The present invention relates to a molding method and apparatus. More particularly this invention concerns a double-mold blow-molding system.
In blow molding a hollow parison is ejected downward from an extruder head into a cavity formed between a pair of mold halves. The parison is a tube whose outer end engages over a blowing lance that is below the mold. The mold halves are closed on the parison, cutting it off from the extruder head and clamping it tight around the blowing lance. Then air is blown into the parison to expand it to fit tightly against the walls of the cavity, imparting the desired shape to it. After a short curing time due to the thinness of the workpiece walls the mold is opened and the cured workpiece ejected. Such techniques can be used for large and small workpieces and can produce shapes of considerable complexity.
In U.S. Pat. No. 5,975,881 of Langos a system is described therein the mold halves are not carried on an array of through-going tie rods, but instead are mounted on respective supports that ride on horizontal guides below a stationary extruder head. In a double system two outer mold halves are provided with respective drives for moving them horizontally on their guides, and the two respective inner halves are mounted on a common support that has no drive but that can slide on the guides and be locked at any position thereon. The inner mold halves are shifted by the outer halves between open and closed positions in a manner that is fairly complex and that does not adapt itself readily to different molding operations.
It is therefore an object of the present invention to provide an improved method of an apparatus for double-mold blow molding.
Another object is the provision of such an improved method of and apparatus for double-mold blow molding which overcomes the above-given disadvantages, that is which is simple and which allows the two molds to be opened and closed exactly as needed for a given product.
A molding system has according to the invention a main slide displaceable parallel to an axis through a relatively long stroke relative to a stationary overhead extruder head, a center support fixed on the main slide, and a pair of end supports displaceable axially through relatively short strokes on the slide between inner and outer end positions. A pair of inner mold halves carried on the center support are directed axially oppositely therefrom toward the end supports. Respective outer mold halves carried on the end supports are axially engageable with the respective inner mold halves in the inner positions of the end supports. An end-support drive connected between the slide and the end supports moves the end supports between their end positions and a slide drive connected to the slide moves the slide axially relative to the head.
With this system the supports, which each normally include a mounting plate to which the inner and outer mold halves are bolted, can therefore be moved to accommodate molds of different sizes. The structure is fairly simple in that the center support is fixed on the slide and it is actually the slide that is moved to position the inner mold halves. Movement of the slide therefore serves initially to move the inner mold half in on a mold being closed and subsequently to move the filled mold off into a curing/unloading station while the other mold is being loaded.
According to the invention each of the end-support drives includes a respective motor mounted on the slide and having a spindle extending parallel to the axis and formed with a threaded portion. A respective nut fixed to the respective end support is threaded on the threaded portion of the respective output spindle. Such a drive is extremely accurate and, when the motor is a stepping motor, makes it very easy for a computer controller to position the mold halves with great accuracy.
According to another feature of the invention a pair of respective blow-molding units are displaceable axially on the support. Respective blowing-unit drives connected between the units and the support displace the blowing units axially synchronously with the respective end supports. This allows the blow-molding units, which typically include a blowing lance or mandrel, to remain centered underneath the extruder head while the respective mold halves are closed on the parison whose upper end is still engaged by the head and whose lower end is fitted over the lance of the blowing unit. When the mold closes the parison is pinched off the extruder head and clamped tightly around the blowing lance.
The blowing-unit drives each include another threaded portion on the respective spindle and a respective nut threaded to the respective other threaded portion and fixed on the respective unit. The portion of each spindle engaging the end-support nut has a pitch (e.g. threads/inch) which is substantially equal to half a pitch of the spindle portion engaging the respective unit nut so that the end supports will move at twice the speed as the respective drive units, thereby covering twice the distance in the same time. This ensures perfectly synchronous movement of each outer mold half and the respective blowing unit. Of course as the blowing unit is moving outward at half speed, the slide is moving in at half speed to keep the blowing unit perfectly centered under the extruder head.
To prevent the end supports from canting on the slide, the slide has an upper surface provided with tracks on which the end supports are movable axially.
The method of the invention therefore comprises the steps of first opening one of the molds by displacing the slide in one direction through a. predetermined distance and the respective outer mold half oppositely through twice the predetermined distance with the respective inner mold half moving jointly with the slide to a position with the respective mold halves spaced apart underneath the extruder head. Then a parison is extruded from the head down between the halves of the one mold and the one mold is closed on the parison. Finally the slide is displaced to position the other of the molds underneath the head, the first three steps are repeated with the other mold.