What Is Injection Molding?

Injection molding, also known as injection molding, is an injection and molding method.

The advantages of the injection molding method are fast production speed, high efficiency, automation of operation, variety of designs and colors, simple to complex shapes, large to small sizes, accurate product sizes, easy updating of products, and complex shapes.

Injection molding is suitable for mass production and complex shape products and other molding processing fields.

At a certain temperature, the fully melted plastic material is stirred by the screw, injected into the mold cavity with high pressure, and then cooled and solidified to obtain a molded product.

This method is suitable for mass production of complex shaped parts and is one of the important processing methods.

Base Information of Injection Molding

The injection molding process can be roughly divided into the following six stages:

Mold closing, glue injection, pressure maintaining, cooling, mold opening and product taking out.

If the above process is repeated, products can be produced periodically in batches.

The molding of thermosetting plastics and rubber also includes the same process, but the barrel temperature is lower than that of thermoplastic plastics, and the injection pressure is higher.

The mold is heated. After the injection of materials, they need to be cured or vulcanized in the mold, and then take off the film while they are hot.

Nowadays, the trend of processing technology is developing in the direction of high-tech.

These technologies include: micro injection molding, high filling composite injection molding, water assisted injection molding, mixed use of various special injection molding processes, foam injection molding, mold technology, simulation technology, etc.

History of Injection Molding

In 1868, Hayat developed a plastic material, which he named celluloid.

Celluloid was invented by Alexander parks in 1851.

Hayat improved it so that it can be processed into finished shapes.

Hayat and his brother Isaiah registered the patent of the first plunger injection machine in 1872.

This machine is relatively simpler than those used in the 20th century.

It basically works like a huge hypodermic needle.

This huge needle (diffusion cylinder) injects plastic into the mold through a heated cylinder.

In the 1940s, the second world war created a huge demand for cheap, mass-produced products.

Low price, mass-produced products.

In 1946, James Watson Hendry, an American inventor, built the first injection molding machine, which made it possible to control the injection speed and quality more accurately.

The machine also enables color or recycled plastics to be thoroughly mixed and injected into raw materials before mixing and injection of materials.

In 1951, the United States developed the first screw type injection machine, which did not apply for a patent, and this device is still in use.

In the 1970s, Hendry went on to develop the first gas assisted injection molding process, which allowed complex, hollow products to be produced and cooled rapidly.

This greatly improves the design flexibility, strength and end-point manufacturing components, while reducing production time, cost, weight and waste.

Injection molding process

Temperature control

1. Barrel temperature:

The temperatures to be controlled in the injection molding process include barrel temperature, nozzle temperature and mold temperature.

The first two temperatures mainly affect the plasticization and flow of plastics, and the second temperature mainly affects the flow and cooling of plastics.

Each kind of plastic has different flow temperature.

For the same kind of plastic, due to different sources or brands, its flow temperature and decomposition temperature are different.

This is due to different average molecular weight and molecular weight distribution.

The plasticizing process of plastic in different types of injection machines is also different, so the barrel temperature is also different.

2. Nozzle temperature:

The nozzle temperature is usually slightly lower than the maximum temperature of the barrel, which is to prevent the “salivation” of molten material in the straight nozzle.

The temperature of the nozzle should not be too low, otherwise the nozzle will be blocked due to the early setting of the melt, or the performance of the product will be affected due to the injection of the early setting material into the mold cavity.

3. Mold temperature:

Mold temperature has a great impact on the internal performance and apparent quality of products.

The temperature of the mold depends on the crystallinity of the plastic, the size and structure of the product, the performance requirements, and other process conditions (melt temperature, injection speed and pressure, molding cycle, etc.).

Pressure control

The pressure in the process of injection molding includes plasticizing pressure and injection pressure, which directly affect the plasticization of plastics and the quality of products.

1. Plasticizing pressure:

(back pressure) When the screw injection machine is used, the pressure that the melt at the top of the screw receives when the screw rotates and retreats is called plasticizing pressure, also known as back pressure.

The magnitude of this pressure can be adjusted by the overflow valve in the hydraulic system.

During injection, the plasticizing pressure needs to be changed according to the design of the screw, the requirements of product quality and the types of plastics.

