1 Injection Molding: Exciting yes, but is it rapid enough?

At its core injection molding is a fairly simple process. It involves heating a wide range of materials in and injection molding machine until a melt is obtained. Then the melt is forced into a split-die chamber/mold where it is allowed to cool into the desired shape. The mold is then opened and the part is ejected. This process is then repeated over and over again until the desired number of parts is obtained.

In this report we will cover:

· Step by step instructions on how to use an injection molder.

· What materials are recommended for injection molded parts.

· The limitations and concerns found in injection molding.

· Various details about injection molding, ranging from cost to accuracy.

· Different sites around the web concerning in injection molding.

2 Step by step:

Injection molding is actually a very simple process as soon as you get the ball rolling, but before the injection molding process can begin the user must tackle a breath of preliminary tasks.

Before any real “injection molding” can begin the user must design and manufacture a mold for their part. Injection molds are essentially a negative of the part being produced, so care must be taken in ensuring that the cuts correctly represent the final product. Because of the high tolerances and the machining skill necessary in making a good injection mold, the bulk of the injection molding costs are found here. Tooling costs can range between $5,000 and $100,000 as the molds must be built to high levels of precision and must be robust enough to withstand the high pressures of the process. Molds are usually constructed of hardened tool steel, but may be constructed of aluminum or other soft materials when tooling life is not an issue. Typically, a hardened steel mold will withstand 500,000 to 2,000,000 molding cycles, without appreciable wear, depending upon the material and processing conditions. Aluminum and other soft molds will only withstand 1,000 to 10,000 molding cycles.

After the mold is manufactured and clamped into the injection molding machine the real fun begins.

First pellets of the desired material are fed into the heated press cylinder where the melt is formed.

The melt is then pushed into the mold through a nozzle by using a hydraulic plunger or a rotating screw, which will continue to apply higher pressure on the melt to fully fill the mold cavity. An example of a plunger and screw injector can almost be seen below.

As soon as the pressure in the mold increases the plunger or screw will stop advancing, but continue to apply high pressure on the melt to fully fill the mold cavity giving the part good dimensional stability and tolerance.

A clamping force is then applied hydraulically opposite to the nozzle to maintain pressure during the cooling stage. The cooling off period can range from 5 seconds to several minutes.

Finally the female and male portion of the mold is separated and you prefect part is ejected. In about two seconds you can have your mold reassembled and back in the injection molding machine ready to go again!

3 Recommended Materials

Although just about any material that can achieve a stable melt can be used in injection molding, the two recommended for rapid prototyping are Thermoplastics and Thermosets.

3.1 Thermoplastics are probably the best material for injection molding.

Thermoplastics don't have strong bonds with each other. Therefore, they tend to be soft and ductile. When heated above glass transition temperature, the bonds weaken and a reduction in strength results. That's why they can be easily molded under shaping forces but when cooled, the plastic returns to its original strength and hardness making the process reversible. They have a low thermal and electrical conductivity making then good insulators. At last, they have a high coefficient of thermal expansion, otherwise known as the shrink rate (see Metalcast Engineering's shrink rate table at http://www.metalcast.com/showroom/materials.html).

3.2 Thermosets are a very good material for injection molding but last the reusability of the Thermoplastics.

Thermosets have strong covalent bonds with each other. That is why they are more rigid and stiff then Thermoplastics, but tend to be more brittle. They also have a low electrical and thermal conductivity making them excellent insulators. Unlike thermoplastics, their properties are not very much dependent on the temperature or strain rate. But if the temperature is high enough, they burn up and disintegrate.

4 Limitations and Concerns

The major concern in injection molding is the ease of separation of the part from the mold. The machinist creating the mold would need to design the mold section to ensure that there is no material preventing the sections of the mold from splitting. To do this he may have to create a mold in multiple sections. Below is an ideal shape for injection molding, notice how easily you can envision the type of mold that would be used for such a part:

Undercuts and overhangs should be avoided as creating and using a mold with a movable section is expensive and time-consuming. The example below could not be done in a one or even three part mold, greatly increasing your tooling cost and production time. Try to envision how many sections the mold would need to produce such a part:

Injection molding is unable to naturally produce hollow cores in its parts, as there would be no way to remove the mold section.

All corners should be rounded (somewhat) to improve the quality of the part.

Because of the high tooling costs, small quantities are not economical.

Large parts are not possible because of the pressure and size constraints of the injection molding machine, not to mention the astronomical machining costs of creating a mold the size of a car.

Large clamping forces are necessary to avoid flash production, which requires more expensive injection molding machines.

5 Miscellaneous Details

· Dimensional Accuracy: +/-0.001 inches per inch

Too Low: 1.6

Easily Achieved: 20 or higher

· Surface Roughness: (in microinches)

Too Low: 7

Easily Achieved: 64 or higher

· Bounding Box Volume: (in cubic inches)

Low: 0.01-0.2

Just Right: 0.2-3000

High: 30,000-40,000

· Wall Thickness Required in Mold:

Max Wall Thickness (per 5-inch span): 0.3

Min Wall Thickness (per 5-inch span): 0.05

· Recommended Batch Sizes:

50-1,000,000 (parts)

· Production Rates:

101-1000 (parts per hour)

· Setup Time:

Weeks to Months

· Setup Cost:

Very High

· Per Part Cost:

Very Low

6 Addition Information

We would like to thank the Manufacturing Advisory Service v2.0 for the majority of the information on injection molding. Additional Information can be found at:

· If you would like to buy a very informative CD-Rom set go to:

http://www.maintenanceresources.com/ProductsShowcase/Plastics/BasicSeries.htm

· The Injection Molding Trade Magazine can be found at:

http://www.immnet.com/index.html

· Nypro offers an online course covering all aspects of injection molding at:

http://www.nyproonline.com/Nypro-UMass/current.html