A manufacturing technique called rapid injection molding lets companies quickly get low-cost and dimensionally accurate prototypes as well as low-volume production parts from plastics. Examples of parts for the medical market include device enclosures, heart-monitoring components, and even portions of automated emergency-response systems for nursing homes. Rapid injection molding is similar to conventional injection molding, yet different enough to affect how parts are designed. A brief description of plastic injection molding in general should prove a useful start.

Regular or rapid?

In general, plastic injection molding uses plastic in the form of resin pellets that are loaded into the hopper of an injection-molding machine. A heated barrel on the machine melts the material and a large screw forces the melt into the mold. The material cools and solidifies. The machine then opens the mold at its parting line and ejector pins push out the part.

Our method automates the design and manufacturing of molds based on 3D CAD models that have been uploaded online. This automation typically cuts lead times for initial parts to one-third that of conventional methods. Naturally, cost savings vary with the number of parts being produced, but rapid injection molding can have a substantial cost advantage in runs of up to thousands of parts.

Rapid injection molding falls between rapid prototyping and conventional injection molding. Hence, it supports prototyping, bridge tooling, and low-volume production. Additive methods such as FDM and SLA work well for testing form and fit, but often such parts are not a substitute for testing with real injection-molded parts. Rapid injection molding is also fast and cost-effective enough to fix problems when initial designs are not quite right.

The technique also works well to make 100, 1,000, or 10,000 parts for pilot production or market testing until production tooling is ready, typically, a three or four month wait. When production volume doesn't justify steel tooling, rapid injection molding can supply production parts. In this scenario, multicavity aluminum molds are a cost-effective way to go.

Inside an injection mold

Rapid injection molding uses aluminum molds milled on 3-axis CNCs instead of the expensive, hardened tool steel molds used with traditional methods. Molds comprise an A-side, sometimes called the cavity side and a B-side, sometimes called the core side. The A-side mounts to the fixed portion of the injection-molding press. Resin is injected through this section of the mold into the part cavity by way of the sprue, the path the resin takes until it reaches the runners. These guide the resin to the gate, the location where the plastic enters the part cavity.

The B-side, on the other hand, mounts to the moving side of the press and includes the steel ejector pins. Before designing the mold, parts are oriented so they will remain on the B-Side when the mold opens.

The mold might also contain a “side action” device, basically a sliding cam arrangement that allows molding part undercuts. Undercuts must be on or connected to the mold parting line, on the outside of the part and in the plane of the parting line. The cam is held in place during the injection process and slides out of the way before part ejection. Rapid injection molding can do up to four side actions per part. Finally, a core is any protruding portion on either side of the mold that enters the opposing cavity when the mold is closed. The void between cavity and core defines the geometry that forms the part.

Design considerations

The use of aluminum molds influences part design. For example, it is difficult to maintain milling accuracy with long cutting tools so it's best to avoid placing small features adjacent to steep walls. Also consider that a milling tool's cylindrical shape cuts part corners with a radius instead of a sharp edge.

Additionally, it is wise to avoid deep and thin ribs because they require clearance for milling tools. Maximum tool lengths average between 8 to 12 times the tool diameters. Note that deep, thin ribs increase mold milling time as well as make hand polishing difficult. To prevent unsightly sink marks, rib thickness should be no more than 60% of the adjacent wall thickness. Rapid injection molding is not suitable for inserts or overmolding.

An important design requirement for good molded parts is maintaining a constant wall thickness. Otherwise, parts tend to sink or warp. Also, it's a good idea to core parts to eliminate thick walls. These parts mold better, can have the same fit and adequate strength, and use less material than non-cored parts. Something to avoid is sharp transitions, because they produce molded-in stress and operating stress concentrations as do sharp corners. Also unacceptable are thick sections with screw bosses, which can cause voids in parts.

It's important to draft or slope vertical walls as much as possible. This allows milling deeper features. In addition, draft helps reduce tool chatter and cosmetic defects when milling deep walls. And part prices tend to be lower on well-drafted molds. When possible, use 1° of draft or more. On core-cavity designs, use 2° or more. A rough rule of thumb is 1° of draft for every inch of depth, up to 3° for 3 inches. Following this guideline makes it easier to eject parts without drag or ejector punch-marks. Additionally, use a core-cavity design instead of ribs when using a draft. This avoids walls with a thick base so molds are easier to mill with better surface finishes. Lastly, parts to get a light texture require 3° of draft minimum while those with a medium texture need 5°.

When it comes to coloring, common stock colors from resin suppliers include white, beige, black and amber. Semi-custom colors are available without additional charge by adding color pellets to natural resins. However, these colors might not provide an exact match to a Pantone or color chip and can create streaks or swirls in parts. For exact custom colors, customers should work with a compounder and then provide us the resin. Finally, many resin additives and fillers are available, such as glass fibers, carbon fibers, and talc, that strengthen composites, make parts self-lubricating, reduce warping and shrinkage, and provide other useful attributes.

Putting rapid injection molding to work

An example of rapid-injection molded parts in a medical device comes from American TeleCare in Eden Prairie, Minn. It wanted to produce an audio-only device it called inLife that guides patients in routinely taking their own pulse, weight, blood pressure, and glucose levels, and in the comfort of their homes. The system also adds a date stamp to each data record. The data then transmits by phone line to a server and uploads into the patient's record for physician review. When developing the device, the company had problems getting parts quickly enough, until it discovered rapid injection molding.

“To accommodate a need for a few thousand test parts, we could either make a lot of RTV molds or pay a high cost to build steel molds for injection molded parts. Either way is expensive. Rapid injection molding filled the gap with the capability of creating 1,000 to 10,000 prototypes of the product's enclosure in a timely and less-expensive manner,” says mechanical engineer John Blomberg at American TeleCare.

“We first needed a model see how the intricate parts fit,” he says. “A stereolithography model was not dimensionally accurate enough to move forward with material testing and verification. To try rapid injection molding, we uploaded a 3D CAD file of the enclosure to protomold.com. Within one business day, we received an interactive Web-based price quote, including useful information such as lead-time options, pricing at various quantities, information on a wide range of molding materials, and even suggestions for design improvements. This experience let us begin low-volume production,” says Blomberg.