For pre reading, please refer to Injection molding materials and mold design professional knowledge sharing (Part 1).
Following describes forms and characteristics of several ejector mechanisms

Ejector rod ejector mechanism is the simplest and most common form of ejector mechanism. Since cross-section of ejector rod is mostly circular, it is easy to manufacture and repair, and has a good ejection effect, and is widely used in production. However, since ejection area is generally small, it is easy to cause stress concentration and penetrate plastic part or deform plastic part, so it is rarely used in pipes or boxes with small demolding slopes and large demolding resistance. Figure 1-13 shows ejector rod structure.

Ejector tube ejector mechanism is also called a hollow ejector rod ejector mechanism. It is suitable for ejection of annular, cylindrical plastic parts or central hole part on plastic part. Since the entire periphery of ejector tube contacts plastic part, force of ejecting plastic part is uniform, plastic part is not easy to deform, and no obvious ejection marks are left. When using ejector, main core and concave mold can be designed on movable mold side at the same time, which is beneficial to improve concentricity of product. For plastic parts that are too thin (thickness <1.5mm), try not to use ejector, because thin ejector is difficult to process and easy to damage. Figure 1-14 shows ejector structure.

 

Push plate ejector mechanism has characteristics of uniform ejection force, good ejection effect and no ejection marks. It is particularly suitable for demolding products with multiple cavities in one mold, round and simple shapes. Disadvantage is that it increases thickness of mold, matching accuracy and processing accuracy of demolding hole position are required to be higher. Figure 1-15 shows push plate ejector structure.

 

Cooling system
Generally, temperature of plastic injected into mold is about 200℃, and temperature of plastic part when it is taken out of mold cavity after solidification is below 60℃. After injection molding of thermoplastics, mold must be effectively cooled so that heat of molten plastic can be transferred to mold as quickly as possible, so that plastic parts can be reliably cooled and shaped, can be quickly demolded, thereby improving shaping quality and production efficiency of plastic parts.
For plastics with low melt viscosity and good fluidity, such as polyethylene, nylon, polystyrene, etc., if plastic parts are thin-walled and small, mold can be cooled naturally; if plastic parts are thick-walled and large, mold needs to be artificially cooled so that plastic parts can quickly condense and shape in mold cavity, shorten molding cycle, and improve production efficiency. Figure 1-16 shows cooling system.

 

Cooling medium includes cooling water and compressed air, and cooling water is often used for cooling. This is because water has a large specific heat capacity, low cost, and water below room temperature is also easy to obtain. Water cooling means opening a cooling water channel around or in mold cavity, using circulating water to take away heat and maintain mold temperature within a certain range.

Try to ensure uniform shrinkage of plastic parts and maintain mold thermal balance.
The more cooling water holes there are, the larger hole diameter, the more evenly water channel is distributed, and the more evenly plastic part is cooled.
Water hole should be at same distance from all parts of cavity surface.
Cooling should be strengthened at gate.
Reduce temperature difference between inlet and outlet water.
Arrangement of cooling water channel should be reasonably considered in combination with characteristics of plastic and structure of part.
Cooling water channel should avoid being close to weld mark of plastic part to avoid loose welding and affecting strength.
Ensure that cooling channel does not leak.
Prevent cooling water channel from interfering with other parts.
Inlet and outlet of cooling channel should be lower than outer surface plane of mold.
Cooling water channel should be easy to process and clean.
Core pulling mechanism
When there are side holes or side recesses on plastic part, part with side hole or side recess must be a movable core. Before demolding, movable core must be pulled out first, and mechanism for pulling out side movable core is called core pulling mechanism.
In actual use, the most commonly used is inclined guide column core pulling mechanism. Action principle of inclined guide pin core pulling mechanism is to use mold opening force and inclination angle of inclined guide pin to force slider to move horizontally, thereby completing core pulling and parting action. Its characteristics are reliable core pulling action and large pulling force, but mold manufacturing requirements are high and actual application is wide.
Parting core pulling distance of inclined slider core pulling mechanism is larger than that of inclined guide pin core pulling mechanism, and lateral injection pressure it can withstand is greater than that of inclined guide pin core pulling mechanism, but it is more difficult to process and has higher mold manufacturing precision requirements. It is generally used for products that require vertical parting or multi-directional core pulling. Figure 1-17 shows inclined guide pin core pulling mechanism.

 

Mold is main tool for plastic part molding. It is very necessary to understand mold structure and its commonly used standard parts. As shown in Figure 1-18, it is a complete set of three-dimensional graphic mold frame structure. There are many forms of mold structure, but in summary, there are two major types, namely molding parts and structural parts. Following introduces some parts in mold.

