Injection Molding Process: A Complete Guide to Plastic Injection Molding

Working Principles of Injection Molding

The working principles of injection molding are likened to syringes used for injection. The operator pours the raw material into the hopper through a feeding device, which goes down to the barrel heater, where the plastic resins are heated past their melting point to liquid. Afterward, a hydraulic ram, resembling a rotating screw, forcefully pushes the molten plastic, injecting it into the closed mold cavity.

This injection of the molten material continues at a high pressure of about 100 to 150 MPa until the mold cavity is filled. The content takes the mold’s shape, after which it begins to cool, solidifying to the desired shape and structure.

Then, the mold opens, using the ejectors to remove it carefully without exerting damage. At the same time, the mold closes for a repeat of the process until the desired volume of parts is injection molded. This process is fast, happening within seconds, supporting the production of thousands of parts per cycle.

Stepwise Guide to Injection Molding Process

While we have already provided a brief overview of the injection molding, below is a more comprehensive process.

1. Mold Design and Creation

The mold is the most significant part of injection molding. When planning to engage in injection molding, the first thing to do is to create the injection mold component, which determines the shape, size, and structure of the intended fabrication.

Machinists often utilize CNC machining (or 3D printing) to manufacture the mold to the desired specification to ensure accurate geometry, dimensions, and draft angles, among other features. A typical injection mold is made from aluminum or steel and features mainly the core and cavities, including components like gates, runners, spruces, and ejector systems.

2. Material Selection

After creating the right mold type for the intended fabrication, the next step is selecting the plastic polymer that best fits your project. You should consider strength, flexibility, chemical resistance, and temperature tolerance, which determine the suitability of the material for the intended application. Standard plastics for injection molding include ABS, polypropylene, and nylon, each offering unique mechanical and chemical properties.

3. Machine Setup

Setting up the injection molding device involves configuring parameters such as temperature, pressure, and cycle time. In addition, the operator must ensure mold alignment, the loading of raw plastic materials, and the testing of the injection system before production begins.

4. Injection Molding Cycle

As mentioned earlier, the injection molding cycle starts by melting the plastic and injecting it into the mold at high pressure. As molten plastic fills the cavity, it begins to take the shape of the mold, forming the part as it cools and solidifies.

In addition, the cooling time for injection molding depends on the selected material’s properties, thickness, and size of the molded part. Cooling channels within the mold cavity may accelerate the process, ensuring the part retains the intended shape and reducing the risk of warping or other injection molding defects.

5. Parts Ejection

Once the molten plastic has solidified, the mold opens, and the finished part is ejected using ejector pins or plates. This step is automated to prevent opening before cooling, speeding up the production process and avoiding damage to the part.

6. Inspection and Post-processing

After the part is ejected, the manufacturer will conduct quality checks to ensure it meets dimensional accuracy and surface finish standards. Inspection methods include visual checks, dimensional measurements, and testing for structural integrity to maintain product consistency.

Afterward, depending on the intended specification, the manufacturer may proceed to post-processing finishes that suit the injection-molded part.

Basic Design Considerations for Injection Molding

All manufacturers must consider the following injection molding basics before engaging in the process.

Tolerances

Injection molding allows manufacturers to create parts with tolerance standards of about ±0.005 inches. While achieving tighter tolerance specifications is feasible, it will significantly increase injection molding costs. In addition, each plastic shrinks at different rates. Therefore, shrinkage tolerance should also be a determining factor when selecting materials.

Wall Thickness

The wall thickness of the part you are molding also influences the accuracy and precision of the process. The recommended practice is to maintain a uniform thickness throughout the molded part. While this may not be feasible for all products, the variation in wall thickness should be kept within 0.04 to 0.15 inches, depending on the party’s size and material. This recommendation will reduce the risk of warping, sink marks, and other defects as the molten polymer cools and solidifies into your desired structure.

Core Geometry

The design of the mold’s core influences the geometry of the part. Machinists should design the core to promote even material distribution and proper cooling. Sharp internal corners should be minimized, as they increase stress, reducing strength and overall part durability.

Draft Angle

The draft angle refers to tapering the vertical walls of the injection-molded part to make it more accessible for release. Without a proper draft, parts can stick or get damaged during ejection. Therefore, the draft angle should be kept between 1 and 2 degrees, though deeper features require a more significant angle for smooth ejection.

Undercuts

Undercuts are complex features characterized by creating an interlock. Such features often make part ejection more challenging. Consequently, reducing undercuts or simplifying the design is often recommended, especially when they are not functional features of the part’s design.

