Better Products at a Lower Cost
Using a variety of methods, insert molding can improve the performance of individual parts and increase reliability.
Advent Tool & Mold, Inc.
In fast-moving global markets, many factors contribute to a product’s ability to compete. Cost is important, but so are performance, reliability and ease of use. Insert molding can impact all of these.
Insert molding is a technique in which a plastic part is injection molded around one or more pre-placed inserts made of metal, plastic, glass, ceramic or other material. Once the molded part cools, the inserts are held firmly and precisely in place and are, essentially, part of the finished piece. This process reduces or eliminates the need for later assembly and can more than pay for itself by offsetting assembly costs.
In addition, it can increase yield by reducing scrap due to error in assembly, significantly improve the consistency of parts, and reduce the tolerance stack-up of multiple inserts in a single part. And because each insert is integral to the finished part, there are no fasteners that can loosen or go out of alignment. As a result, the finished product is simpler, cleaner, more reliable and, in the long run, less costly.
Any Industry, Many Applications
Any industry that uses injection-molded parts can benefit from insert molding. Where cost is the driving factor, insert molding can reduce costs in large-volume production. If precision is key, insert molding ensures the exact placement and alignment of inserted components by compensating for insert variance and reducing tolerance stack-up requirements. And where durability is critical, insert molding virtually guarantees that as long as the molded part remains intact, the insert will stay in place.
Industries benefitting from insert molding include medical, aerospace, automotive, consumer products and computer manufacturing, with application types as varied as the industries. Metal bushings and plastic bearings are inserted into parts to provide lubricity for rotating shafts. Steel inserts are molded into parts to add stiffness, reduce the cost of metal, reduce weight, and encapsulate the steel to prevent corrosion. The addition of steel at critical points also can increase hardness and durability without the cost, weight or need to machine entire components from metal. The use of multiple inserts in various materials, sizes, and shapes in molded assemblies eliminates secondary operations, maximizes retention for improved reliability, and decreases part weight.
Insert molding also frees the designer to move into the third dimension. Standard assembly only allows components to beassembled at their surfaces or more accurately, limited by their surfaces. Insert molding allows components to be addedanywhere in or on the part.
Access to interior locations that in traditional molded components required the assembly of multiple molded parts is made available in a single molded part. This is a very powerful capability, allowing the use of complex components such as a three-dimensional circuit board in place of multiple, separate boards. The possibilities seem unlimited. By adding a variety of other materials to molded parts, designers can expand the capabilities offered by the thousands of moldable resins already at their disposal. The challenge is to make effective use of thesenew capabilities.
Advances in mold building and process technologies have continued to increase insert-molding capabilities. Retracting pins allow precise holding of inserts. Hot-runner innovations have added gate location options, and advanced “logical” electronic controllers have improved repeatability.
Mold-building materials have increased the tolerance latitude required for inserts. Materials such as Hasco’s A4200 sealing elements, also known as MurSeal, allow greater tolerance range of inserts without the risk of flash occurring. An added benefit of the A4200 is the ability to seal off on sensitive, painted or electroplated surfaces without damaging them.
Insert Molding Applications
The range of functions that insert molding can serve is vast and still growing. Some demonstrated uses in the medical technology field include:
• Multi-well plates
• Surgical instruments
• Diagnostic equipment
• Trocar components
• Cannula components
• Surgical jaws
For obvious reasons, insert molding is significantly more complex than standard injection molding. It involves multiple materials, the inserts specially must be designed for integration with the part, and attachment can be molecular (in the case of some plastic inserts), mechanical, or both.
In addition, there is a variety of ways in which parts can be handled in the molding process and in which molding itself can be done.
There are no “one size fits all” solutions, and making the right choices—in design and in process—helps determine the success of the project. Experienced suppliers should be able to help with options and decisions regarding insert molding. And as long as they can offer a full range of processes they will be in a position to help you choose the one that provides your company with the best results.
