Blow molding resin characteristics

Different properties of plastics are preferred for different fabrication methods. The viscosity of the resin melt is typically measured against a standard test referred to as the melt flow index (MFI). The MFI and resin density characteristics of some of the primary applications are shown in Table 1.4. As can be seen, the MFI and density properties vary with fabrication method. To allow for high tolerances and precise definition, injection molding utilizes a resin melt which is runny (MFI=5-400) in comparison to blow mold grade resin (MFI=0.01-0.2) or extrusion grade resin (MFI=0.3-2). Blow mold resin tends to be more taffy like so that it will retain thickness while pressure is applied inside a mold. This viscosity difference limits the type of recycling production methods possible because different melt viscosity resins do not mix homogeneously. If different melt viscosity resins are not separated pror to reuse, the resin properties will not be uniform in production. 

Another important characteristic of blow-molding resins is the degree of parison swell and die swell. The former is the extent to which the molten tube tends to flare out during the extrusion step. An excessively high degree of flare can cause the tube to extend beyond the mold cavity, while too little flare can sometimes cause incomplete filling of the mold structure. Die swell is a measure of how much the tube wall expands as it exits the die under high pressure. The die gap must be adjusted to take die swell into accoimt in order to produce the desired wall thickness. Molders prefer to set this parameter midway and not have to... 

Recycling of degradable plastics is a topic of debate within the plastics industry. Studies have shown that no deleterious effects on tensile strength or ultimate elongation were observed from the addition of 20 wt % embrittled E-CO to a typical LLDPE blow molding resin. Subsequent UV exposure of that mixture showed little difference in degradation characteristics between it and the polyethylene controls [26]. Thus, the presence of E-CO copolymers in the recycle stream should not be a deterrent to recycling of plastics. 

As was the case for ketchup bottles, most uses of coinjection blow molding have the aim of improving the barrier characteristics of the container, to extend the shelf life, and to better preserve the flavor, aroma, or other characteristics of the product. Reheat stretch blow molding is the most common process, with three to five layers of resin forming the parison. The skin layers, both inside and outside. 

Water, with its excellent heat transfer characteristics, is used primarily as the heat transfer medium for polyolefin blow-molding, but for the newer engineering resins a synthetic heat transfer fluid that operates up to temperatures of 121-149°C or more at low pressure is highly desirable because of the safety factor. Because the heat transfer characteristics of the synthetic fluid are not as effective as water, care must be exercised to use proper fluids in a safe manner to obtain as efficient production cycles as possible. Compromises sometimes are necessary, depending upon the required temperatures. 

The results of the sag study provided a unique opportunity to study the changes in melt strength. Sag in blow molding is best described as the extension of the molten parison due to gravitational force. Any variance in the sag properties of a resin will have a direct effect on the shape and physical characteristics of the bottle. 

This restriction implies that only thermoplastic materials have the required physical characteristics to qualify for this type of processing. Resin manufacturers have developed special formulations that exhibit good melt strength and that are predictable in the melted state. Thermosets that are based on unreacted liquids or solids that do not have well-defined behavior during reactions do not have the stability required for blow molding. 

In the low-pressure process, a resin containing a blowing agent is forced into the mold, where it expands to fill the mold under pressures of 690-4100 kPa (100-600 psi). This produces structural foam products with a characteristic surface-swirl pattern caused by the collapse of cells on the surface of molded articles. 

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