Fifteen steps of molding design and design focus

Time:2019-08-30 14:22:46 / Popularity: / Source:

A mold is an industrial product that shapes a material by a certain structure in a specific structure, and is also a production tool capable of mass-producing industrial product parts having a certain shape and size requirement. Large to airplanes, cars, small cups, nails, almost all industrial products must rely on molding. High precision, high consistency, and high productivity of parts produced by mold are unmatched by any other processing method. Molds largely determine quality, efficiency and new product development capabilities of product. Therefore, mold has honorary title of “Mother of Industry”.
Mold designer refers to personnel who are engaged in digital design of enterprise molds, including cavity molds and cold stamping molds, and fully applied digital design tools to improve molding design quality and shorten molding design cycle on the basis of traditional molding design. Mold designer can be employed in the fields of CNC programming, mold 3D design, and product development 3D design.
molding design 

Step 1: Analysis and digestion of 2D/3D drawing of product, content includes following aspects:

1. Geometry of product.
2, Size, tolerance and design basis of product.
3. Technical requirements of product (ie technical conditions).
4. Name, shrinkage and color of plastic used in product.
5. Surface requirements of product.
product development 

Step 2: Determination of injection machine type

Specification of injection is mainly determined by size and production volume of plastic product. When designing injection machine, designer mainly considers plasticizing rate, injection volume, clamping force, effective area of mounting mold (spacing inside injection rod), modulus, top ejection form and determined length. If customer has provided type or specification of injectable use, designer must check parameters and if it does not meet requirements, it must be consulted with customer.

Step 3: Determination of number and layout of cavities

Determination of number of mold cavity is mainly determined by projected area of product, geometry (with or without side cores), accuracy of product, batch size, and economic benefits.
Number of mold cavity is determined mainly by following factors:
1. Production batch of products (monthly batch or annual batch).
2. Whether product has side core and its treatment method.
3. Outer dimensions of mold and effective area of injection molding (or spacing inside injection rod).
4. Weight of product and injection volume of injection machine.
5. Projected area and clamping force of product.
6, Product accuracy.
7, Product color.
8. Economic benefits (production value of each set of molds).
These factors are sometimes mutually constrained, so when designing a design, coordination must be made to ensure that main conditions are met.
After number of cavities is determined, arrangement of cavities and layout of cavity positions are performed. Arrangement of cavities involves size of mold, design of gating system, balance of gating system, design of core (slider) mechanism, design of insert core, and design of hot runner system. These problems are related to selection of parting surface and gate position, so in specific design process, necessary adjustments must be made to achieve most perfect design.
mold cavity 

Step 4: Determination of parting surface

Parting surface has been specified in some foreign product drawings, but in many molding design, it is determined by mold personnel. Generally speaking, parting surface on the plane is easier to handle. Sometimes you should pay special attention to three-dimensional parting surface. Selection of parting surface should follow following principles:
1, Does not affect appearance of product, especially for products with clear requirements on appearance, should pay attention to impact of shape on the appearance.
2, To ensure accuracy of product.
3, Conducive to molding process, especially processing of cavity.
4. Conducive to design of gating system, exhaust system and cooling system.
5. It is beneficial to demoulding product to ensure that product is left on movable mold side when mold is opened.
6, Easy to metal inserts.
When designing lateral parting mechanism, it should be ensured that it is safe and reliable, avoid interference with fixed mechanism as much as possible. Otherwise, a pre-recovery mechanism should be set on mold.
parting surface 

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

After all above contents are determined, mold base is designed according to specified content. When designing formwork, use standard formwork as much as possible to determine form, specifications and thickness of A/B plates of standard formwork. Standard parts include two types: common standard parts and standard parts for molds. General standard parts such as fasteners. Standard parts for molds such as positioning ring, sprue bushing, push rod, push tube, guide post, guide sleeve, mold-specific spring, cooling and heating element, secondary parting mechanism and standard components for precision positioning.
It should be emphasized that when designing molds, standard mold base and standard parts are used as much as possible, because a large part of standard parts have been commercialized and can be purchased at any time, which is extremely advantageous for shortening manufacturing cycle and reducing manufacturing cost.
After buyer has been sized, necessary strength and rigidity calculations are performed on mold-related parts to check that selected formwork is appropriate, especially for large molds.
mold base 

