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Requirements of High Gloss Plastic Injection Mold and Molding

Date: 2021-8-11

At present, most of the appearance parts of household appliances are through tradition injection molding. and it is easy to appear weld marks, gas marks, deformation and other defects; However,here in TKB mold China, we provide professional high gloss plastic injection mold and molding which can solve the above defects.Let’s take a look at the ten key elements of high gloss plastic injection mold design.

Requirements of high gloss plastic injection mold and molding

1 The principle of high gloss plastic injection mold and molding
1). High temperature
Mold forming has a higher temperature requirement (generally about 80℃-130℃). After injection molding is transferred to pressure holding, cooling water is used to reduce the temperature of the mold to 60-70℃.At higher die temperature, pressure holding molding is beneficial to eliminate defects such as weld connection, flow mark and internal stress of products. Therefore, the mold needs to be heated when it is working. In order to prevent heat loss, the heat shield is usually added on the fixed mold side.
2). Mold cavity surface is extremely bright (generally mirror level 2 or higher)
The products produced by the high gloss die can be directly used for installation (assembly) without any surface treatment. Therefore, it has high requirements for mould steel and plastic materials.
3). The hot runner system has more hot nozzle
Each hot nozzle must be equipped with a sealing needle and an independent airway, which can be controlled separately through solenoid valve and time relay to realize time-sharing glue feeding, so as to achieve the purpose of controlling or even eliminating weld marks. The control mode is complex.
4). Heating mode
Mold heating methods usually have water vapor (hot water) heating and electric heating rod (tube) heating. Steam (hot water) heating method is through a specific temperature control machine in the process of injection mold into the steam (hot water), so that the mold rapidly heating; After injection molding, use cold water to cool the mold, so that the mold quickly drops in temperature. The way of electric heating and water heating temperature controller is the same in principle, that is, the heat source is not the same, electric heating is secondary energy, water heating is three times of energy, according to the principle of electric heating energy loss, high utilization rate, good energy saving benefits. Easy to use, so: if it is a flat (surface) product or the use of electric heating real wisdom.
2 Mold Materials
1). Mold materials for common requirements of the product surface can be used: NK80(Datong, Japan), etc.;
2). Mold material selection: S136H(Sweden), CEANA1(Japan), etc.
3).NK80 can not be quenched;S136H should be quenched to 52 degrees after rough machining;CEANA1 itself has 42 degrees and does not need to be quenched (this steel is recommended, because it will not affect subsequent processing or modification);
4). There is also a good choice from German Gestz brand: CPM40/ Gest80
3 mold waterway design
1). Design of the aperture size of the watercourse
The channel adopts the aperture of 5-6mm;Water nozzle with 1/8 or 3/8 teeth (mold side), the other side with 3/4 inch thread (old-fashioned joint);Stainless steel pipe for pipe fitting material; Now we change it to one inlet and one outlet. It is better to make the outlet in the mold. The energy diameter of the interface is connected with DN25, so that the heat loss is less, the operation is convenient and the interface is convenient.
2). Product surface design
One side of the waterway from the product surface is generally 5-6mm;Large selection has an effect on the heating time of the mold, while small selection has an effect on the strength of the mold. The parallel product surface of the waterway shall be evenly arranged (the base center is 15mm equidistant distribution). The thermocouple shall be designed in the middle of the two waterways with a depth of more than 50mm and a maximum of not more than 100mm. It shall be flexible to master according to the structure of the mould. Each set of mold PT100 is one with one, to maintain its accuracy, it must be inserted into the mold cavity core, and to be fixed. Connect the lead to the outside of the mold, and then connect to the socket of the temperature controller.
3). Mold waterway joint design
Mold waterway joints must be designed at the upper and lower side ends or the rear side ends of the mold; No waterway inlet and outlet or water pipe arrangement is allowed on the operating side (the standing side) to avoid pipe rupture and injury to the production personnel. Remember that!
4). Mold inlet and outlet nozzle design
The inlet and outlet of the mold adopts the split-plate design. The temperature control system of the hydrothermal mold has only one inlet and one outlet interface, so as to reduce excessive water pipe connection and unnecessary loss of heat energy. And achieve the purpose of revolution and energy saving. And the bellows surface is wrapped with heat insulation tape, play a role in heat preservation and cleaning.
