Injection Molding and Prototyping

injection molding

What is injection molding?

Injection molding, the most common method of part manufacturing, is the process of producing plastic parts by injecting a material into a mold. The process requires an injection molding machine, raw materials and a mold. Injection molding can be done with many different materials such as metals, elastomers, confections and thermoplastic and thermosetting polymers. The materials are melted and injected into the mold, which then cools and solidifies in the cavity, creating the final product. This process is ideal for producing high volumes of the same product.

Process Cycle

The injection molding cycle is very short and consists of four stages:

  • 1. Clamping – Before any material can be fed into the mold, the mold must be securely closed by the clamping unit. This hydraulically powered until forces both sides of the mold together and keeps the mold secure for the injection of the material.

  • 2. Injection – The raw material is fed from a hopper into a heated barrel. The materials are then slowly moved toward the injection unit by a screw-type plunger and become melted due to the heat and pressure. The melted plastic is then forced through a nozzle and into the mold very quickly. The buildup of pressure during this process tightly packs the material and holds the material to compensate for shrinkage. The volume of material inside of the mold cavity is referred to as the shot. Injection time is estimated by the shot volume, injection pressure and injection power.

  • 3. Cooling – The mold remains cold so the plastic starts to solidify as soon as it makes contact with the interior of the mold. Once the material solidifies, the hold pressure drops to reduce stress, but the mold is still held shut during the cooling process. This allows the temperature to drop to an appropriate level before ejection to minimize shrinking. The cooling time varies depending on the type of plastic being molded.

  • 4. Ejection – After an appropriate amount of time has passed, the mold can finally be ejected from the unit. The ejection system is attached to the rear of the mold and force is applied to push the part out of the mold. Because of the shrinking the part may adhere to the mold, so a release agent may be sprayed into the cavity before the injection process begins. The finished product finally free falls into a collection box or onto a conveyor belt, or it may be removed by an automated system. Once the part is ejected, the mold must be clamped shut in order for the unit to prepare for the next shot. Some post processing may be required, such as trimming of the part or collecting the scrap materials.

Equipment

All injection molding machines use a power source, an injection unit, a mold assembly unit, and a clamping unit to complete the process. Machines are made up of many different components and can vary in configuration.

Injection Unit

The main purpose of the injection unit is to heat the material to a specific temperature at which it will flow smoothly into the mold while under pressure. The unit also contains a hopper, which holds the raw material, and the barrel, which contains a hydraulically-powered screw-type plunger that forces material through a heated section of the unit. A combination of heat, pressure, and friction causes the material to melt. Lastly, the built-up pressure forces the melted plastic through the nozzle and into the cavity. The material is packed tightly into the mold until the plastic is solidified.

Clamping Unit

The two halves of the mold must be securely closed by the clamping unit before any material can be injected. The clamping unit holds the two halves in proper alignment, keeps the mold closed during the injection process, and opens and closes the mold at the appropriate times during the cycle. The clamping unit is mounted on two different platens. The front half of the mold, called the “cavity,” is on a stationary platen that aligns with the nozzle of the injection unit. The rear of the mold, called the “core,” is attached to a movable platen. The clamping motor pushes the movable platen towards the stationary platen while exerting force to keep the mold securely closed. After the appropriate amount of time has been allowed for cooling, the clamping motor opens the mold. Finally, the ejection system pushes the final product out of the unit.

Machine Specifications

Molding machines are generally characterized by two things: shot size, and clamp tonnage. As mentioned before, the shot size is the amount of material that can be injected into the mold in one cycle. Tonnage, which can vary from 5 tons to over 9,000 tons, is determined by the amount of clamping force the machine can exert to keep the mold closed during the injection molding process. The clamp force of a machine is determined by the projected area of the part being molded.

Materials

Many different materials can be used in the injection molding process, such as thermosetting and thermoplastic polymers. Thermoplastics are the most common because they are highly versatile and recyclable.

Prototyping

Prototypes are important for the development process of plastic injection molded components. Functions for prototypes include demonstration and form, fit, and function testing. They save both time and money, since they can be made in less time than injection molding and in smaller runs than full production molds. Prototypes help detect flaws and other issues that may appear in the design phase. They can be produced quickly to meet your exact specifications and product requirements, and products can be painted or decorated to match what the final product will look like. When your production tooling won't be ready for another three or four months, this is an excellent way to make those 100, 1000, or even 10,000 parts for pilot production or market testing. TBC focuses on prototypes because we realize their importance for the design, development, troubleshooting, and marketing of a new product.

The design and engineering of products in the world today is constantly changing and improving, and TBC’s prototyping abilities are an increasingly important resource to our customers. We want to give you the edge over your competitors, and this is achieved through the rapid and efficient development of new products. TBC’s prototype engineering and development experience will help you select the most appropriate technologies and processes to create your prototype.