Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 39 of 75

closes again during the next automatic cycle. If the machine goes into alarm, a lack of training, experience, or keen grasp of the obvious might cause someone to overlook the protruding pin and further damage the cavity by increasing the low-pressure-close setting, or worse, go into manual mode and close the mold under high pressure. High-speed ejection and retraction veloc- ities increase the risk of galling due to an increase in frictional heat. Multiple ejection pulses and excessive ejector-stroke lengths obviously don't help. To be on the safe side, some processors always use one additional ejector pulse than is required to eject the part. I have never agreed with that approach. To me, it's like robbing Peter to pay Paul. If there's a problem with the part not ejecting properly, fix it. The chance of galling can be greatly reduced with proper lubri- cation. In fact, the coefficient of friction for a lubricated surface is four times lower than for a dry surface. Mold-component suppliers offer pins with various types of lubricous coatings, such as thin dense chrome, molybdenum disulfide, DLC (Diamond-Like Carbon), tungsten disulfide (WS 2 ), titanium nitride (TiN), black oxide, and Dicronite dry lube. Tungsten-disulfide-coated pins are not recom- mended for clear or light-colored parts, as they can cause discoloration. I have seen this several times with PC medical parts. In addition to their lubricious char- acteristics, some of these coatings also have the benefit of being extremely hard. Suppliers of the DLC coating claim that the pins do not require any lubrica- tion at all, which is ideal for medical and food-contact applications. Some of these coatings can withstand high tempera- tures, others can't. So, choose the coating that best suits your applica-tion. The chance of galling can also be reduced for any type of pin or coating by ensuring there is proper cooling in the core, because when metal gets hot, it expands. Ejector-plate guide bushings are also no strangers to galling. Some bushings have internal grease grooves, and an external annular depression. There is a small through hole in the side of the bushing connecting these two features. The purpose of this hole is to allow grease to be pumped into the bushing while the mold is still in the press. A grease gun attaches to a Zerk fitting, which is counter-bored in the edge of the ejector plate. The grease flows down a drilled hole or channel leading up to the outer diameter Examples of Zerk Fittings. 38 OCTOBER 2018 Plastics Technology K now How

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