This product is a marine instrument rear cover designed by an enterprise. Shape of product is complicated, and dimensional accuracy is high. Material is PC + ABS, see Figure 1.

 

Figure 1

Inside of instrument rear cover is shown in Figure 2.

 

Figure 2

There is an elastic block (a part of product) on outer wall of product, which will stick to slider when it is demolded, and it will be deformed. Consider placing a thrust mechanism on slider, as shown in Figure 3.

 

Figure 3

Assembly requirements on the top of product are very high, and dimensional accuracy must be strictly controlled, as shown in Figure 4.

 

Figure 4

As can be seen from above 4 pictures, product requires high dimensional accuracy. Mold will be demolded in the form of a 4-sided large slider. Problem is that product has a large clamping force on large core in the middle of movable mold, which requires a lot of ejectors to eject product. However, from inside of product (Figure 2), there can be few positions for ejection. In order to minimize internal stress of product after injection moulding, selection of injection point is also important. Mold flow analysis of this product is provided by Moldex 3D.

First of all, in terms of product's shape characteristics, it is impossible to eject product by many small ejectors, because 4 walls of product have a very strong clamping force on movable mold core. According to conventional method, simply pressing bottom of product will definitely burst product.
I imagine that before four large sliders are opened, middle mold core is retracted a certain distance (20mm), so that shape of inner bottom surface of product is taken off first, product is hooked by shape on slider and bottom edge of product formed on slider, mold core is separated from product, and there are some ejectors in the middle to support product, otherwise ribs and grooves in the middle of product will be deformed or broken.
After mold core is pulled out for a certain distance (20mm), slider is opened, then product is ejected by supporting ejector rod of product when original core is retracted.
A small part of two products formed on slider is called a spring block, which will stick to slider. These two sliders need to be arranged with a thrust mechanism.
Because height of product is large, and distance to eject product is also large, total height of mold will also be very large. Since pin gate is intended to be used, mold is in the form of a three plate mold. It also increases total thickness of mold, so when designing this set of molds, under premise of ensuring that mold has sufficient rigidity, controlling total thickness of mold as much as possible is an important factor in design process.
When injection molding machine opens mold, fixed mold frame and movable mold plate are temporarily not opened, and slider is of course kept in place. Line between movable mold plate carrying four sliders and movable die pad of large core of fixed movable die is separated by 20mm, as shown in Figure 5.

 

Figure 5

It can be seen from FIG. 5 that when movable mold plate and movable die pad are separated by 20 mm, slider is left in place, and large movable core moves with movable die pad 20 mm backward. Because shape around product is on slider, large core of moving mold is surrounded by smooth flat surfaces, and there are 11 jacks in the middle of product. Therefore, large core of movable mold is easily moved backward by 20mm. Shape of top of product is completely removed, and shape of bottom of product is not deformed, as shown in Figure 6.

 

Figure 6

Because general structure uses a simplified small nozzle, guide post is on fixed mold side, positioning of movable mold side is based on two-section guide sleeve arranged between track and movable mold plate. Because movable mold plate and movable mold pad must be separated by 20mm, so a built-in guide post and guide sleeve are arranged between movable mold plate and movable die pad, as shown in Figure 7.

 

Figure 7

Temporary non-separable force of fixed mold frame and movable mold plate is composed of two parts. One is a nylon pull stud arranged on fixed mold frame. Second is that movable mold plate and movable mold system part have strong springs relative to fixed part of injection-molded motor template, which will be introduced in detail later.

Design of moving mold core is beginning of entire design task.
I conceived that this set of molds started from this mold core. Design of mold core is very difficult. To mold so many ribs and grooves, how can this insert be set?
Large core of moving mold is divided as shown in Figure 8.

 

Figure 8

As can be seen from Figure 8, top of mold core is covered with deep grooves, and dimensional accuracy of these grooves is very high. If it is to make whole core, it should be very difficult, and side wall of groove polished by hand is difficult to achieve precise dimensions. Side wall of groove machined by spark machine has a spark machine erosion. It is very difficult to remove it, especially root of groove. Therefore, general consideration is to make an insert. How to make an insert?
After comparing several schemes, I finally adopted this arrangement, as shown in Figure 9.

