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[plastic]Design guides-performance and value with engineering plastic

Reffer:Editor:Author:Hits:-InputTime:2014-11-11 11:29:00

The process of developing thermoplastic parts requires a full understanding of typical material properties under various conditions. Thermoplastics can be categorized by their molecular structure as either amorphous, semi-crystalline plastics, or liquid crystal polymers(LCPs). The microstructures of these plastics and the effects of heating and cooling on the microstructures are shown.

 

Amorphous thermoplastics. Amorphous polymers have a structure that shows no regularity. In an unstressed molten state polymer molecules are randomly oriented and entangled with other molecules. Amorphous materials retain this type of entangled and disordered molecular configuration regardless of their states. Only after heat treatment some small degree of orientation can be observed ( physical aging).

 

When the temperature of the melt decreases, amorphous polymers start becoming rubbery. When the temperature is further reduced to below the glass transition temperature, the amorphous polymers turn into glassy materials. Amorphous polymers possess a wide softening range ( with no distinct melting temperature), moderate heat resistance, good impact resistance, and low shrinkage.

 

Semi-crystalline thermoplastics. Semi-crystalline plastics, in their solid state, show local regular crystalline structures dispersed in an amorphous phase. These crystalline plastics cool down from melt to solid state. The polymer chains are partly able to create a compacted structure with a relatively high density. The degree of crystallization depends on the length and the mobility of the polymer segments, the use of nucleants, the melt, and the mold temperature.

 

Liquid crystal polymers. Liquid crystal polymers(LCPs) exhibit ordered molecular arrangements in both the melt and solid states. Their stiff, rod-like molecules that form the parallel arrays or domains characterize these materials.

The difference in molecular structure may cause remarkable differences in properties. Various properties are time or temperature dependent. The shear modulus, for instance, decreases at elevated temperatures. The shear modulus curve illustrated the temperature limits of a thermoplastic. The shape of the curve is different for amorphous and semi-crystalline thermoplastics.

 

Dimensional stability. The following information on mold shrinkage, thermal expansion, and water absorption relates to the precision of a component both during molding and as secondary effects after molding.

 

Mold shrinkage. Shrinkage or mold shrinkage is the difference between the mold cavity dimensions and the corresponding component dimensions. It is not possible to predict exact shrinkage values for a specific polymer grade. Therefore, the maximum and minimum values for the various DSM thermoplastics are provided.

 

During injection molding the polymer melt is injected into the mold. Once the mold is completely filled the dimensions of the molding are the same as the dimensions of the mold cavity at its service temperature. While cooling down, the polymer starts to shrink. During the holding stage of the injection molding cycle, shrinkage is compensated by post-filling/packing. Both the design of the part as well as the runner/gate should allow for sufficient filling and packing.

 

The process of shrinkage continues even after the part has been ejected. Shrinkage should be measured long enough after injection molding to take into account post shrinkage.