5). The construction hole of the mold
The construction hole of the mold (not the hole) should be blocked with a plug to ensure no air leakage and water leakage. The method is to block it with copper first, and then seal it with a taper throat tooth and high-temperature resistant glue. Highlight mould pays more attention to the arrangement of cooling channels (water and heat mould channels are common). Good channel arrangement can not only greatly improve the injection efficiency, but also play an important role in improving the quality of products. The channel of the highlight mold should not only be uniform but also must be sufficient (must have sufficient quantity).
In this way, the temperature of the mold is very fast; At the same time, the use of extended water pipe directly to carry the water out of the mold core without using the sealing ring, which can prevent the mold from working under high temperature for a long time, resulting in the aging of the sealing ring, but also reduce the maintenance cost of many molds. What is more, the high temperature resistant material (250℃) bellows must be used for the transport pipe of the high light mold.
High pressure 1.6MPa bellows to prevent high temperature and high pressure downpipe from bursting. For circular products using annular water transport; Parallel water channels are used for long – strip products. For products with large height drop, the well form is adopted; For special-shaped products, the three-dimensional water transport mode is one to the product appearance.
4 mold insulation system
1). Mold insert core design
Mold fixed die insert core or moving die insert core four sides to achieve hollowed processing; There should be a certain gap between the mold frame and the inset core (depending on the thermal expansion coefficient of the mold material, 1mm on one side).Prevent the expansion of the mold frame to reduce the contact surface between the mold core and the mold frame, so as to minimize the heat loss; The core and the mold frame are locked in an inclined or similar way, and the front end is made of dust resin or other materials (such as asbestos plate) with obvious heat insulation effect.
2). Mold frame design
The detailed structure of the mold frame and the inlaid core, the cooling water of the mold frame is very important. In order to prevent the heat energy in the mold kernel from being transferred to the mold frame, the water should be transported in a circle arranged up and down near the guide pillar
3). Guide sleeve design
The moving part of the guide sleeve is made of graphite material as far as possible or the front end of the guide column is avoided. The length of 25mm at the matching place is enough.
5 mold gate design
The die gate should be designed to minimize weld marks and to facilitate venting and reduce shearing. For the mold with water heating and temperature control machine, the gate size should be opened larger and the large gate should be used as far as possible. The length, depth and width of the gate should be shortened as far as possible without affecting the product function and forming efficiency.
1). The gate is too small
If the sprue is too small, it is easy to cause insufficient filling (short shot), shrinkage depression, welding wiring and other defects on the appearance, and the forming shrinkage will increase.
2). Too large gate
The sprue is too large, the residual stress around the sprue is too good, which leads to the deformation or fracture of the sprue, and the removal and processing of the sprue is difficult.
It is best to choose a gate unless the flow ratio exceeds the actual limit. The flow length diagram of the resin will provide the flow length of the material under a given molding condition. Multiple gates often produce weld wiring and weld marks. In addition to long and narrow products, the use of a single gate will ensure a more uniform distribution of material, temperature, and pressure retention for a better matching effect.
6 die exhaust
As far as possible in the product around 10mm apart as a section, evenly distributed exhaust slots, depth of 0.15mm;Exhaust design is also required for the middle veneer.
7  Mold parting surface matching
Because the temperature of the high light mold has a large drop, so the veneer with high requirements, at the same time to reduce the area of the veneer, parting surface around 10mm with enough.
8  Highlight mold design for heating rod (tube)
1). there should be electric heating rod (pipe) on the upper and lower sides of the gate, and the cooling water hole is generally 6mm(can be bigger and better);The center distance between the two water holes is 15-20mm;The distance between the heating rod wall and the product surface is 5mm, and the center distance between the two heating rods is 20mm.The distance between the cooling water and the heating rod wall is 6-8mm. If possible, it is best to intercut with the electric heating rod.
2). Water transport in the inner mold cavity can be sealed by high-temperature resistant sealing ring or by hard sealing method.
3). The diameter of the heating rod is 4.92mm, and the mold design is 5mm. Before assembling the heating rod, the burr of the heating rod is removed with the 5mm thimble grinding edge.
4). The inlet and outlet of the mould adopts the same shunt plate design (cooling water) as that of the steam heating mould, because the control system of the electric heating mould has only one inlet and one outlet water pipe.
9  Product requirements of high gloss mold
mold die has strict requirements on the product structure. The brighter the product is, the more sensitive it is to the refraction effect of light. A slight defect on the surface will be found soon. The general product rib thickness does not exceed 0.6mm of the thickness of the main glue level will not shrink, or the shrinkage is not easy to be found, can be ignored. But for high light products, such requirements are far from enough, but also need to reduce the thickness of the product rib to not more than 1 times the thickness of the main glue position, for the screw post must also do a crater type inclined top structure.
10  The choice of plastic material for the high gloss mold
At present, commonly used high-gloss plastic materials are ABS+PMMA and ABS+PC, PMMA, ASA, etc.
As a commonly used housing material, ABS+PC products are better than HIPS in impact resistance, surface gloss and hardness, so when producing high-gloss products, high-gloss ABS materials are usually selected. If you need weather resistance, you can choose ASA, and you may choose PMMA alloy materials in terms of hardness. Now let’s talk about the ABS material specifically.
1). How to control the melt viscosity of ABS?
ABS is an amorphous polymer with no obvious melting point. Due to the various grades of its brand, the appropriate process parameters should be formulated according to the different grades in the process of injection molding. Generally, it can be molded at more than 160℃ and below 270℃.In the molding process, ABS has good thermal stability, a wide range of options, and is not easy to degrade or decompose. And the melt viscosity of ABS is moderate, its mobility is better than polystyrene (PS), polycarbonate (PC), and the melt cooling and curing speed is relatively fast, generally in 5 ~ 15 seconds can be cold solid.
2). How is the bibulous rate of ABS controlled?
The fluidity of ABS is related to injection temperature and injection pressure, among which the injection pressure is a little more sensitive. Therefore, the injection pressure can be used in the molding process to reduce the melt viscosity and improve the filling performance. Because of the different components of ABS, the performance of water absorption and adhesion of water is different, its surface adhesion of water and water absorption rate in 0.2% ~ 0.5%, sometimes up to 0.3% ~ 0.8% between, in order to get more ideal products, before forming for drying treatment, make the water content down to 0.1% below. Otherwise the surface of the parts will appear bubbles, silver and other defects. Usually 1% metal powder is added to the plastic material to improve the high gloss metal effect.
11 Polishing and maintenance of molds
Polishing said in plastic mold processing and other industries in the required surface polishing is very different, strictly speaking: the polishing of the mold should be called mirror processing. It has high requirements not only for the polishing itself but also for the surface smoothness, smoothness and geometric accuracy. Surface polishing generally requires only a shiny surface. The standard of mirror processing is divided into four levels: Ao = ra0.008um, A1= ra0.016um, A3= ra0.032um, A4= ra0.063um, because the methods such as electropolishing and fluid polishing are difficult to accurately control the geometric accuracy of the parts, and the surface quality of chemical polishing, ultrasonic polishing, magnetic grinding polishing and other methods can not meet the requirements. Therefore, the mirror processing of precision molds is mainly based on mechanical polishing.
1). the basic procedures of mechanical polishing, in order to obtain high-quality polishing effect, the most important thing is to have high-quality polishing tools and accessories such as oil stone, sandpaper and grinding paste. The most important and polished working environment, the requirement is a dust-free workshop. The choice of polishing program depends on the surface conditions of the previous machining, such as mechanical machining, electrical discharge machining, grinding, and so on.
2). the general process of mechanical polishing is as follows:
a,  After rough casting, fine milling, electric spark, grinding and other processes, the surface can be polished by rotating surface polishing machine or ultrasonic grinding machine with rotating speed of 35000-40000rpm.Commonly used methods include removing the white EDM layer with a wheel with diameter of 3mm and WA#400.This is followed by hand grinding, with strips of whetstone and kerosene as a lubricant or coolant. The general order of use is #180-#240-#400-#600-#1000.Many die makers choose to start with #400 to save time.
b,  Semi-fine throwing mainly uses sandpaper and kerosene. The number of sandpaper is: #400-#600-#800-#1000-#1200-#1500.In practice, #1500 sandpaper should only be used for hardened die steels (52HRC and above) and not for prehardened steels, as this may cause surface burns to the prehardened steel.
c,  Fine polishing mainly makes diamond grinding paste. If the usual grinding order is 9um(#1800)-6um(#3000)-um(8000).The 9um Diamond Grinding Paint and Polishing Cloth Wheel can be used to remove hairlike scratches from #1200 and #1500 sandpaper. The polishing process was then performed with adhesive felt and diamond grinding paste in the order of 1um(#14000)-1/2um(60,000)-1/4um(# 100,000).Precision requirements in more than 1um (including the number 1um) polishing process in the mold of the polishing must be an absolutely clean space. Dust, smoke, dandruff, and drool can all ruin a highly polished surface after hours of work.