 

Figure 9

As can be seen from Figure 9, I divided inserts into two groups, one in horizontal direction and one in vertical direction. Separated core body is shown in FIG. 10.

 

Figure 10

It can be seen from FIG. 10 that grooves for placing inserts can be processed by wire cutting. Step surfaces of forming grooves can be machined with a spark machine, which is more convenient to operate during polishing. See Figure 11 for first set of inserts.

 

Figure 11

Since it is divided into 5 parts, it can be easily processed by precision grinder and spark machine, and processing is not difficult. See Figure 12 for second set of inserts.

 

Figure 12

It can be seen from Figure 12 that machining of this second group of inserts is also not difficult, but machining accuracy is very high. All ejector rods of movable large core are arranged on this second set of inserts. In addition, these two sets of inserts are designed to be interlocked, see Figure 13.

 

Figure 13

In this way, there is no possibility that inserts can move with each other during injection molding. When two sets of inserts are fixed in core body, a solid whole is formed, as shown in FIG. 14.

 

Figure 14

As can be seen from Figure 14, so many inserts were fixed with only six M6 screws. After a long period of production, it proved that design of fixing method of this insert is very reliable. Large core of movable mold is arranged with ejector rod and waterway, leaving enough space.
Large core of movable mold is fixed on movable mold pad, see Figure 10-15.

 

Figure 15

There is a pond in the center of large core of movable mold, and cooling water is led out by movable mold pad, as shown in Figs. 16 and 17.

 

Figure 16

 

Figure 17

As can be seen from Figure 10-17, in order to avoid ejector rod on center line, positions of entering and exiting water are staggered. See Figure 18 for connection between inlet/outlet waterways and pond.

 

Figure 18

All sliders and rails of this set of molds are arranged on movable mold plate. When injection molding machine is turned on, in order to achieve retracting of large core of movable mold, movable mold plate must be moved forward 20mm in total. This 20mm distance is achieved by 4 limit screws, see Figure 19.

 

Figure 19

Since large core of movable mold is fixed on movable mold backing plate, large core of movable mold is separated and locked once from movable template in each process of mold opening. In order to reduce wear and precise positioning of large core of movable mold, combination of large core of movable mold and movable mold plate must be a set of slopes. Because material of movable mold plate is No. 50, it is not wear-resistant, so I have a wear-resistant insert here, which is called "large core platen", see Figure 20.

 

Figure 20

This core platen is fixed on movable mold plate, see Figure 21.

 

Figure 21

Here is a detail. In order to increase stability of large core of moving mold, inside and outside of this large core mold plate are inclined by 5 °, as shown in Figure 22.

 

Figure 22

As can be seen from Figure 22, inner and outer sides of this core platen are 5 ° inclined planes, one is positive 5 °, the other is negative 5 degrees. Core platen is 0.2mm higher than large surface of movable platen. In this way, when injection molding machine is clamped, large core of movable mold is tightly pressed. This prevents large core of movable mold from being biased when injection moulding machine shoots. On four sides of core platen, there are slider stop blocks (impact blocks) fixed at the bottom of slider, there is a spring between core platen and slider stop block, as shown in Figure 23.

 

Figure 23

Slider 1 is composed of slider 1 insert, slider 1 body, slider 1 stop block, and spring, as shown in FIG. 24.

 

Figure 24

Slider 1 insert is made of 738H, hardness is HRC42, and shape is shown in Figure 25.

 

Figure 25

There is a positioning step on reverse side of slider 1 insert, which is precisely caught in rectangular groove of slider body, see Figure 26.

