Considerations for Injection Molding

Design Components of Injection Molding

One of the earliest stages of the design process, after preliminary ideation, is designing your plastic part for injection molding. A major consideration when designing parts is the manufacturing process. 

Therefore, you’ll likely need to partner with an expert industrial designer to help you design your desired part. After the initial design is complete, you’re ready to start testing its validity through design testing. 


Testing your part for flaws during the design phase will save you a lot of time and money in the long run. Prototyping is an efficient way of testing the design and functionality of your part before proceeding with full production. It is most commonly done through CNC machining or 3D printing. Once you have a successful prototype you can begin paneling consumers. 

Consumer panels can help give you valuable feedback on where your product may be lacking. From there you can make additional design changes and continue paneling until you’re confident you have a strong, well-designed product. At this point you’re ready to hire an engineer to design the mold itself. 

At Tailor Made Products, one of our specialities is helping with the design and development of your product idea, and adapt it for injection molding. Our engineers assist with CAD development and prototype sampling. Request a quote today. 

What materials can be used for injection molding?  

There are three main categories of plastics used in injection molding: Thermoplastic, Thermoset, and Elastomer. In total, there are hundreds of plastic materials to choose from.

The best material for you depends on the desired functionality of your part. The design and complexity of your mold also plays a large role in determining which materials are best suited for your particular injection molding process. Learn more

Injection Molding Financial Considerations

Once you have a good idea of what features and functionality your part needs to include, you can begin looking for a material that meets your needs at the lowest cost possible. The cost of material is an important financial consideration, as it’ll play a major role in the cost to produce your part and, therefore, your overall margins.   

The cost of the mold is an important consideration as well. The cost depends on various factors including the size and complexity of the part, the material used, and the quantities you’ll be producing. 

Another financial consideration is the type of machine you use to mold your part. There are three variations of plastic injection molding machines: Fully Hydraulic Machines, All-Electric Machines and Hybrid Machines. 

Electric vs. Hydraulic Machines

Compared to electric, hydraulic machines are cheaper up-front and cheaper to repair. Their parts are also more resistant to wear and tear, keeping maintenance costs down. However, they consume far more energy than electric and cannot be used for molding parts that require a clean environment because of the machines’ oil leakage. 

All-electric machines are digitally controlled for a more predictable process, high position accuracy, and process replication ability. The machines also require no oil or filters, resulting in less cleanup, and higher injection and clamping speeds than hydraulic versions. Combined, these all add to decreased manufacturing costs. The electric machines, however, have a higher up-front cost than fully hydraulic machines. 

The Hybrid Machines lie between Fully Hydraulic and All-Electric Machines in cost and energy efficiency.

Interested in finding out how much it would cost to mold your product? Request a quote today. 

Injection Molding Production Considerations 

A consideration in designing your mold is the number of parts that can be molded in one shot. This is referred to as the number of cavities. You can have anywhere from a one cavity mold to a 200+ cavity mold. The optimum number of cavities for you depends on your budget and your desired output quantity. The size of your part may limit the number of cavities in your mold as well. 

Multi-cavity molds tend to be more efficient, resulting in a higher output and a lower cost per piece. However, higher cavity molds can sometimes be difficult to process and more difficult to maintain. They can also increase your chance of short shots (when the mold is not 100% filled). 

Vertical vs. Horizontal Injection Molding Machines

Horizontal injection molding machines are most common and tend to be easier to operate and maintain than their vertical counterparts. In horizontal machines, the mold itself opens horizontally, with one half on the right and one on the left. Horizontal machines have a lower center of gravity, making installation more stable. On the other hand, they take up more floor space than vertical machines.

In vertical machines, one half of the mold is up and one is down. For this reason, in vertical molds you aren’t fighting gravity to keep the part in place, as you are in horizontal machines. 

Insert Molding vs. Overmolding 

Insert molding involves inserting a separate part into the mold for the plastic to flow around. In doing this, the two parts are fused together, becoming one piece. Insert molding commonly involves inserting a metal piece into the mold. An example would be inserting a kitchen utensil’s stainless steel handle to be molded to the plastic head. 

Overmolding is another way to combine multiple parts into one single piece. In overmolding, one part is overlaid with another material. It is often used to add a soft or non-slip material to an existing product, such as a nylon utensil overlaid with a silicone handle. Overmolding is also commonly used to create multi-colored products.

Both processes can be done through hand-loading (manual) or automation.

What is the process of injection molding?

In Thermoplastic Injection Molding, the molding process begins as plastic pellets are loaded into a hopper. The hopper then directly feeds the plastic into the barrel of the injection unit. Inside the barrel, a reciprocal screw pushes the pellets forward, heating and melting them as they move through the barrel. By the time they reach the front of the barrel, the plastic pellets have turned entirely molten. 

The screw then pushes the molten plastic out of the barrel and ejects it into the empty part of the mold (also known as the cavity image). Once inside the mold, the plastic is quickly solidified with the help of a coolant (typically water) flowing through tubes within the mold.

When plastic cools, it shrinks and can become stuck to the mold. This is why molds have built-in ejector pins that assist in pushing the plastic part out of the mold. 

The plastic is typically cooled and solidified within a minute, at which point the mold opens and the ejector pins push out the finished plastic part. 

After the part is ejected an operator has to remove the sprue (the small section of plastic that connected the part to the mold). 

Sprues are only attached to parts that are molded a single part at a time. In multi-cavity molds, the sprue connects to a network of distribution tunnels called runners (rather than the part itself).

Runners fan out from the sprue and connect to each cavity in the mold via a small entrance called the gate. 

Parting Line and Draft Angle

Molds typically consist of two parts: the cavity side of the mold and the core side. The cavity side is attached to the fixed side of the press, whereas the core side is attached to the moving clamp side of the press. Where those parts meet is called the parting line

You can see the parting line on the finished plastic product because the two halves of the mold are never perfectly aligned nor at a sharp 90-degree angle.

If the mold were at a 90-degree angle, the plastic part would scrape against the core part of the mold as you tried to eject it. Furthermore, the vacuum between the part and the mold could not break because you wouldn’t be able to get air into the mold. For this reason, molds are designed with a draft angle, allowing air to enter along the sides of the part. 

Ready to take the next step? 

At Tailor Made Products, our core capabilities include: 

  • Injection molding to 1880 tons
  • 2 Shot molding 
  • Insert molding
  • TPE/TPR molding
  • In mold decorating 
  • Component assembly
  • Packaging
  • Warehousing with JIT delivery
  • Engineering and packaging design
  • Sonic welding/Heat staking
  • CAD design and Tooling
  • Quality control management
  • Robotics 

Request a quote or reach out to learn what we can do for you.