Patent Application
20090022615
Powder injection molding (PIM) is a process that uses fine metal or ceramic powder to create a wide variety of parts. The process includes combining the powder with a binder to form a feedstock. The feedstock is injected into a mold and cooled to create a green part. The binder is removed from the green part to form a brown part. The brown part is sintered to form a final part that is ready to be used alone or in combination with one or more other parts.
Although PIM provides a number of advantages, it also suffers from a number of problems. One problem associated with PIM is that it is difficult to prepare parts that have complex shapes such as undercuts and the like. Although attempts have been made to remedy this problem, past approaches still suffer from a number of drawbacks.
One approach is to insert mold the part using a sacrificial component. The sacrificial component is preformed and inserted into the mold manually or robotically. The PIM feedstock is injected into the mold and surrounds the sacrificial component. The part is removed from the mold and processed to eliminate the sacrificial component. The part is eventually sintered to give it strength and rigidity.
Unfortunately, this process suffers from a number of disadvantages. One problem is that it takes a significant amount of time to place each sacrificial component in the mold. This reduces the throughput rate of the part, which adversely affects the economics of the molding operation. Another problem is that a robot or a human is used to place the sacrificial component in the mold. This can be costly and may introduce errors into the process. Yet another problem is that the sacrificial component tends to cause defects such as cracks in the final part.
Other approaches to molding complex structures are even more complicated. These processes often use complex tooling that collapses or is otherwise movable to create an undercut structure. The tooling costs are extremely high for such a process. The potential for something to go wrong is also high, which typically means that the molding machine is subject to additional maintenance and checks which can be costly.
For at least the above reasons, it would be desirable to provide an improved process for molding complex parts.
A number of processes are described herein that can be used to form complex parts. In one embodiment, a multi-shot process is used to form the sacrificial component and inject the PIM feedstock. The multi-shot process involves creating the sacrificial component and the part as one or more shots in the same press. The sequence and number of shots can be varied as desired. For example, the sacrificial component may be formed by injecting sacrificial material in one or more shots. Likewise, the PIM feedstock may also be formed in one or more shots. The shots may be ordered so that the sacrificial component is formed first then the part. Alternatively, the shots may be ordered so that a portion of the part is formed initially, the sacrificial component is formed, and the remainder of the part is formed. Each of these steps may include one or more shots, e.g., the initial portion of the part may be formed in one or more shots.
The green part may undergo further processing to remove the sacrificial component, debind the part, and/or sinter the part. It should be appreciated that the processes described herein may include any number and/or combination of these additional steps in any order. For example, debinding and removing the sacrificial component may occur simultaneously or separately from each other. If these steps are performed separately, they may be performed in any order.
In another embodiment, an insert molding process is used and the sacrificial component is heated before it is placed in the mold. The insert molding process involves preforming the sacrificial component in one press, heating the sacrificial component, placing it in the mold of another press, and injecting the PIM feedstock into the mold. The green part may undergo further processing (e.g., debinding, sintering, etc.).
The sacrificial component may be heated using any suitable method or process. In one embodiment, the sacrificial component may be transported through a furnace or oven that heats it. In another embodiment, the sacrificial component may be heated using a hot plate. The sacrificial component may be heated in a continuous, semi-continuous, or batch-wise process.
In another embodiment, an improved process is described for plastic injection molding. The plastic injection molding process may be similar to any of the PIM processes except that plastic is used instead of metal and/or ceramic powder and the steps of debinding and sintering may be eliminated. The plastic injection molding process may also include heating a preformed sacrificial component before placing it into the mold and forming the sacrificial component and the part in a multi-shot process.
The processes described herein may provide a number of advantages. One potential advantage is that the processes reduce the number and/or severity of defects in the final part. It has been discovered that a large temperature difference between the sacrificial component and the PIM feedstock contributes to the formation of defects. Heating the preformed sacrificial component or forming the sacrificial component in a multi-shot process reduces the temperature difference between the sacrificial component and the PIM feedstock thereby reducing the number and/or severity of defects in the part.
