Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 37 of 75

For more on this subject, there is a 2001 SPE ANTEC paper entitled "Friction Properties of Thermoplastics in Injection Molding," written by Ferreira, Nerves, Muschalle and Pouzada of the University of Minho in Braga, Portugal. It's an interesting read for both mold designers and processors. Next, galling is a form of wear caused by adhesion and friction when two metals slide against each other. The condition is exac- erbated if there is a side load compressing the two surfaces—like when an ejector pin is not perfectly aligned with the through hole in the core. It puts more pressure on one side of the hole then the other. Unless an ejector pin is keyed for a specific orientation, you should be able to freely rotate it when the ejector plates are fully forward. This confirms that the centerlines of the pins are parallel, that they align with the through holes in the core, and that there is sufficient clear-ance around the shafts and heads of the pins. The adhesional aspect of galling causes the material's crystalline structure to slip and tear—depositing, if not welding, microscopic particles on the adjacent surface. Unlike other forms of wear, galling happens quickly. So, if you hear an ejector pin squeal—stop. It's not going to get any better if you continue to cycle the ejector. The frictional aspect of galling is relative to the mating mate- rial's type and hardness. This is the same reason you try to use different types of steel and at least a 10-point Rockwell hardness differential on sliding or angled shutoff surfaces. The hardness differential is the predominant factor. The harder the steel, or at least the surface of the steel, the less it is prone to galling. If you can score lines on the side of an ejector pin with a hand file, which in my experience is not that uncommon for molds built in some Asian countries, you probably shouldn't wait to see if it is going to seize up. Just accept the fact that it eventually will and replace it now. Certain metals are prone to galling due to their atomic struc- ture, especially those with high coefficients of friction, such as aluminum, titanium and, to some degree, stainless steel. Conversely, alloys such as brass or bronze are very resistant to galling, even though they are much softer. Over the years, several mold-component suppliers have offered ejector pins made of various materials such as H-13, M-2, 420 stain- less and copper alloys. Nitrided H-13 pins, or pins with hard surface coatings, are some of the best for reducing the possibility of galling, because their surface hardness is 65 to 74 Rockwell C. Stainless-steel pins, and pins with thin chrome coatings, are well suited for medical, food-contact, and other applications where traces of grease or oxidation can contaminate the parts. The thin- chrome-plated pins are not suitable for extremely high tempera- tures or corrosive molding materials such as rigid PVC. I have some experience with stainless-steel ejector pins— none of it good, due to their propensity to gall. While many molders use core pins made of copper alloys, there are many benefits of using them for ejector pins as well. They have terrific heat-transfer properties and are very resistant to galling. Unfortunately, price isn't one of their benefits. When selecting what type of ejector pins to use, also consider what will wear out, abrade, or corrode first—the pin or the core. Replacing a worn or eroded ejector pin is a lot cheaper than repairing a worn through hole in a core. Preventive maintenance on ejector pins is at least as important as maintenance on the cavities and cores. After all, ejector pins also function to some degree as vents. They need to be routinely cleaned, lubricated and, depending on the molding material, neutralized. It doesn't take long for corrosive gases to tear pins up, which acceler- ates wear and increases the chances of their galling or breaking. A seized pin caused by galling is a real problem for molders. If you're lucky and the pin is fairly large in diameter, the ejector plates may not fully extend or retract, which should cause the machine to stop and go into alarm. That's assuming you tie in the ejector plate to the molding-machine control. If you're not lucky and the pin is fairly small in diameter, the pin will usually buckle and break within the ejector housing. This will allow the ejector plates to fully extend and retract—leaving a portion of the broken pin protruding beyond the parting line. If the low-pressure-close safety was set correctly, you might not have too much damage to the cavity when the mold Unlike other forms of wear, galling happens quickly. When a molding machine goes into alarm, always check for a broken or protruding ejector pin. Corrosive gases can tear up ejector pins. 36 OCTOBER 2018 Plastics Technology T O O L I N G K now How

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