Abstract:
The present invention is directed to a system for thermoforming a batch of dental appliances simultaneously. An embodiment of the invention includes a vacuum plate having an upper surface and a plurality of cups for receiving dental models. A polymer sheet extends over the upper surface of the vacuum plate and over the cups. A heater cap that is movable in proximity to the cups is provided to heat the polymer sheet to be formed onto the dental models. The vacuum plate is configured to create a vacuum seal between an upper edge of the cups and the polymer sheet. A vacuum source is provided to apply a vacuum to the cups below the polymer sheet wherein the pressure contained in said cups is lower than atmospheric pressure whereby at least a portion of the polymer sheet is drawn into each of the cups and drawn over the dental models to form an aligner over each of the dental models. The invention includes the process of drawing a vacuum in a series of steps, or phases, to the cups below the polymer sheet to form the polymer effectively around the dental model.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       REFERENCE TO MICROFICHE APPENDIX 
       [0003]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The field of the invention is dental appliance manufacturing. 
         [0006]    2. Description of the Related Art 
         [0007]    Orthodontic aligners are appliances intended to make a series of discrete tooth position corrections aimed at aligning the teeth correctly. Aligners are equivalent to having bracket/wire braces for orthodontic treatment but they have many advantages. For example, aligners are invisible, comfortable and removable for cleaning and they allow a patient to eat anything they want. The manufacture of aligners begins with making stone models from dental impressions that come from orthodontists. The stone models of the dental impressions provide a positive model of the teeth, also known as the dental arch. 
         [0008]    The stone models are scanned electronically to produce three dimensional computer aided design (CAD) representations to be imported by custom software. The custom software allows the operator to move individual teeth in specific and discrete movements to achieve the final dental arch of aligned teeth. The typical tooth movement is 0.1 mm per week. 
         [0009]    Each aligner is meant to be worn for three weeks, then the patient moves on to wear the next aligner. A patient will typically wear aligners for somewhere between 18 to 36 weeks. For example, a treatment that takes 18 weeks is 6 pairs of aligners, including upper and lower dental arches. 
         [0010]    The software allows the operator to create a stepwise sequence of teeth positions from start to finish via 6 stages, for example for an 18 week treatment plan. In the 18 week example, the software will create 6 pairs of models of dental arches. The resulting adjusted and predictive models of the dental arches are then printed as 3D models. 
         [0011]    The 3D models are washed and then the models are allowed to dry. Once dried, a polymer is thermoformed over the top of the 3D model. 
         [0012]    The thermoformed part is then laser marked with part identification. The laser marked, thermoformed part is then cut by one of several methods so that the aligner that goes to the customer can be separated from the carrier frame of plastic. 
         [0013]    The aligner is then polished in a part tumbling process to remove burrs and sharp edges. The aligners are inspected and then sealed in bags to be shipped to the customer&#39;s orthodontist. 
         [0014]    The aligners are currently thermoformed individually. For example, the Biostar® thermoformer, by Great Lakes Orthodontics, Ltd., uses positive pressure to thermoform a single aligner. The Biostar® thermoformer also does not allow full and variable control of the heater that is used to heat the polymer during the thermoforming process. 
         [0015]    An aligner thermoformer that is capable of producing a series of aligners is shown in U.S. Pat. No. 7,261,533 (“the &#39;533 patent”). The &#39;533 patent is highly complex and does not allow for the production of a batch of aligners simultaneously. It also has one production speed and must be operated in a continuous mode, which does not allow the operator to make just a single aligner. While the temperature set point can be adjusted on the &#39;533 patent, the temperature set point will apply to all aligners as they are produced in series. The &#39;533 patent employs positive pressure to thermoform each aligner while vacuum is used to prevent bubbles from forming during the thermoforming process. 
         [0016]    What is needed is a thermoformer for manufacturing aligners that produces a batch of aligners with the ability to control the temperature set point for each aligner forming location. The number of aligners to be produced could be increased by increasing the number of thermoforming stations. 
         [0017]    The new thermoformer should also only use vacuum to evacuate the gas out of the region between the softened polymer and the dental model to more exactly conform the aligner to the dental model. The more accurate adaptation of aligners to the dental models will result in better performance of the aligners for patients and customers. 
