Abstract:
A clamp is used to secure a heating element to an outside face of a mold. The clamp includes a frame portion which holds a commercially available cartridge heater. The frame portion is sized to mate with the cartridge heater for efficient conduction heating from the cartridge heater. The clamp attaches to the mold via recesses that can be machined into the mold on a three-axis mill, such as cylindrical opening machined in a direction parallel to and slightly intersecting the side face on which attachment occurs. The clamp includes a biasing mechanism for pressing the clamp into heat conducting contact with the outside face of the mold block to which it is attached, for efficient transfer of heat between the cartridge heater and the mold. The preferred biasing mechanism is through a screw lengthening/shortening mechanism, with the clamp attached on two side faces around a corner of the mold block. The biasing force is enhanced by a fulcrum ledge. By turning the screw and shortening the length between two attachment points, the clamp tightly presses into conducting contact with the mold block.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     None.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to the field of molding. More particularly, the present invention relates to the manufacture and use of injection molds involving heating of at least one of the mold blocks to better manage the flow and solidification of the plastic material within the mold block.  
         [0003]     Injection molding, among other types of molding techniques, is commonly utilized to produce plastic parts from molds. Injection molds typically include two plates which are positioned together to form a cavity in which a molten plastic material is injected. The molten plastic material adopts the shape of the cavity and then solidifies. The plates are separated, and the solidified plastic piece is ejected. After ejecting the plastic piece, the plates are again positioned together and the process repeated. Injection molding can be performed thousands of times with the same mold blocks, minimizing the piece price of the resultant parts.  
         [0004]     Proper injection molding requires the plastic to flow in its molten state until the cavity is filled, with solidification occurring slowly enough and in such a way as to allow complete filling. At the same time, solidification should occur as quickly as possible, so the part can be ejected as soon as possible, reducing manufacturing time and increasing throughput of parts.  
         [0005]     Because the temperature of the plastic and the heat transfer from the plastic are so critical to the speedy production of high quality injection molded parts, many injection mold components include structures to generate or absorb heat. For instance, injection mold nozzles and manifolds often include heating elements. Injection mold blocks have also been heated to better control the solidification of the plastic shot. Some prior mold blocks have included passages for flow of a heat transfer fluid through the mold blocks. The temperature of the heat transfer fluid is maintained external to the mold block, and when the heat transfer fluid is pumped through the mold block it heats (or cools) the mold block to the same temperature as the heat transfer fluid.  
         [0006]     Other prior heat control mechanisms for mold blocks involve the use of electrical resistance heater elements. An electrical current is driven through the resistance heater to generate heat within the mold block. A thermostat is used to determine how much current should be driven through the resistance element at any particular time.  
         [0007]     One form of such electrical resistance heater elements are cartridge heaters, wherein the resistance element is contained within a housing. The cartridge heater is separate from the mold block. A recess is formed in the mold block to receive the cartridge heater. When using such a cartridge heater arrangement, the same cartridge heater can be readily changed between different mold blocks. If the cartridge heater breaks or malfunctions, it can usually be replaced without affecting any of the mold blocks.  
         [0008]     While direct placement of the cartridge heater into the mold block provides these advantages, it also has its disadvantages. The recess for the cartridge heater is positioned in a side wall of the mold block, so the cartridge heater doesn&#39;t interfere with the parting line between the two mold block halves. Typically the recess extends perpendicular to the side wall. In such a position, however, machining the recess into the mold block cannot be done with a three-axis CNC mill used to form the cavity and back side detail without reorienting the mold block relative to the mill. Reorienting the mold block in the CNC mill is time consuming, as is machining the recess into the side of the mold block using a different tool.  
