Injection molding and compression molding are two of the most common manufacturing processes for working with plastic and rubber. But what exactly sets them apart? As a professional plastic injection molding manufacturer, I’m going to walk through the key differences between injection vs compression molding so you can determine which process may better suit your production needs.
How Injection Molding Works
Injection molding is a technique that heats up plastic until it liquefies, then forces it into a mold cavity under extreme pressure. Once the plastic cools and solidifies, the end result is a molded part that has taken the shape of the mold.
The actual injection molding process works like this:
First, plastic pellets are fed from a hopper into the injection molding machine’s heated barrel. The pellets are heated to the point that they melt into a liquid which can then be injected.
Next, the now liquified plastic gets injected through a nozzle into the mold cavity with intense pressure—we’re talking around 15,000 to 30,000 PSI here. The mold remains clamped shut against this high pressure to prevent leaks while the plastic flows throughout the cavity, filling every crevice.
Once the plastic has filled the mold, the part gets sufficient time to cool and cure before finally getting ejected from the opened mold. The end result? A perfectly formed plastic part that matches the precise shape of the mold.
Analyzing the Benefits of Injection Molding
There are quite a few reasons why injection molding has become such a massively popular manufacturing technique. Here are some of the biggest benefits to be aware of:
Extremely Fast Production Speeds
Compared to most other plastic molding methods, injection molding offers blazing fast cycle times. We’re talking just seconds in many cases! This makes it easy to produce thousands or even millions of plastic parts on an ongoing basis.
Lower Per-Unit Costs
Thanks to the raw speed of injection molding paired with the reuseability of molds, this process enables very low per-unit costs at high production volumes. The more parts you produce, the less each individual part will cost on average.
Great Consistency and Precision
Injection molding is prized for its consistency. Cycle after cycle, the process can reproduce plastic parts within an extremely tight tolerance range thanks to the precision nature of steel molds paired with automated manufacturing. This consistency enables products with interchangeable part assemblies.
Wide Range of Plastic Options
Injection molding works with both thermoplastics and thermosetting polymers, which provides manufacturers with a broad range of material options like ABS, nylon, polyethylene, polystyrene, epoxy resin, silicone, and more. Certain fillers like glass, carbon, and metal can even be added for increased strength and functionality.
Insert Molding Capabilities
Injection molding enables insert molding, an advanced technique that allows you to embed metal, plastic, or electronic components into the plastic part during molding. This can consolidate post-assembly steps.
Examining the Drawbacks of Injection Molding
Of course, no manufacturing process is completely perfect across the board. Injection molding does come with a few limitations to factor in as well, such as:
High Upfront Costs
Injection molds themselves are typically made from steel or aluminum, and they have to withstand clamping pressure upwards of six tons. That high precision construction doesn’t come cheap, with molds regularly costing $5,000+ at minimum.
Material Waste from Runners
The channels that deliver the liquid plastic to the mold cavity are referred to as runners. These runners leave behind excess hardened plastic after each cycle that gets discarded as scrap.
Limitations on Large Parts
As useful as injection molding can be, it does face limitations when dealing with extremely large components. Anything over 50 lbs or with metal inserts generally has to shift to alternative processes like compression or thermoforming.
What is Compression Molding?
Now that we’ve covered the ins and outs of injection molding, let’s compare it to compression molding. Like injection molding, compression molding uses molds to form plastic or rubber components. But it takes a distinctly different approach:
First, compression molding starts by heating plastic or rubber raw material into thicker, pliable sheets. An operator manually places these softened sheets between two open mold halves.
Next, the molld halves close and mechanical or hydraulic pressure gets applied. This intense pressure compresses the plastic sheets into the mold cavities, spreading the material throughout until it fills every crevice.
At the same time, heat gets applied to the mold itself. This causes the plastic/rubber material to further soften and cure. Once fully cured, the mold halves retract and the final molded parts get ejected or trimmed out.
Essentially, instead of injecting a liquified material like injection molding does, compression molding starts with simple plastic/rubber sheets and squeezes them into shape.
Reviewing the Advantages of Compression Molding
Like injection molding, compression molding comes with its own set of distinct manufacturing advantages:
Lower Tooling Costs
Compression molds are typically made from less expensive metals like aluminum or steel alloys. They also comprise fewer moving components. This makes compression mold tooling cheaper than injection molding in most cases.
Scalability for Larger Components
Compression molding’s manual nature makes it far more suitable for molding very large plastic or rubber parts compared to automated injection molding presses. Aerospace manufacturers, for instance, leverage compression molding for airplane wings and other huge components.
Material Flexibility
This method works exceptionally well with flexible, pliable materials that don’t flow easily in liquefied form, like rubber. It can also handle thermosetting polymers thanks to lower pressures.
Considering the Weaknesses of Compression Molding
As with any other manufacturing technique, compression molding comes with a few disadvantages and limitations worth noting as well:
Excessive Material Waste
All the squishing in compression molding forces material to spill out of the parting line, leaving behind excess “flash” plastic or rubber that gets discarded, contributing to higher material costs per part.
Slower Cycle Times
Unlike the blazing speeds of injection molding, compression molding takes minutes rather than seconds to complete each cycle due to the longer curing requirements. This constrain volume capabilities.
Labor-Intensive Process
There’s much less potential for automation with compression molding compared to injection molding. The manual sheet loading step inherently relies more on human machine operation rather than automated systems.
Key Differences Between Injection Molding vs Compression Molding
Now that we’ve covered the fundamentals behind both manufacturing techniques, let’s directly compare some of the most pivotal contrasts between injection molding and compression molding:
Molding Method
The core molding methods vastly differ—injection molding injects liquefied plastic into a mold, while compression molding compresses plastic/rubber sheets.
Automation Potential
Injection molding can be almost fully automated for hands-free consistency. Compression requires manual loading for each cycle.
Cycle Time Speed
Injection molding measures cycle times in seconds, whereas compression molding cycle times take several minutes.
Part Consistency and Precision
Injection molded components offer tighter dimensional precision and consistency compared to compression techniques.
Material Suitability
Injection works well for thermosoftening plastics; compression handles thermosets and rubber better.
Initial Mold Costs
The tooling price tag runs cheaper for compression molds vs injection molds.
End-Use Applications
Injection molding suits small, intricate plastic parts of all types. Compression molding excels at simpler or larger rubber and plastic components.
Choosing Between Injection Molding vs Compression Molding
Deciding whether injection molding or compression molding makes sense for your application depends on several factors:
Production Volume – Injection molding only makes financial sense for mass-produced plastic components thanks to the higher mold costs. Compression can be ideal for lower production runs.
Part Size – Compression molding allows creating much larger components compared to size-limited injection molding presses.
Part Complexity – If intricate details, tight tolerances and decorative elements are required, injection molding can manufacture highly precise plastic components.
Performance Requirements – Lightweight and exceptionally strong plastic parts are the specialty of injection molding.
Material Type – Thermosetting polymers and rubber often lend themselves better to compression molding rather than molten liquid injection techniques.
No single solution reigns superior in all cases—it ultimately comes down to aligning your production strengths and weaknesses with the ideal molding process for your needs. Identifying those needs takes an in-depth look at your proposed part geometry, performance specifications, budget constraints, and production expectations.