There are a variety of medical grade resins on the market today, including polypropylene, polycarbonate, polyester, polyethylene, polystyrene, thermal plastics Alsatians, polyurethane and liquid silicone rubber.

The key to successful plastic part manufacturing through a hot runner system, is a thorough understanding of the resin properties being processed. Since processing these thermal plastics resins revolves around heating and cooling the resin, we will only concentrate on the thermal properties and how the heat is transferred in and out. All thermal plastics resins can be categorize into four main areas as related to thermal properties: (1) crystalline, (2) semi-crystalline, (3) amorphous and (4) thermoset.

Crystalline solids are a class of solids that have regular or nearly-regular crystalline structures. This means that the atoms in these solids are arranged in an orderly manner. Solids are divided into three main types: (1) crystalline solids, (2) amorphous solids and (3) thermoset solids.

These differentiated on the basis of their physical properties. Crystalline solids are of a definite shape and have a definite melting point, while amorphous solid have no definite shape and melt at a range of temperatures. Crystalline solids are also known for their anisotropic nature, whereas amorphous solids are isotropic. Anisotropy is the property of being directionally dependent; as opposed to isotropy, which means homogeneity in all directions. It can be defined as a difference in a physical property (absorbance, refractive index, density, etc.) for some material when measured along different axes. For example, the light coming through a polarizing lens.

Other generalizations of a crystalline solid include being opaque, where as amorphous solids have clarity. Thermoset solids are heat reactive and remain in some liquid form until heat is applied, whereby a chemical reaction takes place causing the molecules to cross link forming one continuous molecule. Once the reaction takes place it cannot be reversed. Unlike thermal plastics, thermoset materials cannot be reprocessed.

Most medical applications do not allow for regrind material, so hot runner systems are an absolute necessity to reduce manufacturing costs and to remain profitable. There have been several myths surrounding hot runner systems, including hot runner systems degrade resins. The causes of resin degradation are heat, residence time, dead spots and excessive shear.

A properly selected and installed hot runner system does nothing more then convey the resin from the molding machine injection nozzle to the cavity. It does not add or take heat away; it does not induce excessive shear; and, it does not have dead spots where material can degrade. High-quality hot runner systems will manufacture manifolds in such a way to reduce flow-induced shear by having radius flow channels.

General speaking, crystalline-based resins are less shear-sensitive then amorphous-based resins. The reason is that crystalline-based resins have a very specific melting point at which the resin viscosity drops off sharply, reducing the chances of high shear. Amorphous resins are more susceptible to shear because they don’t have a specific melting point and viscosity remains high until flow occurs.