Hoop ratio

In stretch blow molding, there are two ratios that multiply together to provide the blow up ratio BUR. In extrusion blow molding, there is only the hoop ratio (that is the blow-up ratio). In stretch blow molding there is the hoop that is multiplied by the axial ratio. Thus, BUR = Hoop ratio x axial ratio. 

The hoop ratio is the most important. If the product to be packaged is pressurized as soft drinks, carbonated water, and beer, the hoop ratio should be at least 10 or higher. The hoop ratio is defined as the ratio of the largest inside diameter (Di) of the blown article to the inside diameter (Z>2) of the parison or preform. 

The BUR is used to determine the wall thickness that would be necessary in the precursor or preform. If the BUR is 10, and the desired minimum wall thickness is 0.38 mm, then the BUR x the desired wall thickness, would indicate that the minimum wall thickness in the preform should be 3.8 mm since the thickness will be expanding 10 times. The total BUR is equal to the hoop ratio times the axial ratio. 

To check if the orientation is correct, a dog bone can be cut from the stretch blow-molded PET container and a tensile test conducted on an Instron or similar machine. A dog bone shape is cut in the hoop direction and one is cut in the axial direction. PET has a base strength of approximately 46 MPa (6700 psi). If the hoop ratio is 5, then in the container in the hoop direction, the tensile strength should be approximately 231 MPa (33,500 psi). If the axial ratio is 2, then the tensile strength in the vertical direction would approximately be 92 MPa (13,400 psi). 

Two ratios involving preform-to-product dimensions are utilized to describe biaxial stretch molding. The first of these is the hoop ratio H ... 

The BUR for a container holding the contents under pressure is 10 or higher. Hoop ratios (4-7) are usually more important than axial ratios (1.4-2.6). 

The design of the preform in injection blow molding is critical. The preform should be designed to have a wall thickness in the body of the preform anywhere from approximately 0.035 in. ( 1 mm) to approximately 0.200 in. (5 mm). The preform length is designed to clear the inside length of the bottle in the blow mold by approximately 0.005 in. (0.125 mm). Thus there is minimum stretch in the axial direction of the preform when the bottle is blown. The diameter of the core rod is in all practicality determined by the maximum inside dimension (I-dimension) of the finish of the desired container. In determining the wall thickness of the preform in the main body, it is necessary to know what wall thickness is desired in the final blown article plus the maximum inside diameter of the desired blown article. The ratio of the inside diameter of the blown bottle (Dj) to the inside diameter of the preform (D ) is known as the hoop ratio. 

If the wall thickness in the blown article is to be 0.022 in. ( 0.56 mm) and the hoop ratio is 3, then the preform should have approximately 0.066 in. (1.67 mm) wall thickness in the main body. The tip of the preform should be designed for good plastic flow, yet not bullet shaped to permit easy deflection due to the injection pressure. It is best to have at least 0.0455 in. (1.1 mm) flat on the tip of the preform design. This can be greater depending on the gate diameter or orifice size of the gate. The outside radius... 

Each of fee aforementioned materials also has its own stretch ratios. In order to understand stretch blow molding it is necessary to understand the terms orientation temperature, blow pressure, blow-up ratio, axial ratio, hoop ratio, and stretch ratios. 

The hoop ratio is defined as the ratio of the largest inside diameter (D ) of the blown article to the inside diameter (D ) of the parison or preform. 

The total blow-up ratio (BUR) is equal to the hoop ratio times the axial ratio. 

Recently, other kinds of stretch ratios have been defined for blow molding to better define the limits of the process for different materials, The hoop ratio H is defined as the ratio of the maximum outside dimension (diameter) of the finished molded part to the maximum outside dimension of the parison after emerging from the die (parison diameter) ... 

Example 23 A cylindrical polypropylene tank with a mean diameter of 1 m is to be subjected to an internal pressure of 0.2 MN/m. If the maximum strain in the tank is not to exceed 2% in a period of 1 year, estimate a suitable value for its wall thickness. AVhat is the ratio of the hoop strain to the axial strain in the tank. The creep curves in Fig. 2.5 may be used. 

Note that the ratio of the ratio of the hoop stress (pR/h) to the axial stress (pR/lh) is only 2. From the data in this question the hoop stress will be 8.12 MN/m. A plastic cylinder or pipe is an interesting situation in that it is an example of creep under biaxial stresses. The material is being stretched in the hoop direction by a stress of 8.12 MN/m but the strain in this direction is restricted by the perpendicular axial stress of 0.5(8.12) MN/m. Reference to any solid mechanics text will show that this situation is normally dealt with by calculating an equivalent stress, Og. For a cylinder under pressure Og is given by 0.5hoop stress. This would permit the above question to be solved using the method outlined earlier. 

An example of an interference fit design is the maximum allowable interference for a particular hub and shaft. It depends on the types of materials used in the hub and shaft, and on the ratio of the shaft diameter to the hub outside diameter. It is determined to ensure that hoop stress in the interference fit does not exceed the allowable stress of materials used (Chapter 7). 

Ratio of Design Hoop Stress to Minimum Spedfied Yield Strength in the Header Ratio of Nominal Branch Diameter to Nominal Header Diameter ... 

Figure 7. Strain amplification A plot of the strain amplification ratio er as a function of the load frequency for different load magnitudes. Strain amplification ratio is defined as the ratio of the hoop strain in the cell process membrane to the bone surface strain at the osteonal lumen, e is the strain on the whole bone s is the load on the whole bone. Previously published in You et al. (2001).

Hoopes, E. A., Peltzer, E. T., and Bada, J. L., (1978). Determination of anino acid enantianeric ratios by gas liquid chromatography of the N-trifluoroacetyl-L-prolyl-peptide methyl esters. J. Chromatographic Sci.,... 

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