CASTING PROCESS AND SAND MOULD PROVIDED WITH A GATING SYSTEM FOR PRODUCING AT LEAST PARTLY THIN WALLED ALUMINIUM CASTS WITH SAND MOULDING TECHNOLOGY BY MEANS OF GRAVITY CASTING

Abstract

The subject of the invention is a process for the production of at least partly thin-walled and aluminium castings with sand moulding technology by gravity casting, which allows producing casts with 100 times or favourably 200-400 times larger overall dimensions in case of 1-3 mm wall thickness. The main idea of the process is that sand mould containing mould cavity is provided, melt of aluminium content is produced, the melt is introduced into the mould cavity at several points through a gating system of narrowing cross section. A further subject of the invention is a sand mould fitted with a gating system to produce at least partly thin-walled castings with sand moulding technology, by gravity casting. The wall thickness of thin-walled segments is 1-3 mm and the largest dimension is more than a 100 but favourably at least 200-400 multiple of the wall thickness. The main idea behind the sand mould with a gating system is that it contains a mould cavity allowing the production of at least partly thin-walled castings, and is equipped with a gating system, which is composed of at least two sprues and one ingate to each having a porthole into the mould cavity and in liquid contact with the sprues.

Description

The subject of the invention is a casting process to produce at least partly thin-walled aluminium castings by gravity casting and with sand moulding technology. The wall thickness of the thin-walled part is 1-3 mm.

A further subject of the invention is a sand mould and an inlet system (gating system), which will allow manufacturing cast parts of this nature.

In order to produce aluminium casts, sand moulding techniques have been applied for a long time. The basic principle behind this technology is that a die cavity adjusted to the casting's geometry is made from suitable moulding sand. This cavity is then poured with molten aluminium according to the required temperature and other casting conditions. The moulding sand will then be removed from the solidified casting, which will be used for the specific purpose. The sand moulding technology offers the benefit of producing the sand mould in a relatively simple way; its drawback, however, is that a new sand mould is needed for each casting procedure, which requires producing a sand mould for every single casting, thus the mould cannot be reused. Consequently, the sand moulding technology has so far been applied for the manufacture of individual or small range castings. During the casting process the molten aluminium in the mould will spread under the effect of gravitation and will fill in the casting mould. This operation is called gravitation casting. Gravity casting is important as the crude or chemically bonded mould cannot bear high mechanical burden, which means that high pressure cannot be applied. The sand moulding gravity technology having been applied so far does not allow manufacturing thin-walled castings with dimensions larger than 50-100 times of the wall thickness.

The method to be applied in producing thin-walled castings of large surfaces (dimensions) is the high-pressure die casting process, in which the molten metal is fed by means of a pressure casting machine in a short period of time (0.01-0.05 s) and at an extremely high flow rate (20-80 m/s in the conduit) into the cavity of the cooled (cooled in a cooling system to 150-250° C.) where during the solidification process extremely high pressure (500-1500 bar) is applied. The casting machine and the casting mould are complex and costly, which makes them cost-effective only if large series of castings (several ten or hundred thousands, depending on the casting mass) are manufactured.

CN 1709612 Chinese patent description provides information on the process of manufacturing super-thin walled aluminium casts by means of high speed pressure. This process includes the following steps: a model casting mould is fitted into the die-casting machine then the parameters of die-casting are set. The casting pressure is 780 kg/cm², the temperature of the model casting mould is set to 250° C., the dissolvent temperature is 700° C., then the alumina alloy is injected into the die-casting machine. In order to achieve smaller than 1.0 mm wall thickness, 0.23 m/sec injection speed is applied. This is followed by removing the cast, which will be tested if it fulfils the international patent requirements. A casting produced in this way can primarily be applied in 3C products, such as for computer frames, digital cameras and mobile phones.

The conventional sand moulding technology and the pressure technology greatly differ from each other.

The conventional sand moulding technology is a complicated procedure, consisting of several steps, compared to high-pressure casting, in which almost the complete manufacturing process is performed by machines in order to achieve high manufacturing series.

The production of castings with low wall thickness, in the high-pressure casting process, is carried out with very short casting time and at an extremely high flow rate, which cannot be achieved through the gravitation casting process.

