Views: 3
The Basics of Rapid Injection Molding
Designing plastic parts that can be molded has always been important for traditional injection molding processes, but it’s particularly beneficial for parts about to be rapid injection molded (RIM) to ensure speed and quality remain constant during manufacturing. Here’s a look at many of the critical design considerations encountered during rapid injection molding.
With RIM, CAD models are sent directly to the production floor where mold milling begins. In most cases, the mold is made of aluminum instead of steel. This allows for faster and more cost-effective tooling compared to traditional steel molds.
RIM accommodates side-action and hand-load inserts, as well as simple overmolding and insert molding. Selective use of electrical discharge machining (EDM) can improve mold features such as corners and edges. And several surface finish options are available. All of this lets RIM make parts in a few weeks rather than the months needed for traditional injection molding methods.
From left to right, the components of a RIM press include: (1) ram, (2) screw, (3) hopper, (4) barrel, (5) heaters, (6) materials, (7) nozzle, (8) mold, and (9) part.
Here are some common applications for RIM:
Iterate quickly with rapidly built prototypes
Test functions during product development with production-grade parts
Test several different materials
Test several CAD models
Implement bridge tooling
Leverage low-volume production for on-demand parts
Manage demand volatility
Get thousands of parts within days
From wall thickness and radii to ramps and ribs, here’s a quick look at factors designers and engineers should consider if parts will be injection molded.
Wall Thickness: The most crucial design requirement for getting good molded parts is to maintain constant wall thickness. They minimize the possibility of parts warping or deforming.
Note: These are general guidelines, depending on part geometry and molded construction. Larger parts shouldn’t be designed with the minimum wall thickness. General rule for wall thickness is 0.040 to 0.140 in.
Core Geometry: Core out parts to eliminate thick walls. You get the same functions in a well molded part. Unnecessary thickness will change the size of parts, reduce the strength, and require post-processing.
The part on the left is the originally designed part. The part on the right has been cored out to reduce part thickness while still being able to perform all of its necessary functions.
Ramps: Eliminate sharp transitions that cause molded-in stress.
Fillets: Design features that support themselves.
Radii: Sharp corners weaken parts and create molded-in stress from resin flow. Designers should add radii in sharp corners.
Ribs: To prevent sink, ribs should be no more than 60% of the wall’s thickness.
Bosses: do not use the screw boss to make a thicker area, because the thicker area may cause parts to sink and void.
The bosses on the left are poor as they are too thick and might not fill completely, leaving voids. The bosses on the right, however, create strength without having sections that are too thick.
Draft: Draft (slope the vertical walls) as much as possible to make it easier to eject parts without drag marks or ejector punch marks. Draft can also give designers deeper capabilities, and it can also reduce tool jitter and appearance defects when milling deep walls. If you can fit it in, use 1 deg. of draft or more. On core-cavity designs, use 2 deg. or more. A rough rule of thumb is 1 deg. of draft for each of the first 2 in. of depth. From 2 to 4 in. of depth, either 3 deg. of draft or a minimum of 1/8 in. thickness may be required.
Core-Cavity: When you draft, use core-cavity instead of ribs. It provides constant wall thicknesses rather than walls with a thick base.
It also allows machining workshops to grind dies with better surface finish and deliver better parts faster.
Undercuts: An undercut is an area of the part that shadows another area of the part, creating an interlock between the part and one or both mold halves. On the image below, the image (1) shows a clip with an undercut feature. On the image (2), an access hole beneath the undercut lets the mold protrude through the part and provides the needed latch shutoff geometry.
Side-Actions: Side-actions form undercuts on the surface of a neighborhood. The undercuts must get on or connected to the parting line. they need to even be within the plane of the parting line and connected and perpendicular to the direction the mold is opening.
The round blue part is that the side a-action.
Bumpoffs: A bumpoff may be a small undercut during a part design which will be safely faraway from a straight-pull mold without using side-actions. Bumpoffs can solve some simple slight undercuts, but are sensitive to geometry and material.
The green part is that the bumpoff.
Pickouts: A pickout may be a separate piece of metal inserted into the mold to make an undercut. it's ejected with the part, then removed by the operator and re-inserted within the mold. employing a pickout lets designers overcome shape and positioning restrictions but is more costly than sliding shutoffs or employing a side-action.
Steel Core Pins: These holes are often made with steel core pins within the mold. A steel pin is robust enough to handle the strain of ejection and is smooth enough to release cleanly from the part without draft. There shouldn’t be any cosmetic effect on the resulting part; if there’s, it'll be inside the opening where it won’t be seen.
Logos and Text: Textured surfaces, molded part numbers, and company logos look good, but be prepared to pay a touch extra for these and other non-mission critical features. That said, permanent part numbers are requirements for several aerospace and military applications. For text, it's recommended designers:
* Use a mill-friendly (san-serif fonts) font like Century Gothic Bold, Arial, or Verdana.
* Keep the font above 20 pt.
* Don’t go much deeper than 0.010 to 0.015 in.
* Be prepared to extend draft if part ejection may be a concern
Tab Gates: Thin edges restrict flow and may break during gate trimming. Tab gates give injection molders a thick area to put a gate into your part. There could also be alternatives, so please contact the molder’s applications engineers.
Self-Mating Parts: Identical parts that flip and mate to themselves are possible and save the value of a second mold. Elements to allow them to mate include pegs and holes, interlocking rims, and hooks and latches.
Tolerances: Molders can generally hold about ±0.003 in. machining accuracy. Shrink tolerance depends mainly on part design and resin choice. It varies from 0.002 in./in. for stable resins like ABS and polycarbonate to 0.025 in./in. for unstable resins like TPE. There are techniques for getting the foremost accuracy out of injection molding. Contact an application engineer at your injection molder for more information.
Material Selection
When choosing a cloth for a neighborhood, relevant properties might include mechanical, physical, chemical resistance, heat, electrical, flammability, and UV resistance. Resin manufacturers, compounders, and independent resin search engines have data online. Here may be a quick check out some common commodity and engineering resins.
Polypropylene
Soft
Tough
Cheap
Chemical resistant
Makes good living hinges
Polyethylene
Soft
Tough
Cheap
Chemical resistant
High density
Low density
Polystyrene
Hard
Clear
Cheap
Brittle but are often toughened
Engineering Resins
ABS
Inexpensive
Impact resistant
Equipment and handheld housings
Susceptible to sink
Acetal
More expensive
Strong
Good lubricity and machinability
Very sensitive to excess wall thickness
LCP
Very expensive
Very strong
Fills very thin parts
Weak knit lines
Nylon
Reasonable cost
Very strong
Susceptible to shrink and warp, particularly glass-filled
Absorbs water, which results in dimensional and property change
Polycarbonate
Moderate cost
Very tough
Good dimensional accuracy
Susceptible to chemical stress cracking, voids
Other engineering resins include PBT, PET, PPS, PSU, PES, and PEI.
Stock colors from the resin vendor are typically black and natural. Natural could be white, beige, amber, or another color. Semi-custom colors are created when colorant pellets are added to natural resins. For available colors, ask your injection molder. In some cases, there's no added charge for our inventory colors. But they'll not be a match and should create streaks or swirls in parts. Custom colors that require to match a Pantone or color chip got to be compounded with a resin supplier. This process is slower and costlier, but produces a more accurate match.
Short glass fibers are often added to a resin to strengthen a composite and reduce creep, especially at higher temperatures. they create the resin stronger, stiffer, and more brittle. they will also cause warp thanks to the difference in cooling shrink between the resin and fibers.
Long glass fibers are used like short glass fibers to strengthen and reduce creep but make the resin much stronger and stiffer. The downside is that they will be particularly challenging to mold parts that have thin walls and/or long resin flows.
Aramid (Kevlar) fibers are like less-abrasive glass fibers, only not as strong.
Carbon fiber strengthens and/or stiffens a composite and aids in static dissipation. it's an equivalent limitation as glass fibers. Carbon fiber can make plastic very stiff.
Stainless steel fibers are wont to control EMI (electromagnetic interference) and RFI (radio frequency interference) typically in housings for electronic components. they're more conductive than carbon fiber.
Minerals like talc and clay are often used as fillers to scale back costs or increase the hardness of finished parts. Because they are doing not shrink the maximum amount as resins do when cooled, they will reduce warping.
PTFE (Teflon) and molybdenum disulfide are wont to make parts self-lubricating in bearing applications.
Glass beads and mica flakes stiffen a composite and reduce warping and shrinkage. With high loading, they will be challenging to inject.
UV inhibitors prevent parts from breaking down within the sunlight for outdoor applications.
Static treatments make resins dissipate static.
