The sliders for plastic injection mold

Due to the special requirements of some products, the mold release of some part is not consistent with the mold opening direction of the injection machine, which needs the side parting core pulling mechanism to eject the product smoothly.
The side parting core pulling mechanism comes with two types: slider(also known as slide) and lifter.
1. Slider travel calculation:
To ensure smooth product release, the travel distance of the slider has to be sufficient. Usually, the shortest travel distance that can guarantee smooth product release is 2 – 3mm:
AB = AC + (2 – 3)
   sliders stroke
2. All core sliders adopt the press plate + guide pin + spring structure as shown in the diagram (sometimes, when the slider is wider than 100 and yet it is not convenient to adopt the structure, T-plate structure may be considered). However, when the slider is vertically placed and restricted by pin position/mold size, press plate will not be necessary – an integrated mold base may be the option.
 sliders press block
The press plate is a standard self-built part of the company, which shall be located with a locating pin.
3. No matter whether the slider sides are sealed, both of its sides need a gradient design. Usually, the angle of a single side is 3 – 5 °; but when two sliders which travel in vertical direction join with one another, the angle will be 45°. During the design process, if there are sliders joining with one another on the four sides of a product, the ear of one of the sliders may stick out to guarantee accurate location.
4. The ratio of slider height to its length should be no greater than 1, or slider movement will be affected by overturning moment, leading to movement failure. General requirement: L≧1.5H.
5. Usually, the angle of a slider guide pin is 15° – 25°, with the biggest no larger than 25°. The angle of a guide pin is usually 2° smaller than that of the slider. In general, try not to use small guide pins, so as to ensure smooth slider movement.
6. The hole of a guide pin is 1/64" larger than a single side of it, about 0.4. When a guide pin goes through a slider, enough clearance should be kept on the mold plate.
7. Identify the location of a guide pin in a slider: try to place the guide pin in the center of a slider. See the diagram for specific measurements:
guide pin hole for sliders
8. It is required that the wedge surface matched with the slider should be higher than 2/3 of slider height, and the screws used on wedges should as big as possible. The following diagrams show wedges of two different structures. Try to avoid the structure shown in diagram b.
the wedge for sliders in plastic injection mold
9. Identification of slider spring length: Guarantee a sufficient space for spring, so as to avoid spring failure.
Assume slider travel is M and total spring length is L; assume the spring is compressed by 40%, and after the slider quits completely, the spring still bears 10% of the pressure, then:
Space for spring is 0.6L.
When L is too small, to prevent spring failure, spring length is often to be increased.
10. To ensure smooth slider movement, there cannot be obstacles to movement around it, such as pointed angles. Generally, chamfers of R3 – R5 should be designed around it.
11. When a spring is needed to be mounted under a slider (see the diagram for dimensional requirements), to prevent screws from being seized by the spring, Table 4-1 should be referred to for selection of springs and screws.
sliders spring
Spring Matching Screws
ø3/8” M3/16”
ø1/2” M1/4”
ø5/8” M5/16”
ø3/4” M3/8”
ø1” M1/2”
ø1-1/4” M1/2”
12. Large sliders should be cooled separately, and wear blocks should be fitted on the slider or the wedge. At this point, there should be a 0.5 clearance between the slider and the wedge.
See the diagrams.
clearance between slider and wedge

What is the stop pin?

stop pins for plastic injection mold

The function of the stop pin

1.Reduce the contact surface betwwen ejector plates and clamping plates,it is easy to control the flatness of ejector plates by adjusting the thickness of stop pins.

2.Buffer against ejector plates

The material of stop pin

S45C,hardness HRC40°~45°

Position of stop pin

1.An STP must be installed under the RP to prevent deformation caused by large stress,see pic 1#

2. Avoid to interfere with other mold components,such as support,EGP,KO .etc. see pic 2#

The quantity of stop pins

The quantity of stop pin designing

Note:For a large size plastic injection mold, Stop pin is needed to increase the supporting points of the ejector backing plate, so as to prevent ejector plates from deformation


Important Considerations for the Ejecting System(ejectors)

ejecting system ejector pins for plastic injection mold

What should we pay attention to the ejecting system?