         [0018]    The new thermoformer should make parts faster and it should cost less than existing technology. It should also allow more control of the thermoforming process. 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    The present invention is directed to a system for thermoforming a batch of dental appliances simultaneously. An embodiment of the invention includes a vacuum plate having an upper surface and a plurality of cups for receiving dental models. A polymer sheet extends over the upper surface of the vacuum plate and over the cups. A heater cap that is movable in proximity to the cups is provided to heat the polymer sheet to be formed onto the dental models. The vacuum plate is configured to create a vacuum seal between an upper edge of the cups and the polymer sheet. A vacuum source is provided to apply a vacuum to the cups below the polymer sheet wherein the pressure contained in said cups is lower than atmospheric pressure whereby at least a portion of the polymer sheet is drawn into each of the cups and drawn over the dental models to form an aligner over each of the dental models. 
         [0020]    The vacuum seal between the upper edge of the cups and the polymer sheet is created from a rounded surface at an upper edge of the cups where the upper edge of the cups intersects the upper surface of the vacuum plate. 
         [0021]    The heater cap includes a plurality of heating elements with each heating element aligned to provide heat in proximity to each of the cups. Each of the heating elements includes a thermocouple in communication with a controller to individually monitor and adjust the temperature of the heating elements whereby the temperature of each of the cups can be controlled independently. A plurality of cooling fans is typically provided on the heater cap to regulate the temperature of the heater cap. 
         [0022]    An adapter plate having a plurality of orifices is positioned in each of the cups whereby vacuum pressure applied by the vacuum source through the orifices in the adapter plate to effect uniform molding of the polymer sheet about the dental model to form the aligner. The adapter plate comprises a sintered metallic material, such as sintered bronze. The adapter plate includes an alignment feature to orient the dental model in a desired location to enable later processing of the aligner. The alignment feature comprises a peg, pegs or other locating features, integral with the adapter plate, onto which the dental model is secured. 
         [0023]    The heater cap includes a plurality of bosses adapted to insure the polymer sheet is presented to the upper surface of the vacuum plate to prevent air from escaping from between the polymer sheet and the cup when the heater cap is in proximity to the vacuum plate and when the vacuum source draws a vacuum in the cup. 
         [0024]    At least one cylinder is provided to extend and retract the heater cap to position the heater cap in proximity to the vacuum plate. A spring is positioned behind the at least one cylinder to dampen the heater cap when the heater cap is opened. Pneumatic or fluid pressure is applied to at least one cylinder to extend and retract the heater cap. 
         [0025]    A fixed stop is provided on the upper surface of the vacuum plate positioned to engage a back edge of the polymer sheet and an adjustable stop is provided on the upper surface of the vacuum plate positioned to engage a front edge of the polymer sheet, wherein the front edge is substantially parallel to the back edge of the polymer sheet whereby the polymer sheet can be positioned as desired relative to the cups. 
         [0026]    The vacuum source draws a vacuum to approximately 1.0 psia. 
         [0027]    The present invention is also directed to a method of thermoforming a batch of dental appliances simultaneously comprising the steps of: 
         [0028]    a. Providing a vacuum plate with an upper surface and a plurality of cups for receiving dental models; 
         [0029]    b. Extending a polymer sheet over the upper surface of the vacuum plate and over the cups; 
         [0030]    c. Positioning a heater cap that is movable in proximity to the cups to heat the polymer sheet onto the dental models; 
         [0031]    d. Configuring a vacuum plate to create a vacuum seal between an upper edge of the cups and the polymer sheet; and 
         [0032]    e. Applying a vacuum to the cups below the polymer sheet wherein the pressure contained in the cups is lower than atmospheric pressure whereby at least a portion of the polymer sheet is drawn into each of the cups and drawn over the dental models to form an aligner over each of the dental models. 
         [0033]    The method of thermoforming a batch of dental appliances simultaneously can also comprise the additional step of including a plurality of heating elements with each heating element aligned to provide heat in proximity to each of the cups. 
         [0034]    The method of thermoforming a batch of dental appliances simultaneously can also comprise the additional step of applying vacuum in a series of at least one additional phase to the cups below the polymer sheet to form the polymer effectively around the dental model. Multiple vacuum phases may also be applied. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]      FIG. 1  shows a perspective view of an embodiment of the inventive thermoformer. 
           [0036]      FIG. 2  shows a detailed perspective view of an embodiment of the inventive thermoformer. 
           [0037]      FIG. 3A  shows a perspective view of a prior art adapter plate inside of a cup. 
