Patent Application
20120114909
As is known in the art, products can be fabricated by filling a mold containing a part with a liquid material that hardens, such as by heating or exposure to ultraviolet light. This process allows a viscous, uncured material to enter a mold cavity and form into a final shape. During the execution of this type of molding, there are a number of parameters that are defined and managed to yield high quality parts. These parameters include, for example, material type, viscosity, part size, part shape, surface resistance on the mold and part, mold shape, mold and part materials, material fill speed, fill pressure, material and part temperature, surface finish, material thickness, and material pot life.
One issue in the molding process for parts and molds of certain shapes and materials is the formation of voids, e.g., bubbles, which can render the finished product unusable. The disadvantages of voids are well known to one of ordinary skill in the art. In some applications, excessive voids increase manufacturing cycle times and decrease acoustic performance of sensors, for example.
The present invention provides method and apparatus for a mold that minimizes the formation of voids in the molded product by fill port orientation, vent placement, and/or mold assembly manipulation based upon part surface features, fill material, material viscosity, fill rate, and/or volume of material. With this arrangement, higher yields can be achieved since voids in the molded product are reduced. While exemplary embodiments of the invention are shown and described in conjunction with certain part surface features, materials, molded products, mold shapes, mold connect, disconnect features, etc., it is understood that embodiments of the invention are applicable to molding in general, in which it is desirable to reduce bubble formation.
In one aspect of the invention, a method comprises identifying a first surface feature on a part to be molded that can accumulate bubbles in a fill material as a mold of a mold assembly is filled with the fill material, having the mold assembly positioned to a first orientation when the mold assembly is filled at a first amount, and having the mold assembly positioned to a second orientation when the mold assembly is filled to a second amount, wherein manipulating the mold assembly to the first and second orientations minimizes bubbles associated with the first surface feature.
The method can further include one or more of the following features: the first surface feature includes an overhang on a surface of the part, the first surface feature includes an overhang formed by a cap, a fill port for the mold assembly, wherein the part includes a second surface feature that is substantially elongate with a longitudinal axis, wherein a direction of flow material from the fill port is generally parallel to the longitudinal axis of the second surface feature, the second surface feature includes a wire and the fill port is substantially parallel to the direction of flow material from the fill port, a locating a vent in the mold assembly in relation to the first surface feature to provide an exit path for the bubbles created by the first surface feature, and the vent is located on a side of the mold.
In another aspect of the invention, a method comprises: identifying an elongate first surface feature having a longitudinal axis on a part to be molded that can accumulate bubbles as a mold of a mold assembly is filled with a fill material, and locating a fill port for the mold assembly in relation to the longitudinal axis of the first surface feature such that a direction of flow of fill material from the fill port is generally parallel to the longitudinal axis of the first feature to reduce turbulence during the filling process.
The method can further include one or more of the following features: the first surface feature comprises at least one wire extending along a length of the part, and wherein the mold assembly includes an overfill cavity, placing a vent in the mold assembly in relation to a second surface feature of the part, the vent is configured to provide an exit passage for bubbles created by the second surface feature, which includes an overhang, the vent is located on a side of the mold, and manipulating a position of the mold assembly during the fill process to minimize bubbles from the second surface feature.
In a further aspect of the invention, a mold assembly comprises: a mold having a mold cavity, a part located in the mold cavity, the part having first and second surface features, and a fill port to fill the mold cavity with a fill material for fabricating a molded part, wherein the fill material flows from the fill port in a direction generally parallel to a longitudinal axis of the first surface feature on the part.
The mold assembly can further include one or more of the following features: an orientation mechanism to position the mold assembly in a first orientation when the mold assembly is filled at a first amount, and position the mold assembly at a second orientation when the mold assembly is filled to a second amount, wherein manipulating the mold assembly to the first and second orientations minimizes bubbles associated with a second surface feature on the part, and at least one vent located in relation to the second surface feature to provide an exit path for the bubbles associated with the second surface feature.
The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which:
The mold assembly 200 is placed at a fill angle 214 so that the mold cavity 202 fills with the material at an angle. While the surface of the fill material 204 will be horizontal due to the forces of gravity, the fill angle 214 results in a desired angle of the material surface across the mold cavity and surface features of the part to minimize voids associated with complex features of the part during the fill process, as described more fully below.
While the molded product is shown as an acoustic sensor having an elongate geometry, it is understood that exemplary embodiments of the invention are applicable to any product type and shape in which it is desirable to minimize voids.
In step 404, a location is determined for the mold assembly that minimizes turbulence during the filling process. In step 406, a fill angle is determined for the mold. The fill angle is selected to optimize the material flow to reduce material turbulence. Fill angle is determined by identifying surface features, such as overhangs, undercuts, holes or other significant features that may entrap bubbles or restrict flow around the molded part. The fill angle of the mold is selected to provide the cleanest material path during filling. If a single fill angle is insufficient, multiple fill angle adjustments may be made during the fill process in relation to the feature in the molded part.
Multiple fill positions may be required to optimize flow and reduce voids during the filling process. For example, the mold is initially set mold at an angle of 30° in the X,Y plane and filled to 50% of the cavity, as shown in
In an exemplary embodiment, an orientation mechanism 450 manipulates the mold assembly to a desired orientation for a selected amount of time. In one embodiment, the orientation mechanism 450 moves the mold assembly in a relatively low speed constant motion. In general, the orientation mechanism can move the mold assembly at speeds and angles in three dimensions to meet the needs of a particular application.
Referring again to
A variety of material and part quality parameters define the fill rate. Fill rate, which can be controlled by injection fill pressure, should not induce bubbles during cavity filling. For the case where a part contains specific features, such as undercuts, large overhanging features, significant changes in surface plane or surface recesses, the fill rate should be adjusted, e.g., lowered, to avoid bubble generation while the material flows into the mold cavity. Lower viscosity materials, short material pot life and material volume of the filled cavity define the nominal fill rate. The fill material can be ‘pulsed’ during the filling process to agitate entrapped bubbles for freeing the bubbles from entrapped areas or complex surfaces.
In step 414, an overfill amount is selected. The overfill volume (quantity) and duration is specified by determining an amount required for sufficient mold volume “flushing.” Flushing is required to clear bubbles or voids generated in the filling process or from complex surface features. The bubbles are moved through the mold and into the reservoir cavity by the excess material into the cavity during the overfill portion of the fill process.
The overfill cavity should be of sufficient size to accept the required overfill volume. In general, the overfill cavity is designed with significant reserve to be oversized, such as at least one quarter of the final part volume.
In step 416, it is determined whether voids in the mold are less than a size threshold and less than a number threshold. That is, it is determined whether the voids are less than a certain size and whether the number of voids is sufficiently small. In an exemplary embodiment, void volume is determined from visual inspection post cure to determine whether specific molding requirements are met. Measurement techniques include scales, calipers, inspection eye loupes with graduated scales and microscopes. If the voids are less than the size and number threshold, it is determined that the vent is optimized in step 416. If not, the process is adjusted to achieve the desired levels of void size and number.
Vent size (diameter of vent port or overall volume of the vent port) should be at least two times the diameter of the fill port. The vent port can be sized larger than the fill port to ensure the maximum sized bubble that entered the molded cavity from the fill port will easily pass through the molded cavity and exit the vent. Vent size may also be enlarged to allow for material curing, which restricts material flow. The larger vent port improves flow into the overfill cavity.
As noted above, specific part features, such as cavities, undercuts, and complex surfaces can increase bubble or void frequency in molded parts. In general, part features that can generate excessive voids are mapped. A mold is designed to mitigate flow and turbulence of the material during the molding process. The molds are designed to improve material flow into, through and out of the mold. The improvement in material is achieved by identifying to the extent possible optimal fill and vent quantity, sizes, angles and locations after identifying areas in the mold that may restrict or alter the normal flow of the material into the mold. By focusing on these areas, the mold and the molding process are designed with attention given to vent and filling design, as well as mold and part angle, as described below.
In an exemplary embodiment shown in
The mold 650 includes one or more fill ports 652 that decrease flow turbulence by locating the fill port such that material flows from the fill ports in a direction that is similar to the wires 602. With this arrangement, the fill ports 652 direct the material to flow along the length of the wires, and not across the wires. It is understood that material flow across the wires 602, as opposed to along the wires, results in significantly more turbulence during the fill process, which increases likeliness of bubbles and voids.
To minimize bubbles and voids created by the cap overhangs 606, the fill angle of the mold and venting of the mold take into account the location of the overhangs 606. In one embodiment, the cap overhangs 606 are perpendicular to the wires 602 and nominal material flow. As shown in
To further enhance bubble/void reduction, mold vents 610 are located directly above the cap overhangs 606 as shown in
As used herein, a vent refers to a passage, pocket, hole or other opening into the mold cavity to allow excess material, bubbles and/or potential material contaminates to exit the molded volume during the fill process. The vent provides a path for turbulent material and/or contaminates to exit the molded volume during filling. The vent can extend for a relatively small length of the part or for an entirety of the part/mold length to the meet the needs of a particular application.
In general, vents can have any suitable geometry, such as round or oval to enable the use of common machining tools and techniques for making the molds or molded volumes. The vents should ease the flow of material. To this end, the shape of the vent and its transition from the molded volume into the vent cavity/space should be smooth. Transitions with large fillets (rounded corners) generally provide optimal flow characteristics.
In an exemplary embodiment, the vent is located near a feature in the part of the mold that restricts material flow, such as an overhanging feature described below. The vent(s) are located and configured to improve the material path (or flow) around the part to be molded. Vents are typically located on the top or sides of the mold. The top/side locations allow voids and bubbles, which tend to float to the surface of the material, to escape during mold filling.
Side cavities are typically used to allow large pockets or groups of bubbles to escape. These larger cavities are typically located on the sides of the molded volume and provide extended areas for contaminates and bubbles to migrate during filling. These larger side cavities also can be used as an overfill volume during faster filling or so-called ‘flushing’ techniques. This ‘flushing’ process and side vent orientation assists with the more complex molded parts that require the trapped voids to be forced or flushed out of the cavity.
It is understood that a variety of suitable mold materials can be used to mold parts. A wide range of materials that are pourable or capable of being pressure fed will be readily apparent to one of ordinary skill in the art. It is further understood that parts having virtually any geometry can be molded in accordance with exemplary embodiments of the invention. In addition, pressures used to inject the mold material can be selected to meet the needs of a particular application. Exemplary pressures range from about 15 to about 30 psi.
Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.
This invention was made with government support under Contract No. N00014-99-2-0005 and subcontract No. ARL/MSA-043 ITO-01 awarded by the Department of the Navy. The government has certain rights in the invention.
