Patent Publication Number: US-2006012085-A1

Title: Method for blowing objects

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention concerns a method for blowing objects, such as bottles, canisters or the like.  
      2. Discussion of the Related Art  
      It is known that hollow, plastic objects can be formed by what is called “twin sheet molding”, which consists of welding two vacuum-formed pieces together and cooling them. For this technique, use is made of a blowing needle or blowing mandrel in the known manner.  
      It is also known that such objects can be blown out of plastic, whereby a synthetic sleeve is inserted in a mould having the shape of the object to be made and this sleeve is then inflated by means of compressed air, and the formed object is taken out of the mould after cooling off.  
      For blowing, this known technique makes use of a blowing mandrel or blowing needle.  
      Inflating is usually done with pre-blowing, whereby the inflating takes place at a small air pressure which is then increased to the available pressure in the compressed air network.  
      According to a known method, known as “interval blowing”, the compressed air supply is alternately opened and closed while blowing, such that a pulsating pressure is obtained.  
      A disadvantage of this technique consists in that the compressed air network is heavily loaded by alternately opening and closing the compressed air supply, leading to wear of valves and compressors which are part of the compressed air network. It is also known that applying such modulating on/off switches requires a lot of energy.  
      Another disadvantage of this technique is that the heat created while forming the object can only be given off with difficulty.  
      According to another known method, known as “blowing with scavenging air”, in order to cool the formed object, the air is continually refreshed during the blowing by scavenging it at a maximum pressure, as the outgoing air is not shut off.  
      A disadvantage of this technique is that, in order to scavenge at a maximum pressure, only a relatively small flow rate is available, which is disadvantageous in that relatively much time is required for the cooling, and as a consequence the productive capacity is relatively low.  
      Another disadvantage of this technique is that relatively much time is required to de-aerate and to lower the pressure in the blown object from the maximum pressure to the atmospheric pressure without deforming the obtained object, which consequently has a negative influence on the productive capacity.  
      Another disadvantage of this technique is that the air escaping from the blowing mandrel produces noise.  
      According to another known method, known as “blowing with scavenging air while maintaining an adjustable pressure”, after a certain length of time, the air is continuously refreshed during the blowing, whereby a lower adjustable pressure is maintained in the object made.  
      Although the time required to de-aerate from this lower pressure to the atmospheric pressure is shorter than with the above-mentioned other known methods, said de-aeration time still takes relatively long.  
      With this method as well, the flow rate of the scavenging air is still restricted, which is at the expense of the time required to cool the formed object, and which is thus at the expense of the productive capacity.  
      The present invention aims to remedy the above-mentioned and other disadvantages, whereby the scavenging for the cooling requires less time and thus allows for larger productive capacities.  
     SUMMARY OF THE INVENTION  
      To this end, the present invention concerns a method for blowing objects whereby the following steps are successively followed: providing a synthetic sleeve in a mould; inflating this sleeve by means of compressed air up to a maximum pressure; and scavenging the obtained object by means of a compressed air flow whereby the air in the object is continuously refreshed and whereby the pressure of the compressed air varies from a starting pressure to the atmospheric pressure according to an adjustable course which varies as a function of time.  
      An advantage of this method is that the pressure may increase and/or decrease according to any desired pattern, whereby the course of the pressure as a function of time can be selected such that the pressure decreases consistently, for example in a continuous manner or in stages, as a result of which the scavenging flow rate may consistently increase in the same ratio.  
      As a consequence, the de-aeration time is considerably shorter than with known methods, such that the blown product is sooner ready for further processing. As a result, the cycle time decreases considerably and the efficiency of the production process increases.  
      An additional advantage is that scavenging air can be recycled when it leaves the object to be formed, provided a compressor is applied. Since the pressure of the scavenging air, when it leaves the object to be formed, is usually larger than the atmospheric pressure, said recycling may result in a considerable energy saving in order to maintain the compressed air network.  
      Another advantage of the method according to the invention is that the compressed air network is less heavily loaded with less fluctuations, as a result of which there is less wear and energy is saved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In order to better explain the characteristics of the present invention, the following preferred method according to the invention for blowing objects is given as an example only without being limitative in any way, with reference to the accompanying drawings, in which:  
      FIGS.  1  to  6  are successive steps of a method according to the invention;  
       FIGS. 7 and 8  schematically represent a possible course of the pressure as a function of time, during the step represented in  FIG. 6 . 
    
