Plastics Technology: August 2010 Archives

Calculating Residence Time

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By Andy Routsis on August 27, 2010 10:08 AM 0 0 Vote 0 Votes

I was recently asked the question...            

 

Kevin

"Do you have any way of easily calculating the residence time in screw / barrels?"

 

My Response


Most general purpose screws contain approximately 1-2 shots within the flights depending on the geometry. To calculate the residence time you will have to know your part volume, the volume of material in the barrel, and your cycle time. You can use a basic calculation for the barrel volume minus the screw volume (ignoring the flights because they are relatively small) to calculate the material your screw holds.

 

To calculate the volume of the barrel:

3.14 x (Barrel radius)^2 x (Barrel length)

 

To calculate the volume of the screw:

3.14 x (Average root radius of the screw)^2 x (Barrel length) 

 

Volume of material in the barrel/screw: Barrel volume-screw volume


As for the residence time:

If you calculate the volume of material in the barrel as 10 ounces and your part is 5 ounces your screw contains two shots of material. Multiply the number of shots in the barrel by your cycle time and you will get a residence time for the material.

For example: If you have a 25 second cycle time and your screw holds 2 shots the residence time is roughly 50 seconds (two cycle times).

The residence time will be a multiple of your cycle time plus or minus one cycle (depending on the ratio of part volume to barrel volume).

 

-Andy

 

 

Submarine Gating for Polycarbonate...

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By Andy Routsis on August 25, 2010 7:45 AM 0 0 Vote 0 Votes
I received this interesting question through linked-in...

Linked-In Member
Currently we have provided edge gating for appx. 25 dia @ 1.3 thk mm PC part.

We are planning for tunnel or submarine gate... When using polycarbonate material, what kind of gating should be provide?

My Response
When utilizing a submarine gate with PC, there are a few things to consider:

The biggest mistake people make when sub-gating with PC is to use too low an angle. The gate should angle into the parting line at least 60ยบ. This will minimize the amount of deflection necessary to remove the gate from the mold.

Another mistake commonly made is to make the gate too long. The gate should get to the mold cavity as quickly as possible. This will reduce the amount of deflection necessary to remove the gate from the mold, thus reducing the chances of the gate remaining in the mold.

Lastly, if you are having success with a current gate size, we recommend using a smaller cross sectional area for the sub gate since it is very easy to remove steel and enlarge the gate later.

Additional Thoughts
Pay close attention to your runner design, many molders will actually step the parting line behind the gate to allow more room for the runner system to bend as the gate is removed.

-Andy


Correcting for Cavity Imbalance...

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By Andy Routsis on August 23, 2010 1:55 PM 0 0 Vote 0 Votes
I received this inquiry the last week...

Kevin
I have been using the intelligent moulder worksheets and have found poor fill balance in our hot runner tooling. I have had variations from 14 to 22%. How do you get to 6% or less?

note: The Dynamic Cavity Imbalance Test measures the imbalance by comparing the part weights from each cavity using three different injection speeds. 

My Response
You should first look for inconsistencies such as blocked or damaged vents and gates.

You should review your hot runner system and determine if any gates can be adjusted. Often an adjustment can be made, or a him can be placed behind the gate drop to reduce flow in the mold cavities which are not filling well.

If the runner system has sharp turns, you can first try rounding the corners to reduce the stresses.

Additional Thoughts
Each mold can pose it's own unique circumstances. For branched runner systems, the ‘Melt Flipper’ can often be used, even with hot runner molds. This technology, licensed by Beaumont Technologies Incorporated, uses strategically placed inserts at runner branches. The inserts rotate the polymer melt to help manage the uneven stresses resulting from runner turns. I have seen this become very helpful in many different applications.

-Andy

"Steel Safe" Runner Designs

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By Andy Routsis on August 18, 2010 11:22 AM 0 0 Vote 0 Votes
I was recently asked the question...

Jeff
We are about to build a 2 plate family mold but are not sure which molding machine we will use it in. We may also process two grades of material in the mold. How will these factors effect the size we should cut our runners?

My Response
Often, the feed system design is uncertain. You have mentioned two common issues which do effect the runner size.
The capability of your molding machine to fill the mold with a material whose flow characteristics are unknown should always be taken into account. If the runners are cut to large your machine may not be able to fill the mold.
There may be variation between the melt flow rates and pressures to properly balance a family mold, especially when two grades of material are being used.

In this situation I suggest you use a runner dimension that is "steel safe". "Steel safe" means your design should call for the removal of less mold steel than may be ultimately required. You may want to round down your runner dimensions by one or two standard sizes than what you calculated as nominal. This will result in a lower volume feed system with a greater pressure drop. There is a chance the smaller feed system will work and you will save on scrap material. However, if the undersized feed system is not optimal, this approach will allow you to easily machine the runners and improve the performance of your mold.

-Andy


The Need for Safety...

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By Andy Routsis on August 14, 2010 2:12 AM 0 0 Vote 0 Votes
I am travelling overseas and witnessed a roadside welding facility where the workers were not wearing any type of eye protection. As most people know, this is a sure-fire way to lose your eyesight permanently...

My Concerns
Regardless of the country or region in which you live, it is critical that you promote safe working standards for your employees. I understand that different cultures take varying positions on worker safety, but some situations are just unsafe no matter where you live.

-Andy
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