Parison diameter

S. Tanue, T. Kajiwara, K. Funatsu, K. Terada, andM. Yamabe, Numerical Simulation of Blow Molding - Prediction of Parison Diameter and Thickness Distribution in the Parison Formation Process, Polym. Eng. Sci., 36, 2008-2017 (1996). 

The melt parison is extruded from an annular die. The wall thickness of the parison depends both on the annular gap setting and on the shear rate of the melt in the die. The melt will swell after the parison exits from the die and the die swell Increases with increasing shear rate. The die swell is also a function of the temperature, the type of polymer, and its elastic melt properties. Usually the shear rate in the die varies from 10 to as high as 700 s . As a rule of thumb one uses a die land length 8 times the annular gap. The parison should normally not be blown up beyond 3 1 (i.e., bottle parison diameter). 

A very important factor in BM is the effective diameter swell of the parison. Ideally, the diameter swell is directly related to the weight swell and would require no further consideration. In actual practice, the existence of gravity, the finite parison drop time, and the anisotropic aspects (the parison has directional properties) of the BM operation prevent reliable prediction of parison diameter swell directly with the weight swell. After leaving the die, the melt—which has been under shear pressure—undergoes relaxation that causes cross-sectional deformation or swell. 

Webbing in handle Parison walls touch when mold closes Wrong parison diameter Align parison closer to handle side of mold Increase die diameter Reduce melt temperature... 

The ratio of the outer parison diameter to the inner diameter of the die. The swell is influenced by polymer nature, die construction, land length, extrusion speed, additives (viz., external lubricants), and temperature. 

Die-swell ratio (extrudate-swelling ratio) n. In extrusion, particularly in extruding par-isons for blow molding, the ratio of the outer parison diameter or parison wall thickness to, respectively, the outer diameter of the parison die or the die gap. The ratio is affected by the polymer type, its temperature, the die geometry, and the extrusion rate. Some writers have defined the die-swell ratio as the ratio of the cross section of the extrudate shortly after emergence to that of the die opening. 

Drawdown m (1) A film of ink deposited on paper by a smooth edged blade to evaluate the undertone and masstone of the ink. Syn Pulldown. (2) A thin film of even thickness on base material cast by means of a drawdown bar. Syn Casting. (3) In extrusion, the process of pulling the extmdate away from the die at a lineal speed greater than the average velocity of the melt in the die, thus reducing extrudate s cross-sectional dimensions. The term is also used by blow molders to denote the decrease in parison diameter and wall thickness due to gravity. 

In their use, consideration must be given as to the anticipated blow ratio, the ratio of maximum product outside diameter to the parison diameter. Normally, ratios in the range of 2-3 1 are recommended. The practical upper limit is considered to be about 4 1. 

Numerical simulation of blow molding -prediction of parison diameter and... 

In practice, the polymer swells as it leaves the die, giving a wall thickness and parison diameter greater than would be expected. Furthermore, the weight of the parison causes the extruded material to sag under its own weight. This reduces the wall thickness. If the sag is too great, the parison may fail when blown or the wall thickness will vary considerably over the length of the parison. These factors will be dealt with in the next chapter. 

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) ... 

One cmrent rule is the depth of the pocket is determined by multiple factors. These factors are plastic density, weight of part and parison, parison diameter, parison orientation, and pre-blow pressme. When the flash pocket is shallow, it causes extreme pressme at the parting line and machine pressure section and it causes difficulties in trimming off the flash. When the flash pocket is deep, it causes poor cooling. The poor cooling can cause a hot flash which softens and weakens the weld line [1]. 

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