The mold building process is sometimes viewed as a simple manufacturing step. However, it’s more than just replicating an object. True mold building is a discipline, a critical intersection of material science, geometry and cost management. It represents the crucial, often unseen, stage that determines a product’s viability, efficiency and aesthetic quality.

To truly grasp the significance of mold design and building, simply look around. Nearly every plastic product you interact with on a daily basis, from the shell of your vacuum cleaner and the interior panels of your car to the casing of your kitchen appliances and the cap on your water bottle, exists because of a precisely engineered mold.

Mold Design Essentials

When QSI designs a mold, our decision-making process is driven by several key factors. And while they are all important, no two molds are alike, so how they are prioritized varies with each project.

1. Geometric Integrity: 

The core challenge in mold design is turning your object into a cavity that can be reliably filled and, most importantly, released. This process is all about predicting how the material will behave as it cools and solidifies. There are several key factors to this:

• A parting line: where the different sections of your mold meet. It’s much more than a simple seam; it’s a design compromise that balances how easy it is to inject and form the part versus how much time and money you’ll spend cleaning it up later. Placing a parting line requires understanding how the object will be oriented and how the material will flow. A poorly planned line guarantees high labor costs and visible defects on the final product.

• Draft angles: essential, slight tapers (i.e., slopes) built into the walls of the mold cavity. When applicable, they are a necessary component, preventing the finished part from sticking by reducing friction and vacuum when the mold opens. Using draft angles protects the final product from stress and the mold itself from wear, significantly extending its life.
• Undercuts: features that create a lip or hook, making a straight pull impossible. Traditionally, a mold would have a slide feature that is activated when the mold opens or as the part is ejected. QSI can also build complex, multi-part molds with collapsible or expandable cores.

2. Material Dynamics and Thermodynamic Management

A key differentiator in quality mold design is the consideration of material behavior, particularly heat and volume change.

• Shrinkage and Compensation: Every plastic or rubber material shrinks upon cooling or curing. The mold builder must act as a geometric fortuneteller, calculating the expected shrinkage based on the material’s coefficient of thermal expansion and compensating for it in the mold’s dimensions. In situations where a shrink rate isn’t provided, the QSI team looks it up and adjusts the part accordingly. Because a mold is often larger than the desired final product, these calculations are a necessary counter-intuitive step to achieve dimensional accuracy.
• Thermal Management (for industrial molds): In high-volume molding, the mold is an active thermodynamic device. Integrated cooling channels, in the form of water channels or heater rods, are precisely placed and engineered to control the rate and uniformity of cooling. This is critical because uneven cooling leads to warping and internal stress in the part, compromising its structural integrity.

• Balancing Cavities: Ensuring the even distribution of the material (i.e., filling at the same time and pressure) across all mold cavities is crucial because it ensures high-quality and consistent parts. Cavity balance is primarily controlled through the runner system, where the resistance to flow (i.e., pressure drop) is identical to every cavity, ensuring simultaneous filling and similar melt conditions (i.e., temperature, shear rate).

 3. Manufacturing Considerations

Since producing the mold is the ultimate goal, the way it will be manufactured must be considered during the design process. What is the physical capacity of the molding machine compared to the mold itself? For larger molds, can the shop’s handling equipment (e.g., forklifts and cranes) manage the final weight and size?

In addition to what machine or method will be used, QSI looks at whether inserts and/or spare tooling will be needed for wear items. Though an added cost, the benefit is having the ability to easily swap a tool or insert if damage occurs during production.

Finally, the impact on the operator must be considered, ensuring the operation of the machine, as well as the loading and removal of the mold can all be done easily and safely.

4. Longevity, Cost, and Cycle-Time Optimization 

A mold is a financial asset, so its design must align with the intended scale of production. This requires shifting the focus from simply building a mold to building the right mold for the job.

When a project is in the prototyping phase, speed is everything. The mold must be inexpensive, quick to produce and simple. At QSI, prototyping is when we show the customer how the mold will act. And because changes are inevitable, we use soft steel that isn’t expected to last long and allows magnets to hold the mold in place when using surface grinders.

We then move into production, where the focus changes to durability. The mold is now expected to deliver thousands, possibly millions, of consistent cycles, which demands an upgrade to materials like high-quality tool steel. In the production phase, the mold transitions into a high-performance industrial machine with the focus no longer being just durability, but minimal maintenance and the ability to withstand millions of cycles.

Navigating Common Pitfalls

The most common mistakes in mold building are rarely technical, but failures of foresight and holistic planning. QSI has been in the injection molding business since 1997, so we’ve seen and done it all. Here are some common mistakes we’ve seen throughout the years.

• Air Entrapment/Voids: These are the result of not properly mapping the flow and exhaust path of air. When you push plastic into a cavity, the air within it has to go somewhere, so you often need to incorporate tiny vents at the highest points or furthest reaches of the cavity. In insert molding, there could be a wire that’s being over molded or an insert that shuts off the cavity to create a natural vent, and in some molds, there are natural vents from ejector pin holes.
• Material Compatibility Failure: This can occur from neglecting the chemical interaction between pattern, mold and molding material. To avoid this costly error, always verify the chemical release agent’s compatibility with the specific chemistry of the mold material and the molding material’s tolerance for that agent. Some materials can be abrasive, requiring a hard coating, such as titanium nitrite, to create a harder and smoother surface.
• Inaccurate Alignment: In a multi-part mold, if addressing the alignment mechanism is an afterthought, the two halves won’t be centered during molding, resulting in a misaligned final product. The solution is an integrated registration system using keys, or guides, which are deliberate bumps and depressions the perfectly interlock in the mold halves.

Critical Considerations

There’s a lot to think about and consider when building a mold. Before committing resources, the QSI team asks these fundamental questions:

1. Is the object moldable?

Can the intended material reliably flow into and fill all features? Does the object’s geometry require complex coring, resulting in the tool maintenance cost outweighing the production benefit? It’s important to remember that the most complex geometry is not always the best geometry, and though you want to avoid thick areas, it’s not always possible.

2. What is the total cost of ownership (TCO) for the tool?

Don’t just focus on the cost to build the mold. Factor in maintenance, expected lifespan (i.e., number of cycles before degradation), required cycle time, and the cost of wasted material from rejects. Keep in mind, a cheap mold with a high reject rate is the most expensive option.

3. Are there realistic tolerances?

Every step in the mold building and forming process introduces small imperfections, a situation called tolerance stacking. Therefore, the precision, or tolerance, required for your final product must be the first thing guiding your material and process choices. Sometimes the desired tolerance isn’t always the necessary one. You must align your expectations for precision with the capabilities and limitations of your chosen materials and methods.

The Applied Engineering Mindset

At the end of the day, mold building is an exercise in managing controlled variables. Those who are successful, like the QSI team, are not mere technicians; they are applied engineers that anticipate forces and predict material behavior. Our ultimate success lies in designing a robust system that consistently delivers geometric perfection at a profitable industrial scale, all while meeting the customer’s needs.

Authored by: Kent Shultz, Mold Department Manager, QSI Automation