Molds are developing rapidly in production and daily life. Many industries use molds to process products, so mold industry is developing very rapidly. Below is an introduction to complete mold design process. Let's take a look!

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.

 

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 model or specifications of injection molding compound, designer must verify its parameters. If requirements are not met, a replacement must be discussed with customer.

Number of mold cavities is mainly determined based on projected area, geometry (whether or not there is side core pulling), product precision, batch size, and economic benefits.
Number of cavities is primarily determined by following factors: Production batch size (monthly or annual batch); Whether or not there is side core pulling in product and its handling method; Mold dimensions and effective area of injection molding compound (or inner spacing of 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 benefits (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 quantity of high-strength molded parts, layout of cavities are then carried out. Cavity arrangement 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.

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; impact of parting surface on appearance should be carefully considered; It should help ensure precision of product; Facilitates mold processing, especially cavity processing. (First-return mechanism); Facilitates design of gating, venting, and cooling systems; 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 first-return mechanism should be installed on mold.

After determining all above, design mold base accordingly. When designing mold base, choose a standard mold base whenever possible, determining its form, specifications, thickness 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 precision positioning standard components. 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 should be performed on relevant mold parts to verify suitability of selected mold base, especially important for large molds.

Design of gating system includes selection of main runner and 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. The first step in designing gating system is to select gate location. Appropriateness of gate location selection directly affects molding quality of product and whether injection process can proceed smoothly. 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 core or insert deformation; 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 easiest-to-clean area of product, while minimizing its impact on product's appearance.

 

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 ejector 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 ejector plates; 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.

Designing cooling system is a complex task, requiring consideration of cooling effect, uniformity, and system's impact on the overall mold structure. 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.

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 design them specifically according to mold structure. General guiding devices are divided into: 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.

Selection of materials for mold forming parts (cavities, cores) is mainly determined by batch size and type of plastic. For high-gloss or transparent products, martensitic corrosion-resistant stainless steel or age-hardening steel of type 4Cr13 is mainly selected. For plastic products containing glass fiber reinforcement, quenched steel of type Cr12MoV with high wear resistance should be selected. When material of product is PVC, POM, or contains flame retardants, corrosion-resistant stainless steel must be selected.

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.

When drawing cavity or core drawings, it is essential to ensure that given molding dimensions, tolerances, and 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 parts, when using a standard mold base, structural parts other than those specified in standard mold base do not generally require structural drawing drawings.

After mold drawing design is completed, mold designer submits design drawings and related original data to supervisor for proofreading.
Proofreading personnel should systematically proofread the overall structure, working principle, operational feasibility of mold based on design basis and requirements provided by client.

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.

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 clearance without flash; Using mating clearance of core, inserts, ejector pins, etc., or dedicated venting plugs for venting; Sometimes, to prevent vacuum deformation during molding process, venting inserts must be designed.

 

In summary, mold design process outlined above involves various aspects. Some elements can be considered together, while others require repeated consideration. Because these factors often contradict each other, continuous verification and coordination are essential during design process to achieve optimal results. This is especially true for mold structure, which demands careful attention. Often, several solutions should be considered simultaneously, with each structure's advantages and disadvantages meticulously analyzed and optimized. Structural issues directly impact mold manufacturing and use, potentially leading to scrapping of the entire mold. Therefore, mold design is a crucial step in ensuring mold quality, and its process is a systematic engineering undertaking.