How Does the Wrong MFI Affect TPE Materials in Molding?

The melt flow index (MFI) is a common indicator used to assess the flowability of TPE materials. It reflects how fast the material flows through a standard die under specific temperature and pressure. A high or low MFI does not directly indicate the quality of the material. Rather, it determines which molding methods and product structures the material is suitable for. If the compatibility between the MFI and the molding process is overlooked during material selection, avoidable problems may occur during injection molding.

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What happens when the MFI is too high?

TPE materials with a high MFI have good melt flowability and fast mold filling speeds, allowing for a slightly reduced injection pressure. This suits thin-walled products, long-flow structures, and multi-cavity molds. In applications such as overmolding for consumer electronics housings and surfaces with fine textures, high-flow materials can better reproduce mold details.

However, the risk of flash also increases. When the mold parting lines are not precisely aligned, the melt can seep into the gaps and form flash. Stability during the holding pressure phase decreases, and post-cooling shrinkage makes shrinkage compensation difficult, potentially resulting in sink marks or dimensional variations. High flowability is sometimes accompanied by lower molecular weight. As a result, tensile strength and tear resistance may be lower than grades with lower MFI. Careful evaluation is required for load-bearing components.

What happens when the MFI is low?

TPE materials with a low MFI have higher melt viscosity, requiring higher material temperature and injection pressure during injection molding. When product wall thickness is thin or the runner is long, issues such as incomplete filling and prominent weld lines are more likely to occur. Mold venting also becomes more critical.

However, low-MFI materials typically have higher molecular weights and better melt strength, making them less prone to warping during cooling and offering relatively better dimensional stability. In extrusion molding, the tendency of low-MFI TPE materials to swell and sag at the die is more controllable, making them suitable for tubing, wire jackets, and thick-walled profiles. For structural components subjected to mechanical stress, low-MFI grades often provide more reliable mechanical properties.

How should the melt flow index be matched in overmolding processes?

In secondary injection overmolding applications, the melt flow index of the TPE material affects the adhesion to the rigid polymer substrate. If the melt flow index is too low, the melt will not spread sufficiently over the surface of the rigid polymer, leading to false overmolding or localized delamination. If the melt flow index is too high, differences in cooling shrinkage may increase interfacial stress, which is also detrimental to adhesion stability. The melt flow index of the overmolding material should be matched with the substrate type, overmolding thickness, and mold design.

How do you determine the appropriate melt flow index range?

When selecting a grade, work backward from the product structure and molding method. For thin-walled, multi-cavity, or finely textured injection-molded parts, prioritize medium-to-high melt flow indices. For thick-walled structural parts, load-bearing components, or extrusion-grade products, medium-to-low MFI grades are more suitable. During the prototyping phase, adjust the injection speed, material temperature, and holding pressure to observe the filling behavior and surface quality, then compare the results with the MFI data provided by the supplier.

Summary

MFI is a reference for flowability. Actual molding performance is also influenced by temperature sensitivity and mold design, so the results of trial molding should be the final determinant. There is no universal answer regarding whether a high or low MFI is better for TPE materials. The key lies in matching the material to the specific product, mold, and process conditions. When selecting materials, consider the MFI within the context of the entire molding process rather than pursuing a high or low value in isolation. This can reduce uncertainties during mass production.

We hope this post from the Zhongsu Wang editorial team has been helpful to you.