This product is used as a throttle valve on solar water heaters. Material is POM and shape is shown below.

 

As can be seen from above figure, there is thread on the outside of product. This is not difficult, and slider can be used for demoulding. Reverse side has internal threads and central part is not rotatable, see figure below.

 

Product appears to be difficult to demold because middle part of internal thread cannot be turned. Internal thread must be mechanically turned out, middle part of product is thick, and middle part of internal thread must be cooled. These two requirements seem to be a bit contradictory. It is a typical injection molding mold with a threaded rotary core structure, but middle part of rotating part is not turned, non-rotating part is also cooled by water. This case is a big challenge for plastic injection mold designer.

In this case, before considering mold structure, we must first consider how to arrange gate, in what form, where to inject? Because demoulding of this product is to draw core from all sides, it is more difficult to do a multi-cavity mold. Only a small model can be considered to make a cavity mold. However, arrangement of gate is rather cumbersome. Generally speaking, this product has three kinds of feeding methods: 1 hot runner, 2 nozzles, and 3 gates. However, since injection molding machine is relatively small and total thickness of mold is limited. From this point of view, I think that large gate is preferred because mold thickness of large gate is the smallest.
Although big gate is the simplest, but where is glue point? Since optimal position of gate for product is concave, ordinary large gate will be difficult to realize, cutting is also difficult. If large gate is abandoned, gate or hot runner is made, height of injection molding mold is large, obviously It is not reasonable, also cost of mold is increased a lot. Injection molding machine is also larger, which directly increases production cost. Therefore, large gate must be considered. After a period of consideration, I envisioned a variant of large gate, see picture below.

 

As can be seen from above figure, round boss of round pit at upper end of molded product is directly formed as a part of sprue bushing, so that total length of gate is only 10 mm, small head is 2.5 mm, and big head is 3 mm. At this time, resistance of injection molding is the smallest, resistance to demoulding is also the smallest, and it is easy to cut off with a special knife. With this gate design, fixed mold is greatly simplified, thickness of fixed mold is also greatly reduced, which is suitable for small machine type production. To make room for a reasonable demolding mechanism for moving mold. After mold flow analysis, design of this gate is very reasonable, see picture below.

 

Thread core is hollow. For compact structure of mold and sufficient strength of thread core, I have integrated gear and thread core. Other end of thread core has a guiding thread, lead is consistent with lead of shaped thread, see figure below.

 

Guided threads match custom nuts. When gear on thread core is driven, guiding thread rotates while thread core rotates, so that entire thread core side is rotated backward, because nut have a hardness, and reasonable hardness is HRC48~52. At such hardness, processing of internal thread is very difficult. In order to facilitate processing and assembly, I divide nut into two halves. After second half is finished, it is integrated into a corresponding hole. Material of nut is CrWMn, see figure below.

 

Thread is machined from electrode. Electrode of thread of machined nut is machined from a CNC lathe. Fixed end of electrode is milled to a reference surface. After half of nut is machined, electrode is reversed and other half of nut is processed. See figure below.

 

Demolding of this set molds is more difficult to understand. In the first step, before mold is opened, threaded core is driven by hydraulic motor to rotate and move along guide nut, so that thread is separated from product. At this time, thread core is not moving because head shape of thread core is a non-rotating body, as shown in following figure.

 

In the second action, movable mold pad and movable mold bottom plate are separated by 15 mm first, as shown in following figure.

 

As can be seen in above figure, threaded portion of thread core has been unscrewed. Since nylon pull stud is provided on parting surface, main parting surface is not opened first when mold is opened, but 15mm is opened between moving mold bottom plate. At this time, four sliders also tightly wrap product and move forward 15 mm together, so that non-rotatable portion of threaded core is disengaged. Then main parting surface is opened, and two large sliders are separated due to action of inclined guiding column. Third action is to extract cores on both sides of hydraulic cylinder on two sides of mold, as shown in figure below.

 

Since first mold opening is only 15mm, product has been loosened from thread core, but some of thread core extends in the middle of product. This will make product not stick to slider when slider is separated. If second action is opened much more, non-rotating inner core inside threaded core completely leaves product, product may move with core when cylinder pulls core on both sides. That is, it sticks to core and robot cannot pinch product. Fourth action is to take product out with a robot. Entire four movements are coherent.

Core of concept of this mold is lower mold. Lower mold I am talking about here does not refer to entire moving part, but refers to a single part. I haven't seen it before, I haven't designed such a part. It's not a moving mold, but it is also involved in molding. It is core structural part of this set molds. I don't know how to name it. But this is a "whispering idea." I will call it a multi-functional model for time being. See picture below.
Its assembly condition in moving part is as follows, see figure below.
Why is this lower mold called a multi-function lower mold? Let's see how many functions it has.

