Everything you need to know about micro-injection molding

Table of Contents

Medical device, electronics, and biopharmaceutical manufacturers need newer micro-injection molding products to make smaller and more space-efficient microdevices. Micro-injection molded components can be as small as a speck of dust.

What is a micro-injection molded component?

Many new advances in micro injection molding techniques have made possible the design and manufacture of micro molds that enable microinjection molding of thermoplastics, silicones, and metal powders.

This computing has facilitated the development of minimally invasive medical and pharmaceutical micro-devices worldwide.

This paper describes many of the major factors and challenges faced and solutions to the success of microdevice and component conventional injection molding.

Challenges to Micro Molding

Most micro products start with some degree of extreme challenge. They are usually scaled-down versions of similar products on the market.

Micro components become increasingly complex as they are mounted in tiny and delicate arteries, pumps, catheters, or endoscopes, and may have micro components inside that need to work.

They often have challenging geometries because they were once designed as two or more components, but were reduced to one component due to cost pressures so that they did not have to be assembled under a microscope.

These devices may require drugs that are directly compounded with or added to polymers, metals, or membranes and come with working gears, levers and drive mechanisms to make the device work repeatedly and have a reliable lifetime.

Considering these features and the requirement that these devices can be directly implanted in the human body, it is important to develop these devices robustly and to test them well for form, fit, and function.

Microinjection Flow Mold Flow Analysis

Because micro-molds and prototype parts are costly in their development cycle, micro-mold flow simulation analysis can provide a simulation of filling expectations based on a specific design.

When comparing conventionally molded and micro molded parts, a very common assumption is that micro parts can be filled with the same software and the same modeling approach.

For example, a flow analysis involving a typical 500-micron gate would be very different from a simulated flow through a 75-micron gate.

The main difference is that apart from a micro-gate will generate more shear-induced heat when passing through a small orifice.

Therefore, the solid model mesh must have a very high resolution to determine what is happening in the gate and thin wall areas.

Solid model meshes used in mold flow simulations require meshes of a few microns in size, compared to tens of microns in parts.

Gates in micro molded components must be properly sized to avoid excessive thermal stress on the material entering the cavity.

For heat-sensitive materials such as bioabsorbable and biopharmaceutical polymers, it is important to understand the relationship between the material retention time in the barrel, nozzle, and hot runner and the additional heat that may be transferred to the material during the injection process.

Sometimes the material drives the process selection, and sometimes the process drives the material selection.

Some commonly used micro forming materials are PEEK, PLA, PGA, LSR, polyethylene, polypropylene, polycarbonate, LCP, PMMA, cyclic olefin copolymers (COCs), and stainless steel (metal injection molding).

Microinjection Molds

Once the product design and material selection have been determined, it’s time to make the micro injection unit mold.

Whether the material is a thermoplastic, silicone, or metal powder, the mold is the most critical component for success.

Since the product and mold are so small (as shown below), the tolerances of the dimensions become smaller as well. The mold must still meet 25% of the part tolerance to provide a good processing window.

The product tolerance is ±0.01mm and the mold tolerance must be ±0.003mm to achieve a good process window.

Such tolerances are difficult to achieve for the average mold maker for two main reasons.

1.They cannot measure ±0.003mm and therefore cannot verify them.

2.They lack the equipment or skills to achieve these tolerances.

Micromolding runners

In automated assembly operations, they can be used as handles to hold parts in place, or special positioning points can be added to the runner to help us position the part in the assembly nest.

Micro injection molding parting line

The parting line of a micro-injection mold is related to the size of the micro part. A difference of 10 microns on the parting line can easily disrupt the assembly of the product.

Micro injection mold release slope

The more release slopes the better, of course, but the smallest taper can be as small as 0.2 degrees. Any such taper can be troublesome to handle for injection molded parts. Placing a micro part on a taper can create an irregular surface that can interfere with assembly.

Micro-injection Gating Position

As with regular injection molds, the purpose of choosing a gate location for micro-injection molds is to ensure that a uniform flow of plastic is produced in the cavity.

Otherwise, parts may not be adequately filled and may damage the precision pins and cavity components in the mold.

Micro Molding Gate Residue

Most micro molded parts will use edge gates. If so, they need to be removed from the gate properly to avoid problems with small materials causing arterial damage (implanted medical devices) or causing automation and assembly problems.

These problems can be solved in the mold design by placing a dimple in the wall thickness so that the gate residue will be designed under the surface of the guide or mating part in the assembly.

