Blog 27: How does an Injection Molding machine work?

Introduction:

Injection molding is one of the most crucial processes in the mass manufacture of items made of thermoplastics. It is frequently used for processes that create products, without the need for additional finishing steps. Currently, the majority of injection molding machines are universal models that can accept all mound types within certain parameters. 

In the process of injection molding, a thermoplastic polymer is heated past its melting point, turning it from a solid to a fluid that has a relatively low viscosity. This melt is mechanically injected, or pressed, into a mold that mimics the final shape desired. As long as the item is not cooled down below the polymer’s freezing point, the low viscosity of the molten polymer allows for the complete filling of the mold. In the case of semi-crystalline polymers, in-mold cooling of the product at a predetermined cooling rate often controls the crystallinity of the object, which controls its mechanical and aesthetic attributes. The mold is opened and the part is ejected in the last stage.

The two primary mechanisms of injection molding are pressure flow and heat transfer. The two pieces of the necessary equipment in an injection molding machine are an injection molding machine, sometimes known as a press, and a mold, which is also often called a tool or a die. The end result of the process is molding, which is occasionally incorrectly and confusingly referred to as a mold.

Parts of an Injection Molding Machine:

Injection Molding process:

The process cycle for injection molding is very short, typically between 2 seconds and 2 minutes, and consists of the following four stages:

  • Clamping – The clamping unit must first firmly close the two mold halves before the material can be injected into the mold. One half of the mold is let to slide while the other halves are each coupled to the injection molding machine. The clamping unit is hydraulically operated and forces the mold halves together with enough force to maintain the mold shut while the material is injected. The machine will determine how long it takes to close and clamp the mold; larger machines (those with higher clamping force) will take longer. The machine’s dry cycle time can be used to estimate this period of time.
  • Injection – The raw plastic material is fed into the injection molding machine and moved toward the mold by the injection unit, which is typically in the form of pellets. The material gets melted during this procedure as a result of pressure and heat. The fast-injected molten plastic packs and holds the material due to the pressure rise in the mold. The shot is the term used to describe the volume of substance injected. Due to the complicated and erratic flow of the molten plastic into the mold, it is challenging to determine the injection time precisely. The shot volume, injection pressure, and injection power can, nevertheless, be used to predict the injection time.
  • Cooling – As soon as the molten plastic inside the mold makes contact with the internal mold surfaces, it starts to cool. The plastic will harden into the desired part’s shape as it cools. The portion could, however, experience some shrinkage during cooling. During the injection stage, the material is packed to prevent visible shrinkage and allow more material to flow into the mold. The required cooling period must have passed before the mold may be opened. Several thermodynamic characteristics of the plastic and the maximum wall thickness of the part can be used to predict the cooling time.
  • Ejection – The ejection mechanism, which is attached to the back half of the mold, can expel the cooled item from the mold once enough time has passed. A device is utilized to force the part out of the mold when it is opened. Because the object shrinks and sticks to the mold during cooling, force must be used to remove it from the mold. Prior to the injection of the material, a mold release agent can be sprayed over the surfaces of the mold cavity to make it easier for the part to be ejected. The machine’s dry cycle time can be used to estimate the amount of time needed to open the mold and eject the component, which should also account for the time it takes for the part to release itself from the mold. The mold can be clamped shut so that the subsequent injection can take place after the part has been extracted.

Following the injection molding cycle, post-processing is frequently necessary. The substance in the mold’s channels will harden while still being bonded to the part during cooling. It is necessary to remove this extra material and any flash from the part, usually using cutters. For some materials, such as thermoplastics, the leftover trimmings can be recycled by being fed into a plastic grinder, also known as a regrind machine or granulator, which turns the leftover trimmings into pellets. To be reused in the injection molding process, regrind must be blended with raw material in the correct regrind ratio due to some degradation of the material’s characteristics.

How to calculate an injection molding cycle: 

Cycle = Mo+Mc+I+C

Mc = Time to close the mold (this is the time it takes to actually close the tool)

I = Time to inject material into the mold

C = Cooling Time (Time to solidify molten material)

To = Time to open a mold and eject the part (these can overlap and together make up total open time)

The cycle time is multiplied by the number of mold cavities in the tool to determine the injection molding production rate. A tool will typically just have one cavity and little to no automation for prototype and low-scale manufacturing. For highly high-volume applications like caps and closures, full production molds may include dozens of cavities, complete automation, extremely fast cycle times, and extremely high productivity. Once the initial tooling is finished and the process has been stabilized, these features make injection molding incredibly cost-effective.

Advantages

The ability to scale up production to create a huge number of parts is injection molding’s main benefit. The price of manufacturing is quite inexpensive once the upfront costs of the design and the molds have been paid for. As more parts are produced, production costs decrease.

Injection molding also generates less waste than conventional manufacturing techniques like CNC milling, which removes surplus materials. However, there is some waste produced by injection molding, primarily from the sprue, runners, gate positions, and any overflow material that spills out of the part cavity.

The ability to produce several identical parts, which allows for part consistency and reliability in big volume of production, is the last benefit of injection molding.

Disadvantages

While the method of injection molding has several benefits, there are also a number of drawbacks.

When it comes to tooling, injection molding might have significant upfront expenses. A prototype part must be made before any pieces may be produced. A prototype mold tool needs to be made and tested after this is finished. This whole process can be expensive and time- and money-consuming.

The production of large pieces in a single piece via injection molding is likewise not recommended. This is a result of the mold tools and injection mold machines’ size restrictions. Items that are too big to fit within an injection molding machine must be made in many parts and assembled later.

The last drawback is that large undercuts might increase the cost of your project and require expert design to avoid it.

Applications

Numerous applications that call for a repeatable manufacturing process use injection molding. Wire spools, packaging, bottle caps, toys, combs, musical instruments (and their parts), seats, tiny tables, storage containers, mechanical parts, and automotive parts and components are among the manufactured goods that fall under this category.

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