Science of mold making

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*There is not a "single" mold that can be used for injecting molding for "all" plastic materials. Because each plastic has its own chemical, mechanical and thermal characteristics. Therefore, each mold is unique for the material intended to be used. The gate, runner and draft angle may differ from material to material.


Please ask for a quote from Protomold, ICOmold or XcentricMold for finished plastic parts for your own design and get their feedback for your design to see if it even can be molded at all or not. Then please get back to us with your specific mold related questions so that we can reply your questions intelligently.

Traditionally injection molds have been expensive to manufacture. Molds are typically constructed from hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy. Today, aluminum molds cost substantially less than steel injection molded parts. 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. It can also be coated for wear resistance to fiberglass reinforced materials. Today's Mold companies use CNC machining and Electrical Discharge Machining (EDM) in the mold manufacturing processes.

cnc machining mold

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 amount of resin required to fill the sprue, runner and cavities of a mold is called a shot.
injection mold drawing
To properly release the part when the mold opens, the side walls of the mold are tapered in the direction that the mold opens. This tapering is referred to as "draft in the line of draw". The draft required for mold release is primarily dependent on the depth of the cavity. Injection molds are usually designed so that the molded part remains securely on the ejector side of the mold when it opens, and draws the runner and the sprue out of the other side along with the parts. The part then falls freely when ejected from the ejector side. 2-3 degrees draft is required for mold-ability of the parts. The angle should be large enough to allow to eject the part out of the mold. The corners should NOT be too sharp. Otherwise sink marks may occur.
draft angle explained for injection molding

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. 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.
plastic parts made by overmolding

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 cavity mold. A custom mold with 2 or more cavities of the same parts will likely be referred to as multiple cavity (family) molds. When you design a mold for more than one parts (multi-cavity), the part distribution should be so balanced that each part is placed at equal distance to the sprue. That allows the mold flow smooth and consistent.
multi cavity mold
Injection molding can create injection molded parts with complex geometry that many other processes cannot. There are a few precautions when designing something that will be made using this process to reduce the risk of weak spots. First, streamline your product or keep the thickness relatively uniform. Second, try not cramming too many details into one part may cause visual defects in show surfaces or the inability to fill some of the details without sacrificing others.

Molding trial
When filling new or unfamiliar injection molds for the first time, where shot size for that mold is unknown, an injection molding company technician/tool setter usually starts with a small shot weight and fills gradually until the mold is 95 to 99% full. Once this is achieved a small amount of holding pressure will be applied and holding time increased until gate freeze off (solidification time) has occurred on the injection molded part. Gate solidification time is an important as it determines cycle time, which itself is an important issue in the economics of the production process. Holding pressure is increased until the parts are free of sinks and part weight has been achieved. Once the parts are good enough and have passed any specific criteria, a setting sheet is produced for people to follow in the future.

Runner and Gate Design
mold runner and gate design
The runner should be thick enough to carry high amount of plastics without early premature cool down. The gate should be thin enough to have a smooth plastic flow into the cavity.

Wall Thickness
importance of wall thickness in injection molding

Wall thickness and design determine if a part would have a sink or wrap after the injection molding or not. Uneven mold wall thickness is always a problem. Certain materials should also have a minimum thickness for a perfect mold-ability.

Here is the video series for Mold Making on Fusion 360:
Video 1 – How to Create a Mold
Video 3— Create Mold Base