|Figure 1. Proposed mold modifications to uniformly encapsulate a metal ring in one shot. Pneumatics or hydraulics can be used for pin retraction.|
Earlier this fall Paul White, product development engineer for research and development at Finish Thompson Inc. (Erie, PA), posted this challenge on the Internet at the Plastics Network and in this magazine (October 1996 IMM, p. 117): "I need to totally encapsulate a steel ring with plastic in one shot so that there will be no seams where acid can get to the metal ring. The ring must also be centered in the molded part no matter which way you look at the part."
Finish Thompson manufactures, among other things, industrial pumps, the kind that pumps highly corrosive chemicals or acids used often in cleaning, plating, semiconductor, and photo processing applications. The company has two of its own presses, one 225 tons, the other 650 tons. The 4-inch metal ring mentioned by White is a pole piece for a set of rare-earth magnets. The magnets are made of Neodymium, a relatively rare and rarely used 34-karat inert metal you can find at the bottom of the periodic table. This magnet is molded into an impeller hub, an important part in the company's line of seal-less pumps.
The impeller hub comes in direct contact with the chemicals that pass through the pump, and protection from corrosion is paramount to the pump's long life. To protect the ring Finish Thompson currently encapsulates it in 40 percent glass-filled polypropylene, or 15 percent fiber-filled polyvinylidene fluoride (PVDF) in a two-shot process. The first shot encapsulates about three-quarters of the ring; the second shot finishes the last quarter.
The problem with the two-shot process, White says, is that the ring is not always completely sealed. The effect is domino-like: Chemicals violate the seal, the chemicals corrode the ring, the plastic encapsulating the ring swells, the plastic rubs against the barrier surrounding the impeller hub, the barrier is destroyed, the pump leaks, the pump fails. The failure rate is less than 2 percent, but any failure is unacceptable, White says.
White wants to encapsulate the ring in one shot to ensure that the ring remains sealed. Responses to White's query all followed the same vein, with some slight variations in technique. The mold design White wants to use is pictured (Figure 1) and illustrates the concept.
Four spring-loaded pins, located every 90Â°, hold the magnet ring in the mold. The spring pressure is countered by a pneumatic cylinder to maintain pin position. During injection, as the mold fills and molten plastic starts to enclose the ring, cavity pressure will trigger a relief valve in the pneumatic cylinders, retracting the pins. White expects that the melt already in the mold will be stiff enough to keep the ring secure, but still fluid enough to fill in the holes left behind by the retracted pins.
|Figure 2. Encapsulation of a cellular
antennae coil involves a two-shot process.
There are several ways to trigger pin retraction: via a timing mechanism, cavity pressure, machine pressure, or screw position. "I'd rather not have it be a function of time," White says, indicating his preference for cavity pressure retraction. "I'd rather have the plastic itself do the job."
There are also several ways to actuate the pins: hydraulically, pneumatically, with spring action, or a combination, as White chose. White says he thinks the tooling modifications will cost his company as much as $5000. "But," he says "we're saving money by not losing pumps. So it should more than pay for itself."
Although the two-shot process does not work all the time for White, that method still carries merit for many applications. Alex Kondor is senior project engineer at Advance Dial, based in Elmhurst, IL. He was one of the respondents to White's challenge and says the two-shot process works quite well for one of his products, a cellular telephone antenna.
Advance Dial is a custom molder operating four different facilities. The Elmhurst plant in which Kondor works has 33 machines ranging from 20 to 170 tons molding parts for consumer electronics and auto underhood and interior applications. Kondor says about a third of his facility's presses are rotary insert machines.
The heart of the cellular antenna is a gold coil that is fully encapsulated in a thermoplastic or a thermoplastic elastomer. A coil such as this one is not easily positioned in a mold with pins. Kondor says the best option is a two-shot process. In the first shot the "core" is filled and the coil itself is partially encapsulated (Figure 2). The idea is to attach to the coil a series of wart-like positioning posts. These are used in the second shot to position the coil evenly in the mold where it is completely and uniformly encapsulated.
Kondor says both shots are done on one two-cavity mold. In the first run the second cavity is blocked and a series of gold coils is given the first shot. Then the first cavity is blocked and the same coils are given the second shot. Kondor has this advice for molders who are contemplating molding in two shots: "I think the mistake most people make with the two-shot approach is they try to encapsulate as much as possible with the first shot, then finish it off with the second. Instead, use the first shot to mold position posts for the second shot, which does most of the encapsulating."
