Vapor condensation thermoplastic parts finishing

In various embodiments, a vapor condensation thermoplastic part finishing technique is provided that smooths and ensures color saturation of thermoplastic parts. The technique uses nonhazardous vapor condensation to rapidly heat a thermoplastic part to a temperature higher than its melting temperature. The part then may be cooled to a temperature lower than its melting temperature (and preferable lower than its glass-transition temperature. In some cases, evaporation may be employed to rapidly cool the part. Condensation and, where applicable evaporation, may be promoted by pressure changes to the nonhazardous vapor (e.g., increasing pressure to above atmospheric pressure and then decreasing pressure back to atmospheric pressure), exposure of the part to a separately-heated cloud of nonhazardous vapor (e.g., moving the part into and then out of the separately-heated cloud or injecting and then stopping injection of separately-heated vapor), or by other techniques.

BACKGROUND

Technical Field

The present disclosure relates generally to the finishing of plastic parts, and more specifically to techniques and apparatus for smoothing and ensuring color saturation of thermoplastic parts.

Background Information

The formation of parts from solid or particulate thermoplastics is common in a variety of types of industrial manufacturing. Thermoplastic parts may be formed using a wide variety of manufacturing processes, which include removing material from stock by mechanical action, such as machining; deforming stock by the application of force or pressure; melting stock and solidifying the resulting liquid in a mold (commonly referred to as “injection molding”); additive manufacturing that involves consolidating successively layers of material according to a computer aided design (CAD) model (commonly referred to as “three-dimensional (3D) printing”); among other processes.

Irrespective of the process used to manufacture a thermoplastic part, the part typically needs to possess certain visual and tactile characteristics. Such characteristics may be dictated by the part's function or by user expectations (i.e. the part needs to “look right” and “feel right”).

One important visual and tactile characteristic is surface roughness. Surface roughness (Ra) plays an important role in determining how a part will appear and feel. It is quantified by the deviations in the direction of the normal vector of the actual surface of the part from its ideal form. If these deviations are large, the surface of the part is rough; if they are small, the surface is smooth. Some manufacturing processes can inherently provide smoother surfaces than others. For example, injection molding can provide surfaces on parts with a Raof 1 micrometer (μm) or less. In contrast, 3D printing often uses powders with a particle size distribution (PSD) of 10-90 μm, which usually leads to surfaces with an Raof 10 μm or higher. This is a major competitive disadvantage for 3-D printing in applications where it competes with injection molding. However, even manufacturing processes that can inherently provide smoother surfaces do not universally do so. For example, the generally smooth surface of injection-molded parts may have scattered rough scars due to the presence of fixturing devices in the mold, the removal of sprues, or wear to the mold from ongoing use. Further, surface roughness may be introduced at later stages of manufacturing, as a result of parts handing, accident, and the like. Most thermoplastic parts are thereby subject to undesired surface irregularities of one sort or another.

Separate from surface roughness, color quality plays an important role in how a part appears. In particular, if color saturation is lacking, parts can have a hazy look, which distracts from their visual appeal. While this issue may occur in various manufacturing processes, it is particularly prevalent in 3D printing. In 3D printing, unconsolidated particles may adhere to molten particles at the interface that defines the surface of the part being consolidated. These unconsolidated particles may create a thin layer that scatters light, presenting the appearance of a different color, and thus altering the appearance of the part.

Manufacturers have explored a number of post-processing operations in attempts to address surface roughness and color quality of thermoplastic parts but have been met with challenges. A primary challenge is to be able to smooth the surface of the part and eliminate color-affecting material, without altering fine detail or mechanical properties of the part. A secondary challenge is to be able to do so in a manner that requires a minimum of worker and environmental safety apparatus. In particular, this is an issue for 3D printing. A widely touted benefit of 3D printing is that it may be performed in regular office space and other similar design environments. However, unlike typical manufacturing environments, such design environments generally do not have equipment and procedures in place to safely handle hazardous and flammable materials. If such materials are required for post-processing operations, one of the major benefits of 3D printing is lost. A tertiary challenge is to smooth the surface of the part and eliminate color-affecting material via a process that is expedient and relatively inexpensive to perform. Complicated operations that are slow and expensive are not practical for many types of manufacturing.

