Open nozzles

During the processing of amorphous plastics and polyolefins, there is an additional risk of fusion or deformation of the moulded item from the hot nozzle face. 

Gate diameters may be increased according to need. The gate insert may be supplied with a surplus length, which enables it to be adjusted to a sloping or shaped mould 

In the case of open nozzles with an insulating chamber, the nozzle casing is separated from the gate by an insulation gap, which becomes filled with melt during the first injection. This insulation enables quite a large temperature difference to be maintained between nozzle channel and gate. 

A partial-body nozzle does not leave a round witness ring from the nozzle face on the moulding, this is an essential condition of use for many products. 

There are many design variants for open nozzles for specialist applications. 

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Personnel Hazards The following case history illustrates how vacuum can be harmful and dangerous to personnel. A plant superintendent was checkiug an open nozzle on a glass-lined reactor on which there was a vacuum pump pulling vacuum, when suddenly his arm was sucked into the nozzle, up to his shoulder. He could not remove his arm until help arrived to release the vacuum on the vessel. He was injured painfully, though not seriously. The injury could have been veiy serious if help had not been nearby. Personnel hazards can also result from vacuum conveyor systems for solids handling. 

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

Maintenance errors. One common error is the failure to remove an isolation blind in a vent line when returning a vessel to service. Even a thin sheet of plastic placed over an open nozzle may be sufficient to allow a vessel-damaging vacuum to De produced (BP, Hazards of Trapped Pressure and Vacuum, 2003). 

Nonatomizing reverse-jet froth scrubbers using large pluggage-resistant open nozzles allow effective use of higher contacting power to efficiently collect finer dust particles, as described in U.S. Patents 3,803,805, 4,056,371, and 4,057,404. Such scrubbers are provided by... 

Every low-pressure, closed-top tank with an overflow line should be equipped with some type of device to prevent collapse from an overfilling operation. A large open nozzle, a high-level alarm and/or shutdown system, vacuum breakers or separate vent lines are some appropriate safeguards. 

According to the engineering drawings, a 6-inch (150 mm) diameter open nozzle was originally considered the tank vent. Over the seven years of operation, this 6-inch flange... 

An open nozzle can be the primary relief device and should be treated with the same respect as a safety relief valve. However, in the previous two incidents as well as this one, open vents are not always treated with the same respect as safety relief valves. No competent process foreman or chemical process operator would dream of asking a fitter to replace a relief device with a smaller one without requesting that calculations be performed. [7]... 

Strainers are the best defense against nozzle plugging and pump wear. Nozzle screens should be as large as nozzle size permits however, the screen opening should be less than the nozzle opening. Nozzle catalogs specify the proper screen size for each nozzle. Figure 8.16 illustrates strainers, filters or screens. 

Shut-off nozzles cannot be used with certain highly sensitive molding compounds because material could accumulate and decompose at the shut-off point. An open nozzle is used to process molding compounds of this type (PVC, POM) or to minimize the cost of machinery in general. It is impossible to completely prevent the escape of material through an open nozzle. This must be taken into account in the feed operation (by decompressing at the end of the feed phase). 

Tlie use of a shut-off nozzle is not recommended due to high pressure losses and the risk of degradation the use of a shut-off nozzle also results in an increase in injection cylinder pressure and, an increase in water absorption. An open nozzle, together with decompression, is suggested cellulosics will not drool if properly dried and then processed at the correct temperature. 

The screw configuration is as follows thread depth ratio, approximately 1 2, and L/D ratio, 17/1 to 23/1 (standard three-zone screws feed, transition or middle, and metering or feed zones). Screws are equipped with a nonreturn valve to prevent backflow. Decompression-controlled injection-molding machines have an open nozzle. A short nozzle with a wide bore (3-mm minimum) is recommended in order to minimize pressure loss and heat due to friction. Residence time should be as short as possible, and this is accomplished with barrel temperatures at the lower limits of recommended settings. ... 

Accoiding to the engineering drawings, a 6 in. (150 mm) diameter open nozzle was originally considered the tank vent. Over the seven years of operation, this 6 in. flange had been blinded and an unused 3 in. (75 mm) nozzle served as the vent. No one knew when this one-minute modification had occurred. Experience elsewhere in this plant with many similar products had established that a 3 in. vent hole could block sohd with this wax, but it was most likely that a 6 in. nozzle would not clog 18] (see Fig. 8.20). 

A general open nozzle type of injection unit can be used for injection molding of LCP. However, a shutoff nozzle is very effective in preventing dripping down of polymer. 

For all openings, however, nozzles may not be necessary. In some cases we have nozzles and piping that are attached to the openings, while in other cases there could be a manway cover plate or a handhole cover plate that is welded or attached by bolts to the pad area of the opening. Nozzles or openings may be subjected to internal or external pressure, along with... 

A nozzle will help the material to get pumped in the mold there are different types such as shut-off nozzle, etc. Both open nozzles and needle shut-off nozzles can be used. Open... 

Decompression Injection and blow molding machines are fitted with decompression or suck-back. After the screw has finished rotating, it is drawn back so as to suck material away from the nozzle tip. This facility allows the use of an open nozzle. Keep the amount of suck-back as small as is practicable as the introduction of air can cause problems with some materials, for example, with PA 66 where processing temperatures are high. 

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