For previous reading, please refer to Mold Design Guide (Slide Mechanism Design).
Chapter Six: Demolding Mechanism
Demolding is final step in injection molding process, and its quality ultimately determines quality of part. When mold opens, part must remain on half-mold (usually moving mold) equipped with a demolding mechanism, which then ejects part.
Demolding Design Principles: To prevent deformation of part during demolding, thrust should be distributed as evenly as possible and placed as close as possible to core where material shrinks and is tightly packed, or to areas difficult to demold, such as slender pillars. For these areas, a sleeve demolding device is used.
Thrust point should act on part with the highest rigidity and strength, avoiding thin areas. Action surface should also be as large as possible, such as flanges, ribs, and shell wall edges. Cylindrical parts often use push plates for demolding.
To avoid demolding marks affecting part's appearance, demolding position should be located on a concealed surface (inside) or a non-visible surface. Special attention must be paid to selection of ejection position and method for transparent parts. To avoid whitening and deformation of plastic parts due to vacuum adsorption, composite demolding or venting with permeable steel can be used. For example, in demolding with ejector pins and push plates or ejector pins and push blocks, clearance between ejector pins should be appropriately increased for venting. If necessary, an air inlet valve can also be installed.
Demolding mechanism should operate reliably and flexibly, possess sufficient strength and wear resistance. For demolding with rocker arms or lifters, strength and wear resistance of sliding surfaces should be improved, and lubrication grooves should be added to sliding surfaces. Nitriding treatment can also be used to improve surface hardness and wear resistance.
Length of mold return pin should, after mold closing, contact front mold plate or be less than 0.1 mm, as shown in Figure 8.1.1.
Spring return is commonly used for ejector plate return; however, due to its unreliability, it cannot be used as a reliable first return mechanism.

 

Common demolding methods for plastic parts include ejector pins, sleeves, flat ejector pins, and push plates. Because sleeves and flat ejector pins are more expensive (8-9 times more expensive than ejector pins), push plate demolding is mostly used for cylindrical thin-shell plastic parts. Therefore, ejector pins are the most commonly used demolding method. When ejector pins cannot be arranged around plastic part, such as when there are many deep ribs around it (rib depth/15mm), flat ejector pins can be used for demolding. Surface hardness of ejector pins and flat ejector pins should be above HRC55, and surface roughness should be below Ra1.6. Ejector pin and flat ejector pin demolding mechanism is shown in Figure 8.1.1. The key points for setting it are as follows: When ejector pin diameter d£Ø2.5mm, a support ejector pin should be selected to improve ejector pin strength.
Flat ejector pin, support ejector pin K/H.
Top surface is beveled, and a locating pin must be added to fixed end of ejector pin. To prevent ejection slippage, multiple small R-grooves can be machined on the bevel, as shown in Figure 8.1.2.

 

Length L of flat ejector pin and hole mating point should be 10-15mm; for small-diameter ejector pins, L should be 5-6 times diameter.
Ejector pin should be at least 0.15mm from cavity edge, as shown in Figure 8.1.2.
Avoid contact between ejector pin and front mold, as shown in Figure 8.1.3, as this can easily damage front mold or cause burrs. Arrangement principles of ejector pin positions (see also Section 5.5).
8.1.1 Ejector pin and flat ejector pin mating clearance
Mating parts of ejector pins, supported ejector pins, and flat ejector pins are shown in Figures 8.1.4, 8.1.5, and 8.1.6. Mating requirements are as follows:

 

Ejector pin head diameter d and mating dimensions t and w of flat ejector pin should be fitted with rear mold mating section with a mating clearance of 0.04mm. Dimensions of ejector pin and flat ejector pin holes in other non-mating sections are d10.8mm or d110.8mm, clearance between stepped fixed end and hole in face pin plate is 0.5mm.
Bottom end face of ejector pin and flat ejector pin must be flush with bottom surface of face pin plate.
As shown in Figure 8.1.7, top end face of ejector pin should be flush with rear mold surface and protrude 0.1mm above rear mold surface.

