MICRO MOLDING — MEGATREND MINIATURIZATION

With Contributing Expertise From: simtec

Published Date: April 15, 2021

One of the latest trends in LSR manufacturing across many industries is miniaturization, or micro-molding. As components get smaller, manufacturers must deliver incredible accuracy and precision, and liquid silicone rubber (LSR) injection molding, and more specifically LSR micro molding is perfectly suited for these complex, tiny, and critical components.

What is micro molding, and what are its benefits and challenges? We’ll explain more in this guide.

WHAT IS MICRO MOLDING?

Micro molding is a specialized molding process that produces small, high-accuracy, and high-precision parts. These parts are minuscule — generally, they have a diameter of less than 4 millimeters or a weight of less than 1 gram.

The process begins with a mold, also referred to as tooling. The mold contains small cavities in the shape of the required component. During the LIM (liquid injection molding) process, LSR is injected rapidly into the heated cavity where it vulcanizes and forms the part. Some micro molding processes use a runner, a channel used to direct the material into the cavity.

Alternatively, when using direct gating techniques, the material is injected directly into the part, eliminating the runner and the material waste.

Tremendous accuracy and precision are required in both the tool build (mold) and parts production. Because the parts are so small, even minor misalignments can have significant effects on quality. Micro molding LSR also requires extra precision in mixing the material’s A and B components, adding color (if applicable), and dosing to allow process stability and repeatability. Mold, feeding system and molding machine must be capable of controlling mixing, dosing and closing with utmost precision. Then the real challenge starts, controlled demolding, handling and inspection of miniscule tiny tacky parts. Cavity separation is advisable for containment and traceability, and magnifying devices for inspection. All of this requires a team with keen skills and expertise that can innovate in a space where every micrometer is critical.

WHY IS LSR BENEFICIAL FOR MICRO INJECTION MOLDING?

One of the primary reasons LSR is beneficial in micro injection molding is its low viscosity, which makes it ideal to fill in small micro features. LSR’s flexible characteristics make it ideal for applications that demand high elasticity and compressibility for sealing.

Product miniaturization poses many challenges for product engineers, and LSR can help engineers when designing their compact applications.

Since the parts are very small, the shot weight is also small. The material stays in the barrel for a long time, exposed to heat. This heat causes materials like thermoplastic polymers to degrade if the residual time in the molding barrel is too long. LSRs however are maintained at a cool temperature in the barrel and usually do not degrade or prematurely cure in the barrel.

LSR’s low viscosity is also a benefit as it allows for an easier fill of small cavities.

What types of products are commonly produced using LSR micro molding?

Medical parts: Micro molding is useful for medical LSR parts that must achieve high performance in a small space – such as hearing aids, septum, pressure sensing diaphragms, surgical devices, and wearable drug-delivery devices.
Automotive parts: Micro processes in automotive injection molding help create small automotive parts bound by weight constraints — small seals, micro switches, sealed housings, and micro connectors.

WHAT IS SHOT WEIGHT, AND WHY IS IT IMPORTANT FOR MICRO INJECTION MOLDING?

What is shot weight? In injection molding, a shot is the material injected into the mold for each cycle. Shot weight is the weight of this material. The part weight, the runner weight, and the number of cavities in the mold all contribute to determining the optimal shot weight.

Shot weight is particularly important for the micro injection molding process because the tiny parts have specific requirements. Smaller shot weights necessitate smaller injection barrels or stuffer boxes with lower tonnage to match the mold specifications.

CHALLENGES IN MOLD CONSTRUCTION

Mold construction for micro injection molding presents several engineering challenges:

Accuracy and precision: Micro injection molds require exceptionally accurate and precise electrical discharge machining (EDM) to achieve the necessary micro details and avoid mismatch when the mold is closing.
Assembly and disassembly: Because of their small, fragile, easily breakable components and the necessity for accurate alignment, micro injection molds require deliberate, delicate assembly and disassembly methods.
Design: The majority of the challenges of mold construction involve mold design. Mold design must incorporate critical details to ensure consistent alignment of the mold halves and allow for precise demolding (parts removal).
The mold design must also facilitate consistent quality and longevity for high-capacity output — a typical micro injection mold must deliver millions of parts over its lifespan.
Balancing pressures: With micro injection molds, it’s essential to balance cavity pressures throughout the cold deck. Doing so allows for more consistent filling. i.e. a well-balanced cold deck and good valve gate technology ensure a synchronous closing of the needles that avoid short shots or incomplete filling of the component.
Temperature management: Cavity temperature directly affects the curing of the material and formation of the part. Nozzle technology, with good cooling through the very tip/beginning of the cavity helps with the transition from cooling during filling to heating during vulcanization. It is also beneficial in delivering proper fill.
Gating: Direct gating is typically done if the component has adequate landing space for a gate. Because sometimes the part geometry may not permit the use of very small direct gates, engineers must develop alternative in-direct gating solutions like film, edge, or tunnel gates. Overflows can also be used to ensure complete part filling.
Optimizing overflows: The use of overflows can reduce the risk of trapped air, which impedes a mold’s proper function. Runners can also be useful for facilitating part handling during multi-step operations, including in-line slitting, laser marking, pad printing, packaging, and assembly.

CHALLENGES IN LSR MOLDING

LSR molding also presents a particular set of challenges for engineers:

Detecting fluctuations: Monitoring inputs and outputs is critical for detecting fluctuations in the process. Detecting and minimizing fluctuations is necessary because even the slightest deviations can affect the small features of micro molded parts.
Feeding A and B material components: LSR consists of an A & B component – one containing the cross linker and inhibitor, while the other contains the platinum catalyst.
Managing the precise feeding of the materials’ A and B components and maintaining an exact 1:1 ratio is essential for product quality. It allows the proper curing reaction and thorough blending to take place.
Controlling color: For LSRs with color pigments, controlling the third-stream flow for the color pigment is vital, even with low dosing rates.
Demolding and parts handling: Removal of the parts from the cavities can pose a challenge, as LSR parts are tacky. The challenge grows with higher cavitation molds.
Robotic handling: Effective robotic handling is particularly important for parts handling and completing in-line secondary operations. Handling LSR parts present heightened challenges due to its static and tackiness characteristics, and the tiny size of micro parts.
Part inspection: Micro molded parts are very small in size. Orientation and inspection of delicate dimensional features can pose a challenge.

WHY CHOOSE SIMTEC FOR YOUR MICRO MOLDING NEEDS?

Micro molding requires precision molds, advanced molding technologies, and fine-tuned manufacturing cells. A partner like SIMTEC checks all the boxes.

Our proprietary use of cold deck and valve technologies in our molds allow high cavitation even on relatively small platens. Mini valves used for direct gating small, miniature parts, and open gates for indirect gating provide the precision and accuracy demanded for micro molding.

Our advanced LSR injection LSR, LSR 2-shot, multi-shot and overmolding manufacturing cells utilize our fully-automated, proprietary mixing, dosing, molding, demolding and secondary operations technologies.

SIMTEC is well known in the industry for our precision and accuracy delivering consistent part quality for millions of parts, over multi-year projects. We also offer automated, in-line value-added secondary processes such as in-line slitting, laser marking, post-curing, and packaging, and on the frontend provide prototype and design support.

We have extensive experience in a wide array of industries– working with the major players in each.

Our highly skilled engineering team are experienced with intricate part geometries and mass production requirements, offering you the confidence in knowing you’ll receive the best solution to fit your specific requirements.