Screw of injection molding machine

Abstract: A screw of a screw-in-line type plasticizing device of an injection molding machine has a cylindrical main part and a spiral screw flight (1a) on the main part. A portion of the screw flight (1a) of the screw facing a resin supply port has a parallelogrammic axial sectional shape which is inclined towards the discharge end of the screw.

Description: BACKGROUND OF THE INVENTION The present invention relates to a plasticizing device of an injection molding machine according to the precharacterising part of claim 1. Fig. 3 schematically shows a conventional screw 11, in particular the portion around a resin supply port 13. The operation of the conventional in-line screw 11 relies upon frictional force alone so that it could never stably plasticize the material. In particular, platicization is often rendered unstable when a lubricating oil is mixed in the resin material for the purpose of improving lubrication of the molded product. Unstable plasticization is caused when the resin material 4 cannot smoothly be fed forward due to clinging of the resin on the screw 11. This problem takes place when the friction between the resin 4 and the surface 11b of the screw 11 is not substantially different from the friction between the resin 4 and the inner surface 12a of the cylinder. Good plasticizing effect is obtained when the friction between the resin 4 and the inner surface 12a of the cylinder is considerably higher than that between the resin 4 and the screw surface 11b. In order to realise this condition, attempts have been made such as roughening of the inner surface 12a of the cylinder and provision of axial grooves in the inner surface 12a of the cylinder. These attempts such as roughening or grooving of the cylinder inner surface 12a cause problems such as an increase in the driving torque for driving the screw 11 or imperfect plasticization particularly when the speed of the screw is increased. From DE-B-12 83 496 there is known a plasticizing device as defined by the preamble of the claim. In this known device the parallelogramic axial section shape inclined towards the discharge end of the screw is unchanged over the entire length of the screw. By the said construction of the screw flight the catching ability of resin pellets is improved. However due to the fact that said parallelogramic axial sectional shape extends unchanged over the entire length of the screw the kneading and labour efficiency is deteriorated for example in the work of changing the colour of the resin to be treated. Moreover, stagnation of the resin pellets may occur. Accordingly, an object of the present invention is to provide a plasticizing device of an injection molding machine which can overcome the above described problems of the prior art. To this end, according to the invention, a device defined by the preamble of the claim, comprises the features of the characterising part of the claim. A device according to the invention has, as far as the efficiency in the colour changing work is concerned, the same efficiency as conventional screw flight shapes, what is highly desirable, and at the same time the catching ability of resin pellets is improved over such ability of the conventional screws. From US-A-4 185 060 there is known a screw for use in injection molding structural foam plastic products in which the housing or barrel is provided with a vent passageway at an intermediate position of the barrel between the inlet for the mixture of thermoplastic granules and a blowing agent and the discharge passageway for injecting molten plastic. The screw has two compression stages with a decompression stage therebetween. The mixture is compressed, as it advances through the first compression stage and releases moisture vapors and other unwanted volatiles at the decompression stage, the released vapors and volatiles escaping through the vent passageway. Through the second compression stage the mixture is cmpressed and brought in a molten condition suitable for injection molding at the discharge passageway. At the vent zone the forward faces of the screw flights are undercut similar to the screw flight portion in the plasticizing device of the present invention facing the resin supply port having the parallelogramic axial section shape. The said known screw flight shape serves to provide a concavity to minimize the tendency for the plastic mass to flow radially outwardly through the vent. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a critical portion of the screw in accordance with the present invention; Fig. 2 is a sectional view of a critical portion of a known screw; and Fig. 3 is a sectional view of a portion of a plasticizing device which incorporates the screw shown in Fig. 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 shows a screw flight of screw in the conventional in-line type plasticizing device described before in connection with Fig. 3. The flight 11a has a trapezoidal cross-section, so that the resin pellets 4 supplied through the supply port 13 are lifted by the flank of the flight 11a so as to be displaced radially outward. Consequently, plasticizing effect has large dependency on the friction between the resin pellets 4 and the inner surface 12a of the cylinder. Fig. 1 shows the sectional shape of the flight 1a of the screw in accordance with the present invention. As will be seen from this Figure, the screw flight 1a has a parallelogrammic sectional shape when taken along an axial plane, and this axial sectional shape is inclined towards the discharge end of the screw. Therefore, the resin pellets 4 which have been introduced into the space A is prevented by the inclined flank of the screw flight 1a from being displaced radially outward. Consequently, the resin pellets are advanced smoothly towards the discharge end in accordance with the rotation of the screw 1, thus ensuring high stability of plasticization. The above-mentioned inclined parallelogrammic sectional shape is adopted only in the portion of the screw adjacent the resin supply port 13, in order to prevent stagnation of the resin pellets. The screw 1 of the present invention can be produced by machining a single cylindrical blank so as to leave spiral flight 1a such that the angle  of inclination is set to be equal to or less than 90 degrees. The portion of the screw flight having the inclined parallelogrammic sectional shape extends over a length which is substantially equal to the sum of the screw pitch and the axial length of the resin supply port. The in-line type plasticizing device having the screw in accordance with the present invention can effectively plasticize and meter the resin even when the resin contains an oil, without being accompanied by troubles such as increase in the driving torque and imperfect plasticization. Furthermore, stagnation of the resin is avoided because the parallelogrammic inclined sectional shape is adopted only on a portion of the screw flight.

Claims: A plasticizing device of an injection molding machine, comprising a housing (12) having a hollow interior bounded by a substantially cylindrical interior wall (12a), and a resin supply port (13) extending through said cylindrical interior wall (12a) of said housing (12) in communication with said hollow interior; a screw (1) having a screw thread for plasticizing and driving resin pellets axially through said hollow interior of said housing (12), said screw thread extending in a generally radial direction toward said cylindrical interior wall (12a), the screw flight of the screw (11) facing the resin supply port (13) having a prallelogramic axial sectional shape, which is inclined toward the discharge end of the screw (11), characterised in that said parallelogramic axial sectional shape (1a) of the screw flight which is inclined towards the discharge end of the screw (11) is provided only at the portion of the screw flight of the screw (11) facing the resin supply port (13) and extends over a length which is substantially equal to the sum of the screw pitch and the axial length of the resin supply port (13).

Inventors: SUUMEN HIROYOSHI, SUUMEN, HIROYOSHI, SUUMEN, HIROYOSHI, SUMITOMO SANNO GADEN TAUN A-401
Applicants: SUMITOMO HEAVY INDUSTRIES, SUMITOMO HEAVY INDUSTRIES, LTD
Application year: 1991
Main classification: B29C45
IPC codes: B29C45