The Protolis guide to compression molding

When to choose compression molding for prototypes and low volume manufacturing?

• How to prototype rubber parts

Prototyping rubber parts is essential before compression molding, as rubber behaves in ways CAD can’t fully predict. Physical testing helps catch design issues early and reduces the risk of costly tooling changes. The best prototyping method depends on what you need to validate. Some are ideal for form and fit, others for function. Below is a practical comparison of four common approaches, each with its own strengths and trade-offs.

3D printing: fast iteration, limited accuracy

3D printing is usually the first stop in the prototyping process. Common 3D printing technologies include Fused Deposition Modeling (FDM), Stereolithography (SLA), and Selective Laser Sintering (SLS), each offering different strengths in material compatibility, resolution, and durability. With flexible materials like TPU or elastomeric resins, you can produce parts that visually and mechanically resemble rubber, at least to a point. These printed materials typically cover a hardness range from around 27 to 95 Shore A, making it possible to simulate a variety of rubber-like behaviors, from soft and flexible to relatively firm. For compression molded parts, 3D printing is particularly useful for evaluating basic geometry, interface fit, and overall proportions.

The downside is that most printed materials behave quite differently from thermoset or vulcanized rubbers. You’ll often find that the elongation, compression set, and tear resistance are far from production behavior. Still, as a way to quickly explore form and identify obvious design conflicts, it’s hard to beat for speed and accessibility.

Explore more information about vacuum casting: 3D printing in Protolis

Vacuum casting: closest feel, moderate fidelity

Vacuum casting uses a soft silicone mold—typically made from a 3D printed master—to cast parts in polyurethane or silicone-like materials. While it doesn’t replicate the actual compression molding process, it can come surprisingly close in terms of tactile feel, surface finish, and elasticity, especially when using casting resins that approximate the final durometer.

Most vacuum casting materials are available in a hardness range from 30 to 90 Shore A, which covers a broad spectrum of rubber-like behavior. This makes it a strong option for simulating the look and feel of production parts, even if the mechanical performance isn’t an exact match.

The process is particularly useful when you need a small batch of realistic parts for assembly evaluation, field testing, or limited user trials. Just keep in mind that there may be slight property differences compared to compression-molded parts, and some variation between batches due to mold wear or casting conditions.

Explore this section to learn more about vacuum casting: Vacuum casting in Protolis

Compression proto mold

We can develop prototype compression molds much like rapid injection molds. Single cavity molds are typically machined from aluminum or tool steel. At Protolis, these can usually be produced in about two to three weeks, depending on part size and complexity.

While their primary purpose is early part sampling, single cavity molds also provide valuable insight for designing full scale multicavity tools. By isolating one part, you can evaluate geometry, material behavior, and molding conditions with fewer variables. Flow, venting, cure time, and flash are easier to assess, helping catch issues early.

They don’t replicate every production detail, like thermal gradients or pressure loss but in most cases, they reveal enough to guide smarter decisions. Critically, changes to gates, parting lines, or draft are far simpler at this stage.

At Protolis, we use this flexibility to reduce risk, refine designs, and help our clients scale with fewer surprises and faster turnaround.

Explore this section to learn more: Compression molding service in Protolis

Rapid injection molding: high fidelity, higher cost

If you need prototypes that behave nearly identically to the final production parts, rapid injection molding is typically the closest you’ll get before full compression tooling. With aluminum or soft steel molds and actual rubber or TPE materials, you can produce small batches that replicate final part geometry, tolerances, and mechanical behavior with a high degree of precision.

The tradeoff here is time and cost. Tooling, even for rapid molding, still requires careful design, machining, and iteration. It makes the most sense when functional testing is critical, such as in sealing, fatigue, or regulatory scenarios and when your budget allows for a more involved prototyping phase. If you’re still working out basic shape or layout decisions, this may be a bit premature.

CNC milling: best for 2D forms and gaskets

CNC milling can be useful for prototyping solid rubber parts with simple external shapes, such as cylindrical components. The process involves cutting a pre-formed block of material.

Rubber’s elasticity makes it difficult to machine. Milling is only practical for elastomers with a Shore A hardness above 75. Even then, parts often need support to avoid deflection. A collar just above the cutter can help hold the material down, and freezing the part with liquid nitrogen can temporarily increase hardness for cleaner cuts.

Still, even with these tricks, fine detail is limited, and internal or complex shapes aren’t feasible. CNC milling has a place in rubber prototyping, but only for firm materials and relatively simple geometries. For softer rubber or more detailed parts, other methods are typically more effective.

Choosing the right tool for the job

Prototyping is a critical step in developing rubber parts for compression molding. It helps you validate form, function, and performance before committing to production tooling, saving time, cost, and rework.

At Protolis, we’re committed to helping you navigate this process with clarity and confidence. Whether you need guidance on materials, prototyping methods, or next steps toward molding, our team is here to support you. If you’re unsure where to start or want a second opinion, don’t hesitate to reach out. We’re ready to help you move forward with a smarter, more reliable path to production.

