In order to improve injection molding efficiency and quality of molded products, it is also necessary to implement automatic operation. For this reason, it is necessary to be able to automatically and reliably demold sprue, runner and molded products.
In addition, for products produced in small quantities, number of days and prices for mold production are limited, when shape of molded product and type of gate cannot allow automatic demolding, ejection mechanism for molded product, sprue, and runner can be selected to eject the mold most easily.
Molded products of same shape also have different ejection methods due to different material types and gate forms. The most appropriate method must be used in combination.
Ejection of molded products
Decision of ejection method of molded product is of course affected by material and shape of molded product, but in principle, gaps, levels, etc. are made on molded product to achieve the most reliable demolding, and there will be fewer failures. If a failure occurs, it must be easily repairable.
Molded products of same shape also have different ejection methods due to differences in appearance, accuracy, and formability.
Ejection methods generally use ejection pins (straight pins, staged pins), sleeves, scrapers, air, etc., which can be used alone or in combination, depending on life span of mold and difficulty of mold processing.

Processing of tip is the easiest. When hardness is required, it is easier to perform quenching and grinding than other methods. It can be placed at any position of molded product, which is the most commonly used method.
Hole is easy to process, accuracy can also meet requirements, sliding resistance is minimal, and occurrence of locking is less. Because of long mold life and interchangeability, repairs can be easily made when damaged.
However, when ejecting on a small area, ejection stress is concentrated on local part of molded product. Cups and box-shaped items have small shrinkage and inclination, and molded products have high demoulding resistance. In this case, depression and ejection will occur. Most of time, use of ejection pins is inappropriate.

There are no difficulties in processing and heat treatment of plate-shaped ejector plates and other parts, but processing of holes is difficult and requires special processing such as electric discharge machining.
By dividing mold plate and heart-shaped part to form a combined shape, processing becomes easier, but manufacturing time is increased. Since molded product has dividing lines, it has a negative impact on appearance of transparent part, and there are also situations where it is not allowed. Furthermore, sliding resistance is also relatively low, with many round tips and thickness of board is relatively thin, which can easily cause bending and buckling. Avoid using it to extent possible.

Processability of sleeve is relatively good, but sleeve with a small inner diameter and a long length is more difficult to process, and is prone to cracking when used on thin parts.
Since end face of sleeve is fully ejected, molded product is ejected uniformly, which can ensure reliable demoulding and reduce occurrence of cracking of molded product.

Processing of scraper plate and heart shape is not more difficult than ejection pin. Mechanical processing and matching of sliding surface requires more time. Fitting part of sliding surface must be quenched, and heat treatment is more difficult. Furthermore, interchangeability tends to be poor, and repairs require more labor.
If shape of scraper plate and heart-shaped mating surface is round or square, machining and mating processing are relatively easy, but if it is a continuously changing curve, it becomes difficult.
Furthermore, in order to maintain interchangeability after quenching, bushings are used to embed, making repairs easy. Especially if there are several holes, only damaged single piece can be replaced.
Compared with other ejection methods, scraper plate has a large ejection area, molded products can be reliably demoulded. It is effective in demoulding molded products such as cups and hats that have greater demoulding resistance, and is widely used. Furthermore, there are almost no signs of ejection on appearance, which is also one of advantages.

Air extrusion method is to set up a valve, etc., to pass air into gap. Processing is relatively simple, it is an extremely effective method for demoulding molded products with large depths such as cups and boxes.

 

Figure 1 shows case where only ejection pin is used. Position of ejection pins should be arranged at a place with greater demoulding resistance. If demoulding resistance is uniform, it should be arranged evenly. Cups and boxes shown in figure form a molded product, and side resistance is the greatest. In the best case, ejection pin is arranged here. Furthermore, if ejection pin is installed on inner side, it is better to place it near side wall than in the center, otherwise it will easily cause cracking during ejection.

