The design of molded parts is to design the moving mold, fixed mold, and molded parts after determining the overall structure of the mold. The structures of the moving mold, fixed mold, and molded parts come in two forms: integral and insert. The use of insert assembly structure is the biggest feature of injection mold design and manufacturing. Generally, small molds with relatively simple shapes and structures and convenient processing usually adopt an integral structure.
I. The design of molded parts includes the following four aspects:
1. First, it is necessary to consider and determine the parting surface of the moving and fixed molds. During the injection molding process, the surface where the mold is opened to remove the plastic part or the gating system is collectively referred to as the parting surface. In other words, the parting surface is the contact surface between the moving mold and the fixed mold, and its main function is to tightly seal the cavity. The mold designer must determine the mold opening direction when designing the parting surface.
2. When designing molded parts, it is necessary to consider the number of cavities and their layout, the arrangement of the gating system, the core-pulling mechanism, the ejection mechanism, the temperature control system design, the material selection and heat treatment of molded parts, etc. For details, see other chapters.
3. The structural form of the moving mold and fixed mold, whether integral or insert structure, as well as their structural form, dimensions, insert method, and fixing method, need to be determined based on specific conditions, cost calculation, and weighing of pros and cons to make the correct choice. When adopting a combined insert structure, attention should be paid to the strength and rigidity of the mold. For details, see Chapter 16.
4. When designing the dimensions of molded parts, full consideration should be given to the molding shrinkage of the product, draft angle, and the manufacturing process of the parts.
5. Design molded parts according to the customer's design standards and requirements, as well as the technical data provided.
II. Basic principles of mold structure design
1. Clarity
The mold structure principle is clear, the part functions are clear, with no omissions or duplications. The design requirements for each part should be standardized and optimized (design datums should be unified, nominal dimensions should be integers, and arbitrary design is not allowed).
2. Simplicity
The structure should be simple and reliable, facilitating machining and assembly. The advantage of simplicity is that it directly reduces costs. For example, the geometric shape of parts should be kept as simple as possible, minimizing the machined surfaces and number of machining operations, and reducing or simplifying the assembly relationships and adjustment measures with related parts (related parts must not interfere with each other).
3. Safety and Reliability
The mold must have sufficient strength and rigidity, operate continuously without issues, and be easy to maintain and repair.
4. Maintenance
The ejection and demolding mechanisms for the product must be safe and reliable.
The product molding efficiency should be high and quality good. For example: high cooling rate, short molding cycle, and balanced pressure in the gating system. The designed mold should allow for easy removal of runners and gates after the plastic part is formed.
III. Design Steps for Molded Parts
When designing the molded parts for the moving and fixed molds, it is necessary to comprehensively consider relevant issues. After determining the mold structure and gating system, the design can generally proceed according to the following steps.
1. Determine the number of mold cavities and preliminarily establish the cavity layout.
2. Determine the parting line of the plastic part and the parting surface of the mold.
3. Determine the gate type and feed location of the gating system.
4. For plastic parts with concave and convex shapes on the inside and outside, design a side core-pulling mechanism.
5. Determine whether the molding components are solid or insert-type, as well as their assembly method and structural fixation method.
6. After determining the above relevant factors, comprehensively consider the mold structure and overall layout.
7. Determine the internal and external shapes and molding dimensions of the core and cavity, and determine the draft angle of the part.
8. Determine the material, heat treatment, assembly requirements, surface roughness, and other technical requirements for the molding components of the moving and fixed molds.
IV. Methods for Determining the Number of Cavities
There are various methods for determining the number of cavities. During design, the main basis includes the precision of the plastic part, the production batch, the economic efficiency of production, the clamping force of the injection molding machine, and the injection volume. First, consider the precision of the plastic part, then comprehensively evaluate other factors to reasonably select the number of cavities. Generally, for high-precision parts, the maximum number of cavities is 4; for molds with lower precision requirements, the number of cavities should preferably not exceed 24. This is because each additional cavity reduces the corresponding precision.