Products with External Undercuts

External undercuts normally have to be demolded using lateral moving elements. Sliders, splits and core pullers can be used for this application.

The term slider is used when the guide area and the wedge of the locking are not identical or parallel but separated. This sort of lateral movement is advantageous for long moving strokes with a relatively small front surface of the moving element .

The inclined bolt as a cost effective and simple drive is used when the moving stroke is not extremely long. The inclined bolt is a pin which is anchored to the fixed half opening and closing the slider in the way of a control cam . Occasionally, designs can be found where the sliders are arranged between a middle plate and the fixed half and not as usual in the main parting area .

When opening the mold, the bolt f pushes the slider  so far out until the undercut on the molded part is released. A spring- loaded ball catch d is maintaining the slider in open position thus enabling that the bolt later can enter into the drilling of the slider. The molded part remains on the core side until it is ejected from the ejector pins. When closing the mold, the bolt pushes the slider forward until the wedge surface is taking over the last moving phase and the closing itself.

FIGURE 1.14 Ejector half of a mold for vacuum cleaner housing

a: Slider in open position; b: drilling in the slider for the inclined bolt fixed on the opposite side; c: guide bar for the slider (source: Bosch-Siemens Hausgerate GmbH, Giengen)

Injection mold for a container with threaded neck

a: Slider; b: inclined bolt; c: wedge surface; d: middle plate; e: latch for the stroke limited transport of the middle plate; f: stripper plate with stripper ring [1]

Mold for small parts with external undercuts

a: Ejector half with T-slots and sliders; b: fixed half with only one bolt per slider [16]

Elements of the slider design

a: Molded part; b: slider; c: slider stop; d: ball cage; e: wedge surface; f: inclined bolt; g: inclination angle of the bolt; h: inclination angle of the wedge surface [15]

The following experienced rules apply for designing slider molds：

• Arrangement of the inclined bolt and the locking wedge on the fixed half, whereas the slider guidance is on the ejector half of the mold.
• Ensure sufficient guidance length in relation to the slider width: minimum factor of 0.5 or even better 0.7.
• Only one bolt per slider if possible. When using two bolts, what is inevitable for wide sliders, the risk of blocking may be larger.
• Bolt inclination angle should be between 15 to 25°. Chose as small as possible to get a high slider closing force.
• To reach an acceptably short bolt length for the necessary slider stroke (depending on the undercut depth) the bolt inclination angle should be rather bigger than smaller. The optimal design will be a reasonable compromise.
• Wedge surface angles should be 2 to 3° higher than the bolt inclination angle.
• Inclined bolt has to be sufficiently dimensioned to withstand the opening force.
• Slider should be pressed by the wedge to a fixed stop. Counter acting sliders without a fixed stop can, over the course of time, lead to misalignment.
• Guide surfaces of the sliders have to be lubricated. Therefore, they should not lead directly into the cavity but have to be located with offset.
• Cost effective T-slots： Screwed and bolted precision bars.
• Slider should be kept in the open position using a reversible latching mechanism (e.g. using ball catch or slider clip).
• Make sure to have an easy disassembly of the slider, for example using a simple to remove slider clip.
• Tempering the sliders directly which means to provide recesses for elastic coolant hoses.
• The locking wedge has to be sufficiently dimensioned to avoid elastic deformation. Sometimes a counter cone on the ejector half is necessary for support. Such centering surfaces should have an angle of approximately 10° to ensure easy handling and avoid self-locking which can happen at an angle of less than 7°.
• Wedge surface should be provided with an exchangeable hardened plate to prevent wear.
• If a mold needs sliders only on one side, either on the right or on the left, the opposite side should be provided with a sufficient counter support to prevent an offset.

Molds with “core pullers” are used for very deep lateral undercuts mostly driven by hydraulic means rather than using pneumatic drive. Hydraulic cylinders are also used for bigger splits. In each case a mechanical locking against the separating forces is necessary.

Split molds are clearly different from slider molds because their guiding surfaces and locking surfaces of the movable parts are identical. Sliders will mostly move at right angles to the mold axis, or slightly oblique, for example in an angle of 80 or 100° instead of 90°, but the moving angle of the splits is 12 to 20°. This gives another usage area which refers to undercuts with relatively shallow depth. Because of the high clamping force due to the small angle, split solutions are predestined for molded parts where the undercut takes a large extent (e.g. external ribbing of transport box).