Injection molding BrE or Injection molding AmE, is a manufacturing process for producing components by injecting a liquid material into a mold. Injection molding can be carried out with a number of materials including mainly metals, (the process called die-casting), glass, elastomers, confection, and the most common thermoplastic and thermosetting polymers. The material for the part is inserted into a heated barrel, mixed (Using a helical-shaped screw), and injected (Forced) into the mold cavity, where it cools and solidifies to the cavity configuration. Once the product is designed, usually by an industrial designer or engineer, the mold is made by a mold maker (or toolmaker) of metal, usually steel or aluminum, and a precision machine to form a feature of the desired part. Injection molding is widely used for the manufacture of various parts, from the smallest components to all car body panels. Advances in 3D printing technology, using non-melting photopolymers during multiple thermoplastic injection molding of lower temperatures, can be used for some simple injection molds.
The part to be printed in the injection must be carefully designed to facilitate the printing process; materials used for the desired parts, shapes and features of parts, molding materials, and the properties of the printing press shall be taken into account. The flexibility of injection molds is facilitated by the breadth of design considerations and possibilities.
Video Injection moulding
Apps
Injection molding is used to make things like wire coils, packaging, bottle caps, automotive components and components, gameboys, pocket combs, some instruments (and parts), one-piece chairs and small tables, storage containers, mechanical parts (including wheels teeth), and most other plastic products available today. Injection molding is the most common modern method of making plastic parts; is ideal for generating high volumes of the same object.
Maps Injection moulding
Process characteristics
Injection molding using ram or screw-type impeller to force liquid plastic material into the mold cavity; this solidified into a shape that has been in accordance with the contour of the mold. It is most commonly used for processing thermoplastic and thermosetting polymers, with volumes used from previously much higher. Thermoplastics are prevalent because of the characteristics that make them particularly suitable for injection molding, such as recyclable ease, their versatility allows them to be used in a variety of applications, and their ability to soften and flow during heating. The thermoplastic also has a security element on the thermoset; if the thermoset polymer is not removed from the injection tube in a timely manner, chemical crosslinking may occur which cause screws and check valves to seize and potentially damage the injection molding machine.
Injection molding consists of high pressure injection of raw materials into molds that form polymers into desired shapes. Mushrooms can be either a cavity or a lot of cavities. In some cavity molds, each cavity can be identical and form the same part or can be unique and form several different geometries during one cycle. Molds are generally made of tool steel, but stainless steels and aluminum molds are suitable for specific applications. Aluminum molds are typically unsuitable for high volume production or parts with narrow dimensional tolerances, since they have lower mechanical properties and are more prone to wear, damage, and deformation during the injection and clamping cycle; however, aluminum molds are cost effective in low-volume applications, because the cost of mold fabrication and time is greatly reduced. Many steel molds are designed to process over a million parts during their lifetime and can cost hundreds of thousands of dollars to be made.
When thermoplastics are formed, usually the raw material that is dipeletet is fed through the hopper into a barrel heated with reciprocating screws. Upon entering the barrel, rising temperatures and Van der Waals forces that withstand the relative flow of individual chains weaken as a result of increasing the space between molecules at higher thermal energy states. This process reduces its viscosity, allowing the polymer to flow with the driving force of the injection unit. The screws provide the raw material forward, mix and homogenize the thermal and viscous distributions of the polymer, and reduce the required heating time by mechanically cutting the material and adding large amounts of friction heating to the polymer. The feed material progresses through the valve and collects at the front of the screw into a volume known as shot . The shot is the volume of material used to fill the mold cavity, compensate for shrinkage, and provide the bearings (approximately 10% of the total volume of shot, which remains inside the barrel and prevents the screw from coming out from the bottom) to transfer pressure from the screw to the mold cavity. When sufficient material has been collected, the material is forced at high pressure and velocity to the part that forms the cavity. The exact amount of shrinkage is a function of the resin used, and can be predicted relative. To prevent pressure spikes, the process usually uses a transfer position corresponding to a full 95-98% cavity in which the screw shifts from a constant velocity to a constant pressure control. Often the injection is well under 1 second. Once the screw reaches the transfer position the applied packing pressure, which completes the filling of the mold and compensates for the thermal shrinkage, which is high enough for the thermoplastic relative to many other materials. The packing pressure is applied until the gate (inlet cavity) freezes. Due to its small size, the gate is usually the first place to solidify through its entire thickness. Once the gates solidify, no more material can enter the cavity; Therefore, the screws alternate and obtain the material for the next cycle while the material inside the mold cools down so it can be removed and dimensionally stable. This cooling duration is dramatically reduced by the use of cooling water or oil cooling channels from external temperature controllers. Once the required temperature has been reached, the open mold and array pins, arms, strippers, etc. are pushed forward to demould the article. Then, the mold closes and the process is repeated.
