Solutions to common problems in mold design process and mold design

Molds are a technology-intensive and capital-intensive product, and their status in my country's national economic scarf is also very important. The mold industry has been formally identified as a basic industry by my country and has been listed as a key support industry in the "10th Five-Year Plan". In industrial production, various presses and special tools installed on the press are used to make parts or products of the desired shape by pressure by using metal or non-metallic materials. This special tools are collectively called molds. All kinds of tools and products used in daily production and life, from the base of the machine tool and the body shell, to the smallest head screws, buttons and the shells of various household appliances, are closely related to the mold. The shape of the mold determines the appearance of these products, and the processing quality and accuracy of the mold also determines the quality of these products. Due to the different materials, appearance, specifications and uses of various products, molds are divided into non-plastic molds such as casting molds, forging molds, die-casting molds, stamping molds, and plastic molds. Below, the editor of Xianji.com will introduce to you the solutions to common problems in mold design process and mold design.

Mold design process

Step 1: Analyze and digest the 2D and 3D pictures of the product, and its content includes the following aspects:

1. Geometric shape of the product.

2. Product size, tolerance and design reference.

3. Technical requirements of the product (i.e. technical conditions).

4. The name, shrinkage and color of the plastic used in the product.

5. Surface requirements of products.

Step 2: Determination of injection model

The specifications of injections are mainly determined based on the size and production batch of plastic products. When designers choose an injection machine, they mainly consider its plasticization rate, injection volume, clamping force, effective area for installing the mold (inner spacing between injection machine tie rods), modulus, ejection form and ejection length. If the customer has provided the model or specification of the injection used, the designer must check its parameters. If the requirements cannot be met, the designer must discuss replacement with the customer.

Step 3: Determination of the number of cavities and arrangement of cavities

The number of mold cavities is mainly determined based on the projected area, geometry (with or without side core pulling) of the product, product accuracy, batch size and economic benefits. The number of cavities is mainly determined based on the following factors:

1. The production batch of the product (monthly batch or annual batch).

2. Whether the product has side core pulling and its treatment method.

3. The outer dimensions of the mold and the effective area of ​​the injection mold installation (or the inner spacing of the injection machine tie rods).

4. Product weight and injection volume of the injection machine.

5. The projected area and clamping force of the product.

6. Product accuracy.

7. Product color.

8. Economic benefits (production value of each set of molds).

The above factors sometimes restrict each other, so when determining the design plan, they must be coordinated to ensure that their main conditions are met. After the strong quantity is determined, the cavity arrangement and cavity position layout are carried out. The arrangement of the mold cavity involves the mold size, the design of the gating system, the balance of the gating system, the design of the core pulling (slider) mechanism, the design of the insert core and the design of the hot runner system. The above problems are related to the selection of parting surface and gate location, so necessary adjustments must be made during the specific design process to achieve the most perfect design.

Step 4: Determination of parting surface

The parting surface has been specified in some foreign product drawings, but in many mold designs it must be determined by mold personnel. Generally speaking, parting surfaces on a plane are easier to handle. Sometimes you should pay special attention to three-dimensional parting surfaces. The selection of its parting surface should follow the following principles:

1. It does not affect the appearance of the product, especially for products with clear requirements for appearance. More attention should be paid to the impact of the parting surface on the appearance.

2. Helps ensure the accuracy of products.

3. Conducive to mold processing, especially cavity processing. Restore the organization first.

4. Conducive to the design of pouring system, exhaust system and cooling system.

5. It is conducive to the demoulding of the product and ensures that the product remains on the side of the movable mold when the mold is opened.

6. Convenient for metal inserts.

When designing the lateral parting mechanism, it should be ensured that it is safe and reliable, and try to avoid interference with the setting mechanism. Otherwise, a pre-recovery mechanism should be installed on the mold.

Step 5: Determination of mold base and selection of standard parts

After all the above contents are determined, the mold base is designed according to the specified contents. When designing the formwork, try to use the most accurate formwork and determine the form, specifications and thickness of A and B boards of the standard formwork. Standard parts include two categories: general standard parts and mold-specific standard parts. Common standard parts such as fasteners, etc. Special standard parts for molds such as positioning rings, sprue sleeves, push rods, push tubes, guide pillars, guide bushes, special springs for molds, cooling and heating elements, secondary parting mechanisms and standard components for precision positioning, etc. It needs to be emphasized that when designing molds, use standard mold bases and standard parts as much as possible, because a large part of standard parts have been commercialized and can be purchased on the market at any time, which is extremely beneficial to shortening the manufacturing cycle and reducing manufacturing costs. After the buyer determines the size, the necessary strength and rigidity calculations for the mold-related parts should be made to check whether the selected mold base is appropriate, especially for large molds. This is particularly important.

