A plastic part weighing a fraction of a gram may seem straightforward. But shrinking a part changes how material, mold, and process interact: what would be a minor deviation in a conventional part can represent a substantial portion of a microscopic channel, tooth, or wall.
Plastic micro injection molding is not simply conventional injection molding on a smaller scale. Weight alone does not define it, either. A simple sub-gram part could be produced on a properly sized conventional machine; a heavier part with functional microfeatures could require specialized tooling, process control, and metrology.
Before investing in a mold, evaluate the entire system: the part, material, tooling, process, inspection, and handling.
What Is Plastic Micro Injection Molding?
Micro injection molding (µIM) is used to manufacture extremely small thermoplastic components or parts with micron-scale features in production quantities.
There is no universal weight cutoff. A part may require micro injection molding when its mass, geometry, or tolerances make filling, demolding, or measurement more demanding. The evaluation should focus on what must be reproduced consistently.
At DRIMO, our micro injection molding capability covers parts from 0.01 to 1 gram. This range helps identify the types of projects we can evaluate, but feasibility also depends on what must be reproduced and how consistently.
When Does a Part Under 1 Gram Need a Specialized Process?
Weight is often the first number that causes confusion in a project. The machine does not meter material for just one part: the mass leaving the mold includes parts from every cavity and the mold feed system when it solidifies and is ejected.
Mass leaving the mold per cycle = (part weight × number of cavities) + solidified feed-system mass, if any
The machine must be set to meter enough material to supply that mass and maintain a stable melt cushion in front of the screw. That cushion remains in the injection unit after filling and packing; it is not ejected with the parts or runners. The feed system can outweigh the parts themselves, affecting material consumption, shot metering, and residence time.
| Criterion | Conventional molding may be viable | Evaluate specialized micro injection molding |
|---|---|---|
| Geometry | Simple shape and moderate tolerances | Microscopic channels, holes, walls, textures, or cores |
| Mass per cycle | Stable with the available injection unit | Very small for the unit or dominated by runners |
| Material | Wide processing window | Sensitive to moisture, temperature, residence time, or shear |
| Quality | Conventional measurement and ejection | Very small critical dimensions, delicate handling, or specialized inspection |
This matrix supports a preliminary assessment. It does not replace a design for manufacturability (DFM) review, mold design, or process trials.
Why Micro Injection Molding Is Not Conventional Molding in Miniature
A very small shot mass must be compatible with the injection unit. If the unit is too large for the actual shot, it may meter material less consistently and expose the melt to heat for too long. This increases the risk of variation or degradation in sensitive materials.
In a tiny cavity, the material loses heat quickly and may stop flowing before it fills the geometry. Temperature, fill speed, pressure, gating, and venting must work together. Increasing pressure alone will not fix a poorly located gate or trapped air.
The process does not end when the mold opens. A very small part can cling to surfaces because of static, deform, or get lost during ejection. How it will be removed, counted, and packaged must be defined during project design.
Designing a Part for Micro Injection Molding
There is no universal chart of wall thicknesses, draft angles, or aspect ratios. A value that works for one part may fail in another because of flow length, material, surface finish, gate design, or acceptance criteria.
Before releasing the CAD model, review these six design considerations:
1. Wall thickness and flow path. A thin wall may fill near the gate but remain incomplete along a long flow path. Consistent wall thicknesses and gradual transitions are preferable.
2. Feature and tooling. A very small hole may require a steel core that is susceptible to bending, wear, or breakage. The feature must be practical to manufacture and maintain.
3. Radii, draft, and demolding. Sharp corners, deep textures, and undercuts make ejection more difficult. The draft angle depends on geometry, surface finish, and material.
4. Gate, parting line, and venting. Material entry and air escape affect filling, flash, and visible marks. These factors need to be reviewed alongside ejection.
5. Shrinkage and surface requirements. Material and geometry affect distortion. A cosmetic surface, a functional microtexture, and an optical area require different acceptance criteria.
6. Tolerance and measurement. Every critical dimension needs a measurement method that can access the feature without deforming the part and has sufficiently low uncertainty relative to the tolerance. Otherwise, conformance cannot be demonstrated.
Two visually similar parts can require very different solutions. Designate only features that affect function, assembly, or safety as critical-to-quality (CTQ) characteristics. Tightening tolerances on the others increases cost without necessarily improving the product.
The Micro Injection Mold
In a small part, a tiny mold error can consume a substantial portion of the tolerance. In addition to forming the geometry, the mold must allow material to enter, air to escape, temperature to be controlled, and the part to be ejected without damage.
How the microcavity is manufactured depends on the smallest feature, surface finish, expected service life, and insert repair requirements. Every feature must be manufacturable, inspectable, and maintainable.
A feed system that solidifies every cycle simplifies the mold but can consume more material than the part itself. Eliminating those runners reduces waste at the expense of higher cost and complexity. Adding cavities requires proof that every cavity fills and consistently meets requirements.
Before mold acceptance, agree on materials, trial conditions, dimensional reports by cavity, spare parts, maintenance, data ownership, and transfer procedures.
Selecting the Material
Start with the part’s functional requirements: temperature, chemical exposure, friction, transparency, or sterilization. Then choose a polymer family and grade that can fill the geometry while retaining the required properties. Two grades from the same family can behave differently.
| Project requirement | Example polymer family | What to validate |
|---|---|---|
| Filling thin walls or very small passages | High-flow grades, including some LCPs | Mold filling, joints, and grade-specific behavior |
| Transparency and low moisture absorption | COC or another suitable transparent grade | Chemical compatibility, adhesion, and sterilization |
| Low friction in a micromechanism | POM or another low-friction grade | Shrinkage, wear, and assembly fit |
| Demanding temperature or chemical exposure requirements | PEEK or another engineering resin | Processing temperature, residence time, and cost |
Specifying only “nylon,” “polycarbonate,” or “PEEK” is not enough to obtain a quote. The grade, color, fillers, and additives must be defined. In hygroscopic polymers, moisture can degrade the material if conditioning is inadequate. Fillers also affect stiffness, wear, and dimensional stability.
A polymer family’s use in medical devices does not mean it is approved for every application. Biocompatibility, cleanliness, and sterilization depend on the grade, process, and end use.
Quality and Metrology for Microcomponents
Develop the inspection plan alongside the part design. An accessible profile can be measured optically; an internal channel might require X-ray computed tomography; contact measurement can deform a delicate part. The method must be able to access the feature and provide measurement uncertainty appropriate to the tolerance.
Before using capability indices such as Cpk, demonstrate that measurements are repeatable and the process is stable. A Gauge R&R study evaluates variation when measurements are repeated or operators change. Keeping results tied to individual cavities prevents an average from masking an off-target cavity.
ISO 20457:2026 provides a reference for specifying tolerances on molded plastic parts. It does not establish one universal value for every micropart. Tolerances must be agreed on for the actual geometry, material, and function, then verified using an appropriate measurement method.
What Drives Cost and Feasibility
A part is not necessarily inexpensive just because it uses little resin. Engineering, tooling, trials, cavity requirements, material waste, inspection, and handling often contribute more to cost. Delicate ejection requirements can have a greater impact on cost than the resin.
There is no universal break-even volume. Micro injection molding makes more sense when the design is stable, the geometry is moldable, and production volume amortizes the tooling cost. For prototypes or low-volume runs, compare 3D printing, micromachining, or other processes before building a mold.
Evaluating a Micro Injection Molding Supplier
A useful quote should explain the manufacturing and quality-control approach for the part. Ask for evidence covering:
• an injection unit suited to the actual shot size;
• experience with comparable features, materials, and tolerances;
• the proposed gate, runner, venting, and ejection design;
• a method capable of verifying the tolerances, with validation by cavity;
• mold ownership, maintenance, and transfer;
• automation, packaging, traceability, and operational continuity.
Promises of “micron-level tolerances” without identifying the dimension, material, cavity, method, and sample are a warning sign. So is quoting without reviewing the CAD model, CTQs, or end use. For a closer look at these criteria, see our guide to evaluating plastic injection molding companies in Mexico.
Proximity to U.S. customers can make sampling, changes, and support easier, but it does not guarantee better execution. If a mold is developed internationally and production takes place in Mexico, acceptance, data, spare parts, modifications, and ownership must be defined. The technical focus remains the same: control of the project from design through validation.
What to Include in a Request for Quote
The supplier needs to understand both the part and the production program. Provide the following:
• a STEP or STP solid model and a 2D drawing with datums, tolerances, and critical dimensions;
• application, loads, temperature, chemical exposure, and expected service life;
• material grade or required properties, color, fillers, and restrictions on reprocessed material;
• per-order, annual, and total volumes, along with production start and end dates;
• validation, traceability, secondary processing, and packaging requirements;
• permitted locations for the gate, parting line, and ejection marks;
• how parts will be measured, handled, and separated by cavity, if already defined.
Our guide to the information needed for an injection molding quote covers the general documentation. For a micro injection molding project, also include the molded mass per cycle, critical feature, feed-system strategy, and handling conditions.
Plastic Micro Injection Molding FAQs
Does Every Part Under 1 Gram Require Micro Injection Molding?
No. A simple part may be viable in a properly sized conventional molding cell. The decision depends on the total shot size, geometry, material, tolerances, inspection, and handling.
What Is the Minimum Wall Thickness?
There is no single value. It depends on flow length, resin grade, gate design, mold temperature, surface finish, and the feature that must meet the acceptance criteria.
What Tolerances Can the Process Achieve?
A single general value is not a useful answer. The dimension, geometry, material, cavity, measurement method, uncertainty, and evidence of process capability must be defined.
How Are Such Small Parts Inspected?
That depends on the feature. Optical measurement works for accessible profiles, contact measurement for stable geometries, and X-ray computed tomography for internal features. Select the method based on access, tolerance, and uncertainty.
What Does MIM Mean?
In international industrial usage, MIM generally means Metal Injection Molding, a process for metallic materials. For thermoplastics, micro injection molding or µIM is preferred.