At present, most of exterior parts of household appliances are obtained by injection molding. During injection molding process, defects such as weld marks, air marks, and deformation are prone to occur; high-gloss traceless molds can solve above defects. Let's take a look at ten elements of high-gloss, traceless injection mold design.

Mold molding has high temperature requirements (generally around 80℃-130℃). After injection molding switches to pressure holding, cooling water is used to reduce 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 product. Therefore, mold needs to be heated during operation. In order to prevent heat loss, a heat insulation board is usually added to fixed mold side.

Products produced by high-gloss molds can be directly used for assembly (assembly) without any surface treatment. Therefore, it has high requirements for mold steel and plastic materials.

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 purpose of controlling or even eliminating weld marks. Control method is complex.

There are usually two methods of mold heating: steam (hot water) heating and electric heating rod (tube) heating. Water vapor (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 quickly heats up; after injection molding is completed, mold is cooled with cold water to quickly cool down mold. Electric heating method is same as water heating temperature control machine in principle, but heat source is different. Electric heating is a secondary energy, and water heating is a tertiary energy. According to principle, electric heating consumes less energy, has high utilization rate and good energy saving benefits. It is easy to use, so if it is a flat (surface) product, it is better to use electric heating.

 

Figure: Water vapor heating

 

Picture: Heating rod heating

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 German Glitz brand: CPM40/GEST80

 

High Gloss Mold

Water channel uses a hole diameter of 5-6mm; water nozzle uses 1/8 or 3/8 threads (mold side), and the other side uses 3/4 inch thread (old-fashioned connection method); pipe fittings are made of stainless steel pipes; now we change it to one cavity. It is best to make shunt port in mold, and use energy diameter to connect interface with DN25, so that heat loss is less, operation is convenient, and interface is convenient.

Distance between side of water channel and product surface is generally 5-6mm; if it is larger, it will affect heating time of mold, and if it is smaller, it will affect strength of mold. Parallel product surface of water channel must be evenly arranged (distributed at an equal distance of 15mm from center of original material). Thermocouple should be designed in the middle of 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 mold structure. Each set of mold PT100 is matched with one. To maintain its accuracy, it must be inserted into core of mold cavity and fixed. Connect lead wire to outside of mold and then to socket of temperature controller.

Mold water channel joints must be designed at upper and lower sides or rear end of mold; no water channel inlets and outlets or water pipe arrangements are allowed on operating side (station side) to avoid pipe rupture and injury to production personnel. Remember!

Mold inlet and outlet nozzles are designed with a splitter plate. 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; achieve goals of safety and energy saving. Outer surface of corrugated pipe is wrapped with heat-insulating tape to play role of heat preservation and safety.

Construction holes (unnecessary holes) of mold should be plugged with plugs to ensure that there is no air or water leakage. Method is to plug them with copper first, then seal them with tapered throat teeth and high-temperature resistant glue; high-gloss molds are very particular about arrangement of cooling water channels (water channels of hydrothermal molds are shared). Good water channel arrangement can not only greatly improve injection molding efficiency, but also play an important role in improving product quality. Water channels of high-gloss mold must not only be uniform but also sufficient (a sufficient number).
This heats up mold quickly; at the same time, using an extended water pipe to directly transport water out of mold core without using a sealing ring can prevent mold from operating at high temperatures for a long time, causing sealing ring to age, and can also reduce maintenance costs of many molds. It is worth mentioning that water pipe of high-gloss mold must be made of high-temperature resistant material (250℃) 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 product appearance is used.

Four sides of fixed mold core or movable mold core must be hollowed out; there must be a certain gap between mold frame and core (depending on thermal expansion coefficient of mold material, 1 mm on one side). Prevent expansion of mold frame to reduce contact surface between mold core and mold frame to minimize heat loss; mold core and mold frame are locked using an oblique or other similar method, and front end is made of dust resin or other materials (such as asbestos board) with obvious heat insulation effect.

Cooling water of mold frame is very important for detailed structure of mold frame and core. In order to prevent heat energy in mold core from being transmitted to mold frame, a circle of water transport should be arranged up and down near guide pillar.

