Injection molding is one of the most cost-effective ways to produce plastic parts at volume — but getting there requires more design work upfront than most people expect. If you’re developing a product that will eventually be injection molded, the decisions you make at the design stage determine whether the process goes smoothly or turns into an expensive series of tooling corrections.
At Kemperle Industries, we work with product developers and engineers at the design stage — before the molder is ever involved. Our role is to take a concept or early prototype and prepare it for real-world production, which means designing with injection molding constraints in mind from the start.
What Injection Molding Actually Is
Injection molding is a manufacturing process in which molten plastic is injected under high pressure into a closed mold cavity. The plastic cools and solidifies in the shape of the cavity, the mold opens, and the part is ejected. The cycle repeats — typically in seconds to minutes depending on part size and complexity.
The economics of injection molding favor volume. The mold itself is expensive to produce — a simple single-cavity mold might cost a few thousand dollars, while complex multi-cavity production tooling can run into the tens of thousands. Once the mold exists, the per-part cost drops dramatically. This makes injection molding the right answer for parts needed in hundreds, thousands, or millions — not for one-offs or small runs where the tooling cost never amortizes.
For low-volume work, molding and casting or 3D printing are often better fits. The choice depends on quantity, material requirements, and how much surface quality matters.
Why Part Design Determines Injection Molding Success
A part that works perfectly as a 3D-printed prototype or a CNC-machined sample can fail in injection molding — not because the geometry is wrong, but because it wasn’t designed for the process. Injection molding has specific physical constraints that don’t apply to subtractive or additive processes, and ignoring them at the design stage creates problems that are expensive to fix after tooling has been cut.
The most common design problems we see:
Draft Angles
Every surface parallel to the direction the mold opens — the draw direction — needs a slight taper called draft. Without draft, the part grips the mold as it cools and either won’t eject cleanly or will require so much ejector force that it damages the part surface. A minimum of 1° of draft on most surfaces is the standard starting point; textured surfaces need more, typically 3–5° depending on texture depth.
Designing without draft is one of the most common mistakes on first injection-molded parts. It’s easy to miss in CAD because the geometry looks fine — the problem only becomes apparent when you try to pull the mold apart.
Wall Thickness
Uniform wall thickness is one of the most important rules in injection molding design. When wall thickness varies significantly across a part, different sections cool at different rates. Thick sections cool more slowly, and as they cool they can sink inward — creating visible sink marks on the surface, or internal voids that weaken the part structurally.
The target wall thickness depends on the material and part size, but as a general guide most injection-molded plastic parts are designed with walls between 1.5mm and 4mm, with consistency across the part more important than the specific value. Where thickness changes are unavoidable, the transition should be gradual rather than abrupt.
Undercuts
An undercut is any feature that prevents the part from being pulled straight out of the mold in the draw direction — a groove, a hole that runs perpendicular to draw, a snap-fit clip. Undercuts aren’t impossible to mold, but they require additional mold complexity: side actions, lifters, or collapsible cores that move laterally as the mold opens. Each of these adds tooling cost and potential failure points.
The cleanest injection-molded parts are designed to pull cleanly from the mold without any lateral motion. When undercuts are functionally necessary, it’s worth understanding the tooling implications before committing to the geometry.
Gate Location and Parting Lines
The gate is where molten plastic enters the mold cavity. Its location affects how the material flows through the mold, where weld lines form (points where two flow fronts meet, which are structurally weaker), and what surface marks are left on the finished part. Gate location is a design decision, not just a tooling decision — where you put it affects part performance and appearance.
Similarly, the parting line — where the two halves of the mold meet — leaves a witness mark on the part. Placing it deliberately, along an edge or in a non-visible location, is something to plan in the design phase rather than accept wherever the mold maker puts it.
Where Kemperle Fits In
We don’t run injection molds at Kemperle. What we do is take your concept through the design and engineering process and deliver geometry that’s ready for a molder to quote and tool — with draft angles applied, wall thickness reviewed, undercuts minimized or documented, and gate locations considered.
This typically starts with a concept or an existing prototype. We’ll review it for injection molding feasibility, identify any features that will cause problems in tooling or production, and revise the geometry to address them. For parts coming from a physical object that needs to be replicated or adapted, 3D scanning gives us an accurate digital baseline to work from.
The result is a production-ready CAD model — STEP or