Q&A on Injection Molding
What are the advantages of injection molding over other methods of manufacturing a part?
With injection molding, we can mold fairly complex shapes at high production rates while maintaining good dimensional stability and holding fairly tight tolerances. It is possible to reground and reprocess thermoplastic resin used in injection molding, but some of the plastic's physical properties may be lost over each round of regrinding. When manufacturing large volumes, using engineered plastic resins instead of metal can result in many cost savings.
Is injection molding expensive?
Injection molded parts can cost as little as a few cents to as much as several dollars. For injection molded parts, there are many factors that affect the cost, including the size and complexity of the part, the plastic resin being molded, and the number of cavities in the mold.
In what time frame does a plastic part need to be molded?
Injection molded parts typically have a cycle time of a few seconds to a few minutes. Part size, material being molded, mold design, and part design are all factors that determine this. An average mold set-up time is between a few minutes and a few hours.
The setup and start up time can be absorbed into the cost of the part once a setup is established and the process is stable.
What is the cost of an injection mold?
There are several thousand dollars to a few hundred thousand dollars involved in making an injection mold. It is likely that the complexity of the part as well as its size are the biggest factors contributing to this cost. When building molds, parts with a simple appearance may prove difficult to build since they have undercuts that may require moving parts to move to allow free ejection of the part when the mold is opened.
Injection mold ownership: who owns it?
Most injector molds are owned by customers who design and purchase the parts. Unless specific contract terms are agreed upon, customers usually have the right to have their molds molded anywhere; however, making sure the right equipment and process are available in the new location can present some risks.
How long does it take to build an injection mold?
Typically an injection mold can take anywhere form a few weeks to a few months to build. Many factors can contribute to this time such as a supplier’s backlog or amount of resources available as well as the complexity of the part or the complexity of the mold design and the number of cavities needed.
What if I want to change my part design? Can the original injection mold be modified?
In some cases yes depending how the injection mold was designed. A good injection mold will be built using steel inserts that make different features of the part and assembled all together in a mold base. This makes it easier to change one feature of the part when all you have to do is modify or re-build a small steel insert. When
the whole part design is cut into one piece of steel, some design changes can be more challenging and may require building the entire mold over again to incorporate the design change.
What is the typical life expectancy of an injection mold?
Most mold makers build molds to industry standards such as SPI Class 101 which should guarantee a mold life of 1M Shots. This means if your mold has 4 cavities you should get 4M parts from you mold with out any problem. Depending on the complexity of the part and the plastic resin used, this amount may vary. It is not unlikely for you r mold to last for well over 1M shots. If your part is complex and the mold has small intricate steel inserts or you are using an abrasive plastic resin such as a glass filled polycarbonate, mold wear will be more progressive shortening the life of your mold.
What is a mold cavity?
The cavity is the part of the injection mold that forms the part. An injection mold could have one single cavity or multiple cavities of the same part or a family of different parts. Multiple cavities will increase that injection mold cost however lower the cost for the injection molded parts since multiple parts could be made during one molding cycle.
What is a parting Line?
Where the two halves of an injection mold come together is known as the parting line. A witness line of where the mold was parted can usually be seen on the molded part. Sometimes the parting line may be stepped rather than flat all the way around the part.
What is the difference between the “A” half and the “B” half of an injection mold?
The half of the injection mold that sits stationary in the machine and has plastic injected into it is known as the “A” half. The other half that moves back and forth to open and close the mold is known as the “B” half. When the mold is open, the part is ejected from the “B” half.
What is an ejector pin?
These pins actually push the part off of the “B” half of the Mold. Holes are made on the “B” half of the mold and steel pins are ground flush with the mold cavity surface on the “B” half. The other ends of the pins are connected to a plate (known as the “Knock Out Plate”) behind the mold. When the injection mold is open, the molding machine is programmed to push a steel rod (known as the “Knock Out Bar”) forward. This pushes the knock out plate forward moving all of the ejector pins forward simultaneously to eject the part.
What is a gate?
A gate is the opening where the plastics flows into the injection mold cavity.
What kind of tolerances can you hold on an injection molded part?
To hold a tolerance of plus or minus .001 of an inch on a plastic part is not unheard of. Some plastic materials are easier to control variation then others. Injection molding machines today are sophisticated and advanced enough to produce relatively accurate and repeatable results. If you are looking to maintain six sigma quality with a tolerance window of .002 of an inch, you may be starting to push the limit as there may be variations in gauge measurements and other areas that throw off your statistics. Some plastic resins of course are more predictable and shrinkage is consistent and easier to control from part to part.
What does Steel Safe Mean?
When a mold maker builds an injection mold, he has to take into account the plastic shrink rate. So if he were to build an injection mold for a part that was molded from a plastic with a shrink rate of .003 in./in. and needed to have a 1 inch inside diameter then the mold builder would have to build a mold core 1.003 in. in diameter to allow for shrinkage. In order to leave room for error (whether it is in measurement or shrinkage) he would build the core slightly over size because he could always remove steel to make the core smaller if needed after the part is molded. If the part ends up too small it would not be possible to add steel to the mold core to make it bigger so it would require a lot more work to fix the problem by building the mold core over. Please note that if we were trying to control an outside diameter, steel safe would mean making the mold cavity on the small side. If the O.D. of the part was too small, steel could be removed from the mold cavity allowing the part to be molded with a bigger O.D.
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