In Layered Solid Manufacturing (LOM) technology, the selection of appropriate materials is crucial for achieving high-quality prototypes. The process primarily involves three key material components: thin-layer materials, binders, and coating techniques. Thin-layer materials commonly used in LOM are paper, plastic films, and metal foils. Paper is the most widely used material in current LOM processes, while the binder is typically a hot-melt adhesive. The selection of paper, formulation of the hot-melt adhesive, and the coating process must all be carefully considered to ensure the final part meets the required quality standards while keeping costs in check. In this article, we will review the properties of paper, hot-melt adhesives, and the coating process, which play a pivotal role in the success of the LOM process.

1. Paper Properties for LOM

The choice of paper material in LOM is one of the most critical factors that impact the final prototype’s quality. The key properties of paper that are important for the LOM process include:

1) Moisture Resistance

The paper used in LOM must have excellent moisture resistance to ensure that the material does not absorb water over time. Paper that absorbs moisture can lead to deformation during the thermal bonding process, which may result in poor adhesion. The paper’s ability to resist moisture can be measured using its sizing degree.

2) Wettability

Good wettability ensures that the adhesive bonds well with the paper surface. This property is essential for ensuring that the layers bond effectively during the formation of the prototype.

3) Tensile Strength

The paper used for LOM must possess sufficient tensile strength to prevent tearing or breaking during the manufacturing process. This ensures that the paper can withstand the stresses encountered during layer bonding and cutting.

4) Low Shrinkage Rate

A low shrinkage rate is essential for ensuring that the paper does not deform when heated. If the paper shrinks excessively during the bonding process, it can lead to dimensional inaccuracies in the final prototype. Shrinkage can be measured using the paper’s expansion and contraction parameters.

5) Peel Strength

Peel strength refers to the paper’s ability to separate without causing damage during the layer removal process. The paper’s tensile strength in the vertical direction should not be too high to ensure that the paper peels away from the finished layers easily.

6) Smooth and Sandable Surface

The paper used should be smooth and easy to sand. This helps achieve a high-quality finish on the surface of the final prototype, as any rough texture can affect the appearance and functionality of the prototype.

7) Stability

The paper must have long-term stability to ensure that the finished prototype remains intact over time. This ensures that the prototype does not deteriorate or degrade during storage, maintaining its quality and usability.

2. Hot-Melt Adhesive for LOM

In LOM, the bonding between the layers is facilitated by a hot-melt adhesive. There are many types of hot-melt adhesives available, but EVA (Ethylene Vinyl Acetate) is the most commonly used, accounting for about 80% of total hot-melt adhesive consumption. The hot-melt adhesive must meet several key criteria to ensure a successful bonding process:

1) Good Thermal Melt and Solidification Properties

The adhesive must melt at a temperature between 70°C and 100°C and solidify at room temperature. This allows the adhesive to bond layers efficiently during the manufacturing process.

2) Stability Under Repeated Melting and Solidification

The adhesive must be chemically stable under repeated cycles of melting and solidification, ensuring that the adhesive maintains its strength and properties over time.

3) Good Coating and Wettability

The adhesive must have good coating properties when in its molten state, ensuring that it spreads evenly and adheres well to the paper surface.

4) Sufficient Bond Strength

The adhesive must provide strong bonding between the layers to ensure that the prototype holds together securely during the manufacturing process.

5) Easy Waste Separation

The adhesive should allow easy separation of waste materials during the process. Proper waste removal is crucial for maintaining high manufacturing efficiency.

3. Coating Process

The coating process is essential in determining the quality and consistency of the adhesive layer applied to the paper. This process includes both the shape and thickness of the adhesive coating.

1) Coating Shape

There are two primary coating methods: uniform and non-uniform coating. Non-uniform coating can further be divided into stripe and particle coatings. While non-uniform coatings can reduce stress concentration in certain areas, they require more expensive equipment. Uniform coating, on the other hand, is simpler and more cost-effective but may not offer the same benefits in reducing stress.

2) Coating Thickness

The coating thickness refers to the amount of adhesive applied to the paper. It is critical to select the right coating thickness that ensures reliable bonding while minimizing deformation, excess adhesive spillage, and displacement during the bonding process. In general, a thinner adhesive layer is preferred to maintain the accuracy of the prototype and reduce material waste.

Example from KINERGY Company

The paper materials produced by KINERGY, a company based in Singapore, are a good example of high-quality LOM materials. KINERGY’s papers are made using a high melting temperature binder and specially modified additives. These materials produce parts with the hardness of hardwood, a smooth surface, minimal warping during processing, and easy separation of the prototype from the waste. After surface treatment, these materials are moisture-resistant and highly stable, ensuring the long-term durability of the prototypes.

Conclusion

The materials used in LOM—paper, hot-melt adhesive, and the coating process—are all crucial components that influence the quality of the final prototype. By ensuring that each material is carefully selected for its specific properties, manufacturers can optimize the LOM process to produce high-quality, durable prototypes. From moisture-resistant paper to well-formulated adhesives and precise coating techniques, attention to detail in material selection guarantees that LOM technology remains an effective and cost-efficient method for rapid prototyping, especially for large and complex parts.