⚡ electrical engineering

Filling behavior and process analysis of high-temperature-vulcanFilling behavior and process analysis of high-temperature-vulcanized silicone rubber insert injection molding for hybrid glass insulatorsized silicone rubber insert injection molding for hybrid glass insulators

This study demonstrates that optimizing mold temperature and employing graded injection strategies significantly improves filling stability and dimensional uniformity in the insert injection molding of high-temperature-vulcanized silicone rubber for hybrid glass insulators, as validated by coupled thermo-flow-curing simulations and experimental results.

Original authors: Riqing Chen, Xin Luo, Donglei Liu, Kailin Ren, Shaojian Zhang

Published 2026-09-03

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Riqing Chen, Xin Luo, Donglei Liu, Kailin Ren, Shaojian Zhang

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

High-voltage power lines stretch across vast landscapes, carrying electricity over hundreds of miles. To keep this energy safe and prevent it from jumping to the ground or causing outages, engineers rely on insulators. Traditional insulators made of glass or porcelain are incredibly strong, but they struggle when the air is thick with pollution, ice, or moisture, which can lead to dangerous electrical flashes. To solve this, modern engineers often use a hybrid design: a strong glass core for strength, wrapped in a protective shell of silicone rubber. This rubber shell repels water and resists dirt, keeping the power flowing even in harsh weather. However, manufacturing these hybrid parts is a delicate balancing act. The process involves injecting hot, liquid silicone rubber around the glass core inside a mold. If the rubber flows too slowly, it might not reach every corner before it hardens, leaving gaps. If it flows too fast or the mold is too hot, the rubber might start to cure, or harden, too quickly, clogging the path before the part is full. Getting this balance right is essential for creating reliable, safe power infrastructure.

Researchers at Nanchang University in China set out to understand exactly how this filling process works for a specific, large-scale component: a 160-kilonewton hybrid glass insulator. They wanted to see how the temperature of the mold and the speed of the injection affected the final product. To do this, they did not just guess; they first measured the physical properties of the raw silicone rubber. They tested how thick and sticky the material was at different temperatures and pressures, and they measured exactly how fast it began to harden when heated. Using these real-world measurements, they built a detailed computer simulation that could mimic the flow of the rubber, the transfer of heat, and the chemical hardening process all at once. They then tested their computer model by running actual experiments, injecting the rubber into molds at different temperatures to see if the real-world results matched their digital predictions.

The team discovered that intuition can be misleading when dealing with this material. A common assumption might be that heating the mold more would make the rubber flow better, since heat usually makes liquids thinner. However, their experiments showed the opposite. When they heated the mold to 140 degrees Celsius, the rubber flowed smoothly and filled the entire cavity completely. But when they increased the temperature to 160 or 180 degrees Celsius, the rubber failed to fill the mold, leaving empty spaces known as short shots. The reason lies in the race between flow and hardening. While the higher heat did make the rubber thinner initially, it also triggered the hardening process much faster. As the rubber sat in the hot mold, it began to cure and thicken again before it could reach the far corners of the cavity. The computer simulations, which tracked the movement of the rubber front, matched the physical experiments with high accuracy, predicting the exact locations where the rubber stopped flowing. This confirmed that for this specific material and shape, a cooler mold actually allows the rubber to travel further before it sets.

Beyond just temperature, the researchers also investigated how the speed of the injection affects the final shape of the rubber layer. They compared a standard method, where the rubber is pushed in at a constant speed, against a "graded" approach, where the injection speed starts fast and then slows down in stages. The constant-speed method created high pressure inside the mold and resulted in a very uneven rubber coating; on one side of the insulator, the rubber was nearly nine millimeters thick, while on the other side, it was only about three millimeters. This unevenness happens because the constant pressure forces the rubber to squeeze into gaps unevenly as it hardens. When the team switched to the graded injection, slowing the flow as the mold filled up, the pressure dropped significantly. The result was a much more uniform layer of rubber, with the thickness difference between the two sides shrinking by nearly 77 percent. The graded method also reduced the maximum pressure the machine needed to apply by about 8.5 percent and the force required to hold the mold closed by nearly 16 percent.

These findings offer a clear path forward for manufacturing these critical power components. The study demonstrates that simply turning up the heat is not the solution for filling complex molds; instead, controlling the temperature to slow down the hardening reaction is key. Furthermore, slowing the injection speed as the mold fills allows the rubber to settle more evenly, reducing stress on the machinery and ensuring a consistent protective layer. By combining precise material measurements with computer modeling and real-world testing, the researchers provided a reliable guide for engineers. They showed that by carefully managing the heat and the flow rate, it is possible to create hybrid insulators that are fully filled, structurally sound, and uniformly coated, ensuring that the power grid remains resilient against the elements.