           [0038]      FIG. 3B  shows a perspective view of an embodiment of a prior art dental model and a prior art aligner. 
           [0039]      FIG. 3C  shows a perspective view an embodiment of an inventive adapter plate inside of a cup. 
           [0040]      FIG. 3D  shows a perspective assembly view of an embodiment of an inventive adapter plate and a cup. 
           [0041]      FIG. 4A  shows a perspective assembly view of an embodiment of an inventive dental model and the inventive peg from an inventive adapter plate. 
           [0042]      FIG. 4B  shows a perspective view of an embodiment of the inventive thermoformer with three dental models, each in a cup, and one dental model out of a cup but in position to be received in a cup. 
           [0043]      FIG. 5A  shows a side section view taken along line  5 - 5  of  FIG. 4B . 
           [0044]      FIG. 5B  shows a detail view of the L-bracket heating element retainer shown in  FIG. 5A . 
           [0045]      FIG. 6A  shows a perspective view of a polymer sheet. 
           [0046]      FIG. 6B  shows a perspective view of an embodiment of the inventive thermoformer with a polymer sheet covering the surface of the vacuum plate and the cups. 
           [0047]      FIG. 7  shows a side section view taken along line  7 - 7  of  FIG. 6B . 
           [0048]      FIG. 7A  is a detail cross section view of the front stop shown in  FIG. 7 . 
           [0049]      FIG. 7B  shows a detail perspective view of the latch mechanism shown in  FIG. 6B . 
           [0050]      FIG. 8  shows a perspective view of an embodiment of the inventive thermoformer with the heater cap positioned over the vacuum plate. 
           [0051]      FIG. 9  shows a side section view taken along line  9 - 9  of  FIG. 8 . 
           [0052]      FIG. 10  shows a front section view taken along line  10 - 10  of  FIG. 8 . 
           [0053]      FIG. 11  shows a perspective view of an embodiment of the inventive thermoformer with a polymer sheet molded over dental models. 
           [0054]      FIG. 12  shows a side section view taken along line  12 - 12  of  FIG. 11 . 
           [0055]      FIG. 13  shows a perspective view of a batch of aligners after they have been removed from the vacuum plate and cups. 
           [0056]      FIG. 14  shows a perspective view of a single aligner on a disk that has been separated from the batch of aligners shown in  FIG. 13 . 
           [0057]      FIG. 15  shows a timeline of an embodiment of the inventive thermoforming process showing vacuum phases. 
       
    
    
     DETAILED DESCRIPTION 
       [0058]      FIG. 1  shows the thermoformer T as typically used together with a table  2 . The thermoformer T includes generally the heater cap H and the vacuum plate V. A polymer enclosure  7  includes a polymer roll  4 . The polymer roll  4  is mounted on a spool  6 , which allows the polymer  9  to be fed freely from the spool  6 . The polymer  9  is fed through slot  10  to the upper surface  234  of the vacuum plate V. The polymer  9  used with the invention is a polymer such as polyurethane. For example, the proprietary formulation sold under the trademark Zendura® may be used. It is contemplated that alternative polymer formulations or acrylic or other materials may also be used. Polymer sheets  12  (see  FIG. 6A ) are formed from the spool  6  of polymer  9 , but individual polymer sheets  12  may be provided independently from the spool  6 . Also shown in  FIG. 1  is the enclosure  18 , which encloses the controller  19 . The controller includes the electronics and other controls required for the thermoformer T. 
         [0059]    Refer now to  FIG. 2  where the thermoformer T is shown. The vacuum plate V has a plurality of cups  214  extending below the surface  234 . Each cup  214  has a rounded edge  214   a  at the intersection of the wall  214   b  of the cup  214  with the upper surface  234  of the vacuum plate V. Although four (4) cups  214  are shown in  FIG. 2  and throughout the figures provided herein, more than four (4) cups  214  can be provided to increase production as desired. 
         [0060]    The heater cap H includes the heater cap plate  120  and the back plate  122 , which is attached to the heater cap plate  120 . A plurality of heating elements  162  are mounted below the heater cap plate  120 , as will be described in detail later. The heater cap plate  120  is attached to the housing block  329 . The housing block  329  is free to move forward and back along shaft  324  to position the heater cap plate  120  above the vacuum plate V surface  234  so that the plurality of heating elements  162  are positioned in proximity to the plurality of cups  214  in the vacuum plate V. 
