What is injection molding?

March 28, 2022

By siedery

Through an article to help you understand and enter the injection mold industry.

Structure of the injection moulding machine

Injection molding is also known as injection molding, which is a kind of injection and molding method. The advantages of the injection molding method are fast production speed, high efficiency, automatic operation, variety of colors, simple to complex shapes, large to small sizes, accurate product sizes, easy to update products, and the ability to become a complex shape of the parts, injection molding is suitable for mass production and complex shape products and other molding processing fields.

This method is used to obtain molded products by mixing completely molten plastic material by a screw at a certain temperature, injecting it into the mold cavity with high pressure, and then curing it by cooling. This method is suitable for the mass production of complex-shaped parts and is one of the important processing methods.

The injection molding process can be roughly divided into the following 6 stages Stage

Mold closing, injection, pressure-holding, cooling, mold opening, and product removal.

The above process is repeated to produce products in a batch cycle. The molding of thermoset plastics and rubber also includes the same process, but the barrel temperature is lower than that of thermoplastics, the injection pressure is higher, the mold is heated, and the material is cured or vulcanized in the mold after injection, and then the film is removed while it is still hot.

The trend of today’s processing process is developing in the direction of high-tech, these technologies include micro-injection molding, high-fill composite injection molding, water-assisted injection molding, a mixture of various special injection molding processes, foam injection molding, mold technology, simulation technology, etc.

History

In 1868, Hyatt developed a plastic material that he named celluloid. Celluloid had already been invented in 1851 by Alexander Parkes. Hyatt improved it so that it could be processed into finished shapes. Hyatt, together with his brother Isaiah, registered the patent for the first plunger injection machine in 1872. This machine was relatively simple compared to the machines used in the 20th century. It operated basically like a giant hypodermic needle. This giant needle (diffuser cartridge) injected plastic into the mold through a heated cylinder.

World War II in the 1940s created a huge demand for inexpensive, mass-produced products. In the 1940s the Second World War created a huge demand for inexpensive, mass-produced products.

In 1946, American inventor James Watson Hendry built the first injection molding machine, which allowed for more precise control of injection speed and quality of the items produced. This machine also allowed materials(To learn more about the materials commonly used in injection molding please click here.) to be mixed prior to injection so that colored or recycled plastics could be thoroughly mixed for injection into virgin material. 1951 saw the development of the first screw injection machine in the United States, which was not patented, and this device continues to be in use.

In the 1970s, Hendry went on to develop the first gas-assisted injection molding process and allowed the production of complex, hollow products that cooled quickly. This greatly increased design flexibility as well as the strength and endpoint of manufactured parts while reducing production time, cost, weight, and waste.

Injection molding process

Temperature Control

Introduction to the structure of injection moulding machines
  1. barrel temperature: injection molding process needs to control the temperature of the barrel temperature, nozzle temperature, and mold temperature, etc... The first two temperatures mainly affect the plasticization and flow of plastic, while the latter temperature mainly affects the flow and cooling of plastic. Each kind of plastic has a different flow temperature, the same kind of plastic, due to different sources or grades, its flow temperature and decomposition temperature is different, this is due to the average molecular weight and molecular weight distribution is different, plastic in different types of injection machine plasticization process is also different, so the choice of barrel temperature is also different.
  2. Nozzle temperature: The nozzle temperature is usually slightly lower than the maximum barrel temperature, in order to prevent the possible “salivation” of the melt in the straight-through nozzle. Nozzle temperature can not be too low, otherwise, it will cause early condensation of the melt and the nozzle will be blocked, or due to early condensation of the material into the cavity and affect the performance of the product.
  3. The temperature of the mold: the temperature of the mold has a great impact on the intrinsic properties and apparent quality of the products. The temperature of the mold is determined by the presence or absence of plastic crystallinity, the size and structure of the product, performance requirements, and other process conditions (melt temperature, injection speed, and injection pressure, molding cycle, etc.).