Existing post-processing operations generally fail to address one or more of the above-discussed challenges. Some existing post-processing operations involve manually trimming, machining or buffing parts to remove material, using various cutters, coated abrasives (e.g., sandpaper) or solution-born abrasives. However, such operations may alter fine detail, and often are slow and/or expensive.

Other existing post-processing operations involve the use of chemical vapors or liquids to smooth and provide gloss to a part by reflowing its surface (typically referred to as “solvent polishing”). There are two common techniques for solvent polishing. The first technique is to immerse the entire part in a bath of liquid plastic solvent for a period of time selected based on the identity of the solvent and the type of thermoplastic involved. The solvent from the bath penetrates the outer layer of the thermoplastic, thereby causing it to reflow. The second technique is to expose the part to a vaporized solvent. The vaporized solvent may be produced by heating a solvent, for example, in a heated bath disposed below the part. The hot solvent vapor melts the outer layer of the thermoplastic, causing it to reflow. While such operations may be relatively quick and inexpensive, the solvents required are often quite hazardous and flammable, and thereby unsuited for use in a typical design environment.

Accordingly, there is a need for an improved technique for smoothing and ensuring color saturation of thermoplastic parts that may address some or all of the above-described challenges.

SUMMARY

In various embodiments, a vapor condensation thermoplastic part finishing technique is provided that smooths and ensures color saturation of thermoplastic parts. The technique uses nonhazardous vapor condensation to rapidly heat a thermoplastic part to a temperature higher than its melting temperature. The part then may be cooled to a temperature lower than its melting temperature (and preferably lower than its glass-transition temperature. In some cases, evaporation may be employed to rapidly cool the part. Condensation and, where applicable evaporation, may be promoted by pressure changes to the nonhazardous vapor (e.g., increasing pressure to above atmospheric pressure and then decreasing pressure back to atmospheric pressure), exposure of the part to a separately-heated cloud of nonhazardous vapor (e.g., moving the part into and then out of the separately-heated cloud or injecting and then stopping injection of separately-heated vapor), or by other techniques. Because of the short duration of the heat spike and the low thermal conductivity of thermoplastic, the surface of the part heats, melts and reflows, improving its smoothness and color saturation, while the subsurface volume of the part remains at a substantially unchanged temperature, thereby avoiding distortion and/or degradation. Multiple cycles may be performed to achieve a desired level of smoothness and color saturation. Advantageously, the technique may be suited for a typical design environment, avoiding use of hazardous and flammable materials.

In one example embodiment, thermoplastic part finishing is performed by heating the surface of the thermoplastic part to a first temperature higher than a melting temperature of the thermoplastic part by condensing a non-hazardous vapor on the surface of the thermoplastic. The surface of the thermoplastic part is cooled to a second temperature lower than the melting temperature. One or more cycles of heating and cooling are performed until the thermoplastic part has at least one of a roughness or a color saturation that satisfies a predetermined requirement.

In another example embodiment that specifically utilizes pressure changes to the nonhazardous vapor, thermoplastic part finishing is performed using a reactor having a tank that holds the thermoplastic part and a cover that provides an airtight seal. One or more valved ports are disposed in the reactor, the one or more valved ports including a valved port that introduces a non-hazardous vapor into the tank. A movable piston of the reactor moves from a first position to a second position to compress the non-hazardous vapor in the tank to a first pressure sufficient to cause the non-hazardous vapor to condense on a surface of the thermoplastic part at a first temperature that is higher than a melting temperature of the thermoplastic part and thereby melt the surface of the thermoplastic part, and moves from the second position to the first position to decompress the non-hazardous vapor in the reactor to a second pressure sufficient to cause the condensed non-hazardous vapor on the surface of the thermoplastic part to evaporate and thereby cool the surface of the thermoplastic part to a second temperature lower than the melting temperature.