         [0061]      FIG. 3A  shows a prior art adapter plate.  FIG. 3B  shows a prior art dental model  14  and prior art aligner  13 .  FIG. 3C  shows adapter plate  270  of the present invention and  FIG. 3D  shows an inventive adapter plate  270  and a cup  214  of the present invention. Both the prior art adapter plate  268  and the inventive adapter plate  268  include a plurality of indentations  272  and a plurality of mounting screws  274 . Mounting screws  274  are used to attach each adapter plate  270  to the bottom of the cups  214 . The mounting screws  274  attached to the mounting screw holes  274   a  (see  FIG. 3D ). 
         [0062]    Refer now to  FIGS. 3A and 3B . The prior art adapter plate  268  includes both a spear shaped post  275   a  and a spear locator  275   b . The spear shaped post  275   a  engages tab  273  on the prior art dental model  14 . Tab  273  is located on the lingual side of the anterior teeth, tongue side of the front teeth. The spear locator  275   b  is positioned on the lip side of the front teeth. The spear locator  275   b  engages with the prior art spear  273   a  of the prior art dental model  14 . The inventors have discovered that the presence of the spear  273   a  on the prior art dental model  14  creates a reduced area between the top of the spear  273   a  and the gum line, as well as creating a vertical webbing. The vertical webbing, sometimes called a crease or pinch of polymer, must be trimmed away by hand. Sometimes trimming the unwanted vertical webbing from the prior art aligner  13  results in too little gum coverage, which results in a defective, rejected aligner. 
         [0063]    The inventive adapter plate  270  shown in  FIGS. 3C and 3D  includes a peg  276  onto which the socket  276   a  of the dental model  15  is positioned (also see  FIG. 4A ). The peg  276  extends through the tab  276   a . The use of the peg  276  instead of the spear shaped post  275   a  and the spear locator  275   b  simplifies and improves the thermoforming of each aligner  16  by eliminating the undesirable vertical webbing and by reducing the number of components required on the adapter plate  270 . The peg  276  is typically formed by pressing an aluminum plug into the sintered adapter plate  270 , then machining the peg  276  to the desired square or rectangular shape. Other techniques may also be desired to form the peg  276  as desired. 
         [0064]      FIG. 3D  shows how the adapter plate  270  fits into cup  214 . Also shown in  FIG. 3D  is the vacuum port  94  at the base of the cup  214 . 
         [0065]    In  FIG. 4B  an adapter plate  270  is shown positioned in each of the plurality of cups  214 . Each adapter plate  270  is typically constructed of a sintered metal such as sintered bronze, but other porous materials may also be used. 
         [0066]    Also shown in  FIG. 4B  are three (3) dental models  15  positioned in cups  214  on adapter plates  270 . A single dental model  15  is shown with a positioning line  15   a . The positioning line  15   a  illustrates how the dental model  15  is positioned onto the adapter plate  270 . 
         [0067]      FIG. 5A  is a side sectional view taken along line  5 - 5  of  FIG. 4B . In  FIG. 5A , a dental model  15  is shown in cup  214 . Also shown in  FIG. 5A  is an outer side of a heating element  162  and an outer lip  162   a  of the heating element  162 . Each heating element  162  is secured to the underside of the heater cap plate  120  with an L-bracket  188  that extends under the underside of the outer lip  162   a  of the heating element  162 .  FIG. 5B  shows a detail view of the heating element  162  L-bracket  188 . The L-bracket  188  is secured to the inside of the circular wall  184 , which surrounds each outer lip  162   a . The L-bracket  188  includes a slot  186  in the tab  188   a  through which a lock screw  186   a  is positioned. The lock screw  186   a  attaches to the circular wall  184 . The L-bracket  188  also allows the heating element  162  to be removed or replaced from below the heater cap plate  120  as required. 
         [0068]    It is contemplated that alternative heating elements may be used instead of the heating element  162  described and shown. For example, alternative ceramic or composite or bulb type heating elements may be used in square, round, oval or other desired shapes. 
