INJECTION MOULDING OF MARBLED PARTS

Abstract: The moulding is performed by a device incorporating a torpedo having a cylindrical body (1), provided at its front outlet end with a front conical extension (2) and at its rear end with a rear conical extension (3). The diameter of the base of the front conical extension corresponds to the diameter of the body. A plurality of peripheral channels (4) extend longitudinally along the whole length of the body and into the adjacent portion of the front conical extension.

Description: INJECTION MOULDING OF MARBLED PARTS Invention relates to a torpedo, an injection moulding unit incorporating the torpedo and a method of injection moulding of marbled parts. Injection moulding of marbled effect parts has been known for a number of years and various methods and equipment were devised for this purpose. The known methods and equipment have various shortcomings known to persons skilled in the art. The aim of the present invention is to avoid, or least to mitigate these shortcomings. The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which Figure 1 shows a plan view of a torpedo, Figure 2 shows a front view of the torpedo, Figure 3 shows an end view of the torpedo from the right hand end of the torpedo in Figure 2, Figure 4 shows an end view of the torpedo from the left hand end of the torpedo in Figure 2, Figure 5 shows a reciprocating screw for use with a torpedo according to Figures 1 to 4, and Figure 6 shows a torpedo according to Figures 1 to 4, and a screw according to Figure 5 situated in a barrel. The torpedo T (Figs 1 to 4) comprises a cylindrical body 1 provided, at its front (outlet) end with a front conical extension 2 having a vertex angle <*. and at its rear end with a rear conical extension 3 having a vertex angle ft . In the illustrated embodiment the angle ct is substantially of the same magnitude as the angle |3. The diameter of the " base of the front conical extension 2 corresponds to the diameter of the " body 1 while the diameter of the base of the rear conical extension 3 is smaller than the diameter of the body 1. Both the extensions 2 and 3 are coaxial with the body 1 and their ends are rounded. In the illustrated example six peripheral channels 4 extend longitudinally along the whole length of the body 1 and into the adjacent portion of the front conical extension 2. The channels 4 are radially equispaced and each is defined by two parallel planar lateral surfaces and a concavely curved bottom. For the sake of simplicity this curved shape of the bottom is not shown in the drawings. Between each two adjacent channels 4 is a ridge 5 which provides one of said lateral surfaces for each of the adjacent channels 4- The dimensions and shapes of the channels 4 are identical. The width and the depth of each channel 4 decreases from the rear end to the front end of the torpedo, and consequently the width of each ridge 5 increases while its height decreases in the same direction. As is shown in Figure 3 the diameter of the base of the rear extension 3 corresponds to the diameter of a circle having its centre in the axis of the body 1 and touching the lowermost parts of the bottoms of each of the channels 4. As is apparent from Fig.2 the depth of the channels 4 decreases from the rear end to approximately one third of the length of the body 1 , whereupon it becomes nearly parallel to the imaginary cylindrical envelope of the body 1. Figure 5 shows a reciprocating screw S. Its part in contact with the plastics material has a helical ridge 11 which is of a constant pitch and defines a helical groove 12, the width of which is larger than the width of the ridge 11. The height and width of the ridge 11 and the depth and the width of the groove 12 are not constant. The depth of the groove 12 is greater at the rear end and it reduces towards the front end and is known as "flight depth". The front (outlet) end of the screw S is provided according to the invention with a frustoconical recess 13 coaxial with the screw S. The vertex angle^ of the recess 13 is greater than the vertex angle j* of the rear conical extension 3 of the torpedo. The illustrated reciprocating screw S is substantially of a known design, except for the recess 13« In use (Figure 6) the torpedo T and the screw S are situated in a barrel 15 the outlet end of which is provided with a nozzle 16. The outer diameter of the scew S (measured at the ridges 11 ) and the outer diameter of the torpedo T (measured at the ridges 5) is only slightly smaller than the inner diameter of the barrel 15« In the illustrated example the width of the channels 4 at the rear end of the body 1 is substantially the same as the width of the ridges 5, while the depth of the channels 4 at the rear end, i.e. the maximum depth of the channels 4, is substantially the same as the flight depth at the front end, i.e. the minimum depth, of the screw S which is used in connection with the torpedo T, while the length of the torpedo T is approximately equal to one third of the length of the barrel 1 - The outer diameters of the torpedo T and screw S and the inner diameter of the barrel 15 are determined by the size of the injection moulding unit. The nozzle 16 has at its inner end a substantially conical recess 17 the vertex angle of which is substantially of the same magnitude as the vertex angle o: of the front extension 2 of the torpedo T. In the illustrated example the rear extension 3 of the torpedo T is received in the recess 13 of the screw S while the front extension 2 is adjacent, and possibly partly within, the recess 17 of the nozzle 16. The torpedo T is a so-called floating torpedo, i.e. is freely situated within the barrel 15 without being attached to anything. The illustrated spacing of the torpedo T from the nozzle 16 is obtained automatically by the plastics material (not shown) forced through the nozzle 16. In order to produce a part with a marbled effect, the plastics material (usually a so-called crystalline-type resin) is fed to the screw S in a manner known per se. The material may be a premixed mixture of plastics pellets of desired colours and ratio of colours, depending on the desired effect e.g. white and green pellets in the ratio of 40:1. These are then advanced by the scew S as customary to the torpedo T and forced through the longitudinal peripheral channels 4 the cross-sectional area of which decreases in the direction of movement of the plastics material. The material then leaves the barrel 15 through the nozzle 16 and is delivered to one or more moulding cavities (not shown) to produce a part with marbled effect. The parts of the process not specifically mentioned, such as heating of the barrel to obtain regular melting of the plastics moving therethrough, are customary and need not be described. As the material travels through the channels 4 , the cross-sectional area of which decreases in that direction, its volume is gradually reduced, which results in a thermally homogeneous melt without any mechanical mixing action and without the different colours blending together. Change of the material feed from a rotary one (in the screw S) to a rectilinear one (in the channels 4 of the torpedo T) results in controlled blending, so that the marbling effect, is obtained. The space between the front conical extension 2 and the conical recess 17 of the nozzle 16 gradually increases as the material advanced by the rotating screw S is forced into it. When the required quantity of the material has accummulated in this space, the rotation of the screw S is stopped and the screw S is rectilinearly moved forwards, whereby the material is injected through the nozzle 16. Due to the fact that the cross-sectional area of the channels 4 at this space is smaller than at the opposite end of the torpedo T, the pressure generated in this space does not tend the cause counterflow of the material through the channels 4.

