In high-volume footwear midsole molding with styrene-butadiene-styrene grade SBS 1301L and general-purpose polystyrene compounds, the gate freeze-off boundary determines whether the packing phase remains hydraulically connected to the cavity. The boundary is defined by the elapsed time after injection velocity ends at which the gate centerline temperature falls below the compound’s no-flow temperature, or by the point at which solidified skin layers at the gate meet. Commercial SBS 1301L is described in technical literature as a linear SBS with a bound styrene content of approximately 30 wt%; published data for this specific grade configuration is limited, and supplier certificates of analysis must be used for lot-to-lot variation. Because the butadiene midblock does not crystallize at midsole processing temperatures, the freeze-off boundary is governed by styrene endblock vitrification, blend free-volume reduction from added GPPS, and the shear history imposed by the injection screw. The gate freeze boundary is not a single temperature; it is a time-temperature-pressure locus that moves with gate depth, mold temperature, polystyrene content, and packing pressure. Cavity pressure transducers installed behind the sub-gate record a characteristic slope change in the pressure decay curve when the gate freezes. After this point, further hold time only cools the runner and increases cycle time without adding melt to the cavity. In compounds with SBS 1301L/GPPS ratios from 100/0 to 45/55, the no-flow temperature typically lies between 140°C and 175°C depending on styrene content, oil loading, and filler level. Applicable test standards for rheological and thermal characterization include ASTM D3835-16 for capillary viscosity, ISO 1133-1:2022 for melt mass-flow rate, ISO 306:2022 for Vicat softening temperature, and ASTM D3418-21 for glass transition by differential scanning calorimetry.

Why does the no-flow temperature rise when GPPS replaces more than 30 phr of SBS 1301L?

The shift in the gate freeze-off boundary with increasing GPPS content is primarily a consequence of the blend’s continuous styrenic phase becoming richer in high-Tg polystyrene. An SBS triblock of the 1301L type contains styrene endblocks that associate into nanoscale domains; in the melt state these domains are partially disrupted but still impose slow relaxation. Addition of GPPS at levels above 30 phr increases the volume fraction of the hard phase, raises the blend’s zero-shear viscosity at low shear rates, and moves the no-flow temperature toward the Vicat softening temperature of the added polystyrene. In capillary rheometry per ASTM D3835-16 at 190°C, a 100/0 SBS/oil compound may show a shear-thinning power-law index near 0.30 at apparent shear rates between 100 s⁻¹ and 1000 s⁻¹, while a 60/40 SBS/GPPS compound may display a power-law index closer to 0.22, indicating more non-Newtonian flow. The no-flow temperature measured by dynamic oscillatory rheology can be approximated by the temperature at which complex viscosity reaches 10⁵ Pa·s at 1 rad/s; for compounds with 40 wt% GPPS this temperature is typically 10°C to 18°C higher than for unfilled SBS. The practical consequence is that gate freeze time shortens by roughly 0.4 s to 0.7 s for every 10 phr increment in GPPS content when gate depth is held constant at 1.5 mm and mold temperature is 30°C. A hot runner with temperature control accuracy of ±2°C reduces batch-to-batch scatter in the freeze boundary but does not eliminate the formulation-driven shift. Published data for the specific SBS 1301L/GPPS ratio are limited; production trials should therefore verify no-flow temperature by rheometer and cavity pressure decay rather than relying solely on supplier melt index values.

On production-scale reciprocating-screw injection molding machines with clamp forces from 3500 kN to 6000 kN and screw diameters between 55 mm and 80 mm, gate freeze-off failures in SBS 1301L/PS midsoles most often appear as underpacking, sink marks over thicker midsole lugs, and weight variation exceeding ±1.2% across cavities. In multi-cavity molds with geometrically balanced runners, the cavities farthest from the sprue can freeze earlier because the melt spends more time in the runner and arrives cooler. Set-up personnel typically set packing pressure at 35% to 55% of peak injection pressure and incrementally increase holding time while recording cavity pressure with piezoelectric transducers. When holding time is extended beyond the gate freeze-off boundary, the integral of cavity pressure plateaus and no further increase in part weight is observed. The gate freeze-off boundary therefore functions as an operational limit for the packing phase rather than a defect by itself. Compounds containing SBS 1301L and GPPS are usually pre-compounded on twin-screw extruders with L/D ratios of 40:1 to 52:1 and temperature profiles from 120°C at the feed throat to 180°C at the die, depending on the oil absorption capacity of the batch. If the compound is not sufficiently homogenized, localized PS-rich domains freeze prematurely in the gate and produce an irregular boundary that shifts shot-to-shot by more than 0.5 s. That variance is often observed as inconsistent midsole thickness across a production shift. Relevant test methods for incoming compound quality include melt mass-flow rate per ISO 1133-1:2022, procedure A at 190°C/5 kg, and Shore A hardness per ISO 7619-1:2010 after 3 s. Compression set tested under ASTM D395-18, method B at 23°C for 22 h, can indicate whether the hard phase network is sufficient to maintain dimension after demolding.

