Patent Application
20180147789
This invention relates generally to hot plate welding of thermoplastic materials and, more particularly, to a rotary hot plate welding system and method.
The process of hot plate welding uses a heated plate to melt the joining surfaces of the two thermoplastic parts. The part halves are brought into contact with a precisely heated platen for a predetermined period. After the plastic interfaces have melted, the parts are brought together to form a molecular, permanent and often hermetic bond. A properly designed joint welded under precise process control often equals or exceeds the strength of any other part area. A nonstick coating, such as Teflon or chrome, is often applied to the tooling to keep the melted plastic from sticking.
In accordance with one embodiment, a rotary hot plate welding system is provided for joining portions of two parts made of thermoplastic material by inserting the two parts into one of multiple clamping mechanisms that initially hold the parts spaced from each other. The clamping mechanisms are mounted on a turntable that is rotatable around an axis that is parallel to the direction of movement of the parts when they are clamped together by the clamping mechanism. By indexing the turntable, the two parts are moved to at least one stationary heating element extending over a portion of the turntable and between the two parts to be joined, to melt the opposed surfaces of the two parts. Continued indexing of the turntable moves the two parts away from the heating element. The two parts are then clamped together by moving at least one of the parts in a direction parallel to the axis of rotation of the turntable, so that the two parts directly contact each other. The two parts are cooled while they remain clamped together, in direct contact with each other, to solidify the molten thermoplastic material and thus weld the two parts together. The welded parts are then removed from the turntable.
The rotatable turntable includes multiple clamping mechanisms so that multiple pairs of thermoplastic parts can be carried by the turntable at the same time and processed in seriatim through successive stages of the welding process. This permits high production rates to be achieved.
The turntable is preferably indexed to at least two successive heating stations where the opposed surfaces of the two thermoplastic parts are melted, and then to a clamping station where the parts are pressed together to integrate the melted surfaces. Each heating station preferable includes two separate heating elements, each of which heats one of the parts. For example, the heating elements may be plates that are substantially parallel to the direction of movement of the turntable, and are located between the opposed surfaces of the parts, so that successive pairs of the parts can be moved into alignment with the plates by indexing the turntable.
Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to cover all alternatives, modifications and equivalent arrangements as may be included within the spirit and scope of invention as defined by the appended claims.
In the embodiment illustrated in
As the turntable 10 is rotated, each of the clamping assemblies C1-C6 is aligned successively with six different stations S1-S6 spaced around the circumference of the turntable. This enables six different pairs of parts P1, P2 to be carried by the rotatable turntable at the same time.
The first station S1 is a loading station where the thermoplastic parts P1 and P2 are loaded into one of six clamping assemblies C1-C6, and the last station S6 is an unloading station where the welded parts are removed from a clamping assembly. Stations S2 and S3 are heating stations where the opposed surfaces of the thermoplastic parts P1, P2 are heated by stationary heating plates 11a, 11b and 12a, 12b that extend laterally into the space between the parts P1, P2 as the turntable 10 is rotated. The heating plates are supported by a pair of posts 13a and 13b at each of the stations S2 and S3. When the lower fixture (e.g., C1b) is in its partially elevated position (described below), the upper heating plates 11a and 12a are spaced slightly below the upper part P1, and the lower heating plates 12a, 12b are spaced slightly above the lower part P2.
Station S4 is a cooling station where the parts cool and solidify while remaining clamped together, and station S5 is an inspection station. In an alternative embodiment, station S5 is used as a third heating station, rather than an inspection station, so that the number of welded parts produced per unit time can be further increased by reducing the dwell time at each station.
The turntable 10 is rotated intermittently in 60-degree increments, so that each clamping assembly passes successively through the six stations S1-S6 in each 360-degree revolution of the turntable. The turntable stops briefly each time the six clamping assemblies C1-C6 are aligned with the six stations S1-S6. The clamping assemblies thus remain aligned with the stations S1-S6 for a fixed dwell time before the turntable is advanced another 60 degrees. In the illustrative embodiment, with two heating stations and a heating time of 10 seconds per station, the thermoplastic parts are heated for a total of 20 seconds (10 seconds at station S2 and 10 seconds at station S3), which is sufficient time to melt the opposed surfaces of the thermoplastic parts. At least the opposed surface portions of the parts P1 and P2 are preferably made entirely of thermoplastic material so that the entirety of these surface portions are melted quickly, and form a strong weld joint when cooled and solidified.
In the embodiment illustrated in
Vertical movement of the lower fixtures is preferably controlled by a single, servo-controlled actuator 20 that is coupled to each successive lower fixture by a slip joint that allows the actuator and each successive fixture to engage and disengage smoothly and accurately. This servo-controlled actuator 20 is illustrated in
The actuator 20 returns the lower fixture to its lowermost (home) position at the end of each dwell interval, before the turntable is indexed to its next position. The upper end of the actuator 20 is T-shaped to slide within complementary T-shaped slots 21 in the lower fixture each time the turntable is indexed. This permits a single actuator 20 at each of the stations S2, S3 and S4 to service all six clamping assemblies C1-C6 as the turntable 10 is indexed.
At the cooling station S4, the parts with the melted surfaces remain clamped tightly together while they cool and solidify in the absence of the radiant heating, and they remain clamped while they are inspected at station S5. Thus, the weld is completed by allowing the parts P1 and P2 to cool under pressure, thereby solidifying the thermoplastic material that was melted in the heating stations S2 and S3. The final welded product is unclamped by upward movement of the upper fixture, and is then unloaded at station S6. In the present example a welded product is unloaded every 10 seconds.
While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.
