A set of classic molds, difficult in the details!

One, product analysis

This is a medical dropper, which was commissioned by a friend in United States. It has some difficulties and has not been manufactured successfully by several mold factories. Difficulty is that head has a horizontal circular tube, which requires high roundness. It is equipped with a rotary valve and cannot leak liquid. Dropper in the head is 27mm in length, but diameter is very small, only xn--3-5mb.2mm, which is prone to eccentricity during injection.

As can be seen from above figure, there is a convex rib inside round tube of head, which is used to fasten rotary valve. Rotary valve is installed in dripper, as shown in figure below.

Geometry of this product is not very complicated, but mold structure is very complicated. It is mainly difficult to control concentricity between cavity of fixed mold and core of moving mold, and deformation of product caused by pulling of slider when fixed core is pulled, product will be adsorbed on slider. As can be seen from figure, small head of product has a small round tube, which is used to install rotary valve. In sectional view, it can be seen that core of mold is long on one side and short on another. Productis bound to the slider on long side of core. From overall point of view of product, it is difficult to control uniformity of wall thickness of product at large-end cylindrical portion.

Two, concept of overall mold structure

How to avoid mold core drift and elastic deformation in design of mold structure? How to prevent product from running to the side when fixed mold slider moves out? How to arrange feeding form is the key to this case. Through thinking and reasoning, overall structure of mold should look like this, see figure below.

Three, design of moving core

Key point of design of movable mold core is to control rigidity of movable mold core and minimize its elastic deformation. It is considered in two parts, one is fixing method of movable core and mold plate, and the other is consider from steel and heat treatment status.

In the picture above, we can see that fixed part of mold core is longer than molding part. According to my experience, this type of mold must reach such a ratio (not less than 1: 1), otherwise moving mold core will be easily biased by molten plastic. Inside of moving mold core is cooled by water, a water baffle is used to enter and exit water from bottom plate of moving mold.

Hardness and rigidity of moving mold core are also very important. We adopt 8407, and hardness after heat treatment is HRC48. Because product is transparent, surface is polished and chrome-plated (thickness of product is precisely controlled by thickness of coating). Not only that, positioning and tightening of moving mold core is also very important, as shown in figure below.

Positioning sleeve in figure above is tightly pressed into core platen. Positioning sleeve and core platen can also be made as a whole (but it is troublesome to process). We can see that cooperation between core platen and concave wall of movable mold plate is beveled on all sides, inner wall of positioning sleeve and movable mold core are also beveled. In this way, when conical surface of core pressing plate and positioning sleeve tightly presses conical surface of movable mold core, positioning accuracy and rigidity of movable mold core will reach optimal state, as shown in figure below.

As can be seen in above figure, because peripheral inclined surface of core platen is closely matched with inclined surface of concave wall of movable platen, six 10mm hexagon screws can firmly fix movable mold core to movable platen. At the same time, outer circle of blue positioning sleeve in figure above is also positioning of movable mold slider. In addition, main point of processing core platen is that after positioning sleeve is pressed in, inner and outer inclined surfaces are processed at one time. Otherwise, positioning accuracy of movable mold core cannot be guaranteed, entire mold may be severely misaligned.

Four, design of movable mold slide track

For this type of structure, track of slider is “sticked” with hardened materials. I don’t agree with use of thick formwork to dig grooves, because it will deform formwork, seriously affect positioning accuracy of guide posts and guide sleeves. Some manufacturers have to dig grooves in template without attaching hard film. It is possible to test mold, but life of mold is not long. If groove is added and then hard film is added, volume of mold will be enlarged, which is not cost-effective. Therefore, it is an economical practice to attach a hardened track to a flat moving template. Generally speaking, movable mold has a large slider structure, and all functional accessories must be on this track surface, which is often in a tight position. Therefore, we must take into consideration when arranging position, because it is hardened and it is difficult to change, as shown below.

On the track in the picture above are arranged: nylon plastic nails, hex screws, fine positioning, two-section sliding guide sleeve and other parts. This two-section sliding guide sleeve also plays role of precise positioning of orbit and moving mold plate, as shown in the figure below.