Micro molding process Surface Finish

Often overlooked is the importance of the surface finish of the molded part in holding or guiding features into other features during assembly.

For example, some products require a rougher surface for better adhesion. A smooth surface may produce a series of problems when ejecting from the injection mold and requires a compromise.

Micro-injection molding process

Because the accuracy of micro-injection molded products is often in the range of several microns, there are several challenges to achieving good dimensional repeatability in injection molded parts.

It is one thing to create nice sharp corners and cavities in the mold steel (less than 1-micron radius), and quite another to fill these tiny spaces with polymer.

Micro molds require proper venting and sometimes the use of very thin laminates to achieve proper venting and cusp filling.

Typical injection pressures in micro-formed parts range from 30,000 to 50,000 psi, which requires a delicate balancing act to fill at the proper pressure without damaging the tiny, hair-thin core pins.

Dust speck size parts with extremely thin walls (0.001-0.0015 inches) require extreme cavity-to-core alignment accuracy across the parting line.

Micro-core pin damage is likely to occur if the polymer is cooled under unfilled conditions or if the part is filled more on one side than the other.

This challenge can be overcome by filling quickly in short periods (typically <0.1 sec) and at high pressures.

Microformers must be able to inject very small amounts of glue and keep the plastic retention time in the barrel to a minimum. This is especially important for bioresorbable polymers (PLA, PGA) with high shear and heat sensitivity.

Specialized screws, nozzles, and auxiliary equipment are also required to provide the precision to fill, handle, demold, measure, and assemble these tiny devices.

Assembly and Handling

Assembling geometries into the smallest number of parts for micro-assembly is a very worthwhile design effort, as picking them up, assembling them into nests, and attaching them to other parts of similar or different materials can be much more expensive than spending time upfront in the design phase.

Secondary Micro forming

The process of injecting two different materials into two different molds in two different locations, or using a rotating mold to inject two different materials in the same location to achieve a combined geometry and material.

For example, if a pump piston requires a seal or silicone gasket, it is easier to secondary mold the gasket into an o-ring groove in the same mold as the piston than to fit the o-ring into a precision mechanism, clamp the o-ring with scissors and place it on the piston.

Laser welding

If the three-dimensional geometry cannot be combined by secondary forming and the material strength allows it, laser welding is a good way to join miniature parts.

Precisely controlled laser energy and power density can also be used to selectively clean and strip materials such as wire quickly and non-destructively.

Ultrasonic Welding

Ultrasonic welding can also effectively join thermoplastics and compatible metals. Because of the extremely low energy required for strong welding, micro parts require specialized low-energy boosters and ultrasonic generators.

Solvent Bonding

This is often used as a fast, low-capital investment method for joining micro-components. The solvent selected must be compatible with the material being bonded, especially when the component is used for implant applications.

Using solvent bonding to accelerate the high volume assembly process is difficult because the method is not easily automated and reproducible, and is difficult to validate in the high volume range.

Riveting

Micro riveting is a very inexpensive method of joining polymer and metal parts. For example, in battery cans, crimping or locking is a very common practice that produces a good seal and prevents corrosive liquids from escaping from the battery container.

Inexpensive progressive stamping dies to allow a moderately fast method of riveting polymers and metals together by “folding” one material under pressure into another. Material batch-to-batch variation and change can be a drawback of this method.

Testing

An important aspect of automated micro-assembly systems is testing, such as electrical conductivity, leakage or pressure decay, and burst strength. Some of these are destructive and some are non-destructive tests.

The best way to determine if the final assembly or sub-assembly is working properly is to maintain production process control for each component that makes up the assembly.

Statistical verification of each component and revalidation of the assembly will prevent costly testing and inspection later in the automated cell;

however, sometimes these issues may also be unavoidable, especially in implantable and critical drug applications.

Test Measurement

We’ve all heard that “if you can’t measure it, you can’t manufacture it”. In medical and pharmaceutical devices, critical components can be a matter of life and death, which also means “if you can’t verify it, you can’t make it”.

If parts are manufactured consistently and verified, then commercial micro injection molding systems should be avoidable. But it is rarely possible to guarantee 100%.

There are many ways and means to inspect micro featured plastic parts and assemblies. Some can be inspected with a high-resolution camera to verify product features or surface finish.

Some require 3D laser scanning to verify some critical dimensions. Still, others require a high-speed camera to show if the powder or liquid crystal polymer has been dispensed at the correct dosage.

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