Lead is commonly used in x-ray-based medical devices as a housing or component material to limit radiation exposure to operators and patients. Its high density (specific gravity: 11.35) lets it absorb x-rays. It's also manufacturing-friendly due to its relatively low cost and malleability. However, lead's toxicity makes it difficult to handle and dispose. And judging from the increase in regulations limiting the use of lead in other applications, governments may eventually restrict its use in medical devices. The European Union's RoHS (Restriction of Hazardous Substances) directive, for example, already restricts use of lead in solders in electronics. Although medical devices are currently exempt from RoHS regulations, finding alternative solutions is of keen interest.

Other metals and alloys such as tungsten, tungsten alloys, and molybdenum alloys have been investigated as lead replacements, but they do not have lead's manufacturability. One thermoplastic, however, offers a promising alternative. It has the potential to effectively shield x-rays while making it relatively easy to injection mold complex designs.

The material, LNP Thermocomp HSG PH1100B, is a high-density composite from Sabic Innovative Plastics. In fact, HSG in this nylon-based thermoplastic's name refers to high specific gravity. To introduce the material, it's useful to see how it compares to lead as an x-ray shield, review a few design guides for it, and note manufacturing considerations.

To quantify the material's effectiveness as an x-ray shield, researchers conducted several tests, using lead as the baseline material. X-ray measurements were made using 120kV and 140kV at 5mA to match vascular and CT radiography applications. Results showed that at equivalent thicknesses, the HSG PH1100B's performance was equal to or better than lead at the two voltages tested.

HSG PH1100B has a specific gravity of 11 making its viscosity relatively high compared to other filled materials traditionally used in standard injection molded applications. To manage the viscosity, the following ideas and guidelines can help design parts and process the material.

Part and tooling design. HSG materials are special formulations, so mold simulations may be inconsistent. Most mold-flow-calculation models are based on a user-defined range of specific gravity of about 1 to 1.5. HSG PH1100B is roughly 10 times that, which may distort the calculations and provide unrealistic results. Therefore, when designing a part for an HSG formulation, let an experienced tooling engineer review the design.

In addition to standard tool-making practices, use standard tool steels often selected for glass-fiber applications to ensure extended tool life.

The aspect ratio of the PH1100B material is not expected to be beyond 1:1. This means designers should use standard plastic-design principles for unfilled resins without need to consider anisotropic material properties. In addition, the filler has no tendency to align with the flow direction as is the case with materials that use higher aspect-ratio fillers, such as fibers.

Minimum target wall thickness should be about 3 mm. However, runner length, gate design, and vent depth will need special attention to enhance performance from tool and part. For example, flow length is short, requiring short runner systems in larger parts to allow for longer in-cavity mold flow. Small parts can be produced in average-length runners, but this approach should be avoided. Smaller parts using shorter runners should allow longer in-cavity mold flow, though, of course, the total flow length will still be limited.

Pin gates are not recommended because they may promote material separation due to high-shear conditions in the first-stage injection. Edge gates and fans are recommended for enhanced material properties and finished-part aesthetics. Edge gates and fans produce lower peak pressure during first-stage injection due to the generous flow channels. Flow lengths will increase due to less restriction. As a result, the final molded part will likely have better properties and aesthetics. This approach will also result in lower material-shear heating. Shear can cause separation of resin and filler, potentially leading to unsatisfactory aesthetics, performance, and physical properties of the final molded part. Depending upon part and machine, hot runners may be used to maintain melt temperature and reduce shot-to-shot variation.

To manufacture HSG parts, start with standard injection-molding equipment. Be aware, however, that the material can require high injection pressures, sometimes pushing the upper limits of standard machines. First-stage filling works best beyond the 95% mark. Use second-stage filling to control part dimensions. When shopping for new equipment to handle such materials, consider high rate / pressure (HRP) machines.

Machine size and peak pressure are usually part dependent with respect to dimensions and final weight. The rules of thumb are:

• For a smaller part, use a smaller barrel and screw for a higher peak-pressure capability.

• For a larger part, use a larger barrel and screw.

When purchasing a new machine, consider including an accumulation package for an increased range of high peak pressure. Alternately, size the barrel for the part to be molded. It is often preferred to have injection pressure in reserve than have a machine maxed out on every shot. HSG experts can help in sizing the equipment.

Also, general-purpose screws are acceptable when equipped with free flow tips, and low-compression-ratio screws with a target range of 2:1. This ratio indicates the channel depth in the feed zone to the channel depth in the meter zone, and is the final compression ratio of the screw.

Barrier screws and mixing sections cannot be used when running HSG formulations.

HSG is generally easy to process despite its relatively high specific gravity. When processing HSG grades, use middle to high-temperature ranges for melt and mold temperature. For PH1100B, melt temperature would be 270 to 280C and mold temperature would be 90 to 100C.

A few properties for PH1100B

However, when anticipating long in-mold periods, monitor material degradation and adjust temperatures according to the cycle. One approach to monitoring is to take melt temperatures and periodically conduct visual inspections for separation during air shots. The adjustment would consist of lowering the melt temperature in accordance with datasheet recommendations. Excessive high temperatures for long periods may separate the resin from the filler, creating the possibility of locking the screw.

Improvements in engineering thermoplastic compounds let designers consider injection-molded solutions in applications calling for x-ray shielding. HSG materials demonstrate shielding comparable to lead yet avoid machining or other secondary operations ordinarily required. LNP Thermocomp HSG PH1100B composite can be adapted to traditional injection molding processes as long as special consideration is given to its thickness, flow length, and viscosity.

PROPERTY	VALUE
Specific gravity	11.1
Tensile strength	32.5 MPa
Tensile elongation	0.50%
Tensile modulus	8,055 MPa
Flexural strength	84.5 MPa
Flexural modulus	16,655 MPs
Nominal impact strength	1.7 kJ/m2
Ultimate imact strength	15.7 kJ/m2
Heat deflection temperature	154.5 C
Surface resistivity	11.6 ohm/sq
EMI (10 Hz to 1 GHz)	86.9 dB
Thermal conductivity	10.7 to 11.9 W/mK
Glow wire flammability index	825 C
Comparative tracking index	<110 V