A fused deposition process is used to form three-dimensional solid objects from a mixture including a particulate composition dispersed in a binder. The article is formed by depositing the mixture in repeated layers of predefined thickness, with each layer solidifying before the next adjacent layer is dispensed. Following formation and a binder removal step, the article may be at least partially densified to achieve preselected properties. The process permits three-dimensional articles to be formed relatively quickly and inexpensively, without the need for molds or other tooling.

Solid freeform fabrication methods

▪ Abstract The physico-chemical processes that occur during binder thermolysis of porous ceramic bodies are reviewed. The discussion initially focuses on intrinsic and extrinsic mechanisms of polymer degradation, with emphasis on the formation of volatile degradation products and involatile carbonaceous residue. The transport of volatile species (e.g. residual solvents, plasticizers, etc) and degradation products through both empty and binder-filled pores is then considered. The particular case of debinding highly loaded ceramic bodies is addressed in detail. Recent developments in modeling the onset of defect formation and the effects of capillary redistribution of thermoplastic binders on removal kinetics are outlined. On the basis of this fundamental knowledge, the criteria for optimizing binder removal processes is established.

Binder removal from ceramics

Developing a rapid and efficient method for removing polymers (termed binders) from a shaped powder component, know as a green body, is important to forming defect-free metal, ceramic, and cermet structures. The rapid growth in powder injection molding to form complex shapes at high precision in large quantities has increased the need for faster, cleaner, and cheaper polymer removal processes. Binder removal using controlled heating of the component in gaseous atmosphere is the most popular method. This thermal debinding or burnout process is a delicate process, since it is easy to crack, blister, slump, or otherwise damage the component with an improperly designed cycle. To avoid these issues, often long heating cycles are used to remove the binder, but with a loss of productivity. Considerable progress has been made over the past several decades in understanding various phenomena during polymer burnout, resulting in substantial reduction in the thermal debinding time. This article provides an overview o...

Review: Thermal Debinding Process in Particulate Materials Processing

The removal of binder from ceramic or metal moldings by thermolysis involves the transport of degradation products through the parent organic phase and the vacated porous body. A numerical model has been developed to combine an equation which takes into account different gas-flow regimes with an equation for the transport of organic molecules in molten polymers. Computer modeling reveals the critical heating rate above which defects occur due to boiling of the polymer-monomer solution at the center of the molding. The situation in which a porous outer layer of the molding develops, offering resistance to flow of the evolved monomer gas, is then treated. This gives rise to a moving boundary with a variable concentration of diffusant which is dependent on the surface flux, gas transport coefficient, and thickness of the porous layer. The contributions of diffusion and viscous flow to gas transport are considered.

Modeling the effect of gas transport on the formation of defects during thermolysis of powder moldings

The debinding of powder-injection-molded components is critical and time consuming, especially when the sizes of the powders used become relatively small (<5 µm). Mathematical relationships have been derived for predicting the polymer removal times for solvent-, thermal-, and wick-debinding processes by considering one-dimensional mass flow. However, the debinding of a component produced by powder injection molding occurs through all of its surfaces, and, hence, mass flow along all three dimensions needs to be considered. An effective length scale is introduced that considers the entire bulk of the component to compute the solvant-debinding time and the wick-debinding processes. The effective length scale is defined as the volume to surface area ratio of the compact. Introduction of this length scale in the existing equations successfully explains the debinding behavior of various geometries. Furthermore, the length scale helps us to understand the effect of the component's shape and dimensions on the critical heating rates for polymer burnout.

Effective Length Scale for Predicting Solvent-Debinding Times of Components Produced by Powder Injection Molding

A method has been developed for calculating hydraulic pressures induced by thermal expansion of liquid binders early in the removal cycle, when evaporative losses are negligible and fully saturated conditions prevail. Specific results were obtained for flat compacts containing a common wax binder, mixed with varying amounts of low-density polyethylne. In general, these results show how the risk of hydraulic fracture increases with heating rate and compact thickness. Although pressures are minimal when the binder consists entirely of wax, the continual addition of polyethylene eventually gives rise to unacceptable risk levels, even for relatively thin compacts. Binder removal at elevated temperatures is considered subsequently. In this case, vapor pressures eventually approach a critical level, thereby allowing mass removal by evaporation to overcome the effect of thermal expansion in maintaining full saturation. With the onset of void formation, the developing capillary pressure supersedes hydraulic pressure as the driving force in liquid transport. Besides representing capillary flow, the present formulation also accounts for thermal degradation of the binder during removal. The resulting system of equations was solved numerically for a variety of representative debinding conditions. Predictions for flat compact containing a balanced wax/polyethylene binder indicate that thermal degradation of the polyethylene can give rise to a marked improvement in debinding rates. It turns out, however, that this enhancement is far more effective in thinner compacts.

M. R. Barone

Papers

Liquid‐Phase Transport During Removal of Organic Binders in Injection‐Molded Ceramics