德语
With the rapid development of the plastic industry and the continuous improvement of general and engineering plastics in terms of strength, the application scope of plastic products is also expanding, and the use of plastic products is also increasing. Therefore, plastic molds have emerged. Plastic molds are a tool that matches plastic molding machines in the plastic processing industry, giving plastic products complete configuration and precise dimensions. Due to the wide variety of plastic products and processing methods, the structures of plastic molding machines and plastic products are diverse, so there are also many types and structures of plastic molds. The main performance requirements of plastic molds. In plastic film molding, the effect of the quality of the mold on product quality is self-evident. Different types of plastic products and different products have different requirements for the material, performance and shape of the mold. Driven by high-tech and the application demand of pillar industries, my country's plastic molds have formed a huge industrial chain. From the upstream raw and auxiliary materials industry and processing and testing equipment to the downstream major application industries such as machinery, automobiles, motorcycles, home appliances, electronic communications, and building materials, plastic molds have developed vitality. Below, the editor of Xianji.com will introduce to you the principles of selection and design process of plastic mold structure and parting surface.
Plastic mold structure
1. Pouring system
The channel leading the plastic to the cavity through the injection machine nozzle is called the casting system. It consists of a main channel, a split channel, an inner gate, a cold material hole and other structures, and consists of a casting sleeve of parts, a pull rod, etc.
2. Forming parts
It is a variety of parts that directly form the shape and size of plastic parts, consisting of a core (the inner shape of molded plastic parts), a cavity (the outer shape of molded plastic parts), a rod, an inlay block, etc.
III. Structural parts
Various parts that constitute the part structure play a role in installation, guidance, mechanism movement and temperature regulation in the mold. Guide parts: guide column, guide sleeve. Assembly parts: positioning gap, fixed mold base plate, fixed mold plate, moving mold plate, moving mold pad plate, mold foot cooling and heating system
Principles for selecting plastic mold parting surface
1. Overall principle of selection of parting surfaces
The selection of the parting surface not only affects the molding and demolding of the plastic parts, but also involves the mold structure and manufacturing cost. Therefore, it is necessary to pay attention to the selection of the parting surface. Generally speaking, there are three general principles for selecting participle:
1. Ensure the quality of plastic parts. This is the most basic thing, and the quality of the plastic parts must meet the predetermined requirements.
2. Easy to release plastic parts. Easy to release mold, can improve productivity, plastic parts are not easily deformed, and improve genuine product rate.
3. Simplify the mold structure. For the same plastic part, the complexity of the structure is very different due to the different selection of the parting surfaces. If you choose reasonably, you can simplify the mold structure.
2. How to choose a shaped surface
The relationship between the cavity and the mold can be basically divided into three categories: the cavity is completely in the moving mold; the cavity is completely in the fixed mold; the cavity is respectively in the moving and fixed mold. Due to the many shapes of plastic parts, there are many changes in the selection of parting surfaces. In order to give everyone a basic understanding of the choice of parting surfaces, the following are some typical parting surface selections.
1. The typing of long-shaped components is shown in the figure below. If the length of the plastic part is long, such as tubular, cylindrical, and rod-shaped plastic part, if the cavity is placed in a template, the demolding slope will be too large (Figure a). If the cavity is arranged in the moving and fixed molds respectively (Figure b), the demolding slope can be reduced so that the dimension difference between the two ends of the plastic part will not be too large.
a) Partitioning with excessive demolding slope;
b) Partitioning to reduce the demolding slope.
2. The parting of the plastic parts on the moving mold side, as shown in the figure below. Leaving the plastic part on the moving die side makes it easy to arrange and manufacture a mold release mechanism with a simple structure. Therefore, try to leave the plastic parts on the moving mold side. If the cover-shaped plastic parts are selected according to the selection of the parting surface according to the figure a; if the tape insert plastic parts are used, the core is tightened because the insert will not shrink, and the parting surface can be selected according to the figure b; if the core of the plastic parts is symmetrically distributed, the core should be followed. Partitioning forces the plastic parts to stay on the moving mold; if it is a plastic part with side holes, it should be classified according to Figure d to avoid fixed mold core pulling.
a) Cover-shaped plastic parts;
b) Plastic parts with inserts;
c) Core symmetrical plastic parts;
d) Plastic parts with side holes.