Five, design of limit screw

This medical dropper product is relatively long, effective ejection distance is 110mm, so product can be smoothly removed, but space for setting limit screw is only 50mm, (bottom plate + core plate-screw head-step thickness). In other words, space for limit screws is not enough. General treatment method is to add a limit pull plate outside mold, which is more troublesome, and fixing screws of pull plate often break, which often affects production. Here I introduce a very simple and convenient method. For your reference, see figure below.

I added a sleeve to position where limit screw was originally set, and everything became very simple. This method can be extended. If you need to eject a longer distance, you can make a double-layer extension sleeve, or even three layers. For detailed structure, please see figure below.

Six, design of moving mold slider

This slider is a very traditional design, I made a little improvement here, see figure below.

Generally mold designer uses a bevel on the outside of slider, but I leave a little plane here. Advantage of this is that it does not increase processing cost, but with this plane, spark machining is very convenient. In addition, Water joint can be made on this plane, avoiding trouble of tapping thread on inclined plane. Power for sliding slider is realized by inclined guide post fixed on fixed mold side, as shown in figure below.

Seven, design of precise positioning between moving and fixed molds

Design of precise positioning between moving and fixed molds is shown in figure below.

Precise positioning of movable mold side is directly performed on movable mold rail. Movable mold rail is made of Cr12Mov, and hardness is HRC52 ~ 56. Precise positioning of protrusions on fixed mold side is insert type, see figure below.

This precise positioning is very important. It directly determines concentricity of inner and outer circles of product. Method I take is to directly process precise positioning based on cavity after moving and fixed molds are assembled.

Eight, design of gate

Gate setting of this part is a bit special, different from other "test tube" type molds, because diameter of through hole at the end of product is very small, it is not rigid at all, and it will be biased by molten plastic flow. So I used a special gate, which is a variant of "claw" gate, see figure below.

This claw gate is disconnected from cross flow channel when it is ejected from mold and will be connected to product. In the future, there will be a gate cutting process and a special cutting tool. This seems to be troublesome, but I think it is necessary to get a qualified product. I have seen samples of unsuccessful mold trials in the past. Latent gate is used. Although gates will break automatically, residual stress of product is too large. Roundness of round tube on the head is not stable. Although I want to pursue perfection, purpose has not been achieved. I would like to exchange design ideas with young mold designers through this case. Following figure is mold flow analysis result of Moldex 3D company.

The first consideration when designing mold is to ensure that products produced by mold are qualified and acceptable to customers, and second is to reduce costs as much as possible on the premise of ensuring a reasonable life of mold. Look at fixed mold small core, diameter is only 3.2mm, see figure below.

I designed it this way, although diameter of fixed mold core is very small, but because it is clamped by fixed mold slider, it has great strength at the root, which is equivalent to "simple supported beam" calculation formula in material mechanics. Length of support beam is zero. In theory, strength of small core at this point in time is infinite. Then when molten plastic flow enters from end face, hold small core to inject, advanced plastic will hold core tightly, filled plastic makes core have strength in an instant. I have made many similar mold, this kind of gate has very good effect.

Nine, design of fixed mold slider

Fixed mold slider is parted in this way, outer parting line must be in the middle, and inner parting surface must be at the step of core. See below.

According to characteristics of product, outer parting surface of product must be in the middle. However, because side core is long and short, slider 1 is divided as shown below.

lider pins corresponding to slider 1 are relatively short, but slider pins of slider 2 are relatively long, as shown in figure below.

It is certain that when two sliders are separated, product will stick to fixed mold slider 2 and will be pulled along with slider 2. Due to moving mold core hanging in the middle of product, product will definitely be strained. Because of characteristics of shape of fixed mold side slider 2, inner and outer profiles are on one side, and slider needle is not allowed to have a stripping slope (because it is equipped with a rotary valve). This is also main reason for failure of previous molds. Therefore, I set a thrust device on fixed mold side slider 2. First, there is a push tube. When slider is separated, it is immovable, hold against product and forcibly leaving product in the middle, see figure below.

