Method of reducing the permeability of blow-moulded thermoplastic hollow bodies.

The invention relates to a method according to the preamble of claim 1.

The blow-molded hollow bodies are widely used in polyethylene, in particular high molecular weight HDPE, made because this plastic has excellent properties in terms of its resistance to chemical agents, its impact resistance, moldability at low temperatures and also with regard to its impermeability to water vapor. In contrast, hollow body of polyethylene are little resistant to the passage of certain liquids, vapors and gases, such as hydrocarbons, so that the present in the hollow body substance, usually a liquid, can be changed by diffusion from the inside in its nature or amount.

In order to make plastic container for storing a plurality of different materials used, it is known from DE-PS 2401948 to suspend the inner surface of the container some time to the action of a fluorine-containing treatment gas. In this case, a hose or a preform for the adaptation is expanded against the inner surface of a blow mold, in which the hose section is enclosed by the tube section under pressure a mixture of about 0.1 to about 10 vol.% Fluorine and from the rest of an inert gas imports.

However, it was found that the barrier properties of a thus-blown article is far from being uniform over the extent of the wall when working at high speed Blasformsystemen. This means that different areas of the walls to hydrocarbons remain permeable and although varying in the wide circumferential passage speeds. In many cases, the rate of passage substantially achieved those values ​​as they show untreated products. Moreover arise relatively long cycle times, as the number of pieces per unit time produced container depends on the effectiveness of the cooling device in the blow mold.

The invention provides a method for reducing the permeation of blow-molded hollow bodies to be created out of thermoplastics, which avoids these drawbacks and in particular allows a number of pieces increase in blow molding.

To achieve this objective, the measure according to the characterizing portion of claim 1 is proposed.

According to the invention the treatment of the hollow body for reducing the permeation of its walls does not occur in the blow mold, but in a separate Nachkühlwerkzeug. It pressurizing the inner surface of the hollow body is thus performed with the reactive treatment gas in already molded hollow bodies and indeed separated from the blow molding process complicated on the blow molding machine. A threat of expensive facilities of the blow molding machine through corrosion exists. Furthermore, the blow molding process generally onerous precautions are unnecessary.

The hollow body preferably made of high molecular weight HDPE, but taken directly after their formation in a dimensionally stable state from the blow mold and introduced into a Nachkühlwerkzeug. For the subsequent treatment with the reactive gas remaining in the Hohlkörperwandungen heat can be exploited. In this case, a temperature of the inner surface of the hollow body of at least 110 ° C has a total proved expedient. The contact time of the treatment gas is then only a few seconds, for example, 1 to 20 seconds. The treatment gas is preferably introduced under a pressure of 5 to 12 bar in the hollow body. Then, the hollow bodies are flushed and cooled. This can be done with the aid of a dry gas, for example using nitrogen gas, the pressure of the flushing medium is lifted and lowered again at intervals.

According to a further feature of the invention, the reactive treatment gas from fluorine or chlorine or sulfur trioxide and an inert gas. The proportion of the reactive component of the treatment gas may vary within wide limits and is generally about 0.1 to 20 vol.%.

A prepared according to the procedure hollow body showing a uniform barrier effect over the whole extent of its interior surface and to an extent that is sufficient to make the hollow body for the absorption of hydrocarbons suitable. The treatment of the hollow body in a separate Nachkühlwerkzeug itself also results in a significant reduction in cycle time and there are hollow bodies obtained with improved dimensional stability.

The invention is explained in more detail with reference to the drawing.

In the drawing, the blow molding machine and the blow mold are not illustrated. One of removed from the blow mold the hollow body (1) by means of a conveyor belt (2), transported to the work area of ​​a gripping device (3) comprising one or more suction cups (4) and brought by this in a Nachkühlwerkzeug. The Nachkühlwerkzeug essentially consists of an upper (5) and a lower (6) tool half and is mounted in a press (7). The inner contour of Nachkühlwerkzeuges practically corresponds to the blow mold. In Nachkühlwerkzeug closed, the impingement of the inner surface of the hollow body with the reactive treatment gas takes place. For this, a blowing device (8) is provided. The hollow body is about 1 to 20 seconds at a gas pressure of about 5 to 12 bar the action of the exposed treatment gas, wherein the temperature of its inner surface is at least 110 ° C. Subsequently, the reactive gas is replaced by dry nitrogen by the hollow body is flushed via at least one further opening. For this purpose, the gas pressure in the hollow body is reduced either intermittently or permanently to a required for the flushing pressure of 5 to 7 bar. After completion of the rinsing and Nachkühlvorgangs the hollow body can be passed by means of a second, also with suction cups (10) provided gripping means (9) taken from the Nachkühlwerkzeug and on a second conveyor belt (11), more or less simultaneously, the first gripping means another hollow body in the Nachkühlwerkzeug inserts.

For safety reasons, the gripping means (3, 9), the press (7) with the Nachkühlwerkzeug (5, 6) and the second conveyor belt (11) in a gas-tight sealable chamber (12) may be arranged. The openings (13) and (14) of the chamber for the supply and removal of the hollow body (1) are expediently designed in this case in the form of locks. Furthermore, the chamber has at least one air inlet opening (15) and a suction opening (16) which is optionally connected to a gas washer.