Stacked injection molds are a new type of injection mold that is highly efficient, fast, and energy-saving, and are gradually being promoted and used in my country. Unlike conventional molds, the cavities of stacked injection molds are distributed on two or more layers, arranged in an overlapping manner, essentially combining multiple mold sets.
Typically, when injection molding machines are used with conventional molds, their injection volume and mold opening stroke are only used at 20%-40% of their rated capacity, failing to fully utilize the machine's performance. Compared to conventional molds, stacked injection molds only increase clamping force by 5%-10%, but can increase output by 90%-95%, significantly improving equipment utilization and productivity while reducing costs.
Stacked injection molds are best suited for molding large, flat parts, shallow-cavity shell parts, small, multi-cavity thin-walled parts, and parts requiring mass production.
I. Design Considerations for Stacked Injection Molds
Stacked injection molds, as a new type of mold technology, have undergone continuous development, especially with the integration of hot runner technology, making it a cutting-edge technology in the development of plastic molds today. Traditional mold design theories are no longer applicable to stacked injection mold design. Therefore, there is an urgent need to develop a completely new mold design theory to guide the design of stacked injection molds. The following will explain the key design points of stacked injection molds.
1. Maximum Injection Volume of the Injection Molding Machine
Stacked injection molds can use either cold runners or hot runners. When using a cold runner, the amount of plastic used for solidification in the gating system needs to be considered. When using a hot runner, which achieves non-return solidification production, the material in the hot runner plate and the central main nozzle does not affect the required injection volume of the mold and can be ignored. Therefore, the maximum injection volume of the injection molding machine should be determined on a case-by-case basis.
2. Injection Pressure of the Injection Molding Machine
The verification of injection pressure mainly checks whether the injection pressure can meet the molding requirements. For stacked injection molds, which mostly mold thin-walled plastic parts with large projected areas and long flow paths, higher injection pressure and injection speed are required during the filling process. Hot runner molds, due to their hot runner technology, can better transmit injection pressure compared to cold runner molds, thus requiring lower injection pressure. However, due to the increased flow path and projected area, they require higher injection pressure than single-layer cold runner molds. When verifying injection pressure, the injection pressure of the plastic part should be determined based on the injection molding process of various plastics and computer simulation flow analysis, and then compared with the rated injection pressure of the injection molding machine.
3. Maximum Clamping Force of Injection Molding Machine
The cavities of a stacked injection mold are arranged "back-to-back," theoretically allowing any number of stacks to be achieved on the same injection molding machine without increasing the clamping force. However, because the central main nozzle and manifold of a stacked injection mold increase the flow channel, the projected area of the plastic part plus the gating system on the parting surface is larger. Furthermore, the extended runner due to the stacking results in greater pressure loss than a conventional single-layer mold, leading to a corresponding increase in injection pressure and cavity pressure. Therefore, the clamping force is increased. When verifying the clamping force, increasing it by 10%-15% compared to the same single-layer mold is relatively safe.
4. Injection Molding Machine Opening Stroke
Stacked injection molds part and eject the plastic part at two levels. When verifying the opening stroke, for injection molding machines using a hydraulic-mechanical clamping mechanism, mold thickness does not need to be considered. However, when the stacked injection mold has a side-parting core-pulling mechanism, the influence of the core-pulling distance must be considered.
If a synchronous mold opening mechanism, such as a gear and rack or crank-connecting rod mold opening device with the same transmission ratio, is used, the stroke of each layer in a stacked injection mold is not limited by the height of the product. Its mold opening stroke is N times the maximum opening stroke of the layer in the multi-layer mold (N is the number of layers in the stacked injection mold).
5. Main Nozzle Length
The center main nozzle should not be too long or too short. This ensures that when the mold closes, the center main nozzle will not exceed the maximum distance the injection molding machine nozzle can retract or advance on the machine base. Since the center main nozzle moves together with the middle part of the mold during parting, it should be ensured that the center main nozzle remains in the fixed mold section after mold opening to prevent overflow from the head of the center main nozzle from dripping onto the wall of the fixed mold cavity.
