INJECTION MOLDING METHOD AND MOLD FOR INJECTION MOLDING

Abstract: Disclosed are an injection molding method and a mold for an injection molding. A cavity of a mold with one side mold having a stepped portion formed on an inner side of the one side mold facing the back of a mold product with the other side mold, is obtained. The melted resin is injected and fill up into the cavity of the mold through a number of gates. The mold includes a stepped portion formed on an inner side of a mold facing the back of a mold product. The stepped portion is formed around the position where a weld line may be formed without the stepped portion. The mold line is not generated and a separate painting of the mould product as a post-processing is not needed.

Description: INJECTION MOLDING METHOD AND MOLD FOR INJECTION MOLDING DESCRIPTION TECHNICAL FIELD OF THE INVENTION The present invention relates to an injection molding method and a mold for injection molding and particularly, to an injection molding method and a mold for injection molding for preventing the generation of a weld line around the meeting portion where two separated resin portions of a molten resin flow together, by controlling the inner flow of the melted resin in a cavity, which had been injected and filled up in the cavity of the mold. BACKGROUND ART Generally, a weld line is formed as a thin solid line at the mixing portion of two separately flowing molten resins in the mold. The main reason of the generation of the weld line is the incomplete mixing of the two separated molten resins at the meeting portion. The weld line also can be generated by moisture or a volatile matter contained in the resin, or by a mold release used during the injection molding. Particularly, the generation of the weld line is caused when a hole is present in a mould product or two gates are present in the mold. For carrying out the injection molding, one gate for passing the resin is needed. When the mould product is large or complicated, two or more gates are needed. Accordingly, the weld line is liable to be formed in this case. Even though the melted resin is supplied through one gate, the weld line may be generated when an inserting component or a hole is present in the mould product or when the building thickness of the mould product is different. FIGs. 1 A and IB are typical views for explaining the generating process of the weld line. Referring to FIG. 1A, the melted resin is supplied into the cavity through two gates 12 and 14 and the weld line is generated where the two separated portions of the melted resin flow together. At this time, the weld line is not formed even though the separated melted resins flow together near gates 12 and 14, but a weld line 16 is formed at the meeting position far from gates 12 and 14. The melted resin gets cool through the mold and the two separated resin portions far from gates 12 and 14 could not be mixed together completely because of the low temperature of the resins. Therefore the weld line 16 is generated. Meanwhile, in FIG. IB, when a hole 18 is provided in the mould product, weld line 16 is generated even though the melted resin is supplied through one gate 12, because the melted resin is separated around the hole 18 and then flow together. Thus formed weld line deteriorates the exterior view of the mould product and therefore, a painting process for eliminating the weld line is additionally implemented after the injection molding. This makes the working complicated and increases the manufacturing cost. In order to solve above-mentioned problem, PCT/JP 93-001830 with the title of "injection molding method preventing generating of surface defects and metal mold for injection molding" discloses the following technique. A mold cavity is formed with a mold having an adiabatic layer having good adherence and adiabatic property to the melted resin, which is formed on the inner surface of the mold, as one side surface. Then, the melted resin is injected and filled up into the cavity of the mold. The cooling through the mold is restrained by the adiabatic layer and the mold at the other side of the cavity having no adiabatic layer, is controlled so as to grow faster than that of at the side of the cavity having the adiabatic layer. Therefore, the formation of a sink mark and the generation of the weld line during the cooling of the resin and solidification on the surface of the mould product can be restrained. However, in the above-mentioned technique, there is a problem that the lamination treatment for the separate adiabatic layer is difficult. DISCLOSURE OF THE INVENTION Accordingly, it is an object of the present invention considering the above-mentioned problem, to provide an injection molding method for restraining the generation of a weld line around a mixing portion where two separated portions of a melted resin flow together, by controlling the inner flow of the injected and filled up melted resin in the cavity of a mold. Another object of the present invention is to provide a mold for injection molding for restraining the generation of a welding line at a mould product