Insert molding can be done either in horizontal or vertical molding presses; the choice depends on a variety of factors. Horizontal presses are the same ones traditionally used for ordinary injection molding. Vertical presses, on the other hand, specifically are more suited to insert molding processes. A vertical press makes insert loading easier for the operator and gravity helps hold the part in place; in a horizontal press other provisions must be made to hold the insert in place until the mold closes. A vertical press can be configured with either a shuttle or rotary table to help speed the molding process. The press contains a single upper mold half, while the table supports one, two, or more lower mold halves. The shuttle table supports two bottom mold halves and moves back and forth between operators located on opposite sides of the press. The operators take turns unloading parts and reloading the lower mold half with inserts. A rotary table serves one or more operators and rotates one or more mold halves into position under the top mold half, allowing one bottom mold to be loaded with inserts while another, previously loaded with inserts, is being injected with resin and cooled in the press. Shuttle and rotary tables create a virtually continuous process and help maintain a consistent cycle time. Stable cycle time, in turn, helps maintain constant material viscosity during injection, which is critical to achievingpart consistency.
Depending on the nature of the parts and volume being produced, inserts can be loaded into molds either manually or automatically by robots. Each scenario is unique. It is important to weigh the higher capital cost for the robotics against the ongoing labor cost to load inserts manually. Manual labor cost can be minimized by the appropriate use of a rotary table and/or multi-cavity molds to eliminate wasted time and make the process as
continuous as possible.
Another approach uses a standard horizontal molding machine. Inserts are loaded into a loading bar or plate, generally with multiple cavities, and the bar is placed in the mold. The loading bar approach reduces the overall cycle by limiting the time that the mold is open for insert loading. It also allows the operator to be productively loading the inserts while the mold is closed for the molding process. In larger production runs, robots may be the most economical choice.
Choosing the best production model for a particular part depends on a number of factors including the part’s geometry, size, and complexity, the resin being used, the number of cavities in the mold, and the number of parts being produced. The more options a supplier can offer, the greater the likelihood of getting the best and most cost-effective solution.
Types of Inserts
The parts to be molded around also come in a wide array of styles and include:
• Threaded inserts
• Electrical connectors
• Complex machined components
• Mesh or filter media
• EMI/EMF/RFI barrier material
• Machined components
• Molded components
• Sintered vapor barriers
• Machined components
• Composite laminates
• Synthetic weaves
• Filter materials
• Circuit boards
• Mechanical devices
• Coil assemblies
Conceptualizing for Insert Molding
Obviously, insert molding isn’t right for every application, but its capabilities are expanding all the time. Today, more than 10percent of molded parts use the technique, and that figure will continue to grow.
To determine whether it is applicable to your project, consider the following:
1.Consider the overall functional, ergonomic, and cosmetic goals of your project.
2.Determine the costs of using conventional materials and methods for achieving those goals.
3.Evaluate parts such as fasteners that could be eliminated if the components they connect were insert molded, eliminating the additional operation.
4.Contemplate ways in which material characteristics potentially complement one another, e.g., metal needed for rigidity could be protected from corrosion by being encased in plastic, eliminating the need for protective coating as a separate process.
5.Determine if there are ways in which separate parts of a device can be combined into a single injection molded part with inserts to simplify the assembly.
6.Often, material substitutions can be made. For example, replacing solid metal with a smaller metal insert in plastic.
7.Review whether or not there are ways to combine desirable characteristics of two or more resins in a single part by insert molding plastic within plastic.
Making the Transition
The demands on an insert-molded part are different from those that are attached by other methods and often require that the parts be redesigned accordingly.
The following eight tips will provide a good start to planning an insert-molding project:
1. The location of inserts may be different from that of conventionally added parts; they cannot be surface mounted with connectors, but must be surrounded by injected resin. On the other hand, inserted parts can be completely enclosed in resin, which conventional parts cannot. Both of these factors can impact part design.
2. Components typically are attached differently than in conventional assemblies.In conventional assemblies inserts are assembled after molding, and the molded parts are designed with through holes or other means of secondary assembly. Assembly processes include heat staking, gluing, mechanical fastening and so on. Insert molding on the other hand, allows sub-components to be anchored in the molded assembly, and the process allows molten resin to fill undercuts, through holes, or knurling to permanently lock their positions. The group of components must be designed accordingly.
3. While most inserts won’t have size variations that compromise the molding process, some may. For those cases, there are solutions such as overflows or the use of cavity pressure transducers to control the process and adjust for variability in the inserted sub-components.
