With rapid improvement of people's living standards, people have higher and higher requirements for injection molding products, which poses higher challenges to injection molding companies. Vacuum technology can significantly reduce filling injection pressure for injection molds, avoid problems such as trapping air, get rapid promotion and application in applications where filling is difficult. This article will introduce main points of vacuum mold through actual cases.
Figure 1 is an axonometric view of decorative cover plate of opposite sex, which is formed of ABS plastic. Technical requirements of plastic parts are: smooth surface, no pores, shrinkage and other defects. It can be seen from Figure 1 that wall thickness of plastic part is uniform, which is good for filling, but wall thickness is thicker at 5 BOSS holes. Surface is prone to sink marks and pores inside, which will affect injection cycle. Cooling here must be enhanced to improve efficiency. In this paper, vacuum method and process of filling wall thickness at pressure-holding and feeding stage are used to solve sink marks and internal shrinkage defects, cooling time can be shortened, killing two birds with one stone.

 

Comprehensively consider technical requirements of plastic parts, feasibility of molds and injection molding. Vacuum mold is specially used, which can reduce injection pressure, low injection pressure can ensure a small injection residual stress, ultimately ensure that product has a small deformation and warpage. Moreover, cavity is in a vacuum state after being evacuated. With filling of melt, there is no other gas source except decomposition of melt itself which may generate gas. Controlling material temperature and filling speed will improve air trapping problem. Furthermore, in order to achieve a smooth surface, mold temperature must be connected, mold temperature must reach 80℃. Therefore, it is necessary to consider issues such as thermal expansion, mold matching and heat transfer.

Taking into account appearance of product cannot have a gate, gate is designed on the back of product. Considering factors such as flow length, deformation and warpage, filling, etc., a large nozzle is specially used to directly enter center of back of product, so that 4 corners can be filled at the same time, reducing product deformation and warpage caused by unbalanced filling and holding pressure. Center of back just needs to be labeled to cover gate, so that it does not affect aesthetics of product.

Design of cooling water circuit should make mold cooling evenly. Unbalanced cooling can cause deformation. Therefore, in design of waterway, arrangement and method of waterway on movable mold side and fixed mold side are basically same, which can ensure equilibrium of temperature of movable mold and prevent product from being deformed. Diameter of water passage is designed to be ф12, distance from product is balanced to be controlled between 1-2 times diameter of water passage. Some parts of movable mold side are far away from product. Situation of adding water barriers is used to improve cooling effect of water passage.
Sprue bushing is a place where heat accumulates. If it is cooled, cooling time can be greatly increased. Mold is equipped with a cooling water circuit design at sprue sleeve. Note that there is a detail here, cooling must remove air gap in heat transfer channel, otherwise cooling effect is very low. Fitting clearance between sprue bushing and sprue bushing insert is controlled unilaterally at about 0.04-0.1, then heat-resistant grease (a substance that improves thermal conductivity) must be coated during assembly, which can improve heat transfer between sprue bushing and insert. Of course, if cost permits, 3D printed conformal cooling sprue sleeves can be used. I believe that with maturity of 3D printing, this sprue sleeve will be popularized. Since mold must be produced at a temperature of 80℃, movable and fixed mold sides must be designed with heat insulation boards (bakelite material).

Due to gate location restrictions, mold must be in the form of flip-chip molds. That is, ejector plate part is in front mold, and ejection cylinder must be ejected. Furthermore, wall thickness of product BOSS hole is thicker, and this set of molds is injection molded with a special process. Thimble plate must be pushed forward during injection, so that wall thickness at BOSS hole is balanced. In this way, mold can not be arranged with thimble, and air cap is used to assist ejection. Final product is sucked by robot hand, which can realize automatic production.

Vacuum mold has following advantages:
Improve filling effect of product, eliminate influence of air on molding, and fundamentally solve problem of poor exhaust.
Vacuum cavity of mold is easy to fill, which reduces injection pressure and back pressure of injection molding machine.
Speed up injection speed, reduce molding cycle, and improve production efficiency.
Improve product weld line and improve product quality.
Eliminate product defects, burnt, and air bubbles.

5 BOSS holes of plastic part have thicker walls. Surface is prone to sink marks and pores inside, which will affect injection cycle and product yield. This mold adopts injection molding process in which ejector plate is ejected during injection, and ejector plate is retracted when pressure is maintained. Its principle is to realize process of compensating remaining material to wall thickness by means of pressure maintaining and feeding stage. See Figure 2 for details.
(1) In injection and filling stage, ejector plate is pushed to the bottom by oil cylinder to achieve uniform wall thickness injection. As shown in enlarged view of Figure 2A. Under action of pushing thimble plate forward, sleeve sleeve protrudes to same height as cavity, making product wall thickness uniform. It is easy to fill, and will not cause unevenness of surface light color due to instability of melt wave peak due to convex change of product wall thickness.
(2) In pressure-holding stage, ejector plate is retracted to position under pulling of oil cylinder, and sleeve sleeve is retracted downward to make room for glue position of BOSS hole. Finally, material in BOSS hole is filled in by pressure maintaining and shrinking stage. As shown in enlarged view of Figure 2B. Glue position at grid section line is pressed in to achieve final product shape. During injection molding process, glue position of thickest BOSS hole near cavity side has been solidified first, so that surface of product will not shrink, possibility of shrinkage can be greatly reduced, finally yield of the product is improved.
(3) Figure 2C enlarged view of final product ejected state view, supplemented by air cap. Finally, pick-up work is completed with help of manipulator. This is reason why product cannot be equipped with thimble.

 

For precision, thin-walled, large flat parts or plastic parts with complex results, influence of air on molding must be eliminated. Otherwise, product will have defects such as unstable quality, insufficient injection, high injection pressure, deformation, and poor surface quality. When vacuum injection molding is used, surface of plastic part is smoother and molding quality is better. Therefore, application prospect of vacuum injection mold is very broad. For example, emergence of smart phones requires thin walls of products, and some glass fibers are added in order to increase strength.
Generally, conventional injection molding equipment and technology must be produced by high-speed injection molding machine, injection molding process must be adjusted accordingly, high-speed and high-pressure injection must be used. This brings a great test to mold exhaust in a short time. So many must be completed with vacuum. Injection molding is a complicated process, and many factors must be considered in order to produce superior quality, stylish and beautiful products.