SMC/BMC Compression Molding Process and Three Key Control Factors

During the SMC/BMC compression molding process, it is essential to control three key factors: molding temperature, molding pressure, and molding time.

1. Control of Molding Temperature

Molding temperature refers to the temperature of the mold during compression molding. This parameter determines the heat transfer conditions from the mold to the material in the cavity, which significantly influences the material’s melting, flow, and curing processes.

Temperature changes during the SMC/BMC molding process are complex. Since plastics are poor heat conductors, there is a significant temperature difference between the core and edges of the material in the early stages of molding. This results in the curing and cross-linking reactions starting at different times in the inner and outer layers of the material.

The outer layer of the material, which receives heat earlier, cures first and forms a hard shell. As the inner layer cures later and shrinks, it is constrained by the hardened outer shell, causing residual compressive stress on the surface and residual tensile stress in the inner layer. These residual stresses can lead to warping, cracking, and reduced product strength. Therefore, minimizing the temperature difference within the material and eliminating uneven curing are essential for producing high-quality products.

The molding temperature of SMC/BMC materials depends on the exothermic peak temperature and curing rate of the curing system. It is typically set slightly below the peak temperature, usually between 135–170°C, and determined through experimentation.

  • Fast-curing systems: Use the lower end of the temperature range.
  • Slow-curing systems: Use the higher end of the temperature range.
  • Thin-walled products: Use the upper limit of the temperature range.
  • Thick-walled products: Use the lower limit of the temperature range.

However, for thin-walled products with significant depth, a lower temperature within the range may be necessary to prevent material curing during flow.

Raising the molding temperature appropriately, without compromising product strength and other performance indicators, can shorten the molding cycle and improve product quality. On the other hand, too low a temperature results in high viscosity and poor flowability of the melted material, incomplete cross-linking, reduced product strength, dull appearance, and issues like sticking and deformation during demolding.

2. Control of Molding Pressure

Molding pressure, usually expressed as molding pressure intensity (MPa), is the ratio of the total force applied by the hydraulic press to the mold’s projected area in the pressing direction.

The purpose of molding pressure in the compression molding process is to:

  • Ensure tight closure of the mold.
  • Compact the material.
  • Facilitate melt flow and balance the pressure from low-molecular-weight volatiles in the cavity.

Materials with high compressibility require more energy for densification, hence requiring higher molding pressures. For example:

  • Bulk molding compounds (BMC): Require higher pressure compared to sheet molding compounds (SMC).
  • Complex shapes or large, thin-walled, or deep products: Require higher pressure to overcome greater flow resistance.

High molding temperatures accelerate the cross-linking reaction, increasing the viscosity of the molten material, which necessitates higher molding pressures to ensure cavity filling.

While high molding pressure can increase product density, reduce molding shrinkage, and eliminate defects like swelling or air pockets, excessive pressure may:

  • Reduce mold life.
  • Increase power consumption of the hydraulic press.
  • Cause residual stresses in the product.

To avoid excessively high molding pressures when processing thermosetting plastics, techniques like pre-compression, preheating, and moderately increasing molding temperatures are often employed. However, improper preheating conditions (e.g., excessive preheating temperature or time) can partially cure the material, reducing flowability and ultimately requiring even higher molding pressures.

3. Control of Molding Time

Molding time, also called compression molding hold time, refers to the duration the material is subjected to heat and pressure inside the mold, from the moment the mold fully closes (or after the final venting) to when the mold is opened.

The primary function of molding time is to ensure sufficient curing of the material to form a product that conforms to the mold cavity.

Curing is the process of forming a network structure in thermosetting plastics. From a chemical perspective, curing is the progression of the cross-linking reaction. However, in manufacturing, “complete curing” means that the cross-linking reaction has reached a suitable level where the product’s physical, mechanical, or other specified properties meet the desired standards.

  • Under-cured (insufficient curing): The reaction is incomplete, resulting in poor mechanical performance, dull appearance, and warping or deformation after demolding.
  • Over-cured (excessive curing): Prolonged molding time leads to excessive cross-linking, increased shrinkage, internal stresses between resin and fillers, reduced surface quality (e.g., darkening, bubbling), and even cracking.

Factors influencing molding time include the material’s curing rate, product shape and wall thickness, mold structure, molding temperature and pressure, as well as whether pre-compression, preheating, or venting is involved. Among these factors, molding temperature, product wall thickness, and preheating conditions have the most significant impact.

  • Proper preheating: Speeds up material heating and cavity filling, shortening molding time.
  • Higher molding temperatures: Shorten molding time.
  • Thicker product walls: Require longer molding time.

When molding temperature and pressure are fixed, molding time becomes the critical factor determining product performance. A well-controlled molding time ensures optimal curing, reducing defects and enhancing properties like heat resistance, strength, and electrical insulation. However, overextending molding time decreases productivity, increases energy consumption, and may lead to defects as described above.