    
     DESCRIPTION OF THE PREFERED EMBODIMENT  
      FIGS.  1  to  5  represent a mould  1  for blowing synthetic objects, for example in this case canisters for storing liquids, which mould  1  is made in two parts in the known manner, whereby these two mould parts  1 A and  1 B, can be moved away from each other and towards each other by means of a driving mechanism which is not represented in the figures.  
      In the above-mentioned mould  1  is provided at least one opening  2  in which can be provided a blowing needle or blowing mandrel  3  to blow an object  4 .  
      The above-mentioned blowing mandrel  3  in this case consists of two concentric ducts for compressed air, more specifically an inner supply duct  5  and an outer discharge duct  6 .  
      The supply duct  5  is hereby connected to a compressed air network  7  which may for example be provided with an adjustable inlet cock  8 .  
      The outlet of the discharge duct  6  is, in this case, connected to the atmosphere via an adjustable throttle valve  9 , which throttle valve  9  is preferably connected to a control  10  which makes it possible to open and close this throttle valve  9  according to an imposed program which is stored for example in a computer  11 .  
      In this case, each of the mould parts  1 A and  1 B is provided with an internal cooling channel, not represented here, with an inlet  12  and an outlet  13 . The method for blowing objects according to the invention is very simple and as follows.  
      Initially, a sleeve  14  made of plastic or the like is taken, which in this case is realized according to a first known step, starting from a hose which is extruded and which is then cut at a certain length and is squeezed at the cut end  15 .  
      The sleeve  14  is then, as represented in  FIGS. 1 and 2 , provided between the two mould parts  1 A and  1 B, after which these mould parts  1 A and  1 B, as represented in  FIG. 3 , are pressed against each other by means of the above-mentioned driving mechanism, and the sleeve  14  protrudes out of the mould  1  via the opening  2 , as is represented in  FIG. 4 .  
      Next, as represented in  FIG. 5 , the blowing mandrel  3  is provided in the opening of the sleeve  14 , whereby the outlet  16  of the supply duct  5  sticks in the mould  1  and the discharge duct  6  is connected to one far end  17  around the above-mentioned opening  2  on top of the mould  1 .  
      This so-called “shooting” of the blowing mandrel takes place at a low compressed air pressure, such that the compressed air flow rate is restricted as well.  
      Then, as is represented in  FIG. 6 , compressed air is blown into the sleeve  14  via the blowing mandrel  3 , so as to blow this sleeve  14  up against the walls of the mould  1 , such that the sleeve  14  assumes the shape of these walls so as to form the required object  4 . This is done by connecting the maximum available compressed air pressure, such that an increased contact with the walls of the mould is obtained.  
      After a certain adjustable length of time, a decrease of pressure can be realized in the formed object  4  according to a desired variation, by controlling the adjustable inlet cock  8  and/or the throttle valve  9 , whereby this control is preferably such that a pressure variation as a function of time is obtained, as is represented in  FIGS. 7 and 8 , which represent the relative pressure P in relation to the atmospheric pressure as a function of time t.  
      From  FIG. 7  can be derived that, during a first time span A, the sleeve  14  is first inflated at a low pressure which systematically increases from the atmospheric pressure P atm  to a relatively low pressure P 1 , after which, during a time span B, the pressure is quickly raised up to a desired start pressure P start , which is for example the maximum available pressure in the compressed air network  7 , so as to obtain more contact between the sleeve  14  and the mould walls.  
      Next, according to the invention, during a following time span C for cooling, scavenging air is blown through the formed object  4 , whereby, during this phase, the pressure in the object  4  is consistently reduced according to a continuous curve from the start pressure, here P max , to P atm , i.e. for an appropriate control of the throttle valve  9 .  
      It is clear that, since during this time span C the pressure consistently decreases, the flow rate for scavenging will increase in the opposite sense, such that, in a relatively short time span C, a sufficient cooling of the object  4  can be obtained which makes it possible to take the object  4  out of the mould without any unwanted deformations.  
      Additionally, for an even faster cooling of the object  4 , the cooling channel extending through the mould  1  can be connected, during the blowing, to an external cooling circuit  18 , having for example water as a cooling medium.  
      While blowing, the compressed air itself is preferably cooled to an adjustable temperature as well, which may be either higher or lower than zero degrees Celsius. Thus, the decrease of pressure can be faster, since the blown object  4  has to cool less long, and the de-aeration time D thus decreases, and the production speed and the output of the blowing process increase.  
      According to the invention, the decrease of pressure in area C can be obtained by successively going through more than one adjustable discrete pressure value or, when using an unlimited number of such adjustable pressure values, by running through a continuously adjustable curve, whereby the last set pressure value can amount to zero bar relative pressure in relation to the atmospheric pressure.  
      After having run through the area C, the formed object  4  can be removed from the mould  1 , after which any possible excess flashes or the like can be removed from this object  4  by means of punching, grinding or the like.  
      Although in the given example, the start pressure is equal to the maximum pressure P max  that is to be reached in time span C, it is also possible to start time span C at a start pressure P start  that is lower than P max , as is represented in  FIG. 8 .  
      In this case, the set pressure values from the incoming compressed air during the scavenging temporarily increase from a value P start  to a value P max  which is higher than the aforesaid P start  value.  
      Although in the given example, the inlet cock  8  and the throttle valve  9  are automatically adjusted, it is not excluded for such control to be realized manually.  
      The above-mentioned throttle valve  9  and/or inlet cock  8  for adjusting the decrease of pressure is preferably a proportional valve. This offers the advantage that a simple control circuit can be used for controlling the decrease of pressure.  
      It is clear that by “compressed air” should not only be understood air, but that any gas under pressure whatsoever can be used for blowing an object by means of a method according to the present invention.  
      It is also clear that the above-mentioned sleeve  14  can be made of any material whatsoever which is suitable to be inflated.  
      Further, it is clear that the above-described technique can be applied to any production process whereby the object to be formed is made by means of compressed air and must be cooled.  
      Finally, it should be noted that the scavenging air, when leaving the object to be formed, can be recycled, for example by providing a compressor at the outlet of the discharge ducts  6  which compresses the scavenging air to a required pressure and whose high pressure side is connected to the compressed air network.  
      The present invention is by no means limited to the embodiments given as an example and represented in the accompanying drawings; on the contrary, such a method for blowing objects can be made according to different variants while still remaining within the scope of the invention.