A Guide to Insert Injection Molding Processes : Applications, Materials, and Design Considerations

• A Guide to Insert Injection Molding Processes : Applications, Materials, and Design Considerations
  • What is insert molding process ?
  • Workflow of insert injection molding process
    • Insert design and material selection
    • Insert manufacturing
    • Cleaning and surface treatment of inserts
    • Insert preheating (if necessary)
    • Insert​
    • Closed mold
    • Plastic melts and is molded
    • Insert Coverage
    • Pressure holding and accumulating
    • Waiting for solidification and cooling
    • Open the mold and eject the product
    • Post-processing and inspection of products
    • Product Packaging and Delivery
  • Advantages of insert molding
  • Plastic substrate for insert molding and insert material
  • Issues to consider when designing insert injection molded products
  • Three embedding methods for insert injection molded nuts
  • Application of insert injection molding process
  • Future Development Trends and Innovative Technology Applications of Insert Molding
  • Elimold is a professional manufacturer of custom insert molding parts.
  • in conclusion
  • FAQ

Modern technology enables manufacturers to create unique products by fusing metals and plastics. Insert molding is a plastic injection molding process that involves molding plastic (often mixed with other materials). In this process, adding other components to the plastic can impart material properties such as wear resistance, weight reduction, and increased strength. The process involves placing an insert in a mold cavity and then injecting plastic under high pressure to obtain a cured part. Insert molding is particularly suitable for medium-sized products, but for larger parts, the complexity of the molds and positioning increases. Common inserts used in insert molding include studs and screws for mechanical parts, and connectors and terminals for electronic components. This article will explore the process flow, materials, applications, design considerations, and advantages of insert molding.

What is insert molding process ?

Insert molding is a molding method that involves placing prefabricated inserts of different materials into a mold, injecting resin, and then bonding and curing the molten material with the inserts to create a one-piece product. First, prefabricated inserts, such as those made of metal, plastic, or glass, are placed in predetermined positions within the mold. Then, molten plastic resin is injected, bonding it tightly to the inserts and curing it within the mold. Finally, demolding produces a one-piece product with the inserts.

Compared to overmolding, insert molding reduces overall costs and increases productivity by decreasing the number of assembly steps, especially in high-volume production. However, insert molding is relatively more time-consuming because it requires molding another layer on top of the product, i.e., the insert forming the overall product package. Both insert molding and overmolding are more expensive than traditional injection molding.

Workflow of insert injection molding process

Insert molding is essentially the same as injection molding, except that it requires manual or automatic placement of the “insert.” It mainly involves pre-fixing the insert in a suitable position within the injection mold, then injecting plastic to form the insert. After the mold opens, the insert is tightly encased and embedded within the cooled and solidified plastic, resulting in a product with inserts such as threads or electrodes. To fully understand the advantages of insert molding, it is essential to understand its process flow. Insert molding typically includes the following steps:

Insert design and material selection

Inserts (typically made of metal, ceramic, etc.) need to be designed to meet mechanical, electrical, or thermal requirements. Key considerations include material compatibility with the molding resin, coefficient of thermal expansion, surface roughness, geometry, and tolerance accumulation.

Insert manufacturing

Depending on the material and geometry, insert manufacturing processes include CNC machining, stamping, cold heading, die casting, or sintering. Critical dimensions, flatness, burr control, and surface finish are all strictly controlled to ensure repeatability of insert placement and consistency of the encapsulation during molding. Post-processing such as deburring or polishing may be required.

Cleaning and surface treatment of inserts

Cleaning methods for inserts include ultrasonic cleaning, solvent cleaning, or alkaline degreasing to remove oil, oxides, or particulate matter. Surface treatment processes such as knurling, sandblasting, chemical etching, electroplating, or coating can be used to enhance the adhesion, corrosion resistance, or mechanical bonding between the insert and the plastic resin.

Insert preheating (if necessary)

Inserts can be preheated to reduce thermal shock, improve resin flow around the insert, and minimize the formation of internal stress or voids. Preheating also helps prevent molten plastic from freezing prematurely at the insert interface, which is especially useful for large metal inserts or high-temperature engineering plastics.

Insert​

Inserts are precisely positioned into specialized mold cavities or fixtures using manual mold loading, semi-automatic fixtures, or robotic automation. Structures on the mold, such as magnets, pins, grooves, or vacuum suction cups, ensure the correct orientation of the inserts and prevent movement during injection molding. Precise positioning is crucial for dimensional accuracy and functional integrity.

