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October 1, 1998

5 Min Read
Two-stage ejector keeps mould design simple

Injection moulds sometimes require a multistep ejection process in which individual ejection elements have a different stroke in order to safely release the moulded article (moulded parts plus gate) from the mould. Hydromechanical ejector accelerators have the advantage that the ejector device of the mould may have a simple and space-saving design while requiring little effort in terms of assembly.


Figure 1. Hydromechanical ejector accelerators.
(1) ejector accelerator; (2, 3) ejection plates;
(4, 5) ejector pins; (6) ejector bolt; (7) annular piston;
(8) stopping face; (9) oil chamber; (10) central piston;
(11) piston stop; (12, 13) restoring springs.(A) ejector
accelerator prior to ejecting; (B) end of the synchronous
movement of the ejector pins (4, 5) and beginning of
the advance of pin 5; (C) end of the ejection process.

The reliable operation of these hydromechanical ejector accelerators makes it possible to shorten the cycle time. Figure 1 shows the structure as well as its function of the accelerator. The ejector accelerator (1), as well as the ejector pin (4), are inserted into the two ejection plates (2, 3). Part (1) also receives the ejector pin (5). The ejection plates (2, 3) are actuated by the ejector bolt (6).

A stroke H1 is available for the ejection plates. Activating the ejector bolt (6) initially gives rise to an equal forward motion of ejector pins (4) and (5), until--after a stroke H2--the annular piston (7) of the ejector accelerator (1) strikes against the stopping face (8, Figure 1B). Once the ejection plates (2, 3) are moving further, the annular piston (7) is being pressed into the oil chamber (9), where it replaces the existing hydraulic oil.

As a result, the central piston (10) dissociates from its piston stop (1). By the end of stroke H1 of the ejection plates (2, 3), the ejector pin (5) in the central piston (10) has passed a distance exceeding that of the ejector pin (4) by a distance H (Figure 1C). When the ejection plates are retracted, the restoring springs (12, 13) are pushing the central piston (10) and the annular piston (7) and therefore also the ejector pin (5) back into their original positions.

The ejector accelerator is available in two sizes (Table 1, p. 39). The size 1 version can receive ejector pins with a tip diameter of up to 6 mm and a tip height of up to 4 mm; the corresponding tip sizes for the size 2 version are 12 mm in diameter and 10 mm in height, respectively.


Figure 2. Single injection mould for the upper and lower part
of a housing made of polycarbonate. (1, 2) shaped pin;
(3) sprue ejector pin; (4) return pin; (5) ejector
accelerator; (6, 7) ejection plates; (8) coil spring;
(9) boring; (10) ejector bolt. (A) section; (B) section
turned by 90°; (C) top view.

Application: Housing Parts

The mould shown in Figure 2 (p. 38) is used to manufacture housings that have projecting parts--undercuts along their edges. These are moulded with the shaped pins (1, 2). The shaped pins (1, 2), the sprue ejector pin (3), the return pins (4), and the ejector accelerators (5) are mounted in the ejection plates (6, 7).

During ejection, the shaped pins and the ejector accelerators initially push the parts away from the cores. The flatter housing part (on the left-hand side inside the mould), however, still adheres to the shaped pins for a while but is finally pressed out of these. Viewing the cavities from above (Figure 2, bottom left) demonstrates that three ejector accelerators are required for this purpose. Using only one or two ejector accelerators would cause tilting, so that one side of the housing part could get caught. Only the arrangement in one plane (triangle) avoids this danger. Release from the mould is therefore accelerated, since this technique eliminates the need for the commonly used but time-consuming method of multiple ejector strokes.



Ejector force shown above, left; Ejector accelerator force above, right.

A coil spring (8) is sticking on the return pin (4); this coil spring enters a hole (9) as the ejector plates are moving forward. This has the following reason: Table 1 lists two forces F1 and FN for each of the two ejector accelerator sizes. These are the forces exerted by the ejector accelerator immediately after touching the stopping face (8, Figure 1), respectively, after a complete stroke of the ejection plates. These forces are effective even if there are no moulded articles for the ejector pins (4, 5) to push (Figure 1). If--as in this case--the ejector accelerators are positioned nonsymmetrically relative to the ejector bolt (10, Figure 2), then an overturning moment is created, which may cause jamming of the ejection plates in their guideways. This overturning moment is almost entirely compensated by the two springs (8) sitting on the two return pins (4) opposite the ejector accelerators.

Contact information
Mr. Erwin Seifer
An der Halde 41
D-78628 Rottweil
Tel: +49 741 5339-0
Fax: +49 741 5339-99
e-mail: [email protected]

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