One-shot encapsulation, however, is also performed at Advance Dial, Kondor says. One of the parts is a printed circuit board for automotive use. The edges of the board must be completely encapsulated to protect it in the relatively harsh underhood environment. Unlike White's plan at Finish Thompson, for this application Advance Dial performs pin retraction based on timing, and actuates the retraction hydraulically. Kondor says the rule of thumb at Advance Dial is to start pin retraction when the cavity is about half full. He says the tricky part is to not let melt seep into gaps around the pins; this can be alleviated by careful pin placement.
Advice on Encapsulation
IMM ran all of this by a man whose career is encapsulation. He is Tom Boyer, a representative for development technical service at DuPont Engineering Polymers in Wilmington, DE. Boyer's job is encapsulation R&D. He's cut his teeth encapsulating everything from golf balls to antilock brake system wheel speed sensors. His thoughts and advice on the subject follow.
One-shot encapsulation. Almost every encapsulation project can be best molded in one shot. Boyer strongly recommends pins to position and hold the part in the mold. Even coiled parts like the cellular antenna at Advance Dial, he says, may be able to be held with pins by treating the coil with a bonding agent that hardens the material, making it stiff enough to withstand the force exerted by the pins.
Trigger pin retraction. Boyer says he mostly uses cavity pressure or screw position to trigger pin retraction. On a well-tuned machine, cavity pressure is the most reliable. Other machine components can vary in performance, but cavity pressure is the truest indicator of the status in the mold, where the part sits. Boyer also uses screw position. When the screw reaches the cushion, he assumes the part is almost fully packed and pulls the pins; typically the melt is fluid enough to fill the pin holes, but stiff enough to maintain part position and uniformity. In any case, Boyer likes to see the pins pulled as late as possible in the process. Of Advance Dial's strategy of pulling halfway through injection, he says he would be concerned that the part could shift in the mold, causing nonuniform melt distribution. But he says each application should be treated separately. Later pin retraction may minimize nonuniform melt distribution.
Retract with hydraulics. Boyer says pneumatics work but do not provide the precision and predictability inherent in hydraulic systems. He estimates that on a typical mold, adding the pins should cost about $5000 to $10,000, as White indicated. Boyer says the primary parts required are four hydraulic cylinders (pancake cylinders take up the least space), two on top of the mold, the other two on the bottom; these are attached to two plates with the pins attached. Finally, if you are using cavity pressure to trigger pin retraction, install a cavity pressure transducer.
Pin size. If the pins are large and the part is relatively small, the holes left by the pins after retraction may alter flow fronts, creating voids, leak paths, or structural weaknesses. "The smaller the pin, the more likely you are to pack that part out," he says.
Material. Nylon 6/12 is a favorite of Boyer's because its flow properties make it ideal for encapsulation; it's also strong and relatively resistant to corrosive chemicals. No matter what you choose, if you are encapsulating a metal part, Boyer says to be aware that the melt can affect the inductance of the part. High melt temperature and the injection pressure are the two biggest culprits of inductance change.
Get a vertical press. Boyer says anyone who is serious about encapsulation should invest in a vertical injection molding machine. He points out that when encapsulating on a horizontal machine, gravity is your worst enemy. It can lead to nonuniform distribution of plastic around the part. Also, a horizontal machine generally restricts you to a one-cavity tool as the operator cannot position more than one part in the mold at a time.
Boyer recommends a shuttle or rotary vertical press. This provides four main advantages, each of which saves time and money:
- You are restricted to one top half of the mold, but the shuttle or rotary system allows multiple bottom halves of the mold. While parts are being injected and encapsulated on the first mold the operator can remove parts from the second and prep it for injection. Says Boyer, "This is the optimal use of time and material."
- Tooling costs are reduced. Again, you have one top half of the mold, but multiple bottom halves. Every extra bottom half is money saved by not having to machine a top half for it.
- Because the molds lie flat, multiple cavities are easier to handle. The operator lays parts for encapsulation in the mold, instead of trying to position the part vertically in a horizontal press.
- Because the vertical press injects down into the mold, gravity is an advantage. Parts are more likely to be uniformly encapsulated.
For molders who balk at the cost of a vertical press, Boyer says, "The cost savings seen by investing in a vertical machine would more than justify the cost of the machine."
Don't be afraid.
"Encapsulating is not black magic," Boyer says. The most common myth he hears comes from molders who traditionally use epoxy and thermosets to encapsulate parts. He says they're under the false impression that pressures introduced by thermoplastic injection molding are too high. He points out that an injection molding machine can easily be adjusted to mold a good part at reduced pressures with a slower injection velocity. There are few reasons why a part cannot be encapsulated in thermoplastic materials.