For two shot prints, two separate materials are inserted into one section. This type of injection mold is used to add a soft touch on the buttons, to give the product a lot of color, to produce parts with some performance characteristics.
For thermosets, usually two different chemical components are injected into the barrel. These components immediately begin an irreversible chemical reaction that eventually binds cross material into one network of connected molecules. When a chemical reaction occurs, the two liquid components permanently change into a viscoelastic solid. Solidification in the injection and screw barrel can be a problem and have a financial impact; Therefore, minimizing thermoset drying in the barrel is essential. This usually means that the residence time and temperature of chemical precursors are minimized in the injection unit. The residence time can be reduced by minimizing the volume capacity of the barrel and by maximizing cycle time. These factors have led to the use of a thermally insulated cold injection unit that injects chemicals that react into thermally insulated heat mold, which increases the rate of chemical reactions and results in the shorter time required to achieve thermoset compacted components. Once the part is compacted, the valve closes to isolate the injection system and chemical precursor, and the mold opens to remove the printed part. Then, the mold closes and the process repeats.
Pre-mold or machine components can be inserted into the cavity while the mold is open, allowing the material to be injected in the next cycle to form and harden around it. This process is known as Insert printing and allows one part to load some material. This process is often used to make plastic parts with protruding metal screws, allowing them to be fastened and released repeatedly. This technique can also be used for labeling In-mold and film cover may also be attached to printed plastic containers.
Separator lines, canker sores, gate markers, and ejector pin marks are usually located at the end. None of these features are usually desirable, but unavoidable because of the nature of the process. A gate occurs at the gate connecting the liquefaction line (sprue and runner) to the part that forms the cavity. The separator line and ejector pin marks are generated from minute inequality, wear, gas vents, clearance for adjacent parts in relative motion, and/or dimensional differences from the mating surface contacting the injected polymer. Dimensional differences may be associated with non-uniform, induced-pressure induction during injection, machining tolerance, and uniform thermal expansion and contraction of mold components, which undergo rapid cycles during the injection, packing, cooling, and ejection phases of the process.. Printing components are often designed with materials of various thermal expansion coefficients. These factors can not be simultaneously taken into account without the increase in astronomy in the cost of design, fabrication, processing, and quality monitoring. Proficient designers and skilled designers will place these aesthetic losses in hidden areas where possible.
History
The American inventor John Wesley Hyatt, along with his brother Isaiah, patented the first injection molding machine in 1872. The machine is relatively simple compared to the engine used today: it works like a large syringe, using a plunger to inject plastic through a cylindrical heater into print. The industry grew slowly over the years, producing products like fixed collars, buttons, and hair combs.
German chemists Arthur EichengrÃÆ'¼n and Theodore Becker discovered the first soluble form of cellulose acetate in 1903, which was much more flammable than cellulose nitrate. It was finally made available in powder form from where it was easily injection molded. Arthur EichengrÃÆ'¼n developed the first injection molding machine in 1919. In 1939, Arthur EichengrÃÆ'¼n patented the printing of a coated cellulose acetate injection.
