In this article, product to be analyzed is glove box casing of a car. Material is reinforced grade PA66+GF30%. Internal structure of this product is extremely complicated and shape is also complicated. Shape of product should be demoulded by two-side integral slider. Inner part is demoulded by inner core, and ejection distance is long. Total thickness of (138mm) mold is large, and it is produced by a relatively large injection molding machine (Haitian 350 ). If 200-ton injection molding machine can be formed according to weight of product, how to greatly reduce mold thickness while ensuring normal production of product is a challenge to plastic injection mold designer.

 

Figure 1

This product is the main product of a injection moulding moulds manufacturer and has been in production for some time. Since original mold structure is not very reasonable, it is very difficult to take product. Output per shift is very low, and rate of defective products is also high. Main reason is that original mold structure can not arrange secondary ejection, so it is difficult to take product. Operator should shake product to take product and it is easy to injure exposed plastic portion in lower part of product. Secondly, thickness of auto molding is very large (700mm), which is expensive to produce with a large injection molding machine. To this end, we have redesigned mold.

Because raw material of product is with 30% glass fiber, fluidity is very poor. Original mold uses 4 hot runners to feed glue. Diameter of glue point is about Φ3mm. It is not a problem because of this situation. Face of glue is assembled inside dashboard of car and does not affect any appearance. In fact, one of main reasons why total die thickness of auto molding is too thick is use of a hot runner system. Thickness of mold cannot be lowered. After consideration, we have adopted such a gate, which is exactly same as glue of old mold. It is also glued at 4 points, and position of glue is same as original auto molding. But it belongs to large nozzle structure (see Figure 2).

 

Figure 2

Figure 3 below is a mold flow analysis provided by Moldex 3D.

 

  Figure 3

Finally, joint surface of large slider and fixed mold is used to make cross flow path. In order to move glue point to center line of product, we arrange two bosses on bonding surface of large slider and fixed mold insert. In this way, glue point is moved to center line of product (see Figure 4), and hot runner is eliminated. After changing to such a gate, total height of mold is reduced by 120 mm, and injection molding machine is changed from 350 tons to 220 tons. Plastic mold manufacturing cost of mold was also reduced by 40,000 yuan (cost of hot runners for hot runners). Ingeniously, two pieces are raised on large slider, and glue point is moved to middle of product, which greatly reduces difficulty of plastic mold manufacturing.

 

  Figure 4

Large slider is composed of a slider body and a slider seat. Slider body is made of NAK80, and slider seat is made of CrWuMn, which can reduce manufacturing cost of mold. Plastic mold manufacturing is also more convenient, let's look at opposite side of slider body, I have a good layout here, see Figure 5.

 

  Figure 5

As can be seen from Figure 5, two bosses are added to rectangular boss on opposite side of slider body. Why?
Because in order to extend service life of auto molding, slider body uses more expensive steel and heat treatment. In order to save cost of auto molding, slider body is made thinner, the thickest place is 43.6mm, and 15mm positioning step is removed. Remaining 28.6mm. It is more difficult to arrange screw connected to large slider seat, and threaded bottom hole cannot be drilled too deeply, so that actual effective number of teeth of thread is not ideal. Because sealing ring of cooling water passage of whole large slider system is pressed by seven M12 hexagonal threads, if effective number of teeth of screw is not enough, screw can not be tightened too much, or it will slip. Now that these two small square bosses have been added, actual length of thread has increased by 15mm, so there is no worries. From point of view of manufacturing cost of mold, large slider now increases reliability, but does not increase material and plastic mold manufacturing cost. This is a small detail of mold design, but accumulation of these details makes injection moulding moulds manufacturer gradually improve design level of plastic injection mold designer in design process.

As can be seen from Fig. 6, slider body and slider holder are connected by seven M12 hexagon socket screws, and a seal ring is arranged at the position where water is connected to these two.

 

  Figure 6

Melt temperature of PA66 is very high, and shape of product is complicated, and assembly size is required to be stable. Therefore, position of cooling water hole arrangement of large slider system is relatively important. In actual production, mold temperature machine is used, and temperature is constant at 110℃. Now I arrange waterway on slider body, which is led out by slider seat through sealing ring and arranged on two sides of mold. This plastic mold manufacturing is more convenient.
Power of large slider to be opened is force of mold opening by injection molding machine, and large slider is outwardly opened by inclined guide column on fixed frame. Inclined guide column that pushes large slider passes through fixed mold. Gate of large slider and large-sized insert of fixed mold forms a gate, and nozzle of injection molding machine is directly placed on fixed mold. This greatly shortens main flow path and reduces injection pressure during injection molding. Alternating stress experienced by mold itself is reduced, so that life of auto molding is greatly extended.

Fixed mold is designed as a whole without any inserts, see Figure 7.

 

Figure 7

Inner surface of fixed mold is attached to large slider body, and a transverse flow path is formed on bonding surface, and then divided into four flow passages (see Fig. 8). Design of fixed mold is very conventional, but in order to reduce stress after injection molding, design of cooling system is very critical to injection moulding moulds manufacturer.