Other casting methods are also known, in which e.g. centrifugal force is applied to cope with the problems of filling up the mould, however, these processes cannot be applied in manufacturing large-sized, articulated castings. All the above suggests that gravity casting has physical limitations related to the casting value and to decreasing wall thickness of a casting to be produced and to increasing its dimensions.

All this suggests that the gravity sand mould casting process is applied in making casts with wall thickness of above 3.5 mm and with limited surface size related to the wall thickness (maximum 50-100 times larger).

Comparison as Relates to Foundries

The trends in casting technology, such as high-pressure—gravity—sand mould die casting—are becoming more and more specialised. There is no connection between each technology, they are getting increasingly separated, more and more complex technical solutions must be performed, and no technological fault is affordable, or at least at very high cost.

The foundries applying different casting processes tend to develop their own technologies in order to avoid risks which require modelling to make their product effective. Modelling has two different methods. One of them is virtual modelling, which is made by most foundries applying pressure die casting. A program related to the given technical parameters models the casting process and highlights the expected difficulties of casting. This method offers the advantage of quickness, but its drawback is that it will not provide evidence of the behaviour of the mould in real operating conditions.

The other modelling process is sand casting, which means that the actual casting is produced by a sand casting foundry. The casting produced with this method is like the original one as regards its geometry and structure.

In this case the piece allows measurements, helps design the technology, and draws the attention to the technological demand (e.g. running and feeding systems, etc.)

Its drawback, however, is that the process is slow and costly as compared to virtual modelling.

Comparison Regarding the User

Currently the users' demands offer more and more requirements as well as the testing phases are separated.

It is a long time since modelling a product prior to mass production became accepted in car manufacturing; currently it has been the case in other industries as well. This method serves different purposes. Firstly, a prototype is made for different purposes (mechanical, assembly, operation under plant conditions, etc.); however, it can also be used for selling, presenting and marketing purposes.

Nevertheless, it is not all the same for what purposes the product is to be used (pressure, gravity or sand) and what kind of technical content the given cast has.

A given product or casting has basic properties that determine the technology to be applied; these are as follows: expected annual product number, dimensions, raw material, geometry, weight, and prescribed mechanical properties.

Once the customer has decided to employ test manufacture, he has the choice between two ways. One of them is to order the product with testing according to the final technology, which (concerning our case) is basically pressure die casting. The customer orders the pressure die and the casting technology, and waits for it to be prepared. Once the die has been produced, test casting is ordered, and providing it is based on a drawing or a model, the testing phase will be started.

This method is time consuming and costly, and includes a lot of uncertainties and risks on the customer's part until the test results meet the requirements.

Any development by a customer is not allowed to be mass produced without previous physical testing.

There are cases when the ordered product development cannot be modelled by cheap and fast alternative technology because the cast contains technical parameters that are casting technology specific.

In this case the customer will find other alternative solutions such as the piece being manufactured from pig or a plastic model being printed for demonstration purposes.

Until recently no sand die technology has existed which would enable producing prototype pieces of casts designed for pressure casting technology, which includes the following requirements related to the cast: 1-3 mm average wall thickness (this can be even 1 mm walls in certain parts), higher accuracy of the casting size, as well as maintenance of all these in case of large sized casts.

It has been recognised that the most important difficulty is that the above requirements must be fulfilled in a way when there is no die with the required heated, machine-powered closing force, thus the melt is to be fed into the die under atmospheric pressure.

In so far as a stable and technically identical sand cast can be produced by fulfilling the requirements but by neglecting the pressure technology, then an alternative technology has been provided for producing thin-walled casts by means of sand die casting technology. It must be stated that the term technically identical means that the material structure of the casting is different.

Specificity (related to what kind of cast designed for pressure casting requires replacement) is always determined by the requirements of the casting.

To produce a cast designed for pressure die-casting technology with sand moulding technology in which the average wall thickness is 4 mm or larger, the geometry is simple, or tolerated dimensions can be formed and upheld by machining creates no difficulty.

The cast produced in this way will ensure safe and quick testing and assembly according to the customer's demand, provides possibility for the introduction to the market, provides low level of producing costs (related to the cost level of pressure die-casting technology); what is even more important, it ensures quick and cost-effective modification and execution of possible design faults.

This process allows the developer-customer to provide such safe products for mass production, which will not carry technical and design risks and can be adjusted to market competition as related to both quickness and cost efficiency.

