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Custom Mold Design
Sep 15, 2017

Custom Mold Design

        Molds consist of two primary halves, injection molds (A plate) and ejector molds (B plate). First, plastic resin enters the mold through a sprue in the injection mold. The sprue bushing is to seal tightly against the nozzle of the injection barrel of the molding machine in order to allow molten plastic to flow from the barrel into the mold, also known as cavity. The sprue bushing directs the molten plastic to the cavity images through channels that are machined into the faces of the A and B plates. These channels allow plastic to run along them, so they are referred to as runners. The molten plastic flows through the runner and enters one or more specialized gates and into the cavity geometry to form the desired injection molded part.

       The amount of resin required to fill the sprue, runner and cavities of a mold is called a shot. Trapped air in the mold can escape through air vents that are grinded into the parting line of the mold. If the trapped air is not allowed to escape, it is compressed by the pressure of the incoming material and is squeezed into the corners of the cavity, where it prevents filling and causes other defects as well. The air can become so compressed that it ignites and burns the surrounding plastic material. To allow for removal of the injection molded part from the mold, the mold features must not overhang one another in the direction that the mold opens, unless parts of the mold are designed to move from between such undercuts when the mold opens (utilizing components called Lifters or slides).

        Sides of the molded part that appear parallel with the direction of draw (the axis of the cored position [hole] or insert is parallel to the up and down movement of the mold as it opens and closes) are typically angled slightly with (draft) to ease release of the part from the mold. Insufficient draft can cause deformation or damage to the injection molded part. The draft required for mold release is primarily dependent on the depth of the cavity: the deeper the cavity, the more draft necessary. Shrinkage must also be taken into account when determining the draft required. If the skin is too thin, then the molded part will tend to shrink onto the cores that form them while cooling, and cling to those cores or part may warp, twist, blister or crack when the cavity is pulled away. Injection molds are usually designed so that the molded part remains securely on the ejector (B) side of the mold when it opens, and draws the runner and the sprue out of the (A) side along with the parts. The part then falls freely when ejected from the (B) side. Tunnel gates, also known as submarine or mold gate, is located below the parting line or mold surface. The opening is machined into the surface of the mold on the parting line. The molded part is cut (by the mold) from the runner system on ejection from the mold. Ejector pins, also known as knockout pin, is a circular pin placed in either half of the mold (usually the ejector half) which pushes the finished molded product, or runner system out of a mold.

        More complex plastic parts are formed using more complex injection molds. These may have sections called slides, that move into a cavity perpendicular to the draw direction, to form overhanging or undercut part features. When the mold is opened, the slides are pulled away from the plastic part by using stationary angle pins or horn pins on the stationary mold half. These pins enter a slot in the slides and cause the slides to move backward when the moving half of the mold opens(like a cam). The part is then ejected and the mold closes. The closing action of the mold causes the slides to move forward along the angle pins.

        Some injection molds allow previously injection molded parts to be re-inserted to allow a new plastic layer to form around the first part. This is often referred to as overmolding. This system can allow for production of one-piece tires and wheels.

         2-shot or multi-shot injection molds are designed to "overmold" within a single molding cycle and must be processed on specialized injection molding machines with two or more injection units. This process is actually an injection molding process performed twice. In the first step, the base color plastic material is molded into a basic shape. Then the second material is injection molded into the remaining open spaces. That space is then filled during the second injection molding step with a material of a different color.

        Injection molds can produce several copies of the same parts in a single "shot". The number of "impressions" in the mold of that part is often incorrectly referred to as cavitation. A tool with one impression will often be called a single impression (cavity) mold. A custom mold with 2 or more cavities of the same parts will likely be referred to as multiple impression (cavity) mold. Some extremely high production volume molds (like those for bottle caps) can have over 128 cavities.