Gates

Gates refer to the opening in the mold, allowing the easy flow of molten polymer into the cavity. The size and placement of the gates influence the flow, cycle time, and overall quality of the molded part. While gates differ in structure and specific orientation to suit a particular project, precision-focused parts require smaller gates.

Ribs

Ribs are thin, wall-like features that help reinforce the structural strength of injection molded parts while maintaining uniform thickness without increasing material usage. They are often positioned parallel to the mold opening direction, with a 40 to 60 percent wall thickness. If thicker than this, it may cause sink marks.

Ejector Pins

As the name suggests, this feature enables easy removal of the molded structure after cooling. The ejector pins are typically positioned at the B-side of the mold, though the operator must optimize the position to avoid leaving visible marks or deforming the part. However, you may not need ejector pins if the draft angle is sufficient.

Types of Injection Molding Processes

Injection molding includes techniques that involve creating components by turning plastics into liquid before reshaping. Below, we examine the common injection molding types.

Insert Molding

The insert molding method involves placing a pre-made component, such as a metal insert, into the mold before injecting the liquefied plastic resin. In this way, the insert component integrates with the plastic, forming a rigid component of multiple materials. The process is the go-to choice for creating plastic parts that require metal threads.

Blow Molding

Blow molding is the go-to process for creating hollow plastics, such as bottles and containers. The process involves heating the plastic and using air pressure to force it against the walls of the mold, creating a hole. Like in all injection molding processes, the resulting fabrication takes the shape of the mold cavity, creating lightweight and durable containers used in the packaging industry.

Overmolding

Overmolding involves molding one material over another, creating a combined structure. Like insert molding, the resulting part usually comprises two different materials: hard plastic and softer polymer. This process is commonly used to improve grip, where manufacturers add an elastomer, such as rubber, to tool handles.

Liquid Injection Molding

Liquid injection molding, also known as liquid silicone rubber, is a process that uses thermoset polymers to create flexible and heat-resistant parts. The molten material is heated and forced into a mold, curing quickly and forming the desired structure. The process is more common when creating seals and gaskets in the medical industry.

Polymer Resins in Plastic Injection Molding

Before selecting materials for any manufacturing process, manufacturers must examine their physical and chemical properties. Below, we examine standard plastic resins in the injection molding process.

Injection Molding Resins

As mentioned above, below are the typical plastics for injection molding

ABS (Acrylonitrile Butadiene Styrene)

ABS is an excellent plastic for part fabrication because of its toughness and impact resistance. Moreover, it is affordable, dimensionally stable, easy to mold, and offers an excellent appearance. The aggregation of these features makes ABS a common choice for molding parts and components used in the automotive, electronics, and consumer goods industries.

Nylon (Polyamide)

Nylon is known for its strength and wear resistance. These characteristics make nylon readily available in mechanical parts like gears and bearings. In addition, it is easy to mold and offers considerable temperature resistance, making it more resistant to shrinking and wearing during molding. Therefore, manufacturers use nylon to mold complex shapes and structures.

Polycarbonate (PC)

Polycarbonate is a transparent, high-impact resin commonly used in optical applications and electrical housings. It offers excellent heat resistance and dimensional stability but may be prone to scratching.

Acrylic (PMMA)

PMMA is a lightweight, transparent resin known for its glass-like clarity. This feature also makes it suitable for optical and lighting applications, such as lenses, windows, and display screens. However, unlike polycarbonate, PMMA is more straightforward to mold but has lower impact resistance.

Polypropylene (PP)

Polypropylene is a more flexible, lightweight resin, making it more beneficial in packaging, automotive parts, and consumer goods. It is cheap and resistant to chemicals, fatigue, and moisture, making it ideal for high-volume manufacturing.

Polyethylene (PE)

Polyethylene is one of the most widely used plastics because of its favorable properties. It is tough, flexible, and resistant to chemicals. Polyethylene is commonly used in containers, piping, and plastic films, with variants like HDPE offering added strength.

Injection Molding Additives 

In addition to using different plastic options in injection molding, manufacturers and OEMs often utilize various additives to enhance one property or another in the molded part.

PTFE (Polytetrafluoroethylene)

PTFE, commonly marketed as Teflon, is an additive that helps improve lubricity and reduce friction in moving parts. Its high resistance makes it suitable for various applications, such as low-wear and high-friction structures like bearings.

Carbon Fiber

Carbon fiber additives enhance the strength and stiffness of injection molded parts. They are lightweight, ensuring that even as they improve general material characteristics, they do not increase density. Consequently, carbon fibers offer applications in industries that value high strength-to-weight ratios, such as aerospace and sports equipment.