The objective of developing the process applied in the invention is to establish such a casting process, which is suitable for producing articulated, thin-walled aluminium casts by means of gravity sand mould casting supposing 1-3 mm wall thickness and 200-400 times larger sizes are provided.

It has been recognised that the process applied in the invention allows manufacturing larger sized cast parts as compared to the well-known process of gravity die casting. In this way the process is suitable for manufacturing cast parts, which can be applied in practice. The products are mainly indoor and outdoor luminaires, engine parts—spare parts, cylinder-heads, machine components, spare parts for mechanical and precision engineering, fittings, etc. All these products can be produced through this invention in such quality that they will be suited for practical application under operational conditions. This will result in cost-effective development and more effective testing before the manufacture on large scale starts. Aluminium casts for different purposes are to be produced with this process economically, on small to medium scale, even in several hundred items.

This process enables manufacturing larger sized cast parts at a much lower cost—in contrast to pressure casting—which will appear in the initial die cost and production time.

The subject of the invention is the process included in item 1 of the patent claim and the sand mould fitted with an gating system according to item 10 of the claim.

Certain Particularly Favourable Forms of the Invention are Determined in the Sub-Items

Further details are described in examples through figures.

FIG. 1a shows a rough perspective sketch of the inner structure of a sand mould fitted with a gating system according to the invention.

FIG. 1b shows a rough cross sectional sketch of the sprues of the sand mould fitted with a gating system as regards FIG. 1 a.

FIG. 1c shows a perspective image of a casting produced by means of a sand mould.

FIG. 2 shows another rough perspective sketch of the inner layout of the sand mould with a gating system.

Materials, tools and terms applied in the invention:

‘Aluminium casting’: a casting part which is made of Aluminium or Aluminium alloy.

‘Aluminium alloy’: generally ‘silumin’ alloys specifically (ENAC or other) alloy groups according to the patent such as AlSi12MgTi, AlSi7Mg, AlSi10Mg, AlSi9Cu (these are mainly used in the technology relative to the invention, but in certain cases other Al alloys can also be used).

‘Articulated, rich in details’: alloy with complex geometry in which in a given case stiffening ribbing is applied.

‘Thin-walled’: 1-3 mm average wall thickness of the alloy, the size is larger than 50 times or a 100 times of the wall thickness and change in the wall thickness will not exceed 50% of that.

‘Pressure casting’: A casting process in which liquid metal is poured by a casting machine during extremely short times (0.01-0.05 m/s) and at a very high flow rate (20-80 m/s in the channel) into the cooled (150-200° C.) mould cavity where extremely high pressure is applied while the metal is solidified.

‘Gravity casting’: A casting procedure in which liquid metal is poured into the mould cavity by gravity energy under atmospheric pressure. This casting technology operates by the principles of communicating vessels in which no further energy (e.g. centrifugal force effect) aids the molten metal fill in the mould cavity.

‘Sand mould’: refractory sand of 0.2-0.4 mm grain sizes (typically quartz sand but other kind of artificial sand can also occur) hardened with organic or non-organic bonding agent system in cold state (with chemical bond) or by heat (under the effect of being heated)

‘Sand mould casting’: pouring liquid metal into any kind of sand mould.

With the sand mould fitted with a gating system and by the process to be introduced in the following, such—at least partly—thin-walled aluminium castings can be produced by gravity casting, in which the wall thickness of one or more parts is 1-3 mm and the largest dimension is multiplied by 100 times, or even 200-400 times compared to the wall thickness.

The largest dimension means the largest linear dimension of the given part of a cast, i.e. the longest side of the smallest prism which can involve the given part of the cast.

In FIGS. 1a and 1b the inner structure of the sand mould (12) fitted with a gating system (10) is shown. In FIG. 1 a the outer edges of the sand mould are shown only as illustration around the inner formation in a perspective view.

The sand mould (12) includes an upper half (12a) and a bottom half (12b), which are joined in a joint surface (13) and these two parts form the mould cavity (16). In the present structure the mould cavity (16) is completely thin walled and provides casting parts (14) with 1-3 mm wall thickness, which is also separately shown in FIG. 1 c. In FIG. 1 c the cast part is not separated from the complete casting (14′), which means that the solidified parts of the melt in the gating system (10) are joined to the casting part (14), which can be separated, e.g. by cutting them off the casting (14).

