Machine-direction orienter

Figure 19,7 Machine directional orientation using pairs of nip rollers...

Different methods are used. An online ordinary common blown or cast film line uses a machine direction orienter (MDO) on the front end of biaxially oriented film heated chamber extrusion line. If only the machine direction is to be stretched, a series of precision controlled heated rolls can be used. Film is fed through a series of rolls where it is sequentially heated, drawn around rolls that increase in rotational speed providing the stretching action, annealed around larger diameter roll(s), and cooled on a final roll(s). 

Figure 11.2 A cantilever style MDO (machine direction orienter). Courtesy of Marshall and Williams Plastics...

The mechanical properties of PLA can be enhanced by orientation, generally in machine direction orientation with a draw ratio from 2 to 3, and in transverse direction with a draw ratio from 2 to 4 [178]. Especially toughness... 

MDO machine-direction orienter MIL machine interfece level... 

Dartek 0-401. A machine-direction -oriented nylon type 66 film. l... 

A common multilayer PP product is bi-axially oriented polypropylene (BOPP), produced on a line including a machine direction orientation unit, and a tenter frame for transverse orientation. This process commonly uses a three-manifold die that allows thin skin layers of copolymer to be applied on either side of a homopolymer base layer. The copolymer is used in this product so the film can be sealed, which is a requirement in packaging applications. Typically, the skin layer thickness is 5% of the total film thickness. The adhesive property of the copol3mier makes it difficult for the film to be processed through a tenter frame, because the film would stick to the tenter frame clips. The multimanifold die can be designed with narrower width flow channels for the skin layers, to keep the adhesive out of contact with the tenter clips, allowing the product to be extruded successfully. 

The width of the film reduces during the machine direction orientation. As the gap between the low speed roll and high speed roll increases, this width reduction increases. One approach to minimize the width reduction is to shorten the stretch gap. Another approach is to pin the web against the rollers with high-pressure air (16). 

A second major machine direction orientation process involves a very high pressure nip (see Fig. 6). Input cast webs are cold-rolled or compression-rolled... 

SGqUGntial Biaxial OriGntation. The standard process for sequential biaxial orientation is machine direction orientation followed by transverse orientation. This is a continuous process. The machine direction stretch occurs similar to the process described in the monoaxial section (Fig. 5). However, a second draw station is typically not present in this process. In the pure monoaxial orientation case, the process often tries to maximize the molecular orientation in the... 

Reversing the order of sequential stretching (24) is possible. The orientation in the first stretch is usually limited. By reversing the order, this allows the machine direction orientation to be second and hence greater than if it were done first. 

Heaters will soften the tube and the tube is inflated with air. The initial filling of the tube with air requires good timing by the operator. As air is pumped into the expanding tube, the operator pulls the tube away faster than the top nip supplies cast tube. Once the inflated bubble reaches the bottom of the stretch tower, a second nip closes. This second nip seals the air in the tube. The second nip runs at a speed greater than the first and provides the machine direction orientation. The amount of air pumped into the tube before the second nip closes is one of the primary factors in the transverse direction stretch ratio. Other process variables that contribute to the transverse stretch ratio are the web temperature and machine direction stretch ratio. Pressure in the tube may be increased by narrowing the frame used to collapse the bubble. 

The main difference between the flat film methods is how the clips are accelerated through the tenter. In one commercial process, the clips ride in a screw (30). As the pitch of the screw increases, the clips accelerate in the machine direction (Fig. 11). The clips may be passed from one screw to another. This type of process has large constraints on the machine direction orientation. Once the screw is machined, the machine direction stretch ratio is fixed. Changing from one screw to another can demand excessive time. A machine direction orienter prior to the inlet of the tenter oven may be used with this type of tenter (31). This permutation of this process will allow for changing the overall machine direction stretch ratio without changing the screws. 

The elastic nonwoven possesses unique properties useful to applications such as inexpensive elastic leg cuffs and waistbands for disposable diapers. Elastic CCN has the advantages associated with stretchability, but also has further improved wicking properties compared to CCN before heat stretching, due to the greater machine direction orientation of the cotton and PP fibres, making them ideal for feminine hygiene pads and diapers (Christin, Dong, Lary, 2002). 

The final step is yam stretching or drawing to orient the molecular chains to a high degree in the machine direction. Orientation also reduces the diameter of the fibers. The process is performed at a temperature close to, but less than, the melt temperature by stretching between rolls operating at a speed differential. The orientation process has a critical influence on the properties of the finished fiber. The orientation is set by an annealing step and the continuous filament yam is wound on spools or bobbins for subsequent use in textile operations. 

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