What should be considered during the injection molding of TPR (thermoplastic rubber)?
What should be considered during the injection molding of TPR? Depending on material characteristics and supply conditions, the material's appearance and processing properties should generally be inspected prior to molding. Supplied pellets often contain varying levels of moisture, solvents, and other volatile low-molecular-weight substances; TPR grades prone to moisture absorption, in particular, frequently exceed permissible moisture limits for processing.
Therefore, drying and moisture content measurement are essential before processing. Since TPR requires moisture levels below 5% (or even 2–3%) at high temperatures, vacuum drying ovens are commonly used to dry the material at 75°C–90°C for two hours. Dried material must be properly sealed to prevent re-absorption of atmospheric moisture, which would negate the drying effect. Using a hopper dryer allows for the continuous supply of hot, dry material to the injection molding machine, simplifying operations, maintaining cleanliness, improving quality, and increasing injection rates. The capacity of the hopper dryer is typically set at 2.5 times the machine's hourly material consumption.
TPR Coloring
SBC-based TPEs offer superior color characteristics compared to most other TPR materials. Consequently, they require smaller amounts of masterbatch to achieve specific color effects, resulting in purer colors than those produced with other TPRs. Generally, the viscosity of the color masterbatch should be lower than that of the TPR; since TPR has a higher melt index than the masterbatch, this viscosity difference facilitates the dispersion process, leading to more uniform color distribution.
For SBS-based TPEs, polystyrene-based carriers are recommended. For harder SEBS-based TPRs, polypropylene (PP) carriers are recommended. For softer SEBS-based TPRs, low-density polyethylene (LDPE) or ethylene-vinyl acetate (EVA) copolymers may be used. PP carriers are not recommended for softer grades, as they can alter the hardness of the composite material.
In certain overmolding applications, the use of polyethylene (PE) carriers may adversely affect adhesion to the substrate. Cleaning the Barrel Before Injection Molding
The injection molding machine's barrel requires cleaning or disassembly and cleaning before initial use of a newly purchased machine, or during production when changing products, raw materials, or colors, or if signs of plastic decomposition are detected.
Barrel cleaning is typically performed using a heat-based cleaning method. The cleaning material used is generally plastic resin (or regrind). For TPR materials, the virgin material being processed can be used to displace the transitional cleaning material.
TPR Molding Temperatures
During the injection molding process, accurate temperature settings are crucial for the product's appearance and performance. Below are some recommendations for temperature settings when processing TPR. The temperature in the feed zone should be set relatively low to prevent clogging at the feed throat and to allow trapped air to escape. When using masterbatches, the temperature in the transition zone should be set above the masterbatch's melting point to improve mixing. The temperature in the zone closest to the injection nozzle should be set close to the desired melt temperature. Based on testing, the typical temperature ranges for TPR products across the various zones are: 160°C to 210°C for the barrel and 180°C to 230°C for the nozzle.
The mold temperature should be set higher than the condensation temperature of the injection zone; this prevents moisture contamination of the mold, which could otherwise cause streaks on the product surface. While higher mold temperatures generally result in longer cycle times, they improve weld lines and the product's visual quality; therefore, the mold temperature should be set within the 30°C to 40°C range.
Mold Filling, Holding, and Cooling
During the mold cavity filling process, poor filling performance can lead to excessive pressure drop, prolonged filling times, or incomplete filling, resulting in product quality issues. To enhance filling performance and improve the quality of the molded product, the following aspects can be considered:
1) Switch to a different product series from Keyue;
2) Change the gate location;
3) Adjust the injection pressure;
4) Modify the part's geometry. Injection pressure control is typically categorized into primary injection pressure, secondary injection pressure (holding pressure), and sometimes tertiary or higher-stage injection pressures. The timing of pressure switching is crucial for preventing excessive in-mold pressure, flash, or short shots. The specific volume of the molded part depends on the melt pressure and temperature at the moment the gate freezes during the holding phase.
If the pressure and temperature at the transition from the holding phase to the cooling phase remain consistent for each cycle, the specific volume of the part will not change. At a constant molding temperature, holding pressure is the most critical parameter determining part dimensions, while holding pressure and temperature are the key variables affecting dimensional tolerances.
For example, the holding pressure can be reduced immediately after mold filling is complete and then increased once a skin layer of sufficient thickness has formed; this approach allows for the molding of large, thick-walled parts using lower clamping forces while eliminating sink marks and flash.
Holding pressure and speed are typically set at 50%–65% of the peak pressure and speed used during mold filling-meaning the holding pressure is roughly 0.6–0.8 MPa lower than the injection pressure. Because the holding pressure is lower than the injection pressure, the load on the oil pump is reduced during the holding phase, thereby extending the pump's service life and lowering the motor's power consumption.
By precisely adjusting the metering stroke so that a small amount of melt (cushion) remains at the screw tip at the end of the injection stroke, additional injection pressure (secondary or tertiary) can be applied based on the mold filling status to inject a small amount of extra melt. This helps prevent sink marks and allows for the adjustment of part shrinkage.
Cooling time depends primarily on melt temperature, part wall thickness, and cooling efficiency. Material hardness is also a factor; harder grades solidify more quickly in the mold than softer ones. For parts cooled from both sides, the cooling time required per 0.100 inches of wall thickness is typically around 10 to 15 seconds. Overmolded parts require longer cooling times because heat dissipation is limited to a smaller surface area; in this case, the cooling time required per 0.100 inches of wall thickness is approximately 15 to 25 seconds. Impact of Injection Molding Process Conditions
1. Incomplete Molding (Short Shots)
(1) Improper feed adjustment (insufficient or excessive material).
(2) Injection pressure too low, injection time too short, or plunger/screw retracting too early.
(3) Slow injection speed.
(4) Material temperature too low.
2. Flash
(1) Injection pressure too high or injection speed too fast.
(2) Excessive feed quantity causing flash.
(3) Excessively high barrel, nozzle, or mold temperatures reduce plastic viscosity and increase fluidity, leading to flash during mold filling.
3. Silver Streaks, Bubbles, and Voids
(1) Material temperature too high, causing decomposition.
(2) Low injection pressure or short holding time, preventing the melt from making close contact with the cavity surface.
(3) Injection speed too fast: high shear causes plastic decomposition and gas generation; injection speed too slow: failure to fill the cavity in time results in insufficient surface density, causing silver streaks.
(4) Insufficient material, excessive cushion, or low material/mold temperatures affect melt flow and molding pressure, leading to bubbles.
(5) Low back pressure or high screw speed during plasticizing causes the screw to retract too quickly, allowing air to be pushed toward the front of the barrel along with the material.
4. Burn Marks/Discoloration
(1) Barrel or nozzle temperature too high.
(2) Injection pressure or plasticizing back pressure too high.
(3) Injection speed too fast or injection cycle too long.
Key Point: Accurate temperature setting is crucial for the appearance and performance of the molded product.