No doubt, just about everything we come in contact with today has some form of plastic as part of its design. In fact, more of today’s “next-generation” products use plastic to lower cost, decrease size, and reduce weight. This has affected the manufacturing world, specifically in terms of injection molding.
Molders must now produce smaller and smaller micro parts, press manufacturers must build machines to produce these parts, and pressured resin suppliers must support a market that typically does not buy lots of material at a time. Every process from prototyping to assembly has been adjusted. But what does this all mean?
What is micromolding?
It makes sense to start with a definition of micromolding. Obviously, one element of micromolding involves size. Today’s technology can mold parts about as big as a grain of sand, but not all customers need parts that small. Overall, micromolded parts range from about 3.5-in. diameter, or about a 1-oz shot size, on down. This caps the “large” side of micromolding.
Micromolders make such relatively large parts when they have tight tolerances or include micro-features that “general molders” cannot handle. For example, a large part might contain extremely thin wall sections or micro-fluidic channels only a few microns wide.
Experienced micromolders can handle lead-frame molding, over molding, insert molding, and two-shot molding. These processes are important in meeting today’s industry needs and they take any good molder beyond the capability of just making small parts.
The smaller end of micromolding seems to garner the most interest and often we are asked, “Can you provide design guidelines for micro parts?”
Invariably the answer is, “Yes, but…,” because the question is difficult to answer carte blanche. Every detail of any given project can greatly influence the answer. That said, these guidelines can help point you in the right direction.
Molding press or Steinway?
On whole, press manufacturers have responded well to customer demand. Just look at the skyrocketing number of small injection-molding presses on the market. This is good news in some respects. The micromolding industry has become more mature as an independent sector. Therefore, it is easier to find suppliers, resources, and equipment. But just because a company purchases a special press doesn’t guarantee it can make micro parts.
In many ways, buying a micromolding machine is like buying a Steinway piano. It makes for a great piece of hardware but without a virtuoso on staff, it just looks good in the lobby.
That’s because mold design rules when it comes to tiny parts. The process needs the finesse of a great artist who can manage delicate steel and anticipate the subtleties of material flow in situations well beyond that of the data sheet. Virtuoso tool designers combined with a fine piece of equipment can make great music, uh, parts.
Does micromolding = general injection molding?
Although special items come from micromolding, it’s still an injection-molding process. Most of the same principles that apply to larger parts also apply to micromolded parts. For example, you must design parts so the mold can open and the parts eject. Also, parts must have gates so material can enter the cavity.
Consider a 2-mm square part with a few features, tight call-out on surface finish, and a couple of microns tolerance. The design leaves no room for ejection, gate vestige, or parting line exposure. Yet the designer or molder must somehow address these elements. For example, gate vestige to the naked eye might be invisible, but high-powered magnification can reveal 25 microns of potentially unwanted material.
Design considerations like this are important, especially when dealing with such a small surface area. Many micromolded parts are smaller than most gate size recommendations, adding to the design challenge. And where the gate is located can also affect the fill of other features or the weld-line location.
Also, in general, thin wall sections can be as small as 0.004 in.; the feature aspect ratio can be around 6:1 (but aspect is more material-dependant the thinner the feature); gate size can go down to 0.004 in., and ejector pins can be as small as 0.010 in. While not hard-and-fast rules, these are a starting point when designing a part for micromolding.
Material choice makes a big difference
The right material can make or break the design of a micromolded part especially when dealing with fine features, thin wall sections, or long aspect ratios. Liquid crystal polymer (LCP) produces difficult geometries and holds tight tolerances. Materials such as PEEK or Ultem also make good parts but don’t necessarily fill as well as LCP. Material selection for micromolding is about more than just the resin specs.
Accumold recently performed tests to measure effects material selection has on part design. A test mold was built that had a 0.003-in.-thin rectangular section to see how completely different plastics would flow into it. Each material was processed according to spec but the main goal was to see which materials would completely fill the thin-walled area. There were no attempts to over-process the material or force it along.
