GAS-ASSISTED INJECTION MOLDING OF HIGH GLOSS LARGE PARTS

Cruz, Miguel*, Trejo, Adalberto†

Grupo Celanese, S. de R. L. de C.V.*, Industria KI de México, S.A. de C.V.† Av. Insurgentes Sur, 2453-201, Col. Tizapán San Angel, C.P. 01090, Del. Alvaro Obregón, México.

Teléfono: (55) 54-80-94-23. E – mail: mcruz@celanese.com.mx,

RESUMEN El proceso de inyección asistida por gas, permite la fabricación de piezas largas de sección transversal gruesa. En piezas de apariencia, el nivel de brillo es un requerimiento importante en sectores de mercado, tales como el de electrodomésticos. Nuevos materiales plásticos se han desarrollado con el fin de incrementar el brillo y poder alcanzar la apariencia deseada en un producto al salir del molde. Este artículo explora resinas de poliéster como una manera de conseguir alto brillo en piezas largas y como una posible alternativa de sustitución de resinas termofijas, trayendo consigo ahorros en material y tiempo de ciclo. ABSTRACT Gas assist injection molding allows for the production of long rod-shaped parts with thick cross sections. On aesthetic parts, a high gloss level is a main requirement in some markets, like appliances. New plastic materials have been developed for increasing the gloss level and to achieve the desired appearance right out of the mold. This paper exposes polyester resins as a way to get high gloss in smooth large parts, which are commonly used in oven handles. INTRODUCTION Gas assist injection molding (GAIM) is a method of pressurizing an injection molded part with gas, via a network of flow channels, or directly into a part, to provide the necessary packing force to produce a quality injected molded product [1]. The gas is used to form a hollow cavity inside the part. The steps used to produce parts with gas assist injection molding are: the mold closes, a precise shot of resin is injected, the screw is usually bottomed out, pressurized gas (usually N2) is introduced into the melt of a partly pre-filled cavity at a certain delay time, the screw returns for the next shot, gas pressure is held in the part for the entire cooling phase, gas is vented form the part, the mold is opened, and the part is removed.

There are three main categories of molded parts, which are especially appropriate for the production of gas assisted injection molded parts [2]. The first category includes tube or rod-shaped parts. Typical parts for this category are handles and clutches. The second category consists of large cover-shaped structural parts with a network of gas channels often combined with the rib structure of the parts. Examples are automotive panels and business machine covers. The third category covers complex parts with thin and thick wall sections, where the process is used for part integration by consolidating several assembled parts into a single design. Tube or rod-shaped parts have a one dimensional flow path and the part effectively is the gas channel. The wall thickness is a function of viscosity and temperature distribution within the melt when nitrogen gas is injected. Changes in polymer viscosity, temperature, or gas pressure may play havoc on wall uniformity, so a close watch on these variables is imperative [3]. In the mid 1990s, one-piece gas assist textured handles replaced a three-piece plastic and metal handles. Gas assist was chosen to consolidate parts, minimize material and weight, and reduce cost. The weight of a handle dropped by 45-55%. A decade later, a glossy thermoplastic handle for the range industry was developed. The molded parts had better surface aesthetics, even though the oven handle was a high flexural modulus, glass-reinforced PBT/PET blend [3]. This provided Appliance Manufacturers a cost effective alternative to thermoset plastic and painted metal handles for oven doors [4]. A standard oven handle is 760 mm long with a 30 mm by 20 mm cross-section (30 inches by 1.25 inches by 0.75 inches). It is considered to be a large part in gas assisted injection molding. The gloss on the aesthetic side of a large cover-shaped parts injected with gas assist injection molding has been studied [5]. The gloss in large rod-shaped parts has not been examined because the shape and size of the part do not permit to measure it, or it is very difficult to do so, and for those reasons just visual evaluations are performed.

OBJECTIVE The objective of this paper is to investigate the gloss level in gas assisted injection molding large rod-shaped parts. The gloss measurement study examines a new polyester blend consisting of switching PET and BMC in applications with high gloss like oven handles. GLOSS Gloss is an optical phenomenon observed when examining the appearance of a surface. The evaluation of gloss is equal to the capability of a surface to reflect directed light. Gloss is often used as a criterion to evaluate appearance applications, especially in the case of aesthetic products such as appliances in which the OEMs requires high gloss, but in the case the automotive industry, the low gloss appearance is the trend. Measuring instruments, figure 1, have been used to associate reflection behavior and to define measurement values under certain conditions. A visual gloss examination includes many subjective sources of error and is not sufficient when making comparisons in order to select the best material or process for a specific part. Therefore, to be objective, it is necessary to put a measured value on the amount of gloss. The measurement of the reflectometer is a relative measurement compared to a highly polished black glass with a refractive index of 1.567. The glass has an assigned specular gloss of 100 for each illumination angle considered: 20°, 60° or 85°. Since instrument and standard tolerances are tightly controlled, the measurement error should not be more than +/- 1 unit. HOW TO GET HIGH GLOSS There are typically three ways to achieve a high gloss appearance: (1) modify the surface of the mold to high gloss, (2) modify processing parameters to increase gloss, or (3) develop a material which is inherently high gloss right out of the mold. The tool surface can be polished to achieve a high gloss finish on the steel which is transferred to the plastic during molding, figure 2. Sometimes relying strictly on mold texture is generally ineffective and costly (tool maintenance) to meet the high gloss requirements, but it is important in order to get the desired gloss.