3. Parting to ensure the appearance quality of plastic parts, as shown in the figure below. The appearance quality of plastic parts needs to be guaranteed. Carefully consider the parting surface when choosing it. For example, on a smooth surface or an arc surface, try not to set a parting surface. Picture a shows a parting that keeps the surface smooth, picture b shows a parting that can reduce flash, and picture c shows a parting that can reduce flash.
a) Parting to make the surface smooth;
b) Reduce the parting of flash edges;
c) Reduce the type of spillage.
4. Parting that is conducive to exhaust, as shown in the figure below. In injection molds, the parting surface is often used as an exhaust channel. In order to ensure smooth exhaust, the parting surface should be set at the end of the melt flow. Note that there should be no obstruction at the end. The structure of pictures b and d is more reasonable than that of pictures a and c.
5. Parting to ensure coaxiality, as shown in the figure below. Many plastic parts have coaxiality requirements. This requirement should be ensured in the mold design. Generally, the parts with coaxiality requirements in the plastic parts should be designed in the same moving template to meet the accuracy requirements. Picture a can meet the coaxiality requirements, but picture b is not accurate in mold closing, making it difficult to meet the coaxiality requirements. (1-moving mold 2-fixed mold)
a) Parting to ensure coaxiality;
b) It is difficult to ensure the coaxiality of the parting.
6. Parting when there are side holes, as shown in the figure below. When selecting the parting surface, try to avoid using lateral core pulling or parting. If necessary, you can refer to the following principles. In Figure a, the side core is placed on the movable mold to facilitate core pulling. If it is placed on the fixed mold, core pulling is more difficult. In Figure C, it is more reasonable to place the core pulling distance with a long distance in the mold opening direction, and place the core pulling distance with a small distance in the side direction. However, if parting according to Figure d, it will cause difficulty in demoulding. (1—moving mold 2—fixed mold)
7. Ensure accurate parting, as shown in the figure below. Some surfaces on plastic parts require high precision, such as thread surfaces, mating surfaces, etc. At this time, the parting surface cannot pass through these surfaces, otherwise it will affect the accuracy of the plastic parts and even make it difficult to use. As shown in Figure A, the parting ensures the accuracy of the thread. The parting in Figure B passes through the thread axis, so the accuracy of the thread cannot be guaranteed. The parting in Figure C ensures the integrity and smoothness of the circular surface, while d has an obvious seam line around it, which affects the appearance and use.
a) Parting to ensure thread accuracy;
b) The parting accuracy of the thread cannot be guaranteed;
c) Parting to ensure smooth appearance of plastic parts;
d) The parting cannot guarantee the smooth appearance of the plastic parts.
8. Parting of thin-walled plastic parts requiring uniform wall thickness, as shown in the figure below. To make the wall thickness uniform, the common plane is not used as the parting surface, as shown in Figure b, but a tapered stepped parting surface as shown in Figure a.
a) Tapered stepped parting surface;
b) Plane parting surface.
Plastic mold design process
1. Accept the assignment letter
The task statement for molded plastic parts is usually proposed by the part designer, and its content is as follows:
⑴Regular production drawings that have been approved and signed, and indicate the grade, transparency, etc. of the plastic used.
⑵Instructions or technical requirements for plastic parts.
⑶Production output.
⑷Plastic parts samples.
Usually, the mold design task sheet is proposed by the plastic parts craftsman based on the task letter for molding plastic parts. The mold designer designs the mold based on the molding plastic parts task letter and the mold design task letter.
2. Collect, analyze and digest original data
Collect and organize information on part design, molding technology, molding equipment, mechanical processing and special processing for use in mold design.
⑴ Digest the drawings of plastic parts, understand the use of the parts, and analyze the technical requirements such as craftsmanship and dimensional accuracy of the plastic parts. For example, what are the requirements for plastic parts in terms of appearance, color transparency, and performance? Are the geometric structures, slopes, inserts, etc. of plastic parts reasonable? The allowable degree of molding defects such as weld marks and shrinkage holes. Is there any post-processing such as painting, electroplating, bonding, and drilling? Select the dimension with the highest dimensional accuracy of the plastic part for analysis to see if the estimated molding tolerance is lower than the tolerance of the plastic part, and whether plastic parts that meet the requirements can be molded. In addition, you also need to understand the plasticization and molding process parameters of plastics.
