A Practical Guide to Injection Molding Plastics Engineering

A Practical Guide to Injection Molding Plastics Engineering (3)

Introduction:

This blog aims to enhance the understanding of injection molding for individuals involved in the practical aspects of the process, such as machine operators, technicians, and mechanics. Typically, these personnel have learned the process through on-the-job training or trial and error, lacking formal education or training in plastics technology. This blog seeks to provide them with a comprehensive understanding of the fundamentals of injection molding, ultimately improving their job satisfaction and contributing to the profitability of molding operations.

 

Elements of the Injection Molding Process:

The injection molding process consists of various interconnected elements, as illustrated in Figure 1. Upon first encountering this complex system, newcomers to injection molding may find the multitude of wires, hoses, tubes, and pipes overwhelming. It is crucial for operators and technicians to grasp the functioning and significance of each element in ensuring the successful molding of parts. While robotics and additive feeders can enhance efficiency in automated operations, they are not indispensable to the process. Other elements, including the injection molding machine, clamp unit, control unit, hopper, mold, cooling system, and material handling equipment, play critical roles in injection molding.

Injection Molding Elements
Injection Molding Elements

Plastics:

The primary raw material used in injection molding is plastic, which is commonly purchased in the form of pellets. These pellets vary in size, approximately resembling small kitchen match heads or measuring around one-eighth of an inch (three millimeters) in diameter. Plastic can also be obtained in powder form, although it is less prevalent today. Plastics exhibit diverse mechanical properties when molded, making them suitable for different applications. It’s essential to note that not all plastics melt at the same temperature or in the same manner, adding complexity to the molding process.

Moisture Control:

Most plastics are hygroscopic, meaning they can absorb and retain moisture. If the moisture content is not sufficiently removed, it can result in cosmetic or structural defects in the molded parts and lead to corrosive wear of injection unit components. Therefore, many plastics undergo a drying process. Various types of dryers are available, ranging from small units attached to the hopper assembly on top of the press to larger systems capable of drying plastic for multiple machines.

 

Summarizing the Elements:

This introductory overview of the injection molding elements aims to familiarize operators with the components involved in the molding process. Each element will be explored in more detail in subsequent chapters of the blog. Understanding how these elements fit together will aid in comprehending their individual functions. The following table provides a summarized view of the elements:

Plastic Loader: 

The plastic auto loader is responsible for transporting plastic material from storage to the machine hopper. It ensures a continuous supply of raw materials for the molding process.

Dryer: 

The hopper dryer plays a crucial role in removing moisture from the plastic material before it enters the injection unit. Moisture can negatively impact the quality of the molded parts and cause corrosion in the equipment.

Additives: 

Additives are additional ingredients added to the plastic pellets to enhance specific properties or achieve desired characteristics in the final molded parts. These additives can include colorants, lubricants, or other additives.

Additive Feeder: 

The additive feeder is responsible for adding a precise quantity of additives, such as colorants or other ingredients, to the plastic material as it is fed into the injection unit. It ensures accurate and consistent incorporation of additives.

Injection Unit: 

The injection unit is a critical component of the injection molding machine. It is responsible for heating, melting, and injecting the plastic material into the mold. The injection unit applies pressure to fill the mold cavities and create the desired shape of the molded part.

Clamp Unit: 

The clamp unit holds the mold in place during the injection process. It applies sufficient force to keep the mold closed during injection and opens the mold to eject the finished parts once they have solidified.

Control Unit: 

The control unit is the central control system of the injection molding machine. It monitors and controls various parameters of the molding process, such as temperature, pressure, and timing. The control unit ensures precise and consistent molding operations.

Mold: 

The mold is a crucial element in injection molding. It is a combination of tool steel plates that form the cavities and cores required to shape the plastic material. The mold provides the framework for the injection process and determines the final shape and features of the molded parts.

Chiller: 

The chiller is a refrigeration unit that cools the water circulating through the mold’s cooling channels. It helps in rapid cooling and solidification of the plastic material inside the mold, reducing cycle times and improving production efficiency.

Conveyor: 

After the molding process, a conveyor system is used to transport the finished plastic parts to subsequent processing areas or packaging stations. It ensures smooth material flow and efficient handling of the molded parts.

Grinder: 

The grinder is responsible for grinding and recycling runners and defective parts. It reduces them to regrind, which can be mixed with virgin material and reused in the injection molding process. This promotes sustainability and minimizes waste.

Robots: 

In some advanced injection molding operations, robots are used for various tasks. They can perform functions such as picking parts out of the mold, sorting, assembly, or other automated movements, enhancing productivity and efficiency.

A Practical Guide to Injection Molding Plastics Engineering (1)

Injection Molding Elements 

Injection molding involves several interconnected elements that work together to produce high-quality molded parts. Understanding these elements is essential for achieving efficient and successful injection molding operations. Here is an overview of the key elements involved:

Thermoplastics: 

The majority of thermoplastics are made from petroleum and have the unique physical property of being able to be melted, solidified, and remelted again without significantly changing the chemistry of the material (provided they are kept clean and not contaminated). By grinding up the solidified thermoplastic and remelting it, the material can usually be with or without mixing it with virgin (unprocessed) material [l]. Depending upon how many times and under what conditions the thermoplastic material has been melted and solidified (its heat history), some of its properties may be diminished. As a result, most thermoplastic that is reused (and referred to as regrind) is mixed with virgin material where the regrind represents less than 50% of the resulting mixture.