The industry grew rapidly in the 1940s because World War II created a huge demand for cheap bulk products. In 1946, American inventor, James Watson Hendry, built the first screw injection engine, allowing more precise control over injection speed and quality of manufactured goods. The machine also allows the material to be mixed before the injection, so colored or recycled plastics can be added to the virgin material and mixed thoroughly before injection. In the 1970s, Hendry went on to develop the first gas-assisted injection molding process, which enabled the production of complex, rapidly cooled hollow articles. This greatly increases the design flexibility as well as the strength and completion of parts produced while reducing production time, cost, weight and waste. In 1979, plastic production surpassed steel production, and in 1990, aluminum molds were widely used in injection molding. Today, injection injection machines are responsible for most injection machines.
The plastic injection molding industry has grown over the years from producing comb and buttons to produce a wide range of products for many industries including automotive, medical, aerospace, consumer products, toys, plumbing, packaging, and construction.
Examples of polymers most appropriate for process
Most polymers, sometimes referred to as resins, may be used, including all thermoplastics, some thermosets, and some elastomers. Since 1995, the amount of material available for injection molding has increased at a rate of 750 per year; there are about 18,000 materials available when the trend starts. Available materials include alloys or mixtures of previously developed materials, so product designers can select materials with the best set of properties from a wide selection. The main criterion for material selection is the strength and function required for the end, as well as the cost, but also each material has different parameters for printing to be reckoned with. Other considerations when selecting injection molding materials include the flexural elastic modulus, or the rate at which the material can be bent without damage, as well as heat deflection and water absorption. Common polymers such as epoxy and phenolics are examples of thermosetting plastics while nylon, polyethylene, and polystyrene are thermoplastics. Until relatively recently, plastic springs are not possible, but advances in polymeric properties make it now quite practical. Applications include buckles for retaining and disconnecting outdoor equipment.
Tools
Injection molding machine consists of a hopper, injection ram or screw-type plunger, and heating unit. Also known as platens, they hold the mold in which the component is formed. Pressing is assessed by tonnage, which states the amount of clamping force that the machine can provide. This style keeps the mold closed during the injection process. Tonnage can vary from less than 5 tons to over 9,000 tons, with higher numbers used in relatively few manufacturing operations. The required total clamping force is determined by the projected area of ​​the formed part. This projected area is multiplied by a clamping force of from 1.8 to 7.2 tonnes per square centimeter of the projected area. As a rule of thumb, 4 or 5 tons/in 2 can be used for most products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, and thus the clamping tonnage is more to hold the mold closed. The required style can also be determined by the material used and the size of the part. The larger section requires a higher gripping style.
Mold
Prints or off is a generic term used to describe the tools used to produce plastic components in printing.
Since molds are expensive to produce, they are usually used only in mass production where thousands of parts are produced. Typical molds are constructed of hardened steel, hardened steel, aluminum, and/or beryllium copper alloys. The choice of materials to build molds from is primarily one of the economies; in general, the steel mold is more expensive to manufacture, but its longer life will offset the higher initial cost of higher component quantities made before it runs out. Hardened steel molds are less wear resistant and are used for lower volume requirements or larger components; Their typical steel hardness is 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining; this is far superior in terms of wear resistance and age. Typical hardness ranges between 50 and 60 Rockwell-C (HRC). Aluminum molds can be very cheap, and when designed and machined with modern computer equipment it can be economical to print dozens or even hundreds of thousands of parts. Beryllium copper is used in molded areas requiring rapid heat transfer or areas that see the most sliding heat generated. Prints can be manufactured either by CNC machines or by using electrical displacement machining processes.
