The Injection Molding Process Explained

The Injection Molding Process for any particular part can be defined as the “Transformation of Plastic Pellets to a molded product using heat to melt the pellets enabling them to flow so they can be injected in to a mold cavity filling it as fast as possible then packing the melt and holding it under pressure to form the part, then allowing it to adequately cool before being ejected from the mold”

The four critical process parameters in bold listed above are interdependent and will introduce variation in the process. The amount of variation is dependent on the size and complexity of the part as well as other fixed inputs described below.

Molding Process Fixed Inputs

  1. Part Design – An optimal part design for injection molding typically will have uniform wall thicknesses that are properly sized for the material to flow easily and pack into the mold cavity under pressure. Other design features such as rib heights, rib thickness at the base, draft, etc. will also limit the molding process capability.
  2. Material – Material is primarily chosen for the necessary properties required for the molded part in the field but may limit how easy it will be to develop a molding process that will produce dimensionally stable consistent parts. Design guides for commercially available resins can often be requested from the material manufacturer and are very helpful in confirming the integrity of the original part design.
  3. Mold Design/Construction – An optimal mold will be constructed from the proper tool steel and machined to compensate for material shrink properties. Other features may include adequate cooling/heating channels for maintaining a consistent mold temperature, a balanced runner system and adequate gate(s) size and location for optimal flow for the specific material being molded, adequate vents to eject the trapped gases, minimal shut off surfaces, minimal moving parts, a balanced layout, proper ejection, and positive locking to ensure alignment of mold halves when closed.
  4. Machine/Auxiliary Equipment – An optimal molding machine will have the appropriate clamping force for the size mold and the appropriate barrel size for the shot size of the part(s) and runner. Additionally, it should have enough clamping and injection pressures to keep the part from flashing and allow proper filling and injection speeds.   Auxiliary equipment should be able to support the temperatures and other parameters required by the process. A very important requirement is to have a proper dehumidifying dryer for condensation polymers (PC, nylon, etc.)
  5. Production Process Set up – The mold must be properly installed in the machine and the controls set up to achieve optimal flow and packing of the material in the cavity (or cavities). It should also be set up to achieve the optimal cooling condition so the parts can be ejected at the recommended temperature to minimize post molding shrinkage, distortion, or warping. The ability to optimize the molding process with minimal variation will be limited by the first four inputs. When a process is limited by one or more of the first four inputs, machine setting adjustments may need to be made during the molding process to compensate for excessive variation.

The process capability or its ability to produce parts with consistent quality within and across multiple runs is dependent on all these inputs. These inputs are progressively dependent on one another so trying to compensate for any deficiencies from prior inputs at the production process set-up phase becomes increasingly more difficult to achieve.