivity Test measures how much the injection time shifts with a change in resistance.

My Response
Most all-electric molding machines perform very well in the Dynamic Load Sensitivity Test. In most cases, a variation over 1% is very uncommon in newer machines.

In general, load sensitivity is not a concern with all-electric injection molding machines. Such tests are helpful once or twice a year as part of the general machine maintenance program. This will help you maintain a record of the machine's performance, and will help you monitor or detect any degradation in performance over time.

Additional Thoughts
for more about the benefits of all-electric molding machines, please review: Are Hydraulic Machines Going the Way of the Dinasour...?

-Andy
I received a blog response which highlights a growing trend in the molding industry...

Blogger
I am using a pressure sensor at the End of Fill on a new all-electric molding machine. With a DIII process, I am having good success in using the pressure at the end of fill for V/P transfer. This works well with a standard deviation of 0.07mm for cushion.

note: For more on a DIII process, please read: Rules for a DIII Process

Additional Thoughts
The advent of highly-responsive all-electric injection molding machines is allowing people to control the process in very unique ways to fit a specific situation. Using the cavity pressure at the end of fill to transfer from velocity control to pressure control can be a great way of controlling the amount of pressure which is received within the mold cavity during the mold cycle.

The biggest concern with this approach is the response of the machine. Most hydraulic molding machines lack the response time to facilitate the correctly use of an end of fill sensor. 

-Andy
I was asked the following question the other day...

Don
In our main facility, we follow up our training with group discussions with some success. Unfortunately, in our facility overseas, these discussion groups don't work at all. Do you have any ideas why this happens?

My Response
In group settings, there are typically a couple people who follow along with the discussion, while others often struggle to keep up. If the ones who are struggling are uncomfortable in speaking up, then the entire effort suffers.

Additionally, factors such as the company, management, or even culture can make it difficult for a participant to feel comfortable speak up. Overall, most employees prefer to learn at their own pace.

Additional Thoughts
When we establish training programs, we prefer structured self-paced training with focused one-on-one instruction which ensures that the employee learns. In such a setting, the employee does not feel any of the pressures that exist in a group format. This approach has proven successful in virtually all companies and cultures.

For a related discussion, please review this previous entry: Relying on On-The-Job Training Alone

-Andy

I received this follow-up question in a recent blog and decided to answer it in more detail using the blog...

MJ
Which kind of process parameters vary when the screw becomes worn?

My Response
Basically, the most common affects that arise from screw wear relate to the increased back-flow over the flights of the screw. This back-flow increases mixing, but also lowers material conveyance and increases residence time distribution.

As the screw wear increases, the number of revolutions necessary to recover the shot will also increase... thus resulting in more shear heating, a higher melt temperature, and possible polymer degradation.

Additional Thoughts
Since the melting and conveying properties of the screw become compromised, excessive screw wear can also cause a significant increase in the amount of energy which is consumed during screw recovery.

For a related discussion: Tolerance For Screw Wear

-Andy


I received this question today...

AJ
A co-worker told me voids are actually not filled with air or gas. Is this true? Could you explain what voids really are?

My Response
Your co-worker is correct. Voids are sections in the center of a part where material shrinks away from itself and leaves what appears like a small hole within the part.

Although they often appear to be air bubbles, they are actually vacuums within the part where no gas is present. An increase in packing or holding pressure usually corrects this.

Additional Thoughts
Keep in mind, gas entrapment can often be confused with the presence of voids.

-Andy

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
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

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
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
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
I was recently involved in a consulting job and was asked the following question...

Project Manager
The tooling vendor is charging extra for the mold prints and CAD drawings. Should we purchase these?

My Response
You should always obtain prints and CAD files from your tooling vendor... even if you plan on using this vendor for molding your parts, tooling repairs, design modifications, and regular maintenance. Basically, anything can happen, the vendor can go out of business, management could change their practices, ownership can change, or leaking roof could wipe out the data and leave you with a mess.

Just as you have a backup of your computer's critical data... you must obtain prints and files of your molds to serve as a backup of your mold. 

Additional Thoughts
The best approach is to ensure that costs associated with prints and digital files are included in the original quotation. Always spell these specifics out in detail so that there is no confusion.

-Andy
In a recent webinar, I received a common question...

Mark
When tracking the improvements of training, what should I monitor?

My Response
I will answer this is a few parts...

1) Choose from the data that is already available - Do not create new metrics to monitor... if it was important to the management, someone would already be monitoring the parameters.

2) Select 3-4 parameters - Choose a few important parameters... monitoring too many parameters will become overwhelming.

3) Gather data form the last 6-12 months - Without a good baseline for comparison... you will not know how much things have improved.

Additional Thoughts
Don't be afraid to share the credit with other initiatives. In many cases, another improvement initiative will take place while the training is going on. For example, the die setters and technicians could be implementing 5S at the same time you are training them on processing. In such a case, the reduction in downtime and changeover time will most likely be a cumulative result from both initiatives... especially when the initiative is integrated into the training through tasks and customization.

-Andy

About this Archive

This page is an archive of recent entries written by Andy Routsis in April 2010.

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