Hello and welcome to a new Rule of Thumb post on plastics processing. In my previous post we discussed how to locate the maximum shear rates by using injection moulding fill simulations. Now we explore what shear rate limits we need to consider to not harm the processed polymer.
Rheology of polymers
Plastics exhibit non-Newtonian fluid behavior, where viscosity is dependent on the applied shear rate. In certain polymers, shear rate exerts a more significant influence on viscosity than temperature.
Under high stress conditions, such as during processing, polymer molecules align, leading to a substantial reduction and stabilization of the resin's viscosity. This phenomenon is known as shear thinning.
Injection moulding and shear rate / stress limit of polymer melts
In injection moulding, the injection rate or fill time directly correlates with the shear rate experienced by the plastic material. Fill time is a critical process parameter that affects shear heating and shear thinning.
Variations in fill time can alter the viscosity, pressure, and temperature of the polymer within the mould cavity, ultimately impacting the quality of the final part. Maintaining a consistent, optimized fill time is therefore crucial for process stability across different machines.
Excessive shear rates can induce polymer degradation, resulting in a decline in both the aesthetic and mechanical properties of the moulded component.
The shear rate within specific mould geometries, such as sprues, runners, and gates with a round cross-section, can be calculated using the formula:
γ˙=4Q/πr^3, where γ˙ represents the shear rate (1/s), Q (mm^3/s) is the volumetric flow rate, and r (mm) is the radius of the channel.
Shear stress and shear limit control table
Calculated shear rate values can be compared against established material-specific shear rate limitations to identify potential processing issues related to excessive shear. This data facilitates the mathematical determination of optimal flow rates and mould design considerations. Your calculated shear rate should not exceed the shear rate limit for the material. Figure 1 shows the shear stress and shear rates limits of different plastics, based on empirical experiments and literature.
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| Figure 1: Shear stress and shear rate control table. |
Conclusion
In plastics processing, maximum shear rates can reach over 10,000 s⁻¹ in injection moulding and 1000 s⁻¹ in extrusion, with even higher rates (exceeding 1,000,000 s⁻¹) occurring in specific applications like wire coating. Calculating the shear rates of the material during processing and checking if they are below the shear rate limit of the material will lower the risk of polymer damage. Furthermore risk of plastic part failure is reduced since the part will have the desired properties.
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[1] https://s3.amazonaws.com/entecpolymers.com/v3/uploads/pdfs/Rheology-vs-Shear-Rate-RGB.pdf
[2] https://www.findoutaboutplastics.com/2015/04/injection-molding-filling-simulation-my.html
[3] https://www.findoutaboutplastics.com/2022/05/6-benefits-of-injection-moulding.html
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