Injection molding: Tradeshow highlight evolves to truck sidelight

A project shown at the K 2007 tradeshow has blossomed into commercial work in the form of a complete ready-to-install truck sidelight molded by automotive lighting specialist Hella. Electrically conductive plastic, multicomponent molding, high-end hot runner engineering, and more: This project brings together a host of noteworthy issues.

The automotive light project at Hella grew out of the experience gained molding an LED light strip at the K 2007 trade show in a joint project running at the Arburg stand. The project brought together many companies throughout the injection molding value chain, including Arburg, Oechsler, Kiki, Rohwedder, Osram, and Günther. That LED light strip was molded as an upper shell of a pocket-sized flashlight by combining an insert and three-component injection molding. Hot runner manufacturer Günther (Frankenberg, Germany) used the findings from this project in the development and realization of the hot runner engineering for the new Hella tool.

Challenges and more challenges

Shown is the view of the back of the truck’s sidelight . . .

. . . and here the front side, with the overmolded reflector.

A top-end tool keeps multiple steps in the mold: three hot runner systems in one mold for a three-component part.

The ready-to-install truck sidelight consists of three plastics and inserts. Large differences in temperature between the mold and the materials, as well as the confined space for the required eight valve gate nozzles, raised the technology bar. Günther helped integrate complex hot runner engineering into an automotive mold from Finnish moldmaker Sabri Scan for Hella.

That K 2007 project proved critical in terms of generating processing knowledge of the material specified, the electrically conductive compound Schulatec TinCo, a polyamide (PA) 6-based plastic-metal hybrid supplied by compounder A. Schulman (Akron, OH). The material is filled with copper fibers and a low-melting solder. The tin alloy has a melting point similar to PA6, which gives the compound a homogeneous structure and reliable processing characteristics. Its specific electric conductivity is 5 -105 S/m; electromagnetic attenuation is 80 dB in the frequency range of 30 kHz-1.2 GHz, which produces sufficient shielding. Its heat conductivity is 7W. The material can be processed on standard injection molding machines.

Brought in late to the project, Günther’s engineers faced a special challenge: All the critical components in the mold were already designed, finished, and in place, so the hot runner engineering had to be adapted to suit the available (extremely limited) space.

Multistage process for finished part
The truck sidelight is formed in a patent-pending multistage process directly in the mold. After production of the ABS base plate and the PMMA reflector, the electrical components (including an LED and resistor) are inserted. TinCo electrically connects the electric components. The final overmolding of the reflector baseplate combination with ABS simultaneously protects the reflector from splash-water. All insertion and moving procedures are handled robotically.

Günther fitted all of the mold’s hot runners with valve gate nozzles, each with a special shaft version with only minimum surface contact with the mold to keep transmission of temperature to the mold at a low level; this helps ensure stability in the temperature of the mold and the material. The inserted single-needle valves are actuated pneumatically. The direct gating is 2.5 mm in diameter and injection is vertical. The injection pressure amounts to 235 bar at an injection time of 0.12 second, and the holding pressure is 20 bar at a holding pressure duration of 0.3 second.

The temperatures of the manifold and hot runner nozzle are kept at 260°C. The mold temperatures on the nozzle and ejector sides remain constant too, at 55°C. With such a large difference in temperature, an exact thermal separation is clearly essential.

Let’s get molding
Each conductor path element in the base plate is injected directly in order to connect the inserts with each other in serial operation and to ensure that the short-circuit to ground functions. The conductive component has a total weight of 0.5g here.
The TinCo component is injected into the cavities through four 8NMT80VAS valve gate nozzles in a block arrangement with relatively large (8-mm) flow-channel diameters. Channels were kept large to prevent material segregation and blockages.

The nozzles are placed in a group housing in order to attain a compact arrangement of the conductor path elements. An inserted short VA needle guide was used because of the confined space on the TinCo component conductor paths. The four needles are actuated by means of a sliding-cam mechanism connected to a hydraulic lifting cylinder, which permits a plate to be moved on an inclined plane so that the needles can be opened and closed simultaneously.

The ABS component (for the base plate and the overmolded waterproof connection) is injected into the cavity via pneumatically actuated single-needle valves, just as the material for the PMMA reflector is. These are actuated by means of independently operating pistons. The injection of the base plate and the overmolding of the finished parts—the first and last processing step—are achieved by means of a common manifold for the ABS, which divides the material into two nozzle systems.

In the valve gate nozzles that do not work with the TinCo component, the injection point is closed by means of “LA”-type needle guides. The design of these contouring needle guides and the valve gate permits a noncontact and wear-resistant gate closure. The needle guide plunges down as far as the product, sealing at the cavity plate.

The down-stroke depth of the individual shutoff needles can be readjusted when they are installed, which helps with precision and keeping costs down. Plus, separation of the needle guide and the material tube in the nozzle makes for easy replacement of the guide as a wear part.

In the end, Hella’s mold has quite a few moving parts, but the level of complexity of the entire cell is minimal compared to ones used for such parts in the past, which would have required multiple postmolding steps. Matt Defosse

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