WO2009047601A2

WO2009047601A2 - Dense phase powder pump comprising a supply hose having a specific inside roughness

Info

Publication number: WO2009047601A2
Application number: PCT/IB2008/002438
Authority: WO
Inventors: Hanspeter Vieli; Marco Sanwald
Original assignee: Itw Gema Gmbh
Priority date: 2007-10-13
Filing date: 2008-09-18
Publication date: 2009-04-16
Also published as: DE102007049170A1; WO2009047601A3

Abstract

A method to feed coating powder, and powder spraycoating equipment as well as powder spraycoating equipment containing a dense phase powder pump (10) and a flexible powder discharge hose (28) connected/connectable to the powder discharge side (26) of the dense phase powder pump (10) to discharge coating powder, where the hose wall is constituted entirely or at least at the hose inside surface of an electrically insulating material wherein, the hose inside surface exhibits a roughness between 2 and 7 micron.

Description

POWDER SPRAY COATING DEVICE AND POWDER CONVEYING DEVICE OF THE SAME

The present invention relates to a powder spray coating device - hereafter powder spraycoating equipment - or to a powder conveying device - hereafter powder feed apparatus - of powder spraycoating equipment defined in claim 1.

Moreover the present invention relates to conveying coating powder by means of such powder spraycoating equipment or such a powder feed apparatus defined in another claim.

Dense phase powder pumps comprise at least one feed chamber fitted with a powder intake valve and a powder outlet valve. The feed chamber is alternatingly connected to a vacuum source during a suction stage and to a source of conveying compressed air during a discharge stage. The vacuum from said vacuum source aspirates powder through the open powder intake valve into the feed chamber while the powder outlet valve is closed. The conveying compressed air from the source of conveying compressed air discharges powder from within the feed chamber through the open outlet valve while the intake valve is closed. Most dense phase powder pumps comprise two feed chambers operating in mutually time-staggered manner in order that alternatingly coating powder shall be aspirated each time into one feed chamber while the pertinent other feed chamber is discharging coating powder.

Different kinds of coating powder feed apparatus containing a dense phase powder pump are known for instance from the following documents which are incorporated by reference herein: JP 09/071325 A, DE 196 11 533

B4, US 2006/0193704 A1 (= EP 1 644 131 A2), US 7 150 585 B2 (= WO

2004/087331 A1) and US 2005/0178325 A1 (= EP 1 566 352 A2). A vacuum

- 1 - CONFIRMATION CCPY intake of at least one of the two feed chambers and in some embodiment modes also the compressed air intake of the feed chamber is/are fitted with a filter permeable to air but not to coating powder. The preferred filter material is a sintered one. Predominantly the powder intake and outlet valves are pinch valves.

The quantity of powder per unit time - hereafter powder rate - fed by a dense phase powder pump in particular depends on the size (volume) of the feed chamber, on the frequency at which coating powder is aspirated into the feed chamber and then discharged from it, on the magnitude of the vacuum, on the time the powder intake valve is open during suction and on the flow impedances in the powder conduits upstream of the dense phase powder pump and especially downstream of it. The flow impedances depend in particular on the length and the inside cross-section of the powder conduits, mostly powder hoses. The compressed conveying air mixes only little with the coating powder which it pushes through the powder outlet valve out of the feed chamber.

Different conditions apply to dilute phase powder pumps which use an injector as the powder pump to feed coating powder. The mixture of powder and stream of compressed conveying air then moves from the injector to a target site, for instance a bin or a spray tool. The rate of powder conveyed by the injector depends on the rate of compressed conveying air through this injector. Powder spraycoating equipment fitted with an injector as the powder pump illustratively is known from the following documents which are incorporated by reference herein: US 4,284,032; US 4,357,900; EP 0 412 289 B1 and EP 0 636 420 A2.

