Volume 1: A Quick-Reference User Guide for Rapid Injection Molding for Engineers and Designers
Rapid mold fabrication
Prototypes. Refine your design with real molded parts
Markets that use Rapid Injection Molding
The most important design requirement for getting good molded parts: maintain constant wall thickness.
Core out parts to eliminate thick walls.
Recommended absolute wall thickness by resin.
Eliminate sharp transitions which cause molded-in stress.
Sharp corners weaken parts.
To prevent sink, ribs should be no more than 60% of the wall’s thickness.
Draft (slope the vertical walls) as much as possible—this makes it easier to eject parts without drag marks or ejector punch marks. You get better parts, faster.
When you draft, use core-cavity instead of ribs if you can. It allows you to have constant wall thickness rather than walls with a thick base. We can mill molds with better surface finish and deliver better parts faster.
Draft the part as much as possible. This allows us to make deeper features for you. Draft allows us to reduce tool chatter and cosmetic defects when milling deep walls. If you can fit it in, use 1 degree of draft or more. On core-cavity designs, try to use 2 degrees or more. A rough rule of thumb is 1 degree of draft for each of the first two inches of depth. From 2 to 4 inches of depth, either 3 degrees of draft or a minimum of 1/8" thickness may be required.
Proto Labs can add bead-blast texture to the mold for your parts. Light texture requires 3 degrees of draft minimum on vertical walls. Medium texture requires 5 degrees.
Sliding shutoffs are your friend—these features can be made in a straight-pull mold. They do require 3 degrees of draft, but save significant money over side actions.
Side-actions can form undercuts on the outside of your part. Undercuts must be on or connected to the parting line. They must be in the plane of the parting line. Undercuts must be connected and perpendicular to the direction the mold is opening.
A “bumpoff” is a small undercut in a part design that can be safely removed from a straight-pull mold without the use of side actions. Bumpoffs can be used to solve some simple slight undercuts, but are sensitive to geometry, material type, and orientation.
A pickout is separate piece of metal that is inserted into the mold to create an undercut. It is ejected with the part, then removed by the operator and re-inserted in the mold.
* Tip: Using a pickout overcomes many shape and positioning restrictions, but is more costly than sliding shutoffs, or using a side action.
These holes can be made with steel core pins in the mold. A steel pin is strong enough to handle the stress of ejection and its surface is smooth enough to release cleanly from the part without draft. There shouldn’t be any cosmetic effect on the resulting part, if there is, it will be inside the hole where it won’t be seen.
Choose a sans serif font where the smallest feature is at least .020" thick. Serif fonts have small tails which are often too small. Text that is raised above the part is better. We cannot polish around it if the text is cut into your part.
Thin edges restrict flow and can break during gate trimming. We need somewhere thick to gate into your part. There may be alternatives, please contact one of our customer service engineers at 877.479.3680 or firstname.lastname@example.org.
Identical parts that flip over and mate to themselves are possible and save the cost of a second mold.
It’s a Material World
When choosing a material for your part, relevant properties might include mechanical, physical, chemical resistance, heat, electrical, flammability or UV resistance. Resin manufacturers, compounders and independent resin search engines have data online. For resin links, visit protolabs.com/resources#materials.
Stock colors from the resin vendor are typically black and natural. Natural might be white, beige, amber or another color. Semi-custom colors are created when colorant pellets are added to natural resins. For available colors, visit protolabs.com/resources/molding-materials. There is no added charge for our inventory colors. They may not be an exact match and may create streaks or swirls in parts. Custom colors that need to match an exact Pantone or color chip need to be compounded with a resin supplier. This process is slower and more expensive, but produces a more accurate match.
(Short) glass fibers are used to strengthen a composite and reduce creep, especially at higher temperatures. They make the resin stronger, stiffer, and more brittle. They can cause warp due to the difference in cooling shrink between the resin and the fibers.
Carbon fiber is used to strengthen and/or stiffen a composite and also to aid in static dissipation. It has the same limitations as glass fibers. Carbon fiber can make plastic very stiff.
Minerals such as talc and clay are often used as fillers to reduce the cost or increase the hardness of finished parts. Since they do not shrink as much as resins do when cooled, they can reduce warping.
PTFE (Teflon) and molybdenum disulfide are used to make parts self-lubricating in bearing applications.
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 can be particularly challenging to mold parts with thin walls and/or long resin flows.
Aramid (Kevlar) fibers are like glass fiber only not as strong, but less abrasive.
Glass beads and mica flakes are used to stiffen a composite and reduce warping and shrinkage. With high loading they can be challenging to inject.
Stainless steel fibers are used to control EMI (electromagnetic interference) and RFI (radio frequency interference) typically in housings for electronic components. They are more conductive than Carbon fiber.
UV inhibitor for outdoor applications.
Static treatments make resins dissipate static.
Questions? Call your Account Manager or a Customer Service Engineer at 877.479.3680.
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