Gas assist molding offers a variety of process and design advantages for medical equipment applications. It produces parts that are smooth and extremely cleanable, while offering increased strength and rigidity, weight reduction, design flexibility, and improved surface finish. Gas assist may also lower production costs by reducing cycle time compared with producing solid sections.
To take advantage of these process and design advantages, it is important to pay careful attention to gas entry and exit locations, select the geometries that will work best with the design, and avoid sharp corners that may lead to weaknesses. Collaborate with a gas assist injection molding specialist who can help design for manufacturability. This article presents useful tips for making tubular structures that are strong, smooth, and cleanable.
Gas Assist Injection Molding Basics
Gas assist injection molding is a process in which a pressurized gas (usually nitrogen) is injected into the molten plastic melt stream (see Figure 1). The gas produces a bubble that pushes the plastic into the extremities of the mold, coring out the part and leaving a hollow tube-like cross section.
Two kinds of parts can be produced by gas assist injection molding. Closed channel parts include tubes, armrests, handles, and frames. Examples of open-channel parts are access covers, panels, shelves, and chassis. 1
Almost any thermoplastic material can be used in a gas assist application, including polycarbonate, polyphenylene oxide (PPO/Noryl®), polybutylene terephthalate (PBT/Valox), acrylonitrile butadiene styrene (ABS), PC+ABS, polyamide (nylon), and high-impact polystyrene (HIPS), as well as polypropylene, and high-density polyethylene (HDPE).
High-end medical housing applications often use materials such as PC+ABS, Noryl, or polycarbonate. These materials are more rigid and look attractive when painted. They have a great finish and solid feel, but of course are much lighter. Nylon is frequently used for canoe paddles due to its overall ruggedness, while handles for many applications are made out of polycarbonate or PBT.
The ability to create hollow thick parts, or thick sections within parts, enables production of large ribs and results in a higher stiffness-to-weight ratio in structural parts. It facilitates the molding of large cross sections, which may lead to parts consolidation. The gas assist process pushes plastic against the cavity of the mold, producing an attractive flat finish, eliminating such irregularities as ripples or waves — offering a very smooth surface appearance compared with structural foam.
Dimensional stability is also increased with gas assist injection molding. The uniform packing from within the cavity reduces stress within the part, while also reducing warping and sink marks — indentations or dimples that are caused by thick areas on the front of a part.
Making parts using gas assist injection molding can provide incredible strength to plastic parts. Tubular sections produced are stronger than a flat or even a solid plastic piece. At the same time, the process reduces weight. For example, PSI Molded Plastics, which specializes in injection molded component requirements from large parts to small and from low to high volumes, uses gas assist injection molding to produce a variety of handle designs that reduce the weight by as much as 50 percent compared with traditional injection molding. For parts where maintaining strength is a high priority, gas assist designs can reduce weight by 30 percent and increase strength at the same time.
The process offers a great deal of design flexibility. Instead of a solid tube with a 1-in. diameter, gas assist produces a thin wall that is a tube, with all the material removed from the middle. This gives designers the ability to add thick sections as required and core them using gas assist technology.
Producing Tubular Structures
Gas assist has been around for decades, but its use in medical equipment is growing, since plastic works so well where cleanliness and cleanability are crucial. Manufacturers have to produce tubular structures for medical equipment that are smooth, with no kinks, nooks, or crannies that would make them difficult to clean.
Plastic tends to be easier to clean than sheet metal, especially for tubular structures in which internal areas can be hollowed out. Exercise, rehabilitation, and physical therapy equipment is another good candidate, since it is essential to be able to clean the surfaces of such equipment easily.
The latest entries into the gas assist arena are based on new exotic carbon-filled materials. Customers are taking engineered plastic resin — which is already lightweight — and then using gas assist to make it even lighter. They are also combining the use of gas assist with structural foam (using a blowing agent inside of plastic, which also reduces weight and increases strength) to obtain the advantages of both technologies.
Design Considerations. Here are a few key design considerations to keep un mind during this process. First and foremost, follow these two general rules:
Do not try to use gas to solve fundamental design problems. Some designs are not going to work.
Do not split the flow of gas into several different directions because it will be too hard to control. Use one continuous stream.
Gas Entry and Exit Locations. The ideal situation is one in which the gas enters at one end of the part and exits at the other end. Think of the tube as a straw — you want to basically blow the “water” out of the straw to make the tube hollow.
There are other options that also work. The gas can be inserted into the same point as it is vented, akin to blowing a bubble, holding the bubble, and then letting the gas come out from the same location. Another option is to insert a number of gas pins in specific spots to inject the gas.
These spots should be located where there is some kind of thickness problem; inserting the gas there would eliminate the sink mark or blemish that would otherwise appear. Or, one can insert a number of small holes as vents in the back part of the mold, and blow gas in the core side of part, which forces the material against the cavity. These are ways to achieve a clean definition and a nice first surface finish without having necessarily the ideal geometry on the back side.
Typical Geometries. Ideally, design gas channels should be round, or as close to round as possible. The bubble is always going to be round, so it is best to surround it with a round part. They need not be perfectly round — ovals or half-round geometries work well, particularly for reinforcing ribs on the back side of structures. Most handles are designed to be oval for cosmetic or aesthetic reasons. Sometimes, trapezoidal geometries may work; however, these may result in thicker material in the corners, which is usually not desirable.
Sharp Corners and Wall Thickness. Sharp corners should be avoided in gas areas, because the plastic tends to get too thin. Ensure that there is a large enough radius to create a better thickness distribution. Wall thickness can be ¼, or ½ in. thick. Thickness can be changed by varying the gas pressure, or by changing the temperature of the mold or resin.
Gas Assist Molding Success Stories
Gas assist injection molding works very well for medical equipment parts where the outer side is visible to the user and the backside is not visible. The process makes the structure far stiffer than could be done with typical injection molding. Manufacturers can simulate a higher end material by putting the structure on the back side and leaving the visible surface with the desired finish.
PSI Molded Plastics, for example, has experience helping customers successfully use gas assist to solve challenges with medical equipment applications, as well as numerous other industries. In one case, a manufacturer had a long handle with a two-piece clamshell design that was screwed together. The 4-ft tubular handles were expensive and were not aesthetically pleasing, because the two-piece design didn’t fit together properly. The gas assist process was used to combine the pieces into one long tubular part, eliminating all the screws and the seam between the two halves. Consolidating it into one piece eliminated the customer’s appearance and fit problems.
Gas assist can be a great tool for many medical equipment applications and many designs. Every application is different, and it is important to collaborate with a gas assist injection molding specialist to develop a design that is manufacturable — so the mold and the part do not cost a fortune, and the process will run at a reasonable cycle time. The goal is to design a good part that can be made efficiently and have it look good too.
This article was written by Gerry Gajewski, Vice President of Engineering for PSI Molded Plastics, Wolfeboro, NH. For more information, visit here.