The materials required for the 3D printing (3DP) process primarily consist of powder materials and binders. 1. Powder Materials i. Performance Requirements Based on the operating principles of 3DP, the powder materials need to possess good moldability, high forming strength, small particle size, low tendency to agglomerate, good flowability, suitable density and porosity, and quick […]
The materials required for the 3D printing (3DP) process primarily consist of powder materials and binders.
Based on the operating principles of 3DP, the powder materials need to possess good moldability, high forming strength, small particle size, low tendency to agglomerate, good flowability, suitable density and porosity, and quick drying and hardening. Among these properties, particle size is particularly crucial.
Smaller particles provide stronger van der Waals forces between them but have poorer flowability and are prone to dusting, which can clog the print head. Larger particles have better flowability but can compromise the printing precision of the mold. The particle size of the powder can range from 1μm to 100μm depending on the type of printer used and the operating conditions.
Powders that can be utilized in the 3DP process include gypsum powder, starch, ceramic powder, metal powder, thermoplastic materials, and other powders with suitable particle sizes.
Powder materials comprise fillers, binders, and additives.
a. Fillers: Options such as quartz sand, ceramic powder, gypsum powder, polymer powder (e.g., polymethyl methacrylate, polyoxymethylene, polystyrene, polyethylene, paraffin, etc.), metal oxide powder, and starch can be used as the main body of the powder material. These fillers enhance the strength of the parts and help control the deformation after binder-induced shaping.
b. Binders: The inclusion of powder binders can enhance the forming strength of the powder. Polyvinyl alcohol, cellulose (such as polymerized cellulose, silicon carbide cellulose, graphite cellulose, aluminum silicate cellulose, etc.), and maltodextrin can serve as reinforcing agents. However, the length of the cellulose should be less than the height reduction of the powder bed during printing.
The addition of colloidal silica helps the liquid binder to quickly gel upon contact with the powder, increasing printing efficiency.
c. Additives: Besides fillers and binders, various powder additives are required to adjust performance. Solid lubricants like aluminum oxide powder, soluble starch, and talcum powder can improve the flowability of the powder, facilitating the application of thin and even layers. Adding small, high-density particles like silica can increase the powder density and reduce porosity, preventing excessive penetration of the binder during printing.
Adding lecithin can minimize dusting of small particles and maintain the stability of the print shape. Moreover, it’s essential to disperse the powder properly to prevent agglomeration due to small particle sizes.
d. Preparation: Beyond simple mixing, fillers and binders can also be coated. By coating the fillers with a binder (such as polyvinylpyrrolidone) and drying, the two components can be more evenly dispersed within the powder material, allowing for uniform penetration of the ejected binder. Alternatively, the fillers can be divided into two portions for coating: one with an acid-based binder and the other with a base-based binder.
When the two meet through a medium, they can react quickly to form a shape. Coating also effectively reduces friction between particles, enhancing their flowability. However, it’s important that the coating is very thin, ranging from 0.1 to 1.0μm.
Adhesive agents must have suitable viscosity to ensure the formation of individual droplets that can detach from the nozzle of the print head. For the piezoelectric print head SEIKO1020, the recommended viscosity range for adhesive agents is 8-12 mPa·s.
The surface tension should be appropriate to foster good interaction between the adhesive agent and the powder, ensuring effective wetting.
Adhesive agents need to possess enough bonding strength to ensure the integrity of the initial printed structure.
Typically, liquid adhesive agents are ejected through metal print heads and should not cause corrosion.
They should comply with national or relevant organizational environmental regulations, be free from volatile odors, non-toxic side effects, environmentally friendly, and particularly suitable for office environments.
For metal powder applications, adhesive agents usually require sintering and post-processing removal. They should have clean combustion properties, minimal ash residue, and no toxic byproduct emissions.
The aforementioned properties should remain stable for ease of long-term storage and to ensure consistent product quality between batches.
Clearly, liquid adhesive agents primarily consist of a base medium and solvents. Additionally, they must include humectants, quick-drying agents, lubricants, coagulants, flow enhancers, pH adjusters, and other additives (such as dyes, defoamers), none of which should react with the material of the print head.
For example, humectants like polyethylene glycol and glycerol help retain moisture, facilitating long-term stable storage of the adhesive agents, and glycerol also serves as a lubricant to reduce clogging of the print head. Low-boiling-point solutions such as ethanol and methanol can be added to increase the evaporation speed of excess adhesive.
For powder materials that use colloidal silica or similar substances as gelling agents, additives like citric acid can be introduced as coagulants to enhance bonding effectiveness. Small amounts of other solvents (like methanol) or the addition of organic compounds of varying molecular weights can be used to adjust surface tension and viscosity to meet the requirements of the print head.
The surface tension and viscosity greatly affect the formation of droplets during printing; the right droplet shape and size directly impact the precision of the printing process. To improve the flowability of liquid adhesive agents, compounds like diethylene glycol butyl ether, polyethylene glycol, potassium aluminum sulfate, acetone, and sodium polyacrylate can be added as flow enhancers to accelerate the printing speed.
For those adhesive agents with specific pH requirements, triethanolamine, tetramethylammonium hydroxide, citric acid, and others can be used to adjust the pH to optimal levels.
Dyes that disperse evenly may be added for aesthetic purposes or product demands during the printing process. It’s important to note that the quantity of additives should be limited, generally less than 10% by mass fraction, as an excessive amount can affect the outcome of the printed powder and the mechanical performance of the print head.
Adhesive agents used in 3DP processes can be classified in several ways.
a. From a material property perspective, they can be categorized into organic and inorganic adhesive agents. Organic adhesive agents bond powder materials through curing, whereas inorganic adhesive agents bond through colloidal gelation.
b. From a chemical reaction standpoint, they can be divided into acidic and basic adhesive agents, metal salt adhesive agents, and solvent adhesive agents. Acidic and basic adhesive agents bond powder materials through acid-base chemical reactions, metal salt adhesive agents bond through salt recrystallization, reduction, or displacement reactions, and solvent adhesive agents primarily act on polymer powders, dissolving the sprayed or deposited areas and forming specific structures upon solvent evaporation.
c. From a binding mechanism perspective, they can be separated into powder bed adhesive agents, phase change adhesive agents, and sintering inhibiting adhesive agents. Powder bed adhesive agents, differing from standard liquid adhesive agents, bond with the powder bed through the action of sprayed liquid via the nozzle. Phase change adhesive agents bind powders together through the solidification of the adhesive, while sintering inhibiting adhesive agents can control the sintering area by selectively spraying insulating materials.
These three classification methods are shown in Table 5-3.
Table 5-33 Types of Binders for DP Process
| Binder Type | Applicable Materials | Advantages | Disadvantages | ||
| Classification by Material Properties | Organic Binder | Polyethylene, Butyraldehyde Resin, Phenolic Resin | Suitable for most unsintered materials: easy to remove with minimal residue. | Prone to clogging nozzles. | |
| Inorganic Binder | Aluminum Nitrate, Colloidal Silica | Heats the entire powder bed after printing, bonding the powder into parts. | Does not react immediately with powder after deposition. | ||
| Classification by Chemical Reaction Type | Acid-Base Binder | 10% Phosphoric Acid and Citric Acid, Polyvinylpyrrolidone | Almost no residue after heat treatment. | Limited to a few powders. | |
| Metal Salt Binder | Nitrates, Silicates, Phosphates | Utilizes recrystallization, reduction, and displacement reactions for bonding. | Loose powder must resist thermal reduction during the salt reduction process. | ||
| Solvent-Based Binder | Chloroform | High purity of parts. | Commonly used for polymers. | ||
| Classification by Binding Mechanism | Powder Bed Binder | Cement, Gypsum | Non-specific to certain powders, completely removed at high temperatures: simple liquid deposition. | Complex optimization steps. | |
| Phase Change Binder | Dimethylpropane | Applicable to most powders. | Heating restrictions after printing. | ||
| Sintering Inhibitors | Insulating Materials, Chemical Oxidants, etc. | Part boundaries sprayed. | Excess powder contaminates parts. | ||
Liquid binders can also be classified according to their interaction with powder materials, falling into three types:
1. Liquid binders that do not bind on their own, serving only as a medium for the powder materials to bond. These volatilize almost completely after printing, suitable for self-reacting hardening powder materials, such as chloroform, ethanol, etc.
2. Liquid binders that react with powder materials, typically with differing acidity from the powder materials, can achieve binding through the reaction. For example, powders primarily composed of alumina can be solidified by spraying with an acidic binder.
The most commonly used water-based binders, containing water as a major component, can provide a medium and hydrogen bonds in water, allowing gypsum, cement, and other powder materials to gel through hydrogen bonding, volatilizing after forming. For metal powders, metal salts are often added to the binder to induce a reaction.
3. Liquid binders that partially bind on their own, contain substances that bind when added to a liquid solvent, leaving key binding materials after the solvent evaporates. Common additives for binding include butyraldehyde resin, polyvinyl chloride, polycarbosilane, polyvinylpyrrolidone, and other polymers.
Compatible liquid solvents for these binding substances include water, acetone, acetic acid, ethyl acetoacetate, etc., with water-based binders being more popular currently.
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