In yet another example embodiment system that specifically utilizes exposure of the thermoplastic part to separately-heated nonhazardous vapor, thermoplastic part finishing is performed using a reactor that includes a tank that holds the thermoplastic part that is open to atmosphere. The tank has a sump region in which a pool of nonhazardous liquid is disposed, and a vapor cloud region configured to hold a thermoplastic part. A heating assembly heats the non-hazardous liquid to create a nonhazardous vapor having a temperature sufficient to cause the vapor when exposed to the thermoplastic part in the vapor cloud region to condense on a surface of the thermoplastic part at a first temperature that is higher than a melting temperature of the thermoplastic part and thereby melt the surface of the thermoplastic part.

It should be understood that a variety of additional features and alternative embodiments may be implemented other than those discussed in this Summary. This Summary is intended simply as a brief introduction to the reader for the further description that follows and does not indicate or imply that the examples mentioned herein cover all aspects of the disclosure or are necessary or essential aspects of the disclosure.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1is a flow diagram of an example high-level sequence of steps100for vapor condensation thermoplastic part finishing. As used herein the term “part” should be interpreted broadly to include both an object that is a piece or component of a larger object and a stand-alone object that is in-and-of-itself a whole. The thermoplastic part may be made of any of a number of well-known thermoplastics, such as acrylonitrile butadiene styrene (ABS), nylon, polylactic acid (PLA), acrylonitrile styrene acrylate (ASA), polyether ether ketone (PEEK), polyether ether ketone ketone (PEKK), polyetherimide (PEI), thermoplastic elastomer (TPE), thermoplastic polyurethane (TPU), etc. The thermoplastic may have a melting temperature that is in a given range, for example a range from 50° C. to 400° C. Likewise, the non-hazardous vapor may be produced from any of a number of well-known non-hazardous liquids including water, a perfluorocarbon, a hydrocarbon ether, a perfluoropolyether, etc. Selection of a particular non-hazardous liquid may be based on a variety of factors depending on the embodiment. One factor may be the melting temperature of the particular thermoplastic being used (e.g., a thermoplastics with a high melting temperature may require a non-hazardous liquid with a high boiling point). Further, while vapor condensation thermoplastic part finishing may have particular advantages in manufacturing processes that involve 3D printing, it should be remembered that the thermoplastic part may be produced by any of a number of well-known manufacturing processes including machining, deforming, injection molding, etc.

At step110, a user places a thermoplastic part into a reactor or a parts basket thereof. At step120, the surface of the part is heated to a target temperature higher than its melting temperature by a non-hazardous vapor (e.g., water vapor, a perfluorocarbon vapor, a hydrocarbon ether vapor, a perfluoropolyether vapor, etc.) in the reactor condensing upon and heating the surface. As discussed in more detail below, the nonhazardous vapor may be caused to condense upon and heat the surface using any of a number of techniques depending on the implementation, including increasing pressure of nonhazardous vapor about the part to above atmospheric pressure so that it condenses on the cooler part, moving the part into a hot nonhazardous vapor cloud such that it condenses on the cooler part, injecting a hot nonhazardous vapor cloud about the part such that it condenses on the cooler part, or by other techniques. The surface of the thermoplastic part exposed to the hot condensed nonhazardous vapor rapidly heats, melts and reflows, improving its smoothness and color saturation. At step130, the surface of the thermoplastic part is cooled to below its melting temperature (preferably below its glass-transition temperature). Cooling may be promoted in some implementations by evaporation of the non-hazardous vapor. As discussed in more detail below, evaporation may be promoted using any of a number of techniques depending on the implementation, including decompressing nonhazardous vapor about the part, removing the part to a cooler region away from hot nonhazardous vapor, ceasing injection of hot nonhazardous vapor so the part cools in place, or by other techniques. Steps120and130may be performed one or more times (i.e. just once or repeated) until a smoothness and/or color saturation of the part satisfies a predetermined design requirement. At step140, the user removes the finished thermoplastic part from the reactor or parts basket thereof.

