Impulse sealing

The gas quality feeding the dry faee seal should be elean and dry. Due to the possibility of eondensation of the proeess gas in the seal eavity, it was deeided to use a seal gas heater. The heater eontrol was set to provide warm gas at 15°C above the dew point to ensure no eondensate entered the seal eavity. Also, a dual filter in series with 5 and 2 p filtration elements was ehosen to provide an ideal sealing environment and maintain the optimum performanee of the seal. To reduee the risk of seal damage during reverse rotation of the turboexpander, programming logie was set to open the eompressor bypass valve whenever a shutdown impulse was initiated. 

Main uses. Caesium metal is used in the production of vacuum tubes as a scavenger to reduce residual gaseous impurities after the tubes have been sealed. Cs may be generated in situ by heating a pellet of caesium chromate mixed with a metal powder (Zr, Ca, Ba). Cs metal is used as the propellant in ion thrusters (employed in satellites for orientation control) it is ionized in a vacuum chamber, the Cs+ are then accelerated through a nozzle (high specific impulse because of high atomic mass). 

Hydraulic separation depends on a process called jigging, which creates a panicle siratifiealion from an alternate expansion and compaction of a bed of panicles by a pulsating fluid How. As originally developed, a baskel tilled with material was moved up and down in a lank filled with water. The more modem Baum jig process utilizes an air impulse concept in which the water is moved hy air pressure front an adjacent sealed chamber There are several refinements of the process, including the McNally Norton standard washer. 

A small block valve is usually installed at a tank on any instrument impulse line just in case a leak develops on the small tubing. In this unit block valves are generally avoided on overpressure or vacuum relief devices. If a block valve that can defeat an overpressure device cannot be avoided, such a valve is chained and locked open or car-sealed opened and a protocol is developed to administratively control accidental closing of such a prime safety feature. 

ISO 472 defines impulse sealing and thermal impulse sealing thus ... 

The heat is applied to the outer surfaces of the materials being welded, and in order to make the weld it must travel through the material to the interfaces. For many applications thermal sealing has been superseded by impulse sealing but the former process is in use still for welding thick sheets of flexible thermoplastics. The production equipment employs a heated wedge placed between the sheets, over which they are moved continuously. 

The essential difference between machines for thermal sealing and for impulse sealing is that, whereas the former employ electrode bars that are maintained at a constant temperature, the latter employ sealing bars which are heated only during the clamping and welding cycle. 

In some applications, impulse sealers are being replaced by machines which employ a continuously heated knife. In such machines, the film is supported across a roller which is coated with a resilient material such as silicone rubber (to which molten polyethylene will not adhere) and the hot knife edge pressed on to it. A strong bead edge seal is obtained. 

Reaction Bags and Irradiation Chamber. Teflon (50-mil FEP Type C) bags of 32 inches X 48 inches were fabricated by impulse heat sealing. The bags were fitted with glass ball joints to connect them to the TEE Teflon sampling lines. 

To create a total wrap, an L sealer or a fold-heat seal is usually required, while in the case of a sleeve wrap, a single longitudinal seal is used. The method of sealing is usually via a heated wire which may be used in conjunction with an impulse and pressure system or an impulse and radiant heat system. 

Sealing can also be achieved by impulse, ultrasonic and RF/HF (radio or high frequency) methods and by cold sealing (pressure sensitive) type materials. See Chapter 11 for fuller details. 

Electrical, e.g. ultrasonic, high-frequency/radio frequency, impulse, induction sealing. Basically a modification of the heat seal process. 

The ratio of film agea to volume for a 250 ml pack (area/volume) was 0.9 cm/ cm. Film strips were also heat sealed together using an impulse heat sealer and immersed into the juice (7 samples per bottle). In the 1 liter cartons only extrusion coated LDPE was used, as received from PKL,... 

Impulse sealing (reprinted with permission from [6])... 

Impulse sealing produces a narrower seal than bar sealers, resulting in a better looking but weaker seal. Maintenance requirements tend to be heavy, since the nichrome wires often burn out and require replacement. A fluoropolymer tape on the jaws, covering the nichrome wire, is often used to keep the plastic from sticking to the jaws, and may also require frequent replacement. 

Why is radiant sealing, rather than bar or impulse sealing, used for spun-bonded polyolefin ... 

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