Step Six: Design of Gating System

The design of the gating system includes the selection of the main channel and the determination of the cross-sectional shape and size of the runner. If point gate is used, in order to ensure that the runner is detached, attention should also be paid to the design of the degating device. When designing the gating system, the first step is to select the location of the gate. The appropriate selection of the gate location will directly affect the molding quality of the product and whether the injection process can proceed smoothly. The selection of the gate location should follow the following principles:

1. The gate position should be chosen on the parting surface as much as possible to facilitate mold processing and cleaning of the used gate.

2. The distance between the gate position and each part of the mold cavity should be as consistent as possible, and the process should be the shortest (generally it is difficult to achieve with a large nozzle).

3. The gate position should ensure that when the plastic is injected into the cavity, it faces the spacious and thick-walled part of the cavity to facilitate the flow of plastic.

4. Prevent the plastic from flowing directly into the cavity wall, core or insert when it flows into the cavity, so that the plastic can flow into all parts of the cavity as quickly as possible and avoid deformation of the core or insert.

5. Try to avoid welding marks on the product. If they are to occur, make the dissolution marks occur in unimportant parts of the product.

6. The gate position and plastic injection direction should be such that the plastic can flow evenly along the parallel direction of the cavity when injected into the cavity, and facilitate the discharge of gas in the cavity.

7. The gate should be designed in the part of the product that is easiest to clean, while minimizing any impact on the appearance of the product.

Step 7: Design of ejection system

The ejection forms of products can be summarized into three categories: mechanical ejection, hydraulic ejection, and pneumatic ejection.

Mechanical ejection is the last step in the injection molding process. The quality of ejection will ultimately determine the quality of the product. Therefore, product ejection cannot be ignored. The following principles should be followed when designing the ejection system:

1. In order to prevent the product from being deformed due to ejection, the thrust point should be as close as possible to the core or a part that is difficult to demould. For example, for slender hollow cylinders on the product, a push tube is often used for ejection. The arrangement of thrust points should be as balanced as possible.

2. The thrust point should act on the part of the product that can withstand the greatest force and the parts with good rigidity, such as ribs, flanges, wall edges of shell-type products, etc.

3. Try to avoid the thrust point acting on the thin plane of the product to prevent the product from being white or high. For example, shell-shaped products and cylindrical products are mostly ejected by push plates.

4. Try to avoid ejection marks that affect the appearance of the product. The ejection device should be located on the hidden or non-decorative surface of the product. For transparent products, special attention should be paid to the selection of position and ejection form.

5. In order to make the product receive uniform force during ejection and avoid deformation of the product due to vacuum adsorption, a composite ejection or special form of ejection system is often used, such as a push rod, push plate or push rod, push tube composite ejection, or an air inlet push rod, push block and other fixed ejection devices are used. If necessary, an air inlet valve should be installed.

Step 8: Design of cooling system

The design of the cooling system is a relatively tedious task, and the cooling effect, cooling uniformity and the impact of the cooling system on the overall structure of the mold must be considered. Cooling system design includes the following:

1. The arrangement of the cooling system and the specific form of the cooling system.

2. Determine the specific location and size of the cooling system.

3. Cooling of key parts such as moving model core or inserts.

4. Cooling of side slide block and side slide core.

5. Design of cooling components and selection of cooling standard components.

6. Design of sealing structure.

Step 9: The guide device on the plastic injection mold has been determined when using the standard mold base. Under normal circumstances, designers only need to choose according to the specifications of the mold base. However, when a precision guide device must be installed according to product requirements, the designer must make a specific design based on the mold structure.

The general guide is divided into: the guide between the moving and fixed molds; the guide between the push plate and the push rod fixed plate; the guide between the push plate rod and the movable template; the guide between the fixed mold base and the pusher plate. Because general guide devices are limited by processing accuracy or after being used for a period of time, their matching accuracy is reduced, which will directly affect the accuracy of the product. Therefore, products with higher accuracy requirements must separately design precision positioning components. Some have been standardized, such as tapered positioning pins, positioning blocks, etc., but some precision guide positioning devices must be specially designed according to the specific structure of the module.

Part 10: Selection of Mold Steel

The selection of materials for mold forming parts (cavity, core) is mainly determined based on the batch size and plastic category of the product. For high-gloss or transparent products, martensitic corrosion-resistant stainless steel or age-hardened steel such as 4Cr13 is mainly used. For plastic products containing glass fiber reinforcement, Cr12MoV and other types of quenched steel with high wear resistance should be used. When the material of the product is PVC, POM or contains flame retardants, corrosion-resistant stainless steel must be used.

Step 11: Draw the assembly diagram

After the ranking mold base and related content are determined, the assembly drawing can be drawn. In the process of drawing the assembly drawings, the selected pouring system, cooling system, core pulling system, ejection system, etc. are further coordinated and improved to achieve a relatively perfect design from a structural perspective.

Step 12: Draw the main parts of the mold

When drawing the cavity or core diagram, it is necessary to check whether the given molding dimensions, tolerances and draft angle are coordinated, and whether the design basis is coordinated with the design basis of the product. At the same time, the processability of the cavity and core during processing, the mechanical properties and reliability during use must also be considered. When drawing structural parts drawings, when standard formwork is used, most structural parts other than standard formwork do not need to be drawn.

