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Polypropylene (PP) Injection Molding Know-How

2012-9-11 23:50:46

Generally, PP is called polypropylene with its good resistance to fracture property. PP is a kind of semitransparent and half crystal thermoplastic plastic, with features like high strength, good insulativity, low bibulous rate, high temperature, small density and high degree of crystallinity, etc. The modified filler includes glass fiber, miner filler and thermoplastic elastomer, etc.

Mobility of PP with different usage is quite different. Flow rate of the common PP is between ABS and PC.

1.Molding of Plastic
Pure PP is semitransparent and ivory white, which can be dyed to many colors. Coloring of PP can only use color masterbatches on common injection molding machine. There is independent plasticizing element for strengthening mixing effect in Ramada machine, which can be dyed with toner. Product used outdoor is generally filled with UV stabilizing agent and carbon black. Usage proportion of reclaimed materials cannot be over 15%, or the decrease in intensity, decomposition and color change will be caused. Drying process does not require before PP injection molding is processed.

2. Choosing of Injection Molding Machine
There are no special requirements to injection molding machine choosing. As PP is high crystalline, so computer injection molding machines with higher injection pressure and multiple control are needed. Generally clamping force is confirmed by3800t/m2 with the possible injection quantity of 20%-85%.

3. Mold and Gate Design
The temperature of mold is 50-90 Celsius degrees.High temperature is used as to those required higher in size. Temperature of mold core is lower more than 5 Celsius degrees thanthat of mold cavity. Diameter of runner is 4-7mm. Length of needle gate is 1-1.5mm. The diameter can be minimized to 0.7mm. The shorter of the edge gate, the better. Length of the edge gate is about 0.7mm. The depth is half of the thick. Width is two times of the thick and increases gradually with melt flow length of die cavity. The die must possess good exhaust pipe. Depth of vent hole is 0.025mm-0.038mm and the thick is 1.5mm. In order to avoid sink marks, big and round nozzle and runner are needed. And width of dabber should be small (eg. 50-60% of wall thickness). Thick of product made by Co-PP cannot be over 3mm, or bubbles will appear (thick wall product can only use Co-PP).

4. Melting Temperature
Melting point of PP is 160-175 Celsius degrees. The decomposition temperature is 350 Celsius degrees, but the injection processing temperature cannot be above 275 Celsius degrees.The perfect fusion temperature should be 240 Celsius degrees at best. 

5. Injection Speed
High speed injection should be chosen in order to reduce internal stress and transformation. But PP of some levels and modules are not applicable (bubbles and gas lines appears in mantle). If stripe mixed with brightness and darkness spread by gate appears in surface curved decorative pattern, then low injection and higher module temperature should be used.

6.Melting Backpressure 
5bar melt adhesive backpressure can be used. Backpressure of toner materials can be heightened properly.

7.Injection and Packing
Higher injection pressure (1500-1800bar) and packing pressure (about 80% of injection pressure) are adopted. Turn to packing when in the 95% of the whole process with long time for packing.

8.Post-processing of Product
Product needs to be processed by hot water soaking, in order to avoid contraction distortion produced by post crystallization. 


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