Plastic injection molding

Plastic Injection Molding and the fundamentals to quality molded parts

In 1872, American inventor John Wesley Hyatt initiated the process by patenting the initial plastic injection molding machine to substitute ivory billiard balls with wood composite resin ones.

Even though his invention did not fulfill the market demands, the machine itself proved to be successful, so much so that today injection molding is a manufacturing process that is indispensable to producing goods for virtually every vertical market, using hot plastic resins from recycled plastic as well as new plastic to manufacture anything from medical parts, to sutomotive parts to plastic containers for food and beverage purposes. 

Plastic fills many needs in modern life and injection molding is one of the key techniques to use it into endless products, allowing the molding industry to produce thousands of plastic parts. useful for so many solutions for plastic part production injection molding offers.  The  disadvantages of injection molding are few if any.

Most of the plastics materials for injection molding we use on a daily basis are created using different injection molding methods that are used depending on the specific requirements of the product being manufactured.  From its inception, the plastic injection molding process swiftly progressed from producing basic items like buttons to specific product designing a part for injection molding specifically using best practices design to create plastic parts for injection molding  and manufacturing plastic parts through injection molding such as manufacturing complex medical equipment in present times. 
The processes for injection molding work used in injection molding became much more flexible and now is employed for plastic manufacturing using hundreds of plastic materials across a range of industries that use injection molding to create simple as well as complex parts from polymers.  It is truly astounding how many injection molding applications there are that industries use this technique for, making parts by injecting molten plastic into a mould.

plastic injection molded part

Tooling fabrication involves the initial step in the production of injection molding parts, where the tooling is milled from either steel or aluminum. Usually, a metal block is machined to create a negative mold of the plastic part. The tooling can undergo additional treatments such as polishing or laser etching to achieve desired surface finishes.

The production of injection molded parts starts with placing raw material consisting of plastic polymer resin pellets into a hopper from which raw plastic pellets are fed to a barrel which is attached to the injection machine. The temperature of the barrel is increased until the pellets melt, the molten plastic flows and is subsequently pressed. The melted plastic is then injected through the injection unit nozzle(s) that inject plastic into the negative  metal tool via a runner system with varied injection pressure, which directs it into the mold cavity through gates; the injection speed varies. After the plastic cools, and solidifying, the part is pushed out of the tool using ejector pins.

Injection moulded parts are utilized in a wide range of sizes, from small machine components to large car body panels. Injection molding is also commonly used for packaging, automotive parts, containers, toys, plumbing components, and medical products.


Producing and delivering plastic injection molded parts to Original Equipment Manufacturer (OEM) customers entails several steps, each critical to ensuring the quality, accuracy, and timely delivery of the components. When planning to do custom plastic injection molding it is important to be aware of each step of the production chain. 

Here’s a step-by-step overview:

Overview of the injection moulding process

There are many considerations when choosing an injection molding manufacturing process for producing plastic parts, but simply put, the manufacturing process used to produce plastic injection molded parts involves four basic steps but it can change depending on each custom plastic injection project:

1. Melting the material

2. Injecting the molten material into a mold

3. Allowing the molten material to cool and solidify

4. Removing the hardened material from the mold

The processes for injection molding include feeding of plastic pellets of the plastic resin used from a hopper into a barrel, where an internal auger is located.

The auger, a spiral-shaped device, rotates to feed the material while the heater bands on the barrel’s exterior melt the plastic into a liquid plastic setting up for the ideal molding condition.

Upon initiation of the machine’s cycle, the parts of the mold close, and the auger moves forward creating high pressure similar to a syringe plunger, but with much greater force.

In a matter of seconds, the molten plastic is injected by the internal auger into the empty space of the mold known as the cavity.

To facilitate the cooling process, a coolant such as water circulates through channels in and around the mold.

After about 60 seconds, the molten plastic solidifies and takes the shape of the cavity.

Following solidification, the mold opens, and the molded part is ejected.

The plastic injection process then repeats as the machine ejects the solidified plastic part.

Consultation: Communicating with OEM customers to understand their specific requirements, including their existing mold design [if any], part design, material specifications, quantity, and delivery timelines.

Analysis: Analyzing the requirements to develop a production plan.

Quote: Paroviding an accurate quote including cost, time line, terms and conditions in a timely manner, typically withing 72 hours.