 

Figure 26

As can be seen in Figure 26, slider 1 insert and slider 1 body are fixed by four M8 hex socket screws. Positioning is determined by positioning step on slider 1 insert stuck in rectangular groove on slider 1 body. Waterway is arranged on the insert of slider 1 and is led out from water path on slider body. A seal ring is provided at the joint of two. Slider 1 is pulled away by using a "T" shaped block provided on fixed mold frame and a "T" shaped groove on the body of slider 1. Because "T" block and "T" groove have a uniform slope, when injection molding machine opens mold, "T" block slides slider 1 outward.
In order that "T" block can be aligned with entrance of "T" groove on the body of slider 1 when injection molding machine is closed, slider 1 needs to stay in final position, which is achieved by spring in Figure 10-20. Spring is arranged in the hole between core pressure plate and slider stop block. In actual production, spring has a pre-compression of 10 mm. If this spring fails, slider may return to inside, especially upper one of four sliders, which will fall down due to weight. At this time, "T" block cannot be inserted into "T" groove, mold will be crushed when mold is forcibly closed.
In addition to placing a spring and holding slider outward, this slider stop block also acts as a slider to stop inward, as shown in Figure 27.

 

Figure 27

It seems that force of this slider stopper against core platen is insignificant compared to force of injection molding machine clamping. In fact, otherwise, surface of some small bosses on slider 1 insert is larger than movable mold. If these surfaces are worn, holes produced by these small bosses on product will have flash. When injection molding machine is being clamped, carefully observing movement of slider is divided into two stages. The first part is called "fast" clamping, and the second action is high-pressure clamping, which can shorten injection cycle.
During "quick" clamping, inward movement of slider is inertia. If this slider stop is not set, small boss on slider 1 insert will hit large core of movable mold, causing rapid damage of these small bosses. Now with this slider stop block, when slider moves inward due to inertia, these small bosses will not hit large core of movable mold. Only when injection molding machine is under high pressure clamping, surfaces of these small bosses are close to large core of movable mold. In this way, surface of these small bosses will not be damaged. This is a detail that is often overlooked in mold design.
In addition, two contact surfaces of slider insert which are inserted into grooves formed by the other two sliders have a slope of 3 °. See Figure 28.

 

Figure 28

It can be seen from FIG. 24 that abrasion of two surfaces can be avoided because two surfaces have a slope. In addition, there is a detail. Rounded corners at entrance of "T" groove should be as large as possible to ensure that "T" block on fixed mold frame does not collide when inserted into "T" groove, as shown in Figure 29.

 

Figure 29

Structure and method of slider 3 are exactly same as those of slider 1 and will not be described again.

As mentioned earlier, (see Figure 3) there is an elastic block on one side of product, which is molded on slider 3, see Figure 30.

 

Figure 30

It can be seen from Fig. 30 that positions of two elastic blocks of two molded products have a large clamping force on elastic blocks of injection moulding. Since surface of corresponding large core of movable mold is flat, there is no tension on elastic block on the product, so formed elastic block must be left on slider 3, commonly known as "sticky slider". There are many ways to deal with this problem. Now I introduce a simple method, see Figure 31.

 

Figure 31

As can be seen in Figure 31, lower end of "T" block, facing position of slider pin, I designed a small straight surface (length 20mm). When clamping, this straight surface is against hemispherical tip of slider pin. The other end of slider pin is involved in molding in cavity, which is equivalent to a thimble. When moving and fixed molds are separated, due to this small straight surface, slider pin is kept in place for the first 15mm of movement, while slider body and slider 3 insert fixed on the slider 3 body slide outward by 3.88mm.
Because product that has been molded at this time is still set on large core of movable mold, product and slider insert have been separated by 3.88mm. Since elastic block part on product is pushed by slider needle, it is also separated from slider insert by 3.88mm. When injection molding machine continues to turn on and small straight face on "T" block leaves hemispherical tip of slider pin, slider pin will pop out due to action of spring and continue to move outward with slider body, see 32 .

 

Figure 32

This is the simplest slider thrust device that can be used for reference in many places. This example is a special case. One end of slider pin has a shape, so a rotation stop device is required, as shown in Figure 33.

 

Figure 33

In this mechanism, the most ingenious is to open a small straight face at the bottom end of "T" block, see Figure 34.

