TW201738344A - Binder for injection molding - Google Patents

Abstract

Provided is a binder for injection molding, which includes 3 wt% to 20 wt% polyolefine having maleic anhydride groups, 30 wt% to 40 wt% polyolefine compounds, 5 wt% to 20 wt% polyoxymethylene, 1 wt% to 5 wt% stearic acid, and 35 wt% to 60 wt% wax. The binder for injection molding of the present invention is characterized by using the polyolefine having maleic anhydride to bond the polyolefine compounds and the polyoxymethylene so as to increase the compatibility between the components of the binder and pyrolysis duration, thereby reducing the defections and improving the size accuracy and the consistency of the objects.

Description

Injection molding bonding agent

The present invention relates to a binder for injection molding, particularly a binder for metal powder or ceramic powder injection molding.

Powder injection molding is a part manufacturing technology that combines plastic injection molding and powder technology. It can be mainly divided into Metal Injection Molding (MIM) and Ceramic Injection Molding (CIM). In the field of large applications, these technologies have the advantages of good surface condition, fast molding cycle, high precision and uniform quality, suitable for mass production of complex shapes and reduced subsequent processing, etc., and thus have received extensive attention from the industry. The basic step of powder injection molding is to add metal powder or ceramic powder into a binder, use a binder as a carrier, and blend at a suitable temperature to be a plastic to be plastic, and then eject it into a green embryo through an injection machine. The binder in the green embryo is removed by a degreasing process and then sintered into parts. The above process involves the interaction between the binder and the powder, so the nature of the binder is closely related to whether the article can be successfully formed or even further high precision.

The main function of the binder is to provide a carrier, reduce the friction between the powders, make it fluid, and maintain the shape of the embryo when degreasing. The single component binder is not easy to have multiple characteristics at the same time, so the binder is usually a multi-component design, which brings the advantage that the components can be gradually removed during the degreasing process, avoiding the momentary removal of a large amount of binder. The object is deformed or disintegrated. At the same time, the components that are removed first (such as paraffin wax, microcrystalline wax, etc.) can also generate initial pores, so that the gas generated during the degreasing process can smoothly spread out to the raw embryo, and the expansion of the embryo body is avoided. rupture.

The excellent binder must have good fluidity and formability of the plastic to be mixed after mixing, and it needs to have the function of dispersing the powder, avoiding agglomeration of the powder particles, and the embryo is not easily collapsed after degreasing, and is easily removed by heat. Produces the characteristics of residual carbon and the like. However, the polymer used in the multi-component binder usually has the characteristics of crystalline (such as polyoxymethylene (POM)) and amorphous (polyethylene (PE)), due to differences in properties and structure. The difference between them affects the compatibility with each other, resulting in a non-uniform phenomenon in the composition of the binder. This unevenness will make (1) the plastics lack good fluidity, increase the time required for the process, and even incomplete mold filling in the case of complex injection molded parts; and (2 In the subsequent degreasing process, defects such as deformation occur due to inconsistent rate of thermal decomposition. The above is the main reason why the product size and precision are not easily controlled, so it is necessary to strengthen the compatibility between the components in the bonding agent to meet the market demand for high-quality powder injection molded parts.

In view of the above-mentioned problems, it is an object of the present invention to provide a bonding agent for injection molding which enhances the compatibility between components in a bonding agent to satisfy the demand for producing a high-quality powder injection molded article.

According to the above object, the present invention provides a binder for injection molding, which may comprise: a polyolefin having a maleic anhydride group, a polyolefin compound, polyoxymethylene, stearic acid (stearic acid) ) and wax. Among them, the polyolefin having a maleic anhydride group may have an average molecular weight of from about 90,000 to about 110,000.

Preferably, the polyolefin compound and the polyoxymethylene are bonded by forming a bond with a polyolefin having a maleic anhydride group, respectively.

Preferably, the polyolefin having maleic anhydride groups may comprise from about 3 to about 20% by weight of the binder for molding; the polyolefin compound may comprise from about 30 to about 40 of the binder for injection molding. % by weight; polyoxymethylene may comprise from about 5 to about 20% by weight of the binder for injection molding; stearic acid may comprise from about 1 to about 5% by weight of the binder for injection molding; and the wax may It accounts for about 35 to about 60% by weight of the binder for injection molding.

Preferably, the polyolefin compound may comprise a high density polyethylene (HDPE) having a density of from about 0.950 to about 0.965 g/cm ^{3 or} a low density polyethylene having a density of from about 0.915 to about 0.930 g/cm ³ . (low density polyethylene, LDPE); the wax may be selected from the group consisting of paraffin wax, dense wax, Brazilian wax, microcrystalline wax, and combinations thereof.

