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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

Complications From Changing Screw Diameter

I just received this interesting question yesterday from a medical molder…

AP
We are planning to change screw size because we are using less than 20% of the shot size capacity of the press, can you give us a little information how this problem affect our process control and how big is the impact on it if we change the screw size to have it in the advised range please? Also if we change the screw and barrel size do we need to make a lot of modifications to our electric molding machine?
My Response
First, I just wanted to re-iterate the comments I made in a previously blog entitled ‘The 80-20 Rule For Available Shot Size‘.
20% – The typical general purpose screw contains approximately 1-2 shots of material within the flights of the screw. This means that a process running at 50% capacity will have an estimated barrel residence time between (2) and (4) * (cycle time). Likewise, a machine running at 20% capacity has an approximate residence time between (5) and (10) * (cycle time). If you bring this to the extreme, a process running at 5% capacity could have a barrel residence between 2000% and 4000% of the cycle time!
As a medical molder, you can create some potential liability since the residence time will be so high relative to the process. This also creates variability in the melt quality due to longer residence time distributions and very small cushions relative to size of the machine.
When you upgrade to the smaller screw… the biggest changes will be in the melt temperature and shear rate during screw recovery. I strongly recommend taking a melt temperature measurement during the current configuration so that you can better match it after the change. To match the shear during screw recovery, you may want to determine the circumferential speed used during screw recovery so that it can be matched on the new barrel. To calculate this, multiply the RPM times the screw diameter times pi… (RPM)x(d)x(π).
A little more about this topic is discussed on my post entitled ‘The Affect Of Screw Diameter On Shear Rate‘.
As with most processes, you should do your best to match the process outputs such as fill time and melt temperature.
Since the machine is electric, you will have to work with the manufacturer to adjust the settings to ensure the proper plastic pressure is reported on the controller.
Additional Thoughts
You are very likely to encounter a more solid and reliable process with the new configuration… plus you will significantly reduce risks associated with higher residence times and residence time distributions.
-Andy

The Pro’s and Con’s of Mold Insulation

I just received this question regarding adding insulation between the mold and the platens…
Milan

I want ask you if it is a good to put plate between mold and machine platen? Some technician told me yes, cause you need isolate warm mold from cold platen.

My Response
Just to clarify for our readers… many injection molders will position insulation plates between the A and B clamping plates and the stationary and movable machine platens. The purpose of these plates are two fold… (1) to prevent the machine platen from influencing the heating and cooling of the two mold halves in an attempt to stabilize mold tempertaure. (2) to reduce the heating and expansion of the platens… which may affect mold safety on larger machines and clamp tonnage on toggle machines. Over time, this can also affect the squareness of the molding machine.
Most processes can benefit from these plates as they tend to eliminate variables in the process and reduce machine maintenance. There are two ways to go about adding insulation plates to the molds.
Wrong Way – Many injection molders solve these problems with generic plates they just wedge between the mold and the platen during setup. I am not in favor of these plates as they get damaged easily and can shift during production. If they get damaged, they can apply uneven clamp tonnage which will affect part quality and platen alignment. If the plate shifts, you can create a serious risk to both your employees and your equipment.
Right Way – Such plates should be individually fitted and bolted to both the A and B half of each mold. This approach will help prevent damage to the insulation, and ensure consistent performance in the application. This may seem like an expensive option initially, but the reduction in die setting time and uniformity of clamp tonnage should outweigh the initial expense.
Additional Thoughts
These systems most benefit anyone operating a hot runner system, hot sprue, or high-temperature molding application with oil-coolant will benefit from an insulation plate between the clmap plates and the platens.
-Andy

The Importance of Housekeeping

I believe many companies underestimate the importance of housekeeping…

George
My technicians keep telling me that custom-molding facilities are always somewhat messy due to the high number of mold changes. How do I argue against this?
My Response
George, your techs are pulling your leg just to defend their laziness… though many injection molding plants do not emphasize cleanliness and organization.
Good housekeeping will reduce the time it requires to find things and get things accomplished. A systematic and organized workplace is a safer workplace which is much safer, more productive, more efficient, and more profitable working environment.
This is true across the board, whether you are molding containers, heart valves, lawn chairs, or pipe fillings.
Additional Thoughts
Ultimately, the efficiency of an injection molding plant is highly dependent on its cleanliness and organization.
You should approach cleanliness through plant-wide initiatives, employee training, and by personal example.
-Andy

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