Advantages of Continuous Strip Molding
Small electronic components and other parts that combine tiny metal and plastic components have typically been manufactured and assembled on a loose-parts basis. Even with high-speed equipment, handling every part individually was relatively inefficient. Now, more and more, that inefficiency is giving way to fully automated, hands-off processes.
One of those processes is reel-to-reel molding. Although the completed parts give the appearance of being plastic parts with metal inserts, the plastic is actually molded over the part.
The process uses a feed reel and a take-up reel. In a typical version of the process, a continuous metal "carrier" strip is precisely indexed off the feed reel, passes through an injection mold and onto the take-up reel. This metal strip, which is about two inches wide, carries a continuous line of parts on its outer edges. If the part design calls for it, the strip will pass through a blanking die to remove excess metal from the parts before they enter the molding press.
When the continuous metal strip is indexed into the multi-cavity molding press, plastic components are precisely molded over two, four, eight or more metal parts on the strip. The continuous strip is automatically rewound on the take-up reel when a monitor senses sufficient slack between the molding press and the take-up reel.
The molded parts are delivered to customers still on the take-up reel, and placed directly in automated production equipment for hands-off operations that add more components, install covers, or form electrical leads, etc. This technology is ideal for electronic components such as ICs, discrete devices, shunts, DIP switches, and connectors, where the encapsulated metal creates electric current-carrying circuitry.
Insert molding itself is not new. Reel-to-reel processes simply take well-established technology to higher levels of convenience and cost reduction. One of the major gains is in molding precision. Others include:
- Automated handling of components for OEMs.
- Inexpensive in-line stamping or fabricating.
- Lower production costs than manual molding.
- Higher volume throughput of molded insert parts.
- Less process error (high repeatability and quality).
- Consistent part orientation for ease and speed of assembly.
- Less mold maintenance (no operator-related mold damage).
Another advantage is that the process is versatile. Many types of assembly operations for small components are adaptable to continuous strip molding. All it takes is an open mind and an inventive attitude. For example, small parts can be gang-molded in strips. Components could be molded, then formed, stamped or singulated after molding, or simply cut into strips for customer handling.
The carrier strip does not have to be a metal stamping; it could be a Mylar carrier, wires of various types, or other suitable material that can be over-molded with plastic. It is also possible to mold parts and connect them with an integral molded strip of the same plastic.
Even the applications are varied. Beyond electronics, there are medical components that combine metal and plastic elements. In the field of toys, games and dolls, reel-to-reel molding can mass-produce plastic parts with inserts of different plastic. Ranging even farther afield, artificial leaves can be molded over wire stiffeners, detonator plugs over wire conductors, plastic cogs onto stainless steel timing belts, and so on.
For a manufacturer interested in developing an in-house capability for reel-to-reel production, there are sizable obstacles. Number one is the investment involved, which requires high volume production to justify.
Also, the dimensions and tolerances involved are formidable. During molding, each insert must be tightly confined by the mold to avoid flash, yet the mold halves must not close on any part of the metal strip without risking mold damage and downtime. The situation becomes even more demanding as electronic components get smaller and smaller. Center-to-center distances of terminals have gone from 0.100 to 0.050 inch and we continue to develop technology to bring them even closer together.
The plastic resins often present additional challenges. Reel-to-reel systems are used to mold demanding high-performance thermoplastics such as Liquid Crystal Polymer (LCP) and Polyphthalamide (PPA), which has a melt temperature range of 610 to 660F and a mold temperature requirement above 275F.
Quality control is paramount. How do you ensure the quality of incoming stamped strip without spending countless hours on inspection? Or, after molding, how do you check the quality throughout a reel of finished inserts without physically cutting samples out of the strip and splicing it back together? Challenges like these have led to vendor specialization with highly sophisticated inspection equipment so that most custom molders do not even attempt reel-to-reel insert molding.
There are also large tooling and equipment investments. The stamping die and multi-cavity plastic mold for a new project might run up to $200,000, depending on the application, the intricacy of the part, and complexity of the mold. This restricts reel-to-reel molding to high-volume work. Start-up costs can be reduced by using lower-tech approaches, such as semi-automatic shuttle presses.
In addition, after reeled parts are produced, the manufacturer must have equipment that can continue to process them in the reeled form. But the benefits can be worth it. There may be some pain from the initial investment, but there will be a gain later through labor savings.
As electronic components and their tolerances have shrunk, a small number of specialty molding houses have emerged that offer reel-to-reel services to job shops and OEMs. There are about 10 in the United States that engage in reel-to-reel molding. Very few are at the same level of process automation as Spectrum Plastics. This firm, headquartered in Ansonia, Connecticut with a branch facility in Tucson, Arizona, is considered to be one of the two top reel-to-reel injection molders in the country.
