Insert Molding-Combine Metal and Plastic


What is Insert Molding?


Insert molding is a specific case of the wider concept of injection molding. In insert molding, components are inserted into the mold before injecting the molding material, in order to form part of the final product. These components usually consist of materials with properties different from the parent material. A classic case is where a brass fastener is molded into a softer parent material, in order to be used as a structural bearing surface.


Why Insert Molding?


Injection molding is a widely used manufacturing technique with many advantages. The molding material sometimes lack mechanical, chemical or electrical properties with a certain application. By combining multiple materials into one product, an efficient trade-off can often be achieved. This combines, for example,  the low cost of injection molding with the good mechanical properties of a brass nut. It is a process with many advantages over injection molding and assembly, which will be discussed in this article.


The Insert Molding Process


The process of insert molding differs from conventional injection molding in that certain parts are inserted into the mold prior to the molding material. It is very similar in this regard to the process of over molding. The mold is designed in such a way that makes provision for these inserts. The inserts have to be held in their respective positions during the material injection, so as to retain the location and orientation thereof. These components may be inserted into the mold by hand, or with an automated process. A simplification of the process is shown below:


Step 1: The component is inserted onto a retainer inside the mold

Step 2: The molding material is injected into the mold, flowing around the insert

Step 3: The product is removed from the mold, encapsulating the insert


Insertion by Hand


For low production volumes, the components may be inserted by hand. Operator insertion of components is more common than may be expected. As the final product is more complex than a conventional injection molded item, the value is also higher and may therefore warrant a process in which the operator is directly involved. There are, however, disadvantages to this.


Operator insertion of components can never provide the same consistency, repeatability or precision of an automated process. Another factor which can influence this process is the high temperatures often involved in molding. High mold temperatures require operators to wear gloves, which in turn influences their dexterity. As a result, there is often a minimum component size that an operator can insert successfully.


Automated Insertion


Much like conventional injection molding, insert molding lends itself very well to an automated process. With the increased use and availability of industrial robots, components can now be inserted more consistently, precisely and more efficiently than ever before. Robots have many properties that differentiate them from human operators. They withstand much higher temperatures, perfectly repeats a task, can work with very high precision, and do not demand lunch breaks!


The initial capital outlay for an automated process is however much higher, and demands a higher unit count in order to warrant the outlay. Therefore, there exists some breakeven point for each unique situation which makes either a manual or automatic process more cost effective.


Advantages of Insert Molding


Manufacturing Cost


If conventional injection molding is used, a second manufacturing step is required to install inserts of different materials. In the insert molding process, however, the molding and secondary manufacturing steps are incorporated into one super step, saving costs on the additional labor, factory space and managerial capacity that is required.


Quality Assurance

Insert molding in a highly automated process greatly reduces the requirements of a good quality control plan. As there are the minimum manufacturing steps involved, there is much less room for systematic and human errors. By using industrial robots in a well designed molding process, any spatial errors can be eliminated due to the extremely high precision of the equipment.


Optimal Material Utilization


By combining two or more materials into one final product, it is possible to maximize the effect of the material properties. A plastic gear, for example, may only require the mechanical properties of steel at the centre of the assembly. Using insert molding thus enables the manufacturer to only use the expensive materials where absolutely necessary, while saving on cost and weight in other areas.


Structural Integrity


Insert molding provides a much better material marriage than conventional molding, machining and assembly. A threaded brass insert, for example, may resist torque and tensile force much better due to clever use of undercuts and shoulders. The insert can be molded into the parent material in such a way as to completely lock it in – something that is possible with conventional methods like heat staking inserts and ultrasonic inserts which are for samples and prototypes or very low volume production.




Plastics used in insert molding can provide a finish which is visually very appealing. Comparing an insert molded product with one that is machined wholly out of structural steel proves this. A well designed insert molded product is therefore able to provide structural material in hidden areas, combined with a high quality plastic finish in visible areas.


Examples of Insert Molding


Knobs and dials


Plastic user interface areas combined with brass attachment inserts to be used in consumer products such as kitchen appliances


Plastic gears


In certain gear applications, the forces on the teeth may be low enough to use low cost plastics, combined with a steel shaft interface. To reduce weight as well.


Bearing surfaces


Injection molded plastic cover plates and base plates may provide a nice visual appeal, but do not withstand forces well. Combining this with threaded metal inserts can provide the best of both worlds.


Integrated circuits


Traditional ICs can be effectively manufacturing using insert molding. Multiple connector pins can be embedded in the plastic part of the unit in this fashion. This is a good example of using an insert material for its     electrical properties.


Charger cables


Charging and data cables used in the mobile phone industry can be manufactured using insert molding; interfaces such as Micro USB make extensive use of metal/plastic combinations.


