Introduction to Polymer Science and Technology

Polymer processing

machine direction (MD)

die diameter (DD)

Figure 3.31Description of a section of bubble process

Introduction to Polymer Science and Technology

Polymer processing

Typical process parameters include:

The blow-up ratio (BUR) = (BD) / (DD). It is simply the ratio of the final diameter of the blown tube relative to the die diameter, typically 1.5-4. The outside die diameter can be up to 2.5 m. Increasing the BUR is equivalent to working at a higher frost line height.

Frost line (FL) height is the height above the die lips where the polymer crystallizes/solidifies. Therefore, as the temperature of the melt increases, the FL rises further up the bubble. At a given melt temperature, a high FL allows a greater relaxation of surface irregularities, resulting in a glossier and less hazy film. However, too high a FL results in slow cooling and promotion of spherulites/haze. - The bubble ratio (BR) = (lay flat) / (DD)

The ratio of the die-land to die gap varies according to material type or grade. Typically a gap of 0.6 mm for

thin films and 1.6 mm for construction films. The die land is a compromise between the length required for a

satisfactory melt relaxation and the length still enabling an acceptable pressure drop.

The draw down ratio (DDR) is the ratio of the final film gauge to the die-gap opening. It controls the molecular

orientation in the MD.

The machine-direction draw ratio (MDDR), another strain related parameter associated with the film blowing

process, is the ratio of the take up speed relative to the extrudate speed at the die.

The shrinkage of a blown film is affected by the deformation of the melt in the shaping die as well as by the blow-up and the take-off conditions after it emerges from the die. It can be much greater during the formation of the bubble than in the die, particularly if the blow-up ratio is high. Another problem in blown films is the entrapped air between film layers and rollers - this may cause film scratching or wrinkling, or processing problems when winding up the film due to reduced friction. One of the solutions is to use vacuum to remove entrapped air.

Large amounts of blown film are used as geomembranes (Figure 3.32). The most common polymers used for the production of membranes for ponds, lakes and lagoons are HDPE, LDPE, PP and PVC. Figure 3.33 shows an alternative application for these films - a wonderful sight to behold for sure!

Introduction to Polymer Science and Technology

Polymer processing

Figure 3.33Beauties of Ballyclare, N. Ireland

3.2.2.2 Cable coating

In cable/wire coatingbare wire or a bundle of wires that are previously insulated is fed perpendicular to the melt extrusion direction through a mandrel that splits the melt into two. The extruded polymer melt and the wire come in contact within the die, which is a cross-head die.

xtruder

Figure 3.34Crosshead extrusion arrangement for wire coating

3.2.2.3 Cast film/sheet extrusion

In cast film extrusion,the melt is extruded from a slit die onto a chill roll to produce thin film, and in sheet extrusion,to produce precision thin sheet/film, the extrudate passes over and through a series of rolls (Figures 3.35 and 3.36).

Introduction to Polymer Science and Technology

Polymer processing

air knife

chill roll

Figure 3.35Illustration of cast-film process

three-roll stack

Figure 3.36Illustration of sheet extrusion

Sheet extrusion can be set up on-line with thermoforming equipment for the production of various packaging products, particularly for the food industry. Such production lines also include labelling and/or printing facilities, and facilities for the treatment of the plastic surfaces for printability. These include corona-discharge and flame treatment processes that change the polarity and/or the chemistry of the polymer surfaces and hence enhance ink adhesion. The products include various tubs for yoghurt, coleslaw, margarine, and cups for hot and cold beverages. Many thermoplastics can be thermoformed; they include PS, PP, PET, PVC and ABS. In food packaging ethylene-vinyl alcohol (EVAL) is included in multilayer co-extruded films for its superior barrier properties.

Co-extrusionsare commonly used to provide precise properties for specific applications. Multi head extruders feed into the extrusion die with the differing materials, e.g., a 5- layer sheet may consist of inner layer-tie layer-barrier layer-tie layer-outer layer construction. The microstructure of some of the polymers will determine their suitability for certain applications, e.g., amorphous PET sheets are easily thermoformed into containers with excellent clarity but are not sufficiently rigid as packaging trays for ready meals to be heated. Whereas crystalline PET, contain nucleating agents, crystallise readily under heat during thermoforming. Crystallinity renders the tray rigid and therefore prevents it from deformation when being heated and served.

Introduction to Polymer Science and Technology

Polymer processing

Extrusion-thermoforming produces a significant quantity of scrap (it can vary from 15 to 50% of the original sheet depending on the shape of the blanks stamped out for thermoforming). Almost all the skeletal (scrap) can be reused. The chopped and regranulated skeletal is mainly used again (mixed with fresh granules) in the same production. However if this is not acceptable, for reasons of colour matching or contamination, then it can be used as feed stock for other types of productions.

3.2.2.4 Mesh extrusion

Mesh/netting extrusioncan be produced by extrusion of strands through two concentric-ring dies with slots that can be rotated or oscillated with respect to one another in the modes shown in Figure 3.37 for "Netlon" products. During extrusion, when the slots of the inner and outer rings are in register the knots of the mesh are produced, and when they are out of register the strands of the mesh are produced.

outer ring

Figure 3.37"Netlon" process alternatives using a concentric-ring die: mesh with (a) inner ring rotating, (b) outer ring stationary and inner ring oscillated, and (c) rings counter rotating

Date: 2015-12-11; view: 953