plastic part design
A 10 Point Guide to Designing a Plastic Injected Molded Part
Whenever you begin a design project involving plastic parts, where your end goal is to have the component injection molded, you'll want to consider some things on the front end of your design process that will allow for time and money to be saved during the production phase of your project. This foresight at the beginning stages of product design will be useful in understanding the limitations and challenges faced by the manufacturers of your product.
Why do you need to know about the molding process? You need to understand that a well designed part can help with the ease of molding, the ease of tool building, cost factors associated with cycle time optimization, performance of your part, and ease of assembly.
These 10 considerations are by no means exhaustive when considering the best plastic injected part design, but they will for sure get you very close to where you need to be and at the very least, point you in the right direction. So let's check out these helpful tips:
1. Consider the Parting Line
Try and imagine your part being molded. It can be difficult for some to think in the "negative", and by that I mean visualizing the empty cavity space that the liquid plastic will fill in order to mold into a plastic part. Inevitably, once that cavity space is filled, your part will have to be ejected from that space. In order for that to happen, the two mold halves will have to separate, leaving behind your plastic injected part in one side of the mold (preferably the side where you plan on ejecting from). You need to visualize and plan for where this "parting line" will be so as to ensure that your part does not get trapped in the mold.
2. Avoid Undercuts (If Possible)
Undercuts on your part wont necessarily make it more difficult to mold your part, but rather more difficult to demold. The undercut portion of the plastic part will get trapped inside your mold once the part is cooled and hardened, and in turn making it impossible to eject from the mold without other mold actions. Many times, undercuts are a necessity for part function. Side actions and lifting mechanisms will have to be introduced to your tool in order to deal with the ejection of your part. Sometimes this is not avoidable, but if you are looking to save on tooling cost, it might be worth trying to redesign in order to eliminate unnecessary tooling costs.
3. Uniform Wall Thickness
You want to make sure that you try and make the wall thickness of your part as consistent as possible. It's alright if you don't have uniform wall thickness to some extent, but uneven wall thickness greatly increases the likelihood of sink marks, warpage, voids, molded-in stress, longer cooling times, and even material flow restrictions. If wall thickness must be uneven, it is best to have smooth transitions that taper over some distance. The size of the part and the ability of the material to fill will determine the minimum wall thickness allowed for your plastic injected part.
4. Shrinkage and Warpage Considerations
All materials shrink at different rates and at varying degrees within the cooling process of your injection molding operation. Shrinkage and warpage are two different phenomena that can occur. Shrinkage occurs where there is a difference between corresponding linear dimension of the mold and the molded part. Warpage is a dimensional distortion in a molded plastic caused by excessive residual stress in the part. There are various things to consider when trying to control either one of these occurrences. Material considerations, part geometry considerations, tooling considerations, and processing considerations all play a part.
5. Watch The Sharp Corners
Sharp corners are to be avoided at all costs. Sharp edges, such as corners of a square hole, will produce a part with high levels of molded-in stresses. Much of the time these result in weak points that lead to part failure and cracking. Adding radii to sharp corners will reduce the amount of molded-in stress. Radii redistributes the stress more evenly and facilitates the flow of the material and ejection from the mold. Stresses rapidly build whenever the inside corner is less than 25% of the nominal wall thickness of the part.
6. Structural Support
As a product designer you want to minimize the amount of material required to fill your part, while at the same time increasing its structural integrity. Thin walls need some sort of support so that the walls don't warp or collapse. Ribs are commonly employed on injection molded parts in order to stiffen relatively thin parts. Ribs, bosses and other projections on the piece part wall will greatly strengthen your part, but if done incorrectly can contribute to other molding issues such as sink marks and non-fills.
7. Add Draft
Draft is the angling of otherwise vertical walls in order to prevent constant contact with the molding cavity during the mold opening. All vertical walls oriented parallel to the direction of mold pull will need to have draft ideally at 1 degree minimum. There are special consideration for ribs and bosses, and textured surfaces will need to add 1 degree per side for each .001" depth of texture. Draft is an essential element to the part design process.
8. Secondary Operation or Molded In?
Sometimes you have inserts that need to be fixed inside your part. You need to consider whether they get molded in or they are pressed or welded in after the molding has commenced. With this, both options are viable and come down to the economics of the operation. Do you go for a higher priced tool that can accommodate inserts to be molded over? or do you press them into the part after the fact? If you have a low production run, it might be worth considering a post molding operation. For long production runs, it might be more beneficial to have the inserts molded in. It all depends on the customer's preference, the viability, and the project's budget.
9. Gating and Ejection
Gate location is the area where the material will be entering into and filling the cavity of the part. It is important to keep in mind where you intend to gate your part and possibly make provision. Some questions to consider are: Am I allowed to have a gate mark where I am envisioning my gate? and Is this gate at a location where the material will flow from a thick walled to a thin walled region of the part? You must also account for ejector pin marks that will most likely show up on the underside of your molded part. If ejector marks are not allowed then that must be called out on your print so other ejection alternatives can be considered.
10. Material Selection
The material selection process can be as simple as an internet search for the material of an existing part already on the market, or as complex as identifying every single requirement and material property from the ground up. The first step is to define the requirements needed for your particular application. From there it's important to narrow the choices by process of elimination. Do you need it to be rigid, flexible, elastomeric, etc? Is there a specific application? Medical? What specific property requirements are there? Sometimes the best thing to do is to not reinvent the wheel, but rather do a search history for similar commercial applications, then call up the material supplier for recommendations.
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