At present, most of the exterior parts of household appliances are obtained by injection molding. During the injection molding process, defects such as weld marks, air marks, and deformation are prone to occur; high-gloss traceless molds can solve the above defects. Let's take a look at the ten elements of high-gloss, traceless injection mold design.
1. The principle of high-gloss traceless injection molding
1.Higher temperature
Mold molding has high temperature requirements (generally around 80℃-130℃). After the injection molding switches to pressure holding, cooling water is used to reduce the mold temperature to 60-70℃. Holding pressure molding at a higher mold temperature is beneficial to eliminating defects such as weld lines, flow marks, and internal stress in the product. Therefore, the mold needs to be heated during operation. In order to prevent heat loss, a heat insulation board is usually added to the fixed mold side.
2. The surface of the mold cavity is extremely bright (generally mirror level 2 or higher)
Products produced by high-gloss molds can be directly used for assembly (assembly) without any surface treatment. Therefore, it has very high requirements for mold steel and plastic materials.
3. The hot runner system has more hot nozzles
Each hot nozzle must be equipped with a sealing needle and have an independent air channel. It is individually controlled through solenoid valves and time relays to achieve time-sharing glue feeding, thereby achieving the purpose of controlling or even eliminating weld marks. The control method is complex.
4.Heating method
There are usually two methods of mold heating: steam (hot water) heating and electric heating rod (tube) heating. The water vapor (hot water) heating method is to input steam (hot water) into the mold during the injection molding process through a specific temperature control machine, so that the mold quickly heats up; after the injection molding is completed, the mold is cooled with cold water to quickly cool down the mold. The electric heating method is the same as the water heating temperature control machine in principle, but the heat source is different. Electric heating is a secondary energy, and water heating is a tertiary energy. According to the principle, electric heating consumes less energy and has a high utilization rate. Good energy saving benefits. It is easy to use, so if it is a flat (surface) product, it is better to use electric heating.
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Figure: Water vapor heating
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Picture: Heating rod heating
2. Mold material
1. Mold materials with common requirements for product surface are available: NK80 (Datong, Japan), etc.;
2. Material selection for high gloss requirements: S136H (Sweden), CEANA1 (Japan), etc.;
3. NK80 does not need quenching treatment; S136H should be quenched to 52 degrees after rough machining; CEANA1 itself has 42 degrees and does not need quenching treatment (it is recommended to use this steel because it will not affect subsequent processing or modifications);
4. There are also good choices in the German Glitz brand: CPM40/GEST80
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Figure High Gloss Mold
3. Mold water channel design
1. Water channel aperture size design
The water channel uses a hole diameter of 5-6mm; the water nozzle uses 1/8 or 3/8 threads (mold side), and the other side uses 3/4 inch thread (old-fashioned connection method); the pipe fittings are made of stainless steel pipes; now we change One in and one out, the shunt port is best made in the mold, and the interface is connected with a diameter of DN25, so that the heat loss is less, the operation is convenient, and the interface is convenient.
2. Product surface design
The distance between the side of the water channel and the product surface is generally 5-6mm; if it is larger, it will affect the heating time of the mold, and if it is smaller, it will affect the strength of the mold. The parallel product surface of the water channel must be evenly arranged (distributed at an equal distance of 15mm from the center of the original material). The thermocouple should be designed in the middle of the two water channels, with a depth of more than 50mm, and a maximum of no more than 100mm, which can be flexibly controlled depending on the mold structure. Each set of mold PT100 is matched with one. To maintain its accuracy, it must be inserted into the core of the mold cavity and fixed. Connect the lead wire to the outside of the mold and then to the socket of the temperature controller.
3. Mold water channel joint design
The mold water channel joints must be designed at the upper and lower sides or the rear end of the mold; no water channel inlets and outlets or water pipe arrangements are allowed on the operating side (station side) to avoid pipe rupture and injury to production personnel. Remember!
4. Design of mold inlet and outlet nozzle
The mold inlet and outlet nozzles are designed with a splitter plate. The hydrothermal mold temperature control machine system has only one inlet and one outlet interface to reduce excessive water pipe connections and unnecessary loss of heat energy; and achieve the goals of safety and energy saving. And the outer surface of the corrugated pipe is wrapped with heat-insulating tape to play the role of heat preservation and safety.
5. Construction holes of the mold
The construction holes (unnecessary holes) of the mold should be plugged with plugs to ensure that there is no air or water leakage. The method is to plug them with copper first, and then seal them with tapered throat teeth and high-temperature resistant glue; Comparison of the arrangement of cooling water channels in high-gloss molds Pay attention (the water channels of the hydrothermal mold are shared). Good water channel arrangement can not only greatly improve the efficiency of injection molding, but also play an important role in improving product quality. The water channels of the high-gloss mold must not only be uniform but also sufficient (a sufficient number).
