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Compensating for Barrel Residence Time…

I got this question regarding a situation many small-part molders find themselves in…

Arturo
Our Maintenance crew came up with the idea of putting E-Dart systems on our presses. Some of our presses are running with less than 20% barrel capacity. What do you think about this plan without any change in the barrel capacity.
My Response
Regarding eDart Technology… Let me explain a few points for my readers.  In my experience, such technology is best implemented for one of two reasons… 1) to detect and isolate specific part defects such as short shots. 2) to accurately control the transfer from 2nd stage pack to 3rd stage hold using cavity pressure measurements. In both these situations, the mold must be properly instrumented with pressure sensing technology… and your employees need to be trained on the use of such technology before implementation. The learning curve for this technology is steep… so be sure that all your employees have a firm grasp on the fundamentals first.
Regarding Your Question… Based on your line of questioning, my assumption is that you are asking if the eDart will help compensate for the complications arising from running less than 20% machine capacity. A machine running below 20% capacity typically has a residence time of 10 cycles or more. For this reason, the inherent complications resulting from a long residence time such as degradation, property loss, and presence of volatiles will not be affected.
If these were my machines, I would invest in new screws and barrels for the machines before I bought anything else. My approach to technology is as follows… Don’t purchase the latest and greatest until you are 100% sure that 1) your employees are getting the most out of the your equipment, and 2) it will make a significant difference.
With your improperly sized screw and barrel… It is like putting a turbo charger on a car which has a flat tire.
Additional Thoughts
For more information on proper barrel sizing, please read the following entry:
-Andy

Scheduling OJT

I was talking with a Human Resource Manager the other day and got this question…

Frank
My boss wants me to do all the training on-the-job. Unfortunately, this is a scheduling nightmare. What is the best way to do this?
My Response
First, OJT is the most direct method of instruction, but it is not a good training tool. Scheduling is dependant on both the trainer and trainee… requiring equal time spent by each. Ultimately, this training by itself will teach the minimum behaviors necessary to perform their job function.
You should consider using a blended learning model which minimizes and focuses the on-the-job training as a component of an entire employee development initiative.
Additional Thoughts
I know it seems as though I side-stepped the question… but I really believe Frank could not get all his goals accomplished, regardless of how he approached scheduling.
-Andy

Machining Core Threads With a Lathe…

I just received this unique question from a toolmaker in China…

Dodge
We’re trying to make a collapsible core with threads using a CNC lathe.  The specification requires the thread start at half way around the circumference, which is impossible because the lathes have to complete a full circle before making a screw thread.  Is my team wrong on this?  If not what kind of machinery would we need?  We thought about a screw grinder but the part is too big, overall length is 11.25inch.
My Response
Although CNC lathes are typically used to machine threads in typical machining situations… they are not commonly used for machining threads on injection molds. This is especially true for applications, such as yours, where specific thread positions are required.
The most common method of machining such threads is CNC Electric discharge machining (EDM). In such a machining method, a conductive electrode with inverted detail is manufactured using CNC machining processes. This electrode is then placed into the EDM machine and the detail is removed, or burned, from the tool steel using an electric spark between the electrode and the workpiece.
For more information into the EDM manufacturing process… please visit wikipedia:
If you have access to a 4 axis CNC EDM machine, you can create even better threads by rotating the electrode about the core as it moves up or down. This may create more accurate thread detail on the core.
For training on this topic, consider our Mold Design and Mold Making Series.
Additional Thoughts
Mold makers have also used 5-axis CNC machining stations to create this type of detail… but the setup and expense of using such equipment for such a small task is generally not justified.
-Andy

Compensating For Material Variability…

I received this e-mail from a medical molder who is encountering complications with maintaining process controls within their validated standards…

Steve
We a medical molder and monitor and record part weight as a control during production runs. On a particular component the spec is 31.165g +/- 0.045g on a 16 cavity mould and was determined from one validation run. Unfortunately, we are continually having to adjust the ‘Holding Pressure’ to stay within the tolerance and sometimes are having to set a value outside of our verified limits. These adjustments are made at the start of each production run and are generally ok for that run or batch of material. The rest of the process parameters are set to validated point conditions and are identical from run to run.

My question is whether weight is a valid method of control with such tight tolerances and what could cause the weight fluctuation if identical point conditions are used for each run?
Note: After a few follow-up questions it was determined that the part is full during first stage, and changes cannot be made to the shot size, transfer position, or fill time.
My Response
The short answer is… yes, weight can be a great way to control tolerances for a molded part.
Unfortunately, the root of your problem is the fact that the part is completely full during first stage, so the process cannot properly compensate for material variation. In this situation, you must fill the mold completely during first stage to meet the validation requirements. Even with sinks, you are actually packing the part during first stage fill. Since most materials will exhibit viscosity variations of approximately 5-10%… the degree of packing that occurs during first stage fill will vary greatly with material variation. These variations can result from machine changes, ambient temperature changes, humidity changes, as well as the obvious variations in the material.
In the long run, it is much more beneficial to establish a process by separating fill from pack. Filling with a short shot during first stage… and packing out the part during second stage will allow the part to fill a little more or less during first stage. completing fill and packing the out during the second stage will ensure velocity is not being used to perform the packing duties.
In your current process… you have a slight tolerance in your process, and I suggest you do your best to slightly increase the amount of material that enters the mold during fill when the weight is low, and slightly reduce that amount when the part is heavy.
Additional Thoughts
In the long term, you should also try to focus on documenting and validating more process outputs rather than process inputs. When you validate all your process inputs, as well as all your process outputs… you eliminate all your opportunities to compensate for material variation.
-Andy

Blog Archives

Hello Readers,

Our ‘Plastics Training and Technology Blog’ has been active for less than 6 months… yet, we have had thousands of readers and more than a hundred contributions in both questions and comments.

Here at A. Routsis Associates, Inc. we like to thank all of our readers and contributors for their support… but also mention that there are great blog entries in our archives many of the more recent readers may have missed…
A few of the popular archived entries are listed below:


You can also use the search tool on the upper right-hand corner to see if we discussed any topics of interest to you.

As always, feel free to e-mail me at blog@traininteractive.com with any topics that you would like me to address.

-Andy

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