Injection molded plastic components

Plastic injection molding, by definition, is the process of injecting plastic into a mold (or mould in European circles), cooling the plastic, ejecting it from the mold, and packing the finished product for sale to the customer. This sounds simple and in it''s basic form it is, but the process involved with making all this happen is actually quite complicated. Plastic can be injected into the mold with low pressure, but typically is done under high pressure upwards of 30,000 PSI cavity pressure.

There are many diverse types of injection molding and as many different types of plastic injection molding machines to go with it. Some of the more common types of plastic molding are standard mold injection, insert molding, plastic extrusion, blow molding, multi-color or material molding, stack molding, and rotary molding just to name a few. There are custom plastics as well, from common every day plastic resins to the more exotic engineering grade resins used in most automobiles today. There are even injection molding grade "metals" being experimented with and used these days.

There are many types of injection molding machines all made for specific purposes. There are the standard horizontal injection molding machines which range in size from a few tons all the way up to 9000 tons and more. You could drive car between the platens of a press that large. Vertical injection molding machines are often used for insert molding (although this can also be done horizontally) and share a common cover with two ejection halves of the mold. This allows for de-molding and loading of inserts in one half, while the other half is producing the next shot of parts. There are also two and three shot injection molding machines for producing multi-color or multi-material parts such as lenses for tail lights and gear shift knobs with hard plastic inner core and a soft vinyl out covering. The newest machines are "all electric versions" which stray from the standard hydraulic injection machines used now for years. These injection molding machines are much more efficient and repeatable then their hydraulic cousins are. Common injection molding machine manufacturers are Toshiba, Husky, Cincinnati, DeMaag, Engel, Nissei, UBE, Arburg and Boy, just to name a few.

The basic steps to producing a plastic injection molded part on a stand injection molding machine are:

• Heating the plastic resin to the required range of the product being used.
• Developing the shot size through use of a reciprocating screw which conveys the melted plastic to the front of the screw.
• Injecting the plastic into the mold under pressure to fill the cavity of the mold.
• Packing the plastic to create a full part inside the mold.
• Cooling the plastic in the mold through the use of cooling channels most commonly with water.
• Ejecting the cooled part from the mold.
• Repeating the process over and over again.
• Injection molding is a manufacturing method to mass produce plastic parts at a reasonable cost to the producer.
• Plastic modeling deals with very small parts. Plastic injection molds are not very strong and it is easy to destroy detail on the plastic molding if parts aren’t clipped off the sprues properly.

Cutting Injection Molded Plastic Sprues Properly

Several things can be done to avoid damaging the parts on plastic sprues, which are plastic injection molded racks that hold scale modeling parts together. Although a pair of nail clippers or an X-Acto blade can be used, plastic sprue cutters are a good investment for any beginning plastic modeler. Be careful when handling sharp edges!

Even in larger scales, parts attached to sprues are very small and fragile. Fortunately, most parts have small tabs attaching them to the sprues. Small plastic parts have a tendency to bounce around when their cut. Before cutting any pieces from the sprues, always shield the part with a hand, this prevents it from flying away. When removing parts from the sprue, try to leave some of the tab attached; this helps to keep any small parts from being damaged. It might be faster to remove the tab while the part is still attached to its sprue, but the general consensus is that the former helps keep the parts in their best condition.

Keep parts attached to the numbered sprues, by keeping unused parts attached, they are easier to keep track of. Losing small parts is sometimes a plastic modeler’s worst nightmare, especially small, but crucial detail pieces.

Cut out the necessary parts and lay them out on a tray of some sort. The surface should have curved edges to prevent pieces from accidently falling off. Any unused parts should be safely stored away in a small box, away from other clutter. Use sprue cutters and carefully clip away the remaining tabs one at a time.

Finally, use an X-Acto blade to cut off the any remaining excess plastic. A scraping action using the flat edge of the blade works best.

If the edge is still not smooth, try sanding the edges with a smooth grade of sandpaper. An article about sanding plastic models can be found here.

Some Things to Remember About Plastic Sprues:

• Use a pair of plastic sprue cutters or an X-Acto blade to remove parts from sprues
• Do not remove parts from the sprues unless they are needed for immediate use
• Leave the connecting tabs attached when removing parts from sprues
• Keep small parts safe, losing parts may compromise the whole model
• Always be careful when using sharp edges
• Patience is a virtue, especially in plastic modeling