6. Gating System
Stacked injection molds can use either a conventional runner gating system (i.e., a cold runner gating system) or a hot runner gating system. Hot runner gating systems can effectively transfer injection pressure, improving the molding quality of plastic parts and facilitating automated production. However, they have certain requirements regarding the type of plastic used, and hot runner systems are expensive. When using cold runner systems, the molding quality of the plastic parts is slightly lower, but mold processing is easier, resulting in lower costs. Therefore, the choice of gating system depends on the specific circumstances.
7. Mold Temperature Control System
Mold temperature is one of the important factors affecting the molding quality of plastic parts. In the design of stacked injection molds, it is essential to ensure consistent temperature control in each cavity. For stacked hot runner injection molds, to reduce heat loss due to heat conduction, the contact area between the mold and the hot runner plate should be minimized, and appropriate heat insulation pads should be installed.
8. Mold Opening Mechanism
To ensure uniform shrinkage of the plastic parts, the residence time (cooling time) of the plastic parts in each cavity should be equal. Therefore, stacked injection molds must ensure that the parting surfaces of each cavity open simultaneously. Gear and rack transmission mechanisms and mechanical linkage mechanisms are commonly used as opening mechanisms in stacked injection molds. The former offers better technical performance and is more economical, but the latter provides greater flexibility. Hydraulic-assisted mold opening makes it easier to control the opening time, but the structure is larger.
9. Demolding Mechanism
Based on the requirement of equal cooling time, stacked injection molds should eject the plastic parts in each cavity simultaneously. Spring or high-pressure air demolding mechanisms can achieve this.
II. Development and Application of Stacked Injection Molds at Home and Abroad
As early as December 1940, KNOWLESER obtained a patent for stacked molds. Today's stacked injection molds are not only cheaper than traditional single-layer molds, but also increase the flexibility of their application. After decades of research and development, stacked injection molds have evolved through structural changes, including cold runner double-layer injection molds, hot runner double-layer injection molds, 3-layer or 4-layer stacked injection molds, right-angle gate hot runner stacked injection molds, and rotary stacked injection molds.
1. Development Trends of Stacked Injection Molds Abroad
Stacked injection mold technology started earlier and is relatively mature abroad. Well-known stacked injection mold companies include Tradesco, Ferromatik Milacron, Foboha, and Engel. Due to the rapid development of hot runner technology abroad, hot runner stacked injection mold technology is widely used overseas. Furthermore, developed countries are at the forefront of new stacked injection mold technologies, and the recently developed rotary stacked injection mold technology has broadened the application capabilities of stacked injection molds.
In the 1960s and 70s, some foreign companies began developing stacked injection molds. The Swiss company Schottli was the first to develop stacked injection molds for industrial applications.
In 1980, Johnson T. of Germany designed a cold runner double-layer injection mold. This mold consisted of a moving mold section, a fixed mold section, and an intermediate section. The intermediate section was essentially a continuation of the main runner, with branch runners and two separate cavity plates. Ejection mechanisms were installed in both the moving and fixed mold sections, using mechanical, hydraulic, or pneumatic methods to eject the plastic part.
In 1989, D. Gener and Wiesbaden-Delkheim designed a hot runner double-layer injection mold. It also consisted of a moving mold section, a fixed mold section, and an intermediate section. The intermediate section comprised the hot runner, hot nozzles for feeding material into the cavities, and two cavity plates for the finished product.
In 1991, Rozema H. of Tradesco Die & Mold designed a four-layer stacked injection mold. This mold, based on the hot runner double-layer injection mold, extended the hot runner and added an intermediate section, expanding the number of molding layers to four, thus increasing productivity fourfold.
In 1992, Hiroo Kasui and Motoo Yamamoto of Japan invented a hot runner stack injection mold with asymmetrically distributed hot nozzles. However, a reasonable runner design can control the melt flow within the mold cavity to achieve equilibrium.
2. Development Dynamics of Stack Injection Molds in China
Stack injection mold technology was only gradually introduced to my country's mold industry in the late 1980s. Therefore, my country's stack injection mold technology started relatively late, and the proportion of hot runner stack injection molds used in production is small. There is a certain gap in design and application compared to advanced foreign stack mold technology, and in some technical areas (such as rotary stack injection molds), China is still a blank slate. Therefore, facing fierce market competition, my country must quickly improve its stack injection mold technology to gain the initiative in the international market and ensure the survival of its enterprises.