The present invention also provides a composition for producing a molded article, which may comprise from about 30 to about 50% by volume of the binder for injection molding of the present invention; and from about 50 to 70% by volume of the sinterable powder. Wherein, the composition may be obtained by kneading the binding agent for injection molding of the present invention and the sinterable powder at about 170 to about 190 ° C for about 90 to about 150 minutes.

Preferably, the sinterable powder is selected from the group consisting of metal powders, metal alloy powders, metal carbonyl powders, ceramic powders, and combinations thereof.

According to the present invention, the binding agent for injection molding according to the present invention can form a bond with a polyolefin compound and polyoxymethylene by a polyolefin having a maleic anhydride group to improve the compatibility of the bonding agent itself. The effect, and thereby extend the temperature interval of the thermal decomposition of the bonding agent to reduce the probability of defects occurring during sintering. In addition, the improved compatibility of the bonding agent also contributes to the improvement of the fluidity of the green embryo, avoiding short shots during injection molding, and causing unsightly surfaces such as flow marks and welding lines.

A first embodiment of the present invention provides a five-component binder composition for injection molding, which may comprise a polyolefin having a maleic anhydride group, a polyolefin compound, a polyoxymethylene, a hard fat Acid and wax. In this embodiment, the polyolefin having maleic anhydride groups may have an average molecular weight of from about 90,000 to about 110,000, preferably about 100,000. The polyolefin compound may comprise high density polyethylene (HDPE) having a density of from about 0.950 to about 0.965 g/cm ³ or low density polyethylene (LDPE) having a density of from about 0.915 to about 0.930 g/cm ³ , preferably high density. Polyethylene. The wax may be selected from the group consisting of paraffin wax, dense wax, Brazilian wax, microcrystalline wax, and combinations thereof, preferably a group consisting of paraffin wax, microcrystalline wax, and combinations thereof, but is not limited thereto.

According to this first embodiment, the polyolefin, the polyolefin compound, the polyoxymethylene, the stearic acid and the wax having the maleic anhydride group are uniformly mixed at about 170 to about 190 ° C for about 90 to about 150 minutes. Preferably, the mixture is uniformly mixed at about 180 ° C for about 120 minutes to obtain a binder having high compatibility, wherein the polyolefin having a maleic anhydride group may comprise from about 3 to about 20% of the binder for molding. The weight percentage is preferably about 3%; the polyolefin compound may comprise from about 30 to about 40% by weight of the binder for injection molding, preferably about 33%; and the polyoxymethylene may comprise from about 5 to about 10% of the binder for injection molding. 20% by weight, preferably about 11%; stearic acid may comprise from about 1 to about 5% by weight of the binder for injection molding, preferably about 3%; and the wax may comprise about 10% by weight of the binder for injection molding. From 35 to about 60% by weight, preferably about 50%.

Referring to Fig. 1, there is shown a thermogravimetric analysis result of the bonding agent for injection molding, high-density polyethylene, polyoxymethylene, and the first comparative example of the second embodiment of the present invention. In the second embodiment of the present invention, the polyolefin having a maleic anhydride group may be a maleic anhydride modified low-density polyethylene (OREVAC ^® 18302N, modified with maleic anhydride). The density is 0.912 g/cm ³ ), which accounts for about 3% by weight of the injection molding binder; the polyolefin compound can be a high density polyethylene having a density of 0.955 g/cm ³ , which is about 33 for the injection molding binder. % by weight; polyoxymethylene is about 11% by weight of the binder for injection molding; stearic acid is about 3% by weight of the binder for injection molding; and wax is about the binder for injection molding. 50% by weight, wherein the ratio of paraffin to microcrystalline wax is 2:1. The above-mentioned five components are uniformly mixed at about 180 ° C for about 120 minutes to obtain a binding agent for injection molding according to the second embodiment of the present invention. Further, a first comparative example is further provided, except that in the first comparative example, in place of the maleic anhydride-modified linear low-density polyethylene, an ethylene vinyl acetate copolymer (EVA) is used. The remaining composition ratios and mixing methods are the same as those for the injection molding of the second embodiment of the present invention. The thermogravimetric analysis of the injection molding bonding agent, the high-density polyethylene, the polyoxymethylene and the first comparative example of the second embodiment of the present invention by thermogravimetric analysis can obtain the weight loss curves of the composites at different temperatures. . As shown in the figure, it can be known that the weight loss curve A of the single compound, that is, the weight loss curve A of the polyoxymethylene and the weight loss curve B of the high density polyethylene are quite narrow from the start of thermal decomposition to the end of thermal decomposition, and are only about The 80 °C interval (polyoxymethylene is from about 320 to about 400 ° C; high density polyethylene is from about 380 to about 460 ° C). On the other hand, the thermal decomposition process of the weight loss curve C of the injection molding bond of the present embodiment has a wide range (about 200 to 490 ° C) of about 290 °C. In comparison, the weight loss curve D of the first comparative example has only a temperature interval of about 265 ° C (about 200 to 465 ° C).