In the state of the art, the powder discharge conduits feeding the coating powder by means of a powder pump (injector of dense phase) to a target (for instance a powder bin or a spray tool) consist of a flexible hose which in whole or at least at its layer forming the hose inner wall is made of an electrically insulating material. High electrical potentials may arise on account of the friction between the powder particles and the inner hose wall and entail the danger of arc formation and explosions. It is already known, to drain such electric charges and hence to reduce said electric potentials, to integrate electrical conductors into the otherwise electrically insulating hose wall, said conductors being grounded. Illustratively hoses with integrated electric conductor(s) are already known in the following documents which is incorporated by reference herein: DE-G 75 34 723.8, DE-G 79 34 101.8 U1 ; EP 0 974 779 B1 (= US 6,202,701 B); US 4,027,659 and US 3,070,132.

The powder density in a stream of powder in the powder discharge conduit of a dense phase powder pump is substantially larger than that in an injector's powder discharge conduit. Because of the thrust-like, impulsive powder feed by a dense phase powder pump, there is danger that the powder stream - especially in its longitudinal direction but frequently also cross- sectionally — shall exhibit a variable powder density.

The objective of the present invention is to solve the problem to make more uniform the density distribution of a stream of powder discharged by a dense phase powder pump.

This objective is attained in the present invention by the features of claim 1.

Moreover the objective of the present invention is met by means of a method defined in a further claim.

The present invention offers the advantage that the density distribution of a stream of powder delivered by a dense phase powder pump can be made uniform in a simple manner. The present invention offers the further advantage that powder deposits do not arise in the powder discharge conduit or at least that such a danger is no greater than in known powder discharge conduits. Further features of the present invention are defined in the dependent claims.

The present invention is discussed below by means of illustrative embodiments and in relation to the appended drawings.

Fig. 1 schematically shows a powder spraycoating equipment of the invention fitted with a dense phase powder pump,

Fig. 2 is a schematic cross-section of a flexible hose of the invention acting as the powder discharge conduit of the dense phase powder pump,

Fig. 3 is a schematic cross-section of a further embodiment mode of a flexible hose acting as the powder discharge conduit of a dense phase powder pump, and

Fig. 4 is a diagram defining the roughness of a surface.

The coating powder feed apparatus 2 shown in Fig. 1 together with a spray tool 4 constitutes a powder spraycoating equipment though it may also be used to feed coating powder into a bin rather than to a spray tool 4. Preferably the spray tool 4 comprises a high voltage (hv) generator 6 to generate hv from at least one hv electrode with which to electrostatically charge coating powder.

The spraycoating powder feed apparatus 2 contains a dense phase powder pump 10 preferably fitted with two feed chambers 12 and 14 (instead of only one or more than two feed chambers). Seen in the longitudinal chamber direction, each feed chamber 12 and 14 comprises a powder intake 16 with a powder intake valve Q1 respectively Q2 at one chamber end and a powder outlet 18 with a powder a powder outlet valve Q3 respectively Q4 at the opposite chamber end. The valve outlet side 20 of the two powder outlet valves Q3 and Q4 is connected by powder discharge branch conduits 22 respectively

24 preferably by means of a Y-shaped branch element 26 to a common powder discharge conduit 28 of which the other end may communicate with the spray tool 4.

Preferably the powder intake valves Q1 and Q2 are pinch valves. Their design may be known and arbitrary. Preferably they are pinch valves of which the valve duct 40 is constituted by an elastic hose 42 enclosed by a pneumatic chamber 44, said hose being radially pinched shut as needed by compressed air that can be applied to the pneumatic chamber 44.

The powder intake 16, the powder outlet 18 and the valve ducts 40 of the powder intake valves Q1 and Q2 as well as of the powder outlet valves Q3 and Q4 are respectively aligned with the corresponding central chamber lines 32,

34.

In Fig. 1 , the powder intake valve Q1 of the feed chamber 12 is open and the powder outlet valve Q3 is closed while in the other feed chamber 14 the powder intake valve Q2 is closed and the powder outlet valve Q4 is open. The feed chamber 12 is in its coating powder suction stage. The other feed chamber 14 is in its coating powder discharge stage. These stages continuously alternate during powder feeding.

The powder intake side 46 of the two powder intake valves Q1 and Q2 may communicate separately by means of powder feed conduits 48 respectively 50 with a powder bin or, as shown in the preferred embodiment mode of Fig. 1 , they are connected by a preferably Y-shaped branch element 52 to a common powder feed conduit 56 to aspirate spraycoating powder 58 from a powder bin 60.

