Plastic Injection Molding (Its Definition, Process & More)

Plastic Injection Molding

What is Plastic Injection Molding?
Definition: Plastic injection molding (also known as polymer injection molding) is a process in which the molten polymer is forced into a mold cavity under a high pressure, which is then allowed to solidify to get the final component.

By using injection molding process, a high production output rate can be achieved with automated work.

Also the final component ejected from the mold is well finished and it does not require more post production work.

Well, this was just an overview. But there are a lot more things that you should know about injection molding of polymers.

Here I have covered plastic injection molding process, advantages and disadvantages of injection molding process as well as defects of injection molding technique.

So let’s dive right into it!

Injection molding process (steps)

injection molding process steps

The steps for plastic injection molding process are mentioned below;

  • Step 1: Clamping the mold
  • Step 2: Injecting the polymer in the mold cavity
  • Step 3: Cooling the polymer in the mold
  • Step 4: Opening the mold
  • Step 5: Remove the final product from the mold

Let’s discuss these injection molding process steps in detail.

You can see the schematic diagram of injection molding process in the above image.

First of all, the mold is opened and the machine is ready for injection molding. 

After this, the mold is closed and it is clamped so that the mold cavity is formed (see above image).

The polymer (or plastic granules) is then placed in the hopper and it is fed into the barrel.

The polymer is heated in the barrel and it is brought to a required temperature and viscosity. 

After this, the ram is used to inject the molten polymer in the mold cavity.

The plastic starts to solidify when it comes in contact with the cold surface of the mold cavity.

During the solidification process, the ram pressure is maintained to compensate for the shrinkage during solidification.

Now the screw (or ram) is retracted back and the fresh molten polymer enters the forward portion of the barrel.

The solidified plastic is then ejected from the mold cavity by opening the mold.

Injection molding of polymers is very fast and it takes only 10 to 30 seconds to complete.

Sometimes a single mold contains more than one mold cavity so that more components can be manufactured in a single cycle.

The thermoplastics used in injection molding are;

  • Polycarbonate
  • Polypropylene
  • Nylon 6 (N6)
  • Acrylonitrile butadiene styrene (ABS)

Having said that, let’s now discuss the advantages and disadvantages of injection molding process.

Advantages of injection molding

Advantages of injection molding process are mentioned below.

  • The production rate in injection molding process is high. In other words, more components can be manufactured in less time by using injection molding techniques.
  • The advantage of injection molding technique is that you can also use inserts within the mold which forms the combined part with the polymer after solidification.
  • Fillers can also be used with the molten polymer to increase its strength.
  • By using injection molding, high dimensional accuracy can be achieved even for small components.
  • You can also use more than one material at the same time by using co-injection molding.
  • The final component ejected from the mold is well finished and it does not require more post production work.
  • The scarp polymer can be reused, so there is very less wastage of raw material.
  • By using some automated systems, the injection molding process can be made fully automatic.

Disadvantages of injection molding

Disadvantages of injection molding process are mentioned below.

  • The setup cost for injection molding is high.
  • Injection molding is a complicated process.
  • This process is suitable mostly for large scale production work.

Defects in injection molding

Various injection molding defects and their precautions are described below.

#1) Pour short

As the name suggests, pour short is a defect that occurs due to shortage of molten polymer or due to incomplete filling of the mold cavity.

Pour short defects can be prevented by increasing the temperature of the raw material as well as increasing the filling pressure.

The temperature required for the different types of polymers should be determined during the process designing stage.

The barrel used in injection molding machine should be large enough that it can handle more molten polymer than required.

#2) Flashing

Flashing is a defect that occurs due to squeezing of molten polymer between the mold plates.

This defect generally occurs when the injection pressure is very high or the clearance between the mold plates is large.

#3) Sink marks and voids

Ths sink marks defect is caused when the outer surface of the polymer solidifies but the contraction of the internal material causes the skin to depress below its intended surface.

Similarly, Void is a defect that forms internal holes in the solidified polymer due to contraction of internal material during its cooling.

Voids generally occur in the manufacturing of the thicker sections of polymers.

#4) Weld lines

Weld lines are the portion of the solidified plastic which generally occurs when there is a flow of molten polymer around the convex shaped portion of the mold cavity. The two different streams of the molten polymer meet with each other in opposite directions and form a boundary called a weld line.

The mechanical properties of the weld line differs from the rest of the portion of solidified plastic.

This defect can be prevented by maintaining the high temperature of the polymer.

#5) Shrinkage

When the plastic cools down, its volume decreases. This defect is called shrinkage and it occurs in the plastics due to their higher thermal expansion coefficients.

In order to compensate for the shrinkage, the mold cavity is made larger than the actual required dimensions of the solidified plastic.

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