Part of lower mold is to participate informing, bottom of product is formed by this multi-functional lower mold, as shown in figure below.

 

 

As mentioned above, in second movement of mold, movable template and movable mold base are separated by 15 mm. It is quickly placed on movable template, multi-function lower mold is arranged on movable mold bottom plate, that is to say, in this movement, slider will be separated from multifunctional lower mold by 15 mm. When mold is closed, slider has to be returned to multi-function lower mold. This requires precise positioning between slider and multi-function lower mold, and must be inclined on three sides. I have designed 4 grooves with a slope on multi-function die, see figure below.

 

There are also corresponding oblique bumps on slider, see figure below.

 

 

As can be seen from above figure, movable movable mold and movable mold bottom plate need to be accurately positioned, but in the second movement of mold, moving movable mold and movable mold bottom plate are separated by 15 mm, so I designed lower part of multi-functional lower mold to be inclined with inner side of movable mold, so that it can be precisely positioned during mold clamping. Moreover, there is no friction with multifunctional lower mold when movable mold is separated.
Multi-function lower die is fixed on the bottom plate of movable mold, as shown in figure below.

 

As can be seen from above figure, square groove on the bottom plate of movable mold is precisely matched with two squares at the bottom of multifunctional lower mold, thus ensuring precise cooperation between various parts of entire movable mold part, and no mold loss due to friction occurs when moving mold is separated by 15 mm.

 

As can be seen in above figure, joint surface of bottom surface of slider and upper surface of multi-function lower mold is inclined by 3°. It is a detail problem in molding design, and it is a very important detail. Many young plastic injection mold designer don't pay attention to this problem, consequences will be very serious. Because bottom surface of slider is shaped, if joint surface has no slope, joint surface wears quickly and produces a flash on the product.
Why is it worn out quickly? Reason is very simple. Locking of slider is caused by locking block on fixed mold pressing inclined surface of slider during clamping. This squeezing force generates downward pressure, bottom surface of slider is pressed against upper surface of multifunctional lower mold. If opposite direction is parallel to moving direction, friction is generated because magnitude of friction is determined by positive pressure and coefficient of friction, so positive pressure generated from clamping force of injection molding machine generates a large friction force. In this case, wear on the bottom surface of slider and multi-function lower mold is fast.
Now I design opposite side as a bevel, situation is completely different. In process of slider movement, opposite side is empty, only in last moment of injection molding machine clamping, opposite side is encountered. If injection molding mold is made accurately, let opposite side leave 0.008mm for exhausting. Then it is better, and can avoid wear at all.
It can be seen that role of this multi-functional die in entire mold is very important.

Inner core of thread is not rotatable. It is based on relative movement of movable mold and movable mold base to demould. Power is generated by nylon rivet. It is fixed to bottom plate of movable mold. Since part of product facing core of thread is very thick, core of thread must be sufficiently cooled. See picture below.

 

For ease of assembly, pressure plate of threaded inner core is divided into two halves and fixed by four hexagon socket screws. In order to prevent rotation of inner core of thread, step at the last end of thread has two facets, as shown in following figure.

 

As can be seen from above figure, bottom of core of thread is convenient for assembly, diameter of outer circumference of two sections is uniform, otherwise it cannot pass through thread core. In this way, position of bottom surface of thread core is very tight, and it is very troublesome to arrange "O" type seal. See picture below.

 

This copper seal is a good method in this case. It is 0.1 higher than bottom of thread core and can be sealed when screw is tightened.

Design of large slider is shown in figure below.

 

Design of large slider is traditional, but I have placed two positioning blocks here, which is original one, but it is waste utilization. However, it is stuck in multi-function lower mold, which can be accurately positioned to reduce friction between large slider and track.
As mentioned above, movable mold base plate and movable movable mold are separated by 15 mm due to action of nylon rivet, and all sliders are arranged on movable mold, that is, slider has a relative movement of 15 mm between multi-functional lower mold before mold is opened, so positioning block on slider has a small upper slope.

Small slider is pulled by cylinder, see picture below.

 

As can be seen from above figure, core is driven by a small slider, which is pulled by a cylinder mounted on cylinder frame, which is relatively simple and traditional.
Nine, design of transmission mechanism of rotary core
Design of transmission mechanism of threaded core is shown in figure below.

 

As can be seen from above figure, oil motor drives a pair of bevel gears, and second bevel gear is coaxial with transmission gear. Transmission gear drives gear on thread core to rotate thread core. Since threaded core has a guiding thread at lower end, pitch of guiding thread is consistent with pitch of product ( shrinkage rate has been released), so threaded portion of threaded core will be unscrewed from product. Since threaded core has up and down movement, thickness of gear on threaded core is relatively thick.