The present invention relates to the inexpensive manufacture of products having electronic components encapsulated in plastic housings.
Products such as fuel level detectors of the type mounted within the fuel tanks of automobiles or other vehicles incorporate electronic components, which must be protected against the corrosive effects of the fuel stored within the tank. To protect the electronic components from corrosion, it is desirable to embed the electronic components into a molded plastic housing or the like.
The most inexpensive method of making a plastic housing is to injection mold a thermoplastic material in which plastic pellets are heated under pressure until they become a viscous liquid and then inject the viscous liquid into a mold having an inner cavity which is complementary to the shape of the housing to be formed. The problem with injection molding, however, is that the injected plastic is under high pressure and at high temperatures which are harmful or destructive to various electronic components and therefore it is not common to encapsulate electronic components in a thermoplastic material using an injection molding process. In the transfer mold process, however, a thermoset material, in which a resin and a hardener chemically react is utilized. This process does not require that the mold be preheated to temperatures that are destructive to electronic components or require that the liquefied plastic be subjected to intense pressure and is therefore suitable for encapsulating electronic components. The thermoset chemicals used in a transfer molding process, however, are considerably more expensive than the thermoplastic chemicals used in an injection molding process, and as a result, a product requiring a housing made of a thermoset plastic can be considerably more expensive to manufacture than a thermoplastic housing made with an injection molding process.
Although a thermoplastic and injection molding are unsuitable for encapsulating electronic components, the process is suitable for making moveable plastic parts, such as the parts of a fuel level detector to detect the level of a fuel in a fuel tank. It would be desirable, therefore, to provide a method whereby a product having mechanical and electrical components can be manufactured at a minimal expense.
Briefly, the present invention is embodied in a method of manufacturing a product having at least one electronic component. The method is best suited for manufacturing a product having a mechanically functional enclosure, such as the housing of a fuel level sensor. In accordance with the invention, a first mold is provided for forming a first housing that incorporates the mechanically functional features of the device. The first housing, which also has a cavity therein, is formed by injection molding a thermoplastic material into the first mold. Thereafter, a second mold is provided for forming a second housing in which the electronic components of the device are inserted for encapsulating. A thermoset material is then injected into the second mold in a transfer molding process to form a second housing. The second housing is inserted into the cavity of the first housing.
Preferably the first mold for forming the first housing includes an inner mass that is complementary in shape to at least a portion of the outer shape of the second housing such that the first housing is formed with a cavity therein that is complementary to the shape to a portion of the second housing. Prior to injecting material into the first mold at least one lead is inserted therein. In the preferred embodiment, a lead frame that includes including a plurality of leads is inserted into the first mold. A thermoplastic material is then injected into the first mold and allowed to harden, after which the first housing is removed. At least one electronic component is thereafter attached to the lead or leads retained in the first housing.
A second mold is then provided for forming a second housing. The second mold is adapted to receive the first housing including the lead frame molded therein. Upon receipt of the first housing into the second mold, the electronic components connected to the leads of the lead frame will be positioned within the inner opening defined within the second mold. A transfer molding process is used in which a thermoset material is injected into the inner opening of the second mold to form a second housing. Upon removal of the completed part from the second mold, the electronic components will be embedded in a second housing and the second housing will be, in turn, bonded into the cavity of the first housing.
A better understanding of the present invention will be had after a reading of the following detailed description taken in conjunction with the drawings wherein:
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For the purposes of assembling the parts, the leads 90, 92, 94 are retained together by a tie bar 96, which connects the outer ends of the leads 90, 92, 94. After the housing 18 is fully assembled, as further provided below, the tie bar 96 can be broken free of the distal ends of the leads 90, 92, 94 leaving the leads spaced from one another so as not to be electrically connected to one another.
The body 50 includes attachment portions, such as holes in the frame 86 or tabs on the frame 86 or the cylindrical sleeve, not shown, for attaching the fuel level sensor 16 to the inner surface of a fuel tank 14. Within the tubular sleeve 82 is a generally cylindrical inner housing 106 having a planar forward surface 108, and extending axially through the center of the tubular inner housing 106, is an axially hole 114 for receiving the portion of the pin 48 extending axially from the sleeve 28 of the rotor 20. When the parts are assembled, as shown in
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The chemicals in the fuel for motor vehicles are extremely corrosive and damaging to electronic components and therefore it is desirable that the Hall effect sensor 118, including the contacts 120, 122 and the solder joints 95, 96 be hermetically embedded in a suitable plastic inner housing 106. The electronics that make up the Hall effect sensor 118 are easily damaged by excessive heat and pressure applied to a mold, as would be needed to form the inner housing 106 from a thermoplastic using injection molding techniques. It is therefore not desirable to use injection molding to form the inner housing 106. It is customary to embed, and hermetically seal, electronic components in plastic parts made from a thermoset plastic using transfer molding techniques and therefore, the inner housing 106, as shown if
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The molds 130, 132 have mating surfaces 148, 150 respectively which when in contact with each other form a seal allowing a thermoplastic material to be injected through suitable runners 152, 154 extending from a hopper, not shown, and pressure chamber, not shown, in which pellets of thermoplastic material, not shown, are melted. In the immediate proximity of mating surface 148, cavity 134 has an enlarged indentation 160 that is complementary in shape to the perimeter to at least a portion of the lead frame 88 such that the lead frame 88 may be positioned in the indentation 160 before the mating surfaces 148, 150 are engaged to each other.
To manufacture the housing body 50, a lead frame 88 is positioned in the indentation 160 of mold 130 and the molds 130, 132 are assembled to each other with the mating surfaces 138, 140 forming a seal to retain liquefied thermoplastic material within the cavities 134, 136. Thereafter, liquefied plastic thermoplastic material is injected through the runners 152, 154 and into the cavities 134, 136. The liquefied thermoplastic is then permitted to cool after which a formed housing body 50, as depicted in
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To form the inner housing 106, the contacts 120, 122 of a Hall effect sensor 118 are first soldered, forming solderings 95, 96, to connect the contact ends 102, 104 of leads 92, 94 thereto. Thereafter, the tubular sleeve 82 of the housing body 50 is assembled into the cavity 168 of the third mold 164 with the Hall effect sensor 118 extending into the portion of the cavity 170 of the fourth mold 166. The mating surfaces 172, 174 of the third and fourth molds 162, 168 are then engaged with each other after which the resin and hardener of a thermoset material is injected through the feeder lines 180, 182 until the cavities 168, 170 are filled with material thereby encasing the Hall effect sensor 118. After the plastic hardens a completed housing 18 is removed from the molds 164, 166.
While the present invention has been described with respect to a single embodiment, it will be appreciated that many modifications and variations may be made without departing from the true spirit and scope of the invention. It is therefore the intent of the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the invention.