In the present case the gating system (10) consists of two sprues (18), one runner (20) by each sprue and 5 gates (22) with portholes (22) opening from each runner into the mould cavity (16).

The runners (20) allow the liquid metal to run in the joining surface (13) of the mould parts (12a and 12b) or in its surroundings from the sprues (18) to the runner gates (22). According to the process segmented shaping allows complete filling of the mould cavity as well as reduces the formation of turbulence and foaming, and aids the formation of steady flow. These runners are trapezoidal in various sizes; e.g. upper width 10 mm, bottom width 21 mm, height 17 mm.

Ingates (22) are channels connecting the runners (20) and the mould cavity (16) with the aim of allowing the liquid metal to run into the mould cavity, controlling flow rate and eliminating the formation of turbulence and foaming. They come in various sizes; e.g. gate width 42 mm, gate height adjusted to the wall thickness of the cast, e.g. 2 mm, widening towards the runners: width e.g. 10 mm, height 16 mm.

The sprue (18) is composed of the sprue itself, which is formed in the sand mould, and the riser (26) fitted to it from the outside. The upper part of the latter one is a pouring cup (28) to allow easier pouring of the melt into the sprue (18).

The gating system is of narrowing cross section thus the flowing cross section is getting narrower (including even a transitional increase) towards the runners (22a). In this manner the flow rate of the melt will increase towards the runners (22a) and will reach its highest rate there. This arrangement is in contrast to the conventional sand mould technology, in which gating systems of expanding cross sections are applied since slow and laminar flow will result in higher cast quality in case of thick walled casting parts.

In the context of the current invention, a gating system of narrowing cross section (10) is any gating system that can achieve the highest flow rate at the runner (22a) by narrowing the flowing cross sections. For this reason at least the ingates are to be of narrowing cross section, i.e. the inner cross section of ingates is narrowing towards the runner (22) and becomes the narrowest at the runner (22a). The flow rate is at least twice or more advantageously 3-5 times higher than the average flow rate in the runners (20), or when no such are applied, in the down sprues (18). This can be achieved by providing at least twice or even 3-5 times wider total cross section for the runners (22a) than that of the runners (20). In this case both runners (20) have 2 branches respectively, which start from the sprue (18).

Attachments (26) also contribute to the increase of flow rate. The gradient height between the upper port (i.e. the upper edge of the cup) of the sprue (18) and the joining surface (13) of the sand mould (12) is to be 0.3 or even 0.6-1.3 times multiple of the largest dimension of the mould cavity.

Risers (30) are also incorporated in the sand mould (12). Their task is to exhaust from the mould the gases that are formed during casting as well as to exhaust air accumulated in front of the liquid metal. To the purpose they have cylindrical shape. Their typical diameter is double the wall thickness of the cast (2-6 mm).

In the case of thick-walled cast parts (14), a cooling metal insert, e.g. a cooling iron bar is applied (not shown). This cooling iron bar will allow thick walled segments to solidify at an identical rate with the thin walled segments.

Feeders can also be applied to feed the thick walled segments.

In FIG. 2 the inner layout of the sand mould (12) and gating system (10) is illustrated, which includes four sprues (18) and four runners (20). 5 ingates (22) belong to each runner along the longer sides of the mould cavity (16), while along the shorter sides 4 ingates (22) are joined to each runner (20).

The sand mould (12) has 2-5 ingates (22) on each runner (20). The number and layout of the ingates and runners is designed in a way that at least one runner gate (22) should belong to each 100-1000 cm² segment of the mould cavity, which provides thin-walled casting parts. This arrangement will enable the melt to fill in the whole mould cavity (16) before getting solidified.

In order to produce castings of smaller dimensions (14) the runner can even be neglected. In this case the ingates (22) are directly connected to the bottom of the sprue (18). In order to produce casting parts of larger dimensions (14) a number of segmented runners (20) or branching runners may be applied, or in a given case several sprues can be connected to one single runner (20).

The sand mould (12) fitted with a gating system (10) can be applied in the following way.

Plastic patterns and mould cores provide the manufacturing tool, which is applied to produce the mould halves (12) and cores. From moulding sand suitable for pre-heating the mould halves (12a, 12b) are produced, which will form the cavity related to the casting. The sand mould is typically a chemically bonded dry mould which can tolerate heating.