Mold Temperature, Gate Depth, and Cavity Pressure Decay in Midsoles

For SBS 1301L/PS midsole compounds, the gate freeze-off time is a strong function of gate depth and mold temperature because the heat-removal path at the gate is short. The classical conduction model for an amorphous polymer predicts that freeze time scales with the square of gate thickness divided by effective thermal diffusivity. For an unfilled 60/40 SBS/GPPS compound with a thermal diffusivity of approximately 8.0 × 10⁻⁸ m²/s, increasing sub-gate depth from 1.0 mm to 2.0 mm can extend the gate freeze boundary by a factor of about four, assuming identical melt and mold temperatures. In practice, midsole molds use gate depths between 0.8 mm and 3.0 mm depending on local part thickness and targeted cycle time. A gate depth below 0.8 mm shortens the freeze boundary to less than 1.0 s in a 25°C mold, which is usually insufficient for packing a thick midsole path. A gate depth above 3.0 mm delays freeze-off but may require a secondary degating operation and can leave a visible gate tear on the midsole surface. Mold temperature is the second independent variable: raising the mold from 20°C to 50°C extends the gate freeze-off boundary by roughly 0.3 s to 0.6 s per 10°C for a 2.0 mm gate. However, mold temperatures above 45°C for unfilled SBS/GPPS can extend the cooling time required to reach ejection modulus and may increase post-demolding shrinkage. The cavity pressure decay method is the most robust way to locate the boundary on a specific tool. A pressure transducer located behind the gate records a steep linear decay during packing; when the gate freezes, the slope of the pressure-time curve changes and pressure decays more slowly as the cavity cools at constant volume. The freeze time is conventionally identified as the time from injection start to that slope inflection. This measurement should be repeated over at least 30 cycles to account for check-ring leakage, temperature drift, and shot-to-shot viscosity variation.

Rheological characterization of SBS 1301L/PS midsole compounds under production-relevant shear rates is necessary because the gate freeze boundary is influenced by viscous heating during filling. During the injection phase, average shear rate in a 2.0 mm gate can reach 10⁴ s⁻¹ to 10⁵ s⁻¹, causing local temperature rises of 10°C to 20°C above set melt temperature depending on injection speed. This viscous heating temporarily shifts the freeze boundary forward in time, but the effect decays rapidly once the velocity profile ends. Capillary rheometry per ASTM D3835-16 at apparent shear rates from 100 s⁻¹ to 5000 s⁻¹ is used to generate viscosity curves that can be fitted to a power-law or Cross model for process simulation. Injection-molding simulation software based on these data calculates the position of the frozen layer and gate closure time, but the simulation output requires calibration against cavity pressure decay data because grade-specific SBS 1301L data are often incomplete. Dynamic mechanical analysis per ISO 6721-7:2019 or ASTM D5279-21 can identify the glass transition of the styrenic phase, which is frequently reported in the range 82°C to 100°C for SBS/GPPS blends containing 30 wt% to 50 wt% polystyrene. Below this Tg region, the styrene endblocks no longer have sufficient mobility to permit melt flow, and the gate behaves as a mechanically frozen solid. The exact no-flow temperature depends on the time scale of the measurement; a slower cooling rate gives a lower apparent freeze temperature because the blend has more time to relax. This time-scale dependence is particularly strong in SBS/PS compounds because the styrene domains are not fully dissolved in the melt at processing temperatures, and their re-association during cooling is rate-dependent. The following table provides representative values for the general class of linear SBS with nominal 30 wt% styrene/GPPS midsole compounds; grade-specific data must be verified with supplier certificates.

When the hot runner temperature falls below 165°C, the freeze boundary migrates backward into the runner

The location of the gate freeze-off boundary is not limited to the gate itself. In hot-runner-fed SBS 1301L/PS midsoles, a drop in runner temperature to below 165°C creates a zone of high viscosity behind the gate that can freeze during the packing phase even if the gate itself remains open. This backward migration is widely reported in amorphous thermoplastic elastomer compounds with high styrene content, and it produces an apparent short shot or sink despite a packing pressure that is nominally sufficient. The gate freeze boundary then lies inside the runner, and the packing phase cannot transmit pressure to the cavity because the runner has become the frozen plug. Hot runner systems with independently controlled tips are preferred because tip temperature can be held 8°C to 12°C above no-flow temperature while the manifold is kept near 170°C. If tip temperature exceeds 190°C, however, thermal degradation of the butadiene midblock can generate gel particles and black specks in the gate region, which further destabilizes the freeze boundary. The practical control band for hot runner tips in a 60/40 SBS 1301L/GPPS midsole compound is therefore typically 165°C to 190°C, with a recommended set point of 175°C and a control accuracy of ±2°C. Operators should log hot runner tip temperature, manifold temperature, and cavity pressure freeze time together, because a drop of only 5°C in the tip zone can shorten the pack-hold window by 0.3 s to 0.5 s. If the mold is re-started after a weekend shutdown, the first 15 to 20 shots often show a shifted freeze boundary until the hot runner reaches thermal equilibrium. Published data for this specific configuration is limited, but the relationship between runner temperature and gate freeze-off is consistently observed in commercial amorphous TPE processing.

