The plastic injection process is one of the most important processes to produce plastic parts in terms of serial production manner. A very large size range of plastic products can be produced with the plastic injection molding process. The size and weight can range from a few grams to tens of kilograms.
The most basic explanation of the plastic injection process can be like this; Injection of molten polymer into metal molds and solidify inside these molds to obtain required shapes. In each cycle of the plastic injection process, one to tens of parts can be produced. The cycle time changes according to the used material and produced part. But in general, the cycle time of plastic injection molding processes range from 30 seconds to a few minutes.
Injection of thermoplastic materials in injection molding processes is the most common practice. Thermosetting and elastomer polymers can also injection molded if the equipment and toolings of the injection molding machine are changed according to them.
Very complex parts can be produced with plastic injection molding machines. But the shapes of products must be suitable for detachment from the molds. For most plastic injection molding applications, processes are net-shape processes.
General plastic injection machines include three sections. The first section is the injection unit. In here, polymer pellets which are the starting form of molded parts, are provided from the hopper inside the barrel. Inside the barrel, there is a rotating screw that pushes the plastic material forward direction. This screw mechanism is common with the plastic extrusion process. Heaters are also placed around the barrel to heat and produce polymer melt from plastic pellets.
The rotation motion of the screw is adjusted with an electric motor and gear systems at the back-side of the machinery. At the end of the screw, there is a non-return valve that prevents the backflow of polymer melt from the mold side.
There are also other types of injection units with the most used types explained above. In older plastic injection machines, plunger systems are used to push the plastic melt inside the mold cavity. In another alternative is that the starting material is given from hopper to screw section. This screw section melts and homogenizes the polymer melt and pushes this polymer melt inside the plunger system. When the plunger system is fulled with polymer melt, a shut-off valve is closed. Then plunger pushes the polymer to melt inside the mold. This system is generally called a screw-preplasticizer or two-stage system. The first one is called plunger-type injection molding.
The second section of the plastic injection machine is called as ‘clamping unit’. In the clamping unit, there are two plates, one stationary and the other one is moving. Between platens, the mold is placed. The moving platen is moved via the hydraulic system. Movement of moving platen creates required pressure to hold the mold halves during injection and solidification. Several tons of pressure can be applied to injection molds.
Clamping units of plastic injection machines have generally three types; toggle, hydraulic and hydromechanical systems. In toggle systems, crosshead mechanical systems are used to push the movable plate. The advantage of these systems is the application of very good pushing forces at the end of the clamping. In general, they are used in lower tonnages. Hydraulic clamping units are used for higher tonnages which are explained just above. The third type of clamping unit is hydromechanical. In hydromechanical clamping systems, the moving action is done with the hydraulic system and the clamping action is done with a mechanical system. This is very useful when very high tonnages of plastic parts are produced.
In a typical plastic injection molding application, mold is clamped via a hydraulic system with a moving plate. Then the material is supplied from the hopper into the screw-barrel system. With the rotation of the screw, molten plastic is forced into molds. In this stage, the non-return valve is closed to prevent backflow from the mold. The pressure is hold up to the solidification of polymer inside molds take place. When the solidification is complete, molds are opened then the molded part is removed from the mold. Then, the mold is closed and pressurized again. In the solidification stage, new material is provided inside the rotating screw.
Actually, the mold side of the whole system is not considered inside the machinery system. Because molds are designed according to the part geometry. Molds are made from metals, and the cavities to inject polymer inside them carved via machining operations. These operations are somewhat expensive. But, if we think that it is serial production, income will compensate for these machining costs.
Prepared molds are placed on movable and fixed plates. In general, molds are produced in two half designs one of them placed on a movable platen and another one is placed fixed on the fixed platen.
Just like ordinary metal casting processes, plastic injection molds have sprue, gating, and runner systems inside them. Plastic injection is done from the sprue section and directed to the internal cavities by runners. Gatings are the entrance sections of internal cavities of plastic injection molds.
To separate the injected plastic parts from the mold at the end of the cycle, some ejection systems are required. These ejection systems are generally done with the application of ejector pins on the movable platen. Because of the shrinkage nature of plastic materials, after the separation of two plates, the plastic part is clung to the movable plate. With the application of ejector pins, the plastic molded product is separated from the mold.
As we stated above the two-half design is one of the most encountered designs in the plastic injection industry. Also, three half of mold designs are used. In two half molds, runner and sprue systems are separated from mold in unity with the main part. But in the three plate mold design, sprue and runner systems are separated into a separate section. In general, a more uniform distribution of polymer melt inside cavities is done in three plate mold design.
Sprue and runner systems that are produced can be recycled or used in another process. But, if the used polymer melt must be ‘virgin’, sprue and gating systems are discarded. Also, hot-runner systems are designed to hold the runner and sprue systems in the liquid stage, which can be used in another cycle of production.
To solidify and cool the polymer melt inside molds, a water circulation system is designed inside molds. Also, air vents are opened on molds to evacuate the entrapped air inside mold cavities. These vents are in the diameters that polymer cannot fill, but air can pass.
When the polymer is injected inside the mold, weld lines take place around the cores. This is because the polymer melt goes around the core, then meets at the other side of the core. At this meeting point, weld lines can occur.
Solution Of Weld Lines: Smart placement of cores and gating systems, higher melting temperatures and injection pressures, and better venting systems can solve this problem.
Flashing of polymer melt occurs around the parting surfaces of halves of molds.
Solution Of Flashing: Flashing generally occurs because of the insufficient tolerances between touching surfaces of mold halves. Too high injection pressures compared with the clamping force is also another reason. Uneven design of air vents can lead to flashing problems in injection molding processes. So, designing and fixing these problems smartly can solve the flashing problem.
The main reason for sink marks and voids in the internal shrinkage at the thick sections of parts. Voids are the internal spaces inside thick sections and sink marks are the depressions of surface sections because of the internal voids.
Solution Of Voids And Sink Marks: Proper design of parts that has no very thick sections. Uniform thickness along the designed part is very important to solve this problem.
Short shots are the problems in plastic injection processes that solidification of polymer melt without completely filling mold cavity.
Solution Of Short Shot Problem: Short shots take place because of the insufficient shot pressures and injection temperatures. The operator must consider these parameters to solve the short shots problem in plastic injection processes.
Shrinkage is a very important parameter of polymer injection processes that needed to be considered professional. Shrinkage is defined as the linear contraction of a polymer when it cooled from a molten state to a solid-state. Because of this issue, mold design must be made by considering the shrinkage of injected material.
Shrinkage of injected plastic inside mold depends on some factors;
To compensate for the shrinkage in plastic injection, molds are designed bigger than the usual part in volume. So, consideration of shrinkage required very good mastery to evaluate the parameters defined above. But, you can make shrinkage calculations for specific dimensions of mold by using the calculator below.
The use of the above calculator is very simple. You just need to enter the required values; Dimension which must have a length value, and unitless shrinkage value of a typical polymer. Click on the ‘Calculate!’ button to see the mold design value of this dimension. If you want to do another calculation, just click on the ‘Reset!’ button, then enter new values.
You can find shrinkage values of different polymer from polymer suppliers. And also you can find some polymer’s shrinkage values below;
Nylon 6-6: 0.02
We provided the most important data about the conventional plastic injection processes. This article can be a very good guide for plastic injection processes. We included a calculator that you can calculate the mold dimensions by considering the shrinkage inside a mold.
Mechanical Base does not accept any responsibility for the calculations done with calculators by the user. A good engineer must check the calculations again and again.
You can find other useful engineering calculators available in Mechanical Base!
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