4. Inserts must stand up to the rigors of the molding process including significant heat and pressure (although insert molding is typically done at lower pressures than conventional injection molding). These requirements can determine the design of the inserts and even the material from which they are made. Thisparticularly is true of plastic inserts or first shots, which must have a similar or higher melting point than that of the injected resin. The same may be true of other materials such as low-temperature solders.
5. Software packages such as Moldflow can be used to help locate gates, simulate resin flow from insert location, and recognize where voids and knit lines will form. Because of pressures developed in the mold cavity during processing, compromises may need to be made in locations for best results.
6. Inserts must be held in place during the molding process. This typically is achieved with support pins. The pins can be fixed or retractable. Fixed pins leave holes in the finished molded part; these may be left in place or covered in a secondary operation such as over-molding. Retractable pins are pushed out of the way as the mold is filled and leave no holes.
7. Once they are in place, inserts may not be accessible in the ways that “attached” parts normally are. This can affect the use of insert molding for parts that may require adjustment, maintenance, or replacement.
8. Finally, molded inserts can offer benefits that are not possible with conventional methods. While considering how to make the change from attached to integrated components, don’t just focus on equivalent function. Look for additional benefits that can be achieved in the process, including reduced weight, smaller size, greater durability, improved function, and
desirable characteristics that can be added to those of theprimary resin.
From Paper (or Prototype) to Production Parts
Consider suppliers as a partner in turning your design into reality. Depth of knowledge and experience, along with range of capabilities, will help ensure that the potential of your design is fully realized. The following are factors that will help with your development process, maximize results, control costs, keep processes on schedule, and eliminate surprises.
Availability of experienced staff and willingness to work collaboratively during design development helps you make the most of insert molding’s capabilities and technology. In addition to maximizing product performance, a good development team can help you design for manufacturability to maintain yield and stay within budget. Willingness to engage in concurrent engineering keeps your project moving forward, and support for leading computer-aided design formats such as ProE and Solidworks helps ensure that what you want is what you’ll get.
In-house engineering and tooling can ensure that tolerances will be maintained and keep manufacturing on schedule. In high-volume production, mold maintenance and replacement of wear points is critical, and in-house mold manufacturing and mold maintenance help keep manufacturing consistent and avoid production delays.
A range of equipment options and capabilities lets the supplier employ the right technology for your job. Complex, high-volume production runs can benefit from fully automated state-of-the-art equipment, but that same equipment can drive up the costs of producing simpler designs. Most often, you’ll want to find the manufacturing method that weighs the capital costs against the life of program and generates the lowest unit cost over that life. The supplier with more options will be best able to support your needs and tailor a solution accordingly. You also may benefit from having secondary processes (assembly and the like) handled by the same supplier. This helps achieve economies of scale and eliminates costs of transport, communication problems, “finger pointing,” and scheduling issues.
Quality is critical, and there is no right price for poor product. Insist on quality practices such as Lean/Six Sigma and zero-defect. Look for certifications such as ISO 9001, TS16949 and ISO 13485, as well as for the use of quality management tools such as risk analysis.
Look for a range of prototyping capabilities to ensure that what you design will perform to expectations in the real world and allow you to revise your design, if necessary, before committing to full-scale production. And, because production of high-volume tools can take time, look for the ability to produce “bridge” molds that allow you to begin interim production and take your product to market while final tooling is being manufactured.
With thousands of materials available, and added design freedom through insert molding, the sky is the limit. Be sure that your supplier can support a wide range of resins, processes and insert types. The widest range of capabilities ultimately will ensure that you are guided to the best solution for your unique project.
The Bottom Line
Insert molding vastly has increased the capabilities of traditional injection molding. Using a variety of methods it can improve the performance of individual parts, increase reliability, and reduce costs. Realizing its full potential, however, requires a thorough understanding of the available processes, thoughtful design, and careful implementation. An experienced supplier can help designers achieve their goals through insert molding and choose the most cost-effective production methods.
Ken Desrosiers, president of Advent Tool & Mold Inc. in Rochester, N.Y., has been in the mold building and molding industry for 37 years and has spent the last 30 of those years building Advent. The company is an injection molder in the medical, aerospace, automotive and consumer markets. Advent continually invests in educational programs andtraining, most recently completing a Lean/Six Sigma black belt program. The company currently has 14 green belts (Desrosiers is one) and nine black belts.