New compounding extruders give more output for your dollar.

Two unusual designs of compounding extruders are newly available in the U.S. from I-PEC USA, Ramsey, N.J. One looks like a twin screw and a single screw joined end to end. The other has a unusually shaped mixing screw inside a helically grooved barrel, so that it acts more like a twin screw. The machines are built by I-PEC in Japan and have been widely tested in production over the last two years. Both machines are said to provide many of the benefits of a twin-screw extruder or continuous mixer but at lower purchase and operating costs.

SINGLE/TWIN-SCREW HYBRID

You might call it a "one-and-a-half-screw extruder." More accurately, it's a twin-rotor continuous mixer coupled with a single-screw discharge extruder - except in this case, the single screw is an extension of the shaft of one of the mixing rotors. As shown in Fig. 1, I-PEC's Model HTM twin-screw kneading extruder has a 20:1 L/D twin-screw mixing section with two "wing-type" kneading segments. The screws are counterrotating and non-intermeshing (similar to a continuous mixer). One of the mixing rotors extends 16D beyond the other as a conventional single-screw extruder that pressurizes the melt to force it through a die. Between the two sections is an adjustable valve that serves to control residence time and mixing intensity. Note also that the longer screw turns slower than the short one, which is said to increase kneading and help control melt temperature.

The HTM kneader has been on the market in Asia for about a year, and around 20 machines have been sold. Several advantages for this machine are claimed by the builder:

* Output is greater than that of a twin-screw extruder, especially with highly filled compounds, polymer blends, and masterbatches (see Table 1).

* Cost of an HTM on a lb/hr capacity basis is less than for a conventional twin screw and is not much higher than for a single-screw mixer.

* Energy consumption is lower than that of a continuous mixer and comparable to a corotating twin screw.

* Maintenance is easy because the single-screw discharge end (where most of the wear allegedly occurs) can be disconnected from the twin-screw mixing section. The whole single-screw barrel section slides sideways on rails to separate it from the twin-screw section.

* Clean-out is also easier than on continuous mixers.

* The machine is more compact than a continuous mixer with a separate discharge extruder.

HTM models are available with 38 to 90 mm screw diam. and max. speeds of 480-600 rpm for the long screw and 550-660 rpm for the short one. Output rates range from 22 to 3300 lb/hr. All models come with wear-resistant screw and barrel, water-cooled barrel, and a force-feed hopper.

ONE SCREW MIXES LIKE TWO

I-PEC's Model IPM single-screw mixing extruder has been available for two years, during which time over 50 units have been sold in Asia. The IPM has a special two-stage screw and barrel design that reportedly provides 1.5-2 times higher throughputs than other single-screw extruders and handles more sophisticated mixing jobs as well (Table 2). Mixing quality is said to be similar to that of a twin screw although at a somewhat lower output rate. Cost of the IPM machine is much less than that of a twin-screw, and so is the cost per unit of throughput capacity (lb/hr). In addition, the IPM reportedly beats a twin screw in energy efficiency, maintenance cost, and compactness. The IPM is said to be very well suited to filler incorporation and making blends such as ABS/PVC and ABS/PC.

The IPM uses the rotor/stator concept in which the barrel itself serves as a mixing element in addition to the screw. As shown in Fig. 2, a special mixing section of the screw is surrounded by a barrel section with semicircular, helical grooves. This design is said to increase the output per screw revolution and improve mixing while maintaining lower melt temperatures. For that reason, the IPM is said to be better than most single screws for compounding heat-sensitive additives like colorants and flame retardants and to provide less resin degradation as well. The grooved barrel is self-cleaning, I-PEC claims.

The machine reportedly gains flexibility to run a wide range of materials by means of a worm gear at the rear of the machine that causes the screw to move axially in the barrel by up to 15 min. Changing the screw's axial position is said to change the degree to which the mixing zone is filled with melt and the amount of shearing that takes place.

IPM extruders come in 40-100 mm models with 36:1 L/D and capability of 380-460 rpm. Nominal throughputs with ABS are 100-3300 lb/hr. Also featured are a wear-resistant screw and barrel with water cooling, Stellite-tipped mixing section on the screw, and force-feed hopper.

TABLE 1 - PERFORMANCE OF 65-MM HTM

Material RPM(a) Lb/Hr Melt, F

ABS pellet, 25% Br FR(b)            500      1100-1540      410-428
PS pellet, 25% Br FR(b)             500      1100-1540      410-428
PP pellet, 40% talc (7-8[Mu])       450      1100-1320      428-446
PP pellet, 60% talc (2-3[Mu])       400       660-880       446-464
PP pellet, rubber,
30% talc (1-2[Mu])                  450      1100-1320      383-401
PP pellet, 60% Ti[O.sub.2]          500       880-990       383-401
LDPE, 45% carbon black              400       440-550       464-482
HDPE, 60% talc (1-2[Mu])            250       440-550       464-482

a Speed of long screw; short screw turns faster.

b Brominated flame retardant.
TABLE 2 - PERFORMANCE OF IPM SINGLE SCREW

Material                            RPM(a)      Lb/Hr      Melt, F

50 mm Diam. Model

PP pellet (0.5 MFR),
30% talc                             380         198         473
PP pellet (0.5 MFR),
50% talc                             380         176         478
PP pellet (2 MFR), 50% talc          516         205         455
PP pellet (2 MFR), 50%
CaC[O.sub.3] (1[Mu])                 350         178         477
ABS pellet, 25% Br FR(a)             400         515         437
ABS powder, 25% Br FR(a)             380         396         428
PS pellet, 25% Br FR(a)              400         440         437

65 mm Diam. model

PP powder, 14% rubber,
10% talc (5[Mu])                     403        1296         397
PP pellet/powder, peroxide,
10% talc (5[Mu])                     404        1177         405
PP pellet (8 MFR),
50% CaC[O.sub.3]                     350        1019         415
Nylon 6 powder, 3% C black           442         623         477
FR-ABS pellet                        444        1232         466
ABS pellet, 5% silicone              401         825         500

75 mm Diam. Model

FR-ABS                               380        1584         433

a Brominated flame retardant.