Closed mold

After the insert is in place, the mold closes and clamps it with a force sufficient to withstand the injection pressure. Proper alignment ensures the insert is securely in place, preventing flash, misalignment, or damage to mold components. Sensors can be used to confirm the presence of the insert before injection molding.

Plastic melts and is molded

Thermoplastic resin is fed into the injection molding machine barrel, where it is heated and plasticized by the combined action of the heating belt and the shear force of the screw. The melt temperature, viscosity, and uniformity are strictly controlled to ensure good flow and adhesion around the inserts, while preventing material degradation.

Insert Coverage

Molten plastic is injected into a closed mold cavity under controlled pressure and speed. The resin flows and coats the inserts, filling all mold cavities and interlocking structures. Injection parameters are optimized to prevent insert displacement, air bubble retention, short shots, or weak weld lines around the inserts.

Pressure holding and accumulating

After the mold cavity is filled, holding pressure is applied to compensate for material shrinkage during plastic cooling. This step ensures complete insert coverage, dimensional stability, and sufficient bond strength. Proper filling minimizes sink marks, voids, and internal stresses near the insert interface.

Waiting for solidification and cooling

The molded part is cooled within the mold through a controlled cooling channel. The cooling time is precisely calculated to ensure that the plastic around the insert is fully solidified, while avoiding residual stress, warping, or cracking due to differences in thermal shrinkage between the insert and the plastic.

Open the mold and eject the product

After cooling, the mold is opened, and the molded insert is ejected using ejector pins, ejector plates, or a robotic arm. Ejection force and time are controlled to prevent damage to the bond between the plastic and the insert, or deformation of the thin-walled structure.

Post-processing and inspection of products

Secondary processing may include gate trimming, deburring, machining, heat treatment, or surface finishing. Quality inspection includes visual inspection, coordinate measuring machine (CMM), pull-out testing, or electrical testing (if applicable) to verify dimensional accuracy, insert placement, bond strength, appearance quality, and functional performance.

Product Packaging and Delivery

Qualified parts will be cleaned and properly packaged to prevent damage or contamination, and traceable labels will be affixed. Proper packaging ensures that injection-molded components maintain dimensional integrity and functional performance during storage and transportation.

Advantages of insert molding

Insert injection molding simplifies production, saving time and labor costs by eliminating separate assembly steps. By integrating multiple components into a single molded part, no additional assembly is required, reducing production costs. It also reduces the risk of errors or misalignment. Furthermore, it enhances the structural integrity of the final product; the adhesion between the insert and the plastic material strengthens its strength and stability, making it suitable for applications requiring high mechanical strength.

Furthermore, insert molding allows for greater flexibility in product design, enabling specialized manufacturers to incorporate inserts of various materials, shapes, and sizes into the injection molding process to produce complex, custom-made parts. This versatility opens up possibilities for creating products with even more functions.

Plastic substrate for insert molding and insert material

In practical applications, the materials used in insert molding mainly fall into two categories: insert materials and plastic matrices. The appropriate combination of these materials determines the final performance of the product. The table below summarizes commonly used insert and plastic materials and their properties for a more intuitive understanding of material selection:

Material Category	Material	Material Description
Insert Materials – Metals	Carbon Steel	Provides high strength and rigidity at low cost; commonly used for threaded inserts, shafts, and load-bearing features. Requires corrosion protection in humid or aggressive environments.
	Stainless Steel	Offers excellent corrosion resistance and good mechanical strength; widely used in medical, food-contact, and outdoor applications where durability and hygiene are critical.
	Aluminum Alloys	Lightweight with good thermal conductivity and machinability; suitable for inserts requiring weight reduction and moderate structural strength.
	Brass	Excellent machinability and corrosion resistance with good electrical conductivity; commonly used for threaded inserts and electrical terminals.
	Copper	High electrical and thermal conductivity; used for electrical contacts and heat dissipation components, though relatively soft and prone to deformation.
	Zinc Alloys	Good dimensional stability and corrosion resistance; often used for complex insert geometries produced via die casting.
Insert Materials – Non-Metals	Ceramic	High temperature resistance, electrical insulation, and wear resistance; used in specialized electrical or thermal barrier applications.
	Glass	Electrically insulating and chemically inert; typically used for hermetic seals or sensor-related insert molding applications.
	Rigid Thermoplastics	Used when combining different plastic functions in a single part; offers chemical compatibility and controlled bonding with the overmolded resin.
Overmolding Materials – Commodity Plastics	Polypropylene (PP)	Lightweight, chemically resistant, and cost-effective; suitable for consumer products and housings with moderate mechanical requirements.
	Polyethylene (PE)	Flexible and impact resistant with excellent moisture resistance; commonly used for low-load and protective components.
	Polystyrene (PS)	Rigid and easy to process with good dimensional stability; used for non-structural components and electronic housings.
Overmolding Materials – Engineering Plastics	ABS	Good impact resistance and surface finish; widely used for structural housings and assemblies requiring aesthetic quality.
	Polyamide (Nylon, PA6/PA66)	High mechanical strength, wear resistance, and thermal stability; commonly used for insert molding with metal inserts requiring load-bearing capability.
	Polycarbonate (PC)	High impact strength and transparency; used for protective covers and structural components requiring toughness.
	POM (Acetal)	Low friction, high stiffness, and excellent dimensional stability; suitable for precision components with moving interfaces.
Overmolding Materials – High-Performance Plastics	PPS	Excellent chemical resistance and high-temperature performance; used in automotive, electrical, and industrial applications.
	PEEK	Outstanding mechanical strength, chemical resistance, and continuous high-temperature capability; applied in aerospace, medical, and high-end industrial parts.
	LCP	Extremely low shrinkage and excellent flow characteristics; ideal for thin-wall electronic and connector applications with inserts.
Overmolding Materials – Elastomers	TPE / TPU	Provides flexibility, impact absorption, and sealing properties; often used to create soft-touch or vibration-damping features around rigid inserts.
	Silicone Rubber (LSR)	Excellent thermal stability, flexibility, and biocompatibility; commonly used in medical, electrical insulation, and sealing insert-molded components.

Issues to consider when designing insert injection molded products

When designing insert injection molding, the process characteristics of insert injection molding and the product design must be fully considered. DFM analysis must be performed before production begins. The following is a summary by the injection molding engineers team at Elimold regarding design issues to consider when designing plastic products that require insert injection molding.

• The materials for inserts should preferably be copper, aluminum, steel, rigid dissimilar plastics, ceramics, glass, and plastics. Among them, brass is a commonly used material for inserts because it does not rust, is corrosion-resistant, is easy to process, and is reasonably priced.
• The shape of the insert should be as circular or axially symmetrical as possible, and sharp corners or acute angles are not allowed, so as to facilitate uniform shrinkage and prevent local stress or even stress concentration.
• The insert itself needs to take into account its own DFM (Depth Factor Mechanism). Metal inserts are made by cutting or stamping, so the shape of the insert must have good machinability.

• To facilitate placement and positioning within the mold, the protruding portion of the insert (i.e., the part placed in the mold) should be designed as a cylinder, since it is easiest to machine a round hole in the mold.
• Inserts should have structures such as sealing bosses to prevent overflow during injection molding.
• To avoid wavy shrinkage marks caused by an excessively thin plastic base, which would affect both appearance and strength, the minimum distance between the bottom surface of the insert and the plastic wall surface should be taken.
• The distance between the insert and the sidewall of the product should not be too small to ensure that the mold has sufficient strength.
• When inserts are placed in the boss, in order to ensure the stability of the insert and the strength of the plastic matrix, the insert should extend to the bottom of the boss (minimum bottom thickness must be ensured), and the head of the insert should be rounded.
• Small cylindrical inserts can be embedded in a plastic substrate using a central groove or a surface knurled structure, with the knurling groove being 1-2 mm deep.
• Plate and sheet-like inserts can be fixed using the hole and window fixing method, but thin inserts (thickness less than 0.5mm) should be fixed using the cutting or bending method.
• Rod-shaped inserts can be fixed by flattening, punching, bending, or splitting the head, or by flattening the middle part of a round rod.
• Tubular stamped inserts can have bulges machined during stamping to enhance the fastening force.
• In connectors, the secure fixation of terminal inserts within the plastic is a top priority for every product design engineer. Drilling, bending, increasing roughness, adding bumps and recesses, and increasing the plastic wall thickness can all greatly enhance the secure fixation of terminals within the plastic.

