DOI:10.30919/esmm5f713

Received: 30 Mar 2020
Revised: 19 May 2020
Accepted: 20 May 2020
Published online: 22 May 2020

Overview of Injection Molding Technology for Processing Polymers and Their Composites

Hongbo Fu, Hong Xu, Ying Liu, Zhaogang Yang, S. Kormakov, Daming Wu and Jingyao Sun

1College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

2State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, China

3Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH, USA

4Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, USA

5Department of Processes and Devices in Chemical Technology, Moscow Polytechnic University, Moscow, Russia

 

Abstract

Injection molding technology has been in development for almost 150 years. Injection molding is a molding technology that melts the material with the aid of a screw and an external heating device and then injects it into a mold to form the corresponding product as the mold cools. At present, many types of materials can be used to mold products by injection molding. Many requirements and problems in the processing of different materials have also caursed the birth of many molding processes. At present, injection molding has been applied in many fields, from daily necessities that are closely related to our lives to parts and components in the aerospace field. At present, injection molding still faces the challenges of processing new materials, and the continuous improvement of the existing equipment technology is required for the continuous development of injection molding.

Table of Content

This article reviews recent advances of injection molding technology for polymer processing with wide application prospects.

 

 

 

 

 

Keywords: Injection molding; Polymer materials; Processing technology.

1. Introduction

Injection molding technology is one of the main methods of industrial processing. Most injection machines used in industry today are screw-type injection machines. According to the driving mode, injection machines are mainly divided into electric injection machines and hydraulic injection machines.[1-3] It is a powerful molding method that can mold plastic products of various shapes and sizes. It is also the preferred process for products with complex three-dimensional structures. From micron gears,[4-6] micron needles,[7-9] etc. to plastic bottles, plastic barrels and daily necessities that are common in daily life, they can be molded by injection molding. Injection molding technology can be used for a variety of materials, including composite materials, foamed materials, thermoplastic and thermosetting 

plastics and rubber, etc.[10-17] There are also various forms of injection molding such as gas-assisted molding, water- assisted molding, micro-injection molding, injection foam molding, low-pressure molding, injection compression molding, etc.[18-29]

The process of injection molding mainly includes three stages: three processes of filling, filling and holding. During the plasticizing stage of the injection process, the screw conveys the plasticized material forward at a certain speed. As the screw groove of the screw becomes shallower, the material is compacted and continuously conveyed forward. The head is continuously accumulating, waiting for the arrival of the injection instruction, and at the same time, the screw will continuously retreat as the back pressure of the injection machine increases during the injection process. When the injection is started, the screw moves forward, and the material continuously fills the mold. At the same time, as the pressure in the mold increases, the screw moves while injecting. When the material is filled in the mold, the injection machine injects the material into the mold by the action of back pressure. With the temperature of the material continues to decrease, the pressure in the mold cavity begins to decrease. When the injected material can be safely molded without being damaged, the injection mold is opened, and the product is ejected through the mold structure. Products that can be inlaid with the mold, and then the mold is closed for the next cycle.[30-33] The external 

structure and size of the injection product are the exact structure and size of the internal mold of the injection machine, thereby replicating the mold-shaped product. Injection molded products often have many defects, such as short shots, sprays, sags, flow marks, weld marks, and floating fibers. These defects can be treated by subsequent processes such as spray coating, but it will increase the corresponding cost and time. The rapid heating-and-cooling cycle molding has become an effective solution to this problem.[34] At the same time, controlling the parameters in the injection molding process is one of the effective means of manufacturing high-quality precision injection products. At present, injection molding technology has been applied in many fields such as optics, medical plastics, and drug delivery.[35-37] Injection molding technology involes multi-disciplinary backgrounds.

Existing injection molding technology review articles mainly focus on the development and application of injection molding technology, such as the application of metal injection molding in medical equipment, and the applicability of metal injection molding technology to the manufacture of medical equipment. Some also report the limitations and needs for the preparation method, the microstructure and properties of the metal matrix composite materials. There is also an introduction to the theoretical knowledge in the injection molding process, a comprehensive introduction to the melting mechanism and shear mechanism during the injection molding process, and an analysis and introduction to a certain mixed system. In this way, there are many articles about the injection molding process and application, injection molding materials and injection molding mechanism.

The main contribution of this article is a comprehensive introduction to the entire injection molding process including injection molding equipment, injection molding molds, injection molding processes, injection molding materials and some special injection molding technologies. The injection molding of composite materials and micro-injection molding in recent years are also introduced with detailed examples.

 

2. Injection molding equipment, molds and processes

Plastic has gradually replaced many metal materials due to its light weight, stable chemical properties, good insulation properties, and low price, and has been widely used in high-end industries and daily life. At present, new materials represented by polymer materials are indispensable and have become one of the four major basic materials (steel, wood, cement). They are widely used in agriculture, machinery, medical, aerospace, automotive, and construction fields. The plastic industry is developing rapidly. The injection molding process can form a variety of complex shapes, sizes, and precise plastic products. It has the advantages of short cycles, high efficiency, and high precision.