         [0069]    In  FIG. 6A  a polymer sheet  12  is shown. The polymer sheet  12  is comprised of polymer material  9  (see  FIG. 1 ) that has been cut to a size that corresponds to the upper surface  234  of the vacuum plate V. In  FIG. 6B  the polymer sheet  12  is shown positioned on the upper surface  234  of the vacuum plate V. A side sectional view taken along line  7 - 7  of  FIG. 6B  is shown in  FIG. 7 . The polymer sheet  12  is positioned on the upper surface  234  of the vacuum plate V between the front stop  232  and rear stop  232   a .  FIG. 7A  shows a detail of the front stop  232  in relation to the polymer sheet  12 . A compression screw  236  compresses a compression spring  236   a  to provide the desired tension against the front edge of the polymer sheet  12 . As best seen in  FIG. 6B  and the detail view in  7 B, the front stop  232  can be pulled away from the front edge of the polymer sheet  12  by pulling the lever  240 , which rotates about the pivot block  244 , towards the handle  242  thereby urging the front stop  232  away from the front edge of the polymer sheet  12 . The back edge of the polymer sheet  12  is constrained by the rear stop  232   a . The overall compression imposed on the polymer sheet between the front stop  232  and the rear stop  232   a  is controlled by the adjustable compression screw  236  and compression spring  236   a . The rear stop  232   a  is shown as a groove machined into the upper surface  234  of the vacuum plate V, but a separate member attached to the upper surface  234  may also be used to restrain the back edge of the polymer sheet  12 . 
         [0070]    In  FIG. 8  the heater cap H is shown covering the vacuum plate V. A cross side section view taken along line  9 - 9  of  FIG. 8  is shown in  FIG. 9 . After the dental models  15  are in position in the cups  214  and a polymer sheet  12  is on the vacuum plate upper surface  234 , the heater cap H is moved above the vacuum plate V. When the heater cap H is above the vacuum plate V, the heating elements  162  will be in proximity to the cups  214 , which contain the dental models  15 . 
         [0071]    The position of the housing block  329  and heating cap H are controlled by pneumatic pressure. When pneumatic pressure is applied to the closing pressure line  352   a  air cylinder  328  is actuated. When air cylinder  328  is retracted the entire heating cap H moves forward. The housing block  329  moves forward on shaft  324 . As the housing block  329  moves forward, the heater cap plate  120  also moves forward to position the heater cap plate  120  and heating elements  162  above the vacuum plate V. 
         [0072]    After the heater cap H is above vacuum plate V, heat is applied to the polymer sheet  12  with heating elements  162 . Power is applied to heating elements  162  with power conductors  164  (shown in hidden lines connected to heating elements  162 ). The temperature of each heating element  162  can be individually controlled based on information received from the thermocouple  166 , (also shown connected to heating elements  162  with hidden lines). Cooling fans  160   a  with cooling fins  160   b  are shown positioned on the top of the heater cap plate  120 . The cooling fans  160   a  can control the temperature of the heater cap plate  120  during repeated production cycles to manage the temperature of the heater cap plate  120 . The position of the heater cap H can also be controlled hydraulically instead of pneumatically, as desired. 
         [0073]    A vacuum port  94  is shown that opens into the bottom of the cup  214  (best seen in  FIGS. 3D and 9 ). The vacuum port  94  is connected to vacuum source  92 . After the heating elements  162  are activated to cause the polymer sheet  12  to reach the desired temperature set point, a vacuum is drawn by vacuum source  92  to evacuate the cups  214 . The adapter plate  270  includes a plurality of orifices  271  (see  FIG. 3D ) so a vacuum drawn through the vacuum port  94  from a vacuum source  92  draws a vacuum efficiently inside the cup  214 . 
         [0074]      FIG. 10  is a front cross section view taken along line  10 - 10  of  FIG. 8 . Boss  121  is attached to the underside of the heater cap plate  120 . When the heater cap H is moved forward to cover the vacuum plate V, each boss  121  secures the polymer sheet  12  between each cup  214  to prevent air from escaping from between the polymer sheet  12  and the cup  214 . The polymer sheet  12  above each cup  214  can then be drawn into each cup  214  as a vacuum is drawn from vacuum port  94 . 
         [0075]    An effective seal is formed between the polymer sheet  12  and the rounded edge  214   a  that is formed between the cup wall  214   b  and the upper surface  234  of the vacuum plate V so that when a vacuum is drawn through vacuum port  94  the polymer sheet  12  that is above each cup  214  is pulled downwardly into the cup to form onto each dental model  15 . 