In the figure, thrust tube is fixed on fixed mold, and it is immovable relative to product. Slider 2 inlay is fixed on and moves with slider 2. This can effectively prevent product from moving with slider 2 during demolding, which causes product to be strained. Thrust tube on slider 2 is thus fixed, see figure below.

As can be seen from figure above, thrust tube is immovable relative to fixed mold plate, and it is directly fixed to fixed mold (A) with screws. However, inlay is fixed on inlay fixing block which is fixed on slider 2 with screws. It will move with slider 2 and separate from product, as shown in figure below.

This is a very classic slider thrust mechanism. Taken together, this is shown in figure below.

Ten, positioning of fixed mold slider

Generally, large slider mechanism of such an open track easily generates unstable positioning of slider, because slider will be squeezed by inclined surface of locking block, and a very small amount of displacement will also have a great impact on appearance of product. Especially shape of this product, taper part is on fixed mold slider, and straight part is on fixed mold large insert. If deviation is 0.05mm, product shape will be ugly. Therefore, in this set of molds, positioning mechanism of fixed mold slider plays an important role in appearance quality of product.

This positioning mechanism is designed in this way. It has a very simple structure, but it plays role of precise positioning of fixed mold slider and fixed mold kernel, and also makes precise positioning between fixed mold slider 1 and fixed mold slider 2. ,See below.

Eleven, cooling of fixed mold slider

Cooling of fixed mold slider is shown in figure below.

From figure above, you can clearly see cooling water path setting of fixed mold slider. Round pipe with high roundness requirements mentioned above is formed on fixed mold slider. Arrangement of cooling system of fixed mold slider directly affects roundness of circular tube. In fact, cooling system of fixed mold slider not only plays a cooling role (strictly speaking, it should be called a heat exchange system). I think that in order to increase stability of roundness of round tube mentioned above, fixed mold slider must maintain a stable mold temperature, which should be about 50℃. Cooling system of fixed mold slider uses a mold temperature machine, set temperature is 48℃, achieved very good results in actual production.

Twelve, design of fixed mold core

Structure of fixed mold core is shown in figure below.

Design of fixed mold core is very traditional. There is nothing special. As can be seen from figure above, there are three layers of cooling water channels. Because I take into account characteristics of product, cooperation between product and another part (rotary valve) is required to be watertight, and roundness of product is relatively high. Therefore, we must first ensure that mold temperature is very stable, then consider how to shape product. Cavity processing of fixed mold core must be positioned with two large sliders before processing. In final polishing, fixed mold core and two fixed mold sliders are positioned together, otherwise it is difficult to ensure that bonding line is in good position.

Thirteen, take out gate

Gate is taken out in a special way, see figure below.

Judging from above picture, gate cannot be taken out. It will stay in the middle of two sliders. This is very troublesome. But unexpectedly, there was an unexpected surprise. When gates were pulled against each other, one side broke first. This is for sure, because connection between claw gate and cross runner is a point gate. It is a cone with a small end diameter of Φ1mm and a taper of 30 °. Unbroken side comes out with product, see figure below.

Overall structure of mold is like this, figure below shows mold open.

Summary:

This is a set of molds of great practical value. I have puzzled my friends for a long time, and now this problem is finally broken. However, it is not enough to just understand detailed structure of this set of molds simply by text and pictures. I just expressed my design concept and design ideas here, hoping to play a role in attracting new ideas.

Design features

Fixing method of moving model core ensures rigidity and stability of moving mold core, uniformity of product wall thickness;

Thrust structure of fixed mold slider to prevent product from sticking to slider is the key to success of this set of molds;

Precise positioning of fixed mold slider and fixed mold core ensures beautiful appearance of joint line of product appearance, which is characterized by extremely simple structure and convenient processing;

Design of set-type limit screws of movable mold expands ejection space of movable mold in disguise;

Disguised claw-shaped feed inlet effectively protects small core of fixed mold from being offset.