Parison inflation

Blow-molding processes consists of five main operations plastication of the resin, formation of the parison, inflation of the parison, solidification of the part, and removal of the part from the tooling. The best process economics will occur with a part optimized for weight and a minimum cycle time. In order to have a minimum cycle time, the cooling operation must be the rate-limiting step. For the case study... 

Parison inflation is less difficult to model. In general, the parison is inflated very rapidly, and at a predetermined rate such that it does not burst while expanding. An approximate description of the blowing of a cylindrical parison of uniform radius Ri and thickness hi to that of Ro and ho can be obtained by assuming that the flow is planar extension, that the flow is isothermal, and that h/R this instance, the... 

Parison inflation models use a Lagrangian framework with most of them employing the thin-shell formulation and various solidlike or liquid constitutive equations, generally assuming no-slip upon the parison contacting the mold. The first attempts to simulate polymeric parison inflation were made by Denson (83), who analyzed the implications of elongational flow in various fabrication methods, as discussed in the following example. 

Equation E14.2-5 can be solved for any time-dependent or constant inflation pressure to give the radial value as a function of time. For example, if P is constant, the parison inflation time is... 

Petrie and Ito (84) used numerical methods to analyze the dynamic deformation of axisymmetric cylindrical HDPE parisons and estimate final thickness. One of the early and important contributions to parison inflation simulation came from DeLorenzi et al. (85-89), who studied thermoforming and isothermal and nonisothermal parison inflation with both two- and three-dimensional formulation, using FEM with a hyperelastic, solidlike constitutive model. Hyperelastic constitutive models (i.e., models that account for the strains that go beyond the linear elastic into the nonlinear elastic region) were also used, among others, by Charrier (90) and by Marckmann et al. (91), who developed a three-dimensional dynamic FEM procedure using a nonlinear hyperelastic Mooney-Rivlin membrane, and who also used a viscoelastic model (92). However, as was pointed out by Laroche et al. (93), hyperelastic constitutive equations do not allow for time dependence and strain-rate dependence. Thus, their assumption of quasi-static equilibrium during parison inflation, and overpredicts stresses because they cannot account for stress relaxation furthermore, the solutions are prone to numerical instabilities. Hyperelastic models like viscoplastic models do allow for strain hardening, however, which is a very important element of the actual inflation process. 

Schmidt et al. (102) carried out a detailed experimental study of PET blow molding with a well-instrumented machine and compared the results with theoretical predictions using FEM and an Oldroyd B constitutive equation. They measured and calculated internal gas pressure, coupled it with the thermomechanical inflation and performed experiments and computations with free parison inflation. 

PITA parison inflation thinning analysis PO polypropylene oxide... 

Blow molding is complicated by the complex stress field set up in the materials when the parison is inflated. This amounts to a biaxial stretching of the molten polymer and it is difficult to obtain material data under these conditions so that simulation may be performed. Despite this, much work on the inflation stage has been done, mostly with the aim of determining the final thickness distribution. Recently parison inflation has been simulated using three-dimensional finite elements and with remeshing of the parison as it inflates to minimize error from element distortion. ... 

In principle, thermoforming is quite similar to the parison inflation stage of blow molding. A complication is the use of plugs to assist forming. The physics of the interaction between the molten material and the plug is not well understood and is difficult to simulate. As a result, there are some limitations on what can be simulated today. 

Extrusion blow molding is a continuous process capable of high production rates. This process (Figure 8.1) involves three main stages parison formation, parison inflation, and part solidification. It has a number of... 

Figure 6.13 Extrusion blow moulding. Step 2 Parison inflation, (i) Parison in relation to closed mould (ii) inflated parison...

Inflation of the parison is normally accomplished with compressed air, but in some cases the vaporization of liquid nitrogen is used instead. If the air between the parison and the mold wall is not vented as the parison inflates, it will prevent the molten polymer from making uniform contact with the mold surface, resulting in poor surface finish. Slit-type vents may be provided at the mold parting line. 