         [0076]    After heat has been applied to the polymer sheet  12  and a vacuum has been drawn in the cups  214 , the polymer sheet  12  molds about the surface of each dental model  15  as seen in  FIGS. 11 and 12 . After the polymer sheet  12  has been molded about the surface of each dental model  15 , the heater cap H is moved rearward to expose the molded aligners  16  (see  FIGS. 11 and 12 ). Pneumatic pressure is applied to opening line  352   b  to extend cylinder  328  to force heater block  329  and the attached heater cap H to move rearward, away from the vacuum plate V. A spring  333  is provided on shaft  324  to dampen the motion of the heater cap H as it opens by resisting the rearward motion of the housing block  329 . 
         [0077]    Often the dental models come out of the cups  214  together with the aligners  16  and they are then removed therefrom. Each aligner includes formed indentations  272   a  that are formed from the indentations  272  in the adapter plates  270 . The formed indentations may be used for subsequent handling of the aligner  16  before it has been trimmed and buffed. 
         [0078]      FIG. 13  shows a batch of molded aligners  16  that have been removed from the heater cap H. The batch of aligners  16  are separated into individual aligners by punching a disk  17  from the polymer segment  12  with a die cutter or other tool as indicated at lines  12   a ,  12   b ,  12   c  and  12   d . The resulting aligner  16  on disk  17  shown in  FIG. 14  is then ready for further processing to cut, finish and uniquely identify each aligner  16 . 
         [0079]    Process Steps: 
         [0080]    An embodiment of the process steps, including vacuum phases, of manufacturing aligners with the inventive thermoformer as illustrated based on the timeline in  FIG. 15  is as follows: 
         [0081]    1) At T 1  the operator engages two momentary switches (not shown) simultaneously, which causes the heater cap H to close. Each momentary switch requires engagement by each of the operator&#39;s hands to prevent the operator from getting either hand in the moving parts of the thermoformer T. 
         [0082]    2) The polymer sheet  12  undergoes heating above each cup  214  for a predetermined period of time between T 1  and T 5 /T 5 ′. A typical time frame from when the buttons are pressed to when the heater cap H retracts is approximately 25-35 seconds (T 1  to T 5 /T 5 ′). 
         [0083]    3) The polymer sheet  12  above each cup  214  is subjected to a “pre-stretch” at a predetermined time T 2 . The predetermined “pre-stretch” time typically begins about 15-25 (T 2 -T 2 ′) seconds after the switches are engaged at T 1  and lasts for a predetermined period of time, which is typically about 0.5-1.5 seconds (between T 3  to T 3 ′). The “pre-stretch” is produced by opening the vacuum source  92  and achieving an absolute pressure of 13+/−1 in. Hg. The timing of the “pre-stretch” vacuum is established electronically independent of the operator&#39;s commands. The “pre-stretch” is performed to make more polymer  9  available around the adapter plate  270  for the final stretch. If there is too long of a “pre-stretch” undesirable wrinkles may develop in the portion of the polymer sheet  12  in each cup from which air was not able to escape. Too short of a “pre-stretch” and there may not be enough polymer  9  from the polymer sheet  12  introduced and thus the aligner  16  could be too thin. It was determined by the inventors that by adding the “pre-stretch” sequence to the thermoforming process, the thinning problem was solved. 
         [0084]    4) The final pull, in which a vacuum is drawn by the vacuum source  92  to approximately 28+/−1 in. Hg vacuum, is activated at T 4 /T 4 ′ approximately 25-35 seconds after the initial process has started at T 1 . The heater cap H is pulled back at T 5 /T 5 ′, approximately 30-40 seconds after the initial process started at T 1 . 
         [0085]    5) The final pull continues between T 4 /T 4 ′ and T 6 /T 6 ′ for about 3-12 seconds at which time the aligners  16  are completed and ready to be removed from the vacuum plate V. 
         [0086]    The process steps can vary from the time periods specified without varying from the inventive thermoforming process. Also, instead of measuring the time between the process steps shown in  FIG. 15 , a delta or change in pressure between steps at T 1 -T 6  can be used to determine when the process steps are performed. 
         [0087]    The timing and duration of the “pre-stretch” and the final pull and the amount of vacuum provided are approximate ranges and the ranges can vary from the amounts stated without departing from the scope of the inventive system and method. 
         [0088]    Thus specific embodiments and methods of a batch thermoformer for dental appliances have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the disclosure. Moreover, in interpreting the disclosure, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.