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August 21, 2003

3 Min Read
Wall thickness and filling the mold, Part II

An Injection molder looks at wall thickness differently than does the part designer. His concern, in addition to the strength of the part, is whether the mold will fill.

All plastics families have flow characteristics. These are sometimes grouped into categories, called easy, medium, or hard flow, with the easy-flow materials having greater flow lengths.

Rheology is the study of flow, and a couple of important terms are used therein. The first is shear. It is the measure of the internal resistance of a body to applied forces. When the shear rate increases the fluid overcomes molecular resistance to achieve a faster flow rate.

Viscosity is a measure of a fluid's resistance to flow. A fluid (molten plastic) with low viscosity flows more easily due to its molecular makeup. High-viscosity materials resist motion and have a lot of internal friction.

Plastics are called non-Newtonian fluids. Liquids like water are referred to as Newtonian fluids. The basic difference is the length of the respective molecules. Plastics have longer molecules and flow only when pressure is applied.

Plastic flows in the center of the wall, and the molten granules flow through the center of the wall at a greater velocity than does the plastic closer to the wall. Theoretically, plastic flow at the wall is zero, and velocity increases to maximum in the center of the wall.

Due to this plastic-filling phenomenon, the distance a given plastic flows is generally a function of, and proportional to, the square of the wall thickness. It follows that the thicker the wall, the greater the length the plastic will flow if everything else is equal.

The fillers, additives, colorants, stabilizers, and molecular weight of the resin also affect flow lengths. The molder needs to help to determine if the mold can be filled in a given grade of plastic. The table below offers guidelines that can help you determine flow length for a variety of materials.

As an example, a plastic with a flow length of 170/1 means that, in theory, a part with a .1-inch wall thickness will have a flow length of about 17 inches. A thinner wall means that flow length will be less than 17 inches; a thicker wall should increase flow length.

Plastics will only flow the approximate distance illustrated in the table when the flow is uninterrupted and on one plane. Each time a material changes direction, encounters a change in wall thickness, flows around an obstacle, stops, or restarts, the flow length is diminished.

To calculate a new flow length when an existing flow length is known, the following formula gives a quick approximation for wall thickness ranging from .025 to .075 inches:

NFL = PFL

(NWT)2

 EWT

Where: NFL = New flow length, PFL = Present flow length, EWT = Existing wall thickness, NWT = New wall thickness

A reduction in wall thickness reduces the flow length of the plastic part. Increasing the thickness lengthens the flow.

You're usually limited as to what you can do to fill the part better. Your first option is to change gate location. The second option is to use multiple gates into the same part.

Bob Dealey operates Dealey's Mold Engineering, A Plastics Management, Marketing and Technical Consulting Firm, where he consults with the plastics industry. An Honored Member of SPE, he was named Mold Designer of the Year by the Mold Making and Mold Design Division of SPE. He can be reached at 262-245-5800, [email protected] or www.DealeyME.com.

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