Aramid (Kevlar)

Kevlar fibers are less-abrasive glass fibers. Manufacturers incorporate them into plastic resins to increase strength and impact resistance. They are used in high-performance applications, including automotive and aerospace components. However, they may make molded parts less flexible.

Minerals – Talc and Clay

Talc and clay increase rigidity and improve the dimensional stability of resins. They are more commonly used as fillers to reduce costs and hardness. In addition, they do not shrink like resins, so they may help minimize warping while increasing strength without significantly increasing weight. 

Short and Long Glass Fibers

Glass fibers, whether short or long, are added to improve plastic parts’ tensile strength, stiffness, and impact resistance. Long glass fibers offer superior mechanical properties compared to short ones and are used in structural components in automotive and industrial applications. However, glass fibers reduce flexibility, making molding more challenging and may even increase the risk of warping and shrinking.

Pros and Cons of Plastic Injection Molding

We have already examined various aspects of plastic injection molding. Below, we discuss its advantages and disadvantages concisely.

Pros of Injection Molding

• High Efficiency: After mold creation, the production process is quick, allowing for high-volume manufacturing with minimal cycle times.
• Precision and Accuracy: Injection molding can create complex, intricate parts with tight tolerances, which is ideal for industries like automotive and electronics.
• Minimal Waste: Since the process heats plastic resins into molten form before molding, excess plastics in one fabrication may be reused for another, minimizing waste.
• Versatility: It supports various materials, including thermoplastics, thermosets, and other polymers.
• Cost-Effective: While it attracts high initial costs, this gets offset by the low per-unit cost of mass production.
• Automated Process: Injection molding machines may be automated, reducing labor costs and increasing production speed. This feature also enables reproducibility and the creation of identical parts.

Cons of Injection Molding

• High Initial Costs: Mold creation is expensive and time-consuming, making it less ideal for small production runs.
• Design Limitations: It is challenging to change the mold for a particular design after creating it.
• Lead Time for Mold Creation: Creating a custom mold may take weeks or months, delaying production start times.
• Part Shrinkage: Injection molding defects are almost inevitable. Plastics tend to experience shrinkage or warping during cooling, requiring careful mold re-design and material selection.
• Environmental Impact: While plastics are highly efficient, their use contributes to significant environmental concerns, particularly in non-recyclable materials.

Expert Tips and Recommendations for Plastic Injection Molding

While we have extensively discussed plastic injection molding, including design recommendations, we went further to provide tips and suggestions to elevate the result of your injection molding services to reduce costs.

Select Materials that Best Fit Specifications

The choice of material for injection molding influences the part’s performance. Therefore, you should consider factors like strength, flexibility, temperature resistance, and chemical compatibility must be considered. Manufacturers must ensure the chosen material aligns with the part’s functional and environmental requirements.

Consider Bulk Volume Manufacturing

The machine and mold creation cost is high, making plastic injection molding expensive. However, it becomes cost-effective when dealing with high-volume production, the per-part costs drop over the bulk production runs. Therefore, you should consider the expected volume when planning your project, producing small-batch parts does not justify the investment in custom molds.

Optimize Wall Thickness

We have already mentioned the relationship between wall thickness and common injection molding defects, especially warping. Ideally, walls should be as thin as possible without compromising strength to reduce material usage and cooling time. A good rule of thumb is maintaining a consistent thickness throughout the design to ensure an even plastic flow in the mold.

Minimize Undercuts and Complex Designs

Undercuts and intricate geometries add complexity and cost to the mold design, elevating costs. While they may be necessary for certain part functions, minimizing them simplifies molding, reduces production costs, and shortens cycle times.

Modify Gate Location and Size

Poor gate design may lead to defects such as uneven or incomplete filling, warping, and sink marks. Therefore, the operator must modify the gate location and size to optimize flow and reduce pressure within the mold. In fact, larger or more complex parts may require multiple gates to ensure uniform material distribution across the mold.

Optimize Cooling and Venting Systems

Efficient cooling and venting are essential for maintaining part quality and reducing cycle times. Proper cooling channels allow the part to cool evenly, preventing warping or shrinkage. On the other hand, venting systems help remove trapped air and gases from the mold during injection, reducing the risk of gas porosity, voids, and incomplete fills. Both systems should be carefully designed and tested to ensure optimal performance.

Applications of Injection Molded Products

While metal parts and components are common in manufacturing, engineering-grade plastics offer vast applications. These polymers provide more cost-effective solutions for creating complex components.

Many industries make molded parts because it is lighter and safer.