 

8.1.2 Ejector Pin Fixing
Ejector pins are generally fixed by machining steps on ejector plate, as shown in Figure 8.1.4. To prevent ejector pin from rotating, two common methods are used: one is to add a locating pin along axial step edge of ejector pin, as shown in Figure 8.1.8; the other is to add a locating pin laterally, as shown in Figure 8.1.9.
Headless screw fixing, as shown in Figure 8.1.10, is used when there is no backing plate at the end of ejector pin, is commonly used to fix ejector pins and spherical pull rods in three-plate molds.
(2) Headless screw fixing, as shown in Figure 8.1.10, is used when there is no backing plate at the end of ejector pin, is commonly used to fix ejector pins and spherical pull rods in three-plate molds.

 

Ejector sleeve demolding method is shown in Figure 8.2.1. Ejector sleeves are commonly used for demolding cylindrical parts with a length ≥ 20 mm. Standard ejector sleeve surface hardness is HRC ≥ 60, and surface roughness is ≤ Ra1.6. Additionally, ejector sleeve wall thickness should be ≥ 1 mm; when arranging ejector sleeve, ejector sleeve pin fixing position should not interfere with ejector roller hole.

 

8.2.1 Ejector Sleeve Fitting Requirements
Ejector sleeve demolding fit relationship is shown in Figures 8.2.2 and 8.2.3. Fitting requirements are as follows:

 

(1) Length of section where ejector sleeve mates with rear mold is L = 10~15mm, and its diameter D should have a clearance ≤ 0.04mm.
(2) Dimension of remaining non-mate sections is D + 0.8mm.
8.2.2 Large Ejector Pin Fixing
Ejector pin is fixed to base plate, usually using a headless screw as shown in Figure 8.2.1. When ejector pin diameter d > 8mm or 5/16”, fixing end is fixed using a pad, as shown in Figure 8.2.4.

 

Ejector plate demolding mechanism is shown in Figure 8.3.1. This mechanism is suitable for deep cylindrical, thin-walled plastic parts that cannot have ejector pin marks, or small multi-cavity housings (such as button parts). Its characteristics include uniform pushing force, smooth demolding, and minimal deformation of plastic part. It is not suitable for plastic parts with complex parting surface shapes or where ejector plate hole is difficult to machine.

 

8.3.1 Key Points of Mechanism
Key points of ejector plate demolding mechanism:
8.3.1 Key Points of Mechanism
Key points of ejector plate demolding mechanism:
(1) Mating structure between ejector plate and core should be conical; this reduces motion scratches and provides auxiliary guidance; slope of conical surface should be 3~10°, as shown in Figure 8.3.2.
(2) Inner hole of ejector plate should be 0.2~0.3mm larger than forming part (single side) of core, as shown in Figure 8.3.2.

 

When using wire EDM to machine conical surface of core, ensure there is a 0.1mm gap between wire cutter and top of core, as shown in Figure 8.3.3; avoid overcutting core during wire EDM, as shown in Figure 8.3.4. Ejector plate and return pin are connected by screws, as shown in Figure 8.3.1. When ordering mold blank, ensure that ejector plate and side pin mating hole are fitted with straight guide sleeves (straight guide sleeves), and ejector plate material should be same as M202. After demolding, ensure that plastic parts do not remain on ejector plate.
8.3.2 Example of Ejector Mechanism
(1) As shown in Figure 8.3.4, this mold has multiple cavities and uses wire cutting to process core, ejector plate, and fixing plate. Ejector mold usually uses a spherical pull rod, and sprue is only opened in the front mold, as shown in Figure 8.3.5. Wire cutting line of this ejector mold leaves die position inside core to prevent plastic part from being stuck on ejector plate, as shown in Figure 8.3.6.

 

As shown in Figure 8.3.7, fixing plate of this ejector plate mold is inside ejector plate. Features: Makes rear template B smaller, reducing amount of wire EDM machining. Fixing plate on mold is connected to support plate with screws and cylindrical pins, as shown in Figure 8.3.8. Wire EDM line leaves cylindrical part inside core, allowing plastic part to be easily demolded, as shown in Figure 8.3.9.

 

For plastic parts where ejector pin marks are not permitted (e.g., transparent plastic parts), and where high surface finish is required, the entire surface of plastic part can be ejected using an ejector block, as shown in Figure 8.4.1.

