07.20.23 | 6 min read

Understanding the Process of Plastic Parts Manufacturing

While there are various methods for plastic part manufacturing, injection molding is the most popular.

Injection molding is a versatile method for manufacturing mid to large-volume, high-precision plastic parts in a range of sizes. Because of this, it has become the most popular method for plastic parts manufacturing across industries. The process involves injecting molten plastic into a mold, where it cools and solidifies to form the desired part. While it sounds simple, a great deal of work and knowledge goes into creating the perfect part. Below we discuss some advantages and explain the process of injection molding from product design through finished product.

Advantages of Injection Molding

A process doesn’t become a globally relied-upon manufacturing method without proven benefits. Some of its highly recognized benefits include the following: 

  • Highly efficient – Injection molding is a very efficient process, which provides a cost-effective solution for high production volumes. 
  • Precise part dimensions – Injection molding can produce parts with very precise dimensions. This is important for many applications, such as those where the parts need to fit together with other parts or where they need to have a specific size or shape.
  • Customizable – Injection molding can process a wide range of plastics, including Polypropylene, Polyethylene, ABS, Nylon, Polycarbonate, Acetal, and more. Fillers and additives can be incorporated into the plastic to increase strength or improve other properties, such as UV resistance. Colorants can be used to produce parts in virtually any color. This gives designers great flexibility when choosing the material for their parts.

Design For Plastic Parts Manufacturing

Designing and manufacturing complex parts is an involved process that requires planning, experience, and manufacturing capabilities. There are many steps in the design process, and making changes after the initial design phase can add to delays and costs.

First, understand your product requirements. Many crucial decisions about the part design are made early to ensure no essential details are missed. Learning that a critical tolerance was forgotten or the design complexity adds costs will be disheartening and expensive after the mold is built. Some questions to consider are:

  • Materials – Consider all material characteristics, including its shrink rate, to ensure tolerances can be met and that it has the required properties (e.g., strength, durability). In addition to resin, what other materials, inserts, or hardware are required? Are all materials compatible? If overmolding, will materials bond together?
  • What function does the part serve? Will it be assembled with another component, and if so, how? 
  • Are there critical tolerances? 
  • What environment will it be exposed to? How will this impact the resin, formed part, and hardware? 
  • Are there industry or regulatory requirements? 

Ensure your part is designed for manufacturability and assembly. Design elements that must be considered include gate placement, wall thickness, draft angles, ribs and gussets, bosses, and corner radius. Improper design can lead to poor-quality results. Undercuts can add complexity and cost to molds because core pulls or cams must be added to aid in part removal. 

Forms of Injection Molding

Not all products will use the same form of injection molding. Most parts will require standard injection molding, which uses a single color of one type of plastic. 

But if your plastic product requires threaded metal hardware or embedded electrical components, insert molding is the way to go. Insert molding allows manufacturers to encase part of a preformed component that can withstand the pressure and temperature of injection molding (usually metal) in plastic. 

A third form is overmolding. With overmolding, one material is molded on top of another. This is often used for products needing soft touch handles molded from two different materials. The overmolding may happen in the same mold, and components are molded on top of each other or can be done as transfer molding, where one component is molded and then transferred to another mold to complete the part. 

Injection Molding Tooling

High-quality tooling is required for high-quality parts. While molds require an upfront investment, skimping can cost more in the long run. Cavitation can impact mold costs and should be considered carefully. Each cavitation increases the costs; however, high volumes will more than make up for having multiple cavitations in each mold, if the part size is compatible, resulting in lower piece pricing. 

Several grades of materials are used to build molds, and the scope of your project will help the mold builder decide which is best. Aluminum may be used for pre-production prototypes or lower-volume parts, while hardened steel will be used for molds expected to reach one million or more shots. 

The injection mold SPI class, defined by the Plastics Industry Association (formerly called the Society of Plastics Industry), standardizes mold construction based on the number of cycles they are expected to run. The mold classes range from Class 101, the most robust and expensive, to Class 105, intended for very low-volume prototype parts only. 

The Injection Molding Machine Basics

A standard injection molding machine consists of three parts: an injection unit, the mold, and the clamp. The injection unit uses a hopper to feed plastic pellets into it, where a screw transports them through a heated barrel. As the pellets are pushed forward, they melt. The molten plastic is then forced into the mold, where it forms the shape of the part. Within minutes the part solidifies. The mold opens, the part is removed, the mold closes, and the process repeats. 

While this may sound simple, the scientific principles behind producing a quality part are more complex and encompass thermal dynamics, fluid dynamics, polymer and material science, heat transfer, and pressure. Your injection molder must have a firm understanding of how they work together and then apply them in a controlled manner to maintain consistency and manufacture a quality plastic part. Maintaining pressure and fill time is critical to getting the plastic to the proper viscosity so the precise amount of resin enters the mold. 

Trust Your Plastic Parts Manufacturing To LMC

Whether you need plastic injection molding or to combine metal stamping with insert molding or overmolding, we have the capacity (with over 60 molding machines) and the know-how to deliver consistent quality and short lead times. With an in-house tool room, metal stamping capabilities as well as assembly and finishing services, we can be your one-stop partner for your complex parts, saving time and money, reducing lead times, providing supplier stability, and ensuring consistently high-quality products. Contact us to start your quote!

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