Plastic injection molding is used to produce many plastic parts in a cost-effective and timely manner. It’s used to make commercial products such as electrical boxes, mop heads, high-end trash, recycling receptacles, vending machine components, and equipment housings. Storage racks, grill accessories, bird feeders, tackle boxes, and toilet seats are examples of consumer items. Injection Molding Services are also widely employed in the construction, home goods, and food services industries.
What Exactly is Injection Molding?
Plastic injection molding, also known as injection molding, is a manufacturing process used to mass produce plastic parts. It entails injecting molten plastic material into the mold, where it cools and solidifies to its final shape. It is perfect for producing large quantities of plastic products for a variety of companies and people.
During tooling, the mold is adjusted, resulting in the part’s shape. It can produce many identical and dimensionally consistent parts, even on complex designs and parts with limited dimensional tolerance.
Plastic Injection Molding Benefits and Drawbacks
The injection molding process benefits the plastic part producer because of the following benefits:
Productivity is High
The injection molding process is a fast way to create plastic objects. It has a fast production rate and can make a big number of plastic pieces in a single cycle. The mold could have many cavities. The more cavities in the mold, the more components may be produced in a single molding cycle. It can also create thousands of pieces before the tooling needs to be replaced.
Multiple pieces with uniform form and dimensions can be produced using injection molding. Parts manufactured throughout multiple molding cycles are guaranteed to be identical. This benefit is essential when product uniformity is desired.
Scrap Rates are Low
When compared to traditional manufacturing processes and CNC machining, injection molding produces less scrap. If a recyclable thermoplastic material is used, excess polymers from the sprue, runners, and gates after the injection molding process can be heated and remolded.
Labor Costs are Low
Injection molding is a highly automated process in which most of the steps are completed by a machine controlled by an operator with minimum intervention by physical labor. This lowers manufacturing costs.
However, the following are the drawbacks of injection molding:
Before mass production, the mold tooling is designed, produced, and tested, which raises the investment cost. Before an injection molded design is created, a prototype mold tool is created to mold the first parts of the new design. Several iterations and trial runs are required to ensure that the mold produces an accurate item, which can be costly and time-consuming.
Changes to the Part Design are Difficult to Implement
Any change in the shape and dimensions of the part causes changes in the dimensions of the mold cavities. To increase the size of the part, a section of the cavity must be removed so that the molten plastic can occupy a greater volume. Metal fabrication methods must be employed because the tooling comprises sturdy metal. If the part’s size has to be reduced, a new tool with smaller cavities is necessary.
Design Constraints on the Part
As Much as Possible, the Part Design must Include:
- A homogeneous wall thickness allows for uniform cooling while avoiding shrinking and other flaws.
- Because just a limited amount of heated plastic may cool, a low wall thickness and volume are used to produce a consistent and faster cooling period. As a general guideline, the part’s wall thickness should be limited to a minimum of 6 mm.
- External or internal undercuts have been reduced. This is done to reduce tooling costs.
Plastic Injection Molding Process and Equipment
The following diagram depicts the plastic injection molding process cycle. The procedure is carried out in an injection molding machine, which comprises primarily the clamping unit, the injection unit, and the mold.
The clamping process involves closing the mold halves before injecting the molten plastic and holding them closed after the molten material has settled into the cavities. It takes place in the clamping unit, which is in charge of:
- Using enough clamping force to resist the injection force and maintaining the mold halves closed during the injection phase until the dwelling stage.
- After the final stage, eject the molded portion.
- Between molding cycles, open and close the mold plates.
- Maintaining appropriate alignment of the mold plates.
The Clamping Unit Comprises the Following Parts:
- When connected to the injection molding equipment, the platen keeps the mold halves.
- The stationary platen supports the front mold half and houses the injection unit’s nozzle, which is directly aligned with the front mold half. The moveable platen moves the rear mold half by sliding on the tie bar when the mold opens and closes.
- During translation, the tie bar holds the moveable platen in place. It helps to position the mold plates. The tie bar size restricts the size of the mold that may be inserted in an injection molding machine.
- The clamping system is to transfer the mobile platen to the stationary platen. An injection molding machine’s clamping systems are classified into three types:
- Toggle clamps are appropriate for injection molding machines with minimal tonnage demands. It has an actuator that moves the crosshead forward, so extending the crosshead links with the moveable platen attached to their ends.
- Hydraulic clamps have a large tonnage range of 150-1,100 tons that may be readily set and regulated. The moveable platen is translated using hydraulic pressure, and the force required to secure the mold halves during the injection stage is developed using hydraulic pressure.
- Hydromechanical clamps can provide clamping tonnages of over 1,000 tons. The toggle and hydraulic clamping mechanisms are combined into the functioning of hydromechanical clamps. A hydraulic cylinder is used to translate the moveable plate, which is then mechanically fixed in place. Once the mold halves are closed, the tonnage is built using a high-pressure hydraulic cylinder.
The ejection system will be covered further down.
The raw plastic pellets are melted and then fed to the mold in the injection unit during the injection stage. The injection unit is in charge of:
- Providing molten plastic to fill mold voids A shot is the amount of plastic injected into the mold. The volume of the portion determines the shot.