Molds for plastic injections are expensive though valuable assets and must be designed with absolute precision. Making concessions on your product specifications may result in a change in the quality of produced plastic parts. However, a skilled injection molder can still produce high-quality plastics without compromising on key details. In cases where compromises are necessary, your supplier can offer suitable suggestions to minimize the impact of these changes.

This may involve using alternative resins and making slight adjustments to the design, which can help reduce lead time and cut down on production expenses.

CAD Modeling: Using the client’s detailed CAD models of the plastic parts based on the OEM specifications, or creating such models for the client [on a fee basis].

Prototyping: If needed, producing prototypes to test design accuracy and functionality.

Approval: Seeking approval from the OEM customer on the final design after any necessary modifications.

Common injection molding materials

Material Selection: Choosing the appropriate quality of the plastic material as per OEM specifications unless already specified. Colorants and resin additives as well as different plastic can greatly impact user experience and product value of every plastic product.

Resin standard colors are usually black and natural, with natural colors including white, beige, amber, or other options. Semi-custom colors can be achieved by mixing colorant pellets with natural resins. 

Short glass fibers are utilized to enhance a composite’s strength and minimize creep, particularly at elevated temperatures. They enhance the resin’s durability, rigidity, and fragility. They can lead to distortion because of the varying cooling shrinkage rates between the resin and fibers.

Carbon fibers can also be employed to fortify and/or stiffen a composite, as well as assist in static dissipation. It shares similar limitations as glass fibers. Carbon fiber has the capability to greatly increase the rigidity of plastic.

Minerals like talc and clay are commonly added as fillers to reduce costs or enhance the hardness of final components. Due to their lower shrinkage rate compared to resins during cooling, they can help minimize distortion.

PTFE (Teflon) and molybdenum disulfide are utilized to create self-lubricating parts for bearing applications.

Long glass fibers are employed to reinforce and minimize deformation in resin, increasing its strength and stiffness. However, molding parts with thin walls or long resin flows can prove to be challenging.

Aramid (Kevlar) fibers are similar to glass fibers in terms of being less abrasive but not as strong.

The addition of glass beads and mica flakes can be used to enhance the stiffness of a composite and reduce the likelihood of warping and shrinking. However, injecting them at high concentrations can be difficult.

Stainless steel fibers are used in electronic component housings to manage EMI and RFI, being more conductive than carbon fiber.

UV inhibitors are used for applications exposed to outdoor conditions.

Static treatments enable resins to dissipate static electricity.

The plastic injection process for injection molded plastic parts is intricate and engineers have to consider key tooling aspects before production of plastic parts at scale:

Nozzle selection: Various types of nozzles are utilized in plastic injection molding, each tailored to fulfill a particular need. Factors such as the material being used, design of the part, molding parameters, and specific requirements of the injection molding process all play a role in determining the most suitable type of nozzle to use.

Different types of nozzles include:

  1. Open nozzle: An open nozzle is the most basic and commonly used type of nozzle. It consists of a simple channel that connects the barrel to the mold cavity. Open nozzles are typically used for general-purpose applications and materials with low viscosity.
  2. Shut-off nozzle: It is designed to provide better control over the material flow. It prevents drooling or stringing. They incorporate a valve or shut-off mechanism at the nozzle tip that closes when the injection process is complete. This leads to sealing off the flow of molten plastic. The shut-off nozzle helps to reduce part defects and material waste.
  3. Valve gate nozzle: It is also known as hot runner nozzle. It is used in hot runner systems. These systems keep the molten plastic at a consistent temperature throughout the injection molding process. This eliminates the need for the plastic to solidify and re-melt each cycle. They have a valve pin that controls the opening and closing of the nozzle. This precisely controls the flow of plastic into the mold cavity.
  4. Multi-tip nozzle: It consists of multiple nozzle tips. These are arranged in a single assembly. They are used for multi-cavity molds where several identical parts are produced simultaneously. Multi-tip nozzles allow the efficient filling of multiple cavities. This is done with a single injection molding machine which reduces cycle time and increases productivity.
  5. Mix-head nozzle: It is used for injection molding processes that involve mixing multiple components or colors. These nozzles have separate channels for each component. Here the materials are mixed within the nozzle before being injected into the mold. They are commonly used in applications like multi-color parts or parts with special effects.


Quality Assurance: Conducting inspections to ensure each part meets the quality standards and specifications.

Injection Molding: Injecting molten plastic into the molds to form the parts. Parameters like temperature, pressure, and cooling time are optimized for quality and efficiency.

The finish of the surface is a crucial factor to think about when it comes to injection molding, as the parts are usually made for their appearance and ultimate production use. 

