Injection Molding: Mold Processing
- Mold Filling, Packing and Cooling
- Maintaining Cushion
- Effects of Molding Conditions
- Molding Soft TPEs
- Molding Hard TPEs
Drying is not typically required for GLS TPEs. Certain specialty products, such as some Versaflex™, Versollan™, and Versollan™ grades, are hygroscopic, thus they need to be dried prior to molding.
A desiccant dryer with a -40°F dewpoint is strongly recommended for drying hygroscopic materials. Specific drying temperatures and times can be found on the Technical Data Sheets for each individual product.
SBC TPEs have inherently superior color than most other TPEs. Therefore, they require less color concentrate to achieve a particular color and the colors produced are cleaner (less yellow) than other TPEs. Generally, the color concentrate should be lower in viscosity (have higher melt index) than the base formulation. This will promote ease of dispersion.
- Styrenic (PS) color carriers are recommended for the SBS formulations.
- Polypropylene (PP) carriers are recommended for the harder SEBS formulations.
- For soft SEBS formulations low-density polyethylene (LDPE) or ethylene vinyl acetate copolymer (EVA) have been used. PP carrier is not recommended for softer grades, as the formulation hardness will be affected.
Liquid colors can be used but the carrier should be a paraffin type mineral. Polyvinyl chloride (PVC) plasticizers, such as dioctyl phthalate (DOP), should not be used as carriers. Dry colors have also been used but may require more material and time to perform color changes.
The use of a polyethylene (PE) carrier may adversely effect adhesion to the substrate for some overmolding applications. If using a specialty overmolding grade, follow the coloring recommendations given on the individual product Technical Data Sheet.
Up to 20% regrind may be used for SEBS TPEs. High levels of regrind are better tolerated in black materials. Natural, light-colored or clear formulations will more easily show contamination or discoloration. Organic pigments used to produce yellow, red, blue and green colors are more likely to change color after prolonged residence time or high regrind levels. For SBS formulations, the regrind should be kept below 25%. GLS TPEs have high elongation and good tear strength and therefore require the use of a high quality grinder with sharp knives. For lower durometer styrenic formulations the clearances should be set to 0.003" maximum.
Only grinders with high quality support bearings and a rigid frame can maintain the tolerances necessary to achieve the necessary rotor knife to bed knife clearances. The use of a small amount of a dusting agent such as talc or calcium carbonate can minimize agglomeration during the grinding process. Feed small amounts of parts into the grinder at one time to minimize heat buildup, which can lead to agglomeration.
To allow the best incorporation of the regrind into the virgin material, the screen size should be chosen to yield particles that are roughly the same size as virgin pellets.
If the press is down for more than 10 minutes, purge before restarting production. To prevent flashing, restart the machine using a reduced shot size and gradually increase it back to the original shot size. This will help to prevent flashing from occurring behind slides or inserts.
For SBS formulations, if a machine is to be left at temperature longer than one hour, purge it with LDPE or polystyrene before shut down. For SEBS formulations, if the machine is down over the weekend, purge it with a high molecular weight (low or fractional melt flow) LDPE at low temperatures before shut down. On start-up, retract the extruder and air purge it well before attempting to fill the mold.
Setting Barrel Temperatures
Figure 1 shows the typical starting barrel temperatures. Barrel temperatures should be set progressively. The feed zone temperatures should be set fairly low (typically 250°F - 300°F (120°C -150°C)) to avoid feed-throat bridging and allow entrapped air to escape.
Lower temperatures in the transition zone allow proper compression and shearing of the TPE before it fully melts. To improve mixing when using color concentrates, set the transition zone temperatures above the melt temperature of the concentrate. The zone nearest to the nozzle should be set close to the desired melt temperature.
After the process has stabilized, the actual barrel temperatures should be compared to the set points. If the actual temperature exceeds the set temperature, then shear heating has caused the material to overheat. If good parts are being produced, the temperatures should be reset to the actual temperatures being produced by the shear heating.
The heaters should demand power from 25% to 50% of the time. If the heaters are on continuously, there is not enough heat being produced from shear. To increase shear heating, increase the screw rpm and back pressure.
Setting Mold Temperatures
Mold temperatures should be set above the dew point temperature in the molding area. This prevents sweating of the mold and possible water contamination in the cavity. Water contamination usually appears as streaks in the parts. Mold temperatures may be raised if there are long or thin sections of the part that have proven to be difficult to fill. Higher mold temperatures usually result in higher cycle times but may improve weld line integrity and part appearance.
Figure 1. Suggested Initial Start-Up Conditions for Injection Molding.
Setting Shot Size
When starting up a new mold, begin with short-shots, then gradually increase the shot size until all part cavities are 80-90% filled. This procedure can minimize the potential for overpacking and prevent flash in vents. The screw position should be noted and used to set the transfer point. Monitor the cushion to insure that it is maintained during the pack and hold phase.
If there is no cushion, the pack pressure cannot be maintained and there is no control of part densification. After the gate freezes, any additional material volume or pressure will only pack the sprue and runner system, which can cause difficulties with sprue removal during part ejection.
Screw rpm, Back Pressure and Screw Delay Time
The screw rpm should be set so that the screw is fully recovered for the next shot, typically 2 - 3 seconds before the mold opens. Typical screw speeds range from 50 - 150 rpm.
If the screw recovers too fast, and the machine is equipped with a screw delay timer, set the delay time so that there is minimal delay after the screw is fully recovered and the mold opens. This will reduce material residence time at temperature and dead time in the barrel.
