An injection mold is a tool for producing plastic products; it is also a tool for imparting complete structure and precise dimensions to plastic products. Because the production method is to inject the plastic melted at high temperature into the mold through high pressure and mechanical drive.
What is an Injection Mold?
Injection mold is a tool for producing plastic products, and it is also a tool for giving plastic products complete structure and precise dimensions.
What is the injection molding process?
Injection molding is a manufacturing process that allows parts to be produced in large quantities. It works by injecting molten material into a mold. It is often used as a mass production process to make thousands of identical items. Injection molding materials include metals, glass, elastomers, and confectionery, although it is most used with thermoplastic and thermoset polymers.
When to use injection molding?
Injection molding is used to manufacture a wide range of products, including common plastics such as bottle caps as well as remote control housings, syringes, and more. It is also commonly used to make larger items such as body panels. Injection molding is mainly used in situations where thousands or millions of identical parts need to be manufactured from a mold.
What Types of Injection Molds are There?
- According to the molding characteristics, it is divided into thermosetting plastic molds and thermoplastic plastic molds.
- The molding process is divided into transfer mold, blow mold, casting mold, thermoforming mold, hot compression mold, injection mold, etc. Among them, the hot compression mold can be divided into overflow, half overflow, and not overflow.
- The gating system can be divided into cold runner mold and hot runner mold.
- The way of loading and unloading can be divided into mobile type and fixed type.
What are the Components of the Mold Structure?
The injection mold consists of a movable mold and a fixed mold. The movable mold is installed on the moving template of the injection molding machine, and the fixed mold is installed on the fixed template of the injection molding machine. During injection molding, the movable mold and the fixed mold are closed to form a casting system and a cavity. When the mold is opened, the movable mold and the fixed mold are separated to take out the plastic products.
Although the structure of the mold may vary depending on the variety and performance of plastic, the shape and structure of plastic products, and the type of injection machine, the basic structure is the same. The mold is composed of a pouring system, temperature regulation system, forming parts, and structural parts. Among them, the pouring system and the molding parts are the parts that are in direct contact with the plastic and change with the plastic and the product. They are the most complex and variable parts in the mold and require the highest processing finish and precision.
- Gating System:
The gating system is a set of feeding channels that lead the plastic melt from the nozzle of the injection machine to the cavity, usually composed of the main channel, a branch channel, a gate, and a cold material cavity. It is directly related to the molding quality and production efficiency of plastic products. 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 cavity, the runner, and the gate. Molded parts refer to various parts that make up the shape of the product, including movable molds, fixed molds and cavities, cores, molding rods, and exhaust ports.
- Mainstream:
The main channel is a passage in the mold connecting the nozzle of the injection molding machine to the runner or cavity. The top of the sprue is concave for engagement with the nozzle. The inlet diameter of the sprue should be slightly larger than the diameter of the nozzle to avoid overflow and prevent the two from being blocked due to inaccurate connection. The diameter of the inlet depends on the size of the product, generally 4-8mm. The diameter of the sprue should be expanded inward at an angle of 3° to 5° to facilitate the release of the runner debris.
- Cold Material Hole:
The cold slug hole is a cavity at the end of the main channel to capture the cold slug generated between two injections at the end of the nozzle, thereby preventing the blockage of the runner or gate. If the cold material is mixed into the cavity, internal stress is easily generated in the manufactured product. The diameter of the cold material hole is about 8-10mm, and the depth is 6mm. To facilitate demolding, the bottom is often borne by the de-molding rod. The top of the molding rod should be designed as a zigzag hook or a sunken groove so that the sprue can be pulled out smoothly when de-molding.
- Shunt:
The runner is the channel connecting the main runner and each cavity in the multi-slot die. To make the melt fill each cavity at the same speed, the arrangement of the runners on the mold should be symmetrical and equidistant. The shape and size of the runner section have an impact on the flow of the plastic melt, the demolding of the product, and the ease of mold manufacturing. If the flow of the same amount of material is considered, the flow channel resistance with a circular cross-section is the smallest. However, because the specific surface of the cylindrical runner is small, it is not good for the cooling of the runner excess, and the runner must be opened on the two halves of the mold, which is labor-intensive and easy to align. Trapezoidal or semi-circular cross-section runners are often used and are provided on half of the mold with ejector bars. The runner surface must be polished to reduce flow resistance and provide a faster filling speed. The size of the runner depends on the type of plastic, and the size and thickness of the product. For most thermoplastics, the cross-sectional width of the runner is mostly no more than 8m. Under the premise of meeting the needs, the cross-sectional area should be reduced as much as possible, to avoid increasing the shunting duct debris and prolonging the cooling time.