- Injection mold off with pull side
Print design
The mold consists of two main components, injection mold (A plate) and ejector mold (plate B). These components are also referred to as moulder and mouldmaker. The plastic resin enters the mold through the thrush or gate in the injection mold; sprue bushing is to seal tightly the barrel of the injection molding machine and to allow liquid plastic to flow from the barrel into the mold, also known as the cavity. The sprue foam directs the molten plastic into the image cavity through the ducts that are worked into the faces of plates A and B. These channels allow plastics to run along them, so they are referred to as runners . The liquid plastic flows through the runner and enters one or more special gates and into the geometry cavity to form the desired part.
The amount of resin required to fill thrush, runners and mold cavities consists of "shots". The air trapped in the mold can pass through the milled air vents into the mold separation line, or around the ejector pins and slides that are slightly smaller than the holes that hold them. If the trapped air is not allowed to exit, it is compressed by the pressure of the incoming material and squeezed into the corners of the cavity, where it prevents charging and can also cause other damage. The air can even become so dense that it ignites and burns the plastic material around it.
To allow for the removal of molded parts from the mold, the mold features should not overhang each other in the direction that the mold is open, unless the mold part is designed to move from between overhangs as when the mold is open (using a component called Lifter).
The side of the part that appears parallel to the drawing direction (the axis of the inner position (hole) or the insertion parallel to the movement up and down of the mold when opening and closing) is usually a slight incline, called the draft, to facilitate the release of parts of the mold. Inadequate drafts can cause deformation or damage. The draft required for the release of the mold depends mainly on the depth of the cavity; the deeper the cavity, the more concepts are needed. Shrinkage must also be taken into account when determining the required draft. If the skin is too thin, the printed part will tend to shrink to the core formed when it cools and attaches to the nuclei, or it may warp, twist, blister or crack when the cavity is pulled away.
A mold is usually designed in such a way that a reliable printed part remains on the ejector (B) side of the mold when it is open, and draws the runner and canker sores out of side (A) along with the parts. The part then falls freely when removed from side (B). The gate tunnel, also known as a submarine or mold gate, is located below the parting line or the mold surface. The opening is applied to the mold surface at the parting line. The molded part is cut (by mold) from the runner system on ejection of the mold. The ejector pin, also known as pin knockout, is a round pin placed in one half of the mold (usually half the ejector), which pushes the finished mold product, or the runner system out of the mold. Spending articles using pins, arms, strippers, etc., can cause unwanted impressions or distortions, so be careful when designing prints.
The standard method of cooling is to pass the coolant (usually water) through a series of holes drilled through the mold plates and connected to the hose to form a continuous path. The coolant absorbs heat from the mold (which has absorbed heat from hot plastic) and keeps the mold at the right temperature to condense the plastic at the most efficient level.
For easy maintenance and ventilation, cavities and cores are divided into sections, called inserts , and sub-assemblies, also called inserts , blocks , or chase the block . By replacing the replaceable inserts, a single print can create multiple variations of the same section.
More complex parts are formed using more complicated prints. It may have a section called a slide, which moves into a cavity perpendicular to the direction of a draw, to form a feature of the overhanging part. When the mold is opened, the slide is pulled away from the plastic part by using a stationary "pin angle" on half the stationary mold. This pin goes into the slot on the slide and causes the slide to move backward when half moves the print open. The part is then removed and the mold closes. The closing action of the mold causes the slide to move forward along the corner pin.
Some prints allow the pre-cast parts to be re-inserted to allow new layers of plastic to form around the first part. This is often referred to as overmoulding. This system can enable to produce tires and one-piece wheels.
Two-shot or multi-shot prints are designed to "overmould" in a single print cycle and must be processed on a special injection molding machine with two or more injection units. This process is actually a two-time injection molding process and therefore has a much smaller margin of error. In the first step, the basic color material is molded into a basic shape, which contains a space for the second retrieval. Then the second material, a different color, is printed-injection into those spaces. Buttons and buttons, for example, are made with this process having an indestructible mark, and are still read with heavy usage.
