Line Postforming

Information on the technique of forming is covered later in this chapter under Forming. In-line postforming, or postextrusion processing, refers to the special processing that may be done to the extrudate, usually just after it emerges from the die but before the material has a chance to cool. 

OiTTe eriTial pressure sizing unit Vacuum tank 

Blow-molded parts demonstrate that, from the technical and cost standpoints, BM offers a promising alternative to other processes, particularly injection molding and thermoforming. The technical evolution of BM, plus accompanying improvements and new developments in plastics, has led to new BM parts (see Fig. 7-54). With the coextrusion technology now established and the hardware in place, the variety of achievable properties can readily be extended by the correct combination of different materials (see Fig. 7-55). The potential for BM products includes much more than the simple bottles that have been made for many decades. Now the expertise and economics of the method are such that many ideas once deemed futuristic are much closer to realization.  

A significant difference exists between BM and IM. BM usually requires only. 17 to 1.03 MPa (25 to 150 psi) pressures, with possibilities for certain resins of up to 1.38 to 2.07 MPa (200 to 300 psi). For IM, the pressure is usually 13.8 to 137.8 MPa (2,000 to 20,000 psi), and in some cases up to 207 MPa (30,000 psi). The lower pressures generally result in lower internal stresses in the solidified plastics and a more proportional stress distribution. The result is improved resistance to all types of strain (tensile, impact, bending, environmental, etc.). 

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 weight 

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There is off-line (for small quantity of products) or in-line postforming. The in-line refers to forming/shaping the extrudate (tube sheet, etc.) just after it emerges from the extruder die but before the plastic has a chance to cool. It provides specialty products with performance and cost advantages. Upon leaving the extruder s die and... 

Fig. 5-16. In-line postforming with extruder embossing one or both sides with shallow or deep patterns.

Fig- 8-38 Different products that are postformed in line during extrusion. 

This term is used both to describe designs that are etched or engraved in the mold surface and the process of inserting a printed film into the mold, to be produced as an integral component of the finished product. Etched surfaces can be drawn both parallel and perpendicular to a parting line of molds or postforming in an extrusion line.. However, be alert with molds to the fact that parallel to the parting line additional draft is required. A wide selection of patterns is available and new ones can be readily created. 

Different TP pultrusion processes are used. As an example Thermoplastic Pultrusion Technologies (TPT), Yorktown, VA, USA, uses a hot-melt injection process for pultruding RP thermoplastic. Unlike TS pultruded profiles, TP profiles can be postformed and reshaped. Higher continuous use temperatures are possible with some TP matrices, and line speeds are faster with raw materials usually costing less. 

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