Figures 6a-b show the extensional viscosity of the branched samples at 180°C. The measurements were conducted at different extension rates and compared to the linear viscoelastic (LVE) envelope determined by a step strain experiment at the same temperature. The two samples show strain hardening characterized by a deviation of the transient viscosity from the LVE envelope. The deviation at all extension rates confirms the presence of chain branching but more importantly shows a relationship between the extension rate and the extent of hardening. This correlation is considered crucial during parison inflation as it allows better control during the inflation of the parison.

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

Initially a molten tube of plastic called the Parison is extruded through an annular die. A mould then closes round the parison and a jet of gas inflates it to take up the shape of the mould. This is illustrated in Fig. 4.21(a). Although this process is principally used for the production of bottles (for washing-up liquid, disinfectant, soft drinks, etc.) it is not restricted to small hollow articles. Domestic cold water storage tanks, large storage drums and 2(X)... 

The convention extrusion blow moulding process may be continuous or intermittent. In the former method the extruder continuously supplies molten polymer through the annular die. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it or, in multi-mould systems, this would have been picked up by another mould. Alternatively in some machines the mould assembly is fixed and the required length of parison is cut off and transported to the mould by a robot arm. 

Now consider the situation where the parison is inflated to All a cylindrical die of diameter, D. Assuming constancy of volume and neglecting draw-down effects, then from Fig. 4.23... 

Example 4.4 A blow moulding die has an outside diameter of 30 mm and an inside diameter of 27 mm. The parison is inflated with a pressure of 0.4 MN/m to produce a plastic bottle of diameter 50 mm. If the extrusion rate used causes a thickness swelling ratio of 2, estimate the wall thickness of the bottle. Comment on the suitability of the production conditions if melt fracture occurs at a stress of 6 MN/m. ... 

The maximum stress in the inflated parison will be the hoop stress, ae, which is given by... 

They are then forced through a narrow die to form a hollow tube called a parison. A chilled mold is then clamped around the parison and inflated from the inside by air. The air pressure presses the parison against the mold, and it hardens in the shape of the mold. The mold then opens and ejects the HDPE bottle. The bottle is then trimmed and conveyed to the milk filling station. The waste plastic is ground for reuse. GHG emissions associated with the embodied energy of the packaging machinery may be calculated but typically fall near the 1% cutoff line and can be excluded (Cashman et ah, 2009). 

The extrusion blow molding cycle is illustrated in Fig. 14.2. The extrusion component of the cycle is normally continuous. As soon as one length of parison has been captured by the mold, another length starts to form. To allow room for a new length of parison to emerge from the die, the mold moves aside as soon it has captured a parison and the knife has severed it. The mold is rapidly translated to a remote blowing station where inflation takes place. After the product is ejected, the open mold moves back under the die where it surrounds and captures another length of parison. 

We can generally extrude a parison much faster than we can inflate, cool, and eject the product. When this is the case, we employ more than one mold. If we are using two molds, they shuttle back and forth alternately between their individual blowing stations and the parison capture... 

When a parison or preform is inflated, it displaces the air around it within the mold. If no provision is made to vent the mold, compression of the air around the parison or preform can raise its temperature to such an extent that it can scorch the surface of the product. To avoid this problem, we equip blow molds with vents. These can consist of slit vents at the parting line between mold halves, porous plugs of sintered metal, or small holes drilled into the cavity walls. 

Extrusion blow molding. In extrusion blow molding, a parison or tubular profile is extruded and inflated into a cavity with a specified geometry. The blown article is held inside the cavity until it is sufficiently cool. Figure 3.56 [25] presents a schematic of the... 

Heat Transfer in Blow Molding Estimate the cooling time of a 15 cm long, 4 cm in O.D., and 0.3 cm thick HDPE parison at 200°C, which is inflated onto a 10-cm-diameter and 15-cm-long cylindrical bottle mold at 15°C by 5°C cold air. Solve the heat-transfer problem involved. Use the p, k, and Cp data given in Appendix A. Assume that the inner surface of the bottle is at 15°C. 

Fig. 14.15 Schematic representation of the blow molding process, (a) The extruder head with the blowing pin and open mold (b) the extrusion of the parison (c) the mold closed with the parison pinched in the bottom and sealed at the top (d) the inflated parison forming a bottle.

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