The injection mold consists of a moving mold and a fixed mold. The moving die is installed on the moving die plate of the injection molding machine, and the fixed die is installed on the fixed die plate of the injection molding machine. During injection molding, the movable mold and the fixed mold are closed to form a gating system and a cavity. When the mold is opened, the movable mold and the fixed mold are separated, and the plastic product is taken out. In order to reduce the heavy workload of mold design and manufacture, most injection molds adopt standard mold bases.
First, the gating system
The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, the cold material hole, the shunt runner and the gate.
Gating system, also known as runner system, is a group of feeding channels that lead plastic melt from the nozzle of injection machine to the cavity, which usually consists of main runner, shunt runner, gate and cold cavity. It is directly related to the molding quality and production efficiency of plastic products.
Main road 1
It is a channel in the mold that connects the injection nozzle of the injection molding machine to the shunt or cavity. The top of the main channel is recessed to connect with the nozzle. The inlet diameter of the main channel should be slightly larger than the nozzle diameter (0.8mm), so as to avoid material overflow and prevent the closure caused by inaccurate connection between the two. The inlet diameter depends on the size of the product, generally 4-8 mm, and the diameter of the main channel should be expanded inward at an angle of 3 ~ 5 to facilitate the demoulding of the channel debris.
2. Cold hole
It is a cavity at the end of the main channel, which is used to catch the cold material produced between two injections at the end of the nozzle, thus preventing the blocking of the shunt channel or gate. If the cold material is mixed into the cavity, the manufactured product is prone to internal stress. The cold cavity is about 8-6 mm in diameter and 6 mm in depth. In order to facilitate demoulding, the bottom of the cold cavity is often borne by the demoulding rod. The top of the demoulding rod should be designed as a sawtooth hook or set with a sink, so that the main runner can be pulled out smoothly during demoulding.
3. Diversion channel
It is a channel connecting the main runner and each cavity in the multi-groove mold. In order to make the molten material fill all the cavities at the same speed, the arrangement of the shunt channels on the plastic mold should be symmetrical and equidistant. The shape and size of the cross section of the branch channel have an influence on the flow of plastic melt, the demoulding of products and the difficulty of mold manufacturing. If the flow rate is equal, the flow resistance of circular section is the smallest. However, due to the small specific surface area of the cylindrical runner, it is not conducive to the cooling of the shunt, and this shunt must be set on the two halves of the mold, which is labor-consuming and difficult to center. Therefore, trapezoidal or semi-circular cross-section shunt channels are often used to open on the half mold with demoulding rod. The surface of the runner must be polished to reduce the flow resistance and provide a faster filling speed. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the cross-sectional width of the diverter is less than 8mm, the extra-large one can reach 10-, and the extra-small one is 2-3 mm. On the premise of meeting the needs, the cross-sectional area should be reduced as much as possible to increase the diverter and prolong the cooling time.
Step 4: Gate
It is a channel connecting the main channel (or shunt channel) and the cavity. The cross-sectional area of the channel can be equal to the cross-sectional area of the main channel (or the branch channel), but it is usually reduced. So it is the smallest cross-sectional area in the whole flow channel system. The shape and size of the gate have great influence on the product quality.
The function of the gate is:
First, control the material flow speed:
B, during injection, this part of the melted material can be stored and backflow and premature coagulation can be prevented;
C, the passing molten material is subjected to intense shearing and heated, thereby reducing the apparent viscosity and improving the fluidity;
D, it is convenient to separate the product from the runner system. The design of gate shape, size and location depends on the properties of plastics, the size and structure of products. Generally, the cross-sectional shape of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short, which is not only based on the above functions, but also because the small gate is easier to become larger, while the large gate is difficult to shrink. Generally, the gate position should be selected at the thickest part of the product, which will not affect the appearance. The design of gate size should consider the properties of plastic melt. Cavity is the space in the mold for molding plastic products. The parts used to form the cavity are collectively called molded parts. Each molded part usually has a special name.
