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Basic Mold Design Considerations

Posted in Medical Device Development

A well designed and produced mold can make quality plastic parts in large volumes with high tolerance precision, low labor costs, and minimal waste. A poorly designed mold may hinder production and result in blisters, delamination, sink marks, voids, warping, and more. We’ve outlined some basic considerations for mold design that help engineers plan for and avoid product defects.

Mold plates

Typical molds are machined from steel, aluminum, or copper alloy and have two parts: a plate with a cavity and a plate with a core. The cavity is the hollow or concave side of the mold. The core is the convex or mirror of the cavity. Numerous functional and structural considerations go into designing a mold that will form the parts in less time and at the lowest cost. In general, the goal is to produce a light-weight, durable product with less material and little need for secondary operations.

Injection pressure

A mold must be strong enough to resist high pressure and the heat of injected plastic. Injection pressure pushes the plastic through the gap between the mold cavity and core. The higher the melt temperature, the easier the plastic will flow and fill the mold. But higher melt temperatures require more time to harden before the part can be ejected. Most presses heat the plastic with lower temperatures and amp up the force, using a durable clamping unit to ensure the two molds won’t open under injection pressure.

Material shrinkage and draft

Injection mold design must take material shrinkage into consideration. Engineers design molds with shrinkage allowance and draft angles so the part can release from the mold without damage. Angled side walls ensure that the part can break free of the mold as soon as it opens.

Gate types

The gate of a mold is the opening where melted plastic enters, or the boundary between scrap and part. Depending on the capabilities of the injection molding machine, an operator may have to separate parts, manually trimming gates after each cycle. The size and number of gates may necessitate secondary operations or affect the cosmetic appearance of the finished product.


Texture can be added to the mold to eliminate the need for finishing. Textures are often added for design or function, such as to hide finger prints or reduce part wear. Patterns on the mold surface can produce matte, woodgrain, geometric, and many more finishes.

Via Biomedical has designed our own reliable, tough injection machine called the GS-5T molding press. Available for purchase, this table-top press is the perfect molder for assembly lines using a one-piece flow process where small part injection or insert molding is required. Using this technology, we can work with plastic polymers like PEBA (PEBAX®), nylon, polycarbonate, polyethylene, polypropylene, ABS, and other non-corrosive thermoplastics with melting temperatures below 700°F. Instead of gluing parts together, we use insert molding to assemble catheter components with the GS-5T, resulting in a more uniform product.

At Via Biomedical, our skilled engineers can create mold designs to manufacture quality, durable parts that fit your product specifications. We’re experts at considering all aspects of mold design to achieve an efficient production process and flawless outcomes. We’ll be at New England’s premier medical technology event, BIOMEDevice Boston, this May 15-16 at the Convention & Exhibition Center (MCCA) in Boston, MA. Be sure to visit us at Booth #465.

Contact us to design your prototype or purchase your own GS-5T injection machine.