2.1 Keep at least a distance of 3/32” between ejector hole edge and other holes;
2.2 A clearance of 1/64’’ should be kept for ejector holes on the ejector plate;
2.3 A clearance of 1/32’’ should be kept for ejector holes on the  plate of injection mold;
2.4 All ejector pins should adopt the standard dimensions; the ejector base cannot be ground lower;
2.5 For materials like LDPE and PP, the diameter of each ejector pin must be measured, because flash will be likely caused when the clearance between pin and hole is larger than 0.02mm;
2.6 The verticality and glossiness (Ra ~ 0.25μm) of pin holes have to be guaranteed in ejecting system;
2.7 For materials like PP, PE and Nylon, hole diameter = pin diameter + 0.01mm; for materials like HIPS, PC and ABS etc., hole diameter = pin diameter + 0.02mm;
2.8 Ejector pin should pass through the mold base, ejector plate and mold components in a straightforward way;
2.9 Upon installation of all ejector pins, the ejector plate should be able to slide downwards freely;
2.10 Labels in the same direction should be provided near the location of all ejector pins and screw heads, so as to prevent wrong installation;
2.11 All pins need to come with dowels, to prevent wrong installation;
2.12 Upon installation of ejector pins, everything needs to be checked before covering the back panel;
2.13 After installation of the supporter, use a flashlight to examine each rib and hole from the direction of the mold core, to see if there is any problem with pins or sleeve ejectors. Cover the back panel when everything’s OK.
2.14 When designing the location of ejector pins, in addition to guaranteeing sufficient ejection force, it has to be ensured that the product can be ejected in a straightforward way;
2.15 There are two types of ejector pins, i.e. though-hardened and nitrided;
2.15.1 Though-hardened – surface hardness is 65~74HRC, and steel core hardness is 50~55HRC;
2.15.2 Nitrided – the nitrided surface hardness is 65~74HRC.

What are ejector sleeves and how they work?

ejector sleeves for plastic injection mold

Ejector sleeves

1) The application of ejectors sleeves is required when a product features a hollow pillar structure and a great height. The matching relationship between pillars and sleeves is shown in the diagram below:

Ejector sleeves application

When a product features a pillar that is equipped with shockproof rubber, the ø3/16” + ø1.7 sleeve should be selected. If number 5 sleeve option is selected, there will come the stepped sleeve pin, of which the dimensions are shown in the diagram. For mold base below 30 x 40, category A sleeve pins should be applied; for mold base over 30 x 40, category B sleeve pins should be applied. See the diagram:

sleeve pins types in plastic injection mold

The order should be placed in the following format:

ø1/4” x sleeve length x ø3.5 x 30 x ø2.6 x L

ø1/4”x sleeve length x ø3.5 x 50 x ø2.6 x L

If a CD slider needs a sleeve, the sleeve dimensions will be unified as:
ø 6.35 x sleeve length x ø1/8” x sleeve pin length, in consideration of convenient pin replacement.


2) Sleeve pins are often fixed in plastic injection mold by headless screws. However, when several sleeve pins are very close to each other, briquettes and hex socket head cap screws will be used to fix them.

ejector sleeves fixation in plastic injection mold

3) Usually, the fixing holes of a button employ the sleeve ejector, with the standard ø3.57 x ø1.59 sleeve selected without exception. When it is not possible to use the standard size sleeve, it has to be guaranteed that the hole size is kept unchanged while the outer circle is appropriately enlarged, to make it possible for the application of a standard sleeve. The matching relationship between the sleeve pin and the cavity is shown in the diagram below:

sleeve pins design

4) Prevent sleeve from contacting with the knock-out hole.