 

Figure 2 shows situation where there are thin and deep bosses and reinforcing ribs. If ejection pin is used to eject around them, cracking will occur, which will easily cause molded product to be damaged and scrapped. Ejection pin must be set at the bottom of boss and reinforcing rib to ensure reliable demoulding.

 

Figure 3 is an example of using a layered ejection pin. Since it is impossible to use a small ejection pin for small molded products, deflection is reduced because middle section is thicker.

 

Figure 4 is an example of using both ejection pin and scraper plate. Inner surface of heart-shaped mold has a greater resistance to demoulding. If only scraper plate is used to demould, there may be broken residues. In order to prevent this kind of defect from happening, ejection pin is set up, scraper plate is used as main one, and ejection pin is used as a supplement. In this case, ejection pin is set inside heart, causing a heart-shaped cooling failure. Using a small diameter heart can be used for direct cooling, which can eliminate this shortcoming. Shape of molded product must be considered to eliminate possibility of setting an ejection pin, and it can also be demolded.

 

Figure 5 shows some parts with strong demoulding resistance (less and deeper shrinkage). For example, in the case of tubular protrusions, when ejection pin is used to eject peripheral and inner surfaces, molded product may also crack. Leather is ejected with a sleeve, mainly ejection pin, and sleeve is a supplementary mechanism.

 

Figure 6 and Figure 4, Figure 5 takes same consideration and shows an example of using a scraper plate and a sleeve together.

 

Figure 7 shows situation where inner and outer peripheral tools are embedded in movable side of a deep tubular molded product. It is most effective to use a sleeve to push out end face of molded product.

 

Figure 8 shows an example where long sleeves are difficult to process but short sleeves are easy to process.

 

In general, when ejecting basin-shaped products, scraper plate shown in Figure 9 is mostly used. If inclined basin end ejection pin shown in Figure 10 is used, processing is simple. And use a scraper plate to cool heart, as shown in Figure 4 and Figure 5. There are shortcomings﹒

 

 

Figure 11, Figure 12 shows situation of using air to press out. When using a scraper to push out large, deep, and thin molded products (boxes, cups, etc.), molded product may buckle. Furthermore, a vacuum may be formed between molded product and heart shape, causing molded product to break. When used in soft materials such as polyethylene, degree of breakage is more significant. In this case, air pressure is the most effective. Figure 11 shows method of using only air. Figure 12 shows an example of using a scraper with air pressure between heart shapes.
How to take out threaded molded products
If molded product has threads, there are three demoulding methods:
1) Die threaded part die
2) Place insert into threaded part of mold﹒
3) Molded product rotates on threaded part of mold﹒

This method is suitable for external threads (mold structure of male threads is relatively simple to manufacture and can be reliably demoulded).
However, threaded part of molded product has a waste edge that squeezes into parting line. Molded product is processed later, cooperation with fitting part causes a failure.

If mold structure cannot use mold splitting and rotation, inserting fitting method can be used when mold structure is simple.
However, when using this method, after molded product is ejected, insert must be removed. When molded product is externally threaded, it is easy to remove due to shrinkage. However, if it is internally threaded, contact area between inserted insert and molded product is large, making it difficult to take out. Due to material and large contact area of molded product, it is difficult to take out.

Generally, covers, etc. are molded products with internal threads, and most of them are automatically rotated out.
In this case, either one of molded product and mold rotates, makes a withdrawal movement, or one piece only rotates and the other part withdraws, but molded product must have sliding positioning (rotation stop).
When outer circumference of molded product is slidably positioned, mold with dotted gates is opened and rotation starts at the same time, molded product and threaded part of mold are demoulded. Due to pressure of parting surface, demoulding resistance of parting surface is large, and thread of molded product is broken.
In order to prevent this defect, speed of thread withdrawal must be designed to be same as speed of mold opening mechanism of parting line. Furthermore, when there is a place with strong demoulding resistance outside threaded part of molded product, ejection function must be at same speed at the same time as rotation is started.
Although rotary damage can be set on both fixed side and movable side of mold, molding machine is generally set on opposite side of nozzle and ejection mechanism. Due to configuration and ejection relationship between sprue and runner, rotary mechanism is located on movable side, which is advantageous in terms of mold structure and forming efficiency.

