Plastic Degradation During Injection Molding
By: David Rose, Senior Instructor, AIM
When it comes to the injection molding of thermoplastic materials, it is reasonable to assume that most processers give little consideration to the molecular weight (MW) of the materials that they are using to mold parts. Their focus will often be on other factors important to the manufacturing operation, such as: cycle time, spoilage, part dimensions, aesthetics, and performance testing. Although MW is not always at the forefront of thought, it is very important to the performance of the product. Therefore, it is imperative that processors recognize that the injection molding process affords plenty of opportunities to degrade the material.
That is why it is not uncommon to hear Mike Sepe, a prominent plastic materials analyst and a fellow instructor at the American Injection Molding Institute, state that “a processor’s number one responsibility is to preserve the molecular weight of the polymer during molding”. He says this to emphasize the importance of treating the material with an appropriate level of care.
Polymer degradation occurs when the covalent bonds along the backbone of a molecular chain are broken. When this happens, the lengths of the molecular chains are shortened, reducing the MW of the polymer. As the MW declines, the performance properties of the material decay. Polymer degradation can occur during the injection molding process as a result of improper material preparation prior to molding as well as during plasticating and first stage injection.
For many materials, drying is the first step of material preparation. Drying time can vary depending on the temperature and moisture content of material prior to drying. The material must be dried until the moisture content is at a level that is safe for processing. Proper care should be taken during the drying stage to ensure that an appropriate drying temperature is used. It is imperative that drying temperatures are not raised above material suppliers’ recommendations to avoid an oxidation reaction within the polymer that will cleave the molecular chains and reduce the molecular weight of the material. Nylons are particularly sensitive to oxidation during drying, and the drying temperature has a much greater impact on oxidation than the amount of drying time.
Accurate and precise measurement of a material’s moisture content is critical to maintaining the MW of a plastic during molding. If too much moisture is present in the material it may trigger a chemical reaction within the injection unit that degrades the plastic. For plastics that are hydrolyzable (such as PA, PBT, and TPU), the water will react with backbone of the plastic molecules and break them into much shorter chains, significantly decreasing the performance properties of the material. Plastics that are hydrophilic (such as PPS, ABS, and POM) will only experience cosmetic issues because the water reacts with the pendant groups rather than with the backbone of the chain.
Even when plastics are sufficiently dried, the material can degrade within the machine barrel and during injection due to excessive melt temperature, long residence time, and excessive shear rates. The screw’s compression ratio and rpm are working in concert with the heater bands to establish the melt temperature within the barrel. Through compression and shearing, the screw imparts the majority of the heat energy required to get the plastic to its processing temperature. Therefore, the processor needs to pay close attention to the screw recovery settings.
As the material resides in the machine barrel, it will continue to absorb additional heat energy from the heater bands. If material resides in the barrel for too long the energy absorbed may be sufficient to break the covalent bonds within the chains, thereby degrading the material. Additional heat due to frictional heating may be generated within the polymer during first stage injection and may cause the material to degrade as it is flowing into the part, even if had not yet begun to degrade within the barrel.
AIM’S DEGRADATION STUDY
Below is a summary of a study that was conducted at the American Injection Molding Institute related to the influence of certain process factors on material degradation. The variables studied in this experiment were: melt temperature, residence time in the barrel, and moisture content of the plastic material being processed. Two hygroscopic materials, polycarbonate (PC) and polyethylene terephthalate (PET) were studied. Each material was molded under low and high melt temperatures, short and long barrel residence times, and low and high moisture contents. Melt Flow Rate (MFR) is a good indicator of changes in the average molecular weight of a polymer so it was the measurable used to isolate the effects of these variables. The process variables used and resultant change in MFR for each material are outlined in the illustrations below.
The pre- and post-processing MFR of a material can be used as an indicator as to whether or not the material may have been damaged by the process. Additionally, it will also provide a method for expressing the magnitude of any potential damage. For unfilled plastic materials, a 40% increase in MFR is equivalent to slightly less than a 10% reduction in average MW. It is generally accepted that a 10% reduction in MW of a material post processing indicates that the material has not undergone any significant level of degradation or reduction in performance properties. Therefore, a 40% shift in MFR was used as a threshold for these evaluations. However, the reader should consider that the 40% change in Melt Flow Rate is a guideline and by no means a pass / fail number, but rather an indicator to changes in the MW.
This study found, as one might expect, that independently and exclusively increasing either the melt temperature, residence time, or moisture percentage would consequently increase the MFR of both materials that were molded. The study also showed how the increase in MFR would compound if multiple variables, such as melt temperature and residence time, are increased. Even more interestingly, we observed that as long as the PC was dried properly, it held up pretty well. At the elevated temperature and with a lengthy residence time the change in MFR was 37%. Although this was below the 40% threshold, the polymer is clearly been impacted and is approaching the limit. On the other hand, if the PC was wet it would degrade at lower melt temperatures if the residence time was long enough. At the higher melt temperature it rapidly degraded, and with the extended residence time it was significantly compromised resulting in a 614% increase in MFR.
The PET material simply could not be molded wet without significant degradation. Even at a low melt temperature and short residence time, it experienced a 529% increase in MFR. Additionally, the dry PET was not as tolerant as the PC to long residence times. Although it was under the 40% threshold at the higher temperature with a short residence time, it had exceeded the threshold with the extended residence time and the MFR increased by 63%.
The reader should use caution before applying these results to a given process as only one grade of each material was studied and because there are other processing variables that were not included within this study, such as: screw design, screw rpm, and flow channel geometry. What is clear is that increasing melt temperature, residence time, and moisture content increases the risk of material degradation.
Additionally, it should be understood by the reader that for other non-hydrolyzable materials, moisture content will not be the main component to degradation and it is expected that higher melt temperatures and longer residence time will have the greatest impact on potential degradation.
As in most things, it all comes down to the chemistry. The more we understand about the materials that we are molding with and how these materials interact with the part geometry, the mold, and the process, the more productive and innovative we will become.
In order to avoid the risk of degradation, manufacturing personnel should have some science-based foundational knowledge of: plastic materials, polymer rheology, and the intrinsic interactions between the material, mold design, and the injection molding process. Once these interactions are understood, it is relatively easy to implement the appropriate checks within the plant so that polymer degradation can be avoided.