Injection molding process
T rheological properties Rheology
- Because of the different molecular structure of the soft segment between the SBS and the SEBS projectile, the viscosity characteristics ( Viscosity/Shear Rate) of the shear rate are also significantly different.
- But the melt viscosity (Melt Viscosity) of SBS and SEBS bodies is very easy to be affected by the Shear Rate in the barrel, but instead the temperature inside the barrel Not very sensitive.
- SBS projectiles do not rely on high shear rates to achieve plastic flow forces, but SEBS bodies rely on high shear rates to achieve plastic flow.
- Fast injection and High Compression Ratio Screw are more suitable for SEBS processing.
Material Stability Stability (SEBS)
- SEBS elastomer soft segment molecules are saturated, will not be oxidized, high temperature or high shear rate processing does not affect stability.
- General processing temperatures between 200 – 230C can be processed with high shear rates.
- If the material degrades due to high temperature or high shear rate processing, the viscosity of the melt will decrease due to molecular chain breakage
Material Stability Stability (SBS)
- SBS soft segment has unsaturated bonds, which are easily oxidized. High temperature or high shear rate processing is highly likely to cause degradation. /li>
- to avoid material degradation, the processing temperature must be maintained at 150 – 200C, the melt adhesive should not stay in the barrel for too long.
- If the material degrades, the by-products are non-toxic and non-corrosive, and a gel will appear.
- If the processing temperature exceeds 200C, the viscosity of the melt will rise (SBS molecules may have bridging), thus reducing productivity.
Screw Design Screw Design
- Typical SBS and SEBS reciprocating screw-type injection machines are available with a 2:1 to 3:1 compression ratio screw with a screw head angle of 60 degrees.< /li>
- The traditional screw used to inject PVC, PE, PP, PS, and engineering plastics is suitable for injection of SBS and SEBS.
- Two-stage venting screw helps to remove surface moisture from some SBS or SEBS particles
Mainstream Road Sprues
- Using a standard mainstream design with a 3C draft angle is generally available.
- Puller must be designed for materials of different hardness.
- Soft SBS and SEBS materials are not available"Z” type pull pins.
- Undercut fingers”, reverse taper cold wells, and slot bar pullers are typical Design.
Feed Gate Gate
- Normal nozzles are available for SBS or SEBS injections.
- The thickness of the filling port is generally 15% to 25% of the thickness of the part.
- The length of the Land Length should be as short as possible.
- Fan Gate or Edge Gate works best
- As with general plastic injection, SBS/SEBS mold design must consider the shrinkage characteristics of the material.
- SBS or SEBS material shrinkage is based on the formula, in the range of 0.005 to 0.020 inches/inch (i.e. 0.5 – 2%). The shrinkage of the melt into the mold will be higher.
- In general, SBS shrinks less, and SEBS shrinks slightly.
- Higher density (filled material) has a lower material shrinkage than a lower density material.
- Some processing conditions such as mold temperature, melt temperature, injection speed and product thickness can affect product shrinkage
Barrel Temperature Cylinder Temperatures
- The feed zone temperature should be in the range of 65-90C.
- SBS- with 140-180C barrel temperature, can be 20-30C (segmented) gradually from the feeding area to the nozzle area.
- SEBS- General barrel temperature 200-245C (180-190C for transparent materials). Large product up to 265C.
- High-temperature processing can quickly fill the mold, reduce molecular positioning, improve product properties, but increase the cooling time and increase the injection cycle.
Injection Speed Injection Rate
- Smelting viscosity (Melt Viscosity) and Shear Rate of SBS and SEBS materials (especially SEBS materials) are related. Injection SBS materials are usually medium The injection speed. However, the injection rate of SEBS material must use a higher injection speed to avoid the half-way of the melt glue during the filling process. Freeze-Off.
- If the mold has enough exhaust system (Venting), you can use high-speed injection without worrying about Air Entrapment.
- SBS body - the most suitable mold temperature 10-40C.
- SEBS body - the most suitable mold temperature 35-65C.
- The higher mold temperature promotes the flow of the sol in the mold, and the low pressure injection can also fill the mold, which has a more perfect product surface and can also reduce molecular orientation (Molecular Orientation). Make the product have better physical properties.
Barrel Cleaning Purging SBS
- With good thermal stability, there is no need to clean the barrel after each material interruption.
- The thermal stability of the SEBS projectile is very good. The material in the barrel will not degrade even if it is parked for two hours at processing temperature.
- SBS body - can clean the barrel with PS.
- SEBS Projectile - Can clean the barrel with HDPE or PP.
- Under normal storage conditions, both SBS and SEBS bodies will not absorb moisture from the air, so there is no need to pre-bake the ingredients.
- If the appearance of the product is decisive, the material can be placed in a hot hopper at 50°C for 2 to 4 hours.
- Most of the SBS and SEBS elastomers are transparent or opalescent and easy to color. You can use a masterbatch (color) to dry or Inject into the production process.
- Special attention should be paid to the base of the color(Carrier Resin /Base Resin).
- SBS - PS or EVA based color is more appropriate.
- SEBS - PE or PP-based color is more appropriate, EVA-based color is less thermally stable.
- Cannot use PVC-based colors.