 

Figure 13 shows outer point gate, and Figure 14 shows inner point gate. Both use special ejection pins, scrapers, etc. to eject them. After thread (a) rotates several times between mold openings, it breaks away from mold cavity or heart shape, and molded product falls naturally.

 

Figure 15 shows case where molded product is slidingly positioned and mold core is on movable side. After rotation is completed, molded product is ejected from segments H and h, then falls out of mold.
As shown in the figure, when H>h, ejection mechanism of ejection pin (b) must be installed. As shown in Figures 13 to 15, it is suitable for gate to be automatically cut off during demoulding. In the case of lateral gate, runner and molded product move at the same time, and an ejection mechanism with same speed must be installed.

 

Figure 16 shows that there is a sliding positioning on inner flat surface. (a) It rotates and moves at the same time. After thread mold is removed, sliding positioning is still attached. It is necessary to install a separate removal device so that formed part can fall on its own.

 

Figure 17 shows case where there is a sliding positioning external thread on inner surface. (a) Rotate and unscrew thread according to pitch of thread of molded product. Then you must use ejection pin (b) to eject it.

 

Figure 18, Figure 19 There is a sliding positioning on inner side, only (a) rotation and (b) fixed. After thread is unscrewed and demoulded, it is ejected through scraper. Role of scraper is to use various methods such as ejection pins and springs.

 

End faces of molded products shown in Figure 20 and Figure 21 are all slidingly positioned. In order to allow them to fall naturally, an additional ejection device (a) must be installed. When a small molded product has a lateral gate, molded product cannot be ejected directly and must be ejected together with runner. Furthermore, there is no need to set up a special sliding positioning. Gate can be used as sliding positioning set on the end face. However, for soft materials, if there is no fairly large gate, there will be shearing possible.

 

Figure 22 shows a heart-shaped wheel that rotates and reciprocates while being directly connected to the pinion. Figure 23 shows a heart-shaped wheel that is indirectly connected to the pinion. Sliding resistance of reciprocating motion is smaller in Figure 23 and can function smoothly, with a larger reciprocating stroke. In Figure 22, strength is relatively good. In the case where mass has a heart-shaped tip, there is no difference from the one used in Figure 22.

 

Figures 24 and 25 use mold opening stroke.
Figure 26 and Figure 27 are equipped with separate power sources such as hydraulic and electric motors. Advantage is that timing of starting and stopping rotation can be freely controlled, but a power control device is required. When molded product bolts rotate a lot, driving method shown in Figure 27 is the best.
Point gate main injection channel ejection
When lateral gate has a runner and a gate in molding surface, molded product can fall off at the same time, and it is easy to fall off naturally. However, in cases such as point gates, it is difficult to fall off naturally.

 

When there is no runner stripping mechanism as shown in Figure 28, manual demolding is necessary. Mold structure is the simplest, but forming efficiency is low, and it should not be used except for small-scale production.

 

Figure 29 shows a runner scraper, but in stripped state, sprue part is suspended in the hole of flow channel scraper and cannot fall off naturally. In this case, an ejection point should be set in part A, and air pressure should be used to press it off if necessary.

 

Figure 30 shows a simpler structure. Number of completed processing steps is less. Flow channel length L is larger than l. If l is deeper than above situation, when flow channel floats, it will be embedded in the side of mold plate and cannot fall off naturally. Furthermore, if cooling time is long, part of runner is bent, gate part that once floated will still spring back to the side of mold.
Furthermore, there are vertical ridges on the side of Part A, which are attached and do not fall off. In addition, if molded product is placed directly below sprue, sprue positioning cannot be installed, making it difficult to use. However, there is no need to use devices such as springs and extension devices, and gate can be cut off at the same point as mold is opened.

