Step 1: Analysis and Assimilation of 2D and 3D Drawings of Product. This includes following aspects: Geometric shape of product; Dimensions, tolerances, and design datum of product; Technical requirements (i.e., technical conditions) of product; Name, shrinkage, and color of plastic used in product; Surface requirements of product.

 

Step 2: Determination of Injection Molding Compound Type
Determination of injection molding compound specification is mainly based on size of plastic product and production batch. When selecting an injection molding machine, designers mainly consider its plasticizing rate, injection volume, clamping force, effective area of mold mounting (internal spacing of injection molding machine tie rods), mold capacity, ejection form, and ejection length. If customer has provided type or specification of injection molding compound to be used, designer must check its parameters. If it does not meet requirements, a replacement must be discussed with customer.
Step 3: Determining Number and Arrangement of Cavities
Number of mold cavities is primarily determined based on projected area, geometry (whether or not there is side core pulling), precision, batch size, and economic efficiency of product.
Number of cavities is mainly determined by following factors: Production batch size (monthly or annual batch); Whether or not there is side core pulling and its handling method; Mold dimensions and effective area of injection molding machine (or spacing between injection molding machine tie rods); Product weight and injection volume of injection molding machine; Projected area of product and clamping force; Product precision; Product color; Economic efficiency (production value per mold set).
These factors are sometimes interdependent; therefore, coordination is necessary when determining design scheme to ensure that main conditions are met.
After determining number of cavities, arrangement and layout of their positions are then carried out. Arrangement of cavities involves mold dimensions, gating system design, gating system balance, core-pulling (slider) mechanism design, insert core design, and hot runner system design. These issues are related to selection of parting surface and gate location, so necessary adjustments must be made during design process to achieve the most perfect design.
Step 4: Determining Parting Surface
Parting surface is specifically defined in some foreign product drawings, but in many mold designs, it must be determined by mold engineers. Generally speaking, parting surfaces on a plane are easier to handle; however, special attention should be paid to three-dimensional parting surfaces. Selection of parting surface should follow these principles: It should not affect appearance of product, especially for products with specific appearance requirements; It should help ensure precision of product; It should facilitate mold processing, especially cavity processing; It should facilitate design of gating system, venting system, and cooling system; Facilitates product demolding, ensuring product remains on moving mold side during mold opening; Facilitates metal inserts.
When designing a side parting mechanism, ensure its safety and reliability, and minimize interference with stationary parting mechanism; otherwise, a pre-recovery mechanism should be installed on mold.
Step 6: Determining Mold Base and Selecting Standard Parts
After determining all above, design mold base accordingly. When designing mold base, select a standard mold base whenever possible, determining its form, specifications, thicknesses of plates A and B. Standard parts include two main categories: general-purpose standard parts and mold-specific standard parts. General-purpose standard parts include fasteners. Mold-specific standard parts include locating rings, sprue bushings, ejector pins, ejector tubes, guide pillars, guide sleeves, mold-specific springs, cooling and heating elements, secondary parting mechanisms, and standard components for precision positioning.

 

