Undercut refers to the convex or concave area of part. When removing the part from the mold, you need some device so that the part of the mold which contacts to the undercut can be moved to remove the part from the mold since the part will not be removable directly without the device.
This process is called Undercut process.
Undercut process uses different methods depending upon whether the undercut is outside or inside the part.

Here are some examples of parts with undercut.

Push-pin	Grip of a driver	Grip of a cup

External Slide Core Method

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Slide Core Body Unit

The portion where the undercut process is performed linked along with the movement of mold opening. The amount of movement of the slide core and the angle and the length of the angular pin are key factors to this unit.
The locking block installed to the cavity plate receives the pressure from the injection when closing the mold. Upon opening of the mold, the slide core moves backward by the force from the angular pin and the spring, thus releasing the part.
As the hole will be misaligned and the angular pin will not be inserted if the slide core moves too far backward, a core stop block needs to be installed.
This correlation is illustrated below.

(Click each name in the figure to display the description.)

The relief distance and the tilt angle of the angular pin are as follows : S : Undercut amount S1 : Slide stroke amount a 　: Tilt angle of slide core b 　: Tilt angle of angular pin S1 = S + 5mm up a 　= b 　+(2- 5) b 20 In order to prevent the slide core and the locking block from intervening with each other in mold opening and closing, make the tilt angle of the angular pin smaller than that of the locking block. Also, a gap of 0.5mm is added to the hole for angular pin for errors occurring at processing.

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Push Back Spring

The mass when the slide core is sliding varies depending on the direction of sliding.

<Example>

Suppose you install a 5kg slide core on the side parallel to the ground. How long should the spring be? The stroke is 20mm.

D : Spring diameter L : Spring free length (see a catalog by each manufacturer) E : Spring close length (see a catalog by each manufacturer) F : Stroke G : Length of shrinkage by slide weight K : Spring constant (see a catalog by each manufacturer) W : Slide weight

If using light load [K = 0.6kgf / mm] for spring:

<Example of Catalog>

Spring constant ( K )	Free length ( L )	Max. deflection ( F + G )	Outer diameter ( D )	Usage count
0.6kg/mm	65mm	32.5mm	16mm	350,000 times



To figure out the slide mass, it is 1.5 times the weight of the slide core because the slide core slides parallel to the ground:
W = 5*1.5 = 7.5kg


As the length of shrinkage due to the weight of the slide mass is: G = W / K :
G = 7.5/0.6 = 12.5mm


As the free length of the spring is : L = E + F + G :
L = 32.5 + 20 + 12.5 = 65. Referring to the catalog,
Use the spring with L = 65.


Answer D = 16 L = 65mm

Internal Slide Method

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Loose Core Method

(Click each name in the figure to display the description.)

Adapted when there is a small bulge or dent inside the part. The slide unit is installed to the ejector plate and the slide unit is linked to the loose core by the slide rod. As the ejector plate moves forward, the loose core moves to the side pushing the part to remove the undercut. This mechanism employs extremely strong force at an especially fragile part of the mold; therefore, you must be extremely cautious about the mechanism design and material accuracy.

If there are more than two loose cores, be cautious not to have them collide with one another. If they collide, the loose cores can not be used.

(Click each name in the figure to display the description.)

Slide Stroke and Tilt Angle B : Undercut amount a : Tilt angle of slide rod Relief distance A = B + 3mm to 5mm <Example> If stroke h = 150mm, B = 8.1mm, relief = 5mm: A = 8.1 + 5 = 13.1mm What is the tilt a of the slide rod? As a 　= (A / h), a　 = (13.1 / 150) 0.0873 a　 5 Also, if the tilt angle is larger than 15, a support rod needs to be installed.

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Method to push out undercut

(Click each name in the figure to display the description.)

Used for the part which has a side boss inside. The method in which parts are pushed by the block and slid to the side by hand.

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Internal Slide Core Method

(Click each name in the figure to display the description.)

Applied when the part has an inner channel or a rib inside. The method in which a slide core is moved using an angular pin .

Hydraulic Cylinder Method

The undercut process utilizing the hydraulic cylinder method is not related to the opening / closing movement of a mold, and is able to move independently.
This is used when undercut must be processed before mold opening when the undercut is on the cavity plate side.

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Movement of Hydraulic Cylinder

The method in which undercut is processed by the external power of a hydraulic cylinder. In this figure, the oil pressure cylinder is installed to the cavity plate and a hole is made.

(Click within the circle in the figure to display the animation.)

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Calculation of Hydraulic Cylinder

The calculation that determines the size of an hydraulic cylinder based on the size of undercut. Figure out the pressure required for the hydraulic cylinder to calculate the inner diameter of the hydraulic cylinder.


	D : Cylinder inner diameter() a : Cylinder thrust coefficient 0.8 k : Working pressure (generally 60 - 70kgf/) P : Resin pressure 500(kgf/) S1 : Area to receive resin pressure() S2 : Contact area() W : Slide mass (kgf) *The value of k varies depending on molding machine.

The values S1, S2 , W are calculated as follows:

[Detail of Undercut]

First, the area that receives resin pressure (cross section of product) S1 is: Then, the contact area (slide core cross section - cross section of product) S2 is:

Slide Mass (the mass of the part attached to hydraulic cylinder) W is:


[Hydraulic Cylinder: Left Side and Front]

[Profile of Undercut]