The Feeding Hopper

The earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

Many extruder manufacturers now produce dump extruders which are fitted beneath the discharge door of an internal mixer and receive into their feed hopper the full charge of the mixer. The speed of extrusion of the machine is governed by a series of sensors in the feed-hopper, to ensure that the process is continuous and that the screw will not be starved of compound, thus ensuring a continuous production of the mixed rubber compound. Product from such a machine can be slab or pellets. 

The solids conveying device at Dow was built from a standard 63.5 mm diameter extruder. It was built by replacing the standard barrel with a short barrel, providing an L/D ratio of 4.5. The feed hopper casing had an effective barrel length of 1.9 diameters and was water cooled. The feed opening was 1.5 diameters in length ... 

The compression ratio for pellet feedstocks typically ranges from about 1.8 for PMMA resins [2] to 4.5 for LDPE resins [3], although many exceptions are practiced. Compression ratios for several common resins are reported by Giles, Wagner, and Mount [2]. If high levels of a low-density recycle stream are added back into the feed hopper, then the compression ratio and compression rate will need to be increased as follows ... 

Large diameter, melt-fed extruders are commonly used for the final devolatilization and pelletization of LDPE and PE copolymers in resin manufacturing plants. A full description of this type of extruder and process is provided in Section 15.3. Simulation of these processes is complicated by the multiple flights used in the design and the high H/W aspect ratios of the channels. The processes can be simulated from the feed hopper to discharge, however, since they are not required to convey solids and melt resin. This section will show the requirements and difficulties for simulating these processes. 

The source material is supplied from tank (2) to the feed hopper (3) of mixer (4). The feed hopper serving to continuously feed a homogenizing screw of mixer (4) comprises a vertical agitator with blades in its upper portion to prevent sticking of the product on the hopper walls. Arranged in the lower portion of the agitator is a separable screw... 

The angle of the feed hopper is small so that a continued flow of solids to the standpipes can be established without creating a stagnant region in the hopper. 

For a standpipe system, different flow patterns of steady-state flows may exist, depending upon the ranges of operational parameters of the system. This phenomenon is commonly known as the steady-state multiplicity. We introduce the general concepts of the steady-state multiplicity based on the standpipe system in 8.3.3, in which the solids in the feed hopper are kept in a steady moving bed motion. 

If a twin-screw extruder is stopped and opened, several zones can be clearly distinguished [9]. Depending on whether the extruder is fed with a solid or a liquid material, two different situations occur. In the case of a solid feed (a polymer or solid monomer), the channels near the feed hopper are more or less filled with solids. This material melts, resulting in a zone where the channels are only partly filled. At the end of the screw, close to the die, a zone exists where the channels are completely filled with polymer. If the extruder is fed with a liquid monomer, the first part does not necessarily need to be partly empty but, as will be explained later, for reasons of stability it is advisable to create a zone where the channels are not fully filled. In particular, the fully filled zone is very important for correct functioning of the extruder. In this zone the pressure is built up, the mixing and kneading mainly... 

The feed section takes the product from the feeding system and conveys it to the plastification section. At the same time, there must be sufficient free volume to allow gas reflux, e. g., air or nitrogen, to escape from the process section upstream. If the throughput rate of the extruder is less than the input product flow, the product will back up in the feed hopper, indicating that the intake limit has been reached. Along with the known extruder data, such as the screw speed, the screw pitch, and the available volume in the screw channel, other influencing factors, such as the fill rate of the screw channel, the conveying efficiency, the bulk density, and other bulk characteristics of the product sometimes affect the... 

Solids conveying is carried out in two steps the feed hopper and the back (entrance) portion of the screw. The feed hopper is an inverted cone or pyramid, in which solid pellets or powder flow downward from the force of gravity. If they flow poorly ( arcing or bridging ), the problem may be solved by installing a vibrator or a stirrer ( crammer/feeder ) in the hopper, or by coating the particles with a small amount of a chemical that acts as an external lubricant. ... 

The action that pushes solid particles forward in the feed section of a single screw extruder, blow, or injection machine has always been a potential for one of the weakest features of these machines. This forward feeding force near the feed hopper is often weak and erratic and is classified as non-positive. It can be so tenuous that a specific screw/barrel combination will feed virgin but little or no additions of regrind, or one feedstock shape but not another, and often one family of plastics but not another. This action results in non-uniform feed that will in turn result in poor production rates, non-uniform output (surging), and poor product quality.147... 

In the oscillating hopper sample divider [23] ](Figure 1.29), the feed hopper is pivoted about a horizontal axis so that it can oscillate while emptying. Two collectors are placed under the hopper outlet so that the powder falls into them alternately so that at each step the sample is halved. The contents of one box are retained so that at each step the weight of the sample is halved. The oscillating paddle sample divider (Figure 1.30) works in a similar way. 

Screw-presses by other manufacturers employ different means to facilitate feeding the high-pressure chamber of the press. Some use two-speed shafts, employing a quill worm driven by separate gearing that allows the feed worm on the inlet end of the shaft to revolve faster than the rest of the worms. Others use larger diameter barrels with a deeper channel (clearance between the barrel and the hub of the shaft) at the feed end. Sometimes star wheels are used to trap the incoming material within the barrel so it cannot back up into the feed hopper. A star wheel is a... 

During a continuous extrusion process, the feed stock is required to have good flow properties inside the hopper. For the material to demonstrate good flow, the angle between the side wall of the feeding hopper... 

A specially designed compression feed screw and upper arm assembly are shaped to closely conform to the inner wall of the feed hopper. The feed screw (usually non-cylindrical) has uniquely pitched edges. The feed screw design features and gravity aid to drive the powder-stock into the compaction area. There is no vacuum deaeration system. Predensifica-tion and deaeration occur in the screw feed region during operation. 

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