It’s important to note that a textured surface will need more draft compared to a smoother finish in order to ensure the part easily comes off the mold and to properly apply bead blast to the tool.

Trimming: Removing excess material and sprues from the molded parts.

Assembly: If required, assembling multiple parts to form complete components.

Utilizing high-frequency acoustic vibrations, ultrasonic welding is a method of joining two plastic materials together. These vibrations are transmitted to the molded plastic through a horn, while the parts are securely held in place by a fixture. Unlike other bonding methods, ultrasonic welding does not require any additional material to bond the parts together.

In the adhesive method, a material is used to connect two parts without changing the chemical properties of the plastics involved, which is typically suitable for most types of plastics.

On the other hand, solvent bonding involves the use of a liquid material that coats and softens each plastic, allowing them to bond together under pressure. As the solvent evaporates, the plastic molecules blend together to create a permanent bond. However, this method is most effective with thermoplastics.

Ultraviolet (UV) bonding involves the use of high-intensity light to cure adhesives or coatings quickly, creating strong, durable bonds that improve production efficiency and reduce rejected parts. However, for this method to work effectively, it is important to choose a transparent resin that allows UV light to pass through.

If you anticipate needing to separate injection molded parts bonded together, mechanical fastening may be the best option.

Common types of mechanical fasteners include nuts, screws, rivets, and bolts.

To enhance durability, plastic parts that require frequent disassembly often have threaded metal inserts.

Pickouts: Inserts called pickouts are created separately from the tooling and inserted into the mold before material is injected. These are employed to create undercuts on the inside surfaces. Pickouts are released along with the plastic part and reinserted into the mold. While pickouts allow for greater flexibility in shape and positioning, they are more expensive than sliding shutoffs or side-actions.

Part Decoration: Applying surface finishes, painting, or other treatments as specified by the OEM customer.

After being removed from the mold, plastic parts are occasionally left without any decoration. It is advisable to embellish the part if you wish to enhance its appearance or provide instructions or identification. Pad printing, laser marking, and inkjet printing are commonly utilized by manufacturers to decorate parts in traditional ways.

The pad printing process: Uses a silicone pad to transfer 2-D images or text onto a 3-D object. Rigid parts with round surfaces are effectively stamped since the silicone pad wraps around the shape of the item without distorting the text or image. 

Using laser marking techniques provides a quick way to adorn your plastic component.

This approach creates detailed effects on the molded part without the need for any surface preparation beforehand. Other popular laser methods include:

Laser etching: Involves applying a high amount of energy to a small area, causing it to melt and expand and leaving a raised mark on the plastic surface. Typically, etching is used for permanent markings like serial numbers, barcodes, and matrix codes.

Finishing: Applying surface finishes, painting, or other treatments as specified by the OEM customer.

In the United States, plastic injection molding companies and mold builders utilize inkjet printing as a precise, non-contact method of printing. The ink used is specially formulated to enhance the quality of the print. It dries quickly, ensuring it adheres to the surface promptly. Additionally, the ink is designed to resist both oil and heat to prevent any smudging that could obscure the printed images. Colored and pigmented ink is used to create high-contrast, nearly photo-quality texts and images on darker surfaces, making them easier to read.

Inspection: Detailed inspections to ensure dimensions, material quality, and overall part integrity are up to standards.

Testing: Conducting functional and stress tests, if necessary and as required by the OEM client, to validate part performance.

ISO 13485 is the established criteria for the healthcare sector. This encompasses design validation (DV), operational validation (OV), and performance validation (PV).

PPAP is a quality assessment procedure commonly employed in the automotive field. It is utilized to confirm that a manufacturing process can consistently generate a high-quality component. There are 18 elements to a PPAP assessment that span from customer endorsement to prototype part fabrication.

Preparation: Preparing the parts for delivery, including counting, sorting, and quality checking.

Packaging: Utilizing appropriate packaging materials and methods to ensure the parts are protected during transit.

Logistics Planning: Arranging the logistics for timely delivery, considering the OEM’s location and requested timelines.

Shipping: Transporting the parts to the OEM customer’s location using reliable carriers.

Documentation: Including all necessary paperwork, certifications, and documentation required by the OEM customer.


After-Sales Support:

Customer Support: Offering support for any queries, concerns, or additional requirements the OEM customer might have.

Feedback Collection: Gathering feedback to ensure customer satisfaction and improve future processes.

Communication: Maintaining open and transparent communication with OEM customers throughout the process.