Increasing the back pressure increases shear heating of the material. Normal settings for back pressure are 50 - 150 psi. When mixing color concentrates, higher back pressure is preferred to achieve optimum dispersion.
If possible, profile the injection speed to fill the runner system rapidly and then slow down after the material starts flowing through the gate and into the cavity. Maintain this speed until the part is 90% full and then reduce it further to completely fill the cavity without flashing the part.
As stated earlier, GLS TPEs are shear responsive. If a part has difficulty filling, increase the injection speeds before increasing temperature. The injection time to fill the part should be between one and two seconds. Slower fill rates may be required if surface flow defects occur.
Injection and Transfer Pressures
If the machine is not capable of being controlled by fill speed, set the injection pressure high enough to fill the runner system and cavity in about 1 to 5 seconds. Adjust the initial transfer pressure to approximately 50% of the injection pressure required to fill the part cavity. This helps to minimize the pressure during the pack and hold phase of injection. When setting the shot size, monitor the cushion to insure it is maintained during the pack and hold phases.
Transfer From Boost to Pack to Hold
Newer molding equipment provides additional options for transferring from injection boost (first-stage Injection) to the pack and hold phase. The most accurate method to transfer from boost to pack pressure is by screw position. Using screw position allows the processor to consistently inject a specific volume of material to the cavity. It also provides accurate control of part packing and densification, which can help prevent sinks and voids in the part.
Time is another method for controlling transfer but is not recommended. Transfer using cavity pressure is expensive because it involves installing pressure transducers in the part cavity. This process is used when highly accurate molding tolerances are required.
Reducing the transfer pressure from boost to pack and hold will help to control drool at the bushing tip. If the injection unit is equipped with a profiled pack and hold phase, it can be used to reduce the velocity and pressure to the runner.
The optimum time to fill the runner system is approximately 0.5 - 1.5 seconds. It should take another 1 - 5 seconds to fill the cavities. If possible, it is better to control the fill time by controlling the injection speed.
The hold time should be set to achieve gate freeze. Usually, the gate size is the determining factor for hold time. The larger the gate the longer the hold time required to achieve gate freeze.
The cooling time is principally dependent on the temperature of the melt, the wall thickness of the part and cooling efficiency. In addition, the material hardness is a factor. Harder grades (> 50 Shore A) will set up faster in the mold compared to very soft grades (< 20 Shore A).
For an average part and medium hardness SEBS formulation, the cooling time will be approximately 15 to 20 seconds for every 0.100" of wall thickness, provided there is cooling available from both sides.
Overmolded parts will take longer to cool because they can be effectively cooled over a smaller surface area. The cooling time for overmolded parts will be approximately 35 to 40 seconds for every 0.100 inches of wall thickness.
Cushion should be maintained or there will be no control of part densification or compensation for material shrinkage. Inadequate cushion and hold pressures will result in underfilled parts with voids or sinks and poor physical properties. Gates that freeze off too quickly (as a result of being too small or too cool mold temperatures) may also cause these above issues.
A worn or contaminated check ring can limit the machine's ability to hold pressure and maintain a cushion. GLS TPEs have lower viscosity (higher flow) than traditional thermoplastics and will leak back easier than other materials. The sealing capability of the check ring should be verified by observing the machine's ability to maintain a cushion position.
If a part is molded at too low a temperature, it will require excessive pressure to fill the cavity. This will result in high molded-in stresses. This in turn can cause the part to warp during ejection or at a later time when it is exposed to an elevated temperature. There also may be greater than normal post mold shrinkage and a reduction in the ultimate elongation. For clear TPEs, parts that have been processed at too low a temperature will have a frosty surface appearance.
SBS based TPEs will develop a yellow or orange color and a distinctive odor when they have been processed at too high a temperature or held at temperature for too long. The color and odor are strong signs that the material has been degraded. Degradation results in poor appearance and reduction in physical properties. SEBS TPEs that have been processed at too high a temperature will have a burnt odor (degradation) and in the worst case become tacky and bleed oil.
The effects of overpacking the part may include:
- Gate bulge.
- Increased density, thus higher part weights.
- Increased hardness.
The effects of underpacking a part may include:
- Gate pucker.
- Voids and/or surface sinks.
- Reduced physical properties.
- Lower than normal hardness.
Monitoring part weights has successfully been used to verify process stability and consistency. It should be noted that the gate size/location, runner dimensions and other aspects of the mold design may also affect the properties of a part.
Soft GLS TPEs have very low viscosity (high flow), thus they require minimal injection pressure. Typical values for injection pressure are 150 psi - 450 psi.
Most of the GLS soft TPEs are either water-clear or translucent. Clarity in the molded part can be marginally improved by increasing the mold and melt temperatures. A high polish mold surface finish is usually justified with these products, as they duplicate the mold surface quite well.
The softer materials exhibit some tacky behavior. Cleanliness in close proximity to the molding area is important as softer materials attract and retain dust and contaminants. This tackiness also makes part ejection more difficult. In these cases, robotic sprue pickers, runner keepers, or air ejection may be required. The addition of a slight surface texture to the mold can help to mask possible surface blemishes in the molded article.
The harder GLS TPEs usually have higher viscosity and may require slightly higher injection pressure (400 psi - 800 psi) to fill the cavity. Due to their higher modulus, hard formulations require less aggressive sprue-pullers. They also set up faster and are more easily ejected, which may result in reduced cycle times compared to softer materials.