18 Important Tips For Injection Moulders

Table of Contents

Injection Molding is the manufacturing process of injecting heated and melted material into a mold and then cooling and curing it to produce a molded product.

This article explains in detail, If you have an estimate of the press size you will need, you can identify plastic injection molding companies that will meet your requirements. We hope it will be helpful to you.

Injection Nozzles

The melt usually flows from the nozzle into the injection port, but in some molds, the nozzle is part of the mold because it extends to the bottom of the mold.

There are two main types of injection nozzles: open nozzles and closed nozzles. Open nozzles should be used more often in injection molding project production because they are cheaper and have less potential for stagnation.

If the injection molding machine is equipped with a depressurization device, this type of nozzle can be used even for low viscosity melts. Sometimes it is necessary to use a closed nozzle, which acts as a stop valve to hold back the plastic in the injection cylinder.

Make sure that the nozzle is properly inserted into the injection sleeve and that the top hole is slightly smaller than that of the injection sleeve, which allows the nozzle to be withdrawn from the injection mold more easily. 

The hole in the nozzle sleeve should be 1 mm larger than the nozzle, i.e., the nozzle radius should be 0.5 mm thinner than the nozzle sleeve radius. 

Filter and a combination nozzle

Impurities in the plastic can be removed by the filter of the extension nozzle, i.e. the melt and plastic flow through a channel that is divided into narrow spaces by the insert. These narrows and gaps remove impurities and improve the mixing of the plastic.

Therefore, by extension, fixed mixers can be used to achieve better mixing results. These devices can be installed between the injection cylinder and the nozzle to separate and remix the melt, most of which is to make the melt flow through the stainless steel channel. 

Exhaust

Some plastics need to be exhausted in the injection cylinder during injection molding to allow the gas to escape. In most cases, these gases are just air, but they may be molten putout moisture or single molecule gas. 

If this gas is not released, it will be compressed by the melt and carried to the mold surface, where it will expand and form bubbles in the product.

To remove the gas before it reaches the injection nozzle or mold, the melt can be depressurized in the injection cylinder by reducing or decreasing the screw root diameter. Here, the gas can escape through a hole or holes in the injection cylinder. 

The screw root diameter is then increased and the melt, which has been de-volatilized, is directed to the injection nozzle. An injection molding machine equipped with this facility is called a vending machine. 

This kind of exhaust injection molding machine should have a catalytic burner above the good fume exhauster to remove the potentially harmful gases. 

The effect of increasing the back pressure

To obtain a high-quality melt, the plastic must be heated or melted consistently and mixed thoroughly. The correct screw is used to achieve proper melting and mixing, and sufficient pressure (or back pressure) is available in the injection cylinder to obtain consistent mixing and heat.

Increasing the resistance of the return oil creates back pressure in the shot cylinder. However, it takes longer for the screw to reset, so there is more wear and tears in the injection molding machine drive system. Maintaining back pressure and isolation from the air as much as possible also requires a consistent melt temperature and mixing level.

Stop flow valve

Regardless of the type of screw used, the tip is usually equipped with a stop valve. If a stopcock is used, it must be checked periodically, as it is an important part of the shot cylinder.

Currently, switching nozzles are not commonly used because of the tendency to leak plastic and disintegrate within the nozzle equipment. Currently, there is a nozzle type specified for each plastic.

Screw setback

Many injection molding machines are equipped with a screw retraction or suction back device. The screw is hydraulically withdrawn to suck back the plastic at the tip of the nozzle when it stops rotating. 

This device allows the use of open nozzles. The amount of suck-back may be reduced because the ingress of air can cause problems for some plastics.

Screw bedding

Most of the injection cycles require the rotation of the screw village to be adjusted so that when the screw is finished injecting, there will be a small amount of soft cushioned plastic left over to ensure that the screw achieves an effective advance time and maintains a constant firing pressure.

The cushion material for small injection molding machines is about 3mm; for large injection molding machines, it is 9m and must be kept constant regardless of the size of the screw cushion material used. Nowadays, the size of the screw bedding can be controlled by 0.11mm. 

Screw rotation speed

The rotation speed of the screw significantly affects the stability of the injection molding process and the amount of heat applied to the plastic. The faster the screw rotates, the higher the temperature.

 When the screw rotates at high speed, the frictional (shear) energy transferred to the plastic increases the plasticizing efficiency but also increases the unevenness of the melt temperature.

Because of the importance of the surface speed of the screw, the rotational speed of the screw in a large injection molding machine should be less than that of a small injection molding machine, because the shear heat energy generated by a large screw is much higher than that of a small screw at the same rotational speed. The speed of screw rotation varies depending on the plastic.