- Gate:
The gate is the channel that connects the main channel (or branch channel) and the cavity. The cross-sectional area of the channel can be equal to the main channel, but it is usually reduced, so the gate is the part with the smallest cross-sectional area in the entire runner system. The shape and size of the gate have a great influence on the quality of the product.
The function of the gate is to:
- Control the flow rate.
- It can prevent backflow due to the premature solidification of the melt in this part during the injection.
- The passing melt is subjected to strong shearing to increase the temperature, thereby reducing the apparent viscosity, and improving the fluidity.
- It is convenient to separate the product from the runner system. The design of gate shape, size, and location depends on the nature of the plastic, and the size and structure of the product. Generally, the cross-sectional shape of the gate is rectangular or circular, the cross-sectional area should be small and the length should be short.
This is not only based on the above effects but also because it is easier for a small gate to become larger, while it is difficult to shrink a large gate. The gate position should generally be selected at the thickest part of the product without affecting the appearance. The gate size design should consider the properties of the plastic melt.
- Thermostat System:
To meet the requirements of the injection process for the mold temperature, a temperature adjustment system is required to adjust the temperature of the mold. For injection molds for thermoplastics, the cooling system is mainly designed to cool the mold. The common method of mold cooling is to open a cooling water channel in the mold and use the circulating cooling water to take away the heat of the mold. In addition to heating the mold, hot water or steam can be used in the cooling water channel, and electric heating elements can be installed in and around the mold.
- Molded Parts:
The molded part consists of a core and a die. The core forms the inner surface of the article, and the die forms the outer surface shape of the article. After the mold is closed, the core and the cavity constitute the cavity of the mold. According to the process and manufacturing requirements, sometimes the core and the die are composed of several blocks, and sometimes they are made as a whole, and inserts are only used in the parts that are easily damaged and difficult to process.
- Exhaust Vent:
The exhaust port is a groove-shaped air outlet opened in the mold to discharge the original gas and the gas brought in by the melt. When the melt is injected into the cavity, the air originally stored in the cavity and the gas brought in by the melt must be discharged out of the mold through the exhaust port at the end of the material flow, otherwise, the product will have pores, poor connection, Filling the mold is not satisfied, and even the accumulated air will burn the product due to the high temperature caused by compression. Under normal circumstances, the vent hole can be set either at the end of the flow of the melt in the cavity or at the parting surface of the mold. The latter is a shallow groove with a depth of 0.03-0.2mm and a width of 1.5-6mm on one side of the die. During injection, there will not be a lot of molten material oozing out of the vent hole, because the molten material will cool and solidify there and block the channel. The opening position of the exhaust port should not face the operator to prevent accidental ejection of molten material and injury. In addition, the matching gap between the ejector rod and the ejector hole, the matching gap between the ejector block and the stripper plate, and the core can also be used to exhaust air.
- Structural Parts:
Structural parts refer to various parts that constitute the mold structure, including various parts for guiding, demolding, core pulling, and parting. Such as front and rear splints, front, and rear buckle templates, bearing plates, bearing columns, guide columns, stripping plates, stripping rods and return rods, etc.
- Guide parts: To ensure that the movable mold and the fixed mold can be accurately aligned when the mold is closed, guide parts must be set in the mold. In the injection mold, four sets of guide posts and guide sleeves are usually used to form the guide parts, and sometimes it is necessary to set mutually matching inner and outer cone surfaces on the movable mold and the fixed mold to assist in positioning.
- Launch institutions: During the mold opening process, a push-out mechanism is required to push or pull out the plastic product and its condensate in the runner. Push out the fixed plate and the push plate to clamp the push rod. A reset rod is generally fixed in the push rod, and the reset rod resets the push plate when the movable and fixed molds are closed.
- Side core pulling mechanism: Some plastic products with side concave or side holes must be laterally parted before being pushed out, and the lateral core can be pulled out before they can be de-molded smoothly. At this time, a side core pulling mechanism needs to be set in the mold.
- Standard formwork:
To reduce the heavy workload of mold design and manufacturing, most injection molds use standard mold bases.
What is the injection device of the injection mold?
The injection device is a device that melts the resin material by heating and injecting it into the mold. The grease is extruded into the barrel, and the melt is transported to the front end of the barrel by the rotation of the screw. In that process, the resin material in the barrel is heated by heating under the action of the heater, and the resin is melted under the shear stress of the screw, and the molten resin corresponds to the molded product. The main flow channel and the branch channel are retained. At the front end of the barrel, the material is injected into the mold cavity by the continuous advancement of the screw. When the molten resin flows in the mold, it is necessary to control the moving speed of the screw (injection speed) and use the pressure (holding pressure) after the resin fills the mold cavity. When the screw position and the injection pressure reach a certain value, the speed control can be switched to the pressure control.