In 1990, Li Shuzan of Beijing No. 13 Plastics Factory proposed a structural design for a double-cavity injection mold using side-gate feeding. This mold reduces the number of mold parting surfaces compared to point-gate feeding, facilitating sequential mold opening. However, it is unreliable when molding deep cavities or parts requiring significant demolding force.
In 1992, Bu Jianxin of Changzhou Light Industry School introduced a double-cavity injection mold using both side-gate and point-gate feeding. The upper cavity uses side-gate feeding, and the lower cavity uses point-gate feeding. Sequential parting is achieved using limiting hooks and limiting plates, allowing for the molding of different types of plastic parts.
In 1995, Yi Qing designed a special double-cavity injection mold with a two-stage main runner system. The first-stage main runner has a countersunk groove at the top. The moving mold platen drives the ejector plate of the fixed mold to eject the plastic part via a chain drive. Its disadvantages include the need to extend the injection molding machine nozzle to the fixed mold to inject the main runner bushing, and a bulky gating system.
In 1997, Li Shu and Chuan Chengzhi designed a double-layer hot runner injection mold for producing automotive door and window sealing strips. This mold bypassed the center and transferred molten plastic from the mold edge to the runner plate. The mold could mold two sets of plastic parts in one injection cycle, each set containing four sealing strips (front, back, left, and right). The eight sealing strips used on two cars could be molded in one operation.
In 1999, Wang Yuexing of Zhejiang Weixing Group designed a high-efficiency half-type double-layer injection mold. It shared a pair of half-slider blocks, resulting in a simple mold structure, lower manufacturing costs, double the number of cavities, shorter injection molding cycles, and higher production efficiency.
In 2000, Feng Xiaozhong et al. introduced a submerged gate double-layer injection mold for liquor glass caps. This mold allows for in-mold separation of each layer of plastic parts from the runner solidified material, and the parting surfaces of each layer can be ejected simultaneously. This simplifies the mold structure, reduces the requirements for parting distance, and facilitates automated production. However, it requires high reliability of the plastic parts remaining in the mold and a deeply recessed main runner bushing. In 2003, Yan Yalin and Huang Xiaoyan designed a right-angle gate hot runner stacked injection mold. This mold changed the gate's position, placing it in the middle at a right angle to the mold opening direction. While requiring a right-angle injection molding machine, it eliminated the need for a hot runner extension, reducing the distance the molten plastic travels from the injection nozzle to the manifold and simplifying the structural design.
In 2004, Chen Jianling, Liu Tinghua, and others designed a hot runner stacked mold for CD packaging boxes. Using fixed-distance tie rods for sequential mold opening, it features a compact structure, excellent economy, reduced manpower, significantly improved efficiency, and ensured product quality.
In 2007, Shen Honglei and others designed a hot runner stacked mold for CD holders. This mold employs a double-layer hot runner structure, utilizing gears, racks, and hydraulic cylinders to achieve sequential mold opening and part ejection. The produced parts meet dimensional and appearance requirements, significantly improving production efficiency and greatly reducing production costs and scrap rates.
In 2008, Wang Zhenbao et al. applied CAE technology to the design of stacked injection molds. Using Moldflow analysis software, they dynamically simulated the molding process of an air conditioner panel stacked mold by analyzing the plastic filling, holding pressure, and cooling processes. They analyzed the main factors affecting the molding process and optimized the process parameters.
III. Conclusion
Using stacked injection molds, especially hot runner stacked injection molds, can fully utilize the capabilities of injection molding machines, save manpower and equipment resources, and greatly improve production efficiency. Although stacked injection molds have higher design and manufacturing costs, improvements in the following areas can significantly reduce mold costs and expand their application range:
1. Improve the design theory of stacked injection molds and shorten the R&D cycle;
2. Extend the service life of core components (such as heating elements and temperature control elements);
3. Make stacked injection molds compatible with ordinary injection molding equipment;
4. Utilize CAD/CAE/CAM technology for design, analysis, and manufacturing to optimize mold structure;
5. Standardize and commercialize common parts for stacked injection molds;
6. Improve pressure transmission capabilities to make them suitable for the production of thick-walled plastic parts;
7. Optimize the process parameters of stacked injection molding;
8. Achieve full automation of stacked injection molding.