 

Figure 8

It can be seen from Fig. 8 that in addition to four waterways in horizontal direction, central protruding part is also arranged with a “water pond”, so that water path of whole moving mold is relatively uniform and heat exchange is relatively balanced to minimize residual stress in product.
In design of small slider, other side of product is more complicated in shape, and we use a whole slider (see Figure 9). Small slider consists of a slider body and a small slider seat, which is fixed by screws and presses sealing ring to form a combined water path. Small slider is also tilted by inclined guide post. Unlike large slider, oblique guide post is fixed on a dedicated oblique guide post. See picture.

 

Figure 9

Such design processing and assembly are relatively simple. Limit of small slider is realized by two limit blocks, see reverse side of small slider seat (see Figure 10).

 

Figure 10

Appearance of moving mold is the most exciting part of this set of molds. It is quite difficult. Let us first look at appearance of the whole moving mold (see Figure 11).

 

Figure 11

Principle of inner shrinking core (see Fig. 12), inner shrinking core is divided into two parts, middle part is fixed on movable template 2, and moves along with it also. Two sides are arranged on movable die plate 1. Slope of "T" shaped track on the side of lower constriction slides inward as it moves downward. In order to ensure movement of shrinkage block is reliable and has a long life, we have designed a series of corresponding friction mechanism and fixing mechanism, which will be described in detail below.

 

Figure 12

1, Layout of template 1 (active template). In fact, movable template 1 of activity is a special push-plate mechanism, and all kinds of parts are attached to push plate (moving template 1), and only lower contraction core is fixed on movable template 2. Push plate (moving template 1) is opened by plunger of injection molding machine (see Figure 13). Since inner shrinking block has a diagonal "T" block, when inner shrinking core is retracted, inner shrinking slider has an inward movement.

 

Figure 13

2, Design of inner slide slider. Since inner retracting slider only needs to slide 3mm inward, it is enough that inner retracting core only needs to be retracted by 35mm. However, since inner part of product is relatively deep, in fact, moving template 1 needs to be pushed away by 138m to release product. This is a contradiction for retracting slider, because if retracting slider is 138mm forward, two retracting sliders will "figh". How to make retracting slider move only 3mm inward and 138mm forward. Solution to this problem can be seen in Figure 14.

 

Figure 14

To be on safe side, we set retraction slider to move inward by 4mm. Bottom of retraction slider is fixed with a friction plate. When clamping, a protective cross-type friction block has a small step with 4mm wide, which is placed against friction plate connected at the bottom of retractable slider, so that retracting slider does not retreat during injection. As can be seen from Fig. 14, upper part of "foot" of retracting slider has a part called "upper friction piece", which ensures that retracting slider moves smoothly inward, and at the same time, ensures precise positioning of retracting slider during resetting. This "upper friction plate" has another effect. We see a part in Figure 14 called "needle type cylinder", which is used for secondary demoulding, which will be described in detail later. This "upper friction plate" is a "needle type cylinder" fixed block and a compressed air passage.
When lower shrinkage block and retracting slider move relative to each other, due to blocking of small step of 4 mm wide, retracting slider can not be downward, and can only slide inward. When lower constriction is finished 45.72 mm, retracting slider just goes 4mm.
When retracting slider slides 4mm inward and small step is exceeded, friction block at the bottom of retracting slider stops moving inward due to fact that there is nothing blocking retracting slider, and moves downward along with constriction.
When auto molding is closed, inner shrinking core moves forward. Due to frictional force, inner sliding slide will move outward while moving forward. At this time, protective cross-shaped friction block will block restrict slider so that it can only be upward and not outward. When lower shrinking block continues to move upward with shrinking block until foot of inner shrinking block leaves 4 mm step on cross-shaped friction block, lower shrinking block continues to ascend due to blocking of upper friction plate, and inner shrinking block is forcibly to move outward 4mm until final position.

 

Figure 15

This is a very clever design inspired by cross in church. When inner shrinking block moves forward with downward contraction, final position is positioned by small plate, name is called upper friction plate, and is also pin cylinder fixing plate. This small plate has two functions: one is final positioning of retracting slider, and the other is to arrange a set of needle-type cylinders for secondary ejection of product. Since this set of molds does not have a ejector plate and a foot, secondary ejection of product is very difficult. This needle cylinder is a good thing, and it can be ejected at any angle in any part of mold.

 

Figure 16

This is the case in the mold (see Figure 15), so that product can be clipped out with a robot for fully automated production (see Figure 16).

This set of molds has several highlights: first, it is a very headache auto molding to injection moulding moulds manufacturer, which can now be fully automated, and injection molding machine used is smaller, saving energy; secondly, it eliminates hot runner and uses a large nozzle to save plastic mold manufacturing cost of mold. Bright spot is that two bosses are added on joint plane of large slider and fixed mold insert to move glue point to the middle of product; thirdly, it uses a cross-shaped protective friction block to make action of retractable slider safe and reliable.