Step Thirteen: Proofreading of Design Drawings

After the mold drawing design is completed, the mold designer will submit the design drawing and related original materials to the supervisor for proofreading. Proofreaders should systematically proofread the overall structure, working principle, feasibility of operation, etc. of the mold based on the relevant design basis provided by the customer and the customer's requirements.

Step 14: Countersigning the design drawings

After the mold design drawings are completed, they must be immediately submitted to the customer for approval. Only after the customer agrees can the mold be prepared and put into production. When the customer has major opinions and requires major modifications, the design must be redesigned before being submitted to the customer for approval until the customer is satisfied.

Step 15:

The exhaust system plays a vital role in ensuring the quality of product molding. Its exhaust methods include the following:

1. Use the exhaust slot. The exhaust slot is generally located at the last part of the cavity that is filled. The depth of the exhaust groove varies with different plastics, and is basically determined by the maximum gap allowed when the plastic does not produce flash.

2. Use the matching clearance of the core, inserts, push rods, etc. or the special exhaust plug to exhaust air.

3. Sometimes, in order to prevent vacuum deformation of the product at the top, it is necessary to design an exhaust pin.

Solutions to common problems in mold design

1. The side dimensions of the slider seat are larger than the slider

2. The protruding part of the slide block must not interfere with the slideway; the protruding part of the slider seat must not interfere with the board surface;

That is, the shaded part ①: Do not interfere with the slide;

Shaded part ②: Do not interfere with the side of the board;

As shown in the figure:

Suggested solution: Slide height d≤c (slider seat size);

Slider width e≤f (squeezing block width);

As shown in the picture:

3. The back plate of the slider should be fastened to the extrusion block as much as possible. Note: Use countersunk head screws to fasten the slider when it is small; use cylindrical hexagonal sockets when the slider is large. Note the L size tolerance. As shown in the figure:

4. The self-made cone is positioned at 5º on one side and its drawing method is as shown in the figure:

5. Several methods of installation on the water jet plate:

(1) Installation in front of the board:

(2) Installation behind the board is as follows:

（1）

（2）

6. The positioning of the slider must be in single inlay, not as a whole. As shown in the picture:

7. 2015 spline curve problem, outsourced processing is useless, try not to use splines when designing, confirm the spline situation when drawing, and do not use "Use Edge" on arcs when cutting inserts

8. Pattern, mirror finish, etc. must be stated in the technical requirements, even if the customer has not confirmed it.

9. The oil cylinder adopts the unified format "YGC, G type and F type. As shown in the figure below

10. The load-bearing capacity of the sky slider needs to be calculated and cannot be estimated. Touching beads is prohibited.

11. The back expansion of the push rod must be marked after the push rod is marked and cannot be marked elsewhere.

12. The parting surface should be designed as parallel to the X and Y directions as possible, and the angle should be 3º 5º 10º as much as possible. The parting surface should be as simple as possible regardless of molding.

13. The parting surface that has nothing to do with the product must be made into a flat surface and cannot be made into a free-form surface. The extended parting surface must also be made into two dimensions and must not have wrinkles.

14. The ejection stroke should be more than 10mm greater than the height of the product (special exceptions) and the spring preload should be more than 10mm.

15. Use 45 for the back plate locking block (the back plate is usually added to the locking block, material CrWMn, HRC50~55)

16. The distance between water pipe joints is ≥25.

17. All mold clocks produced by customers for sie should adopt Sanmei standards as much as possible.

18. The latent gate shall always adopt the ball head leaning type, as shown in the figure:

19. If the sealing ring is pressed on the lower part of the insert, try to fix it with screws (evenly distributed). If ear stands are used for fixation, try to use bilateral ear stands to ensure pressure balance. As shown in the picture:

20. The effective length of the inner hole of the push tube must be greater than the demoulding distance. As shown in the picture:

Do you all understand the relevant knowledge above? These are the mold design process and solutions to common problems in mold design compiled by the editor of Xianji.com. With the continuous expansion of mold application fields and the increased and higher requirements for molds in applied fields, it has become a general rule that the mold industry develops faster than other manufacturing industries. Currently, the world mold market is in short supply. In recent years, the total market volume has been between 60 and 65 billion US dollars, while my country's mold exports are less than 8%. During the "Eleventh Five-Year Plan" period, the mold exports were completed. This share can be expanded. At the same time, as the trend of economic globalization becomes more and more obvious, the mold manufacturing industry gradually shifts to my country and the trend of multinational groups purchasing molds in my country becomes increasingly obvious. Foreign capital and private capital continue to be optimistic about my country's mold industry. The opportunities in my country's mold industry outweigh the challenges. The future international mold market has broad prospects, and my country's molds still have a lot of room for development.

Note: All pictures in the article are reprinted online, and will be deleted if infringed!