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Tooling and Mold Design: Injection Molding

Views: 6     Author: Site Editor     Publish Time: 2022-08-10      Origin: Site

Plastic injection molding is a precision manufacturing process in which molten resin is injected into a pre-designed mold. As the part cools and hardens, it is removed from the mold for a final touch-up. Tooling and mold design is an essential aspect of injection molding.

 

Mold design and the components involved (often called tooling) is a very complex process requiring a high level of technical expertise. In addition, the process requires engineering skills to produce plastic parts with precise dimensions and design features.

 

Mold engineers and mold designers need to accurately calculate gate sizes (for proper filling) and the best techniques to produce mold durability.

 

In addition, it is essential to carefully design the runner and gate system to distribute the plastic resin in the mold evenly. 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.

 

The article describes the critical components involved in mold design, material selection, and the key steps in tooling.

injection molding 

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.

 

The mold consists of two main parts: the core and the cavity. The "part cavity" is the space or void that receives the injected plastic resin. Various production requirements call for "multi-cavity" or serial molds to create multiple identical parts or different components of a plastic part at once.

 

The following are the critical components involved in mold design.

 

Gates

The runner is the channel through which the molten resin flows. The gate, as the name implies, is the opening at the end of the runner channel through which the resin enters the mold cavity.

 

Various gate types are designed according to different requirements. However, the key factors determining gate type, size, shape, and location are cooling time, tolerance, filling pressure, and optimum flow conditions. In addition, the gate needs to be carefully positioned 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 by using the mold wall's draft angle (taper).

 

The mold designer needs to carefully evaluate the degree of draft angle by considering various factors such as part design and complexity, resin, cavity depth, texture, shrinkage, etc.

 

The draft angle can 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. Tooling and mold design play an important role in determining 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 molded surface to create the desired result. For example, instead of adding symbols to pop-up plastic parts, designers usually include them in the mold design.

 

That said, textures are not just needed for combining design, logo, 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.

 

Material selection for molds

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 for injection molding machines.

 

While steel is more expensive than aluminum and other metals, its high strength and durability are prevalent in the industry and 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.

 

Considering steel's physical properties, such as hardness and brittleness, is essential. The more complex the steel, the more brittle it is. While hard steel is ideal for glass-filled polymers that will wear mold parts, it is not usually a good choice for side-loaded mold parts (because it breaks easily).

 

There are no two opinions about the benefits of steel as a mold design material.

 

However, aluminum also has advantages that are needed in certain situations. Its fast cooling properties make it a good material for tooling. In addition, as a softer metal, it is more accessible to machines so that molds can be built faster (thus reducing production cycle time).

 

Aluminum is often the material of choice for prototypes and short runs due to its cost-effectiveness, faster production cycle times, fast cooling times, etc.

 

Finally, in some areas, hybrid molds (made of steel and aluminum) are also used in the plastic injection molding industry. Copper alloys are also used, but not as often.

 

Other options include coating steel and aluminum molds with materials such as nickel boron or nickel PTFE to improve durability and produce better tool and mold design features.

 

Steps involved in tooling

Tool and die design is a complex process combining the skills of various specialists, such as tools. It dies, mold designers, material engineers, manufacturing specialists, quality inspection specialists, laboratory technicians, etc.

 

The following are the critical steps involved in the tooling.

 

Feasibility

This is where the design and tooling teams work together to determine the tooling materials, features, product design specifications, operational issues, improvement needs, etc.

 

The feasibility phase involves looking at potential problems that may arise from the design geometry. In addition, this phase considers aspects such as special tooling and mold design requirements.

 

In addition, the engineering team works together to understand the physical and chemical properties of the selected plastic resin to select mold materials and review aspects such as mold design, mold flow assessment, gate locations, and cooling conditions.

 

Finally, tooling specifications were 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, the 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 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.

 

Using the tooling for sample preparation

Once the molding process and parameters have been defined, initial samples are 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 structure 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.


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