Claims: CLAIMS 1. A torpedo for the moulding of marbled effect parts comprising a cylindrical body (1 ) provided at its front end, i.e. outlet end, with a front conical extension (2) and at its rear end with a rear conical extension (3), wherein the diameter of the base of the front conical extension corresponds to the diameter of the body, a plurality of peripheral channels (4) extending longitudinally along the whole length of the body and into the adjacent portion on the front conical extension. 2. A torpedo according to Claim 1 , wherein the diameter of the base of the rear conical extension is smaller than the diameter of the body. 3« A torpedo according to Claim 1 or 2, wherein the channels are radially equispaced and each is defined by two substantially parallel lateral surfaces and a bottom, a ridge (5) being provided between each two adjacent channels, the ridge being delimited by lateral surfaces which form the adjacent lateral surfaces of the adjacent channels. - A torpedo according to any one of Claims 1 to 3, wherein the width and the depth of each channel decreases from the rear end to the front end of the torpedo while the width of each ridge increases and its height decreases in the same direction. 5. A torpedo according to Claim 4 wherein the depth of the channels decreases from the rear end to approximately one third of the length of the body, whereupon it becomes nearly parallel to the imaginary cylindrical envelope of the body. 6. A torpedo according to any one of the preceding claims wherein the diameter of the base of the rear extension corresponds substantially to the diameter of a circle having its centre in the axis of the cylindrical body and touching the lowermost parts of the bottoms of the channels.. 7« A torpedo according to Claim 3 and any claim appended thereto wherein the width of the channels at the rear end of the body is substantially the same as the width of the ridges at the rear end. 8. A torpedo according to any one of the preceding claims, wherein the vertex angle (<*) of the front conical extension is substantially of the same magnitude as the vertex- angle (β) of the rear conical extension. 9« A torpedo according to any one of the preceding claims, wherein the ends of the conical extension (2,3) are rounded. 10. A torpedo according to any one of the preceding claims, wheren the dimensions and shapes of all the channels (4) are indentical. 11. A device for injection moulding of marbled effect parts comprising a barrel (15), the outlet end of which is provided with a nozzle (16) the barrel containing a screw (S), a torpedo according to any one of the preceding claims being freely situated as a floating torpedo between the nozzle and the screw. 12. A device according to Claim 11 wherein the front end, i.e. the outlet end, of the screw is provided with a frusto-conical recess (13) which can receive the rear conical extension of the torpedo, the vertex angle (r) of the said recess being greater than the vertex angle (β ) of the rear conical extension. 13« A device according to Claim 11 or 12 wherein the nozzle has at its inner end a substantially conical recess (17) the vertex angle of which is substantially of the same magnitude as the vertex angle ( ) of the front extension of the torpedo. 14. A device according to any one of Claims 11 to 13, wherein the maximum depth of the channels of the torpedo is substantially the same as the minimum flight depth of the screw. 1 ' A device according to any one of Claims 11 to 14, wherein the length of the torpedo is substantially equal to one third of the length of the barrel. 16. A product made with the use of a device according to any one of Claims 11 to 15«

Inventors: PERKINS DEREK MICHAEL, PERKINS, DEREK, MICHAEL
Applicants: HOECHST UK LTD
Application year: 1990
Main classification: B29C45
IPC codes: B29C45