Test Your Rheology IQ

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Test Your Rheology IQ

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Bimodal high density polyethylene, HDPE

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We will try to imagine how bimodal molecular weight distribution rise. We will focus on two main properties: processing and mechanical . We start from normal (unimodal) molecular weight distribution (please, start to analyze the graph). Macromolecules with short chains , are easiest to process, polymer has lower viscosity , long chains are responsible for mechanical properties the grow. Considering, what left (middle), can be replaced. The idea was simple, eliminating average properties, we receive polymer with key properties. Return to the most important, processing and final product properties , considering molecular weight distribution . Short fractions or single monomers are gaseous substance, waxes, their impact is questionable (easy degradation, responsible for bad odor) and appear for unimodal distribution , changing the distribution we eliminate them. Long fractions generally affect mechanical properties , e.g. stiffness , it is possible to reduce wall thickness (sho...
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6. Balance of fill analysis - Validation procedure for injection molds

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The sixth step in validating a injection mold with the overall process shown in injection mold validation flow chart is balance of fill analysis . The steps before: 1. Mold certification 2. Dry cycle mold 3. Process stability test 4. Gage repeatability & reproducibility (R&R) test 5. Mold viscosity test Purpose: The purpose of the balance of fill analysis is to evaluate the thermal and flow balance of the plastic distribution system in the mold. The plastic distribution system encompasses the hot or cold runner system, as well as the core and cavity. Only a naturally or symmetrically balanced manifold should be specified. The cavity-to-cavity weight difference is an indicator of the quality of the hot runner system. It is critical to have the flows balanced to each cavity or the part-to-part variation may be large and process capability may not be achievable. Recall, the purpose of the mold validation procedure is to reduce variation throughout the injection mold. A typical m...
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Head tooling – extrusion blow molding

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Mandrel and die dimensions are estimated based on container dimensional data, container symmetry, blow-up ratio, targeted container weight, neck finish requirements and the type of material (degree of parison swell) that will be used to produce the container. An initial blow-up ratio must be calculated using the container design dimensions and the required parison diameter. The required parison diameter will depend on the relative size of the container, the container design (handle or no handle) and the container neck finish requirements. Initial blow-up ratios may be calculated using the following equation. Blow up ratio = Bd / Nd where: Bd = Bottle diameter, in Nd = Minimum neck diameter, in The blow-up ratio is compared with the maximum recommended blow-up ratio of the selected material. Blow up ratios of 2 or 3 to 1 are considered normal when molding commodity resins such as polyethylene. A blow-up ratio as high as 4:1 is a practical upper limit. The blow up ratio for large contai...
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