Each feed chamber 12 and 14 may be alternatingly connected during a suction stage (in Fig. 1 , the feed chamber 12) to a vacuum source 62 or during a discharge stage (in Fig. 1 , the feed chamber 14) with source 64 of compressed conveying air. The compressed conveying air source 64 receives compressed air preferably by the intermediary of a pressure regulator from a compressed air source 66, illustratively an enterprise's compressed air grid. Coating powder 58 is aspirated by the vacuum from the vacuum source 62 through the open powder intake valve Q1 respectively Q2 into the feed chamber 12 and 14 while the powder outlet valve Q3 respectively Q4 is closed. Coating powder in the feed chamber 12 respectively 14 is discharged by means of the compressed conveying air from the compressed conveying air source 64 through the open powder outlet valve Q3 respectively Q4 while the powder intake valve Q1 respectively Q2 is closed. The two feed chambers 12 and 14 are operated in time-staggered manner, alternatingly coating powder being aspirated into one of the feed chambers (for instance 12) while coating powder is discharged from the other associated feed chamber (for instance 14), and thereafter coating powder is discharged from one feed chamber (for instance 12) while coating powder is aspirated into the other associated feed chamber (for instance 14).

Preferably the powder intake valves Q1 and Q2 and the powder outlet valves Q3 and Q4 are controlled individually.

A control unit 70 drives the dense phase powder pump 10 and comprises several control valves and at least one, preferably several pressure regulators to generate, from a compressed air source 66, the pressure at the output of the compressed conveying air source 64 and the pressure of the control compressed air driving the powder valves Q1 , Q2, Q3 and Q4. Moreover the control unit 70 comprises control valves to load the feed chambers 12 and 14 with compressed air from the compressed conveying air source 64 or with a vacuum from the vacuum source 62.

The branch element 26 of the two powder discharge branch conduits 22 and 24 constitutes the discharge side of the dense phase powder pump 10 which may be connected to the powder discharge conduit 28. As regards a dense phase powder pump fitted only with a single feed chamber 12 or 14, the valve outlet side 20 of the powder outlet valve Q3 respectively Q4 will constitute the powder discharge side of said dense phase powder pump. Also other branch elements may be used than a Y-branch element 26. Instead of one or two feed chambers 12 and 14, a dense phase powder pump also may contain three or more feed chambers 12 respectively 14. By means of the powder discharge conduit 28, a dense phase powder pump 10 may feed coating powder not only to a spray tool 4, but instead also to another chamber. The embodiments modes of the powder discharge conduit 22 described below are suitable for all the aforementioned applications.

The powder discharge conduit 28 consists totally, or at least on its hose inside surface, of an electrically insulating material. This electrically insulating material illustratively may be polyvinyl chloride (PVC) or be a thermoplastic such as a polyethylene or a vinyl polymer or another plastic or a rubber.

According to the present invention, the powder discharge conduit 28 exhibits a roughness Ra between 2 and 7 μ. Preferred ranges are between 2 and 6 μ, also between 3 and 5 μ, preferably a range between 3.5 and 4.5 μ.

The roughness Ra may be defined as indicated in Fig 4. The roughness Ra is the arithmetic mean calculated by means of the averages of the individual measurement segments Lr. The basis of such a calculation is the center line "x" of the roughness profile, that is of the hose's inside surface. The center line "x" bisects the plot in such a way that, within the measurement segment Lr, the sum of the area elements 74 on one side of said center line shall equal the sum of the area elements 76 on the other side. The roughness value Ra of the individual measurement segments is the arithmetic mean of the lot's deviations from the center line "x" regardless of the deviations' signs. The roughness (roughness value Ra) is defined by the standard SN EN ISO 4287. The shaded area elements 74 are situated on the positive side "+y". The broken, shaded- line area elements 76' are the negative area elements 76 folded upward of the negative side "-y". The sums of the solid line defined area elements 74 are equal to the sum of the broken line shaded area elements 76.

The flexible hose constituting the powder discharge conduit 28 may be made of a single wall stratum or of several wall strata.

