The Detail Process Of Plastic Injection Molding

- Jan 26, 2021-

The plastic injection molding is the most common manufacturing method to produce plastic products in large volume, it offers high efficiency, low cost, accurate dimension, and complex part structure.

The plastic injection molding system consists of raw material, injection mold tool, and injection molding machine. During the process, the pre-dried raw material is prepared in the hopper of injection machine, it is melted and plasticized under a high temperature of injection heating nozzle. Then those plasticized raw material will be shot onto the mold cavity where raw material will solidify and cool down to form desired plastic parts, finally the molded parts will be ejected from mold tool.

The whole injection molding cycle is very short, which typically takes only 10 seconds and 2 minutes, and we would like to summarize the injection molding stages as below:


1).  Material Drying

In order to obtain a better quality of plastic injection molded products, it’s necessary to get plastic resin dried before injection molding process. As all of resin pellets normally exist a certain percentage of moisture, which could result in lowering down material physical properties, such as reducing flow rate, tensile, and impact strengths.

For different brand of plastic resin and different supplier, the plastic material has different percentage of moisture, therefore it takes different drying time ranging from 1~5 hours. After material drying, it will greatly avoid most of injection molding defects, such as flowing mark, sink mark, welding line, etc.


 2). Mold Clamping

Injection mold is an essential manufacturing tool to manufacture plastic products, before a plastic injection molding production, we should get a mold tool designed and manufactured.

Once we are ready for plastic injection molding process, we need to get mold tool loaded onto injection molding machine. making sure the two halves of the mold tool must be steadily attached.

For different size of mold tool, it requires different injection machine that offers proper clamping force, because if an improper clamping force used on a mold tool, it could lead to injection defects that can never resolved by injection parameter adjustment.

Meanwhile, a operational trial is necessary to test if the mold tool can open and close smoothly with enough movement stock.


3). Material Feeding

To get the injection molding facility prepared, it’s time to upload those pre-dried plastic pellets into the hopper of injection machine. This action could happen by either manual or automation.

After material is loaded into hopper, the material will be transferred by a nozzle screw forward to injection mold tool, during this stage, the plastic pellet will b melted and plasticized so that it can be injected into mold tool.


4). Injection

During the injection stage, the melted material is shot into mold cavity quickly under a high pressure from injection nozzle. However different size and different part weight requires different injection time. Normally the heavy parts, the longer injection time needed, the complex part structure, the longer injection time needed. However the injection time could also determined by injection pressure, injection speed, and sprue size.  


5). Cooling 

After the molten material injected into the mold tool, it will cool down immediately as the molten material contacts the cool mold steel where the cooling system is designed inside. The cooling system could be cold water or oil, which offers a recyclable cooling channel between mold tool and its outlet.   

Normally it takes few seconds to cool molten material, it will solidify in the mold cavity to form desired plastic product.

It’s very important to have a reasonable cooling system, if not, it could lead to shrinkage issue or deformation issue while injection molding process.


6). Part Release(ejection)

After the ejected material is cooled and solidified, it is the time to get the molded product released by either manual or mechanism structure of mold tool.

At this stage, the two halves of mold tool are opened, a mechanism plate is used to push the part out of the mold. But for some soft parts, we will have to take out of the part from mold.

As soon as the part is released, the mold tool close again for next injection molding cycle.