The above results show that the binder for injection molding of the second embodiment of the present invention is a binder having high compatibility. The basic principle is that a polyolefin with a maleic anhydride group can form a bond with a polyolefin compound and polyoxymethylene, so that the originally poorly compatible polyolefin compound and polyoxymethylene are bonded together by chemical bonds. The compatibility of the binder is manifested so that the thermal cracking interval is significantly prolonged. In addition, compared to the prior art that only uses van der Waals to improve the compatibility between the binder components (such as ethylene-vinyl acetate copolymer and polyethylene), the present case has the chemical bond, so the strength of the green embryo Take advantage of.

Referring to Fig. 2, there is shown a flow chart for producing an injection molded article using the bonding agent for injection molding of the present invention according to a third embodiment of the present invention. In this embodiment, the binder is the binder for injection molding of the present invention. The sinterable powder may be selected from the group consisting of metal powder, metal alloy powder, metal carbonyl powder, ceramic powder, and combinations thereof, but is not limited thereto. As shown, the implementation process can include the following steps:

Step S1: The binder for injection molding of the present invention is uniformly mixed with the sinterable powder. Among them, the binder for injection molding of the present invention accounts for about 30 to about 50% by volume and the sinterable powder accounts for about 50 to about 70% by volume.

Step S2: The mixture obtained in the step S1 is kneaded at about 180 ° C for about 120 minutes to obtain a composition.

Step S3: The composition obtained in the step S2 is cooled, and then further pulverized and granulated to facilitate injection molding of the molding machine.

Step S4: The pellet obtained in the step S3 is injection-molded into a desired object green embryo by an injection molding machine.

Step S5: The object embryo is placed in a degreasing furnace and held at about 800 ° C for about 120 minutes to remove the binder.

Step S6: placing the degreased article embryo into a sintering furnace, holding the temperature at about 1,200 to about 1,360 ° C, preferably about 1,360 ° C for about 120 to about 240 minutes, preferably about 120 minutes, to sinter the green embryo. You can get the items you need.

Please refer to Fig. 3, which is a photograph of the green embryo after sintering in the second comparative example obtained by applying the flow of Fig. 2. The second comparative example is substantially the same as the third embodiment of the present invention, and the binder for injection molding in step S1 accounts for about 40% by volume and the sinterable powder accounts for about 60% by volume, except for the second comparative example. The binder for injection molding consists only of linear low-density polyethylene modified with maleic anhydride, stearic acid and wax, wherein linear low-density polyethylene modified with maleic anhydride accounts for injection molding. The agent is about 47% by weight; stearic acid accounts for about 3% by weight of the injection molding binder; and the wax accounts for about 50% by weight of the injection molding binder, and the ratio of paraffin to microcrystalline wax is 2:1. . As shown in the figure, it can be known that the linear low-density polyethylene modified by maleic anhydride can combine the sinterable powder and form a green embryo with a certain strength, but the deformation state after the green embryo is sintered is not Very ideal, showing a large bending state.

Please refer to Fig. 4, which is a photograph of the green embryo after sintering according to the third embodiment of the present invention. In this embodiment, the binder used is the binder for injection molding of the second embodiment of the present invention, and the binder for injection molding in step S1 accounts for about 40% by volume and the sinterable powder accounts for about 60% by volume. . Comparing Figs. 3 and 4, it can be seen that the deformation state of the article obtained according to the third embodiment of the present invention is significantly smaller than that of the article prepared according to the second comparative example, thereby confirming that the bonding agent of the present invention can solve the aggregation. The problem of poor compatibility between formaldehyde and high-density polyethylene can further improve the occurrence of defects or deformation during sintering.