Cross section, height and width of the gating elements are always determined by the features and casting position of the cast (14).

In order to avoid early solidification, the sand mould (12) is pre-heated at least in the thin walled segment of the casting up to 100° C., better to 100-600° C., or even more advantageous to 300-500° C. (or 0.5-0.8 times of the solidification temperature of the Aluminium alloy). Heating can be performed with gas flame. The moulded cooling iron bars, supposing there are any, are also heated until vapour precipitates and dries from the surface (the surface of the cooling iron bars must remain pure); then the mould surfaces (16), runner gates (22) and the runners (20) and sprues (24) are heated up again before the mould halves are closed.

After the sand mould (12) has been closed and the risers (26) of the gating system have been mounted, the Aluminium melt is produced by heating Aluminium (or Al alloy). Before being fed into the gating system, the melt is over-heated by 100° C., advantageously at least by 200° C., or even by 200-350° C., which will further contribute to avoiding too early solidification.

The liquid metal (melt) is introduced through the gating system (10) into the mould cavity of the preheated mould halves (12a, 12b).

The mould cavity is filled with liquid metal by means of pouring ladles, preferably through the pouring cup (12a) fitted on the sprue of the upper mould half (12a).

Moulding sand and excess parts are removed from the casting (14), which is followed by the casting being applied according to the purpose.

The most difficult task with thin walled castings (14) by gravity casting is to force the molten metal to completely fill in the mould without solidification. How to feed and cool at the same time so that the whole casting should become cooled and evenly solidified approximately at the same time? For this purpose the following methods are applied in order: significant size difference of trapezoidal formation of the ingates (in practice this means a decrease in the flow cross section), segmenting of the runners, increasing pouring height (significant increase in the “liquid” column height relative to the dimensions of the piece), significantly increasing the mould temperature, an increased over-heating temperature of the melt, and application of the above even simultaneously, in the same period of time.

To summarise all the above, prior to the casting process applied in the invention, firstly the manufacturing tool, which enables producing a replica of the part from sand, is made according to a virtual model created by a generally used 3D design program or other suitable programs. This is followed by distributing the tool related to the user's demand, as well as shrinkage of the casting and moulding inclination is concerned. The next step is to determine the pouring position, which can be either vertical or horizontal, according to the geometry of the model then the coring position of the possible cavities of the part will be given. When the part has been assembled from the mould halves (12a, 12b) and the possible cores, the required ingates, sprues, risers, hidden feeders are designed, which will all be installed in the mould frame and be moulded together with the mould halves (12a, 12b), which will allow producing identical parts for the casting. The manufacturing tool is then treated with mould remover and filled in with washed and sized, chemically bonded sand. During the filling up the designed cooling iron is moulded in the sand as well as the upper part of the core is made rigid with iron strands. The mould halves (12a, 12b) are precisely joined by positioning devices, which are also included in the manufacturing tool. Overflow preventers prevent flow-off occurring from the mould buckling on the joining surface, followed by the mould halves (12a, 12b) being treated, heated and closed. In certain cases the upper mould half (12a) is designed with an increased height and balanced. After casting the melt is allowed to cool and then the cores will be carefully removed from the casting, which is then cut off from the ingates and finely purified. This process is followed by checking the main dimensions and delivered to the supplier for testing. After the casting has been tested, small quantity production will be launched.

Advantages of the Application of the Invention Filed for Patent:

A direct economic benefit of the process applied in the invention is that it allows manufacturing castings of almost identical properties related to the technical parameters, with low financial investment and during a fraction of time as compared to the production of large scale casts with metal mould of otherwise high production cost.

In comparison with the castings produced with pressure technology, the quality of casts will be practically identical, however at lower cost and in a simpler way.

The process is suitable for producing cast parts to be made costly even on large scale, which can be applied for practical purposes.

The products of this kind are mainly indoor and outdoor luminaires, engine parts—spare parts, cylinder-heads, machine components, spare parts for mechanical and precision engineering, fittings, etc. All these products can be produced through this invention in such quality that they will be suited for practical application under operational conditions. This will result in cost-effective development and more effective testing before the manufacture on large scale is launched.

Aluminium castings for different purposes are to be produced with this process applied in the invention on small to medium scale, even in several hundred pieces.