3). mechanical polishing should pay attention to the problem, with sandpaper polishing should pay attention to the following points;
a, sandpaper polishing needs to use soft wood or bamboo sticks. When polishing round surface or spherical surface, the use of cork rod can better match the radian of round surface and spherical surface. The harder wood, like cherry wood, is more suitable for polishing a flat surface. Trim the end of the strip so that it matches the shape of the steel surface to avoid deep scratches caused by the sharp Angle of the strip touching the steel surface.
b. When changing to different types of sandpaper, the polishing direction should be changed to 45°-90°, and the stripe shadow left by the polishing of the former type of sandpaper can be analyzed. Before changing to a different type of sandpaper, the polished surface must be carefully wiped with a cleaning solution such as 100% pure cotton and alcohol, because a small gravel left on the surface can ruin the entire finish. This cleaning process is also important when moving from sandpaper polishing to diamond paste polishing. All particles and kerosene must be completely washed before polishing can proceed.
c. In order to avoid abrasions and burns on the workpiece surface, special care must be taken when polishing with #1200 and #1500 sandpaper. It is necessary to apply a light load and to use a two-step polishing method to polish the surface. With each type of sandpaper for polishing should be carried out in two different directions, two sides of the polishing, two sides of the direction between each rotation 45°-90°.
4), diamond grinding and polishing should pay attention to the following points;
a, this polishing must be carried out as far as possible under lighter pressure, especially when polishing pre-hardened steel parts and fine grinding paste polishing. When using #8000 paste, the usual load is 100-200g/cm², but it is difficult to maintain the accuracy of this load. To make this easier, make a thin, narrow handle on a piece of wood, such as a piece of copper. Or add a piece of bamboo to make it softer. This helps to control the polishing pressure to ensure that the die surface pressure is not too high.
b, When using diamond grinding and polishing, not only the working surface is required to be clean, but also the hands of workers must be carefully cleaned.
c, each polishing time should not be too long, the shorter the time, the better the effect. Pitting will occur if the polishing process is carried out too long.
d,  In order to obtain high quality polishing effect, polishing methods and tools that are easy to heat should be avoided. For example; Polishing wheel polishing, the heat generated by the polishing wheel will easily cause orange peel.
e, When the polishing process stops, it is very important to ensure that the surface of the workpiece is clean and carefully remove all abrasives and lubricants. Then a layer of mold antirust coating should be sprayed on the surface.
5), the influence of mold polishing quality factors
Because the mechanical polishing is mainly completed by manual, so the polishing technology is still the main reason affecting the quality of polishing. In addition, it is also related to mold materials, surface conditions before polishing, and heat treatment process. High quality steel is a prerequisite for good polishing quality. If the surface hardness of steel is uneven or the characteristics are different, it will often produce polishing difficulties. The variety and pores in the steel are not conducive to polishing.
a,  Influence of different hardness on polishing process
b,  hardness increases the difficulty of grinding, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve lower roughness increases correspondingly. At the same time the hardness increases, the possibility of excessive polishing phase is reduced.
c, the influence of the workpiece surface condition on the polishing process
During the breaking 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. The machining cutting amount is controlled within 0.05-0.07mm.JN. The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment. Therefore, precision EDM trimming should be used before the end of EDM, otherwise the surface will form a hardened layer. If the EDM finishing specification is not properly selected, the depth of heat-affected layer can be 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, completely remove the damaged surface layer, constitute an average rough metal surface, providing a good basis for polishing processing.
12. Highlight mold maintenance
1. The surface of the mold workpiece must have a high grade of antirust agent or plastic wrap to prevent direct contact with the air and lead to rust;
2, prevent any debris or hand direct contact with the cavity surface;
3, cleaning the mirror work, should use a high-density paper towel spray cleaning agent from top to bottom gently scrub, and can not scrub back and forth; Medical cotton and cloth strips shall not be used. Do not use ** directly on the workpiece blowing, because the debris in the air pipe will lead to the working surface damage.
4. After each mold production or mold test, the mold must be blown clean in the mold channel with ** to prevent the mold kernel from rusting

To conclude, welcome all esteemed friends to contact TKB Mold China Now for high gloss plastic mold and molding solution!

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