It’s important to note the test studies just one design. The mold had a thick-to-thin transition to create an optimal geometry for parts to fill. However, this is not always possible depending on process variables. For example, most available thermoplastics have different grades or versions that are produced to get different results. Common additives such as glass, carbon, or other fibers can affect the resin. All of these situations can affect how the material melts, flows, and fills.
Accumold had hoped to compare results with a simulated flow analysis. Unfortunately, no one could provide one. They either said the analysis would fail to estimate results correctly, or the part was simply not moldable. Our anecdotal experience confirmed their reaction, but we wanted to at least get some hard data. However, many parts we’ve made for decades are not moldable according to common flow-analysis methods. Resin suppliers also lacked confidence that the part was moldable.
The test involved commonly processed engineering plastics such as polyethylene (PE), polypropylene (PP), polyamide (nylon), polycarbonate (PC), polysulfone (PSU), polyoxymethylene (POM), polybutylene terephthalate (PBT), polymethyl methacrylate (PMMA), polyether ether ketone (PEEK), polyetherimide (PEI), and liquid crystal polymer (LCP). Tests produced a wide variety of results, but PE, PP, POM, and LCP did fill the 0.003-in.-thin area.
Prototyping is difficult
When dealing with parts that are as small as a grain of sand or with features that are measured in microns, prototyping might not be simple. Processes like SLA, PolyJet, or 3D printing will probably not reproduce all the fine features. Sometimes micromachining prototypes works.
A second test was run to find the best prototyping method for micromolded parts. The goal was to process the part with as many different rapid prototyping methods as possible. Results with various RP technologies follow:
Stereolithography (SLA) uses liquid UV-curable photopolymer resin subjected to a UV laser beam. The UV laser traces a cross section of the part, hardening the resin and bonding it to the just-completed layer below. This process continues until the machine builds the complete 3D model. Excess resin is drained and the model placed in a UV oven for final curing. The model is finished by smoothing the “stair-step” surface. The method can produce parts quickly with no tooling required and it is inexpensive for low volumes. However, it might not produce all fine features and usually produces representational, not functional parts.
3D printing (inkjet printing) prints tiny droplets of thermoplastic and wax with a decent accuracy and a smooth surface finish. However, parts are often brittle and we could not find a vendor that felt confident its process would be successful with the test part.
Three Dimensional Printing (3DP) distributes a powder and binder to form prototypes. 3DP was developed by M.I.T. and is licensed by various companies. It can create parts from ceramic, metal, polymers, and composites. Depending on what features are needed, this process may or may not work for micromolding.
PolyJet produces thin layers with a jetting head, and cures parts while layering. However, the process might not produce all micro features and it doesn’t represent an actual molded part.
Fusion deposition modeling (FDM). Here, coiled modeling material is extruded through a nozzle. The test part’s size precluded using this option.
Selective laser sintering (SLS) uses a high-temperature laser to melt and fuse, or sinter, powdered plastics or metal into the 3D part. When compared to other methods, SLS has a wide range of material choices including filled thermoplastics.
Cast urethane molds, sometimes called rapid tooling, are produced by forming an impression of a solid part, usually produced by one of the other rapid-prototyping methods, with a room-temperature vulcanizing (RTV) rubber mold. The mold is split and can reproduce 10 to 20 parts before it begins to break down. The cast urethane process is only as good as the prototype part made via some other process by which to cast the impression in the urethane.
Micromachining shapes stock materials like resin through processes such as CNC, micro, laser, or screw machining. Although expensive, this can work well.
Rapid injection molding uses injection-mold tooling made from aluminum. Molds can be produced fairly quickly and make a good source for RP and low-volume production. The process can be a good alternative to standard hard tooling but not all rapid injection molders are set up to handle small parts.
Standard Micro-Mold (Accumold) hard tooling produces the exact same part as a full-production mold. This is the best method to produce a part to spec, however, it’s not technically a rapid process.