Ejector Pin Marks

Ejector pins are used in the moving die to release the casting. The ejector pins will leave small marks on the surface of the casting and should be positioned so that these will not appear on a visible face of the finished casting. If a casting has a face which is to be machined, then, where possible, the ejector-pin marks should be arranged on that face, to be removed by the machining. 

Increasing the size of pins or considering the use of pneumatic air ejection to assist in component release may help to solve the ejection pin marks problem. Flow chart 5.2.17 shows the way to approach the problem of ejector pin marks. 

Problems and Troubleshooting - Injection Molding Flow chart 5.2.17 - Ejector pin marks... 

A diagram of the injection molding process is shown in Figure 6.22. The extruder is similar to other fabrication techniques where the granular resin enters a hopper, where the material is transferred to a heated barrel and a reciprocating screw moves the molten plastic to the molding cavity. The molding cavity is unique to this process. The molten polyethylene is forced xmder pressure into a closed mold that is continually cooled. After the molded part cools, the mold opens and the fabricated part is ejected from the mold. An injection molded article can usually be identified by the ejector pin marks that are usually present on the molded part. 

The test system setup for SDT measurement is shown in Figure 2. Given the interest in only the transparent part, no laser-absorbent parts were used. Specimens were placed so that the ejector pin marks were facing downwards. The focal point of e laser was located at the top surface of the specimen. 

In joining with solvents or adhesives, it is very important that the surfaces of the joint be clean and well matched since poor contact of mating surfaces can cause many troubles. The problem of getting proper contact is aggravated by shrinkage, warpage, flash, marks from ejector pins, and nonflat surfaces. 

Ejector pins often leave compression marks on the molded article. They should not be used on the visible part of the article, if this can be avoided. Another example of an ejector is the sleeve-ejector (Figure 4-12), which is able to transmit greater ejection force. Slccvc-ejectors are primarily employed with moldings, which are axially symmetrical. 

Figure 11-46. Gates and ejector pins create marks and blemishes. Parts drawings should indicate surfaces that can be marred without creating problems, to aid in properly locating gates and ejector pins.

PP is a resilient plastic which stretches rather than cracks if ejector pins of inadequate area are employed. It is not a very hard or rigid material so the rule is to use ejectors that are as large as possible. Ejectors inevitably leave witness marks on the mouldings. Hence, it is preferred for ejectors to operate on side walls, ribs or bosses. When ejector pins are to be positioned in flexible areas of moulding, they should be of ample diameter. 

With a curved tunnel gate (also called submarine gate or cashew gate, Figure 1.38) it is possible to position the gate on the back of the molded part where the appearance is as little affected as by the markings of the ejector pins. 

The position of the ejector pins is regulated by the geometry of the molded part. To prevent markings, it has to be ensured that the pins push against ribs, shoulders, and hidden molded part surfaces. 

Ejectors leave witness marks on mouldings, and hence it is preferable that they operate on sidewalls, ribs, or bosses. They should have sufficient diameters. A stripper plate acts on the entire wall of a part and so distributes the ejection force. Cylindrical ejector pins are most commonly used, although in constricted areas rectangular pins or blade ejectors can be used. For thin-walled articles, air-assisted ejection can be used. 

Ejector pins may leave small marks and should be positioned at points of strength on the casting. 

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