Sequential valve gating aids filling of large parts.

In the minds of most molders, sequential valve gating (SVG) brings up images of small parts, typically in a family mold. Recently, though, SVG showed its worth in a large automotive part - a hard-to-fill PC/polyester bumper for the Ford Escort.

The molder, Nascote Industries of Nashville, Ill., began production of this part just last month on one of its 3500-ton Klockner Windsor presses. Nascote immediately discovered that SVG provided relief from splay and paint-adhesion problems that intermittently affected the part. Even better, SVG has cut the bumper's cycle time from 110 sec to under 75 sec [ILLUSTRATION FOR CHART OMITTED]. "Most of that is due to savings in clamp time," says Todd Wilson, senior molding process engineer.

"Our biggest goal with this project was to increase capacity without additional presses," Wilson adds, pointing out Nascote's success in winning new bumper orders from Ford and other OEMs. However, the evolution of the Escort bumper project forced Nascote to rethink its tooling strategy.

The company has molded PC/polyester bumpers (using Xenoy resin from GE Plastics) for more than seven years. "But the original design was much simpler," explains Tom Skorch, Nascote tooling superintendent. Today's bumper, for example, has thin-wall sections of 3.5 mm, which is about 10% thinner than the initial design. The current version also features more complex geometries - such as integrated fog-lamp openings and ribbed grilles. And the wrap-around bumpers on contemporary Escorts have flow lengths that are longer by more than 6 in.

HELPS FILL BIG BUMPERS

Nascote's first SVG job employs a system from its long-time hot-runner supplier, Kona Corp. of Gloucester, Mass. Retrofitted into one of two existing single-cavity bumper tools, the system has six valve gates controlled by Kona's new hydraulic sequencing unit (see PT, March '94, p. 55). Other than changing the six manifold drops, the three-year-old bumper tool required only a new clamp plate to house the valve-gate hydraulics. Kona's automotive sales manager Joe Lang explains that the new clamp plate can provide access to the gates' adjustable-length pins while the tool stays on the machine - though Nascote's Skorch notes that the gates haven't required any adjustment since their installation.

The bumper fills from six gates. Two in the outer "wing" regions open first. Then at roughly 3.5 sec into the 10-sec injection time, the middle four gates open as well. Nascote determined the sequencing empirically, though future jobs will likely use a mold-filling analysis for this task (see box). "We just knew from experience that the wings needed to fill first," says Wilson.

MULTIPLE BENEFITS

With just 500 parts shipped as of press time, Nascote has already found that SVG promises to answer just about every tough processing and surface-appearance problem the company has encountered with that bumper. Nascote attributes several related benefits to its valve-gating system:

* Splay elimination - This surface defect has completely disappeared from the valve-gated part. GE Plastics senior material processing specialist Jim Betters, who works extensively with large automotive parts, explains that the valve gate's mechanical shutoff - designed to prevent drool - eliminates the need for a decompression stage. Typically, gas bubbles appear in the melt during decompression, causing splay.

* Knit-line control - SVG let Nascote gain unprecedented control over knit lines. Before SVG, the knit line always occurred in the part's center and was visible even after painting. "That center area filled early, and by the end of hold pressure, its wallstock was about 60% solidified," says Wilson. "That's too much to pack out the knit line." By changing the sequence on the middle four gates, the company was able to move the knit line to different locations on the fascia front. The knit line also became fainter, eliminating any read-through after painting. In explanation, Wilson estimates that with the 3-see delay on the center gates, the center region of the part may now be less than 15% solidified by the end of hold pressure.

* Balanced fill - SVG achieves fill balancing through the mechanical action of the gates rather than through runner and manifold sizing.

* Better-fitting bumpers - The balanced fill seems to cut molded-in and localized surface stresses by equally distributing the pressure needed to fill the part's wing regions. With SVG, Nascote no longer needs to mold a "Crown" onto the bumper to compensate for stress and shrinkage effects. The crown shape on previous bumpers would fall into the correct fit as the part stabilized after molding.

* Better paint adhesion - The bumper no longer suffers from a mysterious paint-adhesion defect that Nascote formerly corrected with a secondary operation. "The corrective action was 100% effective," says Wilson, "but it was also expensive." No one knows exactly what caused the problem, but Wilson and Betters share a theory that improved paint adhesion results from lower surface stresses and from better venting fostered by SVG's fill control.

AN INVESTMENT

Nascote's conversion to valve gating for the single bumper tool cost roughly $12,000 for the clamp plate and $32,500 for the new manifold and valve gates. The hydraulic control unit cost another $17,500, but Wilson notes that subsequent jobs could easily make use of the machine controller for hydraulic sequencing - after a bit of programming work. In any case, the cost of SVG retrofitting is dwarfed by the potential market value of the extra bumper-molding capacity that Nascote could realize as a result of the roughly 35% shorter cycle time. Plus, the company has less need to invest in new presses for that job.

For now, SVG bumpers remain in a limited production run, but the method has worked so well that Nascote may add it to around 20 other bumper tools for the next program, which would correspond to the 1997 model year.

RELATED ARTICLE: A Computerized Look at Fill Sequences

Help for molders embarking on SVG applications can be found in today's mold-filling analysis packages, the best known of which all permit simulations based on sequential operation of the valve gates.

For instance, AC Technology of Louisville, Ky., reports use of its C-Mold software in a number of sequentially gated jobs; C-Mold, in fact, was used for an after-the-fact analysis of the Nascote bumper. Moldflow Inc., Shelton, Conn., and Plastics & Computer Inc., Dallas have likewise seen their software used in SVG jobs. For its part, Nascote expects to conduct filling analyses of subsequent SVG jobs - perhaps with some help from its material supplier, GE Plastics.