3. Selection and Control of Injection Molding Process Conditions

There are many factors affecting the quality of injection moulding parts. After the recognition of moulding materials and moulding equipment (including moulds), the selection and control of process conditions in the process of injection moulding have become the key factors. Generally speaking, the process conditions of injection moulding mainly include temperature, pressure and moulding cycle.

3.1 temperature

In the process of injection moulding, the temperature to be controlled mainly includes barrel temperature, nozzle temperature and mould temperature.

1) barrel temperature

When recognizing the temperature of the barrel, the plastics should be plasticized evenly, and can be filled smoothly without degrading the plastics. The acceptance principle of barrel temperature is as follows.

(1) For plastics with poor thermal stability, such as PVC and POM, lower barrel temperature should be chosen; for plastics with wide heating temperature range and high melting viscosity, higher barrel temperature should be obtained. For the same kind of plastics with high average molecular weight and narrow molecular weight distribution, the barrel temperature should be high, whereas the barrel temperature should be low.

For plastics with plasticizers, lubricants and other additives, the barrel temperature can be relatively low because of its low viscosity and good activity. For plastics with glass fiber or other solid fillers, the barrel temperature should be relatively high. In short, the temperature of the barrel should take into account the thermal stability and rheological function of the moulding material.

On the basis of reasonable selection, and in the whole forming process to adhere to stability. In order to prevent the material from overheating and differentiation, besides choosing and controlling the higher temperature of the barrel, the residence time of the material in the barrel should also be strictly controlled, which is particularly important for plastics with poor thermal stability. The residence time of the material in the barrel depends on the relative proportion of the actual injection volume to the larger injection volume of the injection molding machine and the plasticizing ability of the injection molding machine.

(2) The recognition of barrel temperature is also related to the moulding products and die structure. In general, when injection moulding thin-walled, long process, disordered structure, metal insert products, the barrel temperature should be higher; otherwise, there may be defects of products with unequal filling.

(3) Under the same other conditions, the different types of injection moulding machines are selected, and the barrel temperature should be changed accordingly. For example, the barrel temperature of screw injection moulding machine is 10 - 20 C lower than that of plunger injection moulding machine.

(4) The barrel temperature is also bound up with other technological conditions. Sometimes changing the barrel temperature will have the same effect as adjusting other technological conditions. For example, in order to trudge the filling length of melt, the method of trudging barrel temperature can be used, and the corresponding trudging injection pressure and speed can also be chosen. Therefore, the recognition of barrel temperature needs to be considered in combination with other process conditions.

(5) The temperature distribution of the barrel generally increases gradually from the hopper to the nozzle in order to ensure the plastics temperature rising steadily and reach the goal of uniform plasticization. In some special cases, this distribution can also be changed. For example, when the moisture content is high, the temperature of the back section of the barrel can be slightly higher to facilitate water vapor discharge. For example, screw injection moulding machine can generate a lot of shear heat, in order to prevent material differentiation, the temperature of the front section can be slightly lower than that of the middle section.

Whether the barrel temperature recognition is reasonable or not can be judged by the following two methods.

(1) If the material flow is uniform, lubricated, no bubbles and uniform color, the temperature of the feeding barrel is suitable; if the surface of the material flow is rough, there is silver wire or discoloration, the temperature of the feeding barrel is not suitable.

(2) Direct inquiry of the appearance of the product, if there is no defect, that is to say, the temperature of the barrel is appropriate. The following problems should be paid attention to in the temperature control of the barrel. The dispatch of barrel temperature should ensure that plastics are superb in plasticization and can be injected smoothly without causing differentiation. As long as under the condition of full plasticization, the plastic talent entering the cavity fills the cavity with smooth movement and replicates the shape of the cavity close to the requirement of arrival.

Suppose the plasticization temperature is too low and the plasticization is not uniform, the surface of the product will be ripple marks, and the color will be dim. Assuming that the plasticizing temperature is too high, some plastics will be vaporized due to differentiation. The surface of products will be frosted slightly, while the surface will be crazed and foamed heavily.

Sometimes in production, because of poor tolerance to high temperature and other reasons, such as coloring agent, intentionally lowering the plasticizing temperature, forced filling with trekking injection pressure or injection speed and other methods, resulting in increased internal stress of the product, which is simply deformed, split or damaged in storage or application environment. Therefore, in order to take care of the appearance quality of products, the first thing is to ensure the mechanical strength and hardship of products. To achieve this, the sufficient plasticization of materials is the requirement.

(2) The melting temperature of plastics not only affects the processing function, but also affects the surface quality and color of products. The higher temperature near the end of the barrel is higher than the active temperature (or melting point), which is the base guarantee of the forming process.

However, in order to improve the apparent quality of the products, the melting temperature should be trudged before the plastic differentiation temperature is reached, so that the surface gloss of the products will be increased and the color will be more uniform and common. Generally speaking, the melting temperature is better than the temperature required to cover the cavity, which can travel 20-30 degrees Celsius. (3) Because the temperature difference between the material temperature and the barrel temperature is different in different injection moulding machines, the temperature indicated by the thermometer can not be regarded as the practical temperature of the melt in the barrel at any time, but as the reference or scheduling basis of the practical temperature. The temperature indicated by the millivoltmeter of the screw injection moulding machine is close to or slightly lower than the practical temperature of 1I-17 C, while that of the plunger injection moulding machine is 11-17 C higher than that of the plunger injection moulding machine.