Step 6: Design of gating system

Design of gating system includes selection of main flow path, shape of cross-section of runner, and determination of dimensions.
If a point gate is used, in order to ensure separation of runner, attention should also be paid to design of degasser.
In designing gating system, first is to choose location of gate. Appropriate choice of location of gate will directly affect quality of product and smoothness of injection process. Choice of gate location should follow following principles:
1. Position of gate should be selected on parting surface as much as possible to facilitate processing of mold and cleaning of gate.
2. Distance between gate location and various parts of cavity should be as uniform as possible, and process should be the shortest (it is difficult to do with a large nozzle).
3. Position of gate should ensure that plastic is injected into cavity against spacious and thick-walled part of cavity to facilitate inflow of plastic.
4. Avoid plastics from penetrating into cavity wall, core or insert when flowing into cavity, so that plastics can flow into all parts of cavity as soon as possible to avoid deformation of core or insert.
5, Try to avoid production of weld lines. If it is to be produced, make dissolution marks appear in places where product is not important.
6. Position of gate and direction of plastic injection should be, plastic can flow uniformly along parallel direction of cavity when injecting into cavity, and it is beneficial to discharge of gas in cavity.
7. Gate should be designed to be the easiest part to remove on product, while not affecting appearance of product as much as possible.
gating system 

Step 7: Design of ejection system

Ejector form of product can be divided into three categories: mechanical ejection, hydraulic ejection, and pneumatic ejection.
Mechanical ejection is the last link in injection molding process. Quality of ejection will ultimately determine quality of product. Therefore, product ejection cannot be ignored. Following principles should be observed when designing an ejector system:
1. In order to prevent product from being deformed due to ejection, thrust point should be as close as possible to core or part that is difficult to demould. For example, elongated hollow cylinder on product is mostly pushed out by a push tube. Arrangement of thrust points should be as balanced as possible.
2. Thrust point should be applied to part where product can withstand the most stress and part with good rigidity, such as rib, flange, wall edge of shell type product, etc.
3, Try to avoid thrust point on thinner surface of product, to prevent product from whitening, top height, etc., such as shell-shaped products and cylindrical products are mostly pushed out by push plates.
4, Try to avoid appearance of product, ejection device should be placed on hidden or non-decorative surface of product. In particular, attention should be paid to choice of position and ejection form for transparent articles.
5, In order to make product evenly stressed when it is ejected, and avoid deformation of product due to vacuum adsorption, a composite ejection or special form of ejector system, such as push rod, push plate or push rod, push tube composite, is often used. To eject, or use air intake push rod, push block, etc. If necessary, an intake valve should also be provided.
ejector system 

Step 8: Design of cooling system

Design of cooling system is a cumbersome task, considering cooling effect, uniformity of cooling and effect of cooling system on overall structure of mold. Design of cooling system includes following:
1. Arrangement and specific form of cooling system.
2. Determination of specific location and size of cooling system.
3. Key parts are the cooling of the dynamic model core or insert.
4. Cooling of side slider and side slide core.
5. Design of cooling components and selection of cooling standard components.
6. Design of sealing structure.
Layout of Cavity Cooling System 
Layout of Cavity Cooling System
Layout of Core Cooling System 
Layout of Core Cooling System

Step 9: Design of guide device

Guides on plastic injection mold have been determined using standard mold base. Under normal circumstances, designer only needs to select according to mold base specifications. However, when a precision guide is required according to requirements of product, it must be designed by designer according to mold structure.
General orientation is divided into: orientation between moving and fixed molds; guiding of push plate and push rod fixing plate; guiding between pushing plate and moving plate; guiding between fixed die seat and push pirated version. Generally, guiding device is limited by processing precision or after a period of use, precision of matching is lowered, which directly affects precision of product. Therefore, precision positioning component must be separately designed for product with high precision. Some have been standardized, such as tapered positioning pins, positioning blocks, etc., but some precision guiding and positioning devices must be specially designed according to specific structure of module.

Step 10: Selection of mold steel

Selection of materials for mold forming parts (cavities, cores) is mainly determined according to batch size and plastic type of products. For products with high gloss or transparency, martensitic corrosion resistant stainless steel or age hardened steel of type 4Cr13 is mainly used.
For plastic products containing glass fiber reinforced, quenched steel with high wear resistance such as Cr12MoV should be used.
When material of product is PVC, POM or contains a flame retardant, corrosion resistant stainless steel must be used.

Step 11: Drawing fit chart

After aligning formwork and related content is determined, assembly drawing can be drawn. In process of assembly drawing, further coordination and improvement of selected gating system, cooling system, core pulling system, ejector system, to achieve a more perfect design from structure.

Step 12: Drawing of mold main part

When design a cavity or core drawing, necessary molding dimensions, tolerances, and draft angles are coordinated, and design basis is coordinated with design basis of product. At the same time, it is also necessary to consider processability of cavity and core during processing, mechanical properties and reliability during use.
When design a structural part drawing, when a standard formwork is used, most of structural parts other than standard formwork may not be drawn.

