GLS soft TPEs can be overmolded onto rigid plastic, metal, or other substrates using a variety of techniques, including:
- Insert Molding
- Rotary or Shuttle Table Molding
Molding machines with reciprocating screws are recommended. Additional guidelines are:
- Clamp capacity should be 1.5 to 3 tons per square inch projected area
- General-purpose screw with compression ratio of 2:1 to 3:1 can be used
- Shot size should be 25% - 75% of barrel capacity
- Typical maximum residence time should be 8 - 10 minutes at processing temperature.
- Use a small nozzle orifice to help produce shear heating during injection
- Suggested nozzle diameters are 1/16Â¿ (0.0625" or 1.59 mm) to 3/16Â¿ (0.1875" or 4.76mm)
Excessive moisture levels of either the overmold grade or the substrate can adversely affect adhesion. Drying is required for the Versollan family of overmolding products.
For grades which require drying, a moisture level below 0.1% is required. In order to maximize drying efficiency and achieve high quality parts, desiccant or vacuum dryers are strongly recommended. Typical drying conditions are 3 hours at 130 - 150°F and a -40°F dew point.
Refer to the individual Technical Data Sheet for drying recommendations for specific grades.
The color masterbatch carrier should be compatible with the overmold grade selected. For many GLS overmolding grades, the use of a polyethylene (PE) carrier may adversely effect adhesion. High concentration of waxes in the color concentrate can also decrease adhesion.
The most commonly GLS recommended color concentrates are based on polypropylene (PP) and thermoplastic polyurethane (TPU). To make dispersion easier, the color concentrate should have slightly lower viscosity (higher MFI) than the base TPE compound.
For a specific overmolding grade, follow the color carrier recommendations on the individual Technical Data Sheets.
Regrind is not an option for many two-material applications. Clean TPE regrind may be used at levels up to 20%. Higher levels of regrind are tolerated in black materials. Natural products, light colored or clear compounds may show contamination or discoloration unless properly controlled. Organic pigments used to produce yellow, red, blue and green colors are more likely to burnout or lighten after prolonged residence time or high regrind utilization. When possible, use consistent regrind levels.
When starting production, or if the molding press is down for more than 10 minutes, purge it before restarting production. Use low melt flow PP or HDPE. To prevent flashing, restart the machine using a reduced shot size and gradually increase it to the previous setting.
Injection Pressure and Speed
Typically, the required or achieved injection pressures are from 200 - 600 psi. To achieve the benefits of shear thinning the injection speed should be adjusted to fill the mold in 1 - 3 seconds.
To achieve optimal bond strength, higher than normal melt temperatures are often required. In some critical applications, this temperature can be close to the upper processing temperature limits for the TPE. Melt temperatures of 400-460°F are common.
In order to reduce the residence time at high temperature, reduce the temperatures in the rear of the injection unit as much as possible and only keep the last zone and nozzle at the high processing temperature.
Refer to the Technical Data Sheet for the chosen grade to determine barrel temperature settings.
Improved adhesion can be achieved if the substrate temperature is elevated. For insert molding, this is accomplished by pre-heating the substrate prior to inserting it into the mold. Pre-heating can also reduce any surface moisture present on the substrate and thereby improve adhesion.
For two-shot molding, the time between the first and second shots should be kept as short as possible to achieve the best bond. However, it must be long enough for the substrate to develop sufficient properties to resist the injection pressures and not be remelted or distorted by the second shot. The total overall cycle time is dependent on the time it takes to cool the thickest wall sections, rotate the mold and load any inserts.
To prevent the mold from sweating and introducing water contamination into the cavity, the mold temperatures should be set above the dew point temperature in the molding area. Mold temperatures may have to be raised if there are long or thin sections of the part, which cannot be filled by changing other molding parameters. Mold temperatures of 70°F - 120°F are typical.
When using lifter bar ejection systems, differential thermal expansion of the mold may cause the lifter to jam. Therefore, accurate mold temperature is critical when running molds with core lifters.
SEBS compounds release from cool surfaces more easily than hot surfaces. It is common practice to use two mold temperature controllers while running GLS materials. If sticking in the A-half of the tool is a problem, reduce its temperature to improve ejection.
The required cooling time is dependent on the melt temperature, the wall thickness of the part and the quantity of cooling available. Harder grades will set up faster than softer grades and are easier to eject. Overmolded parts will take longer to cool because the TPE is only cooled from one side as the plastic substrate has poor thermal conductivity.
The cooling time for overmolded parts will be approximately 35 to 40 seconds for every 0.100" of overmolded wall thickness.
The adhesion of GLS soft overmolding TPEs to a substrate may be affected by:
- Substrate Type - additives (glass-filled, mineral-filled, heat-stabilized, lubricated)
- Type of color concentrate carrier
- Moisture level in the overmold and substrate
- Regrind quality and loading level
- Substrate cleanliness, preparation and pre-heating
- Process conditions (melt temperature, pressure, injection speed, cooling time)
- Part and mold design (especially shut-off design)