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Polymer Design

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Liquid Resin Casting Ideal For Low Volume Plastics

Liquid Resin Casting (LRC) is an economical way to produce high quality, complex parts. The technique is based on pouring materials that are liquids at, or near, room temperature into various types of molds, and then cured, or solidified, by the addition of heat. The materials used, called thermoset resins, undergo an irreversible reaction.

The process has its greatest commercial value in prototyping and low volume production. Compared to high volume methods, LRC provides lower tooling costs and a generally shorter tooling cycle, though higher per unit costs.

The greatest economic benefits occur with unusual geometries that are too costly to machine and annual volumes that are too low to justify the expense related to high volume procedures.

The viscosity of most thermoset resins enables them to be poured readily into molds without the need to force them in under pressure. And the temperatures required for an initial cure are typically 150 to 200 F. These relatively mild processing conditions allow the use of non-traditional mold media, such as RTV silicone. Other mold materials commonly used include steel, aluminum, epoxy, or polyurethane. These materials can be used in combination with each other to take advantage of their individual attributes.

Polymer Design approaches resin casting from a production, rather than a prototype, standpoint. We use materials that have adequate pot life to permit vacuum degassing, which eliminates entrapped air. This helps ensure that we produce void-free castings.

Formulations of this type need 10 to 12 hours to complete an initial cure. As a result, most molds are limited to one cycle per day. Typical production runs entail the use of multiple copies of molds to support the anticipated production rate.

The various casting resins, such as epoxy, urethane, silicone or a copolymer of one of these, may be compounded with fillers, reinforcement particles or fibers to improve specific properties such as hardness, density, impact strength, resistance to temperature variations, and chemical resistance. Typical filler materials include glass, alumina, aluminum powder, silica, and iron.

Real World Application

Harvest Technologies Inc. of Norwell, MA needed assistance in producing a first-of-its-kind blood filtering device that was still under development. Named 'BloodStream', the unique blood recovery system collects and filters a patient's blood dur ng surgery, while causing little or no damage to red blood cells. Harvest needed three vital parts to begin production of the BloodStream: the pump housing, vacuum socket, and the pump nest.

It was important for Harvest to find a way to get parts produced quickly in low quantities so they could begin the lengthy Food and Drug Administration approval process. With this challenge in mind, Harvest decided to experiment with the LRC process.

Without an in-depth knowledge of LRC, Harvest developed designs for one of the BloodStream's parts, the pump housing, and had a prototype produced. After learning more about the complex designs that could be accomplished through LRC, Harvest completely redesigned and cast the pump housing and the two other components. "LRC showed us some things that we were not aware could be done," said Wes Verkaart, co-founder of Harvest. The result was three parts that were internally more complex than initially planed, operated better, and simplified assembly of the finished device.

The Pump Housing - By using multi-part, interlocking cores, Harvest was able to maintain the critical relationships that were required between the motor, the drive shaft, and the rotor assembly within the pump housing. The use of LRC eliminated what would have otherwise required complex and expensive secondary machining, while allowing the assembly to go together easily and reliably. 

The Vacuum Socket - The vacuum socket was made with elaborate interior thread designs, undercut o-ring grooves, and air channels with 90 and 180 degree bends. These complex interior designs would have been extremely difficult to do with any fabrication method other than LRC. Had another method been used, additional fittings would have been required, increasing production costs. 

The Pump Nest - The pump nest locates and latches down the BloodStream's sterile disposable reservoir that collects the blood. This one part combines many features that would have normally been made by assembling a number of complicated machined parts. The relative ease with which changes can be implemented on aluminum patterns allowed Harvest to try several rounds of radical redesign to determine the best way to achieve their requirements. 

LRC provided Harvest with two other advantages: ease of tooling change and quick turn around. Refinements made to the BloodStream during final product development required umerous changes to the pump housing, vacuum socket, and pump nest before the final parts were approved. Adjustments with other tooling methods would ordinarily pose a very difficult and expensive process. But with LRC, changes can be made relatively easily and inexpensively.

Typically in the LRC process, as was the case with Harvest, an aluminum master that closely resembles the final part is used to create a silicone mold. The master is fairly easy to adjust if changes need to be made. This is unlike other processes, such as injection molding, which use steel molds. The difference between machining an aluminum master and a steel mold is substantial. It is much more expensive and takes longer to adjust a steel mold than to rework an aluminum master. Also, with injection molding, there can never be a 'clean' change made to the mold. Because the injection mold requires significant amounts of heat and pressure, there is always the danger of a seam line opening or part of the mold breaking off after several changes.

"When you are trying to develop a new product at low volume, you can either resort to machining parts, or you can go directly to a process like LRC that you can use for production. So, when the part is dialed in, you are ready to produce," said Verkaart. "With a prototyping process that is not suitable for production, you have to go through two steps -- one is to make the prototype, and then, basically, you have to start all over again for the production process. With LRC, once you're done, you're done."

About a year ago, Harvest had little more than design sketches and an idea for a new blood recovery system. Recently, the BloodStream received FDA approval and will be used in hospital operating rooms around the country later this year.

Liquid Resin Casting offers the low volume manufacturer an opportunity to create and execute first class designs, while minimizing capital outlay and risk often associated with producing tooling for high volume molding techniques.

A wide range of formulations, tooling techniques and compatible secondary operations enables the successful production of parts across a broad range of application areas.

This technical information has been contributed by
Polymer Design

Click here to find suppliers

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