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Injection molding characteristics and precautions of transparent plastic - Maypak

December 27, 2023

Most transparent plastics have high melting points and poor fluidity. Therefore, in order to ensure the surface quality of the product, it is often necessary to make fine adjustments to the process parameters such as machine temperature, injection pressure, and injection speed, so that the mold can be filled with the plastic without causing any Internal stress causes product deformation and cracking. This article talks about matters that should be paid attention to in terms of raw material preparation, equipment and mold requirements, injection molding process and product raw material handling.

PET (polyethylene terephthalate) transparent plastic is usually suitable for the outer packaging of many cosmetics, such as:

Bottles and Containers: Plastic Bottles and Plastic Jars for a variety of perfumes, lotions, skin care products and cosmetics.

Bottle caps and lids: PET plastic is also used to make bottle caps and lids, which effectively seal bottles and containers.

Plastic tubes: Used in tube packaging of lipsticks, eye creams, facial masks and other products.

Cosmetic Jar and containers: PET can be made into transparent cosmetic boxes and containers, which can be used for various cosmetic products.

PET material is often chosen for its clarity, durability, and chemical resistance, making it one of the common choices for cosmetic packaging.

01.  Pay Attention To The Process Characteristics Of Three Commonly Used Transparent Plastics   

PMMA Process Characteristics

PMMA has high viscosity and slightly poor fluidity. It requires high material temperature and high injection pressure. The impact of injection temperature is greater than the injection pressure. Increased injection pressure is beneficial to improving the shrinkage rate of the product

The injection temperature range is wide, the melting temperature is 160°C, and the decomposition temperature reaches 270°C. Therefore, the material temperature adjustment range is wide and the processability is good. Therefore, to improve fluidity, we can start with the injection temperature.

It has poor impact resistance, poor wear resistance, is easy to scratch, and is easy to be brittle. Therefore, the mold temperature should be increased and the condensation process should be improved to overcome these defects.

PC Process Characteristics

PC has high viscosity, high melt temperature, and poor fluidity. It must be injected at a higher temperature (between 270 and 320°C). Relatively speaking, the material temperature adjustment range is narrow, and the processability is not as good as PMMA.

The injection pressure has little effect on the fluidity, but due to the high viscosity, the injection pressure must be higher. In order to prevent the generation of internal stress, the pressure holding time must be as short as possible.

The shrinkage rate is large and the dimensions are stable, but the internal stress of the product is large and it is easy to crack. Therefore, it is better to increase the temperature rather than the pressure to improve the fluidity. The possibility of cracking can be reduced by increasing the mold temperature, improving the mold structure and post-processing. When the injection speed is low, defects such as ripples are prone to occur at the gate. The temperature of the radiation nozzle must be controlled separately, the mold temperature must be high, and the resistance of the runner and gate must be small.

PET Process Characteristics

PET molding temperature is high and the material temperature adjustment range is narrow (260-300℃). However, after melting, the fluidity is good, so the processability is poor, and an anti-flow device is often added to the nozzle. The mechanical strength and performance are not high after injection, and the properties must be improved through the stretching process and modification. Accurate control of mold temperature is an important factor in preventing warpage and deformation. It is recommended to use hot runner molds.

02.  Preparation And Drying Of Raw Materials   

Because any impurity contained in the plastic may affect the transparency of the product, storage and transportation. During the feeding process, attention must be paid to sealing to ensure that the raw materials are clean.

In particular, the raw materials contain moisture, which will cause deterioration of the raw materials after heating, so they must be dry, and during injection molding, a dry hopper must be used for feeding. It should also be noted that during the drying process, the input air should preferably be filtered and dehumidified to ensure that it does not contaminate the raw materials.

03.  Cleaning Of Barrel, Screw And Accessories   

In order to prevent raw material contamination and the presence of old materials or impurities in the recesses of the screw and accessories, especially resins with poor thermal stability, screw cleaning agents should be used to clean all parts before use and after shutdown to prevent impurities from sticking. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.

When temporarily shutting down, in order to prevent raw materials from staying at high temperature for a long time and causing degradation, the temperature of the dryer and barrel should be lowered. For example, the barrel temperature of PC, PMMA, etc. should be lowered to below 160°C. (The hopper temperature should drop below 100℃ for PC)

04.  Mold Design (Including Product Design)   

In order to prevent poor backflow or uneven cooling resulting in poor plastic molding, surface defects and deterioration, generally the following points should be noted when designing the mold:

a) The wall thickness should be as uniform as possible, and the draft angle should be large enough;

b) Transitions should be gradual. Smooth transition to prevent sharp corners. Sharp edges are generated, especially PC products must not have gaps;

c) Gate. The flow channel should be as wide and short as possible, and the gate position should be set according to the shrinkage and condensation process. If necessary, a cold well should be added;

d) The mold surface should be smooth and clean with low roughness (preferably less than 0.8);

e) Vent. The tank must be sufficient to discharge air and gases in the melt in a timely manner;

f) Except for PET, the wall thickness should not be too thin, generally not less than 1 mm.

05.  Injection molding process and injection molding machine   

In order to reduce internal stress and surface quality defects, the following aspects should be paid attention to in the injection molding process:

a) An injection molding machine with a special screw and a separate temperature-controlled injection nozzle should be used;

b) As long as the injection temperature does not decompose the plastic resin, it is appropriate to use a higher injection humidity;

c) Injection pressure: Generally high to overcome the defect of high melt viscosity, but too high a pressure will produce internal stress, causing difficulty in demoulding and deformation;

d) Injection speed: When the mold filling is satisfied, it should generally be low. It is best to use slow, fast and slow multi-stage injection;

e) Holding time and molding cycle: Under the condition that the product is fully filled and no dents or bubbles are generated, it should be as short as possible to minimize the residence time of the molten material in the barrel;

f) Screw speed and back pressure: On the premise of meeting the plasticizing quality, they should be as low as possible to prevent the possibility of degradation;

g) Mold temperature: The cooling quality of the product has a great impact on the quality, so the mold temperature must be able to accurately control the process. If possible, the mold temperature should be higher.

*Maypak packaging is Plastic Bottles Manufacturer and has been focusing on Plastic Bottles for more than 17 years and was established in 2006,the company enjoys a good reputation at home and abroad. If you have any questions about lipstick tubes, please contact us.

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