There are three major factors relating to the resins to be processed that significantly influence the design of a screw and the material that should be used in its manufacture: Degree of Crystallinity, Viscosity and Additives in the resin.
The degree of crystallinity of a resin helps determine the resulting physical properties of a molded part and are important to the parts designer. Equally important to the plastics processor is the fact that crystallinity also influences the manner in which the resin changes from a solid to melt.
The differences in melting characteristics between highly crystalline and less crystalline (or amorphous) resins include their resistance to deformation as heat is applied, their sensitivity to thermal conductivity and their sensitivity to shear, regardless of source.
1. Melting Point - One of the differences between the two types of resin (crystalline and amorphous) is their resistance to deformation as their temperature increases. Both resins soften somewhat at the glass transition temperature but the amorphous resin continues to soften gradually until it reaches a fluid state. Amorphous resins have no defined melting point.
In contrast, the more highly crystalline resins remain in a relatively solid state until the temperature reaches their melting point. At the melting point temperature, crystalline resins quickly change to melt. (See Figure 1)
2. Thermal Conductivity - All plastics are poor conductors of heat. Amorphous resins are especially slow to absorb heat and increases in temperature. Amorphous resins tend to degrade or burn (rather than melt more quickly) when rapidly exposed to higher temperatures. (See Figure 2)
3. Shear Sensitivity - As a consequence of the two differences in melting characteristics discussed above, it can be understood why amorphous resins are considered to be more shear sensitive. High shear rates result in rapidly increased resin temperatures which amorphous resins do not tolerate well.
It is well known that excessive melt temperatures in some resins can cause residual molded-in stresses that detract from part appearance or reduce the mechanical strength of the parts. From these considerations, it may be concluded that amorphous resins should be gradually changed from solid to melt. Screws with longer transition zones and deeper channel depths with lower compression ratios help protect amorphous resins from burning or degrading and help ensure optimum physical properties in the completed parts.
In contrast, the higher crystalline resins can be processed more effectively by screws with shorter transition zones, more shallow channel depths and higher compression ratios.
Viscosity, or the resistance of a melt to flow, is measured by a capillary rheometer (or extrusion plastometer) and is expressed as the Melt Index of a resin. A high melt index (MI) value corresponds to a low melt viscosity, and vice versa. A fractional MI resin refers to a resin with a MI of less than one. MI is also a measure of molecular weight, but because MI is easier to determine, it is often used rather than the molecular weight specification. A lower MI indicates a higher molecular weight, and vice versa. High molecular weight (low MI) resins are more viscous and process differently than medium or high MI resins. High MI resins are somewhat more difficult to melt. The higher the Melt Index, the more shallow the channel depths of the screw. More viscous resins require deeper channel depths.
Additives to thermoplastic resins influence the design of the screw and the materials from which the screw is made and, in addition, bear on the selection of cylinder lining materials. Some additives influence only the screw geometry while other additives affect geometry and materials to be used in making the screw or barrel lining. Reinforcement materials can affect all three considerations. Additives may be grouped into categories based on their impact on screw design and Screw And Barrel material selection.
1. Additives Affecting Screw Geometry
All reinforcements and fillers affect screw geometry. They include fibers made from glass, carbon, graphite and other materials such as calcium carbonate, silica, glass spheres, mica, talc, powdered metals, ceramic, baryte (barium sulfate). anhydrous calcium sulfate and carbon black. Many other inorganic materials are used for fillers.
These additives increase the viscosity of the melt and require screws with deeper channel depths and somewhat lower compression ratios. This design change is especially important with the use of fibers to prevent their breakage and lessen their effectiveness.
2.Additives Affecting Screw & Barrel Material
Special screw materials (and barrel linings) are usually selected to aid in minimizing abrasive or corrosive wear. All of the abrasive reinforcements and fillers require that screws be made from special wear-resistant steels or been capsulated with an abrasion-resistant coating.
The more abrasive additives include glass fibers, calcium carbonate, ceramic and metal powders, and some colorants such as titanium dioxide Premium barrel linings containing vanadium and tungsten carbides (and others) are used to protect those components against excessive wear.
Other additives are corrosive and require screws made with corrosion-resistant alloys or special coatings. Flame retardants and coupling agents can develop a variety of corrosive acids at high temperatures. Barrel linings made from relatively iron-free materials and nickel alloys are necessary to avoid serious corrosive wear.
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