Customization: Tailoring every aspect of the process to meet the specific needs and specifications of the OEM customer.

Compliance: Ensuring all parts are produced and delivered in compliance with industry standards and regulations.

This process is meticulously planned and executed to ensure the plastic injection molded parts are produced to the highest standards of quality and delivered on time to meet the OEM customer’s production and market needs.

Variables that determine the cost of plastic injection molding

Having a comprehensive understanding of all the elements that impact the cost of your injection-molded plastic is crucial. This knowledge is essential for determining which plastic injection molding and mold builder in the USA to collaborate with for producing a cost-efficient component. It also helps in setting the optimal selling price for the final product.
Key factors to take into account include:


Material Used for the Mold

The choice of metal for mold fabrication plays a significant role in determining the overall cost of the mold. Different grades of steel offer varying service lifetimes. Opting for durable steel results in molds with extended lifespans, providing greater value. 
Additionally, you can avoid frequent maintenance or replacements when the mold wears out. When selecting the ideal steel grade for your mold, consider the following factors: 
  • The melting point of thermoplastics 
  • Finishes and post processing
  • Time taken for each manufacturing cycle
  • Total volume of produced parts
The most suitable type of steel for your mold is tool steel, valued for its strength and ability to bounce back.
On the other hand, aluminum could be a more cost-effective option, particularly for injection molding projects with small quantities. We already have a article exploring how they compare. 

Mold manufacturer location

The geographical location of your mold manufacturer can impact the costs of shipping and handling, which in turn impacts the overall expenses of injection molding.
When opting for foreign mold manufacturers, it is important to consider the minimum wage in that specific country, as a higher wage can increase the overhead costs of your injection mold. While the initial costs may be lower when outsourcing to foreign manufacturers, companies often encounter challenges such as language barriers and delays in delivery upon shipping.
There are instances where additional tooling is required for repairs or modifications once the tooling reaches its destination country. To overcome these obstacles, it is advisable to choose a reliable plastic injection molding and mold making company in the United States,
Cindre can help you efficiently manage your injection molding projects and navigate the outsourcing of Chinese production capabilities.
We are strategically located near four major airports.

Resins for Injection Molding

The cost of injection molding is greatly impacted by the specific type of plastic resin utilized. Your choice of resin can affect the price due to factors such as quality, availability, grade, additives, and molding temperature. 
Additionally, the type of resin selected can also impact how frequently mold maintenance is required, ultimately increasing the overall cost. For example, resins with abrasive components like carbon or glass necessitate a sturdier and more costly mold.

Quality Standards

Most injection molding companies understand the importance of maintaining quality control. The reputation of these companies is often judged by the products they deliver to customers. It can be difficult to regain trust if a defective product is released to the market. The automation of injection molding processes can introduce errors.
Failing to catch issues quickly can lead to rejected parts and wasted material.
By implementing reliable quality control systems, problems can be identified and corrected early on. Having an ISO 13485 certification is beneficial for companies producing medical devices.  This certification ensures that medical components are manufactured to meet their intended use.

Key criteria to consider when choosing an Injection Molding Partner

Being a product manufacturer, it can be stressful to choose the top supplier among the many injection molding companies in the U.S.

Collaborating with an injection molder who can provide the required plastic parts as per schedule and within the set budget gives you confidence.

Here are some important factors to keep in mind when deciding on an injection mold partner for your upcoming project:

Volume and Capacity Specialization

When selecting an injection molder, you can limit your choices by considering the size and quantity of your needs. A molder with a low to moderate capacity is best suited for producing less than 10,000 units and is perfect for rapidly creating prototypes for part testing.
On the other hand, high-volume injection molders are capable of producing up to 750,000 units with specially designed small injection molds to accommodate large production runs.

Determine the Required Press Tonnage

Injection molding machines use high pressure to complete the complex plastic production process. Two opposing forces act on the mold to keep it open during the injection of molten resin. The clamping force, measured in tons, holds the mold closed.
Understanding how to select the appropriate press tonnage can enhance the efficiency of plastic part manufacturing. A larger injection molding machine typically exerts more clamping force than a smaller one, but bigger is not always better.
Factors such as surface area, part depth, and melt flow impact the required tonnage for injection molding.
Excessive tonnage can pose risks to the machine, mold, and product.
Flashing may occur, leading to excess material on the edges of the part. Insufficient clamping force can also result in production issues. 

Still have questions about Plastic Injection Molding?

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