 

8.4.1 Key Points of Mechanism
Key points for ejector block demolding:
(1) Ejector block should have high hardness and low surface roughness; selected material should have a certain hardness difference with insert (generally above HRC5); ejector block needs nitriding treatment (except for stainless steel, which is not suitable for nitriding).
(2) Clearance between ejector block and insert should be such that there is no overflow, and smooth sliding is required; a lubrication groove should be provided on sliding side of ejector block.
(3) Mating side of ejector block and insert should be conical, and a straight surface mating is not recommended.
(4) Conical structure of ejector block should meet requirements shown in Figure 8.4.2; ejection distance (H1) should be greater than ejection height of plastic part, and less than half height of ejector block.
(5) Push block should be pushed out stably. For larger push blocks, more than two push rods should be set.

 

8.4.2 Example of Push Block Mechanism
(1) Plastic part is shown in Figure 8.4.3, and push block mechanism is shown in Figure 8.4.4. This mechanism takes into account characteristics of large demolding area and uniform ejection force of push block, uses inner and outer push blocks to eject, so as to achieve demolding balance.

 

(2) Plastic part is shown in Figure 8.4.5. Plastic part must be free of ejector pin marks; pushing mechanism is shown in Figure 8.4.6. This mechanism uses insert push blocks for demolding, and push block marks are uniform.

 

(3) Transparent plastic parts must be free of ejector pin marks. A push block mechanism is used for demolding, as shown in Figure 8.4.7.

 

To obtain a reliable demolding effect, demolding resistance of plastic part is reduced. A secondary demolding mechanism, which completes demolding of plastic part through a secondary demolding action, is called a secondary demolding mechanism, as shown in Figure 8.5.1.

 

Example of a secondary demolding mechanism:
(1) As shown in Figure 8.5.2, plastic part has a semi-circular recess between two ribs, which is tightly enclosed by rear mold cavity. Demolding mechanism is shown in Figure 8.5.3. First demolding causes plastic part to be ejected from rear mold cavity, providing space for strong demolding deformation; second demolding is performed by ejector pins, and semi-circular recess of plastic part is forcefully ejected from core push block. Movement process of this mechanism is as follows: In first demolding, all four ejector plates move, carrying ejector pins and core push blocks to move simultaneously. Demolding distance is ≥h, causing plastic part to be ejected from rear mold cavity, and first demolding is completed. When movement continues until swing block touches upper limit surface, swing block swings, causing two upper pin plates to move rapidly, driving ejector pins to be ejected from plastic part, completing secondary demolding. Note that for this mechanism: h1>h, H>10mm+h1+(secondary demolding movement distance).

 

(2) Sprue and parting line of plastic part are shown in Figure 8.5.5.
Since submersible runner must be located on lifter block and pass through it to enter mold, the mold needs to ensure that runner first exits lifter block. Mold uses a two-stage ejection mechanism, as shown in Figure 8.5.6. During first ejection, pull rod keeps runner stationary, ejector pins and lifters move a distance M from plastic part, separating plastic part from submersible runner. Submersible runner deforms and exits from lifter block, completing first ejection. During second ejection, all four ejector plates move, ejecting plastic part and runner out of mold cavity. Note that to ensure submersible runner exits lifter block, M > S (submersible runner length).

 

Interference occurs when projections of slide core and ejector pin in mold opening direction coincide. Reset mechanism ensures that interference with ejector pin is avoided when slide (core) is reset, as shown in Figure 8.6.1.

 

As shown in Figure 8.6.2, to avoid interference between slide core and ejector pin, following conditions must be met: When top of slide core coincides with projection of ejector pin, there should be a gap between slide core and ejector pin in vertical direction, i.e., F>f; Slide continues to advance a distance C, while ejector pin retracts a distance f; at this time, f³C*ctga8;

 

As shown in Figure 8.6.3, to prevent interference between slide core and ejector pin during mold closing, a pre-reset mechanism of swing block is commonly used. In this mechanism, during mold closing, reset rod first pushes swing block, which forces pressure block to return, thereby driving ejector plate to complete pre-reset. Length of reset rod of mechanism must ensure Z/A + 15mm.