The causes and solutions of air bubbles in injection molding PP

Reasons and solutions for air bubbles in injection-molded PP Products with air bubbles in injection-molded PP.
 

PP bubbles to solve problem one.

1, Improve the mold temperature, reduce the injection time, and cycle because PP bubbles are formed by internal shrinkage due to long cooking time.

2, increase the backpressure, extend the injection time, with high pressure and low speed.

PP material generally does not have bubbles (due to small moisture absorption), it may be a problem with the filler when granulating PP material. If the solution in the process is:
1, add spare material dry baking material;
2, injection back pressure increase, injection injection injection speed slow down;
3, the barrel temperature should not be too high.

Originally PP material is a very good material, 1 first consider your mold structure, where the bubble is formed, No. 2, the bubble barrel temperature should not be too high, 3 the thickness of the product, 4 not pumping rubber to try ha / or less rubber. In fact, injection molding efficiency does not just look at the parameters, that is meaningless, more analysis of the problem is the right way. Sometimes, adjusting the machine is not to look at the molding of the 5 major elements can be.

PP bubble problem solving two.

1, the storage speed is too fast, add some back pressure as appropriate

2, the raw material is not dry enough, containing moisture

3, plastic in the barrel stay too long

4、The amount of screw shot back too long

5, mold exhaust bad

6、Injection pressure is too small, injection speed is too large

7、Melt temperature is too high, mold temperature is too low

Characteristics of polypropylene PP plastic: and the use of polypropylene PP

Non-toxic, tasteless, small density, strength, stiffness, hardness, and heat resistance are better than low-pressure polyethylene, can be used at about 100 degrees. Good electrical properties and high-frequency insulation is not affected by humidity, but it becomes brittle at low temperatures, are not wear-resistant, easy to age. Suitable for making general mechanical parts, corrosion-resistant parts, and insulation parts. Common acids, alkalis, and organic solvents have almost no effect on them, and appropriate grades can be used for eating utensils.

PP physical properties

Polypropylene is usually a translucent colorless solid, odorless and non-toxic. Polypropylene melting point is about 164~170℃, density 0.91g/cm. High strength, hardness, wear resistance, bending fatigue, moisture resistance, and chemical resistance are good, easy to process molding, low price, so it is a large production, widely used in general polymer varieties. The disadvantage is that the low-temperature toughness is poor, not resistant to aging. But it can be overcome by modifying and adding antioxidants respectively.

PP mechanical properties

Polypropylene has excellent mechanical properties due to its high crystallinity and regular structure. However, at room temperature and low temperature, the impact strength is poor due to the high regularity of its molecular structure. The most outstanding property of polypropylene is its resistance to bending fatigue.

PP thermal properties

Polypropylene has good heat resistance, the products can be sterilized at temperatures above 100℃, and will not be deformed at 150℃ under the condition of no external force. The embrittlement temperature is -35℃, and it will be embrittlement at lower than -35℃, and the cold resistance is not as good as polyethylene.

PP chemical stability

The chemical stability of polypropylene is very good, except for the erosion by concentrated sulfuric acid and concentrated nitric acid, it is stable to various other chemical reagents; however, low molecular weight aliphatic hydrocarbons, aromatic hydrocarbons, and chlorinated hydrocarbons can make polypropylene soften and swell, while its chemical stability increases with the increase of crystallinity, so polypropylene is suitable for making various chemical pipes and fittings with good anti-corrosion effect.

PP electrical properties

Polypropylene has excellent high-frequency insulation properties, and because it hardly absorbs water, the insulation properties are not affected by humidity. It has a high dielectric coefficient, and with the rise of temperature, it can be used to make electrical insulation products subject to heat. It also has a high breakdown voltage, which makes it suitable for use as electrical fittings, etc. It has good resistance to voltage and arc, but has high static electricity and is easy to age in contact with copper.

PP weather resistance

Polypropylene is sensitive to ultraviolet rays, adding zinc oxide, dodecyl thiodipropionate, carbon black or similar emulsion filler, etc. can improve its aging resistance.

PP plastic molding characteristics.

1. crystalline material, small wetness, easy to melt body rupture, and easy to decompose with hot metal contact for a long time.

2. Good fluidity, but the shrinkage range and shrinkage value are large, prone to shrinkage holes. Dent, deformation.

3. Cooling speed is fast, the pouring system and cooling system should be slow to dissipate heat, and pay attention to control the molding temperature, the material temperature is easy to orientation when low temperature and high pressure, the mold temperature is lower than 50 degrees, the plastic parts are not smooth, easy to produce bad fusion, flow marks, 90 degrees above the easy to occur warpage deformation. Therefore, the temperature should be controlled at 80 degrees.

4. Plastic wall thickness must be uniform, avoid lack of glue, sharp corners, to prevent stress concentration.

Injection molding process conditions: PP plastic processing technology

Injection molding machine selection: There is no special requirement for the selection of injection molding machines. Because PP has high crystallinity. Need to use higher injection pressure and multi-stage control of computer injection molding machine. The clamping force is generally determined by 3800t/m2, and the injection volume is 20%-85%.

Drying treatment: If stored properly, no drying treatment is required.

Melting temperature: The melting point of PP is 160-175℃ and the decomposition temperature is 350℃, but the temperature setting should not exceed 275℃ during injection processing. The melting section temperature should preferably be 240℃.

Mold temperature: mold temperature is 50-90℃, and high mold temperature is used for high dimensional requirements. The core temperature is more than 5℃ lower than the cavity temperature.

Injection pressure: Use higher injection pressure (1500-1800bar) and holding pressure (about 80% of the injection pressure). Turn the holding pressure at approximately 95% of the full stroke and use a longer holding time.

Injection speed: To reduce internal stress and deformation, the high-speed injection should be selected, but some grades of PP and molds are not applicable (bubbles and airlines appear). If light and dark streaks appear on the engraved patterned surface spread by the gate, use the low-speed injection and higher mold temperature.

Runners and gates: runner diameter 4-7mm, needle-shaped gate length 1-1.5mm, the diameter can be as small as 0.7mm. the shorter the edge-shaped gate length the better, about 0.7mm, depth half of the wall thickness, width twice the wall thickness, and increase with the length of the melt flow in the mold cavity by Ken. The mold must have good exhaust, the exhaust hole is 0.025mm-0.038mm deep and 1.5mm thick, to avoid shrinkage marks, we should use a large and round injection port and round runner, the thickness of reinforcement should be small (for example, 50-60% of the wall thickness). For products made of homopolymer PP, the thickness should not exceed 3mm, otherwise, there will be bubbles.