Definitions:
SHEAR: The movement, in fluid or solid body, of a layer parallel to adjacent layers. Shear degradation: Chain session of a polymer caused by subjecting it to an intense shear field, such as exists in the close clearances of extruders and internal mixers.
Shear Rate: The rate of change of shear strain with time. The universally used unit of shear rate is s־¹.
Previously there was an article on gates, and in our discussion here the shear at the gate is our concern. Understand that if the tool is a family tool than there maybe different gate sizes and if a multicavity tool all gates need to be the same. This means the area of the gate, and the land length.
We will not discuss the pressure drop through the gate area, but will discuss the shear effect on the gate. Pressure drop is important and a real aspect of the gate and fill process.
The basic formula a lot of people will use for shear rate is:
Shear rate = 32Q ÷ [Pi x d³] or 4Q ÷ PiR³
Q = quantity of material per second (volume per cubic)
d = diameter of gate (in same measurement system)
R = radius of diameter of gate (in same measurement system)
What is meant here is that if dealing in meters use meters throughout, if inches use inches through out.
A notification here, if any have recently reviewed an article that now states that there needs to be a factor n in front of the 32Q (4Q) because we are dealing with non-Newtonian flow. At this point I will leave that up to you. What we are looking at is shear rate, and yes the n factor will give real numbers for discussing the max for the material. I have as yet not been able to compile a complete listing of n so I do not include it in this discussion. Many CAE programs use various variations of the formula to come to the conclusions within their programs, since they do not all use the same this may explain why the variation in program numbers and also variation to real world numbers on the press and the need for communications between all parties. Using the n factor for an equation will give a higher shear rate, and one that is more true to what is going on.
So given the formula above we can compute the shear rate through our gate (opening) and from that come to a conclusion as to what is going on.
1- The first thing in the formula to note is that the diameter is cubed.(or radius)
2- The second thing is that the quantity is multiplied by 32. (or 4)
The cubing of the gate diameter is the most severe as to results within our gate.
|
Gate diameter |
fill time seconds |
|
|
|
|
|
1 |
2 |
3 |
4 |
|
0.105 |
87,991 |
43,995 |
29,330 |
21,997 |
|
0.100 |
101,859 |
50,929 |
33,949 |
24,464 |
|
|
|
|
|
|
The above shear rate calculations are based on the two gate sizes shown and the fill volume of 10 cubic inches of plastic. The values are in reciprocal seconds (s־¹)
1- As you slow down the fill time (slower injection speed) you decrease the shear.
2- There is a direct relationship to shear reduction to fill time in that when you decrease by 50% you get a 50% reduction in shear.
a. This is a relationship between time and volume and is used by molders the world over to eliminate issues.
3- The difference in gate size is based on the cubed difference.
4- This is why gate size is critical. 0.005" is not much but in the example above it shows a great deal of difference at the same fill times.
Now where does all this leave us? Basically what it is saying is, if we know our maximum shear values for a material we can calculate how close we are or are we exceeding shear for the materials we are using. For example the material Rigid PVC has a critical shear rate value of 20,000 before it degrades, therefore the data in the example above would indicate that these fill rates and gates sizes would not work for the material. We would either need to use an extremely slow fill, or open the gates to our parts, so that the resulting shear rate is less than a 20,000 value.
This also shows that with multiple gates that if they are not of the same size we achieve different shear rates in the flow of our material, or that is why one cavity may be filling so much faster than the rest. This is not talking about shear imbalance within our tool, only gate shear, which is a component of total shear imbalance.
This does not take into effect the pressure lost though our system and another reason to make all gates the same.
How do you use this data? It should be stated that you use it with care and take the time to build up your data base. For example if we know our cavities all have different gates than this would show some of what is going on. If it was a family mold it may allow for the balancing or at least getting the shear correct in each of our cavities. Further if we had a blush issue and by slowing down the fill rate it was eliminated the data could be worked with to come up with a shear rate. The data than could be used in future tools to predict either rate of fill, gate size, etc. so as to be more competitive and efficient.
It further shows that by profiling the injection speed that shear rates and stresses change within the filling of the part.
A recent example was a medical part which used valve gate system, itself contributing to a high shear rate. The original tool used a valve per gate, and short chicken runner to fill the part. The new mold used same valve system but now filled 4 parts. In maintaining fill rate for the gate area the fill time stayed the same, yet in reality the shear through the valve gate was the highest shear area. This was realized by the extreme buildup of residue in the cavity and vent area, after producing parts for a period of time. Even though they had maintained the fill time and same gate size the area of shear was in the valve gate and not the gate to the part.
The above brings up the point of knowing or realizing that the smallest diameter in the feed system is what institutes the highest shear into your material. In another case the nozzle on the machine was the highest area of shear and by opening the nozzle up to match the O of the sprue the parts could not now be filled.
*** Please note that in balancing flow to a tool, shear can contribute/ be an issue especially in a naturally balanced system. Check below for further reading on this and determination using the 5-Step Process ™