This heats up the mold quickly; at the same time, using an extended water pipe to directly transport water out of the mold core without using a sealing ring can prevent the mold from operating at high temperatures for a long time, causing the sealing ring to age, and can also reduce the maintenance costs of many molds. It is worth mentioning that the water pipe of the high-gloss mold must be made of high-temperature resistant material (250°C) corrugated pipe.
High pressure 1.6Mpa corrugated pipe to prevent water pipe bursting under high temperature and high pressure. For round products, circular water transport is used; for long strip products, parallel water transport channels are used. For products with large height differences, a water well form is used; for special-shaped products, a three-dimensional water transportation method consistent with the product appearance is used.
4. Mold insulation system
1.Mold core design
The four sides of the fixed mold core or movable mold core must be hollowed out; there must be a certain gap between the mold frame and the core (depending on the thermal expansion coefficient of the mold material, 1 mm on one side). Prevent the expansion of the mold frame to reduce the contact surface between the mold core and the mold frame to minimize heat loss; the mold core and the mold frame are locked using an oblique or other similar method, and the front end is made of dust resin or dust resin with obvious heat insulation effect. Other materials (such as asbestos boards).
2. Mold frame design
The cooling water of the mold frame is very important for the detailed structure of the mold frame and the core. In order to prevent the heat energy in the mold core from being transmitted to the mold frame, a circle of water transport should be arranged up and down near the guide pillar.
3. Guide sleeve design
The moving part of the guide sleeve should be made of graphite material as much as possible or the front end of the guide post should be avoided. It is enough to ensure that the length of the fitting part is 25mm;
5. Mold gate design
The mold gate design should reduce weld marks as much as possible and facilitate exhaust and reduce shearing. For molds that use water-heated temperature controllers, the gate size should be larger and large gates should be used to feed glue. Without affecting product function and molding efficiency, the length, depth, and width of the gate should be shortened as much as possible.
1. The gate is too small
If the gate is too small, it will easily cause appearance defects such as insufficient filling (short shots), shrinkage dents, and weld lines, and the molding shrinkage will increase.
2. The gate is too large
If the gate is too large, excessive residual stress will be generated around the gate, resulting in deformation or cracking of the gate, and it will be difficult to remove the gate.
It is better to use a gate unless the flow ratio exceeds practical limits. The resin flow length curve will provide the flow length of the material under certain molding conditions. Multiple gates often produce weld lines and weld marks. In addition to long and narrow products, the use of a single gate will ensure a more consistent distribution of materials, temperatures, and holding pressures for better matching effects.
6. Mold exhaust
Try to space 10mm apart around the product as much as possible, and evenly distribute exhaust grooves with a depth of 0.15mm; the middle veneer of the product also needs an exhaust design.
7. Mold parting surface coordination
Because there is a large temperature difference between high-gloss molds, the veneer coordination requirements are relatively high. At the same time, the area of the veneer must be reduced. A 10mm fit around the parting surface is enough.
8. Heating rod (tube) high-gloss mold design
1. There should be electric heating rods (tubes) on the upper and lower sides of the gate. The cooling water hole is generally 6mm (the larger is better); the distance between the centers of the two water holes is 15-20mm; the distance between the heating rod wall and the product surface is 5mm. The center distance between the heating rods is 20mm; the distance between the cooling water and the heating rod wall is 6-8mm. If possible, it is best to intersperse the electric heating rods.
2. The water transport in the inner mold cavity can be sealed with a high temperature resistant sealing ring or a hard seal.
3. The diameter of the heating rod is 4.92mm, and the diameter of the mold is 5mm. Before assembling the heating rod, use a 5mm thimble to sharpen the edge and remove the burrs of the heating rod.
4. The mold inlet and outlet nozzles use the same manifold design (cooling water) as the water vapor heating mold, because the electric heating mold control system only has one inlet and one outlet water pipe.
9. Product requirements for high-gloss molds
High-gloss molds have strict requirements on product structure. The brighter the product, the more sensitive it is to the refraction effect of light. Slight defects on the surface will be quickly discovered. Therefore, how to solve the shrinkage problem is the primary issue for high-gloss products. Generally, if the rib thickness of a product does not exceed 0.6mm times the thickness of the main glue position, it will not shrink. In other words, the shrinkage is small and difficult to detect, so it can be ignored. But for high-gloss products, such requirements are far from enough. The thickness of the ribs of the product must be reduced to no more than 1 times the thickness of the main glue. The screw columns must also have a crater-type sloped roof structure.