by forming a stepped portion at a predetermined position of the cavity of the mold, which narrows the thickness of the cavity. To accomplish the object, there is provided in the present invention an injection molding method comprising the steps of; obtaining a cavity of a mold with one side mold having a stepped portion formed on an inner side of the one side mold facing the back of a mould product and the other side mold, for controlling the flow of a melted resin through reducing a cross-sectional area of the cavity, the stepped portion being vertical to a joining portion; and injecting and filling up the melted resin in the cavity of the mold through a number of gates; whereby, the flow of the melted resin to an exterior portion of the joining portion can be delayed by the stepped portion, and the mixing of the melted resin occurs first around the gates. At this time, the stepped portion is formed at an outermost portion of a meeting portion formed by the melted and separated resins supplied from neighbored two gates. And the stepped portion is formed from about 60% of the total length of the cavity and is formed to the outermost portion of the cavity so as to reduce the cross-sectional area of the cavity by 20-30%. Another object of the present invention can be accomplished by a mold for an injection molding comprising a stepped portion formed on an inner side of a mold facing the back of a mould product for controlling the flow of a melted resin through reducing a cross-sectional area of a cavity of the mold, the stepped portion being formed around the position where a weld line may be formed without the stepped portion. Meanwhile, the term of "joining portion" means the portion where the melted resin supplied from two separate gates are mixed and join, and the term of "outermost" means the position facing the gates among the joining portion and the joining occurs lately. BRIEF DESCRIPTION OF THE DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: FIG. 1 A is a typical view for illustrating the forming process of a weld line when a melted resin is supplied through two gates; FIG. IB is a typical view for illustrating the forming process of a weld line around a hole; FIG. 2 is a typical view for explaining the injection molding method according to the present invention; FIG. 3 A schematically illustrates the injection molding process of a television case; and FIG. 3B is a cross-sectional view of the end portion of a television case according to a preferred embodiment of the present invention. MODES OF CARRYING OUT THE INVENTION Hereinafter, the injection molding method and the mold for the injection molding according to a preferred embodiment of the present invention will be explained in more detail with reference to the accompanying drawings. FIG. 2 is a typical view for explaining the injection molding method according to the present invention. The flow of the filled up and melted resin from the gates is represented as solid lines in the cavity portion between the upper and lower molds of the mould product. At the inner surface of the lower mold facing the back of the mould product, stepped portions a and b are formed. Accordingly, the cross-sectional area of the cavity formed between the upper and the lower molds is somewhat reduced. This prevents the melted resin supplied from the gates from flowing to the outermost portion. At the mixing portion formed near the gates, a fusion angle of the melted resin is increased, and therefore, the range from 120° to 140° where the weld line is formed, is reduced. Moreover, at the mixing portion formed far from the gates, the flow of the melted resin is disturbed by the stepped portions a and b. The resin flows together before the stepped portions a and b, and the mixed resin proceeds forward to the outermost portion. Therefore, the effect of the prevention of the weld line formation due to the cooling could be obtained. At this time, considering the viscosity according to the temperature of the melted resin and the cooling due to the mold, when the height x of the cavity formed by the stepped portions a and b is above 80% of the height t of the original cavity, the object for controlling the flowing of the melted resin could not be accomplished. And when the height x of the cavity formed by the stepped portions a and b is below 70% of the height t of the original cavity, a difference in the surface gloss is generated due to the difference in the thickness of the mould product. Therefore, the height of stepped portions a and b is preferably in the range of 20-30% of the height of the cavity. The stepped portions a and b are formed between the position where the weld line had been formed because of the incomplete mixing of the two cooled, separated and melted resins by the mold and the gates. And the stepped portions a and b are formed for preventing the generation of the weld line. Further, the stepped portions a and b are preferably formed with the length of 40% of the length from the gates to the outermost portion of the cavity. The stepped portions include an inclined portion a having a predetermined