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How Plastic Injection Molding Works March 16, 2016 By: Matt Guerin
Every day, people take what they have and how it works for granted. Many people have used a zipper almost every day of their lives and still do not know exactly how it works. This is also true for production of plastic parts. Most common household items are made using a plastic injection molding machine. This includes bottles, pens, lids, toys, DVD cases, plates, cups, bowls, automotive dashboards, toothbrushes and so much more. Have you ever wondered how these plastic parts are made?
A variety of common plastic parts made using an injection molding machine. Source: http://img.diytrade.com/cdimg/426325/2424529/0/1153036809/Plastic_Injection_Moulding_Parts_Plastic_components_Plastic_molding_parts.jpg Most plastic parts are made in plastic injection molding machines. These machines are generally very large and powerful. They consist of three main parts: an injection unit, a clamping unit, and an ejection unit. To put it simply, molten plastic is injected into a part-‐shaped mold and then clamped together and cooled to create the part. That part is then ejected from the mold. In reality, an injection molding machine and its processes are not nearly that simple. Below are some of the main components of each unit.
Injection Unit The main functions of the injection unit are to melt the plastic material, inject the molten plastic into the mold, and to build up packing and holding pressure.
Labeled Injection Unit. Source: https://www.xcentricmold.com/images/injection-‐molding-‐machine.jpg
• Hopper – The hopper is where the plastic enters the machine. The plastic enters the hopper in small pebbles.
• Barrel – The barrel houses the injection screw and is wrapped with heater bands. It is a long cylindrical shape.
• Heaters – The heaters are located inside the barrel and melt the plastic that comes in from the hopper.
• Screw – The screw moves the molten plastic forward as it rotates and ejects the plastic out of the nozzle.
• Check Ring – The check ring is located around the screw right before the tip. It prevents back flow of the molten plastic.
• Nozzle – The nozzle is a tapered section at the tip of the barrel. The molten plastic is ejected out of the nozzle and into the mold.
Clamping Unit The main functions of the clamping unit are to support the mold and open and close the mold.
Labeled Clamping Unit. Source: http://www.beejaymolding.com/image-‐files/Clampi1.gif
• Stationary Platen – The stationary platen, also known as the “A” Side platen, holds half of the mold. The nozzle is inserted through the stationary platen and into the mold.
• Moving Platen -‐ The moving platen, also known as the “B” Side platen, contains the other half of the mold. After the part is made and the mold is opened, the part sticks to this platen.
• Toggle Linkages – The toggle linkages are responsible for moving the B-‐Side platen.
• Tail Stock Platen – The tail stock platen is connected to the moving platen by the toggle linkages. It acts as a sturdy base.
• Tie Bars – The tie bars are large rails that the moving platen rides on. They also keep everything in line.
Ejection Unit The main function of the ejection unit is to eject the parts from the mold after it opens.
Labeled Ejection Unit. Source: http://www.imould.com/upload/remote/200941195251066.jpg
• Ejection Cylinder – The ejection cylinder is responsible for moving the ejection platen.
• Ejection Platen – The ejection platen is located directly behind the moving platen.
• Ejector Rods – The ejector rods are long rods attached to the ejection platen. These rods go through small holes in the moving platen to push the part out.
Not only are the machines complex, but so is the process. The process can be broken into five stages. First Stage – Filling In the filling stage, the linear movement of the screw forces plastic through the nozzle and into the mold. The mold cavity is filled with molten plastic to approximately 99% of its capacity.
Plastic injection molding mold. Source: https://www.luxexcel.com/wp-‐content/uploads/2015/03/injection-‐molding.jpg Second Stage – Pack and Hold In the pack and hold stage, additional material flows into the mold to compensate for the shrinkage of the cooling plastic. The pressure is increased during this stage.
Third Stage – Plastication and Screw Recovery After the nozzle has been removed from the mold, the screw rotates and the leftover plastic is reheated for the next cycle.
Injection screw. Source: http://www.xaloy.com/Files/Products/EasyMeltInjectionScrew.jpg Fourth Stage – Cooling The plastic is cooled while in the mold until it is ready to be ejected. Fifth Stage – Part Ejection The mold opens after the part has cooled. The ejector rods push the part out of the mold. Sometimes, there is a bucket or a conveyor belt that the parts drop onto. If it is a larger part, there is often a robotic arm that enters the mold and removes the part. The mold is then closed and starts the next cycle.
All of that may seem complex, so why use plastic injection molding in the first place? Injection molding is great if you need to make complex three-‐dimensional parts that would take too long to 3D print. Injection molding is also beneficial if the company needs the product to be within a tight tolerance. The most significant reason companies use plastic injection molding machines is because they are very economical. They have an extremely fast cycle time and have almost no material waste. The one major drawback is that these machines are expensive upfront, but they pay themselves off quickly.
A plastic injection molding machine. Source: http://image.made-‐in-‐china.com/2f0j00temEGYayvTqB/HDT-‐Plastic-‐Injection-‐Molding-‐Machine-‐HDT-‐160-‐.jpg Plastic injection molding is a technology of ever-‐growing importance. It is one of the few manufacturing methods that the United States does not outsource. In fact, other countries depend on the United States for plastic production. This is an industry that is growing fast, and understanding the basic aspects of the machines and the processes could be very valuable.