Pressure Control

pressure history  in an injection molded part

The pressure in the injection process includes plasticizing pressure and injection pressure and directly affects the plasticization of plastic and product quality.

  1. plasticizing pressure: (back pressure) using a screw injection machine, the top of the screw melt in the screw rotation back pressure is called plasticizing pressure, also known as back pressure. The size of this pressure can be adjusted by the relief valve in the hydraulic system. In injection, the magnitude of the plasticizing pressure varies with the design of the screw, the quality requirements of the product, and the type of plastic. If these conditions and the speed of the screw are said to be constant, increasing the plasticizing pressure will strengthen the shearing effect, i.e., it will increase the temperature of the melt, but it will reduce the efficiency of plasticization, increase backflow and leakage, and increase the driving power.
  2. In addition, increasing the plasticizing pressure often leads to a uniform temperature of the melt, a uniform mixing of the colorants, and the discharge of gases from the melt. General In operation, the decision of plasticizing pressure should be as low as possible under the premise of ensuring the excellent quality of the product, the specific value of which varies with the variety of plastic used, but usually rarely exceeds 20 kg/cm2.
  3. injection pressure: In current production, the injection pressure of almost all injection machines is based on the top of the plunger or screw on the plastic

The pressure applied by the top of the plunger or screw to the plastic (converted from the oil line pressure) prevails. The role of injection pressure in injection molding is to overcome the resistance to the flow of plastic from the barrel to the cavity, to give the rate of molten material filling the mold, and to compact the molten material.

Molding Cycle Time

molding cycle time in detail

The time required to complete an injection molding process is called the molding cycle, also known as the molding cycle. It actually consists of the following parts.

Molding cycle: 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 much as possible under the premise of quality assurance. In the whole molding cycle, injection time and cooling time are the most important, and they have decisive influence on the quality of products. The mold filling time in the injection time is directly inversely proportional to the mold filling rate, and the mold filling time in production is generally about 3-5 seconds. The holding time in the injection time is the pressure time of the plastic in the cavity, which accounts for a large proportion of the entire injection time, generally about 20-120 seconds (up to 5-10 minutes for extra thick parts). Before the melt is frozen at the gate, the amount of holding time has an effect on the dimensional accuracy of the product, but if it is later, it has no effect. The holding time also has a minimum value, which is known to depend on the material temperature, the mold temperature, and the size of the main channel and gate. If the size of the main channel and gate as well as the process conditions are normal, the pressure value that yields the least fluctuation in the shrinkage of the product usually prevails. The cooling time is mainly determined by the thickness of the product, the thermal and crystalline properties of the plastic, and the mold temperature. The end of the cooling time should be to ensure that the products do not cause changes when the mold is removed as a principle, cooling time is generally about 30 to 120 seconds between the cooling time, cooling time is too long unnecessary, not only to reduce production efficiency, complex parts will also cause difficulties in the release of the mold, forced release will even produce release stress. The other time in the molding cycle is related to whether the production process is continuous and automated, as well as the degree of continuity and automation.

Parameters

  1. injection pressure

The injection pressure is provided by the hydraulic system of the injection molding system. The pressure of the hydraulic cylinder is transferred to the plastic melt through the injection molding machine screw. The plastic melt is pushed by the pressure and enters the vertical runner (for some molds it is also the main flow channel), main flow channel, and manifold of the mold, and enters the mold cavity through the gate, which is the injection process, or called the filling process. Pressure exists 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 that the filling process proceeds smoothly.

During the injection molding process, the pressure is highest at the injection molding machine nozzle to overcome the resistance to flow throughout the melt. Subsequently, the pressure gradually decreases along the flow length toward the front end of the melt at the wave front. If the internal exhaust of the mold cavity is good, the final pressure at the front end of the melt is the atmospheric pressure.

There are many factors affecting the melt filling pressure, which are summarized in 3 categories: (1) material factors, such as the type of plastic, viscosity, etc.; (2) structural factors, such as the type, number, and location of the pouring system, the shape of the mold cavity and the thickness of the product, etc.; (3) process elements of molding.

