Flow to Thickness

Definitions: flow 1) to move along in a stream, 2) to precede continuously Thickness: 1) a layer or ply;  Flow :Thickness Ratio 1) a ratio calculated by dividing flow distance by thickness of flow stream.

Flow to thickness (F:T) and or length to thickness both refer to the basic idea of how far can we push the plastic per the wall thickness of the part. These are ratios and written in various ways: F:T  F/T or L:T L/T.  In some publications materials are given a flow to thickness number and  will highlight the definition to explain that it is based on a 0.100" thickness. It is good information and over the years I have come across many variants and discussions on it. From a design point it could mean that if our ratio given for the material was 250/1 we could than use 1 gate to fill a part that is 25" long and 0.1" thick with a gate at the furthest extreme, if in fact 0.100" was given as basis for the ratio.

Thus having these numbers for materials can aid in thinking what you can do in relationship of gate location to flow length.

The author prefers to think in the other realm in what is the flow to thickness ratio of our mold and or design.  Have we design something with a 200:1 ratio and now have to process it and or can we change things and reduce our flow to thickness ratio thus opening up the process window in which to mold our part. What is done is to measure the flow length from the gate and divide by the wall thickness. This than give our flow to thickness ratio for the mold at hand, and we can even check various points within our mold to see if we have balanced fill (this from a purely academic approach.)

When computing what our flow to thickness ratio is by dividing the flow length by the wall thickness, keep the units the same within our division. Another way of putting it is from our gate opening how far does the material have to flow to reach end of fill?

Example:

A flow length of 10 inches with a 0.100 inch wall is a 100:1 Flow to thickness ratio. While a 254 mm flow length with 2.54mm thickness is 100:1 flow to thickness ratio.

What does it mean?

Understanding what the Flow to thickness ratio is and applying it to our processing helps to understand why the mold may process in one machine, and or why we are having issues trying to process the mold.

To put F:T into a practical understanding 3 categories are listed.

 I = F:T less than 100:1

II = F:T greater than 100:1 by less than 200:1

III = F:T greater than 200:1

I      

In the case of F:T less than 100:1 we are in normal molding. Doing what you have always done, using the standard process as provided by your material supplier tends to work out.

II

In the case of being over 100:1 but under 200:1 we are at conditions that typically will use maximum settings as suggested by the material supplier. In some material cases you may even be above suggested settings. Example of this is Polycarbonate, Acrylic, and PVC etc.

III

In the case of being over 200:1 we are in areas of thin wall, and above all settings as suggested by the material suppliers. In these cases, both material temperatures, mold temperature, pressures and speeds need to be higher and further tool construction has to be evaluated as to steel, thicknesses, and compression areas.

The understanding of what the F:T ratio is for the mold can help in processing, and or troubleshooting of the process should it be necessary. An example is an interior car door mold with wall thickness at 0.100". Now not a "thin wall" the flow length that was trying to be achieved made it thin wall. This than allowed all to understand why mold heaters were put into use, material temperature was increased (could not change grade) and pressures / speeds modified.

**There are various formulas for calculations, and it is agreed that F:T should be explored up front at the design stage, in the real world some of these things are still not discussed until we can't make parts.