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Plastic Injection Molding Terms: For your convenience we have listed terms that are used in Plastic Injection Mold Processing and Tooling.
Part The plastic part made with the mold. Mold The injection mold that is built to make plastic parts with the molding process. Shrink Rate How much the plastic material will shrink after cooling. This percentage(%) of shrink is added to the part before the mold is designed. Each plastic material has its own shrink rate ranging from .001 per inch to as much as .060 per inch. Although most fall in between .004 and .022. It is very important when selecting a plastic material that you are well versed in the manufacturers recommendation and have some practical experience to specify the correct shrink rate. Core Out The portion of a part that is gutted out in order to achieve uniform wall thickness. It is important to maintain a consistent wall thickness to reducing warp. Draft Refers to portion of injection molding part that has some sort of taper to it. Generally all plastic components should be designed with draft where possible to facilitate part removal from the mold.
Cavity Refers to the upper half of the injection mold usually the show surface of the finished product but is mainly concave. There usually is not as much standing core outs on this side of the tool. Therefore the part will generally not stick to it when the injection molding machine opens the mold. Core Refers to side of the tool where the plastic part will stick to and is ejected from, also known as bottom half of the tool. Undercut Referring to the portion of the designed component where a side action is required to create holes, windows and other features that cannot be created from the main core and cavity. Side Action Is a method of pulling a core or feature from the side when the main core and cavity movement cannot create the appropriate action. Hydraulic core pulls, mechanical core pulls, and lifters are the most common. If your design requires side action it will probably require one of the above to make the plastic part in order to run in the molding process. Hydraulic Core Pull A side action actuated by a hydraulic cylinder; can use a slide to control the alignment or can simply move a round core in and out of a round hole. Mechanical Core Pull A side action actuated by the motion of the mold opening and closing; most commonly uses a slide to control the alignment and some may have spring loaded returns.
Slide Portion of plastic injection mold that is used in a mechanical or hydraulic core pull to facilitate a side action. Lifter Portion of plastic injection mold that is used with the ejection to facilitate a side action. Ejector Ejectors are used to push the part off the main core of the mold. They are commonly round pins, but can be made in a number of shapes and profiles to ejection the part effectively Sprue The channel that links the injection molding machine nozzle to the runner; usually tapered and round to come out of the mold easily. The size of the sprue is important for proper delivery of material to the part. Runner The channel in which plastic travels from the sprue to the gate filling the part. Runners are commonly full round, half round, and trapezoidal. It is very important when selecting a runner design that you are well versed in the manufacturers recommendation and have some practical experience. The cross section and path of the runner is very important for proper delivery of material to the part.
Gate Where the plastic enters into the cavity of the mold. Types of gates include: Tunnel (Sub), Edge, Fan, Tab, Pinpoint, and Sprue (Direct Gating). Gate size is determined for most efficient fill with adequate shear and minimum cosmetic defects. Vestige Material or witness of material protruding from gate area after gate and runner has been removed from the injection molded part. This vestige is usually trimmed by the molding machine operator. Minimum or no vestige is preferred for cosmetics. Shear Referred to when plastic enters into the mold and the melt is maintained by friction produced by speed and pressure. Too much shear can cause the plastic material to burn, too little can cause the material to freeze off causing short shot. Short Is the result of a plastic part not filling completely, including some or all of the part details. Flash Resulting in a plastic part being filled in a way that creates residual plastic on the edges and at witness areas of the part. Flash can be evidence of mold wear or damage in which it makes it vary difficult to control the flash with the molding process without creating other defects. Commonly the customer supplies a specification for an allowable amount of flash; 0.005-0.010” is typical. Sink Areas of the molded part where it seems to be sunk in, due to inconsistent or improper wall sections. Main wall section to rib or boss thickness ratio can cause sinks.
Warp An area of a molded part that distorts during cooling or molding, causing undesired results in the finished product. Usually caused by inconsistent wall sections resulting in temperature variations during cooling. Mismatch An area of a molded part that appears to have overlaps from incorrect mold alignment. Commonly the customer supplies a specification for an allowable amount of mismatch; 0.005” is typical. Wall Thickness This refers to how thick a given cross section of the plastic part may be, and it is important to maintain a consistent wall thickness to reducing warp. Boss Round protrusions on the plastic part; typically in the core side of the mold. Ribs Thin bladed features on a part that are used for strengthening wall sections and bosses. Commonly used to minimize warp by means of a support feature, but can also cause warp in an improper part design. Full Automatic A mold built to run in the automatic setting of the injection molding machine. Full automatic molds require no operator for part removal from the mold. This is the most desirable for the injection molding process, allowing the machine to cycle consistently with no interruption.
Manual Operation A mold built to run in the semi-automatic setting of the injection molding machine. Manual molds require an operator for part removal from the mold. This is the least desirable for the injection molding process, but required with many molds to protect the parts from damage on part drop or to hand load inserts in the mold each cycle. Cycle The full sequence of actions in the process from the time the mold opens through the time the mold opens again. Steel Safe The amount of metal left on the mold in order to debug a dimension. For example, if you have an inside diameter on the part that is targeted at 0.500”, leave the mold at 0.520 in case you get excessive shrink. It is cheaper to remove metal than it is to add it. The reverse is if you have an outer dimension on the part that is targeted at 0.200”, leave the mold at 0.180”. Cooling Cooling is done with water in most cases, but some materials require elevated temperatures using hot oil or cartridge heaters. Cooling is the most important feature for temperature control during the process.
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