1. The application of injection molding
In manufacturing plastic parts nowadays, we usually prefer injection molding technology that is used to make many plastic products for our life: automotive interior/exterior, containers, electronics, bottle caps, combs and most other plastics available. With multi-cavity injection molds, there are many parts that can be made in high volumes, which can bring many benefits in manufacturing, and help the consumer can get those products with lower price. The advantages of injection molding are: low labor cost, minimal scrap losses, high tolerance precision, repeatably, large material selection, and little need to finish parts after molding. Of course, as all things in this world, this process technology also has its own disadvantages: expensive upfront tooling investment and process limitations.
2. The polymer best suited for injection molding
With injection molding technology, we can use most polymers available today: all thermoplastics, some thermosets and some elastomers. Nowadays, we also can find tens of thousands of different material available for injection molding. Moreover, the product designers have a lot of choices when the available materials can be mixed with alloys or blends of previously developed materials, then there are a vast selection of materials for them to find the one that meets their need or requirements. Choosing materials is based on the strength and factions required for the final part. However, each material has its own different parameters for molding that we will have to consider before deciding.
3. Basic equipment used for Injection Molding
a. Injection Molding Machine:
Injection molding machines (also known as presses) consist of a material hopper, a heating unit and an injection ram or screw-type plunger. There is a platen inside the injection machine, where plastic is injected into the mold through the sprue orifice, which molds will be clamped to. People rate presses by tonnage, which is calculated from the amount of clamping force that the machine can exert.
During the injection molding process, the mold will be kept closed by that force. There are many type of tonnage that can vary from less than 5 tons to 6.000 tons. The higher tonnage presses are also available, but rarely used. The projected area of the custom part being molded will determine the total clamp force to be used. The projected area is multiplied by a clamp force of from 2 – 8 tons for each square inch of the projected areas. As the rule of thumb, the manufacturer will use 4 or 5 tons for most of their products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage is needed to hold the mold closed. The required force can also be determined by the material used and the size of the part with larger plastic parts requiring higher clamping force.
b. Mold
The mold or die refers to the tooling used to produce plastic parts in molding. Traditionally injection molds have been expensive to manufacture and were only used in high-volume production applications where thousands of parts were produced. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. The choice of material to build a mold from is primarily one of economics. Steel molds generally cost more to construct but offer a longer lifespan that will offset the higher initial cost over a higher number of parts made before wearing out. Pre-hardened steel molds are less wear resistant and are primarilly used for lower volume requirements or larger components. The hardness of the pre-hardened steel measures typically 38-45 on the Rockwell-C scale. Hardened steel molds are heat treated after machining, making them superior in terms of wear resistance and lifespan. Typical hardness ranges between 50 and 60 Rockwell-C (HRC).\
Aluminum molds cost substantially less than steel molds, and when higher grade aluminum such as QC-7 and QC-10 aircraft aluminum is used and machined with modern computerized equipment, they can be economical for molding hundreds of thousands of parts. Aluminum molds also offer quick turnaround and faster cycles because of better heat dissipation. They can also be coated for wear resistance to fiberglass reinforced materials. Beryllium copper is used in areas of the mold which require fast heat removal or areas that see the most shear heat generated.
(Reference from 3Dsystems.com)
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