Injection volume

Syringe molding machines are usually evaluated by the amount of PS that can be injected in each shot, which may be measured in ounces or grams. Another scheduling system is based on the volume of melt that can be injected by the injection molding machine.

Plasticizing capacity

An injection molding machine is usually evaluated based on the amount of PS material that can be uniformly melted in one hour, or the amount of PS that can be heated to a uniform melt temperature (in pounds or kilograms), which is called plasticizing capacity.

Estimation of plasticizing capacity

To determine whether the quality of production can be maintained throughout the production process, a simple formula for yield and plasticizing capacity can be used, as follows: t=(total shot volume gX3600)÷(plasticizing volume of injection molding machine kg/hX1000) t is the minimum cycle time.

If the cycle time of the mold is lower than the value of it, the injection molding machine will not be able to plasticize the plastic sufficiently to achieve uniform melt viscosity, so the injection molded parts often have deviations. The deviation often occurs. 

In particular, when it comes to designing custom molded plastic parts, it is important to design for the uniform wall thickness.

Improve proper wall thickness and design to avoid running into the labelled and aforementioned troubles; when injection molding design thin-walled or precision tolerance products, the injection volume, and plasticizing volume must match each other.

Injection cylinder retention time

The rate of plastic decomposition is dependent on temperature and time. For example, the plastic will decompose after some time at a high temperature, but at a lower temperature, it will take a longer time to decompose. Therefore, the retention time of the plastic in the injection cylinder is very important.

The actual retention time can be determined experimentally by measuring the time it takes for colored plastics to pass through the injection cylinder, which can be roughly calculated by the following formula: t = (nominal injection cylinder volume g X cycle time S) ÷ (injection volume g X 300) Note that some plastics have a longer retention time in the injection cylinder than the calculated time because they can clump in the injection cylinder.

Calculating retention time and importance

It is common practice to calculate the residence time of a given plastic in a particular liquid silicone rubber injection molding machine. 

Especially in large injection molding machines with low shot volumes, the manufacturing plastic parts tend to break down, which is not detectable from observation. If the dwell time is short, the plastic will not plasticize uniformly; if the dwell time is long, the plastic properties will decay.

Therefore, it is important to keep the retention time consistent. Method: Ensure that the plastic input to the injection molding machine has a consistent compositio,n, size and shape. Report any malfunctions or losses of the injection molding machine to the maintenance department.

Injection Cylinder Temperature Environment

It should be noted that the melt temperature is important and any injection cylinder temperatures used are only guidelines. I

If you have no experience in processing a particular plastic, start with the lowest setting. Usually the, first zone is set at the lowest temperature to prevent premature melting and sticking of the plastic in the inlet.

The temperature in the other zones is then gradually increased until it reaches the nozzle, often at a slightly lower temperature at the tip of the nozzle to prevent dripping. 

The mold is also heated and cooled, and due to the size of many injection molds, the molds are also differentiated, but unless specified, the zones should be set to the same size.

Melt temperature

The melt temperature can be measured by measuring the nozzle or by the air jet method. When using the latter method, care must be taken to ensure that no accidents occur when cleaning hot melt plastic, as the high temperature of hot melt plastic can burn or even corrode the skin. 

In an injection molding plant, burns can be accidental. Therefore, gloves and a face shield should be worn when handling hot plastic or when there is a risk of hot plastic splashing. 

To ensure safety, the tip of the heat control needle should be preheated to the temperature to be measured.

Each plastic has a specific melt temperature, and the actual injection cylinder adjustment to reach this temperature depends on the screw village rotation speed, back pressure, injection volume, and injection cycle.

Mold temperature

Always check that the injection molding machine is set and running at the temperature specified on the log sheet.

This is very important because temperature affects the surface finish and yield of the injection molded part. All measured values must be recorded and the machine checked at the specified time.

Uniform cooling

The finished injection molded part must be cooled uniformly, i.e., different parts of the mold must be cooled at different rates so that the whole part will be cooled uniformly. 

The injection molded part must be cooled as fast as possible, while ensuring that no defects, such as uneven surface, physical property changes, etc., occur.

The cooling rate of each part of the injection mold tool must be equal, but it means that the mold is cooled unevenly, for example by feeding cold water to the inner part of the mold and using warmer water to cool the outside of the mold. 

This technique should be used in the case of precision flat products with syringe tolerances or large products with a long melt flow at the water gate.

Temperature and Cooling Check

Always check that the injection molding machine is set and running at the temperature specified on the log sheet. This is very important because temperature affects the surface finish and yield of the injection molded part.

All measured values must be recorded and the plastic injection molding machine must be checked at the specified time.

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