When it comes to mold design, every mold designer worth their salt will tell you that taking the necessary time to familiarize yourself with some injection mold design basics — as well as the overall plastic injection molding process — will dramatically increase your chances of running a successfully finished product in the future.

Looking to consistently produce high-quality plastic products and improve yields in the long-run?

In this article we share 5 mold design aspects you need to be considering to kick your next project off in the right direction.


1. The Injection Molding Process


plastic injection molding process

The mold consists of two halves that are hollowed out as a negative image of the part you want to manufacture. Hot, melted plastic or rubber passes through the gate and gets injected into the mold (part) after which it is allowed to cool down. Once cooled, the two halves of the mold release and the part is revealed and removed.


Highlighting The Process

There is a small opening in the mold itself that allows the melted plastic to be injected, known as the gate. Once the part has cooled down, it needs to be removed either automatically or manually from the completed part.

The position of the gate is important in injection mold design — it’s well known in the industry that you would ideally locate the gate at a thicker, intersectional area of the part where it can be removed hassle free without interfering with the structural integrity of your new part.

Removing the gate will likely leave a scar on your part, which should be taken into consideration as it will affect the appearance of the part.


During The Cooling Process

When the liquefied plastic or rubber material cools and solidifies, shrinkage will occur. Remember, this has to be taken into consideration not only when determining part dimensions, but also when adding injection mold design elements such as radiuses to corners and making decisions on wall thickness.

During The Part Release

As the two halves of the part separate to release the molded part you will notice a line that runs through the part. This line is known as the “parting line” and is a normal occurrence as the mold is made out of two halves. It is advised that you design your part to plan for the parting line location.


2. Considering Wall Thickness

Some plastic injection molding shops will tell you that they can only produce injection mold parts with a uniform wall thickness. Although this will make it more convenient to manufacture your parts, it is not essential to the process.

However, different wall thicknesses can make for a more complicated process; this is a result of the cooling process mentioned above. Areas where the wall is thicker will cool down and solidify more slowly than thinner areas.

Now, consider the shrinkage during the cooling down process. This is a direct result of poorly designed molds and products that can lead to hot, liquid resin or moisture shifting to areas of the part where it is not meant to be.

Don’t be alarmed!

You can manage this potential problem by designing your part for manufacturability in mind, otherwise known as DFM.

It’s a matter of thinking practically. Thicker areas can, for example, be located at lower portions of a mold, allowing gravity to help keep still-cooling material where it belongs.


3. Incorporate Draft Angles in Your Mold Design

Whilst many mold designers, engineers and toolmakers might be wary of draft angles as they tend to complicate design and tool manufacture, without them your part becomes a whole lot more expensive and more difficult to produce.

A draft angle is a taper that presents a solution to allow finished parts to leave the walls of the mold much more smoothly with very little friction that can be added to your part depending on the material (see number 5 below) and the plastic injection mold design.

Incorporating draft angles in your design ensures:

  • That the surface of the parts produced remains unscathed
  • The part doesn’t hang-up on internal tool projections
  • Surface damage caused by sliding contact is reduced
  • Molding of textured surfaces (rougher textures need increased draft angles)
  • Warping of the molded part is minimized (by controlling how shrinkage takes place)
  • The entire process becomes that much more efficient


4. Etch or Mill in Texture

Mold design and texturing

Instead of adding a second finishing process after injection molding to create texture on your product, you can incorporate the desired finish, pattern or texture right into the mold.

Surface texturing and in mold design features presents potential enhancements in design, branding, aesthetics, and functionality.

Etching or milling your mold to create a finish gives you a greater degree of control, and also uniformity over the look and feel of your part.

This not only helps you improve production efficiency and save on time, but also helps to reduce injection molding costs by incorporating two processes into one.


5. Get to Know Your Materials

Material selection is one of the most critical considerations in designing your part and impacts every single one of the other aspects mentioned above such as the shrinkage factor, cooling time, flexibility, degrees of draft angles and more.

Here are the top six plastics materials used throughout the industry that you will most likely consider:

  1. Polypropylene (PP)
  2. Acrylonitrile Butadiene Styrene (ABS)
  3. Polyamide (Nylon)
  4. High Density Polyethylene (HDPE)
  5. Polycarbonate (PC)
  6. ABS + PC Blend (mostly used for electronics enclosures)

All these materials differ in wall thickness, hardness, weight and cost so it’s important to understand how using different materials will impact your mold design, and ultimately, your bottom line.

It’s good to keep in mind that if you believe a material is falling short of your expectations for your final product, there is always the possibility of fine tuning through the use of additives and fillers to get what you want.


Final Thoughts

This article merely touches on what you need to know before commencing with your mold drawing and starting your plastic injection molding project, and it can get more complicated as you go along.

The best tip we can give you is to use common sense and to speak to experts in the industry for advice on how to optimise for every part of the plastic injection molding process, from planning and designing, right through to texturing and finishing.

There are literally thousands of plastic injection molding companies in China, and working with a company that speaks your language and understands your needs is paramount.

If you have any questions you’d like to ask our qualified consultants please feel free to leave a comment below and we’ll get back to you.

Contact Provident today to learn more about you can leverage our expert in-house mold design and engineering resources!


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