If these conditions and the rotation speed of the screw are unchanged, increasing the plasticizing pressure will strengthen the shear effect, that is, increase the temperature of the melt, but reduce the plasticizing efficiency, increase the countercurrent and leakage flow, and increase the driving power.

In addition, increasing the plasticizing pressure can often make the melt temperature uniform, the color mixture uniform and the gas in the melt discharged.

In general operation, the plasticizing pressure should be determined as low as possible on the premise of ensuring the good quality of the product.

The specific value varies with the type of plastic used, but usually it rarely exceeds 20kg / cm2.

2. Injection pressure:

In the current production, the injection pressure of almost all injection machines is based on the top of the plunger or screw against the plastic

The applied pressure (converted from the oil circuit pressure) shall prevail.

The role of injection pressure in injection molding is to overcome the flow resistance of plastic from the barrel to the cavity, give the melt a filling rate and compact the melt.

Forming cycle

The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle.

It actually includes the following parts:

Molding cycle: the molding cycle directly affects labor productivity and equipment utilization.

Therefore, in the production process, the relevant time in the molding cycle should be shortened as far as possible on the premise of ensuring the quality.

In the whole molding cycle, injection time and cooling time are the most important, which have a decisive impact on the quality of products.

The filling time in the injection time is directly inversely proportional to the filling rate, and the filling time in production is generally about 3-5 seconds.

The pressure holding time in the injection time is the pressure time of the plastic in the mold cavity, which accounts for a large proportion in the whole injection time, generally about 20-120 seconds (especially thick parts can be as high as 5-10 minutes).

Before the molten material at the gate is frozen, the holding time has an impact on the dimensional accuracy of the product, but not later.

The pressure holding time also has the most favorable value, which is known to depend on the material temperature, mold temperature, and the size of the main runner and gate.

If the dimensions and process conditions of the main runner and gate are normal, the pressure value with the smallest fluctuation range of the shrinkage rate of the product is usually taken as the standard.

The cooling time mainly depends on the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature.

The end point of the cooling time should be based on the principle of ensuring that the product will not change during demoulding.

The cooling time is generally between 30 and 120 seconds.

It is unnecessary to have a long cooling time, which will not only reduce the production efficiency, but also cause difficulty in demoulding complex products.

When demoulding is forced, demoulding stress will even occur.

The other time in the forming cycle is related to whether the production process is continuous and automated and the degree of continuity and automation.

Parameter of Injection Molding

1. Injection pressure

The injection pressure is provided by the hydraulic system of the injection system.

The pressure of the hydraulic cylinder is transmitted to the plastic melt through the screw of the injection molding machine.

The plastic melt, driven by the pressure, enters the vertical runner (also the main runner for some molds), the main runner and the shunt runner of the mold through the nozzle of the injection molding machine, and enters the mold cavity through the gate.

This process is called the injection molding process, or the filling process.

The existence of pressure is to overcome the resistance in the melt flow process, or conversely, the resistance in the flow process needs to be offset by the pressure of the injection molding machine to ensure the smooth filling process.

In the process of injection molding, the pressure at the nozzle of the injection molding machine is the highest to overcome the flow resistance of the melt in the whole process.

After that, the pressure gradually decreases along the flow length to the front end of the melt.

If the internal exhaust of the mold cavity is good, the final pressure at the front end of the melt is atmospheric pressure.

There are many factors affecting the melt filling pressure, which can be summarized into three categories:

  • Material factors, such as plastic type, viscosity, etc;
  • Structural factors, such as the type, number and position of the gating system, the cavity shape of the mold and the thickness of the product;
  • Forming process elements.

2. Injection time

The injection time mentioned here refers to the time required for the plastic melt to fill the cavity, excluding the auxiliary time such as mold opening and closing.

Although the injection time is short and has little effect on the molding cycle, the adjustment of injection time has a great effect on the pressure control of gate, runner and cavity.

Reasonable injection time is conducive to the ideal filling of melt, and it is of great significance to improve the surface quality of products and reduce dimensional tolerance.

The injection time is much lower than the cooling time, which is about 1 / 10 ~ 1 / 15 of the cooling time. This law can be used as the basis for predicting the whole molding time of plastic parts.