Other advantages are that the processes allow for the production of molded parts that have complex shapes and tighter tolerances. Also, molded parts made using the processes described herein may not require as much costly secondary processing. The multi-shot processes may also provide the additional advantage of increasing the throughput rate of the part by eliminating the steps of preforming the sacrificial component and placing it in the mold.
The terms “insert mold,” “insert molding,” and the like are used herein to refer to molding processes where one component of the part is preformed in one molding press and then is inserted, either manually, robotically, or otherwise, into another molding press where the final part is made. The terms “multi-shot,” “multi-shot molding,” and the like are used herein to refer to molding processes where two or more components of the part are formed with the same molding press. Multi-shot molding may use one or more of the following technologies to form a multi-component part—robotic transfer within the same press, coinjection, core toggle, rotary platen, rotary stack, and/or indexing plate. Preferred technologies for multi-shot molding include rotary platen, indexing plate, and/or core toggle.
References to molding the feedstock or a part “around” or “over” a sacrificial component or sacrificial material should not be interpreted as requiring the feedstock to surround the sacrificial component. Rather, references such as this should be interpreted as indicating that the feedstock or part contacts, even in a minor way, the sacrificial component or sacrificial material.
A number of processes are described herein that may be used to mold parts having complex shapes. Although the subject matter described herein is described primarily in the context of PIM processing, it should be understood that the concepts and features referred to may be used in plastic injection molding as well as other settings and situations as would be recognized by those of ordinary skill in the art. Also, it should be understood, that the features, advantages, characteristics, etc. of one embodiment may be applied to any other embodiment to form an additional embodiment unless noted otherwise.
The powder 102, the binder 104, and the additives 106, if any, may be compounded 108 in any suitable manner. In general, it is desirable for the feedstock to have uniform density and to coat each particle of the powder 102 with the binder 104. In one embodiment, the powder 102, binder 104, and/or additives 106 may be combined in a manner that creates a shearing action that helps coat each particle and uniformly distribute the materials. This may be accomplished using a Z blade mixer, planetary mixer, or other suitable equipment.
Once the feedstock has been thoroughly mixed at step 108, the feedstock may be granulated or fed directly to the molding operation 110. If the feedstock is granulated, it is processed to create solid pellets that can be fed into the mold later in the process 100. The solid pellets make it simple and easy to store and transport the feedstock.
Any suitable metal and/or ceramic powders 102 may be used in the process 100. The primary limitation on the powder 102 is that it should be capable of forming a rigid structure upon sintering. Suitable metal powders may include powders of low-alloy steels to stainless steels, soft magnetic alloys to tool steels, Ni-super alloys to titanium, tungsten, ferrous and non-ferrous alloys, or any suitable combinations thereof. Suitable ceramic powders may include powders of alumina, zirconia toughened alumina, zirconia, and other oxides, nitrides, carbides, or combinations thereof.
From an economic standpoint, expensive powders and other raw materials may be especially suited for the process 100. The process 100 has the advantage that very little, if any, powder 102 or other raw material ends up as scrap. In contrast, alternative processes that involve machining and the like produce a significant amount of scrap. The reduced amount of scrap associated with the process 100 helps to make it more economical and efficient.
The powder 102 may include particles having any suitable size and/or shape. In general, it is desirable to incorporate as much powder 102 in the part as possible. Also, it is often desirable to provide a range of powder sizes because although fine powders tend to sinter more readily than coarser powder, the smaller particles are more expensive and may present other limiting factors. In one embodiment, the powder may have a mean diameter of about 5 to 15 microns.
In one embodiment, the powder 102 may have any one or more of the following features. The powder 102 may have a particle size distribution (i.e., a mixture of lower cost large particles and higher cost small particles) that is tailored for high packing density and low cost. The particles may be spherical or predominantly spherical to resist agglomeration in the feedstock. The particle shape may provide sufficient interparticle friction to prevent the part from collapsing when the binder is removed. The particles may also be clean of other substances that may cause unpredictable interactions with the binder 104, additives 106, or other components used in the process 100.
Any suitable binder 104 may be used in the process 100. The binder 104 should provide suitable rheological properties to allow the molten feedstock to be injected into the mold. The binder 104 should also be capable of being removed from the green part at the debinding step 112 of the process 100. The choice of binder 104 may depend on which process is used to remove it.
The binder 104 may be removed catalytically, thermally, or chemically. Binders that can be removed catalytically include polyacetal based binders used with BASF's Catamold feedstock. Binders that can be removed thermally include plastics such as polyethylene and polypropylene as well as stearic acid, micropulvar wax, and/or paraffin wax. Binders that can be removed chemically (e.g., with a solvent) include waxes and the like.
The binder 104 may include a single material or a combination of different materials. The binder 104 may include a polymer material. The binder 104 may also include an organic material, desirably, an organic polymeric material. The binder 104 may be selected to allow it to degrade at different rates during the debinding process 112.
In one embodiment, the binder may be a polyacetal based plastic binder. In another embodiment, the binder may be any one or combination of the binders described in U.S. Pat. No. 5,362,791 to Ebenhoech et al. (the '791 Patent), the entire contents of which are incorporated herein by reference.
Referring to
The insert molding process 120 includes the steps of molding the sacrificial component 122, heating the sacrificial component 124, and molding the part around the sacrificial component 126. In this embodiment, the sacrificial component is preformed in a separate molding press. The sacrificial component may be preformed immediately prior to being placed in the part mold or it may be preformed much earlier and, potentially, at a different geographical location.
The insert molding process 120 is an improvement over conventional insert molding processes because the sacrificial component is heated before it is placed in the part mold. Heating the sacrificial component in this manner reduces the number and/or severity of defects in the part.
It has been discovered that many of the defects that result from conventional PIM insert molding occur due to the temperature difference between the sacrificial component and the molten feedstock. Because the sacrificial component is preformed, it is much cooler when it is placed in the mold than the molten feedstock. When the feedstock contacts the sacrificial component, heat flows from the hotter feedstock to the cooler sacrificial component. As the feedstock cools, it has trouble flowing in the vicinity of the sacrificial component. Also, as the feedstock cools and contracts, the sacrificial component heats up and expands. These opposing forces cause cracks in the part.
The sacrificial component should be heated enough to prevent it from prematurely cooling the feedstock but not so hot that the sacrificial component becomes deformed. If the sacrificial component is heated too much the molten feedstock will heat it too much and deform it resulting in a malformed part. If the sacrificial component is not heated enough, it will prematurely cool the feedstock and cause cracks and/or other defects. A balance should be reached where the sacrificial component is heated to a temperature that is low enough to prevent deformation but high enough to prevent the part from cracking.
The sacrificial component may be heated using any suitable method. In one embodiment, the sacrificial component may be heated in a furnace. In another embodiment, the sacrificial component may be heated on a hot plate. The sacrificial component may be heated in a continuous, semi-continuous, or batchwise process.
Any suitable sacrificial material may be used to make the sacrificial component. In general, the sacrificial material should be capable of being removed from the green part. The sacrificial material may be the same as the binder 104 or may be a separate material. The sacrificial component may also be made of a single sacrificial material or multiple sacrificial materials. Different sacrificial materials may be combined so that different parts of the sacrificial component degrade at different rates. The choice of which sacrificial material depends largely on which method is used to remove the sacrificial material.
The multi-shot process 130 includes the steps of molding the sacrificial component in one or more shots 132 and molding the part in one or more shots 134. In this embodiment, the sacrificial component and the part are molded in the same press. One of the advantages of this process 130 is that the feedstock is injected before the sacrificial component can cool, which fosters temperature stability between the feedstock and the sacrificial component. Thus, the sacrificial component does not cause cracks in the part. However, the shot of feedstock can be delayed long enough to allow the sacrificial component time to solidify.