⑵ Digest the process data and analyze whether the molding method, equipment model, material specification, mold structure type and other requirements proposed in the process task book are appropriate and can be implemented. Molding materials should meet the strength requirements of plastic parts and have good fluidity, uniformity, isotropy, and thermal stability. According to the use of plastic parts, the molding material should meet the requirements for dyeing, metal plating conditions, decorative properties, necessary elasticity and plasticity, transparency or opposite reflective properties, adhesiveness or weldability.
3. Determine the molding method
Use direct pressure method, casting method or injection method.
4. Select molding equipment
Molds are made according to the type of molding equipment, so you must be familiar with the performance, specifications, and characteristics of various molding equipment. For example, for an injection machine, you should know the following in terms of specifications: injection capacity, clamping pressure, injection pressure, mold installation dimensions, ejection device and size, nozzle hole diameter and nozzle spherical radius, gate sleeve positioning ring size, maximum and minimum thickness of the mold, template stroke, etc. For details, see the relevant parameters. It is necessary to initially estimate the mold dimensions and determine whether the mold can be installed and used on the selected injection machine.
5. Specific structural plan:
⑴Determine the mold type
Such as compression molds (open, semi-closed, closed), casting molds, injection molds, etc.
⑵ Determine the main structure of the mold type
Choosing the ideal mold structure depends on determining the necessary molding equipment and the ideal number of cavities, so that the mold itself can work under absolutely reliable conditions to meet the process technology and production economic requirements of the plastic part. The technological requirements for plastic parts are to ensure the geometric shape, surface finish and dimensional accuracy of the plastic parts. The production economic requirements are to make the cost of plastic parts low, the production efficiency high, the mold can work continuously, the service life is long, and the labor is saved. There are many factors that affect the mold structure and individual mold systems, and they are very complex:
①Cavity layout. The number of cavities and their arrangement are determined based on the geometric structure characteristics of the plastic part, dimensional accuracy requirements, batch size, difficulty in mold manufacturing, mold cost, etc. For injection molds, the precision of plastic parts is level 3 and level 3a, the weight is 5 grams, the hardened pouring system is used, and the number of cavities is 4-6; the plastic parts are of general precision (level 4-5), the molding material is partially crystalline material, and the number of cavities is 16-20; the weight of the plastic part is 12-16 grams, and the number of cavities is 8-12; and for plastic parts weighing 50-100 grams, the number of cavities is 4-8. For amorphous plastic parts, the recommended number of cavities is 24-48, 16-32 and 6-10. When the weight of plastic parts continues to increase, multi-cavity molds are rarely used. For plastic parts with level 7-9 precision, the maximum number of cavities increases to 50% compared to the plastic parts with level 4-5 precision.
②Determine the parting surface. The position of the parting surface should be conducive to mold processing, exhaust, demoulding and molding operations, as well as the surface quality of plastic parts, etc.
③Determine the pouring system (shape, location, size of main runner, sub-runner and gate) and exhaust system (exhaust method, exhaust slot location, size).
④Select the ejection method (ejector rod, ejection tube, push plate, combined ejection), and determine the undercut treatment method and core pulling method.
⑤Determine the cooling and heating methods, the shape and position of the heating and cooling grooves, and the installation location of the heating element.
⑥ According to the mold material, strength calculation or empirical data, determine the thickness and outer dimensions of the mold parts, the outer structure and the positions of all connections, positioning and guide parts.
⑦ Determine the structural form of main molded parts and structural parts.
⑧Consider the strength of each part of the mold and calculate the working dimensions of the molded parts.
⑨Draw the mold diagram
It is required to be drawn in accordance with national drawing standards, but it is also required to combine the factory standards and factory customary drawing methods that are not specified by the country. Before drawing the mold assembly drawing, the process drawing should be drawn and must comply with the requirements of the part drawing and process data. The dimensions guaranteed by the next process should be marked with the words 'process dimensions' on the drawing. If no other mechanical processing is performed after molding except for burr repair, then the process drawing will be exactly the same as the part drawing. It is best to mark the part number, name, material, material shrinkage, drawing scale, etc. below the process diagram. Usually the process is drawn on the mold assembly drawing.
