What should pay attention to when injecting polypropylene (PP)?

Polypropylene wax (commonly known as PP wax) is a non-toxic, odorless, odorless milky white waxy solid particulate matter, also known as “pleated glue”.

PP wax is a crystalline polymer. Among the common plastics, PP is the lightest, with a relative density of only 0.92 g/cm3 (smaller than water). It has excellent resistance to ground stress cracking and has a high bending fatigue life. Forming temperature: 160~220 ℃.

Among plastic raw materials, PP has the best temperature resistance, its thermal deformation temperature is 80~100 ℃, and it can be boiled in boiling water. PP products are light in weight, good in ductility and good in acid resistance. PP material has copolymer modification, chemical crosslinking modification, blending modification and filling modification.

Compared with the polycarbonate raw material (PC material), the PP material has no thermal stress to cause the product to crack. Generally, the PP material is modified by adding glass fiber or some metal material additives.

Polypropylene Wax

Pay special attention to the following key points when processing polypropylene wax (PP wax powder) injection molds:

High Melting Point Polypropylene Wax PPW-0931 Name: Polypropylene Wax Model Number: PPW-0931 Chemical Composition: Polyethylene Wax

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1. Plastic solution

Pure PP is a transparent color of hippopotamus milky white and can be stained with particles. The coloring of PP only uses color masterbatches in general plastic machines, but some models have separate melting components to improve the effect of mixing rubber, and can also be colored with pigments.

Products for outdoor applications are generally filled with UV thickeners and carbon black. The application ratio of recycled particles does not need to exceed 15%, otherwise the compressive strength will be reduced and the color will be dissolved. PP injection molds generally do not need special drying before processing.

2. The plastic machine adopts

There are no special requirements for the use of plastic machines. Because PP has a high crystalline form. It is necessary to use a computerized plastic machine with high injection pressure and multi-stage operation. The clamping force is generally determined by 3800t/m2, and the injection is 20%-85%.

3. Design scheme of mold shell and gate

The mold temperature is 50-90 ℃, and the higher mold temperature is specified for the specification. The core temperature is more than 5°C lower than the die temperature, the diameter of the runner is 4-7mm, the length of the needle gate is 1-1.5mm, and the diameter can be as small as 0.7mm.

The less the length of the edge gate, the better, about 0.7mm, the depth is half of the wall thickness, the total width is twice the wall thickness, and it gradually increases with the length of the melt flow in the mold core.

The mold shell must have excellent exhaust pipe performance. The exhaust hole is 0.025mm-0.038mm deep and 1.5mm thick. To prevent the fold marks, it is necessary to use a large and round nozzle and annular runner, and the thickness of the rib should be small ( e.g. 50-60% of the wall thickness).

The product produced by homopolymer PP should not exceed 3 mm in thickness, otherwise bubbles will appear (thick-wall pipe products only use copolymer PP).

4. Melt temperature

The melting point of PP is 160-175 °C, and the melting temperature is 350 °C, but the temperature setting cannot exceed 275 °C during injection production and processing. The temperature of the melting section is preferably 240°C.

5. Injection rate

In order to reduce thermal stress and deformation, rapid injection should be selected, but some grades of PP and mold shells are not suitable (bubbles, air lines). If there is a two-color pattern on the border of light and dark that spreads from the gate on the surface of the engraved pattern, a low-speed injection and a higher mold temperature should be used.

6. Melt adhesive back pressure type

Melt glue back pressure type can be used, and the back pressure type of color pellets can be adjusted appropriately.

7. Injection and pressure keeping

Select higher injection working pressure (1500-1800bar) and holding pressure (about 80% of injection working pressure). Switch to holding pressure at about 95% of the full stroke schedule, and use a long holding time.

8. Post-processing process of the product

In order to avoid the shrinkage and deformation caused by the post-crystal, the product generally needs to be soaked in boiling water.