How to Design a Plastic Injection Mould

Plastic injection molding is a fast and cost-effective method of producing plastic parts. It produces thousands of molded parts per molding cycle. It is also recyclable, which cuts down on waste and costs.
After the molten plastic is transferred into the mold cavities, the screw draws back, and holding pressure is applied to compact the plastic during cooling and shrinkage. This step can range from a few seconds to several minutes.
Thermoplastic
Unlike CNC machining, plastic injection molding can produce parts at a much higher rate. However, if the mold is not properly designed, it can produce defects such as miss sections and bubbles in the molded product.
Thermoplastic polymers are melted and softened by heat, then solidified after cooling. They can be reheated and reformed, making them more versatile than thermoset plastics.
Injection molding is often used with thermoplastics such as acetal, polyethylene, or polypropylene. These commodity plastics are lightweight, inexpensive, and resistant to moisture and bacterial growth.
Thermosetting
Thermosetting plastics are a liquid at room temperature and solidify permanently when heated. These materials are not as versatile as thermoplastics and can exhibit a range of weaknesses, including lack of recyclability and lower impact resistance.
They are available in the form of resin pellets and can be molded into various shapes with aesthetically pleasing surface finishes. They are also highly durable and can be withstand heat, chemical exposure and pressure. They are often used to replace metal parts in automobiles and machinery.
Clamping pressure
Clamping pressure is an important factor in plastic injection molding. It must be sufficiently high to prevent the mold from opening during injection. This requires precise calculations.
The clamping force must be calculated based on the projected area of the molded parts and any runners. This is done by multiplying the length by the width of each cavity. Excessive clamping forces can lead to part defects and damage to the mold and injection machine. This includes fracturing of the hydraulic cylinder and crushing of the mold vents.
Injection speed
A plastic injection molding machine is used to make a wide variety of products. Some are larger and simpler in design, while others require more complex details. Regardless of their size or complexity, all products must be free from flow marks and other defects.
One way to prevent these defects is by increasing the temperature of the plastic melt. However, care must be taken to avoid running too close to the degradation temperature. Sensors are placed throughout the process to track temperatures and provide alerts if they go outside of the set range.
Mold cavity
The mold cavity is the shaped section of the mold that gives plastic parts their shape. It is the most important part of a plastic injection molding process. The cooling channels in the mold cavity influence the molding cycle, especially during the solidification phase of the polymer. A conformal cooling channel design reduces the time needed to cool the molten polymer.
A mold channel (sprue, runners, or gates) is the passageway through which molten plastic flows from the injection nozzle into the mold cavities. It is also the place where local internal shrinkage occurs during cooling and must be ejected from the mold.
Mold tool
A custom plastic part requires a mold that can be designed, fabricated, and tested. This can be costly, but the investment pays off in long-term productivity.
When the plastic is molded, it must be held under pressure until it sets. This pressure, known as clamping pressure, complements the injection pressure and is crucial to a successful part. Insufficient clamping pressure can cause part defects, cracked plates and mold vents, and shear heat damage to the plastic. Excessive clamping pressure can also lead to hydraulic cylinder failure and crushed mold vents.
Mold release agent
Mold release agents act as a lubricating coating to prevent plastic parts from sticking to the mold. They can help reduce the amount of resin that builds up on your equipment, reducing cycle times and improving part quality. Many also contain a lower amount of VOCs than petroleum-based alternatives.
Some release agents are coated directly onto the molding powder. This method is more cost effective than using sacrificial coatings before every injection molding process. It also reduces waste and contamination.
Mold trimming
The plastic injection molding process requires heating and cooling to solidify the molten plastic. If the mold shape or design causes the plastic to cool unevenly, it may not meet dimensional and quality requirements.
The essential equipment for this process is an injection molding machine and a mold, also known as a tool or die. The mold is a metal cube with pathways for the molten plastic called runners and gates. It has holes for coolant, which cools and solidifies the molded plastic.