10. Selection of plastic materials for high-gloss molds
Currently, commonly used high-gloss plastic materials are generally ABS+PMMA, ABS+PC, PMMA, ASA, etc.
As a commonly used case material, ABS+PC products are better than HIPS in terms of impact resistance, surface gloss and hardness, so when producing high-gloss products, high-gloss ABS materials are usually used. If you need weather resistance, you may choose ASA, and in terms of hardness, you may choose PMMA alloy material. Let's talk about ABS material in detail.
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1. How to control the melt viscosity of ABS?
ABS is an amorphous polymer with no obvious melting point. Due to the wide variety of grades and grades, appropriate process parameters should be formulated according to different grades during the injection molding process. Generally, molding can be performed above 160°C and below 270°C. During the molding process, ABS has good thermal stability, a wide range of options, and is not prone to degradation or decomposition. Moreover, the melt viscosity of ABS is moderate, and its fluidity is better than that of polystyrene (PS), polycarbonate (PC), etc., and the cooling and solidification speed of the melt is relatively fast, usually within 5 to 15 seconds.
2. How to control the water absorption rate of ABS?
The fluidity of ABS is related to both injection temperature and injection pressure, with injection pressure being slightly more sensitive. For this reason, the injection pressure can be started during the molding process to reduce the melt viscosity and improve the mold filling performance. ABS has different water absorption and adhesion properties due to different components. Its surface adhesion and water absorption rate ranges from 0.2% to 0.5%, sometimes up to 0.3% to 0.8%. In order to obtain a more ideal product, Drying is performed before molding to reduce the moisture content to less than 0.1%. Otherwise, defects such as bubbles and silver threads will appear on the surface of the product. Usually plastic materials need to add 1% metal powder to improve the high-gloss metal effect.
11. Polishing and maintenance of molds
The polishing mentioned in plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking: the polishing of molds should be called mirror processing. It not only has high requirements for the polishing itself but also has high standards for surface flatness, smoothness and geometric accuracy. Surface polishing generally only requires obtaining a bright surface. The standard for mirror processing is divided into four levels: AO=Ra0.008um, A1=Ra0.016um, A3=Ra0.032um, A4=Ra0.063um. Since it is difficult to accurately control the geometric accuracy of parts by methods such as electrolytic polishing and fluid polishing, However, the surface quality of chemical polishing, ultrasonic polishing, magnetic grinding and polishing and other methods cannot meet the requirements, so the mirror processing of precision molds is still mainly mechanical polishing.
1. Basic procedures for mechanical polishing. In order to obtain high-quality polishing effects, the most important thing is to have high-quality polishing tools and auxiliary products such as oilstone, sandpaper and abrasive paste. The most important thing is the polishing working environment, which requires a dust-free workshop. The choice of polishing procedure depends on the surface conditions of pre-processing, such as machining, EDM, grinding, etc.
2. The general process of mechanical polishing is as follows:
1. The surface after rough polishing, fine milling, EDM, grinding and other processes can be polished with a rotating surface polisher or ultrasonic grinder with a speed of 35000-40000rpm. The commonly used method is to use a wheel with a diameter of 3mm and WA#400 to remove the white spark layer. Then there is manual whetstone grinding, and the strip whetstone is added with kerosene as a lubricant or coolant. The general order of use is #180-#240-#400-#600-#1000. Many moldmakers choose to start with #400 to save time.
3. Semi-finish polishing mainly uses sandpaper and kerosene. The number of sandpaper is: #400-#600-#800-#1000-#1200-#1500. In fact, #1500 sandpaper is only suitable for hardened mold steel (above 52HRC) and is not suitable for pre-hardened steel, because this may cause surface burns on the pre-hardened steel parts.
4. Fine polishing mainly uses diamond grinding paste. The usual grinding sequence is 9um(#1800)-6um(#3000)-um(8000). 9um diamond abrasive paste and polishing cloth wheel can be used to remove hair-like grinding marks left by #1200 and #1500 sandpaper. Then use sticky felt and diamond abrasive paste for polishing, in the order of 1um (#14000)-1/2um (60000)-1/4um (#100000). Polishing processes that require an accuracy of 1um or above (including 1um) require an absolutely clean space for mold polishing. Dust, smoke, dandruff and drool can ruin the highly polished surface you get after hours of work.
