Ejector marks

Slider, Jaw, and ejector Fretting of the elements Contaminations on the molded part surface brown wreaths around the ejector markings... 

Ejector marks are depressions or elevations on the ejector side of the moulded part surface. These wall thickness variations can cause gloss differences and depressions on the visible surface of the moulding as shown in Figure 9.31. 

A distillation system is to operate with a horizontal overhead condenser. Figure 6-19, and pressures are as marked. The estimated air leakage into the system is 7 Ibs/hr. The molecular weight of the product vapor going out the condenser into the ejector (at 80°F) is 53. The vapor pressure of the condensing vapors is 3 mm Hg abs at 80°F. 

The surfaces to be painted should be affected as little as possible by features like mould split lines and the locations of gates, cores, ejectors, and so forth. Otherwise, if marks from the moulding are visible, considerable preparation of the surfaces (to remove irregularities and flash) may be necessary before satisfactory results are obtained. 

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. 

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. 

Ejector locations. Ejection devices for plastic parts can range from screwdrivers used to pry parts out of a hand mold to mechanized stripper plates and elaborate mechanisms which also retract collapsible cores. All of them share one common characteristic they exert pressure on a newly formed part. That pressure can distort the part to the point of disturbing its function or appearance if it occurs while the part is still too soft to withstand it. Therefore, the processor must delay ejection until the moldment can endure it. The more ejectors there are, the more ejection surface there is to distribute that pressure and the sooner the part can be removed from the mold, thereby shortening the molding cycle. However, ejectors cost money and leave marks on the surface of the moldment. Therefore, there is a mold cost associated with a faster molding cycle. (Differences between bidders on a project are often based on variations in cooling and ejection systems.) Additional ejectors leave more marks on the surface and their number and location may be limited by functional and appearance concerns. 

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. 

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. 

Permissibility of marks by gates, ejectors, sliders and splits,... 

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. 

These ejectors, or combinations, are advantageous because the injection molding machines are designed for them speciflcally, and the cycle times are kept short. Gate marks and deformations of thin-walled parts have to be taken into consideration with these solutions. 

Ejection is the name given both to the process of removing the molding from the mold and to the means by which it is done. Ejectors inevitably leave witness marks on the molding, and this alone may determine the disposition of the part in the mold. Once that basic choice has been made, there is usually not much further freedom to position ejectors in cosmetically acceptable positions. The choices are much constrained by other features in the mold and the need to put ejectors where they are most needed to overcome the resistance of the molded part. 

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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