Manufacturers can now use gas-assisted injection molding to produce cost-effective, large aesthetic parts, thanks to a new polyester blend resin that achieves the desired appearance right out of the mold.

Developed to overcome challenges in the gas-assisted injection molding process, the new plastic material is a15 percent glass-filled polybutylene terephthalate (PBT)/polyethylene terephthalate (PET) blend with aneconomical raw material cost.The new material:

• Features high-surface gloss and high flow for long flow channels.
• Develops high gloss parts with no increase in cycle time.
• Is colorable in different colors.
• Maintains a good balance of mechanical properties.
• Fills 760 millimeters (mm) with one gate due to a high-flow rate.

The new resin formulation is designed to allow the PET in the blend to reach the surface before the PBT, which results in the best surface appearance. The formulation, which contains a color package for high gloss, requires a hot mold temperature to achieve the resin’s high gloss and best appearance, and to trigger the crystallization of the PET.

It’s already proven to be a cost-effective alternative to thermoset plastic and painted metal for a standard oven handle that is 760 mm long with a 30- x 20-mm cross-section (30" x 1.25" x 0.75"). Developed for the appliance industry, the glossy thermoplastic handle is considered to be a large part in gas assisted injection molding but has better surface aesthetics than previously possible, even though the oven handle has a high flexural modulus, and is molded with this new glass-reinforced PBT/PET blend.

This glossy large thermoplastic handle is a milestone, especially when you consider that in the mid-1990s a one-piece, gas-assist textured handle was news because it could replace three-piece plastic and metal handles and reduce the weight of the handle by 45 to 55 percent.

Typically, there are three approaches to achieve high gloss:

1. Modify the surface of the mold to high gloss — The tool surface can be polished to achieve a high-gloss finish on the steel which is transferred to the plastic during molding.
2. Adjust the processing parameters to increase gloss — In case of processing parameters, raising mold temperature increases the gloss in unfilled resins. However, in the case of glass fiber reinforced resins, the holding pressure is important too. In gas-assist injection molding the mold temperature and gas packing pressure are the key factors to the desired high gloss when the material is glass fiber reinforced.
3. Develop a material that is inherently high gloss right out of the mold — To develop an inherently high gloss resin, it is important to develop a smooth surface to increase the reflection from the surface. This option is used when tool modifications or processing conditions do not permit achieving the gloss level, or the application requirements are not met.

As a material supplier, the decision was made to develop a polyester blend resin that improves the current level of gloss in commercial applications. The new formulation is colorable and maintains excellent indoor UV stability, scratch resistance, stain resistance, mechanical strength for bearing weight and creep resistance, and chemical resistance to detergents, cleaners and chemicals.

The material colors used to mold the glossy large thermoplastic handle were bisque, black and white. A “60 degree” gloss meter measured gloss levels on a sample size of five parts for each color studied and the average of each was used for the results. It is difficult to measure the gloss level on the backside of the part because of the shape of the grips, so only front “show side” is measured, which provides an 86.7 percent level. The gloss value is very close to a conventional injection molding part with a gloss level from 90 to 100 percent, and greatly exceeds a 55 percent gloss level for parts injected with existing commercial grades.

Now, thanks to the development of this new polyester blend resin, manufacturers can now use gas-assisted injection molding to produce cost-effective, large aesthetic parts that replicate the surface of the mold.They can achieve gloss levels that are very close to conventional injection molding parts with a new high gloss polyester blend that is better than existing commercial grades when injected into the same mold at the same processing conditions.

For additional technical information about this topic, read the Ticona white paper on "Gas-Assisted Injection Molding of High Gloss Large Parts."