Another advantage of the process 130 is that the step of placing the sacrificial component in the mold is eliminated. This may increase the throughput rate of the process 130 as well as eliminate the cost of labor and/or robotic equipment that would otherwise be needed to place the sacrificial component in the mold.
Yet another advantage of the process 130 is that concerns related to shrinkage of the sacrificial component are greatly reduced because the sacrificial component is not allowed to cool significantly. In contrast, shrinkage concerns are much greater in conventional insert molding process because the insert has been allowed to cool before it is put into the mold.
An additional advantage of the process 130 is that part to part variation may be minimized for hydroscopic materials because the components molded by each shot do not absorb significant amounts of moisture. Also, part to part variation may also be minimized for semi-crystalline materials because crystalline formation doesn't occur until the entire part is cooled.
The various shots of sacrificial component and feedstock may occur in any order. In one embodiment, the sacrificial component may be completely formed in one or more shots and then the part may be formed in one or more shots. In another embodiment, the multi-shot process 130 may include forming a portion of the part may first, forming the sacrificial component next, and finally forming the remainder of the part.
The processes 120, 130 may be carried out using any suitable equipment. For example, single screw extruders may be used to inject the sacrificial component and/or the feedstock into the die cavity. The temperature of the screw and the nozzle may be controlled to melt the binder in the feedstock and the sacrificial component so that they can be injected into the die cavity. The die temperature may also be controlled at a lower temperature that allows the part to cool and become rigid.
The tooling for the PIM equipment is similar to that of traditional injection molding equipment with one major difference. The PIM tooling is oversized to allow for shrinkage when the part is sintered.
At step 112, the binder 104 is removed from the green part to form a brown part. The binder 104 may be removed catalytically, thermally, chemically, or using any other suitable process. The preferred method is to remove the binder 104 catalytically.
Catalytic debinding involves feeding green parts into an oven heated to 110-140° C. in the presence of a catalyst. The catalyst may include gaseous nitric acid and/or oxalic acid. The catalyst breaks down the binder 104 into small volatile molecules. These molecules have a relatively high vapor pressure and diffuse rapidly out of the part. Binder removal proceeds from the outside inward thereby preventing pressure build-up in the interior of the part. This promotes a very uniform and rapid debinding from the exterior surface into the center of the part.
The binder decomposition front moves inward at a rate of 1-2 mm per hour making catalytic debinding much faster than other techniques. Furthermore, unlike other debinding techniques, there is no limit on the thickness of the part that can be debound. The small molecules generated by the process have a high vapor pressure, which greatly minimizes the potential for capillary condensation and allows thick part sections to be debound.
Catalytic debinding has another advantage over other debinding techniques. Equipment has been developed for catalytic debinding that allows it to be operated on a continuous basis instead of a batchwise basis. It should be appreciated, however, that the catalytic debinding process can also be operated on a semi-continuous or batchwise basis as well.
Thermal debinding involves passing the green part through a furnace or other heat source that melts, decomposes, and/or volatilizes the binder 104. The debinding process may remove all or substantially all of the binder. Any remaining binder residue may be removed during sintering. If thermal debinding is used, the binder 104 should be capable of melting, decomposing, and/or volatilizing at elevated temperatures. The binder 104 may include several ingredients which melt, decompose, or volatilize at different temperatures to prevent the disruption of the part as it is thermally debound.
Chemical debinding involves dissolving the binder 104 with a solvent. One example of a suitable solvent is trichloroethane. The part may be heated to further volatilize the solvent and/or binder 104. Other examples of suitable solvents include hexane and water.
The debinding process 112 reduces the strength of the green part significantly. Thus, after debinding, care should be exercised when handling the brown part.
The debinding step 112 depicted in
The sacrificial component may be removed using any of the same processes used to remove the binder 104. In one embodiment, the sacrificial component and the binder 104 may be removed catalytically. In this embodiment, the sacrificial component and the binder 104 may both be polyacetal based.