         [0009]     While the cartridge heater is being used, it can come loose from the mold block. In particular, the injection mold press cycles through opening and closing the mold blocks for each shot, and the vibration and movement can loosen the cartridge heater from its hole. The loose cartridge heater can cause damage or melting of the mold block as it presses harder on one section of the hole than another. If the loose cartridge heater comes partially or fully out of its hole, the hot cartridge heater is dangerous to workers and the injection mold press. Thus, further improvements are needed to the “separate cartridge heater” system of controlling temperature of mold blocks.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The present invention is a clamp for securing a heating element to an outside face of a mold, and a method of use of such a clamp. The clamp includes a frame portion which holds a commercially available cartridge heater. The clamp attaches to the mold via recesses that can be machined into the mold on a three-axis mill and in an orientation already used in creation of the mold, i.e., machined in a direction parallel to the side face on which attachment occurs, which could be either the direction used in milling the mold cavity or the direction used in machining the back side of the mold. The clamp includes a biasing mechanism for pressing the clamp into heat conducting contact with the outside face of the mold block to which it is attached, for efficient transfer of heat between the cartridge heater and the mold. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is an exploded plan view of a cartridge heater, clamp and mold in accordance with the present invention.  
         [0012]      FIG. 2  is a perspective view of the cartridge heater, clamp and mold of  FIG. 1  during assembly.  
         [0013]      FIG. 3  is cross-sectional view of the cartridge heater, clamp and mold of  FIG. 1  after assembly and during use. 
     
    
       [0014]     While the above-identified drawing figures set forth one or more preferred embodiments, other embodiments of the present invention are also contemplated, some of which are noted in the discussion. In all cases, this disclosure presents the illustrated embodiments of the present invention by way of representation and not limitation. Numerous other minor modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.  
       DETAILED DESCRIPTION  
       [0015]     As shown in  FIGS. 1-3 , a preferred embodiment of the present invention includes a clamp  10  which is used to attach a cartridge heater  12  to a mold block  14 . The cartridge heater  12  is within the prior art, and includes a resistance heater element  16  (shown in FIG.  3 ) within a housing. The heater element  16  is driven by an electrical current through a power cord  18 . The housing of one preferred cartridge heater  12  is cylindrical, with a diameter of about ⅜ inch and a length of about 4 inches. Preferably the cartridge heater  12  includes a thermostat to enable temperature regulation. Such cartridge heaters  12  are commercially available such as from Watlow Electric Manufacturing Company of St. Louis, Mo. See for instance U.S. Pat. No. 5,247,158, incorporated herein by reference. Because the clamp  10  is specially designed for the particular type of cartridge heater  12  to be used, virtually any shape or size of cartridge heater  12  could be alternatively used.  
         [0016]     The mold block  14  for the invention can be an injection mold block with one or more internal cavities (not separately shown) as known in the art. For instance, typical sizes for mold blocks are 5×5 inches to 10×18 inches, in a rectangular prism shape. Each half of the mold block  14  is about 2 or 3 inches thick. Regardless of the size of the mold block  14 , the mold block  14  has been specially modified to mate with the clamp  10  of the present invention. In particular, the mold block  14  includes two recesses  20 . In a preferred embodiment, one of the recesses  20  is on an exposed side face  22  of the mold block  14 , and the other recess  20  is on a different exposed side face  22  of the mold block  14 .  
         [0017]     The preferred recesses  20  are cylindrical openings which extend vertically (parallel to the side face  22 ) and intersect with the side faces  22 . A side of the recess  20  which is parallel to its longitudinal axis  24  is exposed on a side face  22  of the mold block  14 . The longitudinal axis  24  of each recess  20  is inside the outline of the mold block  14 , so the width of the opening formed by the recess  20  is narrower than the diameter of the cylindrical recess  20 . This construction makes the recesses  20  very easily machinable. Because the recesses  20  extend vertically, they can be machined with the same three axis CNC mill (not shown) and in the same mold block orientation as is used to machine other portions of the mold  14 . For instance, because the recesses  20  extend vertically, they can be machined at the same time and orientation as machining the mold cavity (not shown), or at the same time and orientation as machining features (not shown) on the back side of the mold  14 .  
         [0018]     While the drawings show a single set of recesses  20 , additional sets of recesses could be provided to allow the press operator the option of attaching the clamp  10  in different locations on the mold block  14 . The clamp  10  can also be flipped over (show the surface shown on top becomes the bottom), to attach to a mold block in the opposition direction (with the cord  18  coming off the right rather than the left of the clamp  10 ).  
