Injection molding is suitable for producing a myriad of medical designs in plastic parts such as cardiovascular implants, drug-delivery systems, and product containers. And part counts need not be in the millions. Conventional injection molding involves steel, multi-cavity molds intended for high part counts. Steel is best when surface finish and tolerances are critical. Mold costs can be high, but sufficient numbers can keep costs low. Prototype injection molding, on the other hand, provides just a few workable parts for preproduction validation, and increasingly, low to medium production. So-called “soft,” or aluminum, molds typically make prototype parts.

Regardless of mold material, the process is the same. Resin in pellet form feeds into an injection machine's barrel, which contains a big screw. Heaters around the barrel warm the plastic until it melts into a thick, hot liquid. The screw forces this “melt” into the mold and a press holds the mold closed until the plastic solidifies or freezes. The press opens and ejector pins push out the part.

Precision micromolding in a cleanroom

Plastic parts are classified by weight, according to Scott Herbert, president, Rapidwerks Inc., Pleasonton, Calif., (Rapidwerks.com). Micromolded parts are one gram or less, miniature parts are about one up to four grams, and injection-molded parts are more than four grams. Rapidwerks performs precision micromolding in a class 10K cleanroom.

“We design molds from an engineer's 3D CAD model and look closely for potential problems such as too much draft. On conventional molds, draft is the 3 to 5% taper on mold-cavity walls necessary so parts pop out easily. But with micromolded parts, even 1% of draft can make walls too thin. So it's often necessary to minimize draft to 11/42% or less,” says Herbert.

Additionally, the company proves-out molds with Moldflow Mold Adviser simulation software from Moldflow Corp., Framingham, Mass. Engineering-grade plastics have different viscosities, which affects how the mold fills. Flow analysis shows whether parts will warp and pinpoints spots where parts might freeze before the mold fills. “Analysis also shows sink marks, shallow dimples on the surface of the finished part created by material shrinkage and other variables,” says Herbert. “To fix flow, we might modify gates, where plastic enters the tool, or adjust runners, which are the channels through which melt shoots into mold cavities.”

When the computer simulations look good, the company builds the tool. A production engineer installs it on a machine and takes a few test shots to define such process parameters as material dosing, clamping force on the tool, injection speed, holding time of a part, and temperature settings — all of which are functions of the material being processed.

The company makes sure that all part dimensions fall in tolerance ranges. After the OK, production begins. A vision system on the machine inspects parts while statistical-analysis software monitors recorded values. Parts out of tolerance are scrapped.

Some firms, such as Rapidwerks, lets customers log-on to a company server by Web page or even cell phones, to see jobs running in real time from web cameras mounted on the machines.

The company uses Battenfeld Microsystem 50s, from Austria, because the machines precisely micromold the parts, inspect them, and insert parts into custom packaging such as blister packs. “Positional accuracy on micromolding machines has increasingly improved from year to year. Conventional machines open and close molds with hydraulic systems that could hold plus or minus a couple of thousandths. Today, machines such as the Rapidwerks MicroMolding system work within a few microns,” says Herbert. “This is critical for repeatability of movement in opening and closing molds. These are delicate tools and you don't want molds to slam shut. They should open and close gradually. Similar servo technology ensures accuracy when metering and dosing the exact amount of material.”

Rather than hydraulics, a closed-loop-feedback control servo system keeps the machines running at these tight tolerances. A precision-ground ball screw converts rotary travel to linear travel. Encoders on the servos ensure that linear travel stays in synch with ball screw rotation.

Rapidly prototyping real parts

Rapid injection molding automates the design and manufacture of molds for prototype and low-volume part production. The Protomold Co. Inc., Maple Plain, Minn., (www.Protomold.com), targets prototypes and parts such as electronic enclosures and fluid-handling devices. “We make no implantables and so don't have the expense of a typical medical-device manufacturer's cleanroom and quality certifications,” says Brad Cleveland, president and CEO. “Our part-runs are usually a few hundred to about 10,000 a year. Companies requiring millions of parts invest in high-cost, steel, water-cooled molds that make twenty parts at once at high speeds. Our market focus lets us stay fast and relatively inexpensive.”

In the past, rapid injection molders made mostly simple straight-pull parts. These have no undercuts, part bumps or indentations that impede ejection from a mold. According to Cleveland, today complicated molds with nonlinear parting lines and several side actions are possible. Protomold's operations work like this:

Users upload their 3D CAD models to the company's proprietary manufacturing software via its Web page. The software analyzes the model for part moldability and returns a Web-based quote with design considerations highlighted. These might include sharp corners, insufficient draft, or impossible to mill features such as tall, super-thin ribs. “What's more, parts come out of the mold solidified, but still cooling. So it's necessary to check for consistent wall thickness in a design to help ensure the material won't warp or shrink,” says Cleveland.

The quote also provides options for part quantity, material, and surface finish. Users clicking on the “order” button sets the software in motion to generate a mold design and also tool paths for cutting the mold. High-speed three-axis CNC milling machines cut the mold components from aluminum.

According to Cleveland, a big advantage to their production model is process flexibility. “For example, when customers change their designs, they can get parts from a new mold in days, not weeks, and without spending $30,000,” he says.

Avoiding the sins of stereolithography

Assemblies in particular need prototypes to make sure their parts go together as planned. Molds of tool steel would be prohibitively expensive for such a prototyping task. “And sterolithography can fool you into thinking a design is good,” says Chris Crowley, independent mechanical engineering contractor, Table Mountain Innovation Inc., Golden, Colo., (tablemountaininnovation.com). “So-called ‘working models’ may not, in fact, result in parts that can't be molded. For example, sliding friction is different in prototypes made by 3D printing processes and it's difficult to replicate how moving parts will actually work. Stereolithography also makes it too easy to design parts with undercuts or without draft angles. But take, for example, one small assembly we are working on. The mechanism has eight or nine parts and measures less than one square inch in size. Early feedback with rapid-injection molding lets us correct undercuts, drafts, gate positions, and friction surfaces, and still get workable parts back quickly.

Prototyping a living hinge

Rapid injection molding works well for small medical disposable packaging, says Leon Marucchi, owner, Impact by Design,, Clinton, N.J., (www.whatsyourfunction.com).“Rapid prototype processes generally do not provide enough data for releasing a design to production. Rapid injection molding, on the other hand, provides designers with actual injection-mold data before investing in a $250,000 steel multi-cavity tool.” Marucchi, for example, is designing a polypropylene pharmaceutical container that has what's called a “living hinge,” a membrane only about 0.010-in. thick that lets users flip container tops up and down.

“A single-shot injection tool can incorporate a hinge capable of dynamic movement and without secondary operations or hardware,” says Marucchi. “Model shops have been trying to simulate the dynamics of a real part, using a polypropylene tape in fabrication to hold the container components together. But that generally does not provide accurate dimensional or physical feedback.”

He says rapid injection molding, on the other hand, gave him a complex polypropylene piece with two long, living hinges and multiple snap details, in a few days. “Rapid injection molding lets us see part shrinkage in critical areas. It also makes it quick and easy to redesign the container based on customer feedback,” says Marucchi. “And we get hundreds or thousands of parts up-front to try on assembly lines or in automation equipment.”