Sourcing PU Casting Parts – Topic: [Manual PU casting process – network binder for high rigidity products] – Manufacturers from Vietnam – Vietnam Sourcing
Preparation of Mold / Mold release agent
Remove mold oil / grease with an appropriate washing solvent (Toluen solvent)
Mold release agent: Use a spray or absorbent cloth – coat a thin layer of mold release agent evenly over the surface of the mold cavity.
Preheat the molds at (110-120) ° C in the oven for 1-2 hours.
When starting the casting, the mold temperature must be lowered to (80 ± 5) ° C to ensure the casting process.
For products with metal ring: Metal ring surface treatment by sand blasting / grinding / threaded. After that, coat the ring with PU adhesive (chemlok 218) and put it into the oven.
Preparing Prepolymer (PU)
Prepolymer must be heated to the required temperature at (80 ± 5) ° C.
Before use, Prepolymer must be stirred / stirred to ensure heat dispersion (avoiding local heat).
Heating above the temperature threshold over time will make Prepolymer more susceptible to gel formation.
The leftover Prepolymer left in the container after collection should be pumped with nitrogen for storage.
Preparation of network substances
Solid flocculants such as MOCA, must be dried at (110 – 120) ° C.
The percentage (%) of the weight of the binder is calculated and based on% NCO (see box or COA)
Specifically:% MOCA =% NCO x 3.18 x H (%)
In which: H (%) is the reaction efficiency (90 – 95%)
Use electronic scales to weigh the weight of lattice.
Vacuum (1st time)
Vacuum to remove air bubbles from the Prepolymer (PU)
The pressure of the vacuum machine must reach (-1.0) bar.
The process is carried out once – twice until complete removal of bubbles or very little but unchanged air bubbles.
Prepare the stirring device: Dry, impermeable
If the batch is small, use a flat stainless steel bar.
If the batch is large, using a 2-blade paddle with an engine attached, the paddle must always be submerged and the velocity is relatively low, to avoid bringing air into the PU.
The stirring moves in the form of a figure 8 or a zigzag shape across the rack / container.
Must not create vortex lines.
Vacuum (2nd time) (if any)
After mixing, the vacuum should be conducted again.
Depending on the potlife time of the PU code (see supplier’s TDS) and the volume of the batch, choose the appropriate degassing time.
During degassing, air bubbles often rise very quickly, so the valve can be released / vacuum pressure reduced a little, avoiding overflow.
Stirring time must be controlled, ensuring a sufficient time to allow pouring into the mold before the material is geled again.
Pouring / pouring techniques must be appropriate to ensure:
Pour 1 continuous line (raise the pour from low to high)
Pour the mixture down a surface / mold wall and flow across the bottom and fill the mold upwards.
When pouring can tilt the mold when pouring to improve flow.
Small / large air bubbles emerging on the surface may be gently broken by igniting the gas / hot air gun, care should be taken to avoid damage to the surface.
Curing and shaping
Formation: It means only partially vulcanizing, the material is hard enough to remove the product.
Curing at 100 – 110 oC
Shaping / vulcanizing products: MOCA: 45 – 60 minutes / 16 hours.
The product after removing the mold, has enough features to use (Best stable after 7 days).