 

Figure 34

Since this set of molds has a structure of four large sliders, sliding of four sliders on movable mold plate is pulled and pushed in by four "T" blocks on fixed mold frame. Under action of clamping force, pressure on movable platen at the bottom of slider is very large. Therefore, a friction plate must be provided on movable mold plate to form a friction pair with bottom surface of slider. Design of friction plate is more traditional, see Figure 35.

 

Figure 35

In actual production, surface of friction plate is same height as large core platen, and together forms friction surface. In addition, surface of friction plate and large core platen is 0.2mm higher than surface of movable template. I use Cr12 as material of friction plate, and hardness is HRC60.

For "T" -shaped sliding track of slider, I don't recommend digging grooves in template to achieve it, because digging deep grooves in template will deform template and waste steel. I suggest using cheaper and highly wear-resistant materials to “stick” to moving mold plate after heat treatment. However, for designer, it is necessary to consider in advance. For rails that has been quenched to HRC60, it is difficult to add screw holes and other things. Because positions of four large sliders are removed, contact surfaces of moving and fixed molds are only surface of track, all functional mechanisms and holes can only be arranged on this small track surface. Let's look at arrangement of molds on track, see Figure 36.

 

Fig. 36

As can be seen from FIG. 36, arrangement of functional holes on the surface of this track is very exciting, with a total of 6 functional holes. One is five M10 hexagon socket screws, which are used to fix track on movable mold plate. Second is nylon pull stud hole. Nylon pull stud arranged on fixed mold side is source of power for movable mold shrinking core. Third is precise positioning hole, which cooperates with convex conical shape arranged on fixed mold side, plays an integral and precise positioning of movable and fixed mold. Fourth is a positioning pin arranged at the bottom of fine positioning hole, which plays a role of positioning between track and movable mold plate at the same time as guide sleeve. Fifth is guide sleeve hole. Sixth is retaining ring hole of reset lever, see Figure 37.

 

Fig. 37

By the way, role of retaining ring on the head of reset lever is very important. When movable mold plate and movable die pad are separated by nylon pull studs and 4 large springs, entire ejection system is also followed together due to role of retaining ring. 11 ejectors are always against product. If there is no such action, bottom of product will not have these 11 ejectors. Dense ribs on the bottom of molded products will passively deform and even break large core .
From introduction above, it can be seen that design of this track is very particular. Although it is a simple block with some holes, it reflects depth of mold designer's understanding of mold structure and all actions of mold. Because hardness of track is very hard, it is necessary to consider all functions when designing, once it is changed, it is more troublesome.

Everyone must be wondering why gate design was not mentioned at the beginning of this article.
In fact, key point of this set of molds is not gate. There is no choice of gate form. It must be a three-plate mold gate. why? First of all, due to limitation of total thickness of mold, hot runner is not suitable, shape of fixed mold is precise and complicated. It must be an insert, and position of hot nozzle is not good.
If gate is large, it is not appropriate because dimensional accuracy of top of product is too high. Large gate will cause large stress and uneven density at the top of product, which will cause product to have relatively large deformation and warpage. So I think the only way is pin gate. Since it is a pin gate, position of injection is very important. Although Moldex 3D provides a reasonable mold flow analysis solution, position of injection point is largely affected by form of insert on large core of moving mold. To put it simply, injection point cannot be facing joint line of insert. Even if the most reasonable position is facing joint line, it should be properly removed. So I first consider design of large cores for moving molds.
This set of molds can be designed as long as structure of large core of moving mold is wanted. After consulting with Moldex3D, I finally chose such a gate scheme, as shown in Figure 38-1.

 

Figure 38-1

Results of mold flow analysis are shown in Figure 38-2.

 

Figure 38-2

Shape of fixed mold is shown in Figure 39.

 

Figure 39
As can be seen from FIG. 39, this set of molds is a typical three-plate die-point gate form, and a simplified small nozzle mold base is used. Functional parts such as nylon rivets, fine positioning, limit screws, and "T" blocks are arranged on parting surface fixed mold frame. This is the case when mold is opened, see Figure 40.

