The versatility of plastic polymers keeps them at the top of the list of manufacturing materials of choice. With plastic’s potential to create items for use in industries as diverse as packaging, sports equipment, aerospace, and automotive manufacturing, its strength and cost-effectiveness are tough to beat.
However, creating finished products in plastic requires precise production methods that produce reliable results consistently. What do those techniques involve? While there are many molding processes, we’ll focus on two of the most popular processes in the industry today: injection molding and compression molding. Learn what these procedures involve, what manufacturers use them for, and how they differ.
What Is Plastic Injection Molding?
Injection molding is the other competitor for the top spot among popular plastic manufacturing methods. What makes an injection molding machine different?
Instead of placing plastic polymers into a closed mold, an injection mold injects molten thermoplastic resin into a sealed mold with a part cavity or cavities. Using a hopper filled with raw plastic pellets, the injection molding machine liquefies pellets using a heated screw and barrel. This screw then uses high pressures and forces the molten plastic into the tooling cavity, where the part cures.
Once cured, the tool opens, an ejection pin pushes the parts out from the cavity, and the tool closes again for the next injection cycle. Injection molded parts can fall directly into a box, onto a conveyor, or be removed via an operator or robot, making the injection molding process very precise and efficient.
Advantages of Plastic Injection Molding
With a different method of introducing resins into the mold cavity, injection molding has some benefits over compression techniques. Here are a few of the key reasons why businesses opt for molded parts for consumer products and industrial items so often:
- Rapid cycle times: An injection molding machine has shorter cycle times and fewer secondary operations. Additionally, with multi-cavity tooling, injection molding allows several parts to be produced every cycle, which benefits high-volume part production.
- Superior results for lightweighting projects: Injection molding is ideal for lightweighting projects in sectors such as transportation, as plastics can achieve better results in some applications than even machining parts from aluminum.
- Excellent precision: Injection molds can achieve fine tolerances and exact repeatability from part to part with the help of computer-aided design for specialized items.
- Broad support for many types of thermoplastics: Injection molds include new and more novel materials that can impart specific qualities, such as impact resistance and durability.
- Less waste and more cost-effectiveness: With shorter cycle times and repeatable manufacturing, injection molding lends itself to lower scrap and piece prices due to tooling efficiency.
- Support for highly complex geometry and intricate design elements: Injection molds themselves and the overall molding process ensure that plastics can reach every part of the tool to complete detailed designs.
- The capability to use multiple plastic types in one design: Injection molding is also ideal for “over-molding” and includes additional metal parts, such as elements made from aluminum.
- Excellent for high-volume manufacturing necessary for maintaining stock levels: One injection mold tool could produce multiple plastic parts at once. The injection process takes less time than compression molding, allowing for consistent high-volume production.
Disadvantages of Plastic Injection Molding
As with other molding processes, injection molding has some disadvantages that companies must consider when evaluating their options. A few of the critical drawbacks to note:
- High cost for small producers: Mold design costs can be high for small projects but are easily recovered over long and large production runs.
- Not always suited to substantial plastic parts: Large molds may be necessary in specific contexts, and compression yields better results for larger parts in specific applications.
- Delays if not properly tested: Accurate mold testing is essential to ensure that plastic flows correctly during injection, which can create delays to the start of the manufacturing process.
What Is Compression Molding?
Compared to plastic injection molds, compression molds have fewer moving parts but require more preparatory work. Before polymers can undergo compression molding, manufacturers must heat the material to an appropriate temperature. This heated plastic also undergoes pre-compression to the correct shape to fit into the mold.
After preparing the plastic, manufacturers must insert it into the mold die, which is open at the top. After placement, the top of the mold closes and presses the plastic, forcing it into every area of the mold and overflow regions. The part cures inside the mold, which then separates. The manufacturer removes the part via an operator or robot and readies it for the flash-trimming process and other secondary operations.
Advantages of Compression Molding
Why do manufacturers choose compression molding techniques for specific projects? Several advantages to this process make it the right choice for particular objects. Those benefits include:
- Small-scale cost effectiveness: Compression molding may be more cost-effective for small production runs than injection molds, as tooling is typically less expensive.
- Adaptability to varying thickness requirements: Compression molding yields good results for products with varying thicknesses.
- Additional materials options: Compression molding supports thermoset polymer materials in comparison to the thermoplastic materials required for injection molding.
Disadvantages of Compression Molding
This type of manufacturing is beneficial in specific contexts but may not be more advantageous than plastic injection molding. Here are some of the disadvantages of compression molding.
- Excessive waste and extra processing: The overflow or “flash” on a compressed item must be cut away before the item is ready for use. This flash often cannot be recycled without difficulty.
- Extra startup and maintenance costs: Few manufacturers still run press-type molds, and spooling up new production can be pricey.
- Slower part production time: Compression molds take longer to produce finished parts, and the additional time necessary to remove flash adds to cycle times.
- Poor support for high-volume jobs: Both slow cycle times before part ejection and the time required to trim flash means high-volume production can be difficult, which can lead to missing target deadlines and lengthening overall project turnaround times.
- Thin-walled designs and parts: Compression molds cannot create intricately designed components in the same way that plastic injection molds can.
- Technological and physical limitations: Compression molds remain limited to comparatively simple shapes compared to injection molding.
While the compression molding machine still has a time and place in manufacturing, its limitations mean production with this methodology remains narrower in scope than injection molding.
What Are the Main Differences Between Injection and Compression Molding?
Ultimately, the primary difference between these two methods is how manufacturers introduce the plastic resin into the molds. Another difference is the high flash rate on compressed parts.
With compression molds, plastics enter the mold cavity before anything else occurs. The mold, which has undergone heating, then closes and compresses the plastic material to force it into all parts of the mold cavity. An injection mold, which remains a closed system until the plastic completely cures. Instead of pre-loading the mold, plastic pellets feed from a hopper into a screw-driven heating system that injects the mold with molten plastic at high pressure.
Cost of Compression Molding vs. Injection Molding
Which method is more cost-effective for manufacturers? The cost of compression molding compared to injection molding varies substantially based on the parts required, the materials in use, and other factors.
Generally, compression molding has lower up-front costs than injection molding. But pricing from part to part is more expensive due to longer cycle times and the need for secondary operations to finish the parts before shipment preparation. Compression molding is best for less complex part designs and lower volume production.
While injection molding requires a more considerable upfront investment for mold design and tool build, it is easier to maintain the molds and injection molded parts can be produced at much higher volumes. They also require less human intervention to operate, which lowers manufacturing costs. For complex and high-volume part production runs, injection molding is the quicker, simpler, and more valuable route.
Injection Molding and Compression Molding Examples & Applications
Which objects can you make with these methods? Both solutions can produce various parts and components, especially with other advanced techniques.
You can produce high-quality items with injection molding, such as:
- Finer automotive parts, such as dashboard components, front-end bumpers, and switches and dials
- Kitchenware, including cups, bowls, spatulas, and other food-safe containers
- Medical devices and products that must resist corrosion and bacteria
- A wide variety of everyday consumer products and household goods
- Agricultural and industrial products that withstand high heat and other environmental pressures
For compression molding, examples include objects such as:
- Automotive acoustic parts
- Circuit Breakers
- Large automotive panels and engine components
- Gaskets and rubber components
Experts in Plastic Injection Molding
The type of molding best suited for your project will depend on many factors. When those factors point to injection molding as the most effective option, partnering with an experienced service provider makes a significant difference in the outcome, influencing factors from quality to speed. Get a free quote on your project today from Reliant Plastics and explore available options for injection-molded parts.