The molded parts that make up the appearance of products are called female dies (also called female die), and those that make up the internal shapes of products (such as holes and grooves) are called cores or male dies (also called male dies). When designing molded parts, the overall structure of the cavity should be determined according to the properties of plastic, the geometric shape of the product, the dimensional tolerance and the use requirements. Secondly, according to the determined structure, the location of parting surface, gate and exhaust hole and demoulding method are selected. Finally, according to the size of the controlled product, the parts are designed to determine the combination mode between parts. The plastic melt has a high pressure when it enters the cavity, so it is necessary to choose the molded parts reasonably and check the strength and stiffness. In order to ensure the smooth and beautiful surface of plastic products and easy demoulding, the roughness ra >: 0. And it should be corrosion resistant. Molded parts are usually heat-treated to improve hardness and made of corrosion-resistant steel.
Second, the temperature control system
In order to meet the requirements of injection molding process for mold temperature, a temperature control system is needed to adjust the mold temperature. For the injection mold of thermoplastic, the cooling system is mainly used to cool the mold. The common method of cooling the mold is to open a cooling water channel in the mold, and take away the heat of the mold with circulating cooling water; In addition to using hot water or steam in the cooling water channel, electric heating elements can be installed inside and around the mold to heat the mold.
Third, molding parts.
Molded parts refer to all kinds of parts that constitute the shape of products, including moving dies, fixed dies and cavities, cores, forming rods, exhaust ports, etc. The molding part consists of a core and a female die. The core forms the inner surface of the product, and the female die forms the outer surface shape of the product. After the mold is closed, the core and the cavity form the cavity of the mold. According to the requirements of technology and manufacturing, sometimes the core and die are composed of several pieces, and sometimes they are made into a whole. The inserts are only used in parts that are easy to be damaged and difficult to process.
1, exhaust port
It is to open a slot-shaped air outlet on the mold to discharge the original gas and the gas brought by the melt. When the melt is injected into the mold cavity, the air originally stored in the mold cavity and the gas brought by the melt must be discharged from the mold through the exhaust port at the end of the material flow, otherwise, the product will have air holes, poor connection, insufficient mold filling, and even the accumulated air will burn the product due to the high temperature generated by compression. Generally speaking, the vent hole can be located at the end of the melt flow in the mold cavity or on the parting surface of the plastic mold. The latter is to open a shallow groove with a depth of 0.03-0.2 mm and a width of 1.5-6 mm on one side of the female die. During the injection process, a large amount of molten material will not seep out of the exhaust hole, because the molten material will cool and solidify there and block the passage. Don't point the exhaust port at the operator to avoid accidental discharge of molten material and personal injury. In addition, the gap between the ejector pin and the top hole, the gap between the ejector block and the stripper plate and the core can also be used for exhaust.
Fourth, the structural part.
Refers to all kinds of parts that constitute the mold structure, including: guiding, demoulding, core-pulling and parting parts. Such as front and rear clamping plates, front and rear fastening templates, bearing plates, bearing columns, guide posts, stripper plates, stripper rods and return rods.
1, guide parts
In order to ensure that the moving die and the fixed die can be accurately centered during mold clamping, a guide part must be set in the die. In the injection mold, four groups of guide posts and guide sleeves are usually used to form the guide part, and sometimes it is necessary to set mutually consistent inner and outer conical surfaces on the moving mold and the fixed mold to assist in positioning.
2. Pushing out mechanism
In the process of mold opening, it is necessary to have an ejection mechanism to eject or pull out the plastic products and their solidified products in the runner. Push out the fixed plate and the push plate, and clamp the push rod. Usually, the reset lever is also fixed in the push rod. When the movable die and the fixed die are closed, the reset lever resets the push plate.
3. Side core-pulling mechanism
Some plastic products with side recesses or holes must be separated laterally before being pushed out, and the side core can be successfully demoulded after being pulled out. At this time, it is necessary to set a side core-pulling mechanism in the mold.
The injection device is a device that heats and melts the resin material and then injects it into the mold. The resin is extruded from the material head into the barrel, and the melt is conveyed to the front end of the barrel through the rotation of the screw. In that process, the resin material in the barrel is heated by the heater, and the resin becomes molten under the shear stress of the screw, which will be equivalent to the molded product and the main channel. The molten resin in the branch channel will stay at the front end of the barrel (called metering), and the continuous advancement of the screw will inject the material into the mold cavity. When molten resin flows in the mold, it is necessary to control the moving speed (injection speed) of the screw, and control it by pressure (holding pressure) after the resin fills the mold cavity. When the screw position and injection pressure reach a certain value, we can switch the speed control to the pressure control.