The brown part is sintered to form the finished part at step 114. Sintering refers to the process in which the separate powder particles weld together to provide strength and rigidity to the finished part. Sintering involves heating the brown part to a temperature that is slightly below the melting point of the powder 102. If the temperature reaches the melting point of the powder 102, the brown part will lose its shape and slump. The temperature should be carefully controlled to prevent this from happening.
The part may be sintered in a reducing atmosphere to prevent the powder 102 from oxidizing. This is especially a concern when the powder 102 includes metal. The reducing atmosphere may also reduce any metal oxide that exists on the surface of the powder particles.
The composition of the reducing atmosphere depends on the particular powder 102 being used. A hydrogen atmosphere may be used for many metal parts. If the powder contains an iron/carbon alloy (e.g., carbon steels), then it may be desirable for the atmosphere to contain a carbon compound or compounds so that the carbon content of the atmosphere is in equilibrium with the carbon content of the powder. This prevents the atmosphere from carburizing or decarburizing the metal. The part may also be sintered in a vacuum.
After the binder 104 is removed at step 112, the brown part is very porous. As the part is sintered it shrinks substantially. For example, the part may shrink about 20 wt %. Although the shrinkage is large, it is also uniform, provided that the feedstock was of uniform density. The finished part typically has a density closely approaching the theoretical density for the material. The finished part may have a density of at least about 95 wt %, at least about 97 wt %, or, desirably, at least about 98 wt % of the theoretical density of the material. The mechanical properties of the finished part are also similar to the mechanical properties of a fully dense part (e.g., a wrought metal part).
The part may be sintered in any of a number of suitable furnaces or ovens. In one embodiment, the part may be sintered in a batch vacuum furnace, continuous atmosphere furnace, and/or batch atmosphere furnace.
The size and weight of the final part are consistent from part to part. The variability of dimensions and weights from part to part is minimal. Close tolerances of dimensions and weight may reduce or eliminate the need for secondary machining processes which can be costly and difficult. Tolerances as small as ±0.003 inch per linear inch can be obtained without secondary processing.
After the part has been sintered, it may be removed from the furnace and used without any further processing. Alternatively, the parts may be subjected to additional secondary operations such as heat treatment to harden the part, surface coating the part, hot isostatic pressing until the part reaches full density, glass beading cleaning to remove impurities from the sintered surface, and tumbling to smooth off sharp edges and remove barbs.
The processes 100, 120, 130 or any of the individual steps in the processes 100, 120, 130 may be operated on a continual, semi-continual, or batchwise basis. For example, the molding machines used to create the part and/or the sacrificial component may be highly automated and run continuously. A sintering furnace may also be provided that has a large throughput and is capable of running continuously as well.
Referring to
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The processes described herein are not limited to PIM processes. They may also be applicable to plastic injection molding. The same concepts related to the use of the sacrificial component may be applied to plastic injection molding. It should be appreciated that plastic injection molding processes may not include the steps of debinding and sintering.
The processes may be used to make any of a number of complex features. In one embodiment, the processes may be used to make undercut features in PIM parts. In another embodiment, the processes may be used to make features such as re-entrant angles, multi-shaped blind holes, screw threads, surface profiles, perpendicular holes, intricate cavities, and undercuts.
The processes described herein may be used to make any of a number of plastic, metal, or ceramic, parts. In one embodiment, the processes may be used to make high tolerance parts such as those make in the automotive field for turbocharger, valve train, gearbox, seat-adjustment, fuel injection and sensor systems parts. The parts may also be used in the following fields: medical, dental, firearms, and aerospace, to name but a few.
One technology area where it may be very desirable to use parts made using the processes described herein is pressure sensing systems. In one embodiment, the processes may be used to make a pressure sensor that is used to sense the brake fluid pressure in an automobile.
Reference is made in the following to a number of illustrative embodiments of the subject matter described herein. The following embodiments illustrate only a few selected embodiments that may include the various features, characteristics, and advantages of the subject matter as presently described. Accordingly, the following embodiments should not be considered as being comprehensive of all of the possible embodiments. Also, features and characteristics of one embodiment may and should be interpreted to equally apply to other embodiments or be used in combination with any number of other features from the various embodiments to provide further additional embodiments, which may describe subject matter having a scope that varies (e.g., broader, etc.) from the particular embodiments explained below. Accordingly, any combination of any of the subject matter described herein is contemplated.
The terms used herein should be given their ordinary and customary meaning as determined by reference to relevant entries (e.g., definition of “plane” as a carpenter's tool would not be relevant to the use of the term “plane” when used to refer to an airplane, etc.) in dictionaries (e.g., widely used general reference dictionaries and/or relevant technical dictionaries), commonly understood meanings by those in the art, etc., with the understanding that the broadest meaning imparted by any one or combination of these sources should be given to the claim terms (e.g., two or more relevant dictionary entries should be combined to provide the broadest meaning of the combination of entries, etc.) subject only to the following exceptions: (a) if a term is used herein in a manner more expansive than its ordinary and customary meaning, the term should be given its ordinary and customary meaning plus the additional expansive meaning, or (b) if a term has been explicitly defined to have a different meaning by reciting the term followed by the phrase “as used herein shall mean” or similar language (e.g., “herein this term means,” “as defined herein,” “for the purposes of this disclosure [the term] shall mean,” etc.). References to specific examples, use of “i.e.,” use of the word “invention,” etc., are not meant to invoke exception (b) or otherwise restrict the scope of the recited claim terms. Other than situations where exception (b) applies, nothing contained herein should be considered a disclaimer or disavowal of claim scope. The subject matter recited in the claims is not coextensive with and should not be interpreted to be coextensive with any particular embodiment, feature, or combination of features shown herein. This is true even if only a single embodiment of the particular feature or combination of features is illustrated and described herein. Thus, the appended claims should be read to be given their broadest interpretation in view of the prior art and the ordinary meaning of the claim terms.
As used herein, spatial or directional terms, such as “left,” “right,” “front,” “back,” and the like, relate to the subject matter as it is shown in the drawing FIGS. However, it is to be understood that the subject matter described herein may assume various alternative orientations and, accordingly, such terms are not to be considered as limiting. Furthermore, as used herein (i.e., in the claims and the specification), articles such as “the,” “a,” and “an” can connote the singular or plural. Also, as used herein, the word “or” when used without a preceding “either” (or other similar language indicating that “or” is unequivocally meant to be exclusive—e.g., only one of x or y, etc.) shall be interpreted to be inclusive (e.g., “x or y” means one or both x or y). Likewise, as used herein, the term “and/or” shall also be interpreted to be inclusive (e.g., “x and/or y” means one or both x or y). In situations where “and/or” or “or” are used as a conjunction for a group of three or more items, the group should be interpreted to include one item alone, all of the items together, or any combination or number of the items. Moreover, terms used in the specification and claims such as have, having, include, and including should be construed to be synonymous with the terms comprise and comprising.
Unless otherwise indicated, all numbers or expressions, such as those expressing dimensions, physical characteristics, etc. used in the specification (other than the claims) are understood as modified in all instances by the term “approximately.” At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the claims, each numerical parameter recited in the specification or claims which is modified by the term “approximately” should at least be construed in light of the number of recited significant digits and by applying ordinary rounding techniques. Moreover, all ranges disclosed herein are to be understood to encompass and provide support for claims that recite any and all subranges or any and all individual values subsumed therein. For example, a stated range of 1 to 10 should be considered to include and provide support for claims that recite any and all subranges or individual values that are between and/or inclusive of the minimum value of 1 and the maximum value of 10; that is, all subranges beginning with a minimum value of 1 or more and ending with a maximum value of 10 or less (e.g., 5.5 to 10, 2.34 to 3.56, and so forth) or any values from 1 to 10 (e.g., 3, 5.8, 9.9994, and so forth).
This patent application claims priority to U.S. Provisional Patent Application Ser. No. 60/951,189, entitled “Method of Injection Molding Complex Structures,” filed on 20 Jul. 2007, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60951189 | Jul 2007 | US |
