Injection molding process basic study

Plastic Injection Molding Process- Chapter-1

Definition of Injection molding process:- It is a manufacturing process for producing part by Injecting molten material into Mold Cavity, where material is cools and harden to the configuration of the cavity.


 Well here we should think, what science/Technical study is being used in this entire process. We will learn it later. First see basic details about plastic molding process.

What is plastic?

Plastics are Macromolecular structured Polymer. Polymer is made up of many repeating simple chemical units, known as Monomer

"Polymer is defined as a material made out of large number of repeating units which are linked to each other through chemical bonding. A single polymer molecule may have millions of monomers"

In simple words plastic is one of the family member of polymers. See below picture for understanding.

Note:- All Plastics are Polymers-----But All Polymers are not Plastics, they may be Fibers, Elastomers (rubbers) or adhesives. 

Picture-1 Plastic definition

So from above picture we see plastic is made by polymerization process of monomers, which make polymer, and many polymers on heating give plastic. 

What is polymerization process?

Polymerization is a process in which small monomers are combine chemically to produce a very large chain like or network molecules, which is called polymers. 

 "Atoms together make molecule, many molecules together make compound, compound makes matter and any matter could be a solid, liquid and gas".

Here monomer is nothing but molecule. And molecule is a group of two or more atoms and each molecule has its molecular weight. As we know polymer is made by many many monomers (or by many molecules), so we can say polymer is a compound with high molecular weight

Scientists also called polymer "a giant molecule".

And in chemistry the size of molecules is measured by its molecular weight and molecular weight is equal to sum atomic weight of all atoms that make up the molecule.  In molecule atoms are held together by a chemical bond, that chemical bond is made by sharing electrons. One molecule can attract to another by electrostatic force. And in polymers electrostatic force is more because of high molecular weight which agglomerates the molecules to form permanent shape material-that is plastic. 

Morphology (Structure) of Polymer:- 

As explained polymer is developed by polymerization of monomers. Polymer molecules may exhibit  variety of structures. Common form of these structure are - Linear, Branched and Network. 

Examples- 
   1- Linear structure = High Density polyethylene
   2- Branched Structure = Low Density Polyethylene
   3- Network Structure = Phenol Formaldehyde Resin (PF Resin)

Picture-2- Structure of Polymer

Apart from its polymer chain could be 3 type Isotactic polymer chain, Syndiotactic polymer chain and Atactic polymer chain. 

Classification of plastics

Plastic is classified in many form and type.  Here we will discuss only two type of plastic classification 

 1- Thermoplastic   
 2- Thermosets

Thermoplastics are materials that can be repeatedly heated, melted and converted into a product.

Thermosets undergo a chemical reaction during conversion into a product. The product cannot be re-melted and converted into another product again.

Difference between Thermoplastic and thermosets can be understand by below picture:-

Picture-3 Plastic Classification 

We further study about thermoplastics, since thermoplastics are widely used in industries for Automotive component manufacturing. 

Structure and properties of thermoplastics

There are 2 types of structure of thermoplastics- 1- Amorphous Polymers   2- Crystalline Polymers

In chemistry we have read, there are three states of matter Solid, liquid and gaseous state. And solid posses two different states, namely Amorphous and Crystalline. These state of solid depends upon arrangement of particles. 

Same concept here, thermoplastic is kind of solid plastic which have 2 type of structure -Amorphous and crystalline. 

Amorphous Polymer does not have an organized pattern in its molecular structure. Molecules are oriented randomly and intertwined -Example like-cooked noodles (Meggie). In amorphous polymer random molecules orientation in molten or solid state. This amorphous polymer are made out of  Atactic polymer chain. 

Crystalline Polymers have organized pattern of its molecules, ordered structure and regular line of  polymer molecules. In this type Random molecular orientation in molten phase, but densely packed crystallites occurs in solid phase. This crystalline polymer is made out of syndiotactic and isotactic polymer chain. 

Difference between Amorphous Polymer and Crystalline Polymer

Since in amorphous polymer no crystallinity, which make structure weak. Due to random orientation of molecules there will be weak attraction between polymer chain. Mean low chemical resistance which make it transparent. Also amorphous polymer have low density which make it softer. 

In crystalline polymer ordered structure build strong attraction force between molecules. Mean it is chemical resistance and has a high density which make it translucent (allowing light pass through it but not transparent). 

Picture- Structure of Amorphous polymer and Crystalline Polymer

 Difference between amorphous and Crystalline polymer is as shown in below table.


Common material list  classified based upon polymer structure, see below picture:-


In next article "plastic injection molding process chapter 2", we will following:-

1. injection molding machine

2. Injection molding process parameters

3. Injection molding defects


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