Process for producing stretch blow formed plastic containers

Details Process for producing stretch blow formed plastic containers Process for producing stretch blow formed plastic containers

1999-05-14

2001-11-13

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of electrical or wave energy to work

Direct application of fluid pressure

C264S525000, C264S532000, C264S537000, C264S085000, C264S084000, C425S001000, C425SDIG001

Reexamination Certificate

active

06315939

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is in the field of plastic processing and concerns a method according to the generic part of the independent claim. The method serves for producing plastic containers through stretch blow molding. The plastic containers are made of a plastic material suitable for stretch blow molding and they are e.g. bottles made of polyethyleneterephthalate (PET).
2. Background Information
PET-containers, especially bottles made of this material, are normally produced by means of stretch blow molding. In most cases the bottles are made from an injection molded preform. The preform normally has a diameter which is smaller than the diameter of the bottle to be produced, an axial length which is smaller than the axial length of the bottle to be produced and an opening area (e.g. with thread and neck ring) which already has the form of the opening area of the bottle to be produced. The preform is heated to a temperature suitable for the stretch blow molding process and is positioned in a mold which corresponds to the form of the container to be produced. In this mold, the preform is stretched by means of a stretching rod introduced axially into the mold, whereby an inner pressure in the range of about 5 to 10 bar (stretching pressure) is applied for achieving an increase in diameter also. When the preform has been sufficiently stretched, i.e. when the stretching rod has reached its predetermined end position, the inner pressure is increased, normally up to a range of 40 to 50 bar (blowing pressure) whereby the preform is blown up, its walls being completely pressed against the mold and thus adopting the form of the bottle to be manufactured.
The pre-pressure (stretching pressure) necessary is normally generated by means of connecting the inside of the preform to a network of pressurized air. For generating the blowing pressure a special container for compressed air with a pump is provided which must comply to increased security regulations and thus necessitates a considerable investment.
If the stretch blow molded bottles or containers are used for foodstuffs or pharmaceuticals they are submitted to a sterilization process before filling, i.e. they are substantially freed of living organisms and germs. For this purpose they are treated with liquid or gaseous chemicals (mostly strong oxidants) with a sterilizing effect (poisonous to the organisms to be removed) or they are heated to a temperature at which the organisms cannot survive or they are irradiated, e.g. with ultraviolet light. Sterilization methods in which plasma-enhanced processes are applied for sterilization of containers are also known.
The main disadvantage of sterilization methods with sterilizing gases or liquids such as e.g. ethylene oxide, propylene oxide, chlorine dioxide, hydrogen peroxide, nitrogen oxide or sulfur dioxide is the fact that these substances are also poisonous to humans and that therefore, particular measures must be taken in order to prevent contact with such substances. Furthermore, some of these substances have undesired aromas such that they must be removed carefully after the sterilization process. Several of these substances are explosive under certain conditions which is a disadvantage according to the state of the art.
The main disadvantage of sterilization by means of heat is the fact that it is consuming a large amount of energy and that it requires a long time (time of treatment plus cooling time). This disadvantage is less important for thermo-shock-sterilizing e.g. described in the patent application DE-2,915,659. For thermo-shock-sterilization, the objects to be sterilized are exposed to a fast flowing current of humid air having a temperature of more than 500° C. and effecting sterilization within about two seconds. The time required for cooling is very short compared to traditional heating methods as only the surface of the objects to be sterilized is heated. However, it seems to be difficult to ensure that actually every single spot on the surface of the objects to be sterilized reaches a temperature sufficiently high for sterilization.
SUMMARY OF THE INFORMATION
The object of the invention is to create a method for producing plastic containers by means of stretch blow molding which method is to be more simple than similar methods according to the state of the art and into which method a possibly necessary sterilization of the inner surface of the manufactured containers is integrated.
This object is solved by the method for production of plastic containers as defined by the claims.
According to the inventive method, a preform is introduced into a mold and is stretched in axial direction by means of a stretching rod as is usual according to the state of the art. However, instead of generating the stretching pressure and the blowing pressure with the help of pressurized air as is usual according to the state of the art, an explosive gas mixture is pressed into the preform for creating the stretching pressure. As soon as the preform has been stretched to the desired degree, i.e. the stretching rod has reached a predetermined end position, the explosive gas mixture is ignited with suitable means. On explosion of the gas mixture, a pressure shock and thermal shock is created designed such that the pressure is sufficient for the blowing and the temperature is sufficient for the sterilization of the inner surface. As the pressure shock only lasts for a very short time and as the volume of the container increases to a considerable degree it can happen that the pressure in the container is not sufficient after the explosion to prevent post-explosive shrinking. In such a case it is necessary to continue pressing gas into the container after the explosion and to maintain the pressure in the range of the stretching pressure until the container material in the blown walls is totally stabilized.
Unlike according to the methods of the state of the art, according to the invention, it is not necessary to create a high blowing pressure outside of the container nor to sterilize the container after stretch blow molding. The container manufactured according to the inventive method is advantageously directly supplied to a filling step for being filled and sealed, all within the same arrangement of devices and under sterile conditions.
Temperatures occurring in explosions are very high in relation to the released energy. This means that actually only the inner surfaces of the plastic container are heated and thus sterilized such that containers made of relatively temperature-sensitive materials do not take damage in the method. It is possible that apart from the temperature shock, also the pressure shock or unstable particles with a poisonous effect being formed during the explosion contribute to the sterilizing effect on the surfaces exposed to the explosion.
The explosive gas mixture to be used substantially consists of a gaseous oxidant and an easily oxidizable gas or vapor. The oxidant in most cases is pure oxygen or the oxygen contained in air, the oxidizable component is e.g. hydrogen, methane, methanol ethane, propane, ethylene, propylene or chlorine etc.
The power of the pressure shock, i.e. the maximal pressure to be expected can be controlled, i.e. can be adapted to the conditions determined by the blow molding process, by corresponding composition of the gas mixture. If the gas mixture contains only the reaction partners and these in a stoichiometric relation, a shock-like pressure increase by a factor ten or more is to be expected. For smaller pressure increases, inert gases are added to the explosive mixture.
The temperature reached in the explosion is substantially dependent on the energy set free by the oxidation (i.e. on the kind of reaction partners) and it is dependent on the heat capacity of the gas mixture. This temperature is at its highest when the gas mixture only comprises the reaction partners in a stoichiometrical relation correct for the oxidation. For example, in an explosion of oxyhydrogen in which a stoichiometric oxyg

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