Step 13: Proofreading of design drawings

After mold drawing design is completed, mold designer will submit design drawings and related original materials to supervisor for proofreading.
Proofreading personnel shall systematically proofread overall structure, working principle and feasibility of mold according to relevant design basis and requirements provided by customer.

Step 14: Designing drawing

Once mold design drawings are completed, they must be submitted to customer for approval. Only after customer agrees, mold can be put into production. When a customer has a large opinion that requires major changes, it must be re-designed and then handed over to customer for approval until customer is satisfied.

Step 15: Designing exhaust system

Exhaust system plays a vital role in ensuring quality of product. Exhaust methods are as follows:
1. Use exhaust slot. Venting groove is generally located at portion where cavity is finally filled. Depth of venting groove varies from plastic and is essentially determined by maximum clearance allowed when plastic does not create a flash.
2. Use matching clearance of core, insert, push rod, or special exhaust plug to exhaust.
3, Sometimes in order to prevent vacuum deformation of product in the top, it is necessary to design exhaust insert.
molding design 


Combine above molding design procedures, some of which can be considered together, and some content should be considered repeatedly. Because factors are often contradictory, they must be continuously demonstrated and coordinated in design process to get better treatment. In particular, content related to mold structure must be taken seriously, and often several solutions should be considered at the same time. For each structure, list advantages and disadvantages of each aspect as much as possible, and analyzes them one by one to optimize. Structural reasons will directly affect manufacture and use of mold, serious consequences will even cause entire mold to be scrapped. Therefore, molding design is a key step to ensure quality of mold, and its design process is a systematic project.

Key points in mold design:

1. Reasonable overall layout of mold.
2, Choice of parting surface
3, Layout of flow channel, selection of gate
4, Ejection device
5, Water distribution
6, Choice of exhaust
7. Pay attention to draft angle when mold is divided, extraction of insert, treatment of rubbing angle, and shrinkage of material.
8, Processing drawing should be detailed, but simple.

How do we simplify mold structure to the greatest extent possible?

On one hand, when reviewing products with customers, it is necessary to highlight problems of mold release brought by product to mold opening, whether to improve products: on other hand, whether meeting is held in the company before mold design, soliciting various feasible structural proposals to determine final plan from perspective of customer requirements.
1. Feasibility analysis of products of designed molds. Taking computer case as an example, firstly, assembly drawings of each component are analyzed by design software, that is, set of drawings mentioned in the work to ensure correctness of drawings of each product before molding design. On other hand, you can familiarize yourself with importance of each component in whole chassis to determine key size, which is very beneficial in mold design.
molding design 
2. After product analysis, analyze product using mold structure, carry out process of product, determine content of each process, and use design software to carry out product development. When product is launched, it generally starts from subsequent project, for example, a product requires five processes, stamping is completed from product drawing to fourth, third, second, and first projects, a graphic is copied and then previous project is executed. That is, product of five projects is completed, and then detailed work is carried out. Note that this step is very important and needs to be very careful. If this step is completed well, it will save a lot of time in drawing mold diagram. After stamping content of each project is determined, including in the forming mold, inner and outer lines of thickness of product material are retained to determine size of convex and concave molds.
3. Prepare materials. According to product development drawing, determine size of template in  drawings, including fixed plates, unloading plates, convex and concave molds, inserts, etc., pay attention to preparation directly in product development drawing, which is of great benefit to drawing of mold drawing. I have seen some mold designer directly calculate product drawing to prepare material. This method is too inefficient, draw template size directly on the drawing, and express it in form of a group diagram. In terms of preparation of materials, on other hand, a lot of work is saved in work of various parts of mold, because in work of drawing each component, it is only necessary to add positioning, pins, guide posts and screw holes in preparation drawing.
4. After preparation is completed, mold drawing can be fully entered, and a copy can be made in preparation drawing to draw components, such as adding screw holes, guide post holes, positioning holes, etc. And in punching die. Hole for each hole needs to be cut by wire. In forming die, forming gap of upper and lower molds must not be forgotten, so mold drawing of one product after completion of these work is almost completed 80%, In addition, in process of designing mold drawing, it is necessary to pay attention to: each process, refers to production, such as fitter scribing, wire cutting, etc. to different processing steps have a complete production of layer, which has great benefits for wire cutting and drawing management, such as color distinction, etc. Dimensional marking is also a very important job, and it is also the most troublesome job, because it is too time consuming.
5. After above drawings are completed, drawings cannot be issued. It is also necessary to proofread mold drawings, assemble all accessories, make different layers for each different mold plate, and use same basis as guide post holes to perform mold. Insert product development drawings into group diagrams to ensure that hole positions of template are consistent and gap between upper and lower molds of bending position is correct.
product development 

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