Poker chips


An interesting use of insert molding is the manufacturing of coins that consist of a metal disc embedded into a plastic annulus, much like gambling chips.


Medical instruments


Surgical instruments such as bone saws are manufactured with many metal/plastic interfaces, therefore making it an ideal candidate for insert- and over molding.


Custom products


With clever design and prototyping, the opportunities for using insert molding are truly endless.


What RYD Tooling can offer


Here at RYD Tooling, we have over 15 injection molding machines. Combining that with the skills and expertise learned through 12 years in the field, we can offer the following:


  • Feasibility studies to evaluate the optimal manufacturing strategy
  • Rapid prototyping expertise and experience
  • Prototyping samples using techniques such as 3D printing and CNC machining
  • DFM and flow analysis simulations
  • Mold layout designs for your consideration
  • Injection mold design and manufacturing
  • Full-scale manufacturing
  • Parts assembly
  • Contract management, liaison and coordination with metal insert manufacturers
  • Guidance through the process – from prototyping to final product shipping



Insert Molding – Design Guidelines


Insert molding is a process that provides the user with many advantages, such as faster manufacturing, lower unit costs and higher product quality. If you decide to use this wonderful technology, there are a number of design guidelines to take into account.


Insert Mold Design


  • When designing the mold, it is important to take the nature of the process in consideration. A mold for insert molding differs from that of a conventional injection


  • The mold design must accommodate and ensure the location and orientation of the inserts.


  • The ejector mechanism should take the inserts into consideration. It must not interfere with the inserts when ejecting the final product from the mold.


  • The mold must keep the inserts stable during the plastic injection This is a very important consideration, as even the slightest positional interference from the plastic flow can cause infringement on the design tolerances.


  • In certain cases, a fixture or jig can greatly increase the production rate. When a design contains many complicated inserts, it can make sense to assemble the inserts on a fixture before introducing the set into the mold.


Product Design


The product design must be aligned with the inherent constraints and characteristics of the insert molding process. This design process is analogous to conventional injection molding design, and many of the same principles apply:


  • Sections (especially load bearing bosses) should be sufficiently thick to withstand loading forces.


  • In order to eliminate undesirable effect arising from material cooling, the final product should not exceed the maximum thickness according to simulations in the design process.


  • Inserts should be selected carefully in order to provide sufficient mechanical strength. Correct insert specification would prevent “torque out” and “pull out” (this is where the plastic/insert interface fails mechanically due to excessive torque or tensile force, respectively).


  • The correct engineering plastic must be selected which has high-adhesion can wrap and stick the insert properly and tightly.


Threaded Inserts for Plastic Molding


A very common use of insert molding is to install threaded fasteners into the parent material. Typical applications include computers and terminal boards, automotive products, electronic equipment, business machines, aerospace products, communications equipment and instrument cases.


Various manufacturers now specialize in these threaded inserts. The technology has grown in leaps and bounds and now provides purpose built inserts that provide excellent results.


These fasteners are usually manufactured from brass, but may also consist of stainless steel or aluminium. The fasteners may be molded in (insert molding), inserted using ultrasonic methods, or simply heat staking pressed in. Molding the insert in provides superior mechanical properties, higher precision and a higher production rate.


It is imperative that the fastener and its installation is able to withstand the loading requirements of the design. This is achieved by using undercuts and shoulders to lock the fastener into the parent material. In modern designs, a helical groove design is used to provide both tensile and torsional resistance. This protects the product from “torque out” and “pull out” failures.


RYD Tooling provides the user with a wide variety of cost-effective solutions, in partnership with specialist fastener manufacturers.


A Typical Case of Insert Molding with RYD Tooling


In 2016, RYD assisted a client in Australia to manufacture a product to be used in a Virtual Reality war simulator game. The product consisted of a controller enclosure with 23 instances of threaded brass fastener insertion. The fasteners consisted of ten M3 inserts and thirteen M4 inserts. These tiny fasteners required very high manufacturing precision and dexterity. The case for this design in available to view at the RYD Tooling showroom – a monster mold weighing in at 2150kg, matched with a 1000t injection machine.


Through superior customer service, communications and a cooperative approach, the customer was successfully guided through a tricky development process. RYD Tooling provided feasibility studies, DFM and flow analysis simulations, design suggestions, fastener specification and full-scale production.


We at RYD Tooling understand that the client does not want the technical details to interfere with the creative process. Insert molding is one of our many areas of expertise, and we will get our hands dirty to ensure that your design gets realized.



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Start Your Injection Mold Project with a Free Quote

Choose File
File Name

*Please upload your design or description file (Like:STP,DWG,PRT). Size limit is 20MB.