Injection molding is a cost-effective choice for manufacturing simple and complex parts for prototyping as well as low and high-volume production. Many sizes of parts and components can be successfully injection molded; however, large parts call for extra attention to certain aspects of the process. Factors such as polymer properties, press tonnage, and mold design are critical for minimizing the risk of gaps, errors, and scrap.

Here, we’ll look at some of the details specific to manufacturing large parts with injection molding, the factors that differ from making smaller pieces, and some forward-thinking trends to watch for in this industry.

Large Part Injection Molding Process

Larger parts require higher tonnage presses than small parts. High tonnage is needed to clamp the halves of the mold together. There is no strict definition of “large,” though the rule of thumb is 2 to 8 tons per part square inch. For example, a 10×10 part will require between a 200 and 800 ton press to clamp the halves of the mold. Smaller parts often require anywhere from 5 to 100 tons, medium parts usually need less than 500 tons, and large parts may need from 200 tons up to or beyond 3,000 tons.

In addition to tonnage, the major considerations for large part injection molding include:

  • Shot size: An optimal shot size to adequately fill the mold without excess (a 10% overage is often recommended to reduce the risk of voids or gapping)
  • Injection pressure: This creates an efficient flow of the material to fill all cavities
  • Gates: These allow all cavities and part features in the mold to be properly filled with the molten material
  • Melt temperature: This promotes flow without scorching or overheating the material
  • Cooling regulation: Constructing a mold with sufficient vents and cooling channels to aid efficient cooling while minimizing part shrinkage

Taken together, these factors determine how well the material fills the mold and how the part cools, which is critical to preventing structural weaknesses or poor-quality surfaces. Some trial and error is always involved, though computer modeling and simulation are used to assess these factors and identify potential challenges or failure points before making test parts.

Materials Selection & Distribution Techniques

The specific application is the biggest factor in the material choice for injection molded parts. Options like polypropylene, polystyrene, ABS, nylon, PET, or polycarbonate have different properties that make them suitable for specific end uses.

It’s also important to consider how these materials behave during melting, molding, and cooling. Properties like flow, melting point, viscosity, heat transfer, and estimated shrinkage on cooling affect the manufacturing process and decisions about mold design and tonnage.

Depending on the behavior of the molten material and the complexity or geometry of the part, molds can be created with multiple gates or structured for sequential filling to ensure uniform distribution and void-free material throughout the mold. Striking the proper balance between material choice and distribution techniques is key to achieving optimal results in large part injection molding.

Large Part Injection Molding Challenges

There are some unique challenges involved in large injection molding parts. Identifying them early in the process makes it easier to plan ahead and increases the odds of a successful finished project. Common large part challenges include:

  • Warping and bends due to poor material flow
  • Visible sink marks due to inconsistent material flow
  • Degradation of the material’s mechanical characteristics (e.g., tensile strength, resiliency, stiffness, dielectric constant)
  • Lengthy cooling times and resulting bottlenecks or inadequate cooling
  • Controlling for material shrinkage
  • Cycle time management

Trends in Large Part Injection Molding

Today’s large part injection molding manufacturers use technology, equipment, and materials that make the process more efficient and result in consistent and reliable parts.

Industry 4.0 and the Industrial Internet of Things (IIoT) have introduced technology and capabilities that give manufacturers an edge in all types of manufacturing. Some examples of new and emerging technologies at use in injection molding are:

  • Computer simulation and computer-assisted drafting (CAD) software to help with mold design and predicting potential points of failure.
  • Sensors in presses and molds that monitor temperature, pressure, and filling, and report real-time conditions via wifi. This allows manufacturers to make on-the-fly adjustments to things like shot size, injection pressure, or cooling time.
  • Automation equipment including robotics, vision inspection, and conveyance systems that speed up cycle times, identify and eject defective components quickly, and transfer finished moldings to their next station efficiently.

Large Part Injection Molding Services From Lerner Molded Plastics

At Lerner Molded Plastics, we manufacture injection molded parts of all sizes, including large parts. We operate presses with a range of clamp pressures from 30 to 3,000 tons, and work with shot sizes from 3 to 600 oz. for molds with up to 64 cavities. In addition to injection molding, we offer rotational and blow molding, thermoforming, extrusion, and custom color options. We are ISO 9001:2015 certified and work with production volumes from prototypes to thousands of pieces.

Contact us today to learn more about our large part injection molding capabilities or to request a quote.

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