Prints can produce multiple copies of the same section in one "shot". The amount of "impression" in the section's mold is often mistakenly referred to as cavitation. Tools with one impression will often be called single track prints (cavities). Prints with 2 or more cavities from the same part are likely to be referred to as multiple cavity molds (cavities). Some very high volume production volumes (such as for bottle caps) can have more than 128 cavities.
In some cases, many cavity tools will form a series of different parts in the same tool. Some toolmakers call these molded family prints because they are all interconnected. Some examples include a plastic model kit.
Print storage
Manufacturers strive to protect special prints because of their high average cost. Perfect temperature and humidity levels are maintained to ensure the longest period of time for each custom mold. Special molds, such as those used for rubber injection molding, are stored in temperature-controlled and humidity environments to prevent warping.
Materials tool
Steel tools are often used. Light, aluminum, nickel or epoxy steel is only suitable for very short prototyping or production. The modern hard aluminum (7075 and 2024 alloys) with the right mold design, can easily create prints that can reach 100,000 or more parts of life with proper mold maintenance.
Machining
Mold is built through two main methods: standard machining and EDM. Standard machining, in its conventional form, is historically a method of building injection molds. With technological developments, CNC machines are the primary means of making more complicated prints with more accurate print details in less time than traditional methods.
Electrical displacement (EDM) or spray erosion processes have been widely used in mold making. As well as allowing the formation of difficult shapes for machines, the process allows pre-hardened molds to be formed so that no heat treatment is required. Changes in hardened molds by conventional drilling and milling usually require softening to soften the mold, followed by heat treatment to re-harden. EDM is a simple process in which a shaped electrode, usually made of copper or graphite, is very slowly lowered to the mold surface (over a period of hours), which is immersed in paraffin oil (kerosene). The applied voltage between the tool and the fungus causes splash erosion from the mold surface in the inverse form of the electrode.
Cost
The number of cavities inserted into the mold will be directly correlated with the cost of printing. Fewer cavities require much less tooling work, thus limiting the number of rotated cavities will result in lower initial manufacturing costs for building injection molds.
Because the number of cavities plays an important role in printing costs, so does the complexity of part design. Complexity can be incorporated into many factors such as surface completion, tolerance requirements, internal or external threads, fine detail or number of undercuts that can be entered.
More details, such as undercuts or any features that cause additional tools, will increase the cost of printing. The final surface of the core and the mold cavity will further affect the cost.
The rubber injection molding process produces high yields from durable products, making it the most efficient and cost-effective printing method. A consistent process of vulcanisation involving proper temperature control significantly reduces all waste materials.
Injection process
With injection molding, the granular plastic is fed by the ram being forced from the hopper into the heated barrel. As the grains slowly move forward with the screw-type plunger, the plastic is forced into the heated room, where it is melted. As the plunger progresses, the melted plastic is forced through the nozzle attached to the mold, allowing it to enter the mold cavity through the gate and the runner system. The mold remains cool so the plastic hardens as soon as the mold is filled.
Injection molding cycle
The sequence of events during injection molding of plastic parts is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. After the cavity is filled, holding pressure is maintained to compensate for material depreciation. In the next step, the screw spins, feeding the next shot to the front screw. This causes the screws to pull back when the next shot is set up. Once the part is cool enough, the mold is open and the part is removed.
Traditional and traditional prints
Traditionally, the injection portion of the printing process is carried out at a constant pressure to fill and pack the cavities. This method, however, allows for large variations in the dimensions of cycle-to-cycle. More commonly used now is scientific or separate prints, a method pioneered by RJG Inc. In this case plastic injections are "separated" into several stages to allow for better control of the part dimension and more cycle-to-cycle (commonly called shot-to-sticks in industry) consistency. First the cavity is filled to about 98% full using speed control (speed). Although the pressure must be sufficient to allow the desired speed, the limitations of pressure during this stage are undesirable. After a full 98% cavity, the engine switched from speed control to pressure control, where the cavity "packed" at constant pressure, where sufficient speed to reach the desired pressure was required. This allows the dimensions of the part to be controlled into a thousandth of an inch or better.
