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Top ten elements of high-gloss seamless plastic injection mould design that you must know!

Time：2019-05-08 09:16:31 / Popularity： / Source：

 
At present, most of exterior parts of household appliances are obtained by injection molding. In injection molding process, defects such as weld lines, gas marks, and deformation are prone to occur; high-gloss non-marking molds can solve above defects. Let's take a look at top ten elements of high-gloss seamless plastic injection mould design.
  • Principle of high-gloss seamless injection molding
1, Higher Temperature
Molding requires a higher temperature (generally 80℃ -130℃). After injection into pressure holding, use cooling water to reduce mold temperature to 60-70℃. Pressure-preserving molding at higher mold temperatures is beneficial to eliminate defects such as weld lines, flow marks, and internal stress in the product. Therefore, mold needs to be heat treated during operation. In order to prevent heat loss, a heat insulation board is usually added on fixed mold side.
2, Cavity surface is extremely bright (generally mirror level 2 or higher)
Products produced by high-gloss injection molding can be directly used for installation (assembly) without any surface treatment. Therefore, it has high requirements for mold steel and molding materials.
3, Hot runner system has more hot nozzles
Each thermal nozzle must have a sealing needle and a separate air passage. It can be individually controlled by a solenoid valve and a time relay to realize time-divisional feeding, thereby achieving purpose of controlling or even eliminating welding mark, control method is complicated.
4, Heating method
The way in which mold is heated is usually steam (hot water) heating and electric heating rod (tube) heating. Steam (hot water) heating method is to input steam (hot water) into mold during injection molding process through a specific temperature control machine, so that mold is rapidly heated; after injection molding, mold is cooled by cold water to rapidly cool the mold. Electric heating method and water heating temperature control machine are same in principle, only different if heat source. Electric heating is the secondary energy, and water heating is the third energy. According to principle, electric heating energy loss is small, utilization rate is high, energy saving benefits are good, and easy to use, so if it is a flat product, electric heating method are more suitable.
  • Molding materials
1. Molding materials of general requirements of product surface: NK80 (Japan Datong);
2. High light requirements molding materials: S136H (Sweden), CEANA1 (Japan), etc.;
3. NK80 can be quenched without treatment; S136H should be quenched to 52 degrees after roughing; CEANA1 itself has 42 degrees and does not require quenching treatment (it is recommended to use this steel, as it does not affect subsequent processing or modification);
4. There is also a good choice in the German Gritz brand: CPM40/GEST80
  • Mold waterway design
1. Water channel aperture size design
Water channel adopts a hole diameter of 5-6mm; nozzle uses 1/8 or 3/8 thread (mold side), the other side uses 3/4 inch thread (old-fashion connection); pipe material is made of stainless steel tube; now we change into one out and one out, split port is preferably made in mold, and interface is connected by DN25, so that heat energy loss is small, operation is convenient, and interface is convenient.
2. Product design
Side of waterway is generally 5-6mm away from product surface; large selection has an effect on heating time of mold, and selection of small has an influence on the strength of mold. Parallel product surfaces of waterways must be evenly arranged (15mm equidistant distribution in the center of original material). Thermocouples should be designed in the middle of two waterways. Depth should be more than 50mm, and maximum should not exceed 100mm. Each set of mold PT100 is one-fitted. To maintain its accuracy, it must be inserted into mold cavity and fixed. Use a lead to connect to outside of mold and connect to temperature control socket.
3. Mold waterway joint design
Mold waterway joint must be designed on the upper and lower side or rear side of mold. Operation side (person on station side) is not allowed to have water inlet and outlet or water pipe arrangement to avoid pipe rupture and hurt production personnel.
4. Mold inlet and outlet nozzle design
Mold inlet and outlet nozzle adopts splitter plate design. Water-heat mold temperature control system has only one inlet and outlet interface to reduce excessive water pipe connection and reduce unnecessary loss of heat energy, achieve purpose of energy saving. And outer surface of bellows is wrapped with heat-insulating tape to play the role of heat preservation.
5. Mold construction holes
Construction hole of injection molding should be blocked with plug to ensure no leakage. Method is to block copper first, then use taper throat to add high temperature resistant rubber seal. Layout of cooling water channel for high-gross mold is more particular(hydrothermal mold channel is shared), good water channel layout can not only greatly improve injection efficiency, but also play an important role in improving product quality. Water channel of high-gloss injection molding should not only be uniform but also must be sufficient (to have a sufficient amount).
In this way, mold is heated up quickly; at the same time long water pipe is used to directly transport mold core water without using a sealing ring, which can prevent mold from working at high temperature for a long time, resulting in aging of sealing ring and reducing maintenance cost of many molds. It is worth mentioning that water pipe of high-gloss injection molding must use a high temperature resistant material (250℃) bellows.
High-pressure 1.6Mpa bellows to prevent bursting of water pipes under high temperature and high pressure. For round products, circular water is used; for long products, parallel water channels are used. For products with large height difference, water well form is adopted; for special-shaped products, three-dimensional water transportation method consistent with appearance of product is adopted.
 
