Plastic Material: A Molder’s Perspective

As processors, we are required to mold parts using a large assortment of engineered resins. Each has its own unique characteristics. Polypropylene does not perform the same as Nylon. Amorphous materials do not possess the same processing characteristics as a crystalline. The material topic is broad, and experts such as Mike Sepe have made careers out of their understanding of this complex subject matter.

There are many common factors that if known and practiced will make a molder’s job easier. Although tensile strength and vapor density are important SDS data to the engineer choosing a material for a specific application, the process engineer does not need this information to develop a process. The SDS is an important part of developing a process, and it is important to understand what information is needed for process development. This article will outline basic material properties from a processor’s perspective. It will also explain some of the more complex terminology in ways that the average individual can understand.

VISCOSITY: Viscosity is best described as the thickness of a material in its melted flowing state. This is often found on the SDS as “Specific Gravity”. A good mental comparison for this would be holding a board at a slant and then pouring water and corn syrup down the board. The water flows easily.  The water is thin, with a low viscosity. The corn syrup flows slower. Corn syrup is a thicker liquid at a higher viscosity in its liquid form.

In the world of engineered resins, specific gravity numbers have a direct relationship to the viscosity of styrene. Styrene is considered the middle point (mean) of the material viscosity scale. If a material’s specific gravity number is a positive number, it would be considered higher viscosity. If a material’s specific gravity number is negative, it would be a lower viscosity material. The higher or lower the SG number gets, the higher or lower the viscosity is using styrene as a mean.

MELT TEMPERATURE: Melt temperature is often overlooked as a key SDS variable! One of the keys to establishing primary setpoints for a process is the temperature of the melt coming from the barrel, with the barrel heat soaked to a running state (minimum 30 shots). At the beginning of process development, keep the melt temperature in the lower range of the melt molding window. This will help to assure barrel temperature is less of a factor when establishing cycle time.

BARREL TEMPERATURES: Safety Data Sheets often provide barrel temperature setpoints, and these serve well as temperature starting points. It is important to note that these setpoints may require adjustment to achieve desired melt temperature.

It is important to verify that transition zone actual temperatures do not override setpoints, a sign that barrel shear is greater than what the setpoint is calling for. Also verify that screw rotate time is consistent. Lower feed zone temperatures can sometimes cause poor feeding in the feed throat.

DRYING: Material manufacturers quite often provide material drying information with their materials. This information is paramount to processing requirements. These recommendations should be followed explicitly.

It is important to note that dryers should be evaluated for proper size to match the required drying time throughput. Keep in mind, Materials in the center of the dryer will flow faster than material against the dryer walls. Selecting a dryer with a throughput too close to the desired drying time can lead to processing inefficiencies.

Also, there is always a potential that materials can be overdried. Some materials can tolerate long events in the dryer, but remember that material blends consist of many different bases, additives, pigments, etc. Best practice is to bank dryer temperatures any time that material will be siting idle in the dryer for 8 hours or more.

BACK PRESSURE: Back pressure isn’t always included on the SDS, but it is still a critical parameter in every processing scope. Best practice is to use as little back pressure as possible to reduce the break down of molecular chains. This can not only lead to processing inefficiencies, but in the case of reinforced plastics (glass, metal strand) could result in poor physical/ structural qualities.

INJECTION SPEED: Injection speeds are always a primary consideration during process development. Some materials perform better at faster speeds, some are best suited to slower fill speeds. Best practice is to start injection speeds in the middle of the processing window, and then adjust speeds based on molding variables as they present themselves.

It is also important to note that there is a direct relationship between injection speeds, viscosity and temperature:

Higher Temperature=Lower Viscosity=Faster Fill speeds= Lower Fill Time

Lower Temperature=Higher Viscosity=Slower Fill Speeds=Higher Fill Time

Understanding primary material properties, and what information the material manufacturer provides is key to process development. Every material has its own unique set of physical attributes, and we must use the appropriate information and procedures to assure that our processes are sound and repeatable. Process development is a key component in every company’s success or failure to be profitable.

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