Plastics Technology

FEB 2018

Plastics Technology - Dedicated to improving Plastics Processing.

Issue link: http://pty.epubxp.com/i/929876

Contents of this Issue

Navigation

Page 39 of 91

tions for creep resistance, as we will show in a later article. Another benefit of higher crystallinity that is often overlooked pertains to the way that nylons interact with moisture. As nylon parts absorb moisture they lose strength and modulus. They also exhibit changes in dimensions. As nylon parts take up moisture they swell. But the moisture is only absorbed by the amorphous regions of the part. Therefore, the more crystalline the material in the part, the smaller the effect of the absorbed moisture will be on critical dimensions. Since cooling rate is a critical factor in determining the degree of crystallinity in a semi-crystalline polymer, the mold tempera- ture selected by the processor will be important to part perfor- mance. Part geometry will, of course, be an important aspect of this picture. The wall thickness of the part is a significant element in determining cooling time, and it will always be the case that material that is in direct contact with the mold wall will cool more rapidly than material in the center of the wall. This means that the degree of crystallinity in any molded part will vary with location. But part geometry is a constant. Mold temperature is a variable that is set at the discretion of the processor. Often, the selection of a mold temperature is made without any awareness of its importance to the performance of the part. Higher mold temperatures will typically promote a higher degree of crystallinity if the mold temperature is somewhere on the left side of the curve in the graph on p. 36. But raising the mold temperature is usually considered to bring with it an automatic penalty in cycle time. For some materials this may be true. Polyethylene has a Tg well below room temperature. Studies show that the peak crystallization rate for polyethylene is also below room temperature. For materials like this, colder is better and there is likely no downside to part performance. But with higher performance engineering polymers, where the T g is well above room temperature, lower mold temperatures can intro- duce a wide range of problems that may not be apparent until the part is in use. In our next article we will look at some examples of the unin- tended consequences of attempting to improve productivity by reducing mold temperature. Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS ABOUT THE AUTHOR Mike Sepe is an independent, global materials and processing consultant whose company, Michael P. Sepe, LLC, is based in Sedona, Ariz. He has more than 40 years of experience in the plastics industry and assists clients with material selection, designing for manu- facturability, process optimization, troubleshooting, and failure analysis. Contact: (928) 203-0408 • mike@thematerialanalyst.com. Generally, crystallization will occur at a relatively slow rate at temperatures just below the melting point. The rate will accel- erate as the temperature declines, reaching a maximum crystal growth rate at some point. Beyond this point the rate of crystal- lization will slow, and once the temperature declines below the Tg the process will stop altogether. A generalized pattern for this behavior is shown in the accompanying graph. This graph applies to natural rubber and is therefore not that useful for plastic injection molding. But the overall pattern that it displays is common to all polymers. There are a couple of very important points contained within this graph. First, in a semi-crystalline polymer, cycle time is governed to a significant extent by the rate at which the material achieves a certain modulus. This modulus, in turn, is related to the number and size of the crystals that form. The faster the crystals form, the faster the part can be demolded. The graph shows that at some point, reducing the temperature of the mold, probably the most common tactic that processors use to reduce cycle time, can be counterproductive. Even more important, reducing the mold temperature will reduce the degree of crystal- linity that is achieved within the molded part. This is where the performance element of the discussion comes in. We choose semi-crystalline polymers over amorphous polymers for a variety of reasons. Among these are improved chemical resistance, envi- ronmental stress crack resistance, and fatigue resistance. In addition, a higher degree of crystallinity is associated with higher strength and stiff- ness. If the polymer is not given the opportunity to crystallize, these properties will be compromised. There are other benefits of increased crystallinity. One of these is the degree of modulus retention above the glass transition. One of the key benefits of semi-crystalline polymers is that they possess useful solid-state properties above their Tg. While amorphous polymers essentially soften and become useless as load-bearing materials above their Tg, semi-crystalline materials, even when unfilled, will retain 10-30% of their modulus. For example, an unfilled PBT polyester, with a room-temperature modulus of 340,000 psi (2340 MPa), will still have a modulus of 48,000 psi (330 MPa) at 100 C (212 F). This retention can be improved by incorporating fillers such as glass fibers into the material. But if the material fails to achieve its intended degree of crystallinity, the expected modulus retention at elevated temperatures will be reduced. This can have implica- For every polymer there is a definable relationship between the temperature of the polymer and the rate at which crystals form. 38 FEBRUARY 2018 Plastics Technology PTonline.com M AT E R I A L S K now How

Articles in this issue

Links on this page

Archives of this issue

view archives of Plastics Technology - FEB 2018