 

Advantages and disadvantages of Figure 31 are roughly same. Part A in Figure 30 is not attached, part of flow channel is embedded in the side of mold plate, so that it can float and fall off.
However, sprue ejector is slightly longer than movable side mounting plate, so it cannot be used on a molding machine without a central ejector rod hole or a molding machine with a central ejector hole that is not deep enough.

 

Figure 32 is for slender molded products such as pen covers. An ejection plate is installed in fixed side mold plate, and flow channel is ejected through the pin. This is only used for limited molded product shapes.
Two sections push out

When a scraper is used for ejection, flange portion of molded product is pushed out from inside of scraper. This part is still adhered to scraper. There must be any method to remove it from molded product. In this case, In order to achieve automatic demoulding, a two-stage ejection mechanism is required.

 

Figure 33 shows scraper and ejection pin method. Figure 34 is an example of using two sets of ejection plates to eject a dented molded product in two stages to form a forced ejection.
In the case of two-stage ejection, two groups of ejection strokes must be different, and larger side of stroke will act at the same time or later.
Ejection mechanism of forming machine is a pressure cylinder and its accessories, as well as a positioning rod and its accessories. In the former case, one side is formed by fixed side, using screws, chains, rings, etc., to extend according to mold opening stroke, and the other side is acted by ejection mechanism of forming machine, which can form a two-stage ejection effect.
But in the latter case, two groups must be ejected in two stages from same direction, two groups of ejection mechanisms must have ejection stroke timing and stroke adjustment mechanisms.

 

Figure 35 shows a mechanism in which ejection side is driven by a spring. It is easy to manufacture and has a small installation space. It is the simplest mechanism.
But it cannot withstand excessive force, and its effect is unreliable.

 

Figure 36 shows use of a pressure cylinder instead of a spring. Effect is reliable and timing can be adjusted freely. However, installation location is large and cannot be used when forming machine and mold are small. Furthermore, hydraulic pump and air compressor must have special control facilities.

 

Figure 37 shows use of a cam instead of a spring and a pressure cylinder. It works reliably and does not require other ancillary facilities.
However, cam lifter protruding from fixed side must be installed in a place that does not hinder removal of molded product.

 

Figure 38 shows (a) and (b) two sets of ejection plates. (a) set is ejected directly from (c). (a) set is connected with jaw (d) and (a), performs two-stage ejection through (d) during action stroke.

 

Figure 39 shows scraper plate and ejection pin acting at the same time. There is a sliding member directly below (a). Due to movement of sliding member, scraper plate stops functioning during action of ejection plate, and two-stage ejection is performed.
Furthermore, contact surfaces of (a) and (b) must be smooth.
Ejection plate advance return mechanism
For molds using split molds and lateral heart-shaped molds, if ejection pin is installed, when opening mold, ejection pin must be retracted before sliding part is retracted.
Otherwise, the two will conflict and be damaged.

 

Figure 40 shows an advanced retraction mechanism using a spring as ejection plate. In this case, mold structure and processing are the simplest, and installation location is also small.
However, ejection pin sometimes becomes stuck, which makes advance retraction effect unreliable, sliding part conflicts with ejection pin, and mold is damaged. Furthermore, if ejection stroke is large, spring compression ratio will become larger. Use a spring of considerable strength. Ejection plate will not advance and retreat during action process, and try to avoid action.

 

Figure 41 shows use process. Function is reliable and will not cause malfunction. However, action rod (a) extending from fixed side is long and must be installed in a place where there is no hindrance in taking out molded product.

 

Figure 42 shows use of a joystick. Its advantages and disadvantages are same as those in use phase. However, when ejection plate is fully restored and must be released, more attention must be paid to timing adjustment.

 

When gear is used as shown in Figure 43, function is real and same as link method. Considerations regarding removal of molded product must be same as link method and joystick method. Difference from other methods is function of ejector plate. Reciprocating motion is implemented by rack, and stability of ejector function of ejector plate must be considered.

Advantages and disadvantages of this method of mechanism are: in the case of two-stage ejection, effect is reliable and timing can be adjusted freely; disadvantage is that device is complicated.