Vapor condensation thermoplastic part finishing may take advantage of the heat transfer kinetics for condensation of vapors to liquids, and the physical properties of thermoplastics. The condensation of the non-hazardous vapor to liquid (e.g., water vapor to liquid water, perfluorocarbon vapor to liquid perfluorocarbon, hydrocarbon ether vapor to liquid hydrocarbon ether, perfluoropolyether vapor to liquid perfluoropolyether, etc.) entails the release of a high level of energy.FIG. 2is a table200showing properties of saturated water vapor, illustrating the release of a high level of energy (in BTU/lb) during condensation under various conditions. Further, the heat transfer coefficient for the condensation of many vapors (such as water vapor) is very high (e.g., much higher than the convective heat transfer rate of the equivalent flowing vapor or liquid).FIG. 3is a table300showing estimated heat transfer coefficients for condensation of water vapor in comparison to various convection heat transfer rates. The result is that the energy rapidly released from condensation is transferred to the surface of the part enabling it to melt and reflow. However, the enthalpy of fusion of thermoplastic is quite low, as is its heat capacity. Further, the thermal conductivity of thermoplastic is low.FIG. 4is a table400showing selected properties of various example thermoplastics, illustrating that enthalpy of fusion, heat capacity and thermal conductivity is low in comparison to stainless steel. The result is that energy imparted on the surface briefly is not effectively transferred to the subsurface volume, enabling the temperature of the subsurface volume to be maintained unchanged. It may be noted that these conditions generally cannot be attained if the part is formed from a material with high enthalpy of fusion and heat capacity because the subsurface volume would reach undesirable temperatures. Further, it should be remembered that while some thermoplastics (e.g., nylons) are degraded by long-term expose to high temperature, little degradation occurs over short time periods.

In a first example embodiment, condensation and, where applicable evaporation, may be promoted by pressure changes to the nonhazardous vapor. This may be performed by increasing pressure to above atmospheric pressure and then decreasing pressure back to atmospheric pressure. When saturated vapor is compressed under adiabatic conditions, an increase in temperature and a partial condensation vapor occurs.FIG. 5is a table500showing properties of the adiabatic compression of water vapor, illustrating temperature increase together with partial condensation.

FIG. 6is a flow diagram of an example high-level sequence of steps600for vapor condensation thermoplastic part finishing according to a first embodiment that involves pressure changes. At step610, a user loads a thermoplastic part into the reactor, and then isolates the reactor from atmosphere. At step620, the reactor is decompressed (evacuated) to a residual pressure (e.g., a pressure of less than 10 mbar). At step630, a nonhazardous vapor (e.g., water vapor, a perfluorocarbon vapor, a hydrocarbon ether vapor, a perfluoropolyether vapor, etc.) is preheated to a steady-state temperature lower than the melting temperature of the thermoplastic part and admitted into the reactor to be maintained therein at a steady-state pressure. As part of the preheating step630, residual air in the reactor may be vented to atmosphere. At step640, the non-hazardous vapor in the reactor is rapidly compressed from the steady-state pressure to a greater pressure sufficient to cause the non-hazardous vapor to condense on a surface of the thermoplastic part at a target temperature higher than its melting temperature. The surface of the thermoplastic part exposed to the hot condensed vapor rapidly heats, melts and reflows, improving its smoothness and color saturation. At step650, the non-hazardous vapor in the reactor is rapidly decompressed (evacuated) to a pressure sufficient to cause the condensed nonhazardous vapor on the surface of the thermoplastic part to evaporate. The surface of the thermoplastic part rapidly cools to below its melting temperature (preferably below its glass-transition temperature) and dries. By executing steps640and650in rapid succession, the subsurface volume of the part remains at a substantially unchanged temperature through the heat spike on the surface, thereby avoiding distortion and/or degradation to the part. Steps640and650may be performed one or more times (i.e. just once or repeated) until a smoothness and/or color saturation of the part satisfies a predetermined design requirement. At step660, the user removes the finished thermoplastic part from the reactor.