Technical Summary: Filling Behavior and Process Analysis of HTV Silicone Rubber Insert Injection Molding for Hybrid Glass Insulators

Problem Statement
Hybrid glass insulators, which combine a glass or porcelain core with a high-temperature-vulcanized (HTV) silicone rubber housing, are critical for ultra-high-voltage transmission systems operating in severe environments. However, manufacturing these components via insert injection molding presents significant challenges due to the narrow, non-uniform cavities formed between the glass insert and the mold. The flow behavior of HTV silicone rubber is governed by a complex competition: thermal effects reduce viscosity to facilitate flow, while curing effects increase viscosity, potentially halting flow before the cavity is filled. This coupling often leads to defects such as short shots (incomplete filling), excessive pressure build-up, and non-uniform rubber layer thickness. Current literature lacks systematic investigations into how mold temperature and injection rate strategies influence these specific phenomena in large-scale, narrow-gap HTV overmolding applications.

Methodology
The study employed a multi-faceted approach combining material characterization, numerical simulation, and full-scale experimental validation:

Material Characterization: Rheological, Pressure–Volume–Temperature (PVT), and Differential Scanning Calorimetry (DSC) tests were conducted on DL-SRF-04 HTV silicone rubber to determine shear-thinning behavior, zero-shear viscosity dependencies on temperature and pressure, and curing kinetics.
Numerical Modeling: A coupled thermo–flow–curing simulation was developed using Autodesk Moldflow Insight. The model incorporated the Tait equation of state, the Cross model for base-flow viscosity, and the Kamal–Sourour model for curing kinetics. Crucially, it accounted for curing-induced viscosity growth, activated when the local temperature exceeded a pressure-dependent characteristic threshold ( $T_\eta$ ).
Experimental Validation: Full-scale insert injection molding experiments were performed on a 160 kN hybrid glass insulator using a 160 kN machine. The study compared simulation predictions against physical specimens under varying mold temperatures (140, 160, and 180 °C) and two injection strategies: constant-speed and graded injection (staged reduction in flow rate).

Key Contributions

Parameterization of a Coupled Model: The study successfully integrated experimentally derived rheological, PVT, and curing-kinetic parameters into a numerical framework capable of predicting the competing effects of thermal viscosity reduction and curing-induced viscosity growth.
Mechanism of Short-Shots: The research elucidates that in narrow-gap HTV molding, increasing mold temperature does not guarantee better filling. Instead, higher temperatures accelerate curing, which increases viscosity and flow resistance more rapidly than thermal thinning can compensate, leading to premature flow cessation.
Process Optimization via Graded Injection: The paper demonstrates that a staged reduction in injection rate (graded injection) during the late filling stage effectively mitigates pressure spikes and improves dimensional uniformity without compromising the total fill volume.

Results

Mold Temperature Effects: Complete cavity filling was achieved at 140 °C. Conversely, increasing the mold temperature to 160 °C and 180 °C resulted in short shots, with final filling fractions dropping to 85.0% and 47.6% (experimental), respectively. The simulation predicted these outcomes with relative errors below 8%, correctly identifying the unfilled regions in the distal sheds.
Filling Progression: Analysis of filling stages revealed that flow deceleration is most pronounced in the final 25% of the filling volume (75–100%) at 140 °C, and that the injected mass increment drops sharply in the final 20% of the injection time at 160 °C, indicating the onset of severe flow restriction.
Injection Strategy Impact: Compared to constant-speed injection, graded injection (reducing rates from 49 to 19 mL·s⁻¹ in four stages) yielded significant improvements:
- Maximum injection pressure reduced by 8.5% (from 7.123 to 6.518 MPa).
- Maximum required clamping pressure reduced by 15.9% (from 7.898 to 6.642 MPa).
- The mean thickness difference between measurement points P1 and P2 (representing layer non-uniformity) decreased by 76.9% (from 5.33 mm to 1.23 mm).

Significance and Claims
The authors claim that controlling mold temperature and implementing staged injection rate adjustments are critical for improving filling stability and dimensional uniformity in large-scale HTV silicone rubber insert molding. The study provides a validated numerical tool for assessing short-shot risks and offers practical guidance for selecting mold temperatures that balance flowability against curing speed. Furthermore, it establishes that graded injection is an effective strategy for reducing peak pressures and minimizing rubber layer thickness variations, thereby enhancing the process stability of hybrid glass insulator manufacturing. The authors modestly note that their specific switching points and rate profiles are for comparative evaluation rather than a universal optimum, suggesting future work should focus on establishing general switching criteria based on cavity pressure responses.

More like this