 

Figure 40

Now let’s talk about basic principle of pin gate of fixed mold in three-plate mold, as shown in Figure 41.

 

Figure 41

It can be seen from FIG. 41 that molten plastic is injected from main runner, enters mold cavity through cross flow channel and pin gate. After cooling, melted plastic becomes solid plastic. Parts other than product are called gates. In addition to automatically removing product through various demolding mechanisms, we also need to automatically remove gate part, so that we can smoothly carry out next operation and realize automated production.
First, separation of product from gate is broken by gate pull pin, which has an inverted buckle, as shown in Figure 42.

 

Fig. 42

From Figure 42, the first action of fixed mold opening is to separate fixed mold frame from gate scraper by 100mm. At this time, because gate pull pin tightly pulls gate, product and gate are forcibly broken. Fixed mold continues to open 12mm. At this time, gate scraper forcibly scrapes gate from gate pull pin. Gate is adsorbed on gate scraper. Generally, it is blown down by compressed air. Gate is also possible ejected with a spring block, because injection molding factories are gradually moving towards automatic production mode, and more factories use robotic hands to clamp gate.
Design of two sets of stop screws is very important when gate is pulled away from product and scraped from gate scraper, as shown in Figure 43.

 

Figure 43

Fixed mold inserts consists of a large insert and two sets of small inserts, see Figure 44.

 

Figure 44

Shape of large fixed mold inserts is not very complicated, but dimensional accuracy is very high, as shown in Figure 45.

 

Figure 45

It can be seen from Figure 10-45 that middle part of large die of fixed mold is original. After processing it with a slow wire cutting machine, it it processed with a high-speed machining center and spark machine. Material is 738H, hardness is HRC38. 6 recesses on two sides are useless, but they are for convenience of wire cutting, there is no need to replace them after processing. The first set of inserts consists of 6 identical small inserts, see Figure 46.

 

Figure 46

Second set of inserts consists of two small inserts, see Figure 47.

 

Fig. 47

These two sets of inserts are respectively placed in grooves processed by wire cutting on large inserts of fixed mold. Cooperation of these grooves and small inserts must be very accurate.

Because small inserts in large inserts of fixed mold are small in size, it is difficult to arrange cooling water channel. I use beryllium copper with strong thermal conductivity as small insert, cooling water channel of mold frame contacted by bottom surface of small inserts took away heat. I only arranged cooling water channels on large fixed mold inserts, as shown in Figure 48.

 

Fig. 48

Fixed mold frame is used to lock four slides of movable mold. At the same time, functional parts of fixed mold are arranged on fixed mold frame. It is the most important part of fixed mold system, as shown in Figure 49.

 

Fig. 49

As can be seen from Figure 45, although functional requirements of fixed mold frame are relatively large, there are many parts to be arranged. In general, design of fixed mold frame is very simple. For example, one large insert plus eight small inserts are all fixed with just four M8 hex screws. As mentioned earlier, it is difficult to arrange water channels for 8 small inserts, so 4 sets of cooling water channels are arranged at the bottom of fixed mold frame to effectively cool fixed mold inserts and 8 small inserts. At the same time, 4 point gates also have a direct cooling effect, as shown in Figure 50.

 

Figure 50

See Figure 51 for complete set of molds.

 

Figure 51

1. Design idea is clear. Although shape of product is complicated, all other problems are simplified into problems that can be solved by conventional means around axis of "moving mold shrinking core".
2. Structure is clever, practical, and easy to process. Interlocking and precise positioning of many small inserts on large core of movable mold is a classic in multi-insert structure.
3. Structure is simple. Eight small inserts on large core of movable mold are all fixed by using only six M6 hex screws. Eight small inserts on fixed mold inserts did not use a screw, and only four M8 screws on fixed mold insert were fixed.
4. Slanting surface of large core of movable mold and core platen enables precise positioning between movable mold system and base plate of movable mold. Positioning of entire mold is very reliable.