There are some cases, however, where plastic products are molded from 100% regrind. These are instances where the mechanical and cosmetic properties of the resulting parts are not critical. Thermoplasticstypically have long proper names that relate to their basic chemistry type. The chemistry of thermoplastics is rather complex and may be studied further by reference to some excellent blogs. Persons who are not acquainted with the chemistry of plastics or how plastics are manufactured can refer to them by their “short name.” All plastics have been given an alphabetic symbol that is a “short name” for each plastic’s longer technical name. 

 

  • Polyethylene
  • Polypropylene
  • Polystyrene
  • Acrylonitrile Butadiene Styrene
  • Polyamide (nylon)
  • Polycarbonate
  • Polymethylmethacrylate (acrylic)
  • Polyoxymethylene (acetal)
  • Polyvinylchloride
  • Styrene Acrylonitrile

Thermosets: 

Thermosets are plastics that undergo a chemical change when heated to a certain temperature. These materials, once solidified, cannot be remelted or reused. Any attempt to remelt thermosets simply results in the burning or decomposition of the material rather than returning it to a moldable melt. Thermosets cannot be reprocessed or welded.

The chemical change that occurs in thermosets is often referred to as curing or cross-linking. Cured thermoset polymers cannot be dissolved by organic solvents without decomposition. It is not surprising that thermoset products are well suited for electrical, construction, and household applications where resistance to temperature and various types of wear is critical. The raw materials for thermosets are somewhat different than thermoplastics. Base materials include phenol (a coal tar derivative), formaldehyde, and urea. It is not important to remember these materials, but rather to understand that thermosets are entirely different than thermoplastics, both in how they are made and, as will be illustrated, in how they are processed.

 

How Plastics Affect the Molding Process 

More important to the molder, however, is the fact that crystalline and amorphous materials react quite differently during the molding process. There are at least three vital differences in the way the two types of material respond to the melting and molding process.

Melting Characteristics:

The first major processing difference between crystalline and amorphous materials is the way they melt. As heat is applied, both types of materials soften somewhat at first, but the amorphous material continues to soften gradually until it will flow. The softening point is referred to as the glass transition temperature. Amorphous materials have no defined melting point. In contrast, the more highly crystalline materials remain in a relatively solid state until the temperature reaches their melting point. The melting point of plastics is labeled T. As we will see, this difference in the way the materials melt is an important factor in how the materials are molded.

 

A Practical Guide to Injection Molding Plastics Engineering (2)

Thermal Conductivity:

The ability of plastics to absorb heat (referred to as thermal conductivity) is quite low, about two to three times lower than metals. The low rate of heat absorption influences the speed with which plastics can be heated, melted, and molded. The second important difference in how plastics are molded is the difference in heat-absorbing ability between crystalline and amorphous materials. Amorphous materials have much less ability to conduct heat than crystalline materials. In fact, as the crystallinity increases, the ability to conduct heat also increases. Stated another way, you cannot add more heat to amorphous materials and expect them to melt any faster! In fact, if too much heat is applied to amorphous materials, they will burn and degrade. 

Shear Sensitivity:

After considering the first two differences between the two types of materials, the third difference becomes easily understood. Amorphous materials are more sensitive to shear. Shear occurs when plastic pellets are compressed, or rubbed together causing friction, or are significantly agitated during the molding process. High shear results in rapidly increasing the temperature of the material while being molded which amorphous polymers do not tolerate well. From these considerations, it can be concluded that amorphous materials should be gradually (not abruptly) heated when changing them from a solid to a melt. 

Excessive melt temperatures in some materials (especially amorphous materials) can cause residual molded-in stresses (upon cooling) that detract from part appearance or reduce the mechanical strength of the parts. Unfortunately, in many cases, the loss of mechanical properties (such as impact strength) cannot be determined until the part is subjected to impact tests or fails when performing in its intended use. In later chapters, you will learn how these three factors (melting characteristics, thermal conductivity, and shear sensitivity) are controlled in the molding process. We must remember that all materials have a maximum limiting shear rate, beyond which they will degrade.

Viscosity (Melt Index):

Another property of both crystalline and amorphous materials that affect the molding process is viscosity. Viscosity may be defined as the resistance of a fluid to flow. In other words, if melted plastic is considered viscous, it is thick (like molasses) and will not flow easily. The viscosity of a melted plastic can be measured and given a rating called a Melt Index (MI). A high melt index means that the melted plastic is thin and watery (and has low viscosity).

The lower the melt index, the more thick and viscous the melt is and the less easily it will flow. The melt index of plastics ranges from a fractional MI, meaning that it is less than one (l), to more than a hundred (100). Most common materials have a MI in the range of 2 to 12. There are various test methods and parameters for measuring Melt Index. When comparing materials, it is important that the method and parameters are the same.

The viscosity of plastic is important to the molder. Materials with a very high MI or very low viscosity are more difficult to push or inject and, in some cases, more difficult to mold. Incidentally, it is good to remember that Melt Index is also a measure of molecular weight. A higher MI indicates a lower molecular weight for a given polymer family.

 

Conclusion:

A Practical Guide to Injection Molding Plastics Engineering serves as a valuable resource for individuals involved. The blog fills a gap in the industry by providing practical knowledge. Understanding of the fundamentals of injection molding to machine operators, technicians, and mechanics.

Introduced to the key elements of the injection molding process and gain a deeper understanding of their roles and functions. From the plastic loader and dryer to the injection unit, mold, and control unit, each element is explored in detail, enabling operators to grasp the intricacies of the molding process.

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