• When compression molding may not be ideal

While compression molding remains a solid and dependable method for shaping rubber and silicone materials, it isn’t always the most practical solution for every project. It excels in certain scenarios, particularly with medium to large parts that have relatively simple geometries but there are cases where it can become limiting in terms of speed, consistency, or complexity. Below are some situations where you’ll generally want to consider alternative molding methods.

1. High-volume production

Compression molding tends to be slower than other processes, mainly because each cycle involves manual loading of preforms, followed by extended cure and cooling times. For smaller parts produced at scale, these time costs can add up quickly. The process is reliable, but it doesn’t scale as efficiently when you’re trying to meet high output demands.

Proposal: For high-volume runs, injection molding and in particular liquid silicone rubber (LSR) injection offers tighter cycle times and better compatibility with automation.

2. Tight tolerances or high-precision parts

Maintaining dimensional accuracy in compression molding can be challenging, especially across multiple cavities. Flow variations, shrinkage, and flash formation all contribute to a degree of variability that can be difficult to control, particularly for parts requiring precision fits or seal integrity.

Proposal: In most cases, transfer molding or LSR injection molding is better suited for applications where part-to-part consistency and tight tolerances are critical. For more details on rubber compression part tolerances, refer to this section:

Understanding rubber compression molding tolerances

3. Complex or delicate features 

Rubber compression molding is generally less suitable for parts requiring fine details or thin-walled features. Because the material must flow under heat and pressure to fill the cavity, it can struggle to reach tight or intricate areas, especially in complex geometries. This may lead to issues such as air entrapment, incomplete filling, or visible surface flaws.

Proposal: LSR injection molding handles complex geometries far more efficiently, delivering cleaner finishes and higher fidelity to the original design.

4. Automation and production line integration

Compression molding is often more manual than automated. Operators typically load and unload each mold by hand, and secondary operations, like trimming flash, are usually required. This hands-on approach can slow things down and introduce variability.

Proposal: When automation is a priority, LSR injection molding provides a more streamlined process, with minimal human intervention and higher throughput.

5. Rapid prototyping and short lead times

Tooling for compression molding is relatively straightforward, but it’s not particularly fast to iterate. Making changes to the mold or setup can involve downtime, and that doesn’t lend itself well to projects that need quick turnarounds or frequent design updates.

Proposal: For prototyping or low-volume custom parts, additive manufacturing or vacuum casting are more agile and responsive to design changes. Explore this section to learn more about rubber prototyping:

The Protolis guide to vacuum casting

6. Material waste and efficiency

Flash is an inherent byproduct of compression molding, and while it can be managed, it often requires trimming and adds to material waste. In high-precision environments or when working with expensive materials, that waste becomes a more significant concern.

Proposal: If waste reduction is a priority, injection or transfer molding typically produces cleaner parts with less excess material.

Final thoughts

Compression molding has its place, it’s cost-effective for certain applications, relatively simple to set up, and dependable for a range of part sizes. But like any process, it has its limits. At Protolis, we help clients navigate these decisions every day and we’re always happy to talk through the pros and cons in the context of your specific application.

• Compression molding vs. injection molding

Compression molding and injection molding are two prominent techniques in the manufacturing of flexible parts. Here are five key differences between these methods:

• Material placement: In compression molding, the raw material is placed directly into the open mold cavity and then compressed. In contrast, injection molding involves injecting molten material into a closed mold under high pressure.
• Tooling complexity: Compression molds are typically simpler and less expensive to produce compared to injection molds, which require complex designs to accommodate the injection and cooling systems.
• Cycle time: Compression molding generally has longer cycle times since each phase (heating, pressing, cooling) must be managed sequentially. Injection molding benefits from faster cycle times due to continuous, simultaneous processes.
• Waste management: Compression molding can lead to more material waste in the form of flash (excess material that oozes out of the mold), which must be trimmed. Injection molding usually produces less waste, as the excess material can often be reused immediately.
• Part consistency and detail: Injection molding allows for higher detail precision and more consistent part quality across productions due to controlled injection parameters and material flow. Compression molding may result in less uniformity in part consistency and detail, especially with more complex geometries.
• Costing: Compression molding typically involves lower initial tooling costs, making it economical for low to medium-volume production. However, the per-part cost can be higher due to slower cycle times. In contrast, injection molding has higher initial tooling costs but benefits from lower per-part costs due to faster production rates and efficiency, suitable for high-volume production.

Feature	Compression Molding	Injection Molding
Material Placement	The raw material is placed directly into the open mold cavity.	The molten material is injected into a closed mold under pressure.
Tooling Complexity	Simpler and less expensive molds.	Requires complex molds with more detailed designs.
Cycle Time	Longer due to sequential process phases.	Shorter due to simultaneous processing steps.
Waste Management	More waste in the form of flash, which needs trimming.	Less waste and excess can often be reused.
Part Consistency & Detail	Less uniformity in part consistency and detail.	Higher precision and consistency in details.
Costing	Generally, initial tooling costs are lower, but the per-part cost is higher due to slower cycle times.	Higher initial tooling costs but lower per part cost due to efficiency and faster production rates.

Understanding these differences helps manufacturers select the most appropriate method based on their specific needs. Each technique offers unique advantages that are suitable for different applications in the plastics and rubber manufacturing industries.

Continue your exploration of compression molding in other chapters.