A. Draw the assembly structure diagram
When drawing the general assembly drawing, try to use a 1:1 ratio, start with the cavity, and draw the main view and other views at the same time. The mold assembly drawing should include the following:
①Mold forming part structure
②The structural form of the pouring system and exhaust system.
③Parting surface and parting pick-up method.
④The appearance structure and the position of all connecting parts, positioning and guide parts.
⑤ Mark the cavity height dimension (not required, as needed) and the overall size of the mold.
⑥Auxiliary tools (piece removal and mold removal tools, correction tools, etc.).
⑦Number all parts in order and fill in the detailed list.
⑧Mark technical requirements and instructions for use.
B. Technical requirements for mold assembly drawings:
① Performance requirements for certain systems of the mold. For example, there are assembly requirements for ejection systems and slider core-pulling structures.
②Requirements for mold assembly process. For example, after the mold is assembled, the fitting gap of the parting surface should be no greater than 0.05mm, and the parallelism requirements on the upper and lower sides of the mold should be noted, and the size determined by assembly and the requirements for this size should be pointed out.
③Mold use, assembly and disassembly methods.
④Anti-oxidation treatment, mold number, engraving, marking, oil seal, storage and other requirements.
⑤Requirements related to mold trial and inspection.
C. Draw all parts diagrams
The order of disassembling and drawing parts drawings from the mold assembly drawing should be: inside first then outside, complex first then simple, molded parts first, then structural parts.
①Graphic requirements: Must be drawn in proportion, and enlargement or reduction is allowed. Views are well chosen, projected correctly, and laid out appropriately. In order to make the processing patent number easy to understand and facilitate assembly, the graphics should be consistent with the assembly drawing as much as possible and the graphics should be clear.
② Dimensions must be unified, centralized, orderly and complete. The order of marking dimensions is: first mark the dimensions of the main parts and the draft angle, then mark the matching dimensions, and then mark all dimensions. On non-main parts drawings, mark the matching dimensions first and then all dimensions.
③Surface roughness. Mark the most commonly used roughness in the upper right corner of the drawing, such as 'Other 3, 2'. 'Other roughness symbols are marked on each surface of the part.
④Other contents, such as part name, mold drawing number, material grade, heat treatment and hardness requirements, surface treatment, graphic proportions, free-size processing accuracy, technical description, etc. must be filled in correctly.
D. Proofreading, reviewing drawings, tracing drawings, and sending them for printing
The contents of self-proofreading are:
①The relationship between the mold and its parts and the plastic part drawings. Whether the material, hardness, dimensional accuracy, structure, etc. of the mold and mold parts meet the requirements of the plastic part drawings.
②Plastic parts
Whether the flow of plastic material flow, shrinkage holes, weld marks, cracks, demoulding slope, etc. affect the performance, dimensional accuracy, surface quality and other requirements of plastic parts. Is there any deficiencies in the pattern design, is the processing simple, and is the shrinkage rate of the molding material selected correctly?
③Molding equipment
Whether the injection volume, injection pressure, and clamping force are sufficient, whether there are any problems with the installation of the mold, the core of the plastic parts, and the demoulding, and whether the nozzle of the injection machine and the mouth sleeve are in correct contact.
④Mold structure
a. Whether the parting surface position and finishing accuracy meet the requirements, will overflow occur, and whether the plastic parts can be ensured to remain on the side of the mold with the ejection device after the mold is opened.
b. Whether the demoulding method is correct, whether the size, position and quantity of the extension rod and push tube are appropriate, whether the push plate will be stuck by the core, and whether it will cause scratches on the molded parts.
c. Mold temperature adjustment. The power and quantity of the heater; whether the position, size and quantity of the flow line of the cooling medium are appropriate.
d. How to deal with undercuts in plastic parts, and whether the mechanism for removing undercuts is appropriate, such as whether the slider and push rod in the inclined guide column core-pulling mechanism interfere with each other.
e. Whether the location and size of the pouring and exhaust systems are appropriate.
f. Design drawings
g. Whether the placement position of each mold part on the assembly drawing is appropriate, whether the representation is clear, and whether there are any omissions
h. The part number, name, production quantity on the part drawing, whether the part is made in-house or outsourced, whether it is a standard part or a non-standard part, the processing accuracy of the parts, the correction processing and allowance of high-precision dimensions of the molded plastic parts, and whether the material, heat treatment, surface treatment, and surface finishing degree of the mold parts are clearly marked and described.