Functional Characteristics of Injection Molds:
The temperature in the injection mold is not uniform at various points, and it is also related to the time point in the injection cycle. The function of the mold temperature machine is to keep the temperature constant between 2min and 2max, which means to prevent the temperature difference from fluctuating up and down in the production process or gap. The following control methods are suitable for controlling the temperature of the mold: controlling the fluid temperature is the most used method, and the control accuracy can meet the requirements of most situations. Using this control method, the temperature displayed at the controller and the mold temperature are not consistent. The temperature of the mold fluctuates considerably because the thermal factors affecting the mold are not directly measured and compensated. These factors include changes in the injection cycle, injection speed, melt temperature, and chamber temperature. temperature. The second is the direct control of mold temperature. This method is to install a temperature sensor inside the mold, which is only used when the mold temperature control accuracy is relatively high. The main features of mold temperature control include that the temperature set by the controller is consistent with the mold temperature. Thermal factors affecting the mold can be directly measured and compensated. Typically, mold temperature stability is better than controlling fluid temperature. In addition, mold temperature control has better repeatability in production process control. The third is joint control. Combined control can control the temperature of the fluid and the mold at the same time. In joint control, the position of the temperature sensor in the mold is extremely important. When placing the temperature sensor, the shape, structure, and position of the cooling channel must be considered. In addition, the temperature sensor should be placed in a place that plays a decisive role in the quality of the injection molded part. There are many ways to wire one or more mold controllers to the injection molding machine controller. It is best to use a digital interface from the viewpoint of operability, reliability, and anti-interference.
The heat balance of the injection mold controls the heat conduction of the injection molding machine and the mold is the key to the production of injection molded parts. Inside the mold, the heat brought by the plastic (such as thermoplastic) is transferred to the material and the steel of the mold by thermal radiation, and is transferred to the heat transfer fluid by convection. In addition, heat is transferred to the atmosphere and formwork by thermal radiation. The heat absorbed by the heat transfer fluid is taken away by the mold temperature machine.
The purpose of controlling the mold temperature and the effect of the mold temperature on the injection molded parts in the injection molding process, the purpose of controlling the mold temperature is to heat the mold to the working temperature and keep the mold temperature constant at the working temperature. If the mold temperature is well controlled, the cycle time can be optimized, thereby ensuring the stable high quality of the injection molded parts. Mold temperature affects surface quality, fluidity, shrinkage, injection cycle time, and deformation. Excessive or insufficient mold temperature will have different effects on different materials. For thermoplastics, a higher mold temperature generally improves surface quality and flow, but increases the cooling time and injection cycle time. Lower mold temperature will reduce shrinkage in the mold but will increase the shrinkage of the injection molded part after demolding. For thermosets, a higher mold temperature generally reduces cycle time, which is determined by the time it takes for the part to cool. In addition, in the processing of plastic, a higher mold temperature will also reduce the plasticizing time and reduce the number of cycles.
Precautions for Injection Molds Design:
- High-temperature resistance (as molded molten metal, cast ceramic clay, or some types of plastic).
- Good cooling mold and molding.
- High-pressure resistance (most plastic injection molding).
- Manufacturability (processing technology limitations).
- Mass production of mold material or separate use (cost considerations).
- The number of finished pockets in the mold maximizes output.
- The wall thickness of the plastic parts (the natural technical limitation of the injection mold, the place where the wall is too thin can easily cause subsequent problems. The wall thickness of the mold and the finished product itself are the constraints that need to be considered).
- The location of the parting line or parting line.
- There must be no chamfers or dead corners.
- Dimensions and parameters of the production machine.
- Estimated production and disassembly times of molds (molds are graded, there are low-level molds specially designed to test product designs, and there are high-level molds used for super-mass production, the material grade and design used will be different).
- Interference of mold mechanism (incorrect mechanism stroke or placement will cause bite, friction, and impact between mechanisms, seriously affecting safety and mold life).
Material Suitable for Injection Molds:
The material of the mold directly affects the cooling effect. The commonly used mold materials are P20 steel, H13 steel, P6 steel, S7 steel, beryllium copper alloy, aluminum, 420 stainless steel, and 414 stainless steels.
Its selection principle is:
- Good machinability (machining tools are not easy to wear).
- The material structure is uniform and without defects.
- Good mechanical properties, good strength, toughness, and wear resistance.
- Surface workability is good.
- Easy heat treatment and less deformation.
- Weldability is good.
- Good heat resistance and low coefficient of thermal expansion.
- Fatigue strength is high.
- Easy to get.
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