Pressure-specific volume-temperature behavior of SBS/PS compounds introduces an additional boundary effect during the packing phase. The compressibility of a 60/40 SBS 1301L/GPPS compound at 180°C and 80 MPa is typically higher than that of a semicrystalline polyolefin, because the butadiene-rich matrix has high free volume and the styrenic domains are compressible. During packing, melt is forced into the cavity as long as the gate remains open; once the gate freezes, cavity pressure decays along a constant-volume or constant-mass path. If the freezing boundary is reached too early, the part cannot be packed to the required density and the midsole exhibits microvoids in the central layer, reduced dimensional stability after conditioning, and lower abrasion resistance as tested by ISO 4649:2021, method A. In multi-cavity midsole tools, the gate freeze boundary for each cavity is not identical unless the runner and cooling layout are balanced within 1°C. Cavity-to-cavity differences in freeze time as small as 0.2 s can produce weight variation greater than 1.0% when packing time is set just below the boundary. Process capability studies should therefore measure freeze time and part weight across all cavities, not only at the sprue. The relationship between freeze time and part weight can be fitted with a linear regression; the slope of that regression changes sharply once the freeze boundary is passed. That change point is a practical index for setting minimum holding time. Adding mineral filler, such as calcium carbonate at 5 wt% to 15 wt%, increases thermal conductivity and shortens freeze time by as much as 0.5 s for a 2.0 mm gate, while simultaneously raising compound viscosity and requiring higher injection pressure. The filler effect must be separated from the styrene content effect because both increase apparent no-flow temperature.

The gate freeze boundary is not a single temperature in amorphous SBS/PS compounds

In amorphous SBS 1301L/PS midsole formulations, the boundary that separates an open gate from a frozen gate is a cooling-history-dependent locus rather than a thermodynamic freezing point. A differential scanning calorimetry scan per ASTM D3418-21 may show a styrenic glass transition between 82°C and 100°C, but gate freeze-off often occurs at a higher temperature because flow under pressure ceases before average gate temperature reaches Tg. The no-flow temperature determined by extrapolating dynamic viscosity to 10⁵ Pa·s can be 15°C to 25°C above the DSC Tg, depending on the molecular weight of the polystyrene fraction and the butadiene domain structure. If the compound has been over-sheared in the barrel, the styrene domain network may be partially homogenized, lowering the apparent no-flow temperature and delaying the freeze boundary; this effect is not permanent because the domains re-form during cooling. Conversely, if the compound is processed at too low a melt temperature, the styrene domains remain relatively intact and the melt behaves as a phase-separated system with a higher no-flow temperature and a shorter freeze time. The boundary therefore depends on both set melt temperature and the entire barrel residence-time history. This explains why changing melt temperature from 165°C to 180°C can sometimes extend gate freeze time by more than 0.6 s even though mold temperature is unchanged. It also explains why increasing screw speed without changing barrel temperature can shift the boundary by altering viscous heating and domain morphology. Process engineers should record screw recovery time, melt cushion, and back pressure because these variables influence the thermal and shear history and therefore the gate freeze boundary.

Operational boundaries for SBS 1301L/PS midsole compounds include moisture control and additive compatibility. Although SBS/PS blends are less hygroscopic than polyamide or thermoplastic polyurethane, condensation on cold granules at relative humidity above 60% can introduce enough surface water to produce splay and gate-area porosity. Pre-drying at 70°C to 80°C for 2 h to 4 h in a desiccant dryer is recommended when bulk storage has been exposed to humid air. The use of amine-based stabilizers should be avoided in compounds containing GPPS because amine chemistry can interact with residual peroxide or with unsaturation in the butadiene midblock and lead to premature gel formation in the hot runner. If a color concentrate with a low-viscosity polyethylene carrier resin is added, the freeze boundary can shift backward by up to 0.3 s because the carrier reduces local viscosity and alters thermal conductivity at the gate. That shift is often misdiagnosed as a gate design problem when the actual cause is additive interference. Mold release agents based on silicone can contaminate the gate region and change the heat-transfer coefficient at the steel surface; if release spray is used, the first 5 to 10 shots after application often show an earlier freeze time. These boundaries are reliability limitations rather than product defects. Quality control should therefore monitor gate freeze time as a process parameter alongside melt temperature and holding pressure, using the same injection molding machine and pre-drying procedure to avoid confounding variables.