         [0019]     The clamp  10  includes a frame portion  26  that holds the cartridge heater  12 , and a tightening portion  28 . Each of the frame portion  26  and the tightening portion  28  include a head  30  extending from a neck  32 . The head  30  is slightly smaller than the diameter of the recess  20 , and the neck  32  is slightly smaller than the width of the recess  20  where it intersects with the side face  22  of the mold block  14 . In the preferred embodiment, the head  30  and neck  32  are provided by a cylindrical rod  34  which is permanently joined to the rest of the clamp  10 , and each rod  34  is positioned and sized to mate with the respective recess  20  in the mold block  14 .  
         [0020]     Adjacent the rod  34 , the frame portion  26  includes a planar contacting face  36  which in use abuts against the mold  14 . The contacting face  36  provides ample smooth surface area for conduction to occur between the frame portion  26  and the mold block  14 . In the preferred embodiment, the frame  26  also includes an inside corner  38 , with a mold adjacent side face  40  beyond the inside corner  38 . This mold adjacent side face  40  is spaced from the rod  34  just enough that the side face  40  doesn&#39;t contact the mold block  14  when the rod  34  is in the recess  20 , so the shape of the frame portion  26  doesn&#39;t interfere with placing the rod  34  into the recess  20 .  
         [0021]     As shown in  FIGS. 2 and 3 , the tightening portion  28  is removably attachable to the frame portion  26 . The tightening portion  28  includes a thumb screw  42  which extends through the tightening portion  28  and into a threaded hole  44  in the frame portion  26 . The contacting surface  46  between the frame portion  26  and the tightening portion  28  extends at an angle. The thumb screw  42  is oriented perpendicular to the contacting surface  46 . A slide peg  48  projects out of the tightening portion  28  and is received in the frame portion  26 . The slide peg  48  assists the user to line up the thumb screw  42  with the thumb screw hole  44 . Once the thumb screw  42  is positioned into the thumb screw hole  44 , tightening of the thumb screw  42  advances the tightening portion  28  toward the frame portion  26  with the slide peg  48  sliding into the slide peg hole  50 . The contacting surface  46  also includes a fulcrum ledge  52 .  
         [0022]     A through hole  54  is positioned in the frame portion  26 . The through hole  54  is sized to mate with the cartridge heater  12 . In the preferred embodiment, this mating relationship is a fairly tight fit, so thermal contact is made on all sides of the cartridge heater  12  with the encircling frame portion  26 . The opposite end of the through-hole  54  is closed with a set screw plug  56 , shown in  FIG. 3 . The plug  56  can be removed, if necessary, to gain access to the distal end of the cartridge heater  12  and push the cartridge heater  12  out of the through hole  54 . Having access to push (rather than pull by the cord end) the cartridge heater  12  out of its hole  54  can be important, particularly if the thermal effects of the cartridge heater  12  cause it to firmly seat in the through-hole  54 , or even cause local melting and solidification of the frame portion  26  about the cartridge heater  12 .  
         [0023]     As shown in  FIGS. 1-3 , the top side of the clamp  10  includes several set offs  58 . Similar set-offs (not shown) are positioned on the bottom side of the clamp  10 . In the preferred embodiment, these set offs  58  are simply provided by set screws or cap screws positioned in screw holes  60 . The set offs  58  are useful in vertically positioning the clamp  10  relative to the mold block  14  to which it is attached. The set offs  58  also act as a spacer keeping the clamp  10  from touching/conducting heat into the injection press platens (not shown), which could otherwise act as a massive heat sink when the cartridge heater  12  is intended to be heating the mold  14 . By having set offs  58  on both the top and bottom of the clamp  10 , the clamp  10  can be oriented with either of its major sides facing upward when attached to the mold block  14 .  
         [0024]     In the preferred embodiment, the mold block  14 , the tightening portion  28  and the frame portion  26  are all formed of blocks of metal and particularly blocks of aluminum. Aluminum is readily machined, and has a high heat conductivity. Other metals with high heat conductivity could alternatively be used, such as copper. The rods  34 , the set off cap screws  58 , the plug set screw  56 , and the thumb screw  42  are all formed of steel for strength purposes. The steel has a lower heat conductivity, which is preferred for the set off cap screws  58 . The lower heat conductivity is largely inconsequential for the rods  34 , the plug set screw  56  and the thumb screw  42 . Metals other than steel could alternatively be used.  
