5 Common Mistakes to Avoid When Designing Plastic Molded Parts
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Designing plastic molded parts is a high-stakes process. Once the steel is cut, making changes to the tool is difficult, time-consuming, and costly. To ensure your product moves smoothly from the drawing board to the factory floor, you need to apply Design for Manufacturability (DFM) principles early in the process. Whether you are an experienced engineer or a product designer new to the medium, avoiding these five common mistakes when designing plastic molded parts will save you time, money, and frustration.
1. Non-Uniform Wall Thickness
Inconsistent wall thickness is one of the most common culprits of “bad design” for plastic parts. With machining or 3D printing, there is a lot of freedom when deciding how thick a section should be or how thin a section will be. With injection molding, however, the physics of the situations are a lot more severe.
When plastic is injected into a mold, the plastic cools down immediately. Thicker sections will take longer to cool. When a section of the plastic is a varying thickness, it will cool unevenly. This uneven cooling creates internal stresses, pressure will build up, and as the material cools and expands, inwardly it will begin to sink. This will cause the part to warp, twist, and sink mark.
How to avoid this:
Consistent wall thickness is the ideal situation of any design. When considering increased strength or stiffness, don’t just go to thickening the wall, as this is not ideal. Make ribs or gussets. These will help keep the part lightweight while improving internal structure. If uneven thickness is unavoidable, a strong, gentle transition is best. Consider a chamfer or ramp to smooth the transition so the material can flow more evenly.
2. Ignoring Draft Angles
Think of the tray with straight walls and an ice cube in the tray. It is sealed it with a vacuum and getting it out will require a tremendous amount of force. This is also true when it comes to the frost.
As the plastic in a blister cavity cools off, it will shrink onto the core of the mold. If the walls of the frost are a perfect 90 degrees, the vacuum between the plastic and the mold will be severe in the pull and will cause scuffs in the top surface, which is cosmetic. This will drag across the surface, and, in the worst case scenario, the ejector pins of the die will tear off the plastic in a effort to hypertrophy the piece of plastic out of the mold.
How to avoid this:
Make sure you put draft angles on the artwork. A draft is an essential taper that is a part of any mold to remove the part out of the mold. If you have a horizontal draft, it will be less of a headache in the end.
How much of a draft: For standard texture of the mold, 1.5 to 2 degrees of draft is usually sufficient.
First, let’s not ignore the design of the part. Does the part possess a leather-like texture or a stipple finish? Are the patterns on the part’s surface especially deep or pronounced? If the answer to these questions is yes, then the part will require increased draft angles. Expect to add a 3 to 5-degree increase to the draft angles to prevent texture patterns from getting damaged or scraped off when ejecting the part from the mold.
3. Creating Unnecessary Undercuts
An undercut is a feature of the part that prevents the part from being removed from the mold by a straight pull. These include side holes, snap-fit latches, and threads.
While undercuts are a requirement for the part being molded for its purpose, the undercuts complicate the molding process. To pull a part free with undercuts, the mold has to include expensive and time-consuming designs, such as side-action slides, lifters, or collapsing cores. These mechanisms add to the mold cost, increase the maintenance time, and can extend the cycle time that is needed to produce each part.
How to avoid this:
Look at each undercut feature of your design and determine whether it is needed or can the design be altered in such a way that it can be removed as a solid?
Use of slots: You can sometimes create a “shut-off” whereby the core and cavity are brought together at a central plane by a hole in the part, removing the requirement for a slide.
Redesign snap-fit: Sometimes adjusting the geometry of a snap-fit can place it in the line of draft.
If an undercut must be included, design it such that it can be easily accessed by a side-action. But be warned: the simpler the mold, the better it works and the more cost-effective it is.
4. Sharp Corners and Stress Concentrations
When it comes to designs created digitally, making a perfect 90-degree box is simple and easy. However, when it comes to plastic molded components, sharp corners pose a great risk.
When a mold is filled with molten plastic, it is in a liquid state. Liquids do not like to move in a straight line, and when they do, turbulence is created. The plastic is subjected to a 90-degree corner during a molding cycle, creating a situation with excess pressure and molded-in stresses. When that section of the mold is cooled and subjected to a load, that corner is already a “stress concentrator” where most of the energy is collected. The risk of having that area crack is tremendously increased. Hence, a failure of that part is likely.
How to avoid this:
Smooth all corners and add a radius to internal and external corners.
-Internal Radii: Must be at least 50% of the wall thickness.
-External Radii: Must be the internal radius added to the thickness of the wall.
Doing this ensures that there will be a consistent wall thickness that surrounds the corner and it allows for better fluid flow of the plastic during molding, and, in effect, a stronger, better part is created.
5. Poor Gate Placement
Gates provide entry into the cavity of the mold, where the molten plastic is poured. Once the mold is full, the plastic solidifies, and gate marks appear on the perimeter of the finished part, requiring extra finishing steps to remove them. But, the issues tied to gate placement is more than just an aesthetic outcome; there are implications tied to the overall design of how the cavity is filled.
Gates, when placed incorrectly in filling pattern, can lead to knit lines. These are pockets of plastic solidified mid-injection when flow fronts stagnate. Knit lines can appear in critical areas, which is detrimental to the overall integrity of the part. Additional issues, including jetting, result in poor gate placement. The plastic can enter rapidly enough to cause the filling front to become turbulent, and fill the cavity like a snake, back and forth, rather than a smooth jet.
How to avoid this:
Thinking of gate placement early in the design, rather than an afterthought.
- Ideally, the plastic should flow from the thickest section of the part to the thinnest, as this lends to easier packing of the cavity, and a reduction of sink marks.
- The gate should be on a cosmetic surface to improve the overall visual integrity of the part.
- The filling of the mold should be analyzed to ensure that air traps are removed, knit lines are avoided in critical areas, and plastic is freed from the cavity as quickly as possible.
How Design for Manufacturability Provides a Competitive Advantage
The difference between an idea and a successful product is the element of Design for Manufacturability (DFM). If one understands and anticipates the fundamentals of the injection molding process—shrinkage, draft, and flow, one will be able to avoid costly iterations and production headaches.
Designing plastic parts is an iterative process. You should not wait until your design is “finished” to consult with your manufacturer. Engaging with your manufacturer’s team of mold engineers early in the process allows one to avoiding the following five common mistakes while they are still lines on a screen instead of becoming irreversible problems in a costly steel tool.
Frequently Asked Questions
Q: Why is wall thickness so critical in plastic molded parts?
A: Uniform wall thickness ensures even cooling. Uneven cooling leads to warping and internal stress. Additionally, thicker walls increase the cycle time (the time it takes to make one part), which directly increases the unit cost of production.
Q: Can I design a part with zero draft?
A: Generally, no. While there are rare exceptions for flexible materials or very shallow features, attempting to mold a rigid plastic part with zero draft usually results in parts sticking to the mold, surface damage, and ejection issues.
Q: What is the difference between a boss and a rib?
A: A boss is a cylindrical feature used for mounting screws or accepting inserts. A rib is a thin, wall-like feature used to add stiffness and strength to a part without increasing overall wall thickness. Both must be designed carefully to avoid sink marks on the opposite side of the part.