 

Mold frame is composed of parts such as mold plates, guide pins and guide sleeves, but cavity is not processed. In order to improve efficiency of mold design and manufacturing, some large professional mold frame factories produce various types of standard mold frames for customers to use, as shown in Figure 1-19.

Core is also called male mold. This structure is installed on B plate. When mold is closed, it bears pressure of injection molding machine and cooperates with cavity. During injection molding, core molds inner wall shape of plastic part. Its structural form is shown in Figure 1-20.

 

Cavity is also called female mold. This structure is installed on A plate. When mold is closed, it bears pressure of injection molding machine and cooperates with core. During injection molding, cavity molds outer wall shape of plastic part. Its structural form is shown in Figure 1-21.

Part that slides upward along guide to drive side core to complete core pulling and reciprocating motion, as shown in Figure 1-22.

 

It cooperates with guide sleeve (or hole) installed on other half of mold to determine relative position of movable and fixed molds and ensure precision of mold movement guidance. It is a cylindrical part, as shown in Figure 1-23.

It cooperates with guide pillar installed on other half of mold to determine relative position of movable and fixed molds and ensure precision of mold movement guidance. It is a circular sleeve-shaped part, as shown in Figure 1-24.

 

Before using UG NX 4 to design plastic injection molds, steps and sequence of mold design must be fully analyzed to simplify mold design process. In actual production, design and manufacturing process of a complete mold is generally shown in Figure 1-25.

 

Analyze plastic product drawing, understand purpose of parts, analyze processability of plastic parts, dimensional accuracy and other technical requirements. For example, what are requirements for appearance, color transparency, and performance of plastic parts? Are geometric structure, slope, and inserts of plastic parts reasonable? Allowable degree of molding defects such as weld marks and shrinkage holes? Are there any post-processing such as painting, electroplating, gluing, and drilling?
Select size with the highest dimensional accuracy of plastic parts for analysis, estimate whether molding tolerance is lower than tolerance of plastic parts, and whether plastic parts that meet requirements can be molded. In addition, it is necessary to understand plasticization and molding process parameters of plastic.
Analyze process data, analyze whether molding method, equipment model, material specification, mold structure type and other requirements proposed in process task book are appropriate and can be implemented.
Molding material should meet strength requirements of plastic parts, and have good fluidity, uniformity, isotropy, and thermal stability. According to purpose of plastic parts, molding materials should meet requirements of dyeing, metal plating, decorative properties, necessary elasticity and plasticity, transparency or opposite reflective properties, adhesiveness or weldability, etc.

Mold design is carried out according to type of molding equipment, so performance, specifications and characteristics of various molding equipment must be familiar. For example, for injection molding machines, specifications should include injection capacity, clamping pressure, injection pressure, mold installation size, ejection device and size, nozzle hole diameter, nozzle spherical radius, gate sleeve size, positioning ring size, mold plate stroke, maximum and minimum thickness of mold, etc. Preliminary estimate of mold size to determine whether mold can be installed and used on selected injection molding machine.

There are many structures to consider for injection molding, mainly following items:
Cavity layout, determine number of cavities and their arrangement according to geometric structure characteristics of plastic parts, dimensional accuracy requirements, batch size, mold manufacturing difficulty, mold cost, etc.
Determine parting surface. Position of parting surface should be conducive to mold processing, exhaust, demoulding, molding operations and surface quality of plastic parts.
Determine pouring system (shape, position, size of main runner, sub-runner and gate) and exhaust system (exhaust method, exhaust groove position, size).
Select ejection method (ejector, ejector, push plate, combined ejection), determine side concave treatment method and core pulling method.
Determine cooling and heating methods, shape, position, and installation location of heating element of heating and cooling grooves.
According to mold material, strength calculation or empirical data, determine thickness, shape size, shape structure and all connection, positioning, and guide parts of mold parts.
Determine structural form of main molding parts and structural parts.
Consider strength of each part of mold and calculate working size of molding parts.
After making a comprehensive consideration of above issues and clarifying which mechanism to use for design, use UG to develop mold splitting and other structural designs.
Process from product design to mold design
In process of actual product to mold design, as a product and mold designer, you should first understand flow chart of actual product to mold design, as shown in Figure 1-26.
According to flow chart, to design a product, you must first conduct market research, followed by solution design, then technical design, manufacturing design, and finally put it into production. In above process, many design steps and phased results must be differentiated. Therefore, it is not difficult to see that links from product design to mold design are inseparable, and these links directly affect structural design of each part.