TPV injection molding causes inlet burns mainly due to excessive material shear. At moment when material passes through gate during injection, shear rate reaches its peak, so temperature at gate reaches its highest value, and excessive shearing in this place will cause burn marks on gate.
The reason why TPV injection molding causes burning at gate:
1. The injection speed is too high. Most TPVs recommend a higher injection rate and fill times are around 0.5-1.5 seconds. But too high an injection speed increases shear rate of material at gate.
2. The back pressure is too high. Retracting injection molding screw too fast will also cause excessive shearing of material. The last point where melt fills mold is near sprue, which also increases likelihood of burn marks near sprue.
3. The sprue size should be as small as possible, but not too small. Small gate size and high shear rate can reduce TPV melt viscosity and improve TPV flowability. We recommend that gate size be as small as possible, but not too small to cause excessive shear; in general, gate size is preferably 0.5 to 3 mm.
4. The mold temperature at gate is too high. Lower mold temperature on gate to avoid burning gate.
5.The temperature of barrel near nozzle is set too high. Reduce temperature of barrel near nozzle so that there are no burn marks on bolt.
6. Butterfly valve The wall thickness transition is typically less than 15%. The butterfly valve reduces material flow velocity near valve, which increases likelihood of shear for SPE. Usually a sudden change in wall thickness of product will cause throttling, so we recommend that change in wall thickness does not exceed 15%. Burrs and sharp corners on mold accessories can reduce material flow, so mold area must be polished to reduce throttling.
By way, let me tell you how to solve problem of burns on TPV injection molded products?
1. Injection molding screw speed is too high. We recommend a higher screw speed of 100~200rpm to ensure that TPV can be subjected to high shear during plasticizing, but if speed exceeds a certain limit, it will result in SPE. Subject to excessive shear, which can char TFE material.
2. If back pressure is too high, we recommend using a back pressure that can make TPV plasticize. The recommended minimum back pressure requirement is 0.3 MPa (50 psi). Excessive back pressure will damage TPV. too much shear force, even air ingressa, results in charred marks where product was last poured.
3. Too much clamping pressure Too much clamping pressure can cause clogging of outlet port, and commonly accepted clamping pressure is 4.0 to 6.9 kN/cm2.
4. Exhaust The use of TPV usually requires fast injection molding, so exhaust volume must be relatively sufficient. When molten TPV material enters high pressure cavity quickly, air in cavity must be released as soon as possible. Burn marks near gate are usually result of improper ventilation as trapped gas ignites (naturally).
5.Injection rate too high.Small gate size and high shear rate can reduce TPV melt viscosity, thereby improving TPV fluidity. We recommend that gate size be as small as possible, but not too small to cause excessive shear; in general, gate size is preferably 0.5 to 3 mm.
6. Recommended TPV exhaust size
Recommended maximum clearance
3.8 to 5.0 mm
5.0 to 12.0 mm
3.8 to 5.0 mm
5.0 to 12.0 mm