It is important to emphasize that when designing molds, standard mold bases and standard parts should be used as much as possible. Many standard parts are commercially available and readily available on the market, which is extremely beneficial for shortening manufacturing cycle and reducing manufacturing costs.
After buyer's dimensions are determined, necessary strength and rigidity calculations must be performed on relevant mold parts to verify suitability of selected mold base, especially important for large molds.
Step 7: Design of Gating System
Design of gating system includes selection of main runner, determination of cross-sectional shape and dimensions of branch runners.
If a point gate is used, attention should also be paid to design of gate removal device to ensure removal of branch runners.
When designing gating system, the first step is to select gate location. Appropriateness of gate location selection directly affects molding quality of product and smoothness of injection process. Selection of gate location should follow these principles: Gate location should be selected as close as possible to parting surface to facilitate mold processing and gate cleaning; Distance from gate to all parts of mold cavity should be as consistent as possible, and flow path should be minimized (this is generally difficult to achieve with large gates); Gate position should ensure that plastic is injected into spacious, thick-walled parts of mold cavity to facilitate plastic flow; Avoid plastic from directly hitting cavity walls, core, or inserts during injection. Ensure plastic flows quickly into all parts of cavity and prevent deformation of core or inserts; Minimize weld lines on product. If weld lines are unavoidable, ensure they occur in non-critical areas; Gate position and direction of plastic injection should allow plastic to flow evenly along parallel direction of mold cavity and facilitate venting of gases; Gate should be designed in the easiest-to-clean location on product, while minimizing its impact on product's appearance.
Step 8: Ejection System Design
Ejection methods can be broadly categorized into three types: mechanical ejection, hydraulic ejection, and pneumatic ejection.
Mechanical ejection is final step in injection molding process, and its quality ultimately determines quality of finished product. Therefore, ejection is crucial. Following principles should be followed when designing an ejection system: To prevent product deformation during ejection, ejection point should be as close as possible to core or areas difficult to demold, such as slender hollow cylinders, which often use push tubes. Ejection points should be arranged as evenly as possible; Ejection point should act on part of product that can withstand the greatest force and has good rigidity, such as ribs, flanges, and edges of shell-shaped products; Avoid applying ejection point to thinner surfaces of product to prevent whitening or excessive ejection height. Shell-shaped and cylindrical products often use push plates for ejection; Minimize ejection marks affecting product's appearance. Ejection device should be located on a concealed or non-decorative surface of product. Special attention should be paid to selection of ejection location and ejection method for transparent products; To ensure uniform force distribution during ejection and prevent deformation due to vacuum suction, composite ejection or special ejection systems are often used, such as push rods, push plates, or a combination of push rods and push tubes, or fixed ejection devices with air intake push rods or push blocks. Air intake valves should also be installed when necessary.
Step 9: Cooling System Design
Designing cooling system is a complex task, requiring consideration of cooling effect, uniformity, and system's impact on the overall mold structure. Cooling system design includes following: Arrangement and specific form of cooling system; Determining specific location and dimensions of cooling system; Cooling of key areas such as moving mold core or inserts; Cooling of side slides and side slide cores; Design of cooling elements and selection of standard cooling elements; Design of sealing structure.
Step 10: Guiding devices on plastic injection mold are already determined when using a standard mold base. Generally, designers only need to select mold bases according to specifications. However, when precision guiding devices are required based on product requirements, designers must create specific designs based on mold structure.
General guiding mechanisms are categorized as follows: guidance between moving and fixed molds; guidance between ejector plate and ejector rod fixing plate; guidance between ejector rod and moving mold plate; guidance between fixed mold base and ejector rod. Due to limitations in machining accuracy or a decrease in fit after a period of use, precision of general guiding devices directly affects precision of product. Therefore, for products with high precision requirements, precision positioning elements must be designed separately. Some are standardized, such as tapered positioning pins and positioning blocks, but some precision guiding and positioning devices must be specially designed according to specific structure of module.
Part 11: Selection of Mold Steel
Selection of materials for mold forming parts (cavities, cores) is mainly determined by batch size of product and type of plastic. For high-gloss or transparent products, martensitic corrosion-resistant stainless steel or age-hardening steel of 4Cr13 type is mainly selected.
For plastic products containing glass fiber reinforcement, quenched steel with high wear resistance, such as Cr12MoV, should be selected.
When product material is PVC, POM, or contains flame retardants, corrosion-resistant stainless steel must be selected.
Step 12: Drawing Assembly Drawing
After mold base and related details are determined, assembly drawing can be drawn. During assembly drawing process, selected gating system, cooling system, core-pulling system, ejection system are further coordinated and improved to achieve a more perfect structural design.
Step 13: Drawing Main Mold Parts
When drawing cavity or core drawings, it is essential to ensure that given molding dimensions, tolerances, draft angles are consistent, and that design datum is consistent with product's design datum. Manufacturability of cavity and core during processing, as well as their mechanical properties and reliability during use, must also be considered.
For structural component drawings, when using a standard mold base, most structural components other than standard mold base do not require structural component drawings.

 

Step 14: Design Drawing Proofreading
After mold design is completed, mold designer submits design drawings and related original documents to supervisor for proofreading.
Proofreader should systematically proofread the overall structure, working principle, operational feasibility of mold, based on design basis provided by client and client's requirements.
Step 15: Design Drawing Countersigning
After mold design drawings are completed, they must be submitted to client for approval immediately. Only after client's approval can mold be prepared for production. If client has significant objections requiring major modifications, a redesign must be submitted to client for approval until client is satisfied.
Step 16: Venting system plays a crucial role in ensuring quality of molded product. Venting methods include:
Using venting grooves. Venting grooves are generally located in last part of cavity to be filled. Depth of venting grooves varies depending on plastic, and is basically determined by maximum allowable gap without flash; Utilize clearances of core, inserts, ejector pins, etc., or special venting plugs for venting; Sometimes, to prevent vacuum deformation caused by work-in-process during assembly, venting inserts must be designed.
Conclusion
In summary, above mold design process involves some aspects that can be considered together, while others require repeated consideration. Because these factors often contradict each other, continuous argumentation and coordination are necessary during design process to achieve a better outcome. This is especially true for mold structure, which must be taken seriously. Often, several solutions should be considered simultaneously, listing advantages and disadvantages of each structure, analyzing them one by one, and optimizing accordingly. Structural factors directly affect mold manufacturing and use, and in severe cases, can even lead to scrapping of the entire mold. Therefore, mold design is a crucial step in ensuring mold quality, and its design process is a systematic engineering project.