  • Mold insulation system
1. Mold core design
Mold set core or movable mold insert shall be hollowed out on all four sides; mold frame and insert core shall have a certain gap (depending on thermal expansion coefficient of mold material, 1 mm on one side). Prevent expansion of mold frame to reduce contact surface of mold core on mold frame, so as to minimize loss of heat energy; lock of mold core and mold frame adopts oblique type or something like that, and front end adopts dust resin with obvious heat insulation effect or other materials (such as asbestos board).
2. Plastic injection mould design
Detailed structure of mold frame and core, cooling water of mold frame is very important. In order to prevent heat energy in the mold core from being transmitted to mold frame, a water circulation is arranged at a position close to guide column.
3. Guide sleeve design
Moving part of guide sleeve should be made of graphite material as far as possible or front end of guide post should be avoided. It is sufficient to ensure length of joint is 25mm.
  • Mold gate design
Mold gates are designed to minimize weld lines and to facilitate venting and shear reduction. For mold using water heating temperature control machine, gate size should be larger as much as possible. Gate should be shortened as much as possible without affecting product function and molding efficiency.
1. Gate is too small
If gate is too small, it is likely to cause defects such as insufficient filling (short shot), shrinkage depression, and weld line, and molding shrinkage will increase.
2. Gate is too large
If gate is too large, residual stresses may occur around gate, resulting in deformation or cracking of product port, and difficulty in removing gate.
It is best to use a gate unless flow ratio exceeds actual limit. Flow length plot of resin will provide flow length of material under a defined molding condition. Multiple gates often produce weld lines. In addition to long, narrow products, use of a single gate will ensure a more consistent material, temperature, and pressure distribution for better fit.
  • Mold exhaust
Exhaust groove is evenly distributed as far as possible by a section of 10mm around product, depth is 0.15mm; intermediate veneer of product also needs exhaust design.
  • Mold parting surface matching
Because temperature of high-gloss injection molding has a large drop, veneering requirements are higher, and at the same time area of veneer is reduced. It is enough to have a 10mm fit around parting surface.
  • Heating rod (tube) high-gross plastic injection mould design
1. There should be electric heating rods (tubes) on the upper and lower sides of gate. Cooling water holes are generally 6mm (it can be bigger and better); distance between two water holes is 15-20mm; distance between heating rod walls and surface of product is 5mm, distance betwwen two center of heating rods is 20 mm; distance between cooling water and heating rod wall is 6-8 mm, and it is better to intersperse with electric heating rod if necessary.
2, Inner mold cavity water can be sealed with high temperature sealing ring or hard sealing.
3. Diameter of heating rod is 4.92mm, and plastic injection mould design is 5mm. Before assembling heating rod, burr of heating rod is removed with a 5mm thimble grinding edge.
4. Mold inlet and outlet nozzles adopt same splitter plate design (cooling water) as steam heating mold, because electric mold control system only has one inlet and outlet water pipeline.
  • High-gross injection molding requirements for products
High-gross injection molding has strict requirements on product structure. The brighter the product is more sensitive to light refraction effect, surface is slightly defective and will be discovered soon. Therefore, how to solve shrinkage problem is primary problem of high-gloss products. Thickness of general product ribs does not exceed 0.6 mm of thickness of main body glue and does not shrink, or shrinkage is small and is not easy to be found, which can be ignored. However, for high-gloss products, such requirements are far from enough. Thickness of product ribs should be reduced to not more than 1 times thickness of main body glue. For screw column, a crater-type sloping roof structure must also be used.
  • Selection of plastic molding materials for high-gloss injection molding
At present, commonly used high-gloss plastic molding materials are generally ABS+PMMA and ABS+PC, PMMA, ASA, etc.
As a commonly used casing material, ABS+PC products are better than HIPS in terms of impact resistance, surface gloss and hardness, so high-gloss ABS materials are usually used in production of high-gloss products. If weather resistance is required, ASA can be selected and PMMA alloy material may be selected for hardness. Let's talk about ABS material in detail.
 
1. How to control melt viscosity of ABS?
ABS is an amorphous polymer with no significant melting point. Due to wide variety of grades, appropriate process parameters should be formulated according to grades in plastic molding process, generally above 160℃, below 270℃. In plastic molding process, ABS has good thermal stability, and range of options is large, and it is not easy to degrade or decompose. And ABS has a moderate melt viscosity, its fluidity is better than polystyrene (PS), polycarbonate (PC), etc., melt solidification speed is relatively fast, and generally it can be cooled in 5 to 15 seconds.
2. How to control water absorption rate of ABS?
Fluidity of ABS is related to both injection temperature and injection pressure, with injection pressure being slightly more sensitive. For this reason, it is possible to start from injection pressure during plastic molding process to lower melt viscosity and improve filling performance. ABS has different properties of water absorption and adhesion to water due to different components. Surface adhesion water and water absorption rate are between 0.2% and 0.5%, and sometimes between 0.3% and 0.8%. In order to obtain a more ideal product, drying is carried out before molding to reduce water content to less than 0.1%. Otherwise, bubbles, silver, etc. will appear on the surface of part. Usually plastic materials need to add 1% metal powder to improve effect of high-gloss metal.
Last article：How to set your injection molding machine parameters? Next article：Mold exhaust is so important, you don't even know?

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