1)Incorrect material selection – The coefficient of thermal expansion for all steel, hardened steels, copper & aluminum, must be taken into consideration when designing and building tools. Large differences can cause damage and uneven wear to all components.
2) Incorrect back plates – Most molds have 2 back plates, one on the fixed side and one on the moving side. Back plates have 3 main functions:
To hold the mold in the molding machine using clamps
To form part of the runner system
To support the entire mold against excessive platen deflection
If the back plates are too thin, the resultant repetitive deflection during each cycle eventually causes the following part quality problems:
Flashing /Short shots / Weight variation / Voids / Sink marks
Balancing issues in the runner system
These quality problems occur because the plates do not provide enough support to stabilize the mold against cavity injection pressure and clamp tonnage.
3) Incorrect runner design – A runner system that is not balanced will cause filling issues and inefficiency in running the tool. Some parts may be over-packed and other under-filled; both scenarios could cause quality issues with the part.
4) Incorrect gate design – Selecting the incorrect gate has a similar effect with filling issues and inefficiency in running the tool. The gate is the area where the polymer transits from the runner to the part, and if this is wrong the gate could cause restrictions, incorrect polymer flow, turbulence, or other defective filling issues.
When designing a plastic part keep in mind that while the plastic goes into the mold as liquid it cools rapidly and solidifies into the shape of the cavity. Designing a part that can be molding using a “straight pull” is key to keeping the cost of the injection mold down. A straight pull mold is designed so that when the two halves (A side and B side) separate from each other, there is no Metal blocking the path of the plastic in the direction of the pull. This means that if there is an undercut like a snap detail, hole on the side or otherwise; that once the part cools it will not eject from the mold unless the undercut is relieved. This is done using slides and lifters in the injection mold. Slides can be either mechanical or hydraulic and are expensive features to add to any mold. Lifters are typically smaller and are always mechanical.
Vents are areas designed into the injection mold to allow for gases to escape during the molding process. Lack of proper venting will cause excessive injection pressure, short shots, burn marks, sink and splay. There are many ways to vent a mold, but is typically done by machining numerous shallow channels at the parting line. The size of the gate will depend upon the material selection. Other ways to vent a mold are ejector pins, vent pins and runners. Vents can be cut into the ejector pins to allow the gas to escape as the cavity is filled. Vent pins are placed within the geometry of the part when venting is required to remove gas from the mold during filling and when vents solely on parting line are inadequate.
Adding a draft in your molded part design requires special attention to make it easy to eject the part from the mold. Adding a few degrees of draft means that parts will leave the mold much more smoothly, with minimum friction and scraping between the finished, cooled product and the walls of the mold.
There are minimum draft requirements needed for any part based on the depth of the part, the material chosen, and the amount of texture applied to the mold.