RHEOLOGY VS SHEAR RATEAND IDEAL PROCESSING PARAMETERS RELATING TO SHEAR

E N T E C P O L Y M E R S | 1 9 0 0 S u m m i t T o w e r B l v d . , S u i t e 9 0 0 | O r l a n d o , F L 3 2 8 1 0 | P : 8 3 3 . 6 0 9 . 5 7 0 3 | E n t e c P o l y m e r s . c o m

RHEOLOGY VS SHEAR RATEAND IDEAL PROCESSING PARAMETERS RELATING TO SHEAR

10/25/2019The information presented in this document was assembled from literature of the resin product producer(s). The information is believed to be accurate however Entec Polymers (“Entec”) makes no representations as to its accuracy and assumes no obligation or liability for the information, including without limitation its content, any advice giv-en, or the results obtained. ENTEC DISCLAIMS ALL WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING FITNESS FOR A PARTICULAR PURPOSE. The customer shall use its own independent skill and expertise in the evaluation of the resin. product to determine suitability for a particular application and accepts the results at its sole risk.

Plastics are non-Newtonian, meaning they change viscosity with changes in shear rate. For some plastics, viscosity is influenced more by shear rate than by temperature. When a polymer is forced to flow quickly by large amounts of stress (pressure), the molecules are forced to align themselves in a more parallel fashion which allows them to flow more easily. This alignment results in a significant drop in the viscosity of the resin. As the viscosity becomes lower, it also becomes more stable.

In injection molding, shear rate translates to injection rate or fill time which is one of the most important process parameter to establish for an injection molding process. Fill time affects how much shear heating and shear thinning the plastic experiences which in turn affects the materials viscosity, the pressure and temperature of the plastic inside the cavities, and overall part quality. Any change in fill time may adversely affect the molded part. So, once the ideal fill time has been established, that fill time should stay with the mold forever regardless of which machine it may be run on.

Using an injection molding machine, you can calculate this effective critical shear rate by generating a viscosity curve for a particular process. This critical shear rate will provide the processor the minimum speed allowable to produce consistent parts. We first start by setting the hold phase to zero, injection pressure to maximum available and the cooling time to safe value. Filling the part 90% with a cushion, we then run a series of parts at different fill speeds without changing any other processing parameters. Fill speeds should range from fast to slow. You can then use the injection pressure at transfer and fill speed to develop the viscosity curve. Figure A below shows a typical viscosity curve generated at the molding machine.

0 10 20 30 40 50 60 70 80 90

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Injection Speed

App

aren

t Visc

osity

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sec)

The critical shear rate is defined as the point where the slope changes from shallow to steep. For process stability, the plastic likes the flat section of the curve. Looking at the curve, note that the viscosity stays fairly constant after about 60% of the machines injection speed. Therefore, setting the injection speed to 70% would ensure that the filling stage of the process will stay consistent. Any small natural variations (i.e. lot to lot MFI values) will not cause large changes in viscosity which could result in shot to shot variation.

Although being on the flat section of the curve is ideal, it is not always possible and compromises may need to be made based upon part geometry, mold, machine and/or thermal limitations of certain materials (i.e. PVC). Excessive shear rates can lead to degradation of the polymer resulting in aesthetic and mechanical property deterioration. If you suspect a process may be introducing excessive shear into the polymer, the following equations can be used to calculate shear rate through specific geometry or sections of the mold (i.e. sprue, runner, gate).

Figure A

E N T E C P O L Y M E R S | 1 9 0 0 S u m m i t T o w e r B l v d . , S u i t e 9 0 0 | O r l a n d o , F L 3 2 8 1 0 | P : 8 3 3 . 6 0 9 . 5 7 0 3 | E n t e c P o l y m e r s . c o m

RHEOLOGY VS SHEAR RATEAND IDEAL PROCESSING PARAMETERS RELATING TO SHEAR

10/25/2019The information presented in this document was assembled from literature of the resin product producer(s). The information is believed to be accurate however Entec Polymers (“Entec”) makes no representations as to its accuracy and assumes no obligation or liability for the information, including without limitation its content, any advice giv-en, or the results obtained. ENTEC DISCLAIMS ALL WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING FITNESS FOR A PARTICULAR PURPOSE. The customer shall use its own independent skill and expertise in the evaluation of the resin. product to determine suitability for a particular application and accepts the results at its sole risk.

The resultant value may be then cross referenced to published shear rate values to determine if the current process exceeds shear rate limitations of the material. This data can then be used to mathematically determine ideal flow rate and/or mold geometry. See Table Below.

Shear Rate through a round channel: ẙ = 4Q or ẙ = 32Q

πr3 πd3

Shear Rate through a rectangular flow channel: ẙ = 6Q wh2

Where “ẙ” is shear rate, “Q” is flow rate, “r” is radius of a round channel, “d” is the diameter of a round channel, “w” is width of a recatangular channel, and “h” is the heigh (or thickness) of the rectangular channel.

MATERIAL TYPE DESCRIPTION SHEAR STRESS LIMIT SHEAR RATE LIMITpsi MPa 1/S

ABS Acrulonitrile Butadiene Styrene 43.5 0.30 50,000GPPS Polystyrene (General Purpose) 36.3 0.25 40,000HIPS High Impact Polystyrene 43.5 0.30 40,000LDPE Low Density Polyehylene 14.5 0.10 40,000HDPE High Density Polyehylene 29.0 0.20 40,000PA6 Nylon 6 72.5 0.50 60,000

PA66 Nylon 66 72.5 0.50 60,000PBT Polybutylene Terephthalate 58.0 0.40 50,000PC Polycarbonate 72.5 0.50 40,000PET Polyethylene Terephthalate 72.5 0.50 Unknown

PMMA Polymethyl Methacrylate (Acrylic) 58.0 0.40 40,000PP Polypropylene 36.3 0.25 10,000

PVC Flexible Polyvinyl Chloride 21.8 0.15 20,000RPVC Rigid Polyvinyl Chloride 29.0 0.20 20,000SAN Styrene Acrylonitrile 43.5 0.30 40,000PSU Polysulphone 72.5 0.50 50,000