Different types of injection molding process
Although most of the injection molding process is covered by conventional process descriptions above, there are several important print variations including, but not limited to:
- Die casting
- Metal injection mold
- Injection of thin-wall injection
- Injection molding of liquid silicone rubber
- Injection mold reaction
A more complete list of injection molding processes can be found here: [1]
Process troubleshooting
Like all industrial processes, injection molding can produce defective parts. In the field of injection molding, problem solving is often done by examining the defective parts for certain defects and overcoming these defects with mold design or characteristics of the process itself. Trials are often performed before full production runs in an attempt to predict defects and determine the exact specifications for use in the injection process.
When filling a new or unknown print for the first time, where the size of the shot for the printout is unknown, a technician/tuner tool can test before the full production runs. They start with a small weight shot and fill in gradually until the mold is 95 to 99% full. Once this is achieved, a small amount of retaining pressure will be applied and the holding time increases until the gate freezes (compaction time) has occurred. The freeze gate time can be determined by increasing the hold time, and then weigh the part. When the section weight does not change, it is known that the gate has frozen and no more material is injected into the section. Gate compaction time is important, because it determines the cycle time and quality and consistency of the product, which itself is an important issue in the production process economy. The holding pressure is increased until the free part of the sink and part weight has been reached.
Mold defect
Injection molding is a complex technology with possible production problems. They can be caused by defects in the mold, or more often by the printing process itself.
Methods such as industrial CT scans can help find these defects both externally and internally.
Tolerance
Printing tolerance is the allowance specified in deviations in parameters such as dimensions, weight, shape, or angle, etc. To maximize control in tolerance settings there is usually a minimum and maximum limit on the thickness, based on the process used. Injection molding is usually capable of tolerance equivalent to IT Grade around 9-14. The possible tolerances of thermoplastics or thermosets are Ã, Â ± 0.008 to Ã, Â ± 0.002 inches. In special application tolerances as low as Ã, Â ± 5 Âμm on both diameter and linear features are achieved in mass production. The final surface of 0.0500 to 0.1000 Âμm or better can be obtained. Rough or pebbled surfaces are also possible.
Power requirements
The power required for the injection molding process depends on many things and varies between the materials used. Manufacturing Process Reference Guide states that power requirements depend on "material density, melting point, thermal conductivity, part size, and printing speed." Below is a table of page 243 of the same reference as mentioned earlier that best describes the characteristics relevant to the power required for the most commonly used material.
Robotic molding
Automation means that smaller section sizes allow a mobile inspection system to inspect some parts faster. In addition to installing a check system on automatic devices, multi-axis robots can remove parts of the prints and position them for further processing.
Specific examples include removing parts of the mold immediately after parts are made, as well as applying machine vision systems. The robot holds the part after the ejector pin has been extended to free part of the mold. This then moves them to the holding location or directly to the inspection system. The choice depends on the type of product, as well as the general layout of the manufacturing equipment. The vision system installed in the robot has greatly improved the quality control to include the mold parts. A moving robot can more precisely determine the accuracy of metal component placement, and check faster than humans can.
Gallery
See also
References
Further reading
Lindsay, John A. (2012). Practical guide for rubber injection molding (Online-Ausg. ed.). Shawbury, Shrewsbury, Shropshire, U.K.: Smithers Rapra. ISBN: 9781847357083.
External links
- Internal Display of Injection Printing Process - How Injection Print Works
- Shrinkage & amp; bouncy
- Manufacturing engineering and mechanical properties of plastic parts - INTEMA (Research Institute), Universidad Nacional de Mar del Plata - CONICET
- Interactive injection molding video
Source of the article : Wikipedia
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