Spectrum has learned how to deal with the challenges of continuous strip molding by using automatic feed and indexing equipment, specially modified molding presses, 'smart' robots, and vision inspection systems. Mastering these technologies has enabled the company to add a complete new reel-to-reel line every six months for the last three years. And they expect to continue that rate of growth into the future. A look at Spectrum shows the level of capabilities required to be successful in the process.
Spectrum's strip feed system is servo-driven and fully programmable. The indexing distance can be stepped in increments of 0.0002 inch for extremely accurate positioning. The company also uses traversing robots that pick runners out of operating three-plate molds, saving cycle time and raising output rates.
Spectrum has learned a great deal about metals and metalworking while developing its reel-to-reel insert molding capabilities. Its engineers can provide expert advice about carrier strip design, insert placement on the strip, pilot hole requirements, tolerances, etc. The company also performs stamping and piercing operations, on-line or off-line.
To handle moisture-sensitive engineering resins such as LCP, PPA, nylons, and polyesters, Spectrum has a closed-loop central drying system. This system can deliver -40F dew point air heated to temperatures up to 300F. It is linked to the molding machines by a just-in-time dry air conveying system. Rather than a bulky hopper, each press has a hopper that holds about a pound of material. The system can be programmed to send small batches of material from any dryer hopper to any machine at specified intervals. Dry air conveying ensures that the material stays ultra-dry throughout the system.
Spectrum's reel-to-reel production lines are equipped with vision systems to safeguard the integrity of the molding operation. Each system has one or two cameras dedicated to 100 percent inspection at speeds up to 12,000 parts per minute. They can check the part surface, shape, position, dimensions, and the presence or absence of a critical feature.
In one premolding setup, a vision system inspects the incoming strip just before it enters the mold. It automatically checks the delicate stamped leads of each component for correct position and width tolerances. More than making spot checks here and there, or the first foot off the roll, this system checks every insert and contact before molding. If anything is out of place, the system will either shut down or index the bad section through the press without molding plastic around it. A visual record of each bad section is captured and stored in a buffer area, available for immediate or later study.
Beyond protecting against mold damage and downtime, vision systems can look for defects such as bent leads outside the molding zone that might short out and cause major problems later. In this way, the systems provide additional quality control dividends for customers.
Let's say you have a metalworking job shop with plenty of progressive die stamping experience. Yet, your normal blueprint tolerances may be inadequate if insert molding follows the stamping. Tighter tolerances may be necessary to meet the dimensional specifications of the molded inserts. In one obvious example, it's impossible to hold 0.0002 inch tolerances on molded parts if the incoming strip has 0.0002 inch tolerances on the pilot holes. Spectrum's answer is to work closely with the stamping vendor, which may also be its molded parts customer, and write specs that are sufficiently tight up-front. For customers who are not stampers, Spectrum can outsource the stamping to vendors that are familiar with the specialized tolerance requirements.
Spectrum's Optical Measurement Inspection System (OMIS) is accurate, reliable, and fast. It combines the features of an optical comparator, toolmaker's microscope, and coordinate measuring machine in one video-based system. For post-production inspection, OMIS can verify that finished reels of parts are in compliance with customer specifications.
OMIS can also be used to check incoming reels of stamped strip to ensure they meet insert molding requirements. Once the system is programmed with the critical physical parameters for a strip, it will automatically inspect a sample from an entire reel--covering in 10 minutes the number of features it would take an inspector two days to review.
Spectrum has many satisfied reel-to-reel customers. For one, the firm produces dip switches with 2,4,6, 8, 10 and 12 leads--all from the same strip stock--simply by blanking out the unneeded leads before insert molding. This costs virtually nothing because it is done in-line with the molding.
For another customer, Spectrum molds a medical cover, which originally had two separate stainless steel components, and a loose-piece molded part. Assembly involved eight employees using adhesives in a slow, difficult operation until Spectrum recommended a redesign that allowed the substitution of unattended insert molding. After molding, parts are removed from the reel automatically and shipped in bulk. By eliminating the labor input of eight people, this customer got a substantial payback that more than justified the tooling cost. A customer engineer commented, "We earned back the entire sum the very first year and saved thousands of dollars besides."
One customer stopped its in-house strip molding and moved five machines to Spectrum. In time, their original one- and two-cavity molds and machines were replaced with eight-cavity molds fitted to Spectrum equipment, and the molding was made totally automatic. What once took five machines to produce annual product requirements, now needs only 1.5 machines. The company found that its people and time could be better applied elsewhere in their organization by subcontracting with a specialist.
Each new insert molding project requires proper setup of the project as well as the machines. Everyone involved--customer, molder, plater, stamper, etc.--should participate in a design review meeting before the first tool is made. This is the time to share information about the project specifications, establish tolerances, and draw up complete prints that reflect the requirements of each party. This advance coordination will help assure a successful marriage of insert molding and automation.
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