Thermoforming thick wall

We use plug assisted thermoforming when we wish to make thick-walled products or thin-walled, deep draw products. The principal steps of the process are illustrated in Fig. 16.2. The first two stages are similar to those of vacuum forming. Once the sheet has been clamped, a plug made from a material with low heat conductivity is thrust downwards into the cavity,... 

As the final mold equipment for BM consists of female molds only, it is possible simply by changing machine parts or melt conditions to vary the wall thickness and the weight of the finished part. If the exact thickness required in the finished product cannot be accurately calculated in advance, this flexibility is a great advantage from the standpoint of both time and cost. With BM, it is possible to produce walls that are almost paper-thin. Such thicknesses cannot be achieved by conventional IM, but, with certain limitations, can be produced by thermoforming. Both BM and IM can be succesfully used for very thick walls. The final choice of process for a specific wall section is strongly influenced by such factors as tolerances, reentrant curved shapes, and costs. 

This is applicable for thick-walled parts and multilayer thermoforming [162, 163]. 

If a thermoplastic sheet is softened by heat and then pressure is applied to one of the sides so as to generate a freely blown surface, it will be found that the shape so formed has a uniform thickness. If this was the case during thermoforming, then a simple volume balance between the original sheet and the final shape could provide the wall thickness of the end product. 

Example 4.8 A small flower pot as shown in Fig. 4.56 is to be thermoformed using negative forming from a flat plastic sheet 2.5 mm thick. If the diameter of the top of the pot is 70 mm, the diameter of the base is 45 mm and the depth is 67 mm estimate the wall thickness of the pot at a point 40 mm from the top. Calculate also the draw ratio for this moulding. 

Elden, R.A. and Swann, A.D, Calendering of Plastics, Plastics Institute Monograph, Iliffe, 1971. Rosenzweig, N., Markis, M and Tadmar, Z., Wall Thickness Distribution in Thermoforming Polym. Eng. Sci. October 1979 Vol 19 No. 13 pp 946-950. 

An overview is presented of TrexeTs MuCell microcellular foam extrasion technology and a description is given of its application to the manufacture of thermoformable PP sheet. The results of pilot scale thermoforming trials carried ont to optimise forming conditions and obtain data abont potential cycle time effects, measurements of the densities, flexnral modnlns and wall thicknesses of the sheets and containers made from the sheets and cell stmcture analysis are presented and discussed. 

Research carried out by the Queen s University of Belfast in the thermoforming of PP and PP foam is reviewed. Studies were made of the influence of crystallinity on thermoformability and the nse of chemical blowing agents in the production of foams having lower density and optimum cell stracture. Finite element modelling was nsed in the prediction of wall thicknesses and an optimised process control system was developed. BELFASUQUEEN S UNIVERSITY... 

In the plastication step of the process, melting is critical in controlling cycle time. Also, during thermoforming, the heating of the sheet is the most time consuming step of the whole process. For example, if we consider the melting of an infinite slab, at an initial temperature of To, as presented in Fig. 6.61, the heat supplied by thehot wall, set at a heater temperature of Th, will create a layer of molten polymer of thickness, X (t). 

Derive the equation that describes the thickness distribution after thermoforming the constant diameter tubular geometry described in Fig. 6.75. Use the notation presented in the figure. Assume that the sheet stops stretching once it touches the mold wall. 

By using NatureWorks PLA instead of polypropylene, Autobar was able to reduce the wall thickness of the containers, from 460 microns down to 330 microns. The downgauging allows Autobar to use less material, which helps reduce the production costs, without compromising the quality of the thermoformed container. 

Fabricators will use t]Q as a metric of melt strength for creeping flows. For example, selection of resins with higher values of tj0 should minimize sag in thermoformed parts, and allow a more uniform part wall thickness. Similarly, t and n are metrics of the onset and rate of shear thinning. They are used to identify low-f/(y) conditions (easy flow) in an extruder, which are often critical to fast part fabrication rates. 

Thermoforming shows two important drawbacks. First, the pattern of deformation results in highly uneven wall thicknesses in the finished article. In a cup, for example, the lower brim may be half as thin as the middle or the upper rim. Second, the fact that deformation takes place in the rubbery condition. In this condition, straining is accompanied by chain orientation, which is frozen in upon rapid cooling. When the article is heated to above its Tg, it tends to recover its original shape and deforms, but at lower temperatures it shows a strong tendency to shrink. 

One of the most common problems In thermoforming is nonuniform walls. If a draw is too deep, a corner too sharp, etc., nonuniform thinning may result. A number of thermoforming variations are directed towards alleviating problems associated with nonuniformity of wall thickness. Other thermoforming variations are designed primarily to increase production speeds. Some do both. 

In the blow moulding and thermoforming processes, solidification commences only when the melt contacts the cold mould wall, and flow stops. The time available for melt stress relaxation depend on the thickness of the product, and how close the melt is to the cold mould. There is likely to be high orientation in the thin side walls of a thermoformed disposable cup, made from a highly viscous glassy polymer that has solidified rapidly (hence the heat reversion shown in Fig. 1.16). Hence, if an empty cup is squeezed flat, vertical cracks tend to occur in the side walls, in the direction of orientation. 

The production of technical parts is the other main application for thermoforming. Vehicle dashboards and interior parts, fridge inner liners, bath tubs, sanitary modules, swimming pools, and surfboards can be produced by thermoforming. For large-scale products and products with a significant wall thickness mainly sheet... 

Furthermore, thermoforming is used to produce thin wall parts ectMiomically. The required force to deform a semi-finished product in the thermoforming process is reduced with decreasing thickness, while the shear force in the competing injection molding process increases. 

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