In case of processing parameters, raising mold temperature increases the gloss in unfilled resins, but 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 in order to have high gloss in the part when the material is glass fiber reinforced, figure 3. The disadvantage of raising mold temperature is that cooling time is longer and the cost of the part is increased. The third option is to develop a resin which is inherently high gloss, and achieving the desired appearance 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, figure 4. This option is used when tool modifications or processing conditions do not permit achieving the gloss level, or the application requirements are not meet. New engineering resins have been developed which improve the current level of gloss in commercial applications. MATERIAL An experimental grade of polyester blend with high gloss was selected throughout the study. It is a 15% glass-filled polyester blend (PBT/PET) featuring high surface gloss and high flow for long flow channels. The formulation of this grade is proprietary. The challenges in a polyester blend for a gas assist injection molding process are to develop a high gloss material with no increase in cycle time, it is colorable in different colors, maintains a good balance of mechanical properties, high flow in order to fill 760 mm with one gate, and all at a reasonable raw material cost. In order to have a high gloss, the PET in the polyester blend reaches the surface quicker than the PBT and it provides the best surface appearance. The formulation contains a color package for high gloss. A hot mold temperature is necessary for high gloss and best appearance, and for crystallizing the PET. For polyester blend, an inherently high gloss resin can be formulated to achieve gloss values of 90 to 100 right out of the mold in the conventional injection molding. 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.

EQUIPMENT AND PROCESS The rod-shaped part was produced in a mold of two cavities with a combination of hot runner and cold runner systems. Figure 5 shows the molded part, the front “show side” is smooth. The back side has “bumps” or grip features. The front side must look glossy for best appearance. Figure 6 shows the cross sections of the part. The cavities are highly polished (SPI-A2 as minimum) in stainless steel, preferably for best appearance. The part is 760 mm long, and the cavity is filled with one gate and one gas pin. Gas assisted injection molding experiments were conducted with a 350 ton clamping force. The injection molding machine was utilized in conjunction with a gas injection control unit where the gas and polymer entered the mold at separate locations. The mold cavities were cleaned and polished very well before starting up. The processing conditions for the polyester blend followed the recommended material guidelines, but during the material trials, some parameters were adjusted for the new polyester blend. The drying temperature was increased from 121 to 135°C in order to have a moisture level below 0.01%, required for the PET in the polyester blend. A high surface mold temperature of 128°C was required for best appearance and to crystallize the material. The A side, or stationary side of the mold, was hotter because it is the show side of the part versus the moving side. The material required high melt temperature of 270°C for covering the fiberglass on the surfaces of the parts. The temperatures of the hot runner system were setup slightly lower than the melt temperature. In the case of temperatures, the goal was to find a balance between appearance, cycle time, and avoiding ejector pin push on the part because the material was too soft coming out of the mold, figure 7. At times, the gas pin got dirty and clogged with material and the gas could not vent out of the part and the parts ruptured or “popped” when the mold opened. This gas assist process was also optimized in order to produce the highest quality part, by minimizing the cycle time and avoiding pin push. The gas pressure was 85bar and the hold pressure in conventional injection molding was 950bar. Shot to shot consistency was maintained as accurately as possible for achieving the desired weight of the part. Parts were weighed to verify consistency. The material colors for this analysis were bisque, black and white. A “60 degree” gloss meter was used

to measure gloss levels on the samples. A sample size of five parts was taken for each color studied and the average of each was used for the results. EXPERIMENTAL RESULTS Due to the shape of the grips on the backside of the part, it was a difficult to measure that area, so only the gloss level of the front “show side” is reported. The gloss level was 86.7% on the “show side” of the part. The gloss value was very close to the conventional injection molding parts with a gloss level from 90 to 100%. The parts injected with current commercial grades have a gloss level of 55%. The gas assisted parts replicated the surface of the mold.

CONCLUSIONS

On rod-shaped parts, the gloss level was very close to the conventional injection molding parts when a high gloss polyester blend is used, and it is better than current commercial grades injected into the same mold at the same processing conditions. REFERENCES [1]Paul Dier, Richard Goralsky, Gass Assist Injection Molding, Bauer, 2000, USA. [2]Hansen, Michael, “Application Examples for Gas-Assisted Injection Molded Parts”, Journal of Injection Molding Technology, V0l.3, No.3, September 1999. [3]http://machinedesign.com/article/gas-assist-molding-not-just-for-hollow-parts-0809 (2010). [4]http://kiindustries.thomasnet.com/item/all-categories/injection-molding/item-1003?&forward=1 [5]Cooper, C., Experimental Studies in Gas-Assist Injection Molding to Investigate Residual Wall Thickness and Gloss Levels, SPE Technical paper, 2005.

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