Continuous extrusion plastics

Production rates are high but are dependent on size. Continuous lengths up to 60m/min for some tube sections and profiles, up to 5 m/min for sheet and rod sections. 

Extruders are often run below their maximum speed for trouble free production. 

It can have multiple holes in die for increased production rates. 

Extruder costs increase steeply at the higher range of output. 

Lead times are dependent on the complexity of the 2-dimensional die, but normally weeks. Material utilization is good. Waste is only produced when cutting continuous section to length. Process flexibility is moderate. Tooling is dedicated, but changeover and setup times are short. 

---

ROTATIONAL MOULDING [2 8] CONTACT MOULDING [2 9] CONTINUOUS EXTRUSION (PLASTICS)... 

CC Continuous extrusion (plastics) process capability chart. 

The success of any continuous extrusion process depends not only upon uniform quality and conditioning of the raw materials but also upon the speed and continuity of the feed of additives or regrind along with virgin plastic upstream of the extruders hopper. Variations in the bulk density of materials can exist in the hopper, requiring controllers such as weight feeders, etc. 

Profiles, was recently opened to make plastic lumber profiles using a proprietary, continuous-extrusion process developed in-house. 

Figure 7.81 Schematic diagram of rotating mold system used in continuous extrusion blow molding. Reprinted, by permission, from A. B. Strong, Plastics Materials and Processing, 2nd ed., p. 489. Copyright 2000 by Prentice Hall, Inc.

Fig. 14.16 Schematic view of (a) continuous extrusion blow molding (b) reciprocating screw blow molding and (c) ram-accumulator blow molding. [Reprinted by permission from Modem Plastics Encyclopedia, McGraw-Hill, New York, 1976-1977.)...

Due to the plastic s viscosity, a melt shear effect is developed throughout the process. This shear is of prime importance between the calender rolls. The calender forms the web as a continuous extrusion between the rolls (Chapter 1). Unlike when processing just through a conventional extrusion line, the plastic mass cannot be confined when being calendered. Because of the lack of confinement, the shear effect and a broad melt band are essential aspects of calendering. 

From packaging materials, through fibers, foams and surface coatings, to continuous extrusions and large scale moldings, plastics have transformed almost every aspect of life. Without them much of modem medicine would be impossible and the consumer electronics and computer industries would disappear. Plastic sewage and water pipes alone [see Figure 1-24] have made an immeasurable contribution to public health worldwide. 

The quasi-continuous extrusion of plasticized materials, mostly mixtures of steam-conditioned animal feeds and binders, such as molasses, through bores in differently shaped dies by press rollers (see Figure 180), was developed during the first part of this century. Today, the process, which is called pelleting, is used for the manufacture of cylindrical extrudates from many pasty, plastic, or deformable materials. 

Corrugated plastic can have a double-faced or single-faced construction. The double-faced construction has two linerboards separated by a fluted center. The single-faced construction allows one to form tubes or to shape the material around most cylindrical or round products. Profile plastic is produced as a continuous extrusion with two outer linerboards separated by a vertical flute. Inhibited corrugated and profiled plastics are used for boxes, dividers and partitions between metal surfaces. 

Like the extrusion of metals, the extrusion of plastics involves the continuous forming of a shape by forcing softened plastic material through a die orifice that has approximately the geometric profile of the cross-section of the work. The extruded form is subsequently hardened by cooling. With the continuous extrusion process, such products as rods, tubes, and shapes of uniform cross-section can be economically produced. Extrusion to obtain a sleeve of the correct proportion almost always precedes the basic process of blow molding. 

A survey of extrusion heads for fabrication of profiled plastic items and a study of the rheological performance of melts of polymers in channels when ultrasound was used, was carried out [1]. The construction of a many stream extrusion head for the fabrication of continuous extrusion using ultrasonic oscillations has been produced [2]. 

FIGURE 2.32 Continuous extrusion blow molding, (a) Rotating horizontal table carrying mold sets, (b) Continuous vertical rotation of a wheel carrying mold sets on the periphery. (After Frados, J. ed. 1976. Plastics... 

The continuous extrusion of plastic netting started in the 1950 s (26). Extruded netting is netting in which the strands are extruded from a die, the joints being formed either within the die or immediately outside the die. A variety of configurations are known, such as square, diamond, twiU, etc. Common materials used for the preparation are PP, PP, and poly (amide). 

FIGURE 1.33 Continuous extrusion blow molding with parison transfer. The transport arm cuts the extruded parison from the die head and lowers it into the waiting mold. (After Morgan, B.T., Peters, D.L., and Wilson, N.R. 1967, Mod. Plastics 45, lA, Encycl. Issue, 797.)... 

Westover [94] designed a continuous ram extruder that combined four plunger-cylinders. Two plunger-cylinders were used for plasticating and two for pumping. An intricate shuttle valve connecting all the plunger cylinders provides continuous extrusion. 

Goldsworthy, W. B. (1979), New technology for continuous reinforced plastics processing It s called Pulforming , and it permits extrusion of variable cross-section parts, curves, or both . Modem Plastics, 56, 82-83. 

Fig. 39. Bekum BM04D continuous extrusion blow molder for the manufacture of biax-ially oriented PVC bottles. Production rate for 1-L bottle is 2000 per h maximum bottle size is 2 L. Courtesy of Bekum Plastic Machinery, Inc.

The compression properties of amorphous PLA foamed in a continuous extrusion process using CO2 as a blowing agent are summarized in Table 17.12. Correlations were found by plotting the compressive modulus E) and stress at yield (tr) as a function of CO2 content. The decreased modulus and strength with the CO2 content is due to the high level of plasticization induced by the residual CO2 for specimens foamed at higher CO2 content [73, 74]. 

The constant pressure of the continuous extrusion enables the manufacture of materials with different viscosities and compressibilities into an optimal plastic tube laminate. In the accumulator head system (intermittent parison generation), pressure differences of materials with different viscosities and compressibilities would lead to layer thickness problems. 

The orifice angle of the cone can signiflcantly influence the flow anomalies that are frequently found with high molecular polyethylene (PE). In such cases, changing the melt temperature has little effect. Computer programs are available to calculate the distribution of shear rate in the extrusion flow channel. It is important to ensure that the critical shear rate occurs near the die exit resp. is confined to as narrow a region as possible. With the accumulator head, the effects of flow anomahes can be influenced by the extrusion velocity. To achieve the shortest cycle times with continuous extrusion, the plastic usually exits from the orifice at close to critical shear rate. 

---