 

2.1 Introduction of injection molding machine

Injection molding machine, also known as plastic injection molding machine or injection machine, is used to add granular or powdery polymer raw materials to the injection molding machine barrel, melt and plasticize into polymers with good fluidity under the action of external heating and mechanical shear the melt, followed by the plunger or screw, quickly enters the mold cavity with a lower temperature, and is cooled and solidified to form a plastic product consistent with the shape of the mold.[38] The injection machine usually consists of an injection system, a mold clamping mechanism, a hydraulic system, an electrical control system, a heating part and other auxiliary parts such as a cooling part and a feeding part. The following mainly introduces the injection system, mold clamping mechanism, hydraulic and electrical control systems.

As one of the most important components, the injection system is mainly composed of three parts: the plasticizing part, the injection part and the pressure driving part. Its functions are as follows: (1) Plasticizing- under the combined action of the screw and the heating ring, the material is melted and plasticized uniformly; (2) injection-  one under the action of the screw, the plasticized material is injected into the mold cavity at a set pressure and speed; (3) the pressure is maintained- one molten material is injected into the cavity. Inside the cavity, the screw stays still to replenish a part of the molten material into the cavity to eliminate shrinkage caused by cooling, ensure product quality, and prevent material from flowing back.

The injection part is mainly composed of a pressurizing device and a driving device. When the melt in the metering section reaches the required amount, the screw stops rotating and remains stationary. Then, under the action of the injection cylinder, the molten material in the metering chamber is injected into the closed mold cavity under the setting of the injection process parameters, and the injection ends. The pressure device mainly provides power to make the screw exert pressure on the material. There are mainly two power sources: hydraulic pressure and mechanical force. At present, most of them use hydraulic pressure, and use self-sufficient hydraulic system to supply pressure.

The driving device mainly provides power to rotate the screw to complete the plasticization of the material. The most common drive devices are AC motors and hydraulic motors. Hydraulic motors are more commonly used. The advantages of using a hydraulic motor drive are: (1) softer transmission characteristics, smaller starting inertia, and protection of screw overload; (2) smooth screw speed adjustment; (3) small size, simple structure, and easy service.

As one of the main components of the injection molding machine, the mold clamping device is mainly composed of a mold closing device, a mold adjusting device and an ejection device. The function of the clamping system is as follows: (1) to support the forming mold and ensure that it has an ideal speed change process when reciprocating between the open and closed positions, to avoid strong vibration during operation, to prevent the mold from being damaged by impact, and to achieve smooth ejection of products and increase productivity. (2) Provide sufficient clamping force to ensure that the mold can still be reliably locked under the action of the melt pressure to prevent the phenomenon of cracks in the slit and thus to ensure the quality of the product. To this end, the clamping parts and molds must also have sufficient strength and rigidity. (3) The product is cooled, the mold is opened, and a method for removing the product is provided. The clamping device is mainly composed of a fixed template, a draw rod, a power source, etc.

The working quality of the hydraulic system not only determines the technical performance of the injection molding machine, but also directly affects the quality of the product and the energy consumption of the injection molding machine. The composition of the hydraulic system is different, and is mainly determined by the operating cycle and performance requirements of the injection molding machine, which generally consists of power source (hydraulic pump), control elements (pressure valve, directional valve, flow valve, etc.), actuators (hydraulic cylinder, hydraulic motor, etc.) and auxiliary devices (oil tank, oil gauge, etc.). The actuator is controlled by the hydraulic circuit to complete the adjustment of each action program and parameters (pressure, flow) of the injection molding machine.

The injection molding machine has the following characteristics: the molded product has a complex shape, high quality, and high precision; it can form metal insert products with good assembly and interchangeability; convenient and efficient product mold replacement, which is conducive to increasing product competitiveness; automation, simple operation and high efficiency. Modern injection molding machines are developing in the direction of high quality, high efficiency, and low consumption. Small and medium-sized injection molding machines are biased toward precision molding; while large-scale injection molding machines are biased toward low energy consumption, quietness, stable operation, and high efficiency.

In 2015, Engel developed the duo 500 pico series injection molding machine (Fig. 1). This series of injection molding machines uses a multi-variable pump system and some relatively advanced control technologies, so the injection molding time is very short. This shortens the empty cycle to 2.6 s, which has a greatly increased production efficiency, is currently the most advanced injection molding machine in the world. In addition, because the duo 500 pico series uses multi-variable pump technology, its energy loss is very small. In 2016, Battenfeld developed the HM400/ 3400 Advantage model (Fig. 1), with a clamping force of up to 400t. In the design process of HM400/3400 Advantage model, the high-efficiency and energy-saving double pump system is used. The maximum injection speed of this device has obvious advantages compared with other similar types of equipment, and can reach up to about 300 mm/s. In order to improve the working efficiency and meet a variety of work requirements, the Arburg S series injection molding machine is designed with dual pump and multi-pump control technology to meet the needs of simultaneous operations.