Analysis of the Influence of Hot-Press Temperature on Product Quality in Brake Pad Production

Introduction

In automotive brake systems, brake pads serve as core safety-related consumables, whose performance directly determines vehicle braking efficiency and driving safety. As the automotive industry raises increasingly stringent requirements for environmental protection, noise reduction and braking stability, the manufacturing processes for brake pads are also confronted with greater challenges. In the production workflow of brake pads, hot-press forming is a critical procedure that defines the final physical and tribological properties of finished products. Precise control of hot press temperature not only governs the curing degree of resin inside friction materials, but also exerts profound impacts on the bonding strength of fiber networks, density distribution of workpieces and removal efficiency of volatile substances. Any temperature deviation may trigger structural defects or performance degradation of brake pads. Therefore, in-depth analysis of the mechanism by which hot-press temperature influences brake pad quality carries important engineering guiding significance for improving manufacturing process levels and guaranteeing product consistency.

Overview of Hot Pressing Process

Hot-press forming is the core procedure that converts mixed friction materials into finished brake pads with specified shapes, dimensions and properties. During this process, the hot press applies high temperature and high pressure to induce complex physical and chemical changes within friction materials.

First of all, hot pressing acts as the major driving force for resin curing. Phenolic resin and other binders in brake pad formulations undergo cross-linking reactions at specific temperatures, transforming from linear structures into three-dimensional network structures, so as to endow brake pads with sufficient mechanical strength. Secondly, hot pressing facilitates tight bonding between fibers and friction powder materials, densifying loose raw materials to meet design-specified density standards. In addition, appropriate temperature enables smooth discharge of moisture and low-molecular volatiles inside materials, avoiding internal pores or delamination in finished products. It can be said that the hot-pressing process functions as a “smelting furnace” converting raw materials into high-performance friction materials, and temperature stands as the most pivotal parameter governing this transformation.

Impacts of Excessively High Hot Press Temperature on Brake Pads

Although high temperature accelerates resin curing, excessively high hot-press temperature can cause irreversible damage to brake pads. When mold temperature exceeds the upper limit specified in the formulation design, the following severe quality defects commonly occur:

1. Excessive carbonization and burning-loss of resin: Under excessive temperature, resin undergoes thermal degradation and even carbonization, which deprives binders of their original bonding performance. This not only reduces the shear strength and impact strength of brake pads, but also causes scorching on friction-material surfaces, seriously impairing appearance and functional performance.
2. Sheet brittleness and crack generation: Thermal degradation and excessive cross-linking of the resin matrix trigger internal stress concentration within brake pads, making workpieces brittle. Microcracks or even macroscopic fractures readily emerge during demolding, subsequent processing or in-service operation.
3. Burning-loss of friction materials and performance deterioration: Some organic friction powders and rubber modifiers are temperature-sensitive. Overheating causes premature decomposition or volatilization of these components, altering the originally designed formulation ratio. As a result, unstable friction coefficients and even thermal fade may occur.
4. Oxidation of metal components and surface blistering: Excessively high temperature accelerates oxidation of brake pad backplates, undermining adhesion for subsequent spraying operations. Meanwhile, excessive temperature leads to rapid skin-formation on material surface layers, while volatiles generated inside cannot escape in time, consequently forming bubbles inside or on the surface of brake pads and giving rise to serious structural defects.

Impacts of Insufficient Hot Press Temperature on Brake Pads

Contrary to excessive temperature, inadequate hot-press temperature also prevents brake pads from achieving target performance, whose core root cause lies in incomplete chemical reactions:

1. Incomplete resin curing: When temperature falls below the activation-energy threshold for resin curing reactions, cross-linking reactions cannot proceed sufficiently. Consequently, abundant unreacted free resin remains inside brake pads, resulting in low hardness and poor wear resistance of finished products.
2. Insufficient bonding force and particle shedding: Since binders fail to form dense network structures, bonding forces between fibers and powder materials become weak. Block-falling or particle shedding of friction blocks may easily take place during bench tests or vehicle service, posing substantial safety hazards.
3. Loose sheet structure and uneven density: At low temperatures, materials exhibit poor plastic flow capacity and fail to reach target density under identical pressure. High porosity and loose internal structure not only compromise mechanical strength, but also degrade thermal conductivity and aggravate heat accumulation during braking.
4. Excessive moisture content and poor heat resistance: Brake pads with incomplete curing usually retain high residual volatiles and moisture content. Under high-temperature braking conditions in practical service, rapid vaporization of such moisture brings about drastic fluctuation of friction coefficients (namely water fade), and the overall thermal-fatigue resistance of products drops significantly.

Exploration of Optimal Temperature Ranges for Different Formulations

Brake pads adopt a wide variety of formulations, and different base materials show markedly divergent sensitivity to hot press temperature. Scientific setting of temperature intervals serves as a prerequisite for guaranteeing product quality:

Semimetallic formulations: Containing large quantities of steel wool or copper fibers with fast thermal conductivity, and generally adopting resin systems with favorable heat resistance. Their hot-press temperature is normally controlled within 180°C~220°C to ensure adequate resin curing and favorable bonding between metallic fibers and the matrix.

Ceramic formulations: Ceramic fibers and long fibers are comparatively temperature-sensitive, and modified resins are widely adopted in such formulations. To avoid fiber damage and resin degradation, hot-press temperature is kept relatively low, generally ranging from 160°C to 200°C.

Organic / NAO (Non-Asbestos Organic) formulations: Such formulations incorporate substantial organic fibers and rubber particles with low upper temperature limits. Hot-press temperature shall be strictly controlled, usually within 150°C~180°C, to prevent scorching of rubber components and carbonization of organic fibers.

Comparison of Impacts Caused by Excessively High and Insufficient Temperature on Brake Pad Quality

To facilitate intuitive understanding of consequences induced by temperature deviation, major impacts are compared as follows:

Suggestions for Temperature Uniformity and Collaborative Control

In practical production, merely setting a standard temperature value is insufficient. Temperature uniformity as well as collaborative control with other process parameters are equally critical.

Importance of temperature uniformity: Temperature differences across different mold zones constitute a major contributor to poor product consistency. When temperature discrepancy between mold center and edges, or between upper and lower molds exceeds ±5°C, uneven curing occurs within one batch or even within a single brake pad. Manufacturers shall regularly verify flatness of heating plates and accuracy of thermocouples, and optimize design of mold heating flow channels to guarantee uniform heat conduction.

Synergy among temperature, pressure and time: Hot pressing represents a multi-parameter coupling process.

1. Heating-up rate: Over-rapid heating-up gives rise to excessive internal-external temperature gradients within materials and generates thermal stress; over-slow heating-up reduces production efficiency. Reasonable heating-up curves shall be formulated according to material thickness.
2. Holding time: Temperature and time are functionally equivalent. When temperature is slightly low, holding time can be appropriately prolonged to compensate for curing degree, yet risks of poor volatile removal shall be watched out for; when temperature is relatively high, holding time must be shortened to avoid over-curing.
3. Cooling rate: The cooling phase after hot-press completion is also critical. Pressure-maintained cooling prevents spring-back deformation of workpieces before resin is fully shaped, and avoids residual internal stress induced by sharp cooling.

It is recommended to adopt SPC (Statistical Process Control) for real-time monitoring of hot-press temperature, and determine the optimal temperature-pressure-time window for specific formulations via DOE (Design of Experiments), so as to realize refined control of process parameters.

Conclusion

Hot-press temperature acts as the “lifeline” in brake pad production. Neither excessively high nor excessively low temperature is desirable; precise process windows shall be identified according to specific formulation systems, mold structures and equipment performance. Excessively high temperature triggers resin carbonization, sheet cracking and surface blistering, whereas insufficient temperature leads to incomplete curing, particle shedding and poor heat resistance. Technical and quality management personnel in manufacturing industries must thoroughly understand the physical-chemical mechanisms behind temperature effects, strictly control temperature uniformity, and emphasize coordinated matching among temperature, pressure and time. Only through scientific and rigorous process control can brake pads with stable performance and reliable safety be manufactured, providing solid guarantees for vehicle driving safety.