Automotive Industry

Injection-molded plastics have improved cars’ safety, cost, functionality, and performance. This is because plastics, a lighter alternative to metals, improve fuel efficiency.

Automotive part manufacturers use injection molding for functional parts with complex geometries like plastic fuel tanks, bumpers, and door panels.

Medical Industry

Some injection molded parts are biocompatible and have good optical clarity. Hence, manufacturers use them in the medical and optical fields. With plastics, medical practitioners can easily and quickly sterilize equipment.

Additionally, molded plastic parts are safer. Examples include sterile gloves, IV tubes, and urine bags. They all allow for one-time use because the cost of production is low.

Food Packaging Industry

Conventional food packaging materials are glass, metal, or paper. Plastics are slowly replacing them because they are cheaper, safer, and lighter. Furthermore, most molded plastics can withstand harsh chemicals and extreme hot and cold environmental conditions. This protects your food or beverage from odors, unwanted reactions, and moisture. Examples are water and soft drink bottles, pouches, sachets, and chip bags.

Aerospace Industry

For years, plastics have been used to make interior and exterior components in the aerospace industry. In addition to their lightweight benefits, there is little or no need for maintenance, so the aircraft stays longer in the air.

Polyetheretherketone(PEEK) is the most common plastic aerospace part manufacturers use because it has low flammability properties and can withstand UV and sun rays. Turbine blades and housings, chassis components, panels, enclosures, and containers are all injection molded products.

Electronics and Electrical Industry

Plastics are poor conductors of heat and electricity. So, manufacturers use them extensively when their insulating properties are needed. Also, they reduce the risk of burns and electric shock.

Injection molding plays a crucial role in manufacturing components used in electronics, evident in their use in household items, fuse boxes, telecommunication equipment, and many more.

When and When Not to Use Injection Molding Process？

As we have reiterated, injection molding is one of the most common methods for creating plastic components, especially for bulk-volume production. You should use this technique when manufacturing plastic consumer products, requiring consistency, simple plastic components, etc.

However, it is unsuitable for low-volume production because of the high costs required for mold creation. Similarly, when product designs frequently change, the fixed nature of molds makes injection molding impractical. Therefore, when creating plastic prototypes or low-volume intricate designs requiring flexibility, you should consider alternative processes like CNC machining and 3D printing that offer greater efficiency and cost-effectiveness.

Bonus 1: Plastic Injection Molding Machinery

Like any sophisticated device, plastic injection molding machinery consists of several components. However, several injection molding systems exist, such as hydraulic, electric, and hybrid machines. Each device has unique properties and slight variations in its parts.

That said, below is an overview of the standard parts of an injection molding machine.

• Clamp Unit: This holds the mold in place, allowing it to remain closed during the injection molding cycle.
• Injection Unit: This is the main component of the machine. It melts the plastic resin and forces the injection into the mold cavity. It consists of the screw and barrel that melts and mixes plastic pellets, creating a uniform mixture, and the nozzle that injects the molten plastic into the mold.
• Mold: This is the precision tool that shapes the molding. It is where the molten plastic cools and solidifies, forming the desired structure.
• Control System: This component regulates machine functions, such as melting temperature, pressure, and flow rate.
• Cooling System: It is responsible for rapid cooling and eventual solidification of the molten resin.
• Ejection System: It removes the finished part from the mold.

Bonus 2: Common Defects in Injection Molding Plastics

We have already mentioned most of the common defects in plastic injection molding. Let’s quickly review them.

• Warping: This deformation results from uneven cooling or poor material flow.
• Sink Marks: Indentations form when the material cools unevenly in thicker areas.
• Flash: Excess material seeps from the mold, creating thin protrusions along the edges.
• Short Shots: Incomplete parts occur when the mold doesn’t fill entirely.
• Burn Marks: Discoloration or degradation results from excessive heating of the material or trapped air.

Addressing these defects often involves adjusting the mold design, material selection, or process parameters to ensure higher precision.

Bonus 3: How to Reduce Injection Molding Costs?

Reducing injection molding costs involves reducing things that will increase costs. Optimizing the part’s design and reducing overly complicated features, geometries, and undercuts are practical solutions to reducing costs.

Other practical methods of reducing injection molding costs include:

• Maintaining a consistent wall thickness will help reduce material wastage and mold complexity, reducing costs.
• Bulk production is an effective strategy for reducing cost, as the cost-per-unit benefits economies of scale.
• Mold and machine maintenance will help extend tool life, reducing the need for expensive replacement.
• You can also reduce costs by using cost-effective materials, such as recycling resins, and minimizing the use of additives if they are not required.