Please refer to Fig. 5, which is a graph showing the results of viscosity and shear rate of the composition of the fourth embodiment of the present invention and the third comparative example. In the fourth embodiment of the present invention, the composition can be produced in accordance with steps S1 to S2 in Fig. 2, wherein the binder for injection molding can be the same as the binder for injection molding of the second embodiment of the present invention, and can be sintered. The powder may be a metal powder (Mitsubishi 17-4PH). In the third comparative example, the fourth embodiment of the present invention is the same except that the M3 binder of Japan ATECT Co., Ltd. is used instead of the binder for injection molding of the second embodiment of the present invention. As shown, the viscosity of the composition of the fourth embodiment is smaller or substantially the same as the viscosity of the third comparative example within the range of the measured shear rate. It can thus be seen that the composition of the fourth embodiment of the present invention has a higher or substantially the same fluidity as the composition prepared using a commercially available binder, whereby the subsequent injection molding process can be performed. A better or substantially the same fill rate is achieved.

As described above, the composition composed of the binder for injection molding of the present invention can be solved by the formation of a bond between the polyolefin having a maleic anhydride group in the binder and the polyolefin compound and polyoxymethylene. The problem of poor compatibility between formaldehyde and high density polyethylene increases its compatibility, resulting in a significant extension of its thermal cracking interval compared to prior art techniques using a binder of ethylene-vinyl acetate copolymer. By utilizing this feature, the raw embryo can be slowly and evenly removed during the degreasing and sintering process, thereby avoiding deformation or disintegration of the object due to the removal of too much binder in a short time, so that the defects of the object can be effectively reduced. Improve the accuracy and consistency of its dimensions. In addition, the improved compatibility of the bonding agent also contributes to the improvement of the fluidity of the composition, and it is possible to avoid formation of bubbles inside the green embryo during filling or to create holes or corners in the corners of the mold due to incomplete filling. Finally, the composition composed of the binder for injection molding of the present invention can remove the binder only by high-temperature sintering, and does not require the occurrence of contamination and the dissolution and acid stripping process of subsequent waste disposal problems.

The above is intended to be illustrative only and not limiting. Any equivalent modifications or alterations to the spirit and scope of the invention are intended to be included in the scope of the appended claims.

S1~S6‧‧‧ steps.

Fig. 1 is a graph showing the results of thermogravimetric analysis (TGA) of the bonding agent for injection molding, high-density polyethylene, polyoxymethylene, and the first comparative example of the second embodiment of the present invention.

Fig. 2 is a flow chart showing the manufacture of an injection molded article using the bonding agent for injection molding of the present invention in the third embodiment of the present invention.

Fig. 3 is a photograph of the green embryo after sintering of the second comparative example obtained by applying the flow of Fig. 2.

Fig. 4 is a photograph of the green embryo after sintering according to the third embodiment of the present invention.

Fig. 5 is a graph showing the results of viscosity and shear rate of the fourth embodiment and the third comparative example of the present invention.

Claims (6)

A binder for injection molding, comprising: a polyolefin having a maleic anhydride group; a polyolefin compound; polyoxymethylene; stearic acid; and a wax, wherein the maleic anhydride group is provided The polyolefin has an average molecular weight of from about 90,000 to about 110,000. The binding agent for injection molding according to the above aspect of the invention, wherein the polyolefin compound and the polyoxymethylene are bonded by being bonded to the polyolefin having the maleic anhydride group, respectively. The binding agent for injection molding according to claim 1, wherein the maleic anhydride group-containing polyolefin accounts for about 3 to about 20% by weight of the injection molding binder; The compound comprises from about 30 to about 40% by weight of the binder for injection molding; the polyoxymethylene comprises from about 5 to about 20% by weight of the binder for injection molding; and stearic acid comprises from about 1 to about 10% of the binder for injection molding. A weight percentage of about 5%; and the wax comprises from about 35 to about 60% by weight of the binder for injection molding. The bonding agent for injection molding according to claim 1, wherein the polyolefin compound comprises a high density polyethylene having a density of from about 0.950 to about 0.965 g/cm ³ or a density of from about 0.915 to about 0.930 g/cm. A low density polyethylene of ³ ; the wax is selected from the group consisting of paraffin wax, dense wax, Brazilian wax, microcrystalline wax, and combinations thereof. A composition for producing a molded article, comprising: from about 30 to about 50% by volume of the binding agent for injection molding according to any one of claims 1 to 4; and from about 50 to about 70% by volume a sinterable powder, wherein the composition is mixed with the sinterable powder by about 90 to about 90% to about 190 ° C of the binding agent for injection molding according to any one of claims 1 to 4 It takes about 150 minutes. The composition of claim 5, wherein the sinterable powder is selected from the group consisting of metal powders, metal alloy powders, metal carbonyl powders, ceramic powders, and combinations thereof.