slope for preventing the generation of the turbulent flow of the melted resin and a narrow portion b which reduces the cross-sectional area of the cavity. At this time, since the stepped portions are formed at the outermost portion of the cavity, the melted resin running in the cavity proceeds from each gate with a concentric circle shape. The completely mixed resins near the gates keep the high temperature and do not generate the weld line. At the mixing portion apart from the gates with a predetermined distance, the advance of the melted resin is disturbed by the stepped portions a and b, and accordingly, the shape of the advancing melted resin is not the concentric circle any more. The separated melted resins advance right and left and two filled up resins from different gates can be completely mixed. The completely mixed resin has the concentric circle shape and proceeds toward the outermost portion of the cavity. That is, two melted resins supplied from two gates can be mixed before the cooling to a lower temperature by the mold, because of the presence of the stepped portion. The mixed resin fills up in the outermost portion of the cavity while forming a horizontal line. Accordingly, the generation of the weld line due to the mixing of two separated resins at a low temperature, can be prevented. FIG. 3A schematically illustrates the injection molding process of a television case. At the front surface portion of a television case 30, a spool 31 and four gates 32, 34, 36 and 38 are connected through a runner 40. The melted resin supplied from spool 31 is supplied to the four gates 32, 34, 36 and 38 through the runner 40 and flows from the center portion to the outermost portion of the cavity. FIG. 3B is a cross-sectional view of the end portion of a television case according to this preferred embodiment of the present invention. As illustrated in the cross-sectional view in FIG. 3B, a stepped portion 42 is formed at the side surface portion of the television case. The flow of the melted resin supplied from gates 32, 34, 36 and 38 to the stepped portion 42 is disturbed and two separated resins can be mixed before the cooling of the resins. The mixed resin fills up in the outermost portion of the side surface portion with the velocity of the concentric circle to prevent the generation of the weld line. As described above, the stepped portion for reducing the cross-sectional area of the cavity is formed at the outermost mixing portion of the separated resins from two or more gates. The flowing characteristic of the resin in the cavity is controlled so as to reduce the generation of the weld line. As a result, a separate painting of the mould product is not needed. Although the preferred embodiment of the invention has been described, it is . understood that the present invention should not be limited to the preferred embodiment, but various changes and modifications can be made by one skilled in the art within the spirit and scope of the invention as hereinafter claimed.

Claims: WHAT IS CLAIMED IS: 1. An injection molding method comprising the steps of; obtaining a cavity of a mold with one side mold having a stepped portion formed on an inner side of said one side mold facing the back of a mold product and the other side mold, for controlling the flow of a melted resin through reducing a cross-sectional area of said cavity, said stepped portion being vertical to a joining portion; and injecting and filling up said melted resin into said cavity of said mold through a number of gates; whereby, said flow of said melted resin to an exterior portion of said joining portion can be delayed by said stepped portion, and the mixing of said melted resin occurs first around said gates. 2. The injection molding method as claimed in claim 1, wherein said stepped portion is formed at an outermost portion of a meeting portion formed by said melted and separated resins supplied from neighbored two gates. 3. The injection molding method as claimed in claim 2, wherein said stepped portion is formed so as to reduce said cross-sectional area of said cavity by 20-30%. 4. The injection molding method as claimed in claim 3, wherein said stepped portion is formed from about 60% of the total length from said gates, to said outermost portion of said cavity. 5. The injection molding method as claimed in claim 4, wherein said stepped portion comprises an inclined portion and a narrow portion. 6. A mold for an injection molding comprising a stepped portion formed on an inner side of a mold facing the back of a mould product for controlling the flow of a melted resin through reducing a cross-sectional area of a cavity of said mold, said stepped- portion being formed around the position where a weld line may be formed without said stepped portion. 7. The mold for an injection molding as claimed in claim 6, wherein the height of said stepped portion is 20-30% of a height of said cavity of said mold.

Inventors: KIM JONG KAB, KIM, JONG, KAB
Applicants: DAEWOO ELECTRONICS CO LTD, DAEWOO ELECTRONICS CO., LTD.
Application year: 1999
Main classification: B29C45
IPC codes: B29C45