Moving part of guide sleeve should be made of graphite material as much as possible or front end of guide post should be avoided. It is enough to ensure that length of fitting part is 25mm;

Mold gate design should reduce weld marks as much as possible, facilitate exhaust and reduce shearing. For molds that use water-heated temperature control machines, gate size should be larger and large gates should be used to feed glue. Without affecting product function and molding efficiency, gate should be as short as possible in length, depth, and width.

If gate is too small, it will easily cause appearance defects such as insufficient filling (short shots), shrinkage dents, and weld lines, and molding shrinkage will increase.

If gate is too large, excessive residual stress will be generated around gate, resulting in deformation or cracking of gate, and it will be difficult to remove gate.
It is better to use a gate unless flow ratio exceeds practical limits. Resin flow length curve will provide flow length of material under certain molding conditions. Multiple gates often create weld lines and weld marks. In addition to long and narrow products, use of a single gate will ensure a more consistent distribution of materials, temperatures, and holding pressures for better matching effects.

Try to space 10mm apart around product as much as possible, and evenly distribute exhaust grooves with a depth of 0.15mm; middle veneer of product also needs an exhaust design.

Because there is a large temperature difference between high-gloss molds, veneer coordination requirements are relatively high. At the same time, area of veneer must be reduced. A 10mm fit around parting surface is enough.

1. There should be electric heating rods (tubes) on upper and lower sides of gate. Cooling water hole is generally 6mm (larger is better); distance between centers of two water holes is 15-20mm; distance between heating rod wall and product surface is 5mm, and center distance between two heating rods is 20mm; distance between cooling water and heating rod wall is 6-8mm. If possible, it is best to intersperse electric heating rods.
2. Water transport in inner mold cavity can be sealed with a high temperature resistant sealing ring or a hard seal.
3. Diameter of heating rod is 4.92mm, and diameter of mold is 5mm. Before assembling heating rod, use a 5mm thimble to sharpen edge and remove burrs of heating rod.
4. Mold inlet and outlet nozzles use same manifold design (cooling water) as water vapor heating mold, because electric heating mold control system only has one inlet and one outlet water pipe.

High-gloss molds have strict requirements on product structure. The brighter product, the more sensitive it is to refraction effect of light. Slight defects on the surface will be quickly discovered. Therefore, how to solve shrinkage problem is primary issue for high-gloss products. Generally, if rib thickness of a product does not exceed 0.6mm times thickness of main glue position, it will not shrink. In other words, shrinkage is small and difficult to detect, so it can be ignored. But for high-gloss products, such requirements are far from enough. Thickness of ribs of product must be reduced to no more than 1 times thickness of main glue. Screw columns must also have a crater-type lifter structure.

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.

 

ABS is an amorphous polymer with no obvious melting point. Due to wide variety of grades and grades, appropriate process parameters should be formulated according to different grades during injection molding process. Generally, molding can be performed above 160℃ and below 270℃. During molding process, ABS has good thermal stability, a wide range of options, and is not prone to degradation or decomposition. Moreover, melt viscosity of ABS is moderate, and its fluidity is better than that of polystyrene (PS), polycarbonate (PC), etc., cooling and solidification speed of melt is relatively fast, usually within 5 to 15 seconds.

Fluidity of ABS is related to both injection temperature and injection pressure, with injection pressure being slightly more sensitive. For this reason, injection pressure can be started during molding process to reduce melt viscosity and improve 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 moisture content to less than 0.1%. Otherwise, defects such as bubbles and silver threads will appear on the surface of product. Usually plastic materials need to add 1% metal powder to improve high-gloss metal effect.

Polishing mentioned in plastic mold processing is very different from surface polishing required in other industries. Strictly speaking: polishing of molds should be called mirror processing. It not only has high requirements for polishing itself but also has high standards for surface flatness, smoothness and geometric accuracy. Surface polishing generally only requires obtaining a bright surface. Standard for mirror processing is divided into four levels: A0=Ra0.008um, A1=Ra0.016um, A3=Ra0.032um, A4=Ra0.063um. Since it is difficult to accurately control geometric accuracy of parts by methods such as electrolytic polishing and fluid polishing, However, surface quality of chemical polishing, ultrasonic polishing, magnetic grinding and polishing and other methods cannot meet requirements, so mirror processing of precision molds is still mainly mechanical polishing.

To obtain high-quality polishing results, 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 polishing working environment, which requires a dust-free workshop. Choice of polishing procedure depends on the surface conditions of pre-processing, such as machining, EDM, grinding, etc.

1. 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. Commonly used methods include using a wheel with a diameter of 3mm and WA#400 to remove white spark layer. Then there is manual whetstone grinding, strip whetstone is added with kerosene as a lubricant or coolant. 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. 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 pre-hardened steel parts.
4. Fine polishing mainly uses diamond grinding paste. 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 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 highly polished surface you get after hours of work.

When polishing with sandpaper, you should pay attention to following points:
1. Polishing with sandpaper requires use of soft wooden sticks or bamboo sticks. When polishing a round or spherical surface, using a cork stick can better match curvature of round or spherical surface. Harder strips of wood, like cherry, are more suitable for polishing flat surfaces. Trim ends of wooden strips so that they match surface shape of steel parts. This will prevent sharp angles of wooden strips from contacting surface of steel parts and causing deep scratches.
2. When using different types of sandpaper, polishing direction should be changed by 45°-90°. Stripe shadow left by previous type of sandpaper after polishing can be analyzed. Before changing to a different type of sandpaper, you must carefully wipe polishing surface with 100% pure cotton dipped in alcohol or other cleaning fluid, because a small grit 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 surface of workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is necessary to apply a light load and polish 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 two sides and three directions.

1. This kind of polishing must be carried out under lighter pressure as much as possible, especially when polishing pre-hardened steel parts and polishing with fine abrasive paste. When using #8000 abrasive paste, common load is 100-200g/cm², but it is difficult to maintain accuracy of this load. To make this easier, you can make a thin, narrow handle on wooden strip, such as adding a piece of copper; or you can remove part of bamboo strip to make it softer. This can help control polishing pressure to ensure that pressure on mold surface is not too high.
2. When using diamond grinding and polishing, not only working surface must be clean, but workers' hands must also be carefully cleaned.
3. Each polishing time should not be too long. The shorter time, the better effect. Pitting can occur if 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, heat generated by polishing wheel can easily cause orange peel.
5. When polishing process is stopped, it is very important to ensure that surface of workpiece is clean, to carefully remove all abrasives and lubricants. Then a layer of mold anti-rust coating should be sprayed on the surface.

Since mechanical polishing is mainly done manually, polishing technology is still main factor affecting polishing quality. In addition, it is also related to mold material, surface condition before polishing, heat treatment process, etc. High-quality steel is a prerequisite for good polishing quality. If surface hardness of 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. Influence of different hardness on polishing process
2. Increased hardness makes grinding more difficult, but roughness after polishing decreases. As hardness increases, polishing time required to achieve lower roughness increases accordingly. At the same time, hardness increases and possibility of over-polishing decreases.
3. Influence of workpiece surface condition on polishing process
During crushing process of steel cutting machinery, surface will be damaged due to heat, internal stress or other factors. Improper cutting parameters will affect polishing effect, so high-speed CNC finishing is required, and processing cutting amount is controlled at 0.05-0.07mm. Surface after EDM processing is more difficult to grind than surface after ordinary machining or heat treatment. Therefore, precise EDM dressing should be used before end of EDM processing, otherwise a hardened layer will form on the surface. If EDM finishing specifications are improperly selected, depth of heat-affected layer can reach up to 0.4mm. Hardness of hardened layer is higher than basic hardness and must be removed. Therefore, it is best to add a rough grinding process to completely remove damaged surface layer and form an evenly rough metal surface, which provides a good foundation for polishing.

1. Surface of 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 cavity surface;
3. When cleaning mirror surface, high-density paper towels should be sprayed with cleaning agent and scrubbed gently from top to bottom, cannot be scrubbed back and forth; medical cotton and cloth strips cannot be used; a gun cannot be used to blow directly on workpiece, because air in trachea is Debris and moisture can cause damage to work surface.
4. After each mold production or mold trial, water channel of mold must be blown clean with a gun to prevent mold core from rusting.