Counter-rotating screws

The desired self-cleaning function led to the development of the intermeshing twin screw. The counter-rotating screw was discarded, because it tended to get blocked by solids and was a poor mixer attention focused instead on the intermeshing, co-rotating extruder. 

Figure 2.26 Energetic optimization, i.e., "minimal stress" for co-rotating and counter-rotating screws...

Fig. 8 Traditional compounding counter-rotating screw geometry. (Courtesy of Leistritz Corporation.)...

In counter-rotating screws, the roll-off process between the screw crest and screw root and between the screw flanks creates a calendar effect. The shear velocity available to wipe the boundary layers is proportionately lower due to the small relative velocity between the surfaces. 

In counter-rotating screws, the rolling motion of the screws and the resultant calendar effect produces... 

In ICRR the roll-off process between the screw crest and screw root and between the screw flanks simulates the action of a calender. The necessary shear velocity required to wipe the boundary layers is proportionately lower because of the low relative velocity. Counter-rotating screws require greater clearances between them since their mode of action is rather like a two-roll mill, passing material through the nip between them [Schoengood, 1973]. The material is drawn into the roller gap and is squeezed onto the... 

Figure 6.18 Intermeshing counter-rotating screws in a twin-screw extruder.

When two counter-rotating screws are conjugated, there is little or no opportunity for polymeric material that is being extruded to remain in a channel of either screw in the region in which the screws intermesh. This is a result of the minimal clearance of the flight of one intermeshing screw within the channel of the other screw. Screws arrayed in such manner are said to wipe each other because polymeric material that is carried in the chaimel of either screw will be prevented from remaining in that... 

Fig. 4. Counter-rotating screws (a) fully intermeshing (b) partially intermeshing and (c) nonintermeshing (tangential).t...

On counter-rotating screw extruders, screw tips are known to cause hot spots in the melt. This problem has been investigated. Since the temperature has the major effect on the foaming process, this situation can result in circumferential heterogeneity of the parison. Thermal conditioning of the screw tips or appropriate lubrication, reducing the temperature increase of the melt, can solve the problem. 

The intermeshing point for counter-rotating screws acts as a calender nip forcing material through it. Only a small projjortion of the material passes through the nip between the screws, while the remainder is carried along axially in closed chambers. Since the relative speeds of... 

The first technique consists of using a twin screw extruder to mix, formulate, and finally extrude polymer melts according to a principle further described in Section 13.3.3. In the latter case, the single screw is replaced by two counter-rotating screws whose opposite rotation ensures the total homogenization of the material. Such a double-screw machine is represented in Figure 13.8. This technique is particularly well-suited to the continuous production of granulates from extruded tubes. 

PHO was reacted with 0.2, 0.375, and 0.55 wt.% Hexamethylene diisocyanate using a counter-rotating screw torque rheometer (Haake) at 40 rpm for 2 minute at lOO C. PLA and PHO with 0.55 (wt.%) HDI (mPHO) were melt-blended using Haake torque rheometer in the ratios of 100/0, 80/20, 60/40, 50/50, 40/60, 20/80 and 0/100 (wt/wt %). These blending were carried out at 40 rpm for 3 minutes at 175 °C. After blending, these samples were dried in a vacuum oven. The blended samples were prepared using a compression molding machine for various characterizations. 

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