Three embedding methods for insert injection molded nuts

There are three ways to embed a nut into a plastic part:

Thermofused Nuts	Hot melt embedding is the most common and typical embedding method, generally using a hot melt machine and a manual soldering iron to embed the nails;
Insert injection nut	Injection molding generally has very strict requirements for the hole diameter of the nut support, which must be controlled within 0.05mm. This is because the product is placed in the injection mold after being fixed by the Modifying Pin, and the nut hole diameter must be controlled by the size of the PIN pin of the injection molding machine.
Ultrasonic Nuts	Ultrasonic embedding is a process that uses ultrasonic vibration to cause friction between the nut and the surface and internal molecules of the workpiece, raising the temperature at the interface. When the temperature reaches the softening temperature of the workpiece, the nut is embedded in the rubber part. When the vibration stops, the workpiece is simultaneously cooled and shaped under a certain pressure.

Application of insert injection molding process

It is widely used in industries that require robust, durable, and functionally complex parts, where a single material may not provide all the necessary properties. Here are some common applications of insert molding:

Industry	Representative Insert-Molded Products	Notes on Application
Automotive	Sensor housings; dashboard controls; threaded fasteners; mounting brackets; interior trim with molded-in clips; under-hood connector assemblies	High vibration and thermal demands favor metal inserts for strength and durability; reduces assembly steps.
Aerospace & Defense	Avionics housings; connector blocks; structural clips and brackets; communication assemblies; instrument panels	Lightweight parts with embedded mounts and fasteners improve reliability and reduce fallout.
Consumer Electronics	Electrical connectors; control panels; smartphone/charger components; knobs and switches; wire encapsulations	Precise integration of conductors and mechanical features enhances performance and miniaturization.
Medical Devices	Surgical tool handles; diagnostic equipment housings; catheter components; prosthetic connectors; embedded electrodes or sensor elements	Requires biocompatibility, sterilizability, and reliability for regulated environments.
Industrial & OEM Equipment	Industrial sensors; bushings and bush guides; threaded inserts; tool handles; pneumatic/electromechanical components	Insert molding improves durability and reduces secondary assembly steps.
Agriculture & Heavy Equipment	Durable equipment components; control grips with metal reinforcement; weather-resistant housings	Components benefit from enhanced mechanical strength and environmental resistance.
Hand Tools & Power Sports	Reinforced tool bodies; grips with embedded supports; fastener assemblies	Enhances ergonomic performance and structural integrity.
Consumer & Household Products	Appliance knobs; switches; threaded features in plastic housings; molded handles	Simplifies design and improves product longevity through integrated inserts.
Electrical/Power Systems	Power connectors; terminal blocks; insulated housings with metal contacts; high-reliability blocks	Combines electrical functionality with structural plastic enclosures.

Future Development Trends and Innovative Technology Applications of Insert Molding

Currently, insert molding processes increasingly utilize automation and robotics. This is because automation and robotics improve production efficiency, reduce labor costs, and enhance product quality. Robotic systems can be used for tasks such as insert placement, part removal, and quality inspection. Furthermore, advancements in materials science and injection molding technology have made it possible to use multiple materials simultaneously in a single molding process. This allows manufacturers to create parts with different material properties (such as stiffness, flexibility, or conductivity), further expanding design possibilities.

In response to growing environmental concerns, many specialized manufacturers have begun exploring the use of environmentally friendly materials, such as bioplastics or recycled plastics, in insert molding processes. Larger manufacturers are implementing energy-efficient equipment and production technologies to minimize their environmental impact. As insert molding continues to evolve and adapt to industry needs, it is expected to play an increasingly important role in the production of high-quality, innovative products across various industries.

Elimold is a professional manufacturer of custom insert molding parts.

Want to use insert molding services but unsure if the manufacturing process is correct? All you need to do is contact Elimold, as we have a professional team of engineers locally. Our talented engineering team will be happy to provide the best advice on the ideal manufacturing process for your parts. Our engineering team always adheres to specifications to produce parts that meet customer requirements, something proven by our many clients. What are you waiting for? Contact Elimold today for your custom insert molding parts.

in conclusion

Insert molding is a common and widely used industrial process, offering benefits such as reduced labor costs, improved precision, and simplified assembly processes. By integrating metal inserts, this process enhances the overall strength of molded parts, improves their mechanical properties, and provides greater design flexibility. Insert molding is widely used in industries such as automotive, electronics, and medical devices, serving as a versatile solution to meet diverse manufacturing needs.

FAQ