Green strength is a term used to describe a molded or cast products ability to withstand handling, mold ejection, and machining before it is completely cured or hardened. This physical characteristic is a critical criterion in the productivity levels processes such as injection molding and powder metallurgy. The quicker cast or molded products can be removed or ejected from the mold and handled the higher the process turnover becomes. This process must not, of course, damage or alter the finished product so a high green strength rating in any product is a desirable characteristic. Good green strength values play a vital role in many areas including the ceramics, thermoplastic, and metal powder molding industries.
The term “green” is used to describe a finished product that has not yet cured completely or been sintered or hardened. Although the final form is established, the full resilience and strength rating of the distributable product has not yet been reached. This may take a fairly long time to achieve; if the product were to be left in the mold for that period, production levels would suffer. Machining metal powder molded products prior to sintering or final hardening is also a quicker and easier process. For these products to be ejected from their molds and handled or finished requires an acceptable level of green strength to prevent breakage.
Injection molding products are a good example of the importance of acceptable green strength characteristics in materials. The molds used in injection molding are expensive, and it is impractical to make hundreds of molds even for high production run products. To maintain good production levels, the molded product needs to be removed as quickly as possible to allow for the next item to be molded. Most injection molding materials have a fairly protracted full cure cycle; they need to exhibit a fair amount of resilience to be ejected and transported quickly. This is where the green strength of the materials is crucial to quick turnovers.
Powder metallurgy is another industry that benefits from this type of quick production cycle. That was not possible until recently due to limitations in the materials used; advancements in the field, however, have introduced new raw materials and processes that ensure higher green strengths. It is now possible to produce metal powder molded products that exhibit pre-sintering strengths of between 3,500 and 6,000 psi pounds per square inch (psi). This allows these products to be handled or even machined prior to final hardening. Another powder molding industry that benefits from the high green strength of its materials is the ceramics industry where quick turnovers are possible with minimal product losses.