Injection molding is a precision manufacturing process that allows the molten resin to be injected into a pre-designed mold. As the part cools and hardens, it is removed from the mold for final touch-ups. Tooling and mold design is an essential aspect of injection molding. Mold design and the various components involved is a very complex process that requires a high level of technical expertise. In addition, the process requires engineering expertise to produce plastic parts with precise dimensions and design features.
Tooling engineers and mold designers must accurately calculate gate sizes and the best techniques to produce tooling durability. It is also essential to carefully design the flow path and gate system to distribute the plastic resin evenly in the mold. It is also essential to consider the proper placement of cooling channels along the mold walls to create a homogeneous product and to eliminate defects common in plastic injection molding.
Complex plastic parts require complex mold designs. To do this, various components need to be added to the mold. These may include rotating devices, hydraulic cylinders, multiform slides, floating plates, and other features. This article describes the critical components involved in mold design, the selection of materials, and the key steps in mold processing.
Mold Design – Key Components
The mold is an essential part of the plastic injection molding process. Tooling and mold design determine the success of the entire project.
A mold consists of two main parts: the core and the cavity. The part cavities are the spaces or voids that receive the injected plastic resin. Various production requirements call for using multiple cavities or series molds to make multiple identical parts or different components of a plastic part at one time.
The following are the critical components involved in mold design.
Gates
The runner is the channel through which the molten resin flows. As the name implies, gates are the openings at the end of the runner through which the resin enters the mold cavity.
A variety of gate types have been designed depending on the requirements. However, the key factors determining gate type, size, shape, and location are cooling time, tolerance, filling pressure, and optimum flow conditions. In addition, careful positioning of the gate is required to avoid defects such as flow marks, warpage, and shrinkage.
Mold Draw Slope
Once the resin has been successfully injected into the mold cavity, allowing for cooling/hardening time, the next step is to remove the product from the mold without damaging the part. This is accomplished using the mold pull angle (taper) of the mold walls.
The mold designer must carefully evaluate the release taper, considering various factors such as part design and complexity, resin, cavity depth, texture, shrinkage, etc.
The draft angle may vary between 1 and 5 degrees depending on the part. The deeper the mold cavity, the greater the draft angle required to remove the finished part.
Surface Finish
Surface finish is an essential aspect of part design. Tool and die design play an essential role in determining the surface finish.
The surface finish also depends on mold cooling, part cooling, and general temperature control. Plastic resins require different mold temperatures and cooling times to achieve the desired finish.
In addition, designers often add patterns and textures to the mold surface to create the desired result. For example, designers usually include symbols in the mold design instead of adding symbols to a pop-up plastic part.
That said, textures are optional to merge designs, logos, and symbols. They are also needed for functional purposes, such as improving the grip of a plastic handle. Various textures, such as matte, grain, gloss, patterns, etc., are included in the tool and mold design.
Tooling material selection
Molds are made of metals such as steel and aluminum. Aluminum molds are typically used for plastic injection molding. However, steel has quickly become the preferred choice of injection molders.
While steel is indeed more expensive than aluminum and other metals, its high strength and durability are prevalent in the industry and more than make up for its high cost.
Steel is used in hardened (heat-treated) or pre-hardened form. Hardened steel, as the name implies, is superior in strength and has higher wear resistance.
It is essential to consider steel’s physical properties, such as hardness and brittleness. The more complex the steel, the more brittle it will be. While hard steel is excellent for glass-filled polymers that will wear out mold components, it is usually not a good choice for side-loaded mold components (because it breaks easily).
There are no two opinions about the benefits of steel as a mold design material.
However, aluminum also has its advantages that are needed in certain situations. Its fast cooling properties make it a good material for molds. In addition, as a softer metal, it is more accessible to the machine, so molds can be built faster (and thus reduce production cycle times).
Aluminum is often the material of choice for prototypes and small production runs due to its cost-effectiveness, faster production cycle times, and fast cooling times.
Finally, hybrid molds (in some areas made of steel and aluminum) are also used in the plastic injection molding industry. Copper alloys are also used, although less commonly.
Other options include coating steel and aluminum molds with nickel-boron or nickel-tetrafluoroethylene to improve durability and produce better tool and mold design features.
Steps involved in tooling
Tool and die design is a complex process that requires a combination of skills from various specialists, such as tooling engineers, mold designers, material engineers, manufacturing specialists, quality inspection specialists, lab technicians, etc.
The following are the critical steps involved in tooling.
Feasibility
This is the phase where the design and tooling teams work together to determine the tooling materials, features, product design specifications, operational issues, enhancement needs, etc.
The feasibility phase involves looking at any potential issues that may arise due to the geometry of the design. In addition, aspects such as special tooling and mold design requirements are also considered during this phase.
In addition, the engineering team works collaboratively to understand the plastic resin’s physical and chemical properties to select the mold material and review aspects such as mold design, mold flow evaluation, gate location, and cooling conditions. Finally, tooling specifications are finalized to purchase the required components.
Design
Designs are created in 2D and 3D to understand the mold geometry and dimensions accurately. Once the preliminary design is reviewed and approved, a final design is created. The final design is created using the tool builder. After final adjustments are made, specifications are entered into the tool designer to create the mold.
Building the primary and secondary tools
Tooling drawings are prepared along with a review of construction standards. Once the drawings are verified at all engineering levels and specifications and entered into the tool builder, their progress is closely reviewed until the molds are complete. The completed tooling is then checked for final approval.
Preparing Samples with the Tooling
Once the molding process and parameters have been defined, initial samples can be produced. These are prepared using the defined molding practices. The sample parts are then sent for final inspection and qualification.
Final Tooling Correction
After inspection of the produced samples, new adjustments to the tooling can be recommended. The tool construction is verified and documented for future production if the sample is approved. Plastic parts are created using these tools and submitted to the customer for approval before starting the final mass-production process.