2. 1. Issues to pay attention to during mechanical polishing. When polishing with sandpaper, you should pay attention to the following points;
1. Polishing with sandpaper requires the use of soft wooden sticks or bamboo sticks. When polishing a round or spherical surface, using a cork stick can better match the curvature of the round or spherical surface. Harder strips of wood, like cherry, are more suitable for polishing flat surfaces. Trim the ends of the wooden strips so that they match the surface shape of the steel parts. This will prevent the sharp angles of the wooden strips from contacting the surface of the steel parts and causing deep scratches.
2. When using different types of sandpaper, the polishing direction should be changed by 45°-90°. The stripe shadow left by the previous type of sandpaper after polishing can be analyzed. Before changing to a different type of sandpaper, you must carefully wipe the polishing surface with 100% cotton dipped in a cleaning solution such as alcohol, because a small gravel left on the surface will destroy the entire subsequent polishing work. This bucket cleaning process is equally important when switching from sandpaper polishing to diamond abrasive paste polishing. All particles and kerosene must be completely cleaned before polishing can continue.
3. In order to avoid scratching and burning the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is necessary to apply a light load and polish the surface using a two-step polishing method. When polishing with each type of sandpaper, polishing should be carried out on two sides and three times in two different directions, with each rotation of 45°-90° between the two sides and three directions.
3. The following points should be paid attention to when grinding and polishing diamonds;
1. This kind of polishing must be carried out under lighter pressure as much as possible, especially polishing
When polishing pre-hardened steel parts with fine abrasive paste. When using #8000 abrasive paste, the common load is 100-200g/cm², but it is difficult to maintain the accuracy of this load. To make this easier, you can make a thin, narrow handle on the wooden strip, such as adding a piece of copper; or you can remove part of the bamboo strip to make it softer. This can help control the polishing pressure to ensure that the pressure on the mold surface is not too high.
2. When using diamond grinding and polishing, not only the working surface must be clean, but the workers' hands must also be carefully cleaned.
3. Each polishing time should not be too long. The shorter the time, the better the effect. Pitting can occur if the polishing process is carried out for too long.
4. In order to obtain high-quality polishing results, polishing methods and tools that are prone to heat should be avoided. For example; when polishing with a polishing wheel, the heat generated by the polishing wheel can easily cause orange peel.
5. When the polishing process is stopped, it is very important to ensure that the surface of the workpiece is clean and to carefully remove all abrasives and lubricants. Then a layer of mold anti-rust coating should be sprayed on the surface.
4. Factors affecting mold polishing quality
Since mechanical polishing is mainly done manually, polishing technology is still the main factor affecting polishing quality. In addition, it is also related to the mold material, surface condition before polishing, heat treatment process, etc. High-quality steel is a prerequisite for good polishing quality. If the surface hardness of the steel is uneven or there are differences in characteristics, polishing difficulties will often occur. Various debris and pores in steel are not conducive to polishing.
1. The influence of different hardness on polishing process
2. Increased hardness makes grinding more difficult, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve lower roughness increases accordingly. At the same time, the hardness increases and the possibility of over-polishing decreases.
3. The influence of workpiece surface condition on polishing process
During the crushing process of steel cutting machinery, the surface will be damaged due to heat, internal stress or other factors. Improper cutting parameters will affect the polishing effect, so high-speed CNC finishing is required, and the processing cutting amount is controlled at 0.05-0.07mm.JN The surface after EDM processing is more difficult to grind than the surface after ordinary machining or heat treatment. Therefore, precise EDM dressing should be used before the end of EDM processing, otherwise a hardened layer will form on the surface. If the EDM finishing specifications are improperly selected, the depth of the heat-affected layer can reach up to 0.4mm. The hardness of the hardened layer is higher than the basic hardness and must be removed. Therefore, it is best to add a rough grinding process to completely remove the damaged surface layer and form an evenly rough metal surface, which provides a good foundation for polishing.
12. Maintenance of high-gloss mold
1. The surface of the mold workpiece must usually be covered with a high-grade anti-rust agent or sealed with plastic wrap to prevent direct contact with air and cause rust;
2. Prevent any debris or hands from direct contact with the cavity surface;
3. When cleaning the mirror surface, high-density paper towels should be sprayed with cleaning agent and scrubbed gently from top to bottom, and cannot be scrubbed back and forth; medical cotton and cloth strips cannot be used; a gun cannot be used to blow directly on the workpiece, because the air in the trachea is Debris and moisture can cause damage to the work surface.
4. After each mold production or mold trial, the water channel of the mold must be blown clean with a gun to prevent the mold core from rusting.
