Automotive headlight housings have evolved from round, stamped steel assemblies to thermoset parabolic reflectors with diamond-like angles, covered with clear lenses. Today‘s BMC (bulk molding compound) reflectors are cosmetic parts with complex curves and shapes. Their parabolic surfaces are the aiming mechanisms for the light. With the move to clear lenses, any imperfections in the surface of the parabola can easily be spotted. In the U.S., Europe, and Japan, forward lighting must adhere to stringent specs regarding the luminous intensity distribution along the forward direction of the driver‘s view. Any imperfection in the parabolic surface can cause problems with metallizing the reflector and potentially distort or alter the intensity and direction of the light beam.
1. Fast fill. Finished reflectors require a super high-gloss surface finish that‘s uniform across all areas of the parabola. Any variations or blemishes in this finish will be highlighted when the reflector is metallized. To force resin to the surface to achieve a better surface gloss and consistency, higher-than-normal mold temperatures are used. But the material may start setting up as soon as it hits hot tool steel.
Therefore, the BMC must be injected at a very high speed to ensure even curing across the entire molding surface. Typical fill rates should be below 2 seconds. The press must be equipped for such high-speed injection, with no lag time between injection start and complete fill.
2. High pressure. High-speed filling literally puts additional pressure on a machine. BMC is stationary in front of the screw, and must be started and moved quickly into the mold to prevent premature crosslinking. So, a press that is molding headlight reflectors must be capable of generating a minimum of 20,000 psi of injection pressure to hasten the BMC along to yield quality surface finishes. I recommend a machine capable of producing injection pressures of at least 22,000 to 23,000 psi.
3. Accurate shot size repeatability. If shot sizes vary, cavity pressures will vary shot to shot. That means surface finishes also will vary shot to shot.
With all the bosses, fastener holes, and bulb holes in today‘s reflectors, there’s a lot of potential for flash. If shot sizes vary, so will the flash. Variations in the flash level and thickness make it difficult to use cost-effective flash removal automation. Also, if the flash is not properly removed, it can break off when the part is metallized or assembled, resulting in the scrapping of the finished product.
So, closed-loop injection unit control（either with servoelectric valves on a hydraulic press, or a-c servomotors on an all-electric)is a must. It’s also essential that the screw design meets the requirements of the material used and the screw motor output.
4. Mold heating accuracy and recovery time. Mold surface temperature is critical for producing high gloss. If mold temperatures are not high enough, the polyester molecules in the BMC won’t completely cure, resulting in craters and pits in the finished part flaws highlighted by metallization.
The mold heating source must be sufficient and mold temperatures must be maintained to within ±5 deg F. it‘s best to have the mold heaters integrated into the machine control for easy monitoring.
When a molded part is ejected, heat is removed from the mold. The mold‘s heating circuitry must be capable of quickly recovering the post-ejection heat lost to maintain shot-to-shot repeatability.
5. Hold pressure profile. Post injection material flow in the mold creates stress within the molded part that can cause surface imperfections. The thin vents and lands in the reflectors must be cured prior to applying the higher hold pressure needed for forcing resin to the part‘s surface to create a high-gloss finish.
However, the material will move in the mold, and flow lines or surface blemishes will be evident if this pressure is prematurely applied. It‘s critical to profile hold pressure to first create a seal and then stage the necessary pressure. Therefore, I recommend machinery with closed-loop hold and pack pressure control.
6. Barrel cooling. Variations in barrel temperature will vary shot size and material pressure, thereby impacting the surface quality of the molded part and the level of flash produced. Water-cooled barrels are a must. Water channels machined into the barrel wall provide the best heat transfer. having at least two independent zones controlled by water temperature controls accurate to within ±5 deg F. As with the mold heating controls, these should be interfaced with the press controller. In both cases, such interfaces allow alarm limits to be set and can halt production before scrap parts are produced.
7. Nonreturn valve. A nonreturn valve specifically designed for thermoset molding is essential for the shot control accuracies required to mold headlight reflectors.
If the clearance between the check ring and barrel wall is not large enough to process BMC, premature curing can occur, because there‘s too much shear heat in the material as it passes the nonreturn valve. This can spoil the surface finish. Conversely, if the clearance is too large, material will flow past the nonreturn valve during injection, compromising shot-to-shot consistency.
8. Machine clamp. With the high injection rates and pressures involved, clamp design is key. It must automatically compensate for tie bar temperature changes to ensure even and consistent lockup. It also must be sturdy enough to eliminate movement during injection. Any movement will create a higher level of flash and release the so-called “pressure seal”， mentioned above, which is needed to create a cosmetic surface.
Because headlight assemblies are increasingly becoming an aerodynamic feature of car designs, headlight reflectors are getting bigger. So, the press must have adequate tie bar spacing.
9. Ejector. Ejector pins, natural vents for releasing trapped gases, can become worn out and not allow proper pressure build-up in the mold. the ejector system must be accurately guided and have accurate position control to minimize this phenomenon.
Most thermoset presses use ejector-actuated gate cutters to create a clean gate vestige when degating parts. Machines must be capable setting and actuating the pressure, speed, and positioning of the ejectors while the mold is closed.
10.Vacuum adaptability. To help eliminate the voids and charring problems caused by air trapped in the mold, I highly recommend inmold vacuum assist, as well as using vents.
The vacuum system is tied into a vacuum ring on the mold. Vacuum is applied before and during injection until the material has filled out the part. The machine specified must be capable of signaling the vacuum system when to turn itself on and off.
Every year hundreds of millions of headlights go onto new vehicles and into the automotive aftermarket. BMC has performed very well in newer halogen applications, and continues to offer the kind of price/performance profile that makes it the material of choice for high-intensity discharge lighting.