By this innovative process castings can be produced with lower machining tolerance and with less moulding incline, therefore dimensional accuracy is in compliance with the related standard, moreover this process allows achieving higher dimensional accuracy compared to that of gravity sand mould casting.

Claims

1: Casting process for producing at least partly thin-walled aluminium castings with sand mould technology by gravity casting, wherein wall thickness of a thin-walled part is 1-3 mm, characterised by:

providing a sand mould comprising a mould cavity,

producing a melt of aluminium content,

introducing the melt into the mould cavity at multiple points through a gating system of narrowing cross section.

2: The process according to claim 1 characterised by producing a casting the largest overall dimension of which is more than 100 times of the wall thickness.

3: The process according to claim 1 characterised by providing a gating system of narrowing cross section for the mould cavity which contains at least two sprues each being in liquid communication with at least one gate respectively, each gate having an inlet opening into the mould cavity.

4: The process according to claim 3 characterised by providing liquid communication between at least two gates and at least one sprue with a runner, and the overall cross section of the inlets of the gates opening from one runner is at least two times smaller, more preferably 3-5 times smaller than the overall cross section of branches of the given runner.

5: The process according to claim 3 characterised by providing 2-4 gates per each runner, and selecting the number of gates and runners such that a thin-walled casting portion having a dimension of 100-1000 cm2 is cast from one gate.

6: The process according to claim 2 characterised by selecting the height of the sprue such that the drop between the upper inlet opening of the sprue and a parting plane of the sand mould is at least 0.1 times greater, preferably 0.6-1.3 times greater than the largest dimension of the casting to be produced.

7: The process according to claim 1 characterised by preheating the sand mould before starting the casting at least in the segments of the thin-walled parts of the casting to at least 100° C.

8: The process according to claim 7 characterised by providing a cooling metal insert in the sand mould and heating the cooling metal insert prior to starting the casting until vapour is precipitated and the insert's surface becomes dry.

9: The process according to claim 1 characterised by that prior to being poured into the gating system, the melt is overheated by at least 100° C. with respect to melting point of the melt.

10: Sand mould having a parting plane and provided with a gating system for producing at least a partly thin-walled aluminium casting with sand moulding technology by gravity casting, wherein wall thickness of a thin-walled portion of a casting is 1-3 mm and the largest dimension of the casting is at least 100 times greater, characterised by that the sand mould defines a mould cavity for producing the at least partly thin-walled casting, and is provided with a gating system of an overall narrowing cross section, which contains at least two sprues each being in liquid communication with at least one gate respectively, each gate having an inlet opening into the mould cavity.

11: Sand mould provided with a gating system according to claim 10 characterised by having a runner providing liquid communication between at least one sprue and at least two gates.

12: Sand mould provided with a gating system according to claim 11 characterised by that the inlets of gates from a runner have a cross section at least two times smaller than the overall cross section of branches of the runner.

13: Sand mould according to claim 11 characterised by comprising 2-4 gates per runner and at least one gate is provided for producing a thin-walled casting portion having an area of 100-1000 cm2.

14: Sand mould according to claim 10 characterised by that the sprue comprises a sprue part formed in the sand mould and an attachment joined to it from above.

15: Sand mould according to claim 10 characterised by that the drop between the upper inlet opening of the sprue and the parting plane of the sand mould is at least 0.1 times greater than the largest dimension of the mould cavity.

16: Sand mould according to claim 10 wherein the largest dimension of the casting is at least 200-400 times greater.

17: Sand mould according to claim 10 wherein the drop between the upper inlet opening of the sprue and the parting plane of the sand mould is 0.6 to 1.3 times greater than the largest dimension of the mould cavity.

18: The process according to claim 1 characterised by that prior to being poured into the gating system temperature of the melt is at least 200° C. above melting point.

19: The process according to claim 1 characterised by that prior to being poured into the gating system temperature of the melt is at least 350° C. above melting point.

20: Sand mould for producing at least partly thin-walled aluminium casting with sand moulding technology by gravity casting, wherein wall thickness of a thin-walled portion of the casting is 1 to 3 mm and the largest dimension of the casting is at least 100 times greater, which comprises a mould cavity defined by the mould, and a gating system provided with at least one gate having an inlet opening into the mould cavity, and at least one sprue in liquid communication with each gate;

said gate having a progressively smaller cross sectional area toward said inlet opening.