When products/parts are designed, the type of resin chosen for the application is typically driven by the part requirements as they relate to performance and environment.  These requirements narrow down the selection of resin choices very quickly.  If the resin choice has been narrowed down to more than one type or family of resins, it is very important to understand that the selected resin can have an impact on cycle time.  One resin may cause the molding of the part to have a cycle time 25% longer, or more, than the other resin due to its properties.  The property which can control cycle time is Thermal Diffusivity.

To provide some insight to injection molding and the functions of the injection mold, we will broadly discuss the functionality of the injection mold.

The injection mold has several functions:

· Act as heat exchanger (cool the resin from 400F to 650F or more down to 100F in mere seconds)

· Act as a high pressure vessel (must contain a resin pressure of 1000 psi to 10,000 psi and more)

· Lastly....Form a part

Each function is extremely important, but for this discussion we will focus on the importance of why the tool must act as a heat exchanger. When the resin is injected into the tool, the tool must extract the heat out of resin as quickly as possible in order to keep the cycle time as short as possible.

In order for the tool to extract heat from the resin, we must understand as much as possible about the properties of the resin. 

We all know that resins are not created equal. One of the differences between resins is their ability to give off heat. This property is called thermal diffusivity. This property seldom, if ever, shows up on a resin data sheet - but is responsible for cycle time.

To understand this concept, thermal conductivity is a property that determines how much heat will flow from a material. Thermal diffusivity determines how quickly heat will flow from it. For example, if a copper rod is dropped into boiling water how hot the other end feels is determined by the copper's thermal conductivity. How quickly the heat is felt is determined by the copper's diffusivity.

By taking the density, thermal conductivity, and the specific heat from a data sheet, it is easy to calculate the thermal diffusivity. A high diffusivity resin will cycle faster than a low diffusivity resin.

The equation for Thermal Diffusivity is as follows:

                              k

α =       -------------

  (ρ) (Cp)

Where:

α = thermal diffusivity            - has the units of ft2/hr

k = thermal conductivity         - has the units of BTU/(hr ft oF)

ρ = density                              - has the units of lbs/ft3

Cp = specific heat                   - has the units of BTU/(lb oF)

If you are using a slow cycling, low diffusivity resin and know it's cycle time, you could make a very rough estimate (i.e. not significantly accurate) of how much cycle time could be saved by going to a higher diffusivity resin:

The values for density, thermal conductivity, and specific heat change as the resin cools from the melt temperature to ejection temperature. 

In order to have a vivid understanding of how one material may improve cycle time over another material, a flow analysis must be conducted.  Bozilla Corporation utilizes flow analyses to optimize material selection, part design, tool design, process settings and ultimately cycle times. 

Today, everyone is looking for faster cycles - choose wisely !

This article was written in conjunction with:

Robert Beard, P.E.

Robert A. Beard & Associates Inc.

www.Plastic-Solvers.com