         [0025]     To use the clamp  10 , first the various portions are assembled as illustrated in  FIG. 2 . The cartridge heater  12  is slid all the way in to the cartridge heater hole  54 . The slide peg  48  is inserted into the slide peg hole  50 , and the thumb screw  42  is started in the thumb screw hole  44 . With the thumb screw  42  still loose, there is some play between the frame portion  26  and the tightening portion  28 . With this play, the clamp  10  is positioned onto the mold block  14 . The rods  34  are slid down into the recesses  20 . When the clamp  10  is at the desired vertical height, the thumb screw  42  is further tightened. This tightening pulls the frame portion  26  tighter and tighter against the mold block  14 . The angle of the contacting face  36  and the fulcrum ledge  52  further assist in transferring the force of the thumb screw  42  into a directional orientation that presses the frame portion  26  into tight contact against the mold block  14 . In particular, the distance between the two recesses  20  is fixed, while the distance between the two rods  34  can be shortened by tightening the thumb screw  42 . The shortening length is achieved by biasing and moving the frame portion  26  toward the mold block  14 . While two recesses  20  that were positioned along the same single side of the mold would have some affect in tightening the frame portion  26  against the mold block  14 , positioning the recesses  20  on different sides  22  separated by the corner further helps generate a strong force biasing the frame portion  26  against the mold block  14 . Thus, a worker can very simply turn the thumb screw  42  to bias the frame portion  26  tightly against the mold block  14 . The fulcrum ledge  52  further helps to transfer the pull force of the thumb screw  42  into a force pushing the frame portion  26  into the mold block  14 .  
         [0026]     If desired, the tight fit between the cartridge heater  12  and cartridge heater hole  54  can be the sole mechanism to hold the cartridge heater  12  in place. However, in the preferred embodiment, once the thumb screw  42  is fully tightened, the edge of the tightening portion  28  extends over the proximal end of the heater cartridge  12  to positively hold the cartridge heater  12  into place in the frame portion  26 . The cartridge heater  12  thus cannot be removed from the frame portion  26  without loosening of the thumb screw  42 .  
         [0027]     Electrical current is driven through the resistance heater element  16  as known in the art, as desired to generate heat. Heat is then conducted as shown by the dashed arrows in  FIG. 3 , from the resistance heater element  16 , through the frame portion  26  and into the mold block  14 . The tight contact provided by the clamp  10  between the planar contacting face  36  and the side face  22  of the mold block  14  minimizes thermal resistance to the heat in conducting between the frame portion  26  and the mold block  14 .  
         [0028]     When molds  14  are being changed in the injection mold press, all that the worker need do is loosen the thumb screw  42  and remove the clamp  10  from the mold  14 . Even without fully removing thumb screw  42  from the thumb screw hole  44 , the worker can attach the clamp  10  to the new mold  14 .  
         [0029]     Because the tightening portion  28  holds the cartridge heater  12  into place in the frame portion  26 , the prior art problems associated with the cartridge heater  12  coming partially or fully out of the cartridge heater hole are avoided. Further, the cartridge heater  12  need only be changed between frame portions  26  when the cartridge heater  12  fails. Because the cartridge heater  12  is not moved from mold to mold but rather stays protected within the frame portion  26  as the frame portion  26  is moved from mold to mold, the likelihood of damage to the cartridge heater  12  is decreased. Changing the clamp  10  from mold to mold is much simpler than pulling the cartridge heater  12  from its hole and inserting it in a new hole in the new mold.  
         [0030]     The recesses  20  for the clamp  10  can be formed in the mold block  14  with the same three axis mill used to machine the cavity or back side detail on the mold, and without reorienting the mold block  14 . This shortens the time required for machining of the mold block  14  and lowers costs.  
         [0031]     If the cartridge heater  12  does fail during use, the clamp  10  can be quickly removed and replaced with a different clamp/cartridge heater combination. Similarly, if the cartridge heater  12  gets permanently stuck within the cartridge heater hole  54 , the stuck cartridge does not affect the mold  14  because the clamp  10  can be quickly removed and replaced with a different clamp/cartridge heater combination.  
         [0032]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.