In the mold flow analysis, the injection time in the analysis result is equal to the injection time set in the process conditions only when the melt is completely pushed by the screw rotation to fill the cavity.

If the pressure maintaining switch of the screw occurs before the cavity is full, the analysis result will be greater than the setting of the process conditions.

3. Injection temperature

Injection temperature is an important factor affecting injection pressure.

The barrel of the injection molding machine has 5-6 heating sections, and each raw material has its proper processing temperature (refer to the data provided by the material supplier for detailed processing temperature).

The injection temperature must be controlled within a certain range.

If the temperature is too low, the melt plasticization is poor, which affects the quality of the molded parts and increases the difficulty of the process;

If the temperature is too high, the raw materials are easy to decompose.

In the actual injection molding process, the injection temperature is often higher than the barrel temperature, and the higher value is related to the injection rate and material performance, up to 30 ℃.

This is due to the high heat generated by the shear of the molten material when it passes through the injection port.

There are two ways to compensate for this difference in mold flow analysis, one is to try to measure the temperature of the melt during air injection, and the other is to include the nozzle in modeling.

4. Pressure maintaining pressure and time

At the end of the injection molding process, the screw stops rotating and just advances forward. At this time, the injection molding enters the pressure retaining stage.

During the pressure maintaining process, the nozzle of the injection molding machine continuously feeds the cavity to fill the volume left by the shrinkage of the part.

If the pressure is not maintained after the cavity is full, the part will shrink by about 25%, especially the shrinkage marks will be formed at the rib due to excessive shrinkage.

The pressure retaining pressure is generally about 85% of the maximum filling pressure, which of course should be determined according to the actual situation.

5. Back pressure

Back pressure refers to the pressure to be overcome when the screw reverses and retreats to store materials.

The use of high back pressure is conducive to the dispersion of colorants and the melting of plastics, but it also prolongs the screw retraction time, reduces the length of plastic fibers, and increases the pressure of the injection molding machine.

Therefore, the back pressure should be lower, generally not more than 20% of the injection molding pressure.

When injecting foam plastic, the back pressure should be higher than the pressure formed by the gas, otherwise the screw will be pushed out of the barrel.

Some injection molding machines can program the back pressure to compensate for the reduction of screw length during melting, which will reduce the input heat and reduce the temperature.

However, as the result of this change is difficult to estimate, it is not easy to adjust the machine accordingly.

Defect Resolution of Injection Molding

The process of injection molding is a complex processing process that involves many factors such as mold design, mold manufacturing, raw material characteristics, raw material pretreatment methods, molding process, injection molding machine operation, and is closely related to processing environment conditions, product cooling time, and post-processing process.

Therefore, the quality of the product is not only determined by the injection precision and measurement precision of the injection molding machine, or only determined by the quality of the mold design and the precision level of the mold processing.

Generally, it is also affected and restricted by the above other factors.

Under the constraint of so many complex factors, the occurrence of defects in injection molded products is inevitable.

Therefore, it is particularly important to seek the internal mechanism of defects and predict the positions and types of defects that may occur in products, and use them to guide mold design and improvement, summarize the rules of defects generation, and formulate more reasonable process operating conditions.

We will explain the mechanism and solutions of injection molding defects from three main factors that affect the plastic material characteristics, mold structure, injection molding process and injection molding equipment in the injection molding process.

Classification of common defects of injection molded products

The plastic raw materials used in the injection molding process are diverse, and the types and forms of mold design are also diverse. In addition, the familiarity of operators with specific injection molding machines, the operating skills and practical experience of workers are also different.

At the same time, the objective environment (such as ambient temperature, humidity and air cleanliness) will also vary with the seasons.

These objective and subjective conditions jointly determine the occurrence of defects in injection molded products.

Generally speaking, there are three aspects to evaluate the performance of plastic products:

First, appearance quality, including integrity, color and luster;

Second, the accuracy between size and relative position, that is, size accuracy and position accuracy;

Third, the mechanical properties, chemical properties and electrical properties corresponding to the use, that is, functionality;

Therefore, if there is a problem in any of the above three aspects, it will lead to the production and expansion of product defects.

Classification of common defects of injection molded products:

Appearance defects: craze discoloration and weld marks;

Process problems: flash, shrinkage and lack of glue;

Performance problems: warpage and embrittlement.

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