⑤ Working dimensions and matching dimensions of main parts, molded parts. Size figures should be correct and do not require manufacturer conversions.
⑥ Check the view position of all parts drawings and assembly drawings, whether the projection is correct, whether the drawing method complies with the national drawing standards, and whether there are any missing dimensions.
⑦ Check the processing performance: (Whether the geometric structure, view drawing method, size marking, etc. of all parts are conducive to processing)
⑧Main working dimensions of complex calculation auxiliary tools
In principle, professional proofreading is carried out according to the designer's self-checking project; however, it should focus on structural principles, process performance and operational safety. When tracing, you must first digest the graphics, draw them according to the national standard requirements, and fill in all dimensions and technical requirements. After tracing, self-correct and sign. Submit the traced base drawing to the designer for proofreading and signature. The common practice is for the relevant technical personnel of the tool manufacturing unit to review, countersign, and check the manufacturing process before sending it to the public.
⑨Write manufacturing process cards
The technical personnel of the tool manufacturing unit prepare the manufacturing process card and prepare it for processing and manufacturing. In the manufacturing process of mold parts, inspection should be strengthened and the focus of inspection should be on dimensional accuracy. After the mold is assembled, the inspector will inspect it according to the mold inspection form. The main thing is to check whether the performance of the mold parts is good. Only in this way can the manufacturing quality of the mold be known.
⑶Mold trial and mold repair
Although the mold design is carried out under the expected process conditions when selecting molding materials and molding equipment, people's understanding is often imperfect. Therefore, after the mold processing is completed, a trial mold test must be conducted to see the quality of the molded parts. After the discovery is made, the mold will be repaired to eliminate the error.
There are many types of defects in plastic parts, and the reasons are also very complicated. There are reasons for molds and process conditions, and the two are often intertwined. Before repairing the mold, a detailed analysis and study should be conducted based on the actual situation of the adverse phenomena in the plastic parts, and the causes of the defects in the plastic parts should be found out and remedies should be proposed. Because the molding conditions are easy to change, the general approach is to change the molding conditions first. When changing the molding conditions cannot solve the problem, repair the mold is considered.
You should be more cautious when repairing molds, and do not act rashly if you are not very sure. The reason is that once the mold conditions are changed, major modifications and restoration to the original state cannot be made.
6. Organize and archive data
1. After the mold is tested, if it is not used temporarily, the demoulding residue, dust, oil, etc. should be completely wiped off, coated with butter or other anti-rust oil or anti-rust agent, and kept in a storage place.
2. From the beginning of designing the mold to the successful processing and inspection of the mold, the technical data generated during this period, such as the mission statement, part drawings, technical instructions, mold assembly drawings, mold parts drawings, base drawings, mold design instructions, inspection record sheets, mold trial and mold repair records, etc., shall be systematically organized, bound, numbered and archived according to regulations. This may seem troublesome, but it will be very useful for repairing the mold and designing new molds in the future.
The above is the plastic mold structure, parting surface selection principles and design process introduced by the editor of Xianji.com. Here, the editor would like to remind everyone that the heat resistance of plastic molds has accelerated with the emergence of high-speed molding machinery, and the production speed of plastic products has accelerated. Since the molding temperature is generally between 160~350℃, some have poor fluidity and fast molding speed, which will cause the surface temperature of the mold part to exceed 400℃ in a very short time. When the working temperature of the mold is high, the hardness and strength of the mold will decrease, causing early wear or plastic deformation of the mold and failure. In order to ensure the accuracy and minimal deformation of the mold during use, the mold steel should have high heat resistance. To reduce temperature rise, mold steel should have good thermal conductivity and low thermal expansion coefficient. Most of the working conditions of molds with sufficient temperature at working temperature are very harsh, and some often bear large loads, resulting in brittle fracture. The extrusion molding pressure is relatively high, generally 10~35MPa. Therefore, in order to prevent mold parts from suddenly breaking during operation, the mold must have high strength and toughness.
Note: All pictures in the article are reprinted online, and will be deleted if infringed!
