Case analysis: Funnel medical injection molding

Time:2019-03-18 10:13:31 / Popularity: / Source:

Case analysis: Funnel medical injection molding
  • Product analysis
Geometry of this product is not very complicated, but mold structure has certain difficulty, mainly to control concentricity of inner wall, and deformation of product caused by pulling of slider when core is fixed.
This type of product has a wide range of uses, especially in the field of medical devices. Generally, for plastic injection molding factory that has not made a similar product, if plastic injection molding factory does not attach great importance to its characteristics, it is difficult to control wall thickness of product.
How to avoid drift and elastic deformation of core in plastic injection mould design of mold structure for this medical injection molding? How to avoid product not running sideways when fixed mode slider moves outward? This is the key to this case.
Illustration: 3D drawing of product:
Overall structure of mold structure
1.01 Dynamic mold core design.

The key is to control rigidity of moving core and minimize its elastic deformation.
As can be seen from above figure, fixed part of core is longer than molded part. According to my experience, such a mold must reach such a ratio (not less than 1:1), otherwise movable core is easily deflected by molten plastic. .
Inside of movable core is made of cooling water, and water is taken in and out of bottom plate of movable mold by water blocking piece.
Hardness and rigidity of dynamic core are also very important. We take 8407 and hardness is HRC48. Since product is transparent, surface is polished and titanium-plated. Not only that, but positioning and fastening of dynamic core is also very important.

1.02 Core plate design
Positioning sleeve in above figure is pressed into core platen, and these two can also be made into a whole. (But processing is troublesome.) We can see that core plate is matched with concave wall of movable template, and all sides are beveled. Inner wall of positioning sleeve and surface of movable mold core are also inclined, so that when core pressing plate is pressed tightly against movable mold core, positioning accuracy and rigidity of movable mold core are optimal.

As shown above, six 10mm hexagon socket screws can firmly fix movable core to movable mold. At the same time, outer circle of blue positioning sleeve in above figure is also positioning of movable mold slider. In addition, processing point of core plate is that inner and outer slopes are processed in one time after positioning sleeve is pressed in. Otherwise, positioning accuracy of movable mold core cannot be ensured, and entire mold may be biased.
1.03 Design of movable mode slider track
For this kind of medical injection molding, track of slider is attached with hardened material. I don't agree to use a thick template to dig groove, because it will deform, seriously affecting positioning accuracy of guide post and guide sleeve. If groove is not attached, life of mold is not long. If groove is opened and hard piece is added, volume of mold will be enlarged, which is not cost-effective. Generally speaking, moving mold has a large slider structure, and all functional mechanisms are to be on this orbital surface, and position is very tight. Therefore, we must take care of layout when it is placed, because it is quenched and it is very difficult to change. Track on the following figure is arranged with: nylon glue nails, hexagon socket screws, fine positioning, guide sleeves and other parts.

1.04 Limit screw design
"Funnel" product of medical injection molding is relatively long and effective ejection distance is 110mm, so product can be taken out smoothly, but space for setting limit screw is only 50mm (floor plate + core plate - screw head - step thickness). That is to say, space of limit screw is not enough. General treatment method is to add limit pull plate outside mold, which is troublesome, and fixing screws of pull plate often break, which often affects production. I am here to introduce a very simple and convenient method for reference. See below:
I added a sleeve to place where limit screw was set, and everything became very simple. This method can be extended. If ejection distance is longer, you can make a double-layered extension or even three layers. Please see figure below for detailed structure.

1.05 Design of movable mode slider
This slider is a very traditional plastic injection mould design, I made a little improvement here. General plastic injection mould design is a bevel on the outside of slider, but I have left a little plane here. Advantage of this is that it does not increase processing cost, but with this plane, spark machine is very convenient to process, and water joint can be on this plane, trouble of tapping thread on slope is avoided.

1.06 Positioning between fixed modes
See below :

Fine positioning on movable mold side is directly made on movable mold track, and dynamic mold track is made by Cr12Mov, and hardness is HRC52~56.

2.01 Gate design
Gate setting of this part is a bit special, unlike other "test tube" molds, because diameter of through hole at the end of roduct is very small, it is not rigid at all, and molten plastic flow is biased. So I used a special gate, which is a variant of "claw" gate, as shown below:

Although diameter of fixed mold core is small, due to clamping of fixed mold slider, there is still a great strength at the root. When molten plastic flow enters from end surface, core is fed inwardly by core, advanced plastic holds core. Filled plastic allows core to have strength in an instant. In calculation formula of material mechanics, L value of simply supported beam is at a minimum. However, amount of deformation of simply supported beam is proportional to fourth power of length L. We have done a lot of similar molds, and this gate has a good effect.
Once gate is determined, fixed-mode slider can be designed.
2.02 Fixed mold slider design
Fixed-mode slider is shaped like this, outer parting line is definitely in the middle, and inner parting surface must be at the step of core. See below:

According to position of slider, side of slider is short, as shown below:
It is certain that when two sliders are separated, product will stick to fixed mold slider 2, and will pull along with slider 2, and product will be pulled due to movable mold core hanging in the middle of product. Because of shape of fixed mold slider 2, inner and outer profiles are on one side, slider needle is not allowed to have a stripping slope, and wrapping force is large. So I set thrust device on the side of fixed die 2. First, there is a push tube that does not move when slider is separated, see figure below.

Thrust tube of slider 2 is fixed like this

As can be seen from above figure, thrust tube is stationary relative to A plate. However, insert is fixed to slider 2, which moves outward with slider 2 and is separated from article.
This classic slider mechanism is like this:

2.03 Positioning of fixed mode slider
Generally, large slider mechanism of such an open track can easily cause positioning of slider to be unstable, because slider is squeezed by inclined surface of locking block, and a very small displacement will also have a great effect on the appearance of product. In particular, shape of product, taper portion is on fixed-die slider, and straight portion is on large-sized insert of fixed mold. If deviate from 0.05mm, shape of product will become ugly. Therefore in this set of molds, positioning mechanism of fixed-die slider plays an important role in appearance quality of product.
I designed this positioning mechanism in this way, very simple structure, but it plays a precise positioning role of fixed-mode slider and fixed-die, and also makes precise positioning between fixed-die sliders 1, 2.

2.03 Fixed-mode slider water

2.04 Design of water transport of mold

Design of fixed mold is very traditional in plastic injection molding factory. There is no special place. As can be seen from above figure, there is a cooling water circuit divided into three layers. Because I consider characteristics of product, cooperation between product and another part (rotary valve) is required to be watertight, and roundness requirement of product is relatively high. Therefore firstly make sure that mold temperature is very stable, and then consider how to shape product. Cavity machining of fixed mold is to be positioned with two large sliders before machining. Final polishing is also like this.
2.05 Gate removal
Looking at picture below, it is reasonable to say that gate is not available. It will stay in the middle of two sliders. This is very troublesome.
However, unexpectedly there were unexpected surprises. When gates were pulling each other, one side was disconnected first, and side without broken side came out with products.
Overall structure of mold opens as follows:

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