Injection Moulding Process Guide

Plastic moulding is a high-precision manufacturing process, which is an indispensable step in the product design or product development. In the moulding machine polymer granules are firstly melted and then injected into a mold under pressure. Next, they are cooled and solidified in a mold. The available materials can be colored or filled with other materials. Specifically, the injection molding process is a technique popular in the manufacturing of plastic products and materials. It involves injecting melted plastic into a mold cavity which has been designed as the required shape. Moreover, injection molding is so popular in the plastic mould design, because of the lower unit cost when manufacturing in large quantities. Today, let’s take a quick look at the moulding process.

Basic Step 1: Product Design

  Product design is one of the most significant facets of the moulding process. An ideal design is one that adjusts the required objectives to a satisfactory level under the premise of a lot of creativity. Moreover, the successful implementation of this part draws support from a series of computer-aided design (CAD) software, like SolidWorks. Proficient operation of CAD software is also critical because it allows for quicker iterations and more accurate prototyping if necessary. During the plastic injection mould design, there are specific means to avoid unnecessary costs are to design uniform wall thickness of products and to gradually shift from one thickness to another if changes in the wall thickness are not avoidable. It is also important to avoid building stress in the plastic injection mould design process. Product design in the plastic moulding process involves many aspects, including function, aesthetics, manufacturability, assembly, etc. This part can minimize the need for expensive tooling changes in the following process. Moreover, in the injection moulding design, you can design whatever color you need your plastic to be, even including multiple colors in one product when overmolding processes are used.  

Basic Step 2: Prototype Design

Injection molds prototype in the moulding process  can manufacture real injection molded components for functional testing. They can also serve as a bridge to fill the manufacturing gap between the production with an injection molds prototype and the production with an injection mold. Prototyping designed in the moulding process can be effectively used to test a plastic prototype part in terms of form, fit, and function. But it may be not comprehensive in functionality. For example, it may be a lack of fine details such as color preference or surface finish. Despite these shortcomings, a prototyping manufacturing is an indispensable and important part. Nowadays, 3D printed parts for the prototype are popular in the manufacturing industry. Nonetheless, due to the limitations of 3D printing, this method can produce less than opium results. Even so, a prototype tooling is definitely necessary to test the characteristics of plastic material in many ways: Design Feedback
  • With the help of a prototype tooling, you can make early adjustments before prototyping begins.
  • Talking timely with applications engineers to discuss any design challenges and manufacturing questions that arise in the product produced by a prototype tooling.
Iterative Development
  • Produce multiple prototypes to compare and optimize designs simultaneously.
  • Reduce overall time to market and save unnecessary costs.
Design Validation
  • Test the form and fit of components with small quantities of molded parts.
  • Perform functional tests with prototypes made of materials in accordance with the standard of production.
  • Full library of online design resources including guidelines, tips, white papers, and videos.
Market Testing
  • Produce a pilot run injection-molded parts to validate market interest before launch.
  • Minimize financial risk at market launch by implementing affordable aluminum tooling before starting full-scale production.

Basic Step 3: Plastic Injection Mold Design

Just as with the product design, mold design is another vital opportunity to prevent defects during the moulding process. The plastic injection molds tooling covers jigs, fixtures, gauges, and other equipment applied in the injection molds process. These “secondary” instruments are often critical to the success of an molding process. With the help of suitable plastic injection tooling, the efficiency and overall quality of the injection molding can be improved greatly. There are six stages of the plastic injection mold tooling: Stage 1: Manufacturability and Feasibility In this initial stage, design engineers, material engineers, and quality engineers collectively work to determine mold specification, mold component functionality, mold materials, operational constraints, and any other needed product improvements. In this stage, you can confirm the standard plastic design practices and other details, which are conducive to create the most robust design possible. Stage 2: Design Before determining mold sides and steel sizes, 2D and 3D design models should be constructed. Once the 2D and 3D design models are approved, the detailed design is finalized. Stage 3: Final Design Specifications The design specifications are critical for mold construction. Final adjustments and modifications are conducted indoors, with the priority given to manufacturability and critical dimensional requirements. Stage 4: Construction of Primary and Secondary Molds After the completion of tool drawings and the verification of construction standards, the tool builder’s progress is closely monitored and on-site meetings are held. The completed mold is inspected in accordance with a comprehensive checklist.
Moreover, the most commonly used plastic material in the injection mold design are as follows:
  • Polypropylene (PP): accounting for about 38% of global production
  • ABS: accounting for about 27% of global production
  • Polyethylene (PE): accounting for about 15% of global production
  • Polystyrene (PS): accounting for about 8% of global production
Stage 5: Bring the Molds Indoors for the Initial Sample
An initial sampling will be produced by using scientific molding practices. Cavity pressure transducers equipped in the mold can accurately determine the filling profile as time flying. Sample parts are qualified. Stage 6: Make Final Corrections and Put on Production The preliminary verification of mold construction will be useful in minimizing the setup time in the future. Perfect parts are submitted to the customer for check. Upon the receipt of the final approval, the production process is launched.

Basic Step 4: The Manufacturing Process

When a product has been reasonably designed and approved, it’s time to start the actual manufacturing in large quantities. Here are six stages of plastic injection molding. Stage 1: Clamping
You should close both halves of a mold before they are injected with material. The two halves are closed by a clamping unit, which consists of metal plates (or platen).
And then both halves are attached to the injection molding machine and one half can slide. Above all, you should feed material granules from the hopper into the heated barrel & rotating screw. Larger machines with more clamping power take longer to close and clamp the mold.
Stage 2: Injection
Having been forced forward by the materials piled up at the front, the screw is put forward by a hydraulic ram. This action will make sure all the mold’s cavities are crammed in the closed mold tool.
These materials will be firstly heated by heat, friction & shear force and then are sent to the front under pressure through either a screw or check device.
The volume of material injected is called the ‘shot’. When 95%-99% or the mold is filled, you should stop the injection.
The flow of the materials is always changing and dynamic. Hence, It is difficult to calculate exactly the injection time. However, injection time can be controlled and estimated by other factors such as injection pressure, power, and shot volume.
The injection pressure can range from 35-140 MPa. The rate of injection and the pressure which can be reached are determined and controlled by the hydraulic system in the machine.
Stage 3: Cooling Period
In this stage, the cooling time is determined by the type of resin used and the thickness of the part. Each mold is equipped with internal cooling or heating lines where water is cycled through the mold to keep a constant temperature.
Stage 4: Plasticizing the resin
While the part cools, the barrel screw retracts and draws new plastic resin into the barrel from the material hopper. The heater bands maintain the needed barrel temperature for the type of resin being used. Stage 5: Ejection
The mold opens and the ejector rod puts the ejector pins forward. The part falls and then is captured in a bin placed below the mold. Stage 6: Removing  the runner and packaging The machine operators regularly separate the usable parts from the leftover runners. In many cases, the runners are squashed and recycled to reduce unnecessary costs and environmental impact. Those usable parts and components are then weighed, counted and packaged for assembly or shipping.
Stebro Mold is a full-service injection mold company in China. We specialize in manufacturing plastic mold and the molding service. Our services include outdoor power equipment, medical devices, and industries of electrical, electronic, and automotive. Moreover, as an injection mold company in China, we are also good at making difficult-to-process engineering resins, high-temperature material like PEEK and PSU, metal replacement, and precision plastic components. Having learned the molding process, if you want to know more detailed information about the plastic injection molding, please log in to our official website and then contact us directly.

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