- Before inserting the plastic pellets into the mold, heat is used to melt and homogenize them.
- Using enough injection pressure and speed to drive the molten plastic into the mold cavities.
The Injection Unit Comprises the Following Components:
The hopper is a huge container that receives raw plastic pellets. It has a bottom hole via which pellets are introduced to the threads of the reciprocating screw within the barrel.
- The barrel houses the reciprocating screw and is jacketed on all sides with heaters.
- The heaters generate thermal energy, which is used to melt the plastic pellets into a molten, viscous condition.
- The reciprocating screw rotates and slides axially to push the plastic across the length of the barrel. The injection pressure is supplied via a hydraulic cylinder. Because of the combined heat, pressure, and friction, the plastic gains fluid qualities as it goes along the length of the barrel. A non-return valve prevents the backflow of molten plastic from accumulating in front of the screw. The most prevalent injection system in current injection molding machines is the reciprocating screw.
A screw pre-plasticizer is another injection mechanism. Separate barrels in this injection system melt and inject the plastic into the mold. The mechanism in the first barrel is like that of a reciprocating screw. After passing through the first barrel, the plastic is transferred to the second barrel, which uses a plunger to deliver the molten plastic to the mold.
To melt and inject the plastic, older injection molding machines use a single-barrel, plunger-type injection system.
- The nozzle directs molten plastic into the mold chambers. It is positioned in the stationary platen and is parallel to the front mold half.
Cooling and Dwelling
After the molten plastic has been put into the mold, it is allowed to settle inside the cavities. In this step, the holding pressure to condense the molten plastic during solidification replaces the injection pressure.
When the molten plastic contacts the surface of the cavities, cooling begins. A coolant system inside the mold removes heat, which aids in cooling. Part shrinkage may occur while cooling. Additional melt may flow to compensate for shrinkage during cooling. The mold halves are split and the molded object is ejected after it has cooled sufficiently.
The cooled portion is separated from the mold during the ejection stage. The clamping unit’s ejection method allows the molded object to be removed from the mold cavities more easily.
The ejection system comprises an actuating ejector bar that uses ejecting pins to push the ejector plate. After the molding cycle, the ejecting pins force the hardened part out of the open mold plates. Because the item sticks to the mold during cooling, sufficient ejecting power is required.
Mold-release agents are used to help remove molded items from mold cavities. It can be reapplied before the clamping stage after a few molding cycles, or it can permanently adhere to the surface of the mold cavities.
Trimming is the ultimate phase in the manufacture of injection-molded plastics, in which excess polymers from the flow of the molten plastic are removed from the molded part. Each molded unit is isolated from the other molded pieces. Trimming is done on different equipment.
The mold channels (sprue, runners, and gates) are filled with molten plastic during the injection. The molten plastic in those channels hardens alongside the melt within the cavities. Flashes may also be observed on the part’s edges. Excess plastic materials attach to the section that needs to be cut after cooling.
- The mold comprises two plates that are connected to clamping plates. The injection unit is right next to the front mold half. The rear mold half is coupled to a moveable plate that allows the mold to open and close and is also next to the clamping unit’s ejection system. The two plates must be clean and free of contamination before the molding cycle begins.
- The mold cavity is the formed portion in the mold plates that gives the final shape to the plastic pieces. When the molten plastic flows into the cavity, it takes on the shape and volume of the hollow space. The front mold half has most of the volume. A cavity in a mold might be one or multiple.
- The parting line is a line that represents the separation of the closed mold halves. In sophisticated tooling designs, a separating line might be a straight line or a curve. Because air is easiest to vent at the parting line, molten plastic prefers to travel in this region. Some unalterable parts may have a line or a curve that exposes that the two sides of the part are created on different plates.
- The locating ring holds the nozzle in place against the front mold plate.
- The sprue bushing aligns the nozzle hole with the front mold plate. That is the location of the nozzle.
- The sprue is the initial melt channel between the injection unit’s nozzle and the front portion of the mold. It is the main channel to which several runners are linked.
- The runner is responsible for distributing the molten plastic to the mold cavity.
- By limiting the flow channel of the molten plastic, the gate directs it into the cavities. It is positioned at the end of each runner and is where the molten plastic enters. A cavity can have one or many gates.
- Other mold took characteristics include air vents that evacuate trapped gases inside the mold and a cooling channel that allows heat to be dissipated to a coolant.
What are the Primary Cost Drivers in the Plastic Injection Molding Industry?
For small volumes, the cost of a plastic injection mold tool can be a considerable part of the whole order. This expense, however, can be amortized over higher volumes and thus avoided. For larger volumes, plastic resin and cycle times become the primary cost drivers.
You must choose the top plastic Injection Molding Services to get unique quick prototypes and end-use production parts. As a reputable manufacturer, we guarantee the highest quality plastic injection molded components and mold tools. To match your requirements, they employ top-tier material verification and quality standards. All you have to do is send them your product design details.
3ERP has a long history of quick tooling and low-volume injection molding. Contact us today for a free quote.