Plastic Injection molding is a precise manufacturing process where molten resins are injected in a predesigned mold. As the part cools and hardens, it is then taken out of the mold for adding finishing touches. Tooling and mold design are an important aspect of injection molding.
Mold designing along with the various components involved (commonly known as tooling) is a very complex process that requires high levels of technical expertise. Additionally, the process calls for engineering know-how to produce plastic parts with accurate dimensions and design features.
Tooling engineers and mold designers need to work out accurate calculations on gate sizing, (to get proper filling) and the best techniques that produce tooling durability.
Additionally, it is important to carefully design the system of runners and gates to get an even distribution of plastic resin through the mold. It is also important to consider proper placement of cooling channels along the mold walls to create a homogenous product and eliminate defects that are common in plastic injection molding.
Complex plastic parts require complex mold designs. Various components need to be added to the mold for this purpose. These could include features like rotating devices, hydraulic cylinders, multi-form slides, floating plates, etc.
The article talks about the key components involved in mold designing, material choices and the key steps of tooling.
Mold Designing- Key Components
Mold is the most essential part of the plastic injection molding process. Tooling and mold design define the success of the entire project.
Molds consist of two main parts: the core and the cavity. A “part cavity” is the space or the void that receives the injected plastic resins. Various production requirements use “multi-cavity” or family molds to create multiple identical parts or different components of a plastic part in one go.
Following are the key components involved in mold designing:
Runners are the channels through which the molten resin flows. Gates, as the name suggests, are the openings at the end of the runner channels through which the resin enters the mold cavity.
Various gate types are designed depending on different requirements. However, the key factors determining the type, size, shape, and location of a gate are cooling time, tolerance, fill pressure and optimum flow conditions. Additionally, gates need to be carefully located to avoid defects like flow marks, warping, and shrinkage.
Once the resin is successfully injected in the mold cavity, allowed the cooling/hardening time, the next step is to remove the product from the mold without damaging the part. This is done by using a draft angle (taper) to the mold walls.
Mold designers need to carefully assess the degree of draft angle by taking into account various factors like part design and complexity, resin, depth of cavity, texture, shrinkage, etc.
Draft angles can vary from 1 to 5 degrees depending on the part. The deeper a mold cavity, the higher is the draft angle required for taking out the finished part.
Surface finish is an important aspect of part design. Tooling and mold design play an important role in determining the surface finish.
The surface finish also depends on factors like mold cooling, part cooling, and general temperature control. Different plastic resins require different mold temperatures and cooling time to acquire the desired finish.
Additionally, designers often include patterns and textures on the mold surface to create the desired end result. For example, rather than working on adding a symbol on an ejected plastic part, designers often include it in the mold design.
That said, textures are not just needed for the incorporation of design, logos, and symbols. They are also required for functional purposes like improving the grip of a plastic handle, etc. Various textures like matte, grains, gloss, patterned, etc are included as a part of tooling and mold design.
Material Choices for Molds
Molds are made out of metals like steel and aluminum. Aluminum molds were conventionally used for plastic injection molding. However, steel has fast become the preferred option for injection molders.
While it is true that steel is more expensive than aluminum and other metals, its high strength, durability are very desirable within the industry and more than make up for its high cost.
Steel is used in hardened (heat-treated) or prehardened forms. Hardened steel, as the name suggests, is superior in terms of strength and has higher wear resistance.
When considering steel, it is important to take into account its physical properties like hardness and brittleness. The harder a steel, the more brittle it is. While hard steel is great for glass-filled polymers that can wear down tooling components, it is often not a good choice for mold components that are sideloaded (as it can easily crack).
There are no two opinions about the benefits of steel as a mold design material.
However, aluminum too has its advantages which are required in certain scenarios. Its rapid cooling properties make it a great tooling material. Additionally, being a softer metal, it is easier to machine and hence makes it faster to build molds (thereby reducing the production cycle time).
Owing to its cost-effectiveness, faster production cycles, quick cooling time, etc, Aluminum is often a preferred material for prototypes and short runs.
Finally, hybrid molds (made of steel as well as aluminum in some areas) are also used in the plastic injection molding industry. Copper alloys are also used, although not as commonly.
Other options include coating steel and aluminum molds with materials like nickel-boron or nickel-teflon to improve the durability and produce better tooling and mold design functions.
Steps Involved in Tooling
Tooling and mold design is a complex process that combines the skills of various experts like tooling engineers, mold designers, material engineers, manufacturing experts, quality check experts, lab technicians, etc.
Following are the key steps involved in tooling:
This is the stage in which the design and tooling team works together to determine the mold materials to be used, functionality, product design specifications, operational issues, need for enhancements, etc.
The feasibility stage involves looking at any potential issues that may come as a result of the geometry of the design. Additionally, aspects like special tooling and mold design requirements are considered at this stage.
Further, engineering teams work together to understand the physical and chemical properties of the selected plastic resins in order to select the mold material and review aspects like mold design, mold flow evaluation, gate location, and cooling conditions.
Finally, tooling specifications are finalized to purchase the required components.
Designs are created in 2D and 3D to give an accurate idea of the mold geometry and sizes. Final designs are created once the preliminary designs are reviewed and approved.
Final designs are created using a tool builder. Specifications are fed into the tool designer to create a mold after final adjustments have been made.
Constructing Primary and Secondary Tools
Tool drawings are prepared along with a review of the construction standards. Once the drawings are verified at all engineering levels and specifications fed into the tool builder, its progress is closely reviewed until mold completion. Completed molds are then inspected for final approvals.
Using the tool for preparing samples
Once the molding process and the parameters are established, the initial samples are produced. These are prepared using defined molding practices. Sample parts are then sent for a final check and qualification.
Final tool corrections
Upon inspection of the sample produced, new adjustments can be recommended for the tools. If the samples are approved, tool construction is verified and documented to be used for future productions. Plastic parts are created using these tools and submitted to the customer for approval before starting the final large scale production process