Do you know the whole process of PET injection molding?

Abstract: PET plastic injection molding process is one of the very important technology for plastic processing.This blog systematically introduces PET processing by plastic injection molding machines. The content includes PET property, features, injection molding process, molding machines, process conditions,  processing ways,etc. It will help PET making factory get useful knowledge in few minutes.

PET plastic injection molding process is one of the very important technology for processing plastic, PET plastic bottles, most of the plastic packaging plant will adopt blow molding production process, but the injection molding process has especially the unique processing advantages. For PET for injection molding, technical personnel need to consider many factors.

PET plastics introduction

PET chemical name is Polyterephthalate alcohol ester, also known as polyester. At present, the most used by customers is the GF-PET, mainly for the bottle preform. 

The rheological property of PET is better in the melting state, and the influence of pressure on the viscosity is larger than that of the temperature. Therefore, the property of flow of melted material is changed mainly from the pressure. 

PET features & property: 

The glass conversion temperature of PET is about 165℃, and the temperature range of the material is 120~220℃. PET has a strong hygroscopicity at high temperature. For glass fiber reinforced PET materials, bending deformation is very easy to occur at high temperature. 

The crystallinity of the material can be improved by adding a crystalline reinforcing agent. The transparent products made with PET have glossiness and heat distortion temperature. Special additives such as mica can be added to PET to minimize the bending deformation. If lower mold temperature is used, transparent products can be obtained by using non – filled PET materials. 

Injection molding process: 

The injection molding method is mainly used to enhance the molding of PET. A screw type injection machine is usually used. The screw usually needs to be hardened to avoid wear after long term use. The length of the nozzle hole of the injection machine should be as short as possible, and the diameter of the injection machine should be controlled around 3mm. 

The melting point of the enhanced PET is up to 260 ℃. In order to prevent the nozzle clogging, a higher power heater should be installed. In addition, the tip of the nozzle hole is best processed as a reverse cone as shown in figure -1, so that the molten material in the flow channel and the nozzle can be cut off conveniently. 

Injection molding machine: 

Injection molding is mainly used to enhance the molding of PET. PET can only be molded by a screw type injection molding machine in general.

It is best to choose a mutant screw with a reverse ring at the top, which has a large surface hardness and wear resistance, and the ratio of the length to diameter ratio not L/D = (15~20):1 compression is about 3:1.

L/D too large materials stay in the barrel for too long, excessive heat can cause degradation and affect the performance of products. The compression ratio is too small to produce less heat, easy to plasticization and poor performance. On the other hand, the fracture of glass fiber will be more, and the mechanical properties of the fiber will be reduced. When the glass fiber reinforced PET is reinforced, the inner wall of the barrel is badly worn, and the barrel is made of wear-resistant material or lined with wear-resistant material.

As the nozzle is short, the inner wall requires polishing and the aperture is as large as possible. The hydraulic brake valve type nozzle is good. The nozzle should have heat preservation and temperature control measures to ensure that the nozzle will not be frozen and blocked. But the nozzle temperature is not too high, otherwise cause salivation. The low pressure PP material must be used before starting the molding, and the barrel is cleaned.

The main injection molding conditions of PET:

1, the temperature of the barrel. The forming temperature range of PET is narrow, and the temperature will directly affect the performance of the product. If the temperature is too low, it is not good to make plastic plastic parts, depression, lack of material defects; on the contrary, the temperature is too high, will cause the spill, nozzle salivation, deep color change, the decline in mechanical strength, even cause degradation. Usually, the barrel temperature is controlled at 240~280℃, and the barrel temperature of glass fiber reinforced PET is 250~290℃. The temperature of nozzles should not exceed 300 ℃, and the temperature of nozzles is generally lower than that of barrel temperature.

2, mold temperature. The mold temperature has a direct effect on the cooling rate and crystallinity of the melt, and the crystallinity is different, and the properties of the plastic parts are different. Usually, the mold temperature is controlled at 100~140℃. When the thin-walled plastic parts are formed, it is advisable for small value; when the thick wall plastic parts are formed, it is advisable to have a large value.

3. Injection pressure. The fluidity of PET melts is good and easy to be formed. Medium pressure is usually used, pressure is 80~140MPa, and the injection pressure of glass fiber reinforced PET is 90~150MPa. The determination of injection pressure should consider the viscosity of PET, the type and quantity of filler, the location and size of gate, the shape and size of plastic part, the mold temperature, the type of injection molding machine and so on.

How much do you know about the processing method of PET plastic?

 1, the treatment of plastic

Because the PET macromolecule contains lipid base and has a certain hydrophilicity, the pellets are sensitive to water at high temperature. When the moisture content exceeds the limit, the molecular weight of PET decreases, and the products are colored and crisp. In this case, the material must be dried before processing. The drying temperature is 150, 4 hours, which is generally 170, 3-4 hours. The air – ejection method is used to test whether the material is completely dry. 

2. Selection of injection molding machine

PET because of its short time after melting point and high melting point, it is necessary to choose injection system with more temperature control section and less self heating when plasticizing, and the actual weight of the product can’t be less than 2/3 of machine injection volume. Based on these requirements, in recent years, Ramada has developed a small and medium-sized series of PET special plasticizing systems. The clamping force is selected by more than 6300t/m2.

3. Design of mold and gate

PET bottle embryo is usually molded by hot runner mold. The insulation board between mold and injection molding machine is best insulated with 12mm thickness, and the heat insulation plate will withstand high pressure. The exhaust must be sufficient to avoid partial overheating or fragmentation, but the depth of the exhaust port is generally not more than 0.03mm, otherwise it is easy to produce the fly edge.

4. Melting temperature

It can be measured by air ejection method. At 270-295 ℃, the enhancement level GF-PET can be set to 290-315℃.

5. Injection speed

The general injection speed is fast, which prevents the early solidification of the injection. But too fast, the high shear rate makes the material fragile. The ejection is usually completed in 4 seconds.

6, back pressure

The lower the better, so as not to wear. Generally not more than 100bar. It is usually not used.

7. Detained time

Do not use too long detention time to prevent the decline of molecular weight. Try to avoid the temperature above 300 ℃. If the shutdown is less than 15 minutes. It is only required to do air – fire treatment; if more than 15 minutes, the viscosity PE should be cleaned and the barrel temperature is reduced to PE temperature until the machine is reopened.

8. Precautions

If the mold temperature control is not good or the material temperature control is undeserved, easy to produce “white and opaque”. The mold temperature is low and uniform, the cooling speed is fast, the crystallization is less, and the product is transparent.