Fig. 2 is a cross-section of a preferred embodiment of a hose that may constitute the powder discharge conduit 28. This preferred embodiment mode contains at least one, preferably two electrical conductors 80 partly imbedded into the hose wall and running in this wall in the hose's longitudinal direction. Fig. 2 shows two such electrical conductors 80 and 82 configured diametrically in said hose and protruding into the hose duct, though being mostly imbedded into the hose wall. Unlike the embodiment mode shown in Fig. 2, the radially inward tip 81 respectively 83 of the electric conductors 80 and 82 also may be flush with the hose inside surface 86, though being freely accessible, that is, not being covered by the insulating material of the hose 22.

Fig. 3 shows a cross-section of a further design of a flexible hose of the present invention that may be used as a flexible hose and, be in the form of a powder discharge conduit 28 being denoted in this Fig. 3 by 28/2. At least one and preferably two mutually diametrically opposite electrical conductors 90 and 92 are fully imbedded in, that is enclosed by, the hose wall and as a result will neither pierce the hose inside surface 86 nor the hose outside surface 96.

All electrically conductors 80, 82 or 90, 92 illustratively may be electrically conducting plastics made electrically conducting by admixed carbon. Said conductors also may be in the form of metal wires or stranded conductors or other materials. Preferably the electrical conductors shall be flexible. The presence of one or more electrical conductors 80, 82 or 90, 92 allows grounding electric charges generated by the friction of the powder at the rough inner hose surface 86.

In the preferred embodiment mode of the present invention, the hose inside diameter 98 always is between 5 and 8 mm depending on the powder rate fed by the dense phase powder pump.

A hose of the present invention such as designed or provided for the powder discharge conduit 28 also may be advantageously used as a powder feed conduit 56 (suction conduit).

Claims

1. A powder spraycoating equipment, or a powder feed apparatus of a powder spraycoating equipment, containing a dense phase powder pump (10) and a flexible powder discharge hose (28; 28/2) connected/connectable to the powder discharge side (26) of the dense phase powder pump (10) to discharge coating powder, the hose wall being made totally or at least at the hose inside surface of an electrically insulating material, characterized in that the roughness of the hose inside surface (86) is between 2 and 7 μ.

2. Powder spraycoating equipment or powder feed apparatus of powder spraycoating equipment as claimed in claim 1, characterized in that the roughness of the hose inside surface is between 2 and 6 μ, preferably 3 and 5μ and again preferably between 3.5 and 4.5 μ.

3. Powder spraycoating equipment or powder feed apparatus of powder spraycoating equipment as claimed in one of the above claims, characterized in that the wall of the powder discharge hose (28; 28/2) as a whole, or at least the hose stratum constituting the hose inside surface (86), is made of at least one of the following electrically insulating materials: plastic, in particular polyvinylchloride or a thermoplastic, in particualr a polyethylene or a vinyl polymer.

4. Powder spraycoating equipment or powder feed apparatus of powder spraycoating equipment as claimed in one of the above claims, characterized in that at least one electric conductor (80, 82; 90, 92) is at least partly imbedded in the electrically insulating hose wall and runs through the hose wall in said hose's longitudinal direction.

5. Powder spraycoating equipment or powder feed apparatus of powder spraycoating equipment as claimed in claim 4, characterized in that more than half the periphery of the electric conductor (80, 82) is imbedded into the electrically insulating hose wall and held by it and in that the remnant small peripheral portion of the electric conductor (80, 82) is in the hose inside surface but uncovered or projects from the hose wall inside surface (86) into the hose duct.

6. Powder spraycoating equipment or powder feed apparatus of powder spraycoating equipment as claimed in one of the above claims, characterized in that the hose inside diameter (98) is between 5 and 8 mm.

7. A method to feed coating powder, characterized in that powder spraycoating equipment or powder feed apparatus of powder spraycoating equipment is manufactured as claimed in one of the above claims, which contains a dense phase powder pump (10), and in that the coating powder is moved by the dense phase powder pump (10) through a flexible powder discharge hose (28; 28/2) as claimed in one of the above claims.