How to Leverage Low-Volume Injection Molding

What is On-Demand Manufacturing?

At Protolabs, our on-demand, low-volume production offering with injection molding—which uses aluminum tooling—is a fast, cost-effective way to produce hundreds of thousands of end-use molded parts.

Use this process as the primary production method for your products. On-demand manufacturing is also an excellent way to move from prototyping to low-volume production for molded parts. We may have “Proto” in our name, but we are actually a full-service manufacturing provider. Low-volume runs can also help you validate part design and manage inventory overhead with production of parts only when demand dictates. In addition, even if you need to eventually shift into mass production of plastic parts, you can still use our cost-effective aluminum tooling as a bridge before committing to a capital expense with steel tooling. Finally, this on-demand approach also optimizes your supply chain, making it more adept in managing demand volatility.

This design tip outlines our quoting system, which has new upgrades; reviews several design considerations that can improve injection molding part design for on-demand manufacturing; and briefly looks at how this on-demand approach can help manage your supply chain.

Our Digital Quoting Platform Provides Manufacturing Analysis

First things first. Part designs can be uploaded directly online, day or night, seven days a week at Protolabs. An interactive quote, which really serves as the keystone of our digital manufacturing platform, is typically sent within hours. Once you receive a quote, you can set primary manufacturing requirements such as desired material, surface finish, and initial production quantity. This free, automated, design for manufacturability (DFM) analysis is provided with every quote. This includes recommendations on how and where to modify part geometry to improve mechanical strength, reduce sink, adjust wall thicknesses, and other considerations that can improve cost, production, and tool life. In addition, recent upgrades to our system have integrated our on-demand manufacturing option. You now can find prototyping and production options for injection molding under one quoting roof. Also, our new Price Curve tool compares prototyping vs. on-demand manufacturing options so you have full visibility to total cost of ownership on molded parts throughout the product life cycle.

Optional Consultative Design Service Preps Parts for Production

You can also get free design help with our Consultative Design Service (CDS) if your CAD model isn’t quite suited to our system, or you need help getting it to meet our guidelines. CDS will help you update your CAD model to address the manufacturability feedback you received with your quote. One of our injection molding applications engineers will work with you on your part design to ensure your part is moldable.

Designing Molded Parts: A Quick Review

Next, let's review a few key design considerations for injection molding.

Finesse a Part's Finish
A common mistake made with injection-molded parts is calling out a better surface finish than is necessary for part function. A very smooth SPI-A2 (read: shiny) finish, for example, requires mold cavity surfaces to be hand-polished with diamond buff to 1-2 Ra. This drives up the cost of mold manufacturing and increases lead-time. The least expensive part finish is a non-cosmetic PM-F0, an as-machined surface that will probably show some tool marks. In between are two options such as a 10-12 Ra SPI-C1 finish, made using a 600 grit stone, or a PM-T1 bead-blast textured finish. Other finishes are available as well. Use the lowest-cost finish that is compatible with your application. 

Use Draft for Easier Part Ejection from Mold
Injection-molded parts need draft for the same reason that ice cubes aren’t true cubes. Ice cube trays are angled, which create ice cubes with tapered edges (or draft) so the cubes release easily from the tray. Molded parts should, too. Draft allows for easier ejection of the part.

Scale Wall Thickness Up and Down
Wall thickness is important. Imagine you're designing a simple box. At first glance, thick walls might seem like a good way to keep the box's contents safe. But the plastics used with injection molding are, well, plastic.

Walls thicker than what is recommended for the particular thermoplastic family being used will be subject to warp and sink during cooling. A general rule of thumb for wall thickness is somewhere between 0.040 in. to 0.140 in. (1 to 3.5mm), applied consistently across the entire part.

Use Radii to Solve Corners that Weaken Part Integrity
Since aluminum molds are machined using end mills, sharp internal corners are a concern. For that matter, sharp corners on any molded part tend to create stress and weaken structural integrity. As such, we recommend generous use of radii (rounded edges or vertexes) in part design. For the inside of a 4 in. by 6 in. by 2 in. box, for example, it might be best to plan on 0.25 in. (6mm) radiused corners for a strong part with improved moldability.

Consider Multi-Cavity Molds As An Option
Multi-cavity molds involve more than one cavity cut into the mold to allow for multiple, identical parts to be formed in a single shot; family molds involve more than one cavity cut into the mold to allow for multiple, different parts to be formed in one shot. Customers can use multi-cavity molds to simultaneously produce two or more part designs with minor iterations to quickly test different product designs in parallel. They are also a good option when quantities beyond the initial sample are required as they can help reduce cost per part. But bear in mind that while multi-cavity and family molds produce more parts per cycle, their use is highly dependent on geometry.

Choose a Material for Your Part
We have hundreds of commercial-grade manufacturing materials. Our thermoplastic resins are available in many different colors and additive filler options. Attributes such as durability, strength, chemical and thermal resistance, and others vary by plastic. In addition, liquid silicone rubber (LSR) injection molding, which requires less draft due to its flexible nature, is available. It uses a process similar to plastic injection molding, with comparable pricing and delivery. You may also want to check out our alternative materials guide for finding alternate materials that will work for your parts during supply shortages.

Call out Critical-to-Quality Features

When you use our on-demand manufacturing option, you’ll be able to indicate in your part design the dimensions that are most critical to the part’s function. By calling out these Critical-to-Quality (CTQ) features in your 3D CAD model, you can be assured that the parts ordered are consistent with the model. In addition, that means the next time you order those parts, we can follow those precise steps to manufacture consistent, repeatable parts that adhere to your CTQ specifications.

Inspections are key pieces of this CTQ process. Accordingly, our applications engineering team will review your model and email you an Inspection Statement of Work (ISOW), which lets you know if any features you circled have issues with tolerances and moldability. Once we send the ISOW, we manufacture and inspect your order at Protolabs speeds.

Once we have completed molding process development, which ensures we have a consistent and repeatable process to produce quality parts, we will inspect the first three shots from the tool, using an automated Coordinate-measuring Machine (CMM), and provide you with a three-part First Article Inspection (FAI) report, and a 30-part Process Capability Report.

Advantages of CTQ include:

• In-process quality feedback at the press with no impact to lead time
• Reduction in cost and time by eliminating in-house measurements
• Receipt of a dimensional report to validate samples
• Gain critical design and material performance lessons for current and future iterations
• Improved part accuracy and dimensional adherence on customer-specified critical dimension(s)

Gain Supply Chain Flexibility

These days, as we all continue to navigate a global pandemic, supply chain considerations are critical. An on-demand manufacturing approach can help you create a more nimble supply chain by:

• Procuring parts on demand without any Minimum Order Quantity (MOQ)
• Lowering inventory costs and warehousing expenses with on-demand sourcing
• Managing increases in demand volatility without going on back-order
• Mitigating the risk of domestic and global shipping delays
• Minimizing downtime and reducing the risk of stock-outs when large-scale tools are being repaired

Ultimately, whether you choose us for prototyping or on-demand manufacturing, please know that we are here to help. We always have applications engineers available to assist with any questions at [email protected] or 877-479-3680. Ready to start your next project? Just upload a CAD file to our website.