What role does HPMC play in the injection molding of honeycomb ceramics and can it improve molding efficiency? ​

During the injection molding process of honeycomb ceramics,Hydroxypropyl methylcellulose (HPMC)As a key additive, it mainly exerts multiple effects by regulating the rheological properties, stability, and strength of the slurry, and can significantly improve the molding efficiency. The specific mechanism is as follows:

1、 The core role of HPMC in injection molding

Improve the rheological properties of the slurry and enhance the grouting filling ability

The molds of honeycomb ceramics (such as gypsum molds) have complex porous structures and fine channels (especially thin-walled honeycomb cell grids, with wall thickness often less than 1mm), requiring the slurry to have good fluidity and thixotropy:

The hydroxyl and ether groups in the HPMC molecular chain can form hydrogen bonds with water molecules, increasing the viscosity of the slurry (usually controlled at 500-2000 mPa · s) and avoiding rapid settling of solid particles (such as alumina and cordierite). At the same time, its "pseudoplasticity" characteristic (shear thinning) enhances the fluidity of the slurry under injection pressure (such as 0.1-0.3 MPa), which can quickly fill the fine pores of the mold, reduce bubbles and unfilled defects.

For high solid content slurries (solid volume fraction 60% -70%), HPMC disperses particles through steric hindrance effect, reduces inter particle friction, and maintains injectability of the slurry at low water content, reducing subsequent drying shrinkage.

Enhance the strength of the billet and reduce molding damage

The strength of the blank formed by grouting (especially wet blank) is low, and it is prone to cracking or deformation during demolding and transportation

HPMC molecular chains can adsorb onto the surface of ceramic particles, forming a continuous "adhesive film" that connects dispersed particles through intermolecular entanglement, significantly improving the wet green strength (usually increasing the wet green flexural strength by 30% -50%).

For thin-walled honeycomb structures, HPMC can enhance the flexibility of the billet and reduce edge damage caused by stress concentration during demolding (such as cell collapse and edge slagging).

Regulating moisture migration and optimizing demolding time

Gypsum mold grouting relies on the water absorption of the mold to gradually solidify the slurry into a billet, and HPMC adjusts the water migration rate through water retention:

moderate amountHPMC(Usually added at a dosage of 0.2% -0.8%) can delay the rapid absorption of moisture by the mold, avoiding the formation of a "hard shell" on the surface of the billet due to rapid dehydration, which makes it difficult to discharge internal moisture and leads to delamination or cracking.

At the same time, the hydrophilicity of HPMC ensures uniform migration of moisture, allowing the green body to solidify synchronously from the surface to the interior, reducing deformation after demolding caused by uneven solidification.

Hinder particle agglomeration and improve the uniformity of the body

Ceramic raw materials (such as nanoscale powders) are prone to agglomeration due to high surface energy, resulting in uneven density of the green body and defects after firing

The ether groups of HPMC can disperse and aggregate particles through electrostatic repulsion and steric hindrance effects, making the distribution of solid particles in the slurry more uniform, thereby ensuring the consistency of the microstructure of the billet and reducing local shrinkage differences during firing.

2、 The improvement effect of HPMC on molding efficiency

Shorten the grouting cycle

High fluidity slurry can quickly fill the mold (especially complex honeycomb channels), reducing the waiting time caused by slow flow of traditional low viscosity slurry (usually shortening the grouting time by 20% -30%).

Uniform moisture migration ensures a more stable solidification speed of the billet, avoiding prolonged demolding waiting time due to local uncured areas, and reducing post demolding repair processes (such as filling missing corners and repairing cracks).

Reduce waste rate and minimize repetitive operations

The improvement of wet billet strength and billet uniformity reduces the damage rate during demolding and transportation (usually from 10% -15% to below 5%), reducing the waste of raw materials caused by waste and the time cost of re grouting.

Obstructing particle agglomeration can reduce defects such as cracking and deformation after firing, lower rework rates, and indirectly improve overall molding efficiency.

Adapt to automated production lines

Stable slurry performance (with small viscosity fluctuations and controllable thixotropy) is more suitable for automated grouting equipment (such as quantitative grouting machines), reducing equipment adjustment time caused by unstable slurry performance and improving production line continuity.

3、 Key parameter selection

To maximize efficiency, HPMC parameters need to be adjusted according to the material (such as cordierite, silicon carbide) and structure (pore density, wall thickness) of honeycomb ceramics:

Viscosity: Thin walled high pore dense honeycomb (such as over 600 pores per square inch) should choose medium low viscosity HPMC (100-500 mPa · s) to ensure high fluidity; Thick walled structures can choose medium to high viscosity (1000-3000 mPa · s) to enhance wet billet strength.

Addition amount: Excessive addition amount (more than 1%) can lead to high viscosity of the slurry, which in turn prolongs the grouting time, and during firing, it is easy to produce pores due to carbon residue. The optimal range (usually 0.3% -0.6%) needs to be determined through experiments.

Etherization degree: HPMC with high methoxy content (such as 28% -30%) has moderate hydrophilicity, better compatibility with ceramic slurry, more stable dispersion effect, and is suitable for most ceramic systems.

summary

HPMCIn the injection molding of honeycomb ceramics, by optimizing the flowability of the slurry, enhancing the wet billet strength, regulating water migration and dispersing particles, not only the quality of the billet is improved (reducing defects and improving uniformity), but also the molding efficiency is significantly improved by shortening the injection cycle, reducing waste rate and adapting to automated production. In practical applications, it is necessary to select the viscosity, addition amount, and etherification degree of HPMC according to the specific structure and raw material characteristics of honeycomb ceramics, in order to achieve a balance between performance and efficiency.