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 very short and has little influence on the molding cycle, the adjustment of injection time plays a great role in controlling the pressure of gates, runners and cavities. A proper injection time helps to fill the melt ideally and is very important to improve the surface quality of the product and reduce the dimensional tolerance.

The injection time is much lower than the cooling time, about 1/10 to 1/15 of the cooling time, and this rule can be used as a basis for predicting the full molding time of the part. In the mold flow analysis, the injection time in the analysis is equal to the injection time set in the process conditions only if the melt is completely filled by the screw rotation. If the screw pressure switch occurs before the cavity is filled, the analysis result will be greater than the process conditions.

3. Injection temperature

The injection temperature is an important factor affecting the injection pressure. The injection molding machine barrel has 5 to 6 heating sections, and each raw material has its own suitable processing temperature (detailed processing temperature can be referred to the data provided by the material supplier). The injection molding temperature must be controlled within a certain range. If the temperature is too low, the melt will be poorly plasticized, affecting the quality of the molded parts and making the process more difficult; if the temperature is too high, the raw material will be easily decomposed. 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 the performance of the material, up to 30 ℃. This is due to the high heat generated by the shearing of the melt through the injection port. There are two ways to compensate for this difference in mold flow analysis: by trying to measure the temperature of the melt during empty injection, or by including the injection nozzle in the modeling.

4. holding pressure and holding time

Near the end of the injection process, the screw stops rotating and simply advances, at which point the injection enters the holding pressure phase. The nozzle of the injection molding machine is constantly replenishing the cavity to fill the volume left empty by the shrinkage of the part. If the cavity is filled without holding pressure, the part will shrink about 25%, especially at the tendons due to excessive shrinkage and the formation of shrinkage marks. The holding pressure is generally about 85% of the maximum filling pressure, but of course it is determined according to the actual situation.

5. back pressure

Back pressure is the pressure that needs to be overcome when the screw reverses and recedes to store the material. The use of high back pressure is good for the dispersion of color and melting of plastic, but at the same time it prolongs the screw retraction time, reduces the length of plastic fiber and increases the pressure of the injection molding machine, so the back pressure should be lower, generally not more than 20% of the injection pressure. When injecting foam, 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 in screw length during melting, which will reduce the heat input and bring down the temperature. However, since the result of such a change is difficult to estimate, it is not easy to adjust the machine accordingly.

Defect Resolution

The injection molding process is a complex process involving mold design, mold manufacturing, raw material characteristics and raw material pretreatment methods, molding process, injection molding machine operation and other factors, and is closely related to the processing environment conditions, product cooling time, post-treatment process. Therefore, the quality of the product does not only depend on the injection accuracy of the injection molding machine, metering accuracy, or just by the merits of the mold design and the accuracy level of the mold processing, usually, it will also be affected and constrained by the other factors mentioned above.

Under the constraints of so many compound factors, the appearance of defects in injection molded products is inevitable, so it is especially important to seek the inner mechanism of defects and predict the location and types of defects that may be generated in the products, and use them to guide the mold design and improvement, summarize the laws of defects, and formulate more reasonable process operating conditions. We will explain the mechanism and solution 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, the types and forms of mold design are also varied, in addition, the familiarity of operators with specific injection molding machines and the differences in operating skills and practical experience among workers also vary, at the same time, the objective environment (such as ambient temperature, humidity, air cleanliness) also varies with seasonal changes. These objective and subjective conditions together determine the generation of defects in injection molded products.

Generally speaking, there are three main aspects in the evaluation of the performance of plastic products.

First, the quality of appearance, including integrity, color, and luster.

Second, the accuracy between dimensions and relative positions, i.e., dimensional accuracy and positional accuracy.

Third, the mechanical properties, chemical properties, electrical properties, etc. corresponding to the use, i.e., functionality

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

Classification of common defects of injection molded products

Appearance defects: silver stripe discoloration, fusion marks

Process problems: flying edge, shrinkage, lack of glue

Performance problems: warpage, brittleness

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