LPIM (Low Pressure Injection Molding) – Structural Web Injection Molding

Structural Web molding technology is a variation of Low Pressure Injection Molding (LPIM) that was developed by Milacron as a proprietary molding process on our LPIM machines. It is a process that is well suited for large part/large shot sizes with good part surface aesthetics and low molded in stresses. The process also creates a part with a dual wall section (hollow) allowing for 10% to 30% part weight savings over a solid molded part with the same part geometry.

With Milacron’s LPIM machines being multi-nozzle machines, Structural Web molding is possible with an option added to the standard LPIM machine. The process is a multi-point gas assist process that introduces nitrogen into the molten resin after injection into the mold. Unlike other gas assist processes, no gas pins are necessary in the mold for the nitrogen as it is introduced through one or more Structural Web LPIM nozzles.

The Structural Web Process Sequence:

1. Nozzles open to partially fill the mold cavity

2. Nozzles close, then gas is introduced into the molten material in the cavity

3. The nitrogen gas pushes the molten plastic against the cavity walls and fills out the cavity

4. The nitrogen gas is allowed to vent out of the part

5. The finished part is ejected from the mold

Benefits of the Structural Web Process

The Structural Web Process allows for part designs with thick or varying wall thicknesses such as a thicker rib intersection with the main structure of the part, allowing for a stronger part, but without the worry of sink marks opposite the thicker rib (see photo on the left). Also, since Structural Web is not a foaming process like Structural Foam, the surface finish of the part is like an injection molded part that does not have swirl marks on the surface that are normally associated with the structural foam process. Molded in color and texturing that are possible with typical injection molding are used extensively with this process, producing parts that do not need to be painted and can be decorated like a typical injection molded part.

Additional Benefits
The Structural Web Process is also a light-weighting process where between 10% and 30% of the part weight of a solid molded part can be reduced. LPIM machines are offered with very large shot sizes from 200 lbs and up allowing for ultra large parts not possible in typical injection molding. Many types of resins can be used, HDPE & PP are the most popular, but engineering resins have been successful with this process. Many Structural Web applications are families of parts such as control housing panels for an irrigation controller, mop buckets and wringer components, or shelf type utility carts. Additionally, large rubbish carts and rubbish receptacles are also made using this process.

Parts molded using the Structural Web process exhibit higher strength to weight ratios than a solid molded part due to the part design flexibility that allows for thicker ribs and the unique dual wall thickness characteristics of the molded part. This dual wall thickness contributes to improved impact resistance and sound deadening properties in many applications such as ATV components or equipment housings. Since the parts are packed out with the nitrogen gas, they have lower molded in stresses than the same parts molded in typical injection molding at much higher pressures. Structural Web products exhibit many of the same properties and benefits of a roto-molded part, but at a much faster cycle time. Typical cycle times for Structural Web parts are between 120-200 seconds, depending on the thickest wall thickness cross section of the part.

In summary, the Structural Web Process is another unique process that Milacron LPIM machines can be utilized for to make ultra large, cosmetic parts that have special requirements. Due to the multi-nozzle technology of Milacron’s LPIM machines and the ability to run multiple molds simultaneously, higher productivity and lower cost is seen by molding families of large parts together in one machine producing a matched set, or making a single large part with a large part weight and projected area that would not be possible in a typical injection molding machine.

Typical markets served by Structural Web are; Material Handling (Pallets), Institutional (Utility Carts, Rubbish Carts, Mop Buckets), Construction (Drainage Chambers, Telecommunications Enclosures), ATV (Cargo Bins, Trim Pieces), Lawn and Garden (Yard Carts, Utility Trailers), and Medical Equipment (Cabinet Enclosures & Housings).

Milacron has set the standards, driven innovation and led the industry in Multi-Nozzle Low Pressure Injection Molding Technologies for more than four decades. To learn more about the Low Pressure Injection Molding (LPIM) process, check out our previous blog post here. If you are interested in learning more about Milacron’s low pressure injection molding technologies, please visit our website.

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