Technical Field

This invention relates in general to blow moulding of plastic products, and more particularly to a method of attaching appendages to blow moulded articles. Background Art

Extrusion blow moulding is a well known process for fabricating plastic products such as containers, fuel tanks, etc. In operation, a parison is extruded through a die at a given melt temperature appropriate for the raw material. A mould is shaped in the form of a negative image of the desired final product outer surface. The extruded parison is pinched off at a bottom portion thereof while a narrow blowpipe is inserted from the bottom into the internal cavity of the tube. Next, high pressure gas is introduced into the parison (i.e. via a needle or blow pin) such that the extruded parison expands against the mould, for shaping the plastic to the desired product geometry.

The mould is then opened and the final plastic product is removed therefrom.

It is often desirable to incorporate one or more appendages such as handles, harness clips, etc., to the extruded plastic product.

According to prior art techniques, such additional appendages have been installed via post- moulding processing steps using expensive dedicated plastic weld machinery. The use of additional down-line processing steps and machinery has been found to be labour intensive, and the cost of a typical down-line plastic welding machine (per weld) can be in the vicinity of $100,000 Cdn (1989). Disclosure of the Invention According to the present invention, a simple and inexpensive process is provided for attaching

appendages such as handles or harness clips, etc. onto extrusion moulded products without additional expensive post-moulding welding equipment. In accordance with an aspect of the present invention, there is provided a method for creating a blow moulded product comprising the steps of extruding a parison tube (3) into a cavity formed between opposite halves of a mould (5) , pinching off a topmost portion of said parison via crimping apparatus (7 and 9) , admitting high pressure compressed gas into the inner portion of said parison tube via an gas blow pin (11) for inflating the tube against said opposite halves of said mould and thereby create said blow moulded product, the improvement comprising the steps of projecting an undercut dowel (13) from an inner wall of at least one of said opposite halves of said mould such that said parison forms around and conforms to the shape of said dowel to thereby create said blow moulded product, separating said product from said dowel thereby leaving an undercut in said parison in the shape of said dowel, removing said blow moulded product from said mould and inserting pin portions (19 and 21) of an appendage (17) into said undercut prior to cooling of said product whereby said undercut shrinks around said pin portions during cooling to form a secure connection of said appendage to said product.

Best Mode of Carrying Out the Invention

Figure 1 is a cross sectional view of standard extrusion blow processing equipment incorporating a hydraulically retractable dowel in accordance with the present invention;

Figure 2 is an enlarged cross sectional view of the dowel illustrated in Figure 1;

Figures 3A-3D illustrate a handle for attachment to a plastic product in accordance with the present invention; and

Figure 4 is a detailed view of the dowel illustrated in Figures 1 and 2. Best Mode For Carrying Out the Invention

Turning to Figure 1, standard extrusion blow processing equipment is shown comprising an accumulator head 1 for extruding a heated fluid plastic parison 3 into a mould 5 consisting of two cavities. The parison is pinched off at top and bottom ends thereof via crimping apparatus 7 and 9 (e.g. pinch plates). A needle or blow pin 11 extends into the extruded parison for admitting a high pressure flow of compressed gas for inflating the plastic tube against the shaped walls of the mould 5.

The above described process and apparatus is well known prior art.

However, in accordance with the present invention, a dowel 13 extends through an aperture in the mould 5 such that the blown plastic tube envelops and conforms to the shape of the dowel, forming an undercut in the surface of the moulded plastic product, as shown best with reference to Figure 2.

Once the parison has been blown against the mould walls and around dowel 13, the dowel 13 is hydraulically retracted via a hydraulic cylinder 15, the mould 5 is opened, and the product is removed.

It is known that blow moulded plastic products shrink approximately 2-4% during cooling. Thus, in accordance with the present invention, upon removal of the plastic product from the mould, any appendages such as the handle 17 shown in Figures 3A-3D, may immediately be attached to the plastic product by inserting pin portions 19 and 21 into corresponding positioned undercuts in the product while the plastic is still shrinking. Thus, the undercut effectively shrinks during cooling around the pins 19 forming a secure connection of the handle or other appendage,

without requiring expensive post-mould welding equipment, etc.

Figures 3A-3D show a preferred form of handle for attachment to a plastic product (e.g. vehicle fuel tank, container, etc.), in accordance with the process of the present invention. However, the process and apparatus of the present invention may advantageously be used to attach virtually any other type of appendage such as clips, straps, etc. Also, whereas the dowel 13 is shown in Figure 1 as being hydraulically operated, other mechanical or electro-mechanical dowel operation means may be used.

With reference to Figure 4, a detailed view of the undercut dowel 13 is shown with optimized undercut forming diameters of B and D in the range of from B = D (upper limit) to D < 4B (lower limit) . For dimensions of B greater than D, there will be no lip on the undercut such that the undercut will be essentially cylindrical and therefore insufficient to retain the associated pins 19 of the appendage after shrinkage. If the dimensions of D are greater than 4B, the lip on the undercut will be too great, resulting in a portion of the product being ripped out upon separation from the dowel 13 within the blow processing equipment. According to a successful prototype, the dimensions were D = 8 mm and B = 6.5 mm. Industrial Applicability

In summary, the process of the present invention makes use of the approximately 2-4% total shrinkage of a blow extruded plastic product during cooling, to implement a process for creating an undercut in the plastic product during blow extrusion and attaching appendages thereto by means of appropriate dimension pins inserted into the undercut while the product is cooling, such that the undercut shrinks around the appendage pin forming a secure connection.

Other embodiments and variations are contemplated.

For example the dimensions of D and B can be greater or less than as discussed with reference to the successful prototype, provided the relation therebetween conforms to the above discussed upper and lower limits.

As a further alternative, one or more ejector pins may be housed within the wall of the moulding and ejected therefrom for forcing the product away from the dowel 13 and the mould wall within the vicinity of the dowel, instead of retracting the dowel into the wall.

All such modifications and embodiments are believed to be within the sphere and scope of the present invention as defined by the claims appended hereto.