Injection molding is one of the most versatile and cost-effective manufacturing processes used today. From small plastic trinkets to large automotive body panels, injection molding can churn out complex and detailed parts in mass quantities.
However, despite its versatility, there are limitations to what can and cannot be injection molded. As useful as injection molding may be, it simply isn’t suitable for every application.
So if you’re wondering “what can’t be injection molded?”, as a professional plastic injection molding manufacturer, I am writing this complete guide for you.
What is Injection Molding?
Before jumping into the list, let me briefly explain what injection molding is and how it works.
Injection molding is a manufacturing process where a polymer material, usually a type of plastic or rubber, is heated until it becomes molten. This molten material is then injected under high pressure into a mold cavity, where it cools and solidifies into the desired shape.
The mold itself consists of two halves that form the cavity – often made of tool steel or aluminum – along with small channels for the molten plastic to be injected into it. Some key components of an injection mold are:
- Mold Cavity: Hollowed out area in the mold that forms the outside surface of the part.
- Sprue: Channel through which molten plastic flows, usually ending at the gates.
- Gates: Inlets that connect the sprue to the cavity, allowing plastic to enter mold.
- Ejector Pins: Pins that push finished part out of the opened mold.
Limitations of Injection Molding Process
Now, let’s dive into the key things that can’t be easily injection molded.
1. Extremely Large Parts
One of the biggest limitations with injection molding is part size. Although there’s theoretically no size limit, in practice very large parts are challenging to mold.
This is because as part size grows, the clamping force required increases exponentially. Extremely massive molds require specialized high-capacity injection molding machines that are scarce and expensive.
As a guideline, conventional injection molding machines typically can’t mold parts larger than around 2 square meters comfortably. Larger parts would need expensive custom equipment.
2. Deep Hollow Cavities or Cylinders
Another limitation is the ability to form deep hollow cavities, cylinders or holes. Because the molten plastic has to completely fill the mold, having an overly deep cavity is problematic.
As plastic flows into narrow, restricted spaces, it tends to cool and harden earlier. So the deeper the intended cavity, the harder it is for the plastic to completely fill before freezing off.
3. Excessively Thin or Delicate Geometries
Molding thin-walled or fragile geometries can also be an issue with injection molding.
As hot plastic is injected at high pressures, extremely thin cross sections can warp or get crushed. Delicate features like blades or spikes can get distorted without enough underlying support too.
Generally, walls thinner than 0.75mm or 0.03 inches will be challenging to mold properly.
4. Interlocking, Rotating or Enclosed Moving Parts
Having interlocking, rotating, or fully enclosed and independently moving components in a single part is very difficult to injection mold as well.
The injection molding process requires designing the mold tool to separate from the part in a straight, unimpeded manner. This allows easy ejection of the cooled plastic piece.
So parts with complex articulations, rotating joints, or fully sealed internals can’t be easily extracted out of a simple two-piece rigid mold.
Material Limitations for Injection Molded Parts
Aside from design constraints, certain types of materials are also not suitable for injection molding processes.
1. Low Melting Point Materials
Substances with excessively low melting temperatures generally can’t be molded. Materials need to liquefy enough under high heat and pressure without burning or degrading.
For example, modeling clay would simply start scorching before becoming fluid enough to inject into a mold.
2. Porous Materials
Similarly, porous materials with tiny air gaps like bread dough or wet concrete wouldn’t work. When forced into a mold under pressure, the air pockets would compress in unpredictable ways.
After curing, the material would likely be filled with unwanted dents and dimples from the pores collapsing.
3. Multiphase Combinations
While some exceptions exist, trying to injection mold heterogeneous, multiphase mixtures leads to separation. Dissimilar densities under the heat and pressure causes segregation.
For example, attempting to mold a smoothie-like blend of granola chunks, fruit bits, and yogurt would just lead to globs of ingredients clustered in the cured material.
4. Flexible Rubbers
Although some specialized injection molding setups for rubber exist, highly flexible and elastic materials are problematic. Their stretchiness makes it difficult to control dimensionally or extract out of metal molds.
As a result, soft rubbery materials usually get compression or transfer molded instead. The one exception is thermoplastic elastomers (TPE), which act like flexible plastics.
Key Takeaways on Injection Molding Limitations
The main things that can’t be easily injection molded include:
- Extremely large parts
- Deep hollow cavities
- Thin or fragile geometries
- Interlocking assemblies
- Low melting point materials
- Porous substances
- Multiphase mixtures
- Highly flexible rubbers
Understanding these injection molding restrictions allows you to design parts better suited to the process. It also helps guide early decision making when selecting the optimal manufacturing methods.
And if you need help further refining your injection molded components, Protolabs offers exceptional design analysis services. Their experts can review your 3D CAD models and provide feedback to improve moldability.
Frequently Asked Questions
Now let’s wrap up with answers to some common frequently asked questions about injection molding limitations:
Can You Injection Mold Metal?
Actually yes, injection molding metallic parts is possible through a process called metal injection molding (MIM), which uses a metallic powder + binder mixture.
However, potential limitations when injection molding metal include costs rising for complex geometries, minimum wall thicknesses around 0.02 inches, and possible sintering defects.
What Geometric Features Should You Avoid?
Its best to avoid the following features which can make injection molding more difficult: overly thin ribs or walls, narrow yet deep ribs, ball joints, snap fits in deep recesses, bosses in corners, and sharp transitions between sections.
How Small of a Part Can You Injection Mold?
In theory, injection molding can produce extremely tiny micro-parts down to 0.003 inches (0.075mm) in length. However, getting accurate micro mold tooling and handling minuscule components gets exponentially harder as parts shrink.
As a practical minimum, molded parts as small as 0.02 inches (0.5mm) across can be done but require extra precision.
So there you have it – a complete guide covering key limitations to be aware of with injection molding manufacturing processes. Let me know in the comments if you have any other questions!
