Polymer Processing เทคโนโลยีการแปรรูปพอลิเมอร์มีอะไรบ้าง #2 เทอร์โมเซต (Thermoset)

Polymer Processing: Thermoset

• Thermosetting or Thermoset materials are materials that can be molded only once. They cannot be recycled by melting and reusing like thermoplastics because the processing of these materials changes their molecular structure into a network, with chemical crosslinking occurring during molding. Breaking these crosslinks requires a very high amount of energy or a change to chemical methods. In any case, it is not possible to restore them to their original shape or characteristics prior to molding.
• Examples of materials in this group include resin, rubber, or elastomers, which are also classified in this group when considering their post-molding behavior.
• Therefore, if exposed to heat, materials in this group generally do not soften; they can maintain their shape at high temperatures until they eventually burn into soot or ash.

• The process of crosslinking requires chemicals that contain a Crosslink Agent. For example, resin for casting must be mixed with a hardener for it to harden (reaction), or in the case of natural rubber, sulfur must be added.
• The catalyst or initiator for the reaction can be achieved through various means such as mixing with a crosslinking agent and leaving it for a specific duration, applying heat, or using UV light, etc.

1. Injection Molding

This is similar to general thermoplastic molding, but differs in that it is divided into two main stages: the injection phase, which requires preparing the material to have low viscosity so it can flow into the desired cavities, and the crosslink/curing phase, which requires the material to maintain the desired shape, achieved through heat (for natural rubber) or chemicals.

1.1 Reaction Injection Molding (RIM)

RIM is an injection technique suitable for thermosetting materials with very low viscosity, such as polyurethane. Two types of components are mixed and injected into a mold at a temperature of approximately 80-90 degrees Celsius to allow for curing.

1.2 Rubber/Elastomer Injection Molding

For natural and synthetic rubber, during the injection stage, the temperature of the injection unit is maintained at approximately 80-90 degrees Celsius so that the rubber can flow into the mold. The mold itself is set to a temperature of approximately 150-180 degrees Celsius to facilitate crosslinking, which differs from the standard plastic injection process.

2. Compression Molding

• The thermosetting compression molding process does not differ significantly from injection molding, except for the material loading phase. Instead of injection, the initial material—referred to by various names such as charge, preform, or compound—is placed into the mold, and pressure is applied to force it into the mold cavity. Sometimes, to reduce the required pressure, air vents may be created to expel air while closing the mold.
• The mold temperature depends on the type of material being molded and can range from 90 to 200 degrees Celsius.

3. Casting

This is a familiar process for producing items from resin or other thermosetting materials that have an initial liquid or viscous state. It usually consists of main ingredients, a hardening agent, and additives to achieve special properties, which are then mixed and poured into a mold of the desired shape.

4. 3D Printing

All the aforementioned processes require molds for forming, which may not be suitable for producing a single prototype or for low-volume production. Another case is when the product has a very complex shape that cannot be demolded. 3D printing technology is another interesting alternative nowadays, with machines available ranging in price from thousands to millions of Baht.

4.1 Resin 3D Printer

This technology has a starting price in the thousands. Most of these form objects by projecting UV light onto the resin layer by layer until the desired model is achieved. There are several patterns or methods for light projection, which can be read further in the article "How many types of Resin 3D printers are there, and how do they differ?".

4.2 Silicone 3D Printing

Silicone is a synthetic material that is widely used and has various properties depending on the formula adjustment. The 3D printing process often uses an extrusion syringe to deposit the material layer by layer into the desired shape. Higher-end machines offer greater precision and volume capacity. Read more details about printing this group of materials at "3D Printing Silicone and PU". Companies with expertise and industrial-grade machines, such as the Germanreprap model L320, support silicone and many types of viscous materials.