How a design review can help you avoid part quality problems

Conducting a design review early in your part design process is a critical step to avoid manufacturing and quality problems later. 

What is a design review? A comprehensive analysis of your part and tooling design to optimize them for manufacturability. It also involves deciding on the most cost-effective material and process to produce your parts.

Why do you need to do a design review? Because your part and tool designs may contain features that could significantly add to the cost of your parts, or make it challenging to produce them at a consistently high level of quality. 

A savvy molder can also help you to reduce waste and remove unnecessary costs from your part and tool designs, enabling them to deliver higher-quality parts at a more affordable price.

Here are some tips to help you work with your molder to identify and eliminate potential part quality problems.

Part design 

The design of your part is the first place to look for potential problems and inefficiencies. Most of these have to do with the nature and behavior of thermosets and other plastic resins during the molding process. They behave differently than you may expect:

Avoid sharp corners inside the mold cavity: Sharp corners are where two surfaces of the part meet at or close to a 90-degree angle. As the compound flows into the mold to fill it, sharp inside and outside corners may not fill completely, causing cosmetic defects and may sometimes affect the structural integrity of the part. Rounded corners fill more consistently and add strength to the part.

Include adequate draft in your part and tool designs: If the walls of your part and tooling are straight up and down, it will be difficult if not impossible to eject a freshly-molded part from the tool. Inadequate draft – the amount of taper applied to the faces of a part – can cause scuffing, warping or damage to your parts. In a worst-case scenario, it can even cause damage to the tool. For best results, your part and tool walls need to be designed with at least 1.5 degrees of draft.

Wall thickness: For machined parts, the thickness of walls isn’t much of a factor. But it’s a big deal for molded plastic parts. Thick walls take longer to cure. That means longer process and cycle times and additional time on the press to complete your job. Plus, thick walls use more compound. In other words, they increase the cost of your part and may cause moldability issues.

How can you solve this problem? Thick walls can be hollowed out and supported by internal ribs. They provide the same amount of structural strength but use less material and cure faster.

Holes in the line of draw: Plastic molds are composed of two halves. When joined together, they create a cavity in the shape of your part that gets filled with compound under pressure. The line of draw refers to the direction in which the moving half of the tool travels as the mold cavity opens. Holes in the line of draw are challenging to create during molding. They require more complex tooling that can add significant cost to your parts. 

If you’re producing a large number of parts, molding the hole is probably the only cost-effective way to create it. But if you’re working with a lower-volume part, the investment in complex tooling may be cost-prohibitive. In cases like these, it may be less expensive to have your molder machine the holes after the parts have been molded.

Over-tolerancing of part dimensions: Engineers who are accustomed to designing parts for machining routinely specify tight tolerances for some of their dimensions. Although a CNC machine can accommodate tolerances down to several thousandths of an inch, plastic molding can’t produce parts at that level of precision. Why? Because molded parts can shrink up to several percent as they cure and cool.

Therefore, your part designs need to be modified to incorporate realistic tolerances for the molding process and the material you’re using for your parts. Your molder can help you work through this process.

Cosmetic issues: A variety of tooling design features can cause cosmetic problems. For example, parting lines may need to be hidden so they’re not visible. Ejection pins can leave slight marks on the surface of parts. Your molder can help you tweak your part and tool designs to minimize or eliminate these issues.

Material selection

Material selection can also have a significant impact on the quality and cost of your parts. Thermoset materials are generally stronger than thermoplastic materials and are well-suited to high-temperature and high-strength applications. 

What type of thermoset is best for your part? That depends on its end-use application. Key questions that can help your molder with material selection include these:

• What forces will the part be exposed to?
• What environment will the part “live” in?
• What is the life expectancy of the part?

The goal is to select the right material that meets your application requirements and is affordable. Most molders have a lot of material expertise and may provide you with options you hadn’t considered during your design process.

Tooling

Last but not least is the tooling that will be used to mold your parts. Here are several key factors to keep in mind:

Fix tool issues during the design phase: Trying to chase down problems and fix them after the steel tooling has been built can be mighty expensive – and time-consuming. Precise fixes to steel tooling require specialized expertise. Getting access to the artisans who can get the job done right can easily delay the production of your part by weeks or months.

Don’t cut corners on tool quality: Thermoset materials can be very abrasive, which means they can shorten the lifetime of inferior-quality grades of tool steel. Use the highest-quality steel you can afford. You’ll thank yourself later!

How long is your production run? If it’s a longer run, your molder may recommend a multi-cavity tool, which can produce more than one part per cycle. A multi-cavity tool can help you meet your production targets in less time, helping to reduce your per-part cost.

Simplify tooling where possible: Simplifying the design of your tool can reduce both its cost and the time required to build it. Once your part goes into production, simple tools tend to run more smoothly, with fewer issues and less maintenance costs. 

Trust an experienced team

Producing highly-engineered parts and tooling is a field where the depth of the molder’s experience can be a major asset. A seasoned team has solved many problems in its lifetime. Chances are, they’ve produced parts similar to yours; they know what potential problems and issues to look out for. 

Ci-Dell’s seasoned sales, engineering and production teams have a wealth of experience and knowledge in part and mold design, material selection and molding processes. We have long-tenure employees in key positions. They can help you achieve a successful outcome for your next project.

Contact CI-Dell today to discuss your specialty molding needs.