Various systems may be used to implement the first embodiment that involves pressure changes.FIGS. 7A-7Care schematic diagrams of an example system700for vapor condensation thermoplastic part finishing that may implement the sequence of steps600ofFIG. 6. At the core of the system700is a reactor that includes a tank705(e.g., a cylindrical vessel) and a cover710(e.g., a domed, cylindrical assembly) that fits over an opening715in the tank, being fastened in place to isolate the reactor from atmosphere.FIG. 7Afocuses on the cover710and structures attached thereto.FIG. 7Bfocuses on the tank705and structures attached thereto.FIG. 7Cillustrates the combination of the tank705and the cover710.

Referring toFIG. 7A, the cover710includes a piston715(e.g., a rigid plate whose diameter is slightly smaller than the diameter of the cover) that is free to move inside the cover between a pair of piston stops720disposed near a top end and a bottom end of the cover (e.g., ring-shaped stops). A space is defined between the piston715and the top end of the cover710, which includes two valved ports: an air supply port regulated by valve V-6coupled to a compressed air supply725, and a vent port regulated by valve V-7coupled to a vent line730that leads to atmosphere. The pressure in the space above the piston may be measured by a pressure sensor735.

Referring toFIGS. 7B and 7C, the tank705includes a parts basket737(e.g., an open wire mesh basket that sits above the bottom of the tank) that is designed to hold a single or multiple parts. The dimensions of the tank may be determined based on the dimensions of the parts basket737and that in turn by the parts it needs to hold. The tank705includes four valved ports: an vacuum port regulated by valve V-1coupled to a vacuum pump740, and a boiler port regulated by valve V-2coupled to a low-pressure boiler745, a drain port (preferably disposed at a lowest point in the tank) regulated by valve V-3coupled to a drain750, and a vent port regulated by valve V-4coupled to another vent line755that leads to atmosphere. The tank may also include a pressure sensor760, a temperature sensor765and a pressure release valve770that releases excess pressure by venting gas to the atmosphere.

The low-pressure boiler745may be any of a variety of commercially available boilers (e.g., a Chromalox® low-pressure generator that delivers 9 to 45 pounds (lb) of non-hazardous vapor (e.g., water vapor) per hour at 0-90 pounds per square in gauge (psig)). The low-pressure boiler745may include an integral valve or a separate valve V-5may be provided, coupled to a water supply line747. Generating non-hazardous vapor (e.g., water vapor) at pressures less than 90 psig eliminates many safety issues associated with boilers that operate at significantly higher pressures, rendering the system700better suited for a design environment. To provide further safety, an additional pressure relief valve749may be provided.

The vacuum pump740may be any of a variety of commercially available vacuum pumps that is capable of lowering the air pressure in the reactor from 1 bar to less than 10 mbar, preferable in an elapsed time of 5 minutes or less.

The system700ofFIGS. 7A-7Cmay be utilized to implement vapor condensation thermoplastic part finishing involving pressure changes.FIG. 8is a table showing the status of components of the system ofFIGS. 7A-7Cin various system states. Initially, the system700is in an Off-Line state. The tank705of the reactor is closed with the cover710in place, all valves V-1to V-7are closed and the low-pressure boiler745and low-pressure boiler745are powered off. Thereafter, the system700enters a Standby state. The low-pressure boiler745is filled with water, for example, by opening valve V-5(or an integral valve if present) and powered on to produce saturated non-hazardous vapor (e.g., water vapor) at a desired pressure and temperature. Valves V-4and V-7are opened to vent the reactor to atmosphere. Then, in an operation that generally corresponds to part of step610ofFIG. 6, the system enters Load Parts state. The cover710is removed to open the tank705of the reactor. A user loads a thermoplastic part (or multiple parts) into the parts basket737. Valve V-5(or the integral valve is present) is closed. Next, in an operation that also may be part of step610ofFIG. 6, the system enters a Close Tank state. The user puts the cover710in place and valves V-4and V-7to the vent lines are closed to vent the reactor to atmosphere.

Subsequently, in an operation that generally corresponds to step620ofFIG. 6, the system enters an Evacuate Tank state. Valve V-1is opened and the vacuum pump740is activated to decompress (evacuate) the reactor to a residual pressure of less than 10 mbar. Thereafter, in an operation that generally corresponds to step630ofFIG. 6, the system enters a Preheat Tank state. Valve V-2is opened to admit non-hazardous vapor (e.g., water vapor) from the low-pressure boiler745into the tank705of the reactor and the cover710below the piston715. The piston715is moved from a bottom position against the bottom stop to a top position against the top stop due to the difference in pressure between the non-hazardous vapor (e.g., water vapor) admitted below the piston715and residual air above the piston715. Vent V-5may be opened to vent the residual air above the piston715to atmosphere. Valve V-2is closed once the reactor has reached a steady-state temperature lower than the melting temperature of the thermoplastic part.

Further, in an operation that generally corresponds to step640ofFIG. 6, the system enters a Polish Parts state. Valve V-6is opened to admit high-pressure air into the cover710above the piston715. This causes the piston715to move rapidly from against the top stop to the bottom stop, rapidly compressing the non-hazardous vapor in the reactor. The non-hazardous vapor is compressed from the steady-state pressure to a greater pressure sufficient to cause the non-hazardous vapor to condense on a surface of the thermoplastic part at a target temperature higher than its melting point, thereby heating, melting and reflowing its surface to improve its smoothness and color saturation. After a predetermined cycle time, valve V-6is closed.

Thereafter, in an operation that generally corresponds to step650ofFIG. 6, the system enters a Cool and Dry Parts state. Valves V-3and V-7are opened to reduce (evacuate) the pressure in the reactor to atmospheric pressure. Compressed nonhazardous vapor and condensate in the tank705drain through valve V-3. The rapid des compression causes the condensed non-hazardous vapor on the surface of the thermoplastic part to evaporate, rapidly cooling the thermoplastic part to below its melting temperature (preferably below its glass-transition temperature) and drying it. Compressed air above the piston715is vented to atmosphere through valve V-7.

The Polish Parts and Cool and Dry Parts states may be cycled between one or more times (i.e. just once or repeated) until a smoothness and/or color saturation is achieved. Finally, in an operation that generally corresponds to step660ofFIG. 6, the system700enters a Remove Parts state. After closing valve V-4and opening valve V-4, a user removes the cover710, allowing the parts basket737to be accessed and the finished thermoplastic part removed from the reactor. The system700is now ready to accept one or more additional thermoplastic parts for finishing, and the steps repeated.

The system700ofFIGS. 7A-7Cmay be constructed according to a number of design parameters.FIG. 9is a table900showing the effect of steam pressure on operating temperatures and adiabatic heat generation in an example 1 cubit foot (ft3) reactor with a 4× compression ratio.FIG. 10is a table1000showing estimated heat requirements for the example reactor ofFIG. 9.

In a second example embodiment, condensation and, where applicable evaporation, may be promoted by exposure of the part to a separately-heated cloud of nonhazardous vapor, and then removing such exposure. This may be performed by suspending the thermoplastic part in a vapor cloud above a pool of heated (e.g., boiling) nonhazardous liquid (e.g., water, a perfluorocarbon, a hydrocarbon ether, a perfluoropolyether, etc.) for a predetermined cycle time, and then moving the part to a cooler area. The vapor may be maintained at a constant pressure (e.g., atmospheric pressure) throughout the process. Alternatively, this may be performed by injecting superheated nonhazardous liquid to create a vapor cloud about a suspended thermoplastic part for a predetermined cycle time, and then ceasing the injection. The vapor cloud may be maintained at a constant pressure (e.g., at atmospheric pressure) while about the thermoplastic part.

Since pressure is maintained constant, the target temperature may be achieved by selection of the an appropriate nonhazardous vapor.FIG. 11is a table1100showing the melting point for various example thermoplastics, target temperate ranges based on such melting points and examples of commercially available liquids that have a boiling point suitable for the target range at atmospheric pressure. In general, liquids that have a boiling point 20° C. to 40° C. higher than the melting points are suitable.

FIG. 12is a flow diagram of an example high-level sequence of steps1200for vapor condensation thermoplastic part finishing according to a second embodiment that involves exposure of the part to a separately-heated cloud of nonhazardous vapor. At step1210, a user loads a thermoplastic part into the reactor or a parts basket thereof. At step1220, a non-hazardous liquid (e.g., water, a perfluorocarbon, a hydrocarbon ether, a perfluoropolyether, etc.) is heated. Depending on the implementation, the nature of the heating may vary. For example, a pool of non-hazardous liquid may be boiled at atmospheric pressure. Alternatively, a non-hazardous liquid may be preheated in a pool at atmospheric pressure to a temperature below its boiling point at atmospheric pressure and then superheated under higher pressure to above its boiling point at atmospheric pressure. At step1230, the thermoplastic part is exposed to a vapor cloud resulting from the heating for a predetermined cycle time. Nonhazardous vapor condenses on a surface of the thermoplastic part at a target temperature higher than its melting temperature. The surface of the thermoplastic part exposed to the hot condensed vapor rapidly heats, melts and reflows, improving its smoothness and color saturation. Depending on the implementation, the exposure to hot nonhazardous vapor may vary. For example, the thermoplastic part may be moved into a vapor cloud above a pool of boiling non-hazardous liquid. Alternatively, superheated non-hazardous liquid may be injected about a stationary thermoplastic part to create a vapor cloud around it.

At step1240, the thermoplastic part is removed from exposure to the vapor cloud, such that surface of the thermoplastic part cools to below its melting temperature (preferably below its glass-transition temperature). Depending on the implementation, the removal may vary. For example, the thermoplastic part may be moved out of the vapor cloud above a pool of boiling non-hazardous liquid and into a cooler area Alternatively, the thermoplastic part may remain stationary and injection of superheated non-hazardous liquid may cease, such that that thermoplastic part cools in place.

Steps1230and1240may be performed one or more times (i.e. just once or repeated) until a smoothness and/or color saturation of the part satisfies a predetermined design requirement. At step1250, the user removes the finished thermoplastic part from the reactor or parts basket thereof.

Various systems may be used to implement the second embodiment that involves exposure of the part to a separately-heated cloud of nonhazardous vapor.FIG. 13is a schematic diagram of a first example system1300for vapor condensation thermoplastic part finishing that may implement the sequence of steps1200ofFIG. 12. At the core of the system1300is a reactor that includes a tank1310in which a parts basket1315(e.g., an open wire mesh basket that sits above the bottom of the tank) that is designed to hold a single or multiple parts is suspended. The dimensions of the tank may be determined based on the dimensions of the parts basket and that in turn by the parts it needs to hold.

The tank1310includes three regions: a sump region1320in which a pool1345of nonhazardous liquid is disposed; a vapor cloud region1330; and a cooling region1340. The sump region1320and/or vapor cloud region1330may include a heating assembly, for example, a heating mantel1325. The cooling region1340may include cooling coils1350. The top of the tank1310may be open to atmosphere. The pool1345of nonhazardous liquid is boiled by operation of the heating assembly to create a vapor cloud of hot nonhazardous vapor in the vapor cloud region1330. Operation of the heating assembly may be regulated by a first thermometer1360. Hot nonhazardous vapor that travels upward into the cooling region1340may re-condense and travel back down to the pool1345, by dripping down the sides of the tank1310or by one or more dedicated condensate return lines (not shown), thereby suppressing loss of vapor into the atmosphere. The cooling coils1350may circulate a cooling fluid (e.g. cool water) with their operation regulated by a second thermometer1370.

To finish a thermoplastic part, the parts basket1315may be lowered into the vapor cloud in the vapor cloud region1330so that nonhazardous vapor condenses on the surface of the thermoplastic part at a target temperature higher than its melting temperature, causing it to heat, melt and reflow. After a cycle time has elapsed, the parts basket1315may be raised into the cooling region1340so that the surface of the thermoplastic part cools to below its melting temperature (preferably below its glass-transition temperature). This process may be performed one or more times (i.e. just once or repeated).

FIG. 14is a schematic diagram of a second example system1400for vapor condensation thermoplastic part finishing that may implement the sequence of steps1200ofFIG. 12. Again, at the core of the system1400is a reactor that includes a tank1410in which a parts basket1415that is designed to hold a single or multiple parts is suspended. The dimensions of the tank may be determined based on the dimensions of the parts basket and that in turn by the parts it needs to hold. The tank1410includes three regions: a sump region1420in which a pool1445of nonhazardous liquid is disposed; a vapor cloud region1430in which the parts basket1415is disposed when the system is in use; and a cooling region1440having cooling coils1450. The top of the tank may be open to atmosphere.

The pool1445of nonhazardous liquid disposed in the sump region1420is preheated to a temperature below its boiling point at atmospheric pressure and then superheated to above its boiling point at atmospheric pressure by a heating assembly. The heating assembly may include control heaters1460that preheat the liquid in the pool1445, a pump1465and a a check valve1467that draw the preheated liquid and a liquid super heater1470that further heats the liquid under higher pressure. Superheated nonhazardous liquid may be stored in a liquid accumulator1480. Operation of the liquid superheater1470may be regulated by temperature and pressure sensors1475,1477.

To finish a thermoplastic part, the parts basket1415may be lowered into the vapor cloud region1430and then held stationary. Superheated nonhazardous liquid is injected via a nozzle system1480connected through a pressure relief valve1485into the vapor cloud region1430for a predetermined cycle time. The nonhazardous vapor condenses on the surface of the thermoplastic part at a target temperature higher than its melting temperature, causing it to heat, melt and reflow. After the cycle time has elapsed, the injection may cease, so that the surface of the thermoplastic part cools to below its melting temperature (preferably below its glass-transition temperature). Hot nonhazardous vapor that travels upward into the cooling region1440may re-condense and travel back down to the pool1445, by one or more dedicated condensate returns1490, thereby suppressing loss of vapor into the atmosphere. To promote cooling, cooling coils1450may circulate a cooling fluid (e.g. cool water). This process may be performed one or more times (i.e. just once or repeated). After all cycles have completed, the parts basket1415may be raised and the part removed.

With the systems ofFIG. 13andFIG. 14, a finished thermoplastic part may still be coated with a thin film of nonhazardous liquid, that may need to be removed for performance and aesthetic considerations. If a nonhazardous liquid with a low vapor pressure at room temperature is utilized, special procedures may be required to remove the film. For example, the finished thermoplastic part may be subject to a vapor degreaser that operates with a low boiling point fluorinated liquid to remove (and potentially recover) the remaining nonhazardous liquid.

The foregoing has been a detailed description of several embodiments for thermoplastic part finishing utilizing condensation of a nonhazardous vapor. Further modifications and additions may be made without departing from the disclosure's intended spirit and scope. For example, while it is discussed above that in one embodiment movement of a piston internal to the reactor may be used to compress and decompress the nonhazardous vapor, it should be understood that other compression and decompression mechanisms may alternatively be used, including external compressors, pressure vessels, pumps, and the like. Likewise, while several example non-hazardous liquids/vapors are discussed above, it should be remembered that a variety of other non-hazardous substances may alternatively be used, including various polyglycols, polysilicones and other high boiling point substances below their combustion point. Accordingly, it should be remembered that the above descriptions are meant to be taken only by way of example, and the invention is not restricted to any one particular embodiment, configuration or implementation discussed above. Rather, the invention is defined by the following claims.