INJECTION
MOlDINg
Injection molding is a manufacturing process for producing parts by injecting molten material into a mold. Injection molding can be performed with a host of materials mainly including metals, (for which the process is called die-casting), glasses, elastomers, confections, and most
commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed (Using a helical shaped screw), and injected (Forced) into a mold cavity, where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, molds are made by a mold-maker (or toolmaker) from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection molding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars. Advances in 3D printing technology, using photopolymers which do not melt during the injection molding of some lower temperature thermoplastics, can be used for some simple injection molds.
Parts to be injection molded must be very carefully designed to facilitate the molding process; the material used for the part, the desired shape and features of the part, the material of the mold, and the properties of the molding machine must all be taken into account. The versatility of injection moulding is facilitated by this breadth of design considerations and possibilities.
WALL THICKNESS
What is the Minimum Wall Thickness Required for my Injection Molded Parts?
High-quality injection molded plastic casing pulls your whole product together into a neat package, and any walls within your product must be stable in order to maintain its overall structure and quality.
It could be something like a fingernail slot that can easily be pried open to change a battery, for example, or something similar to the back of a cell phone with thin outer casing that looks sleek and modern, but also holds a battery in place.
There are many factors that can help you decide on the appropriate wall thickness for your product.
So, what is the minimum wall thickness required on a plastic part? The question about how thin or thick a product’s walls or casing should be is one that only leads to more questions. Here are a few to keep in mind when first exploring the idea of wall thickness:
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Is the wall used for structure?
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Will the part be fragile in that section if it is made thin?
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If a specific plastic material is needed, how will the thinness be affected by the material choice? (The chart below discusses this in more detail.)
Your answers to these questions, along with a few other important variables, will help you determine whether a thin wall or part will compromise the integrity of your product or if it will help it function better.
Important Variables to Consider
Material composition, flow rate and part yield — are key to the plastic injection molding process, so it’s helpful to know a bit more about them when deciding on wall thickness.
Material composition alone can be the distinguishing factor in wall thickness because of a material’s stiffness and strength characteristics. Stiffness refers to when and how far a certain part will bend when force is applied, while strength refers to the part’s ability to resist fracturing or breaking when force is applied. Each material has its own stiffness and strength and both are affected by part thickness. So, as a part’s wall gets thinner, it bends more when it shouldn’t and is more susceptible to breaking. This may sound straightforward, but in order for stiffness and strength to work in your favor, thickness must be chosen according to material composition. We elaborate on this point in the chart at the bottom of this article.
Flow Rate
The rate at which a material moves or flows into a mold — is partially dependent on material composition and will make a difference in the manufacturing process.
Because pressure is required for the injection molding process, thin wall areas in a mold can cause material flow rate issues no matter what material is chosen. In turn, flow rate issues can lead to costly flaws, such as voids and sink marks.
Proper wall thickness will help you avoid extra costs that can result from the need for more pressure. Think about trying to drink a smoothie through a straw. The thinner the straw, the more pressure you’ll need to apply to suck the smoothie through the straw.
So, if you have a thin area in the mold that the plastic has to move through, you might need a larger machine that can push more pressure. That’s where it can become more expensive. If you need more than 200 tons of pressure for a machine with a range of 100–200 tons, then you’ll need to use a bigger machine, and bigger machines have higher operating costs.
Also, if you need higher pressure, the molds will need to be made of higher-quality steel and the mold bases will be larger. The increases in size will increase the price for the mold and price per part.
Voids can also develop on the inside of a part, making it look complete, but creating bubbles or hollow sections that affect the structure. Voids may not be visible from the outside. They occur because of something called short shot, which happens when a material cools before it can completely fill the mold. It’s similar to pouring too little batter into a waffle machine then closing it.
Part yield is the ratio of good to bad parts that come out of the mold. Thin walls will contribute to low part yield, making it necessary for part yield to be considered prior to budgeting and manufacturing.
Let’s say you made four parts: two came out fine, one has a void where the material didn’t fill and one broke as it was removed. That would mean the process had a 50 percent part yield.
As the manufacturer might have to discard every other piece in order to achieve the minimum wall thickness, the costs of trial and error will fall on the person or company paying to have the part made. By keeping the wall thickness to a proper range for the chosen material, however, you can avoid unnecessary costs and delays.
Some manufacturers set a consistent wall thickness regardless of the material being used, but most will adjust accord to the material.
The ultimate takeaway is this: working with your manufacturer to select the proper material for your part and the recommended thickness for that material — is the best way to ensure that the part will be produced in a cost-effective manner that enhances the quality of your product. Below are the guidelines manufacturers typically follow:
