The basics'

Materials … part 2

In part 1 materials were broke down to the thermosets, thermoplastics, and than the amorphous and semi-crystalline types of materials.  Most people would say so what and why must we know what the material is. The key is knowing what you run.

When we look at the processing conditions for a material and optimize the press or extruder, knowing what we have means everything.

In taking the first statement of softening upon heating or melting, these two things say the world of what we are processing.

With amorphous materials, they only soften resulting in low shrink, and as we add heat, we can influence in certain cases the flowabilty of the material. As we add or take away heat/energy, it is usually done via straight-line type profile, i.e. energy in (BTU) and gain or loss of temperature to the melt. The viscosity can change in slow fill cases by temperature increases. Over-packing can be an issue with these materials, but warpage is usually not.  Shrinkage is usually low and if we do not meet spec or have too high a rate, we need to look at pressure, gatefreeze, and temperatures. Hot runners, heated sprue bushings tend not to be issues of great concern.

With semi-crystalline materials, they actually melt though they do have a softening stage, where it is possible to flow the material. By melting it results in the high rate of shrinkage we see in the materials, this can cause warpage too. When we add heat / energy to the material it softens and gains in temperature to a point. We than need to add additional BTU's to melt the polymer this is call the latent heat of fusion. Once the material is melted, we do not change the viscosity by adding additional heat; we only change the heat content of the polymer. By heat content we talk of the temperature of the melt, thus being hotter, it does flow further but this is because of the rate of cooling not the viscosity. Under-packing is usually an issue with these types of materials.  Hot runners and heated sprue bushing can be a problem because we now need to pull out the heat of fusion energy. (Those additional BTU's to set up the material so that we do not string.) Thus cooling around the gates and sprue bushing must be thought out from the get go. The additional problem / opportunity with semi crystalline materials is the mold temperature. The cooling rate or rate at which we take away the BTU's from the plastics effects the crystallization rate (rate at which the crystals form). This is from the mold temperature, and the faster the rate of cooling the fewer crystals, part doesn't shrink as much (cold mold), the slower the more crystals (larger crystals), parts shrink more (hot mold.).

I hope that it is understood somewhat now why understanding the difference between the two types of thermoplastics is helpful. Yes you must now know the type, but in a broad sense.
To be continued…….

Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Distribution Services

A request,
Over the years (yes too many) I have written and commented on various aspects of the industry and have heard at times that people enjoy the column. My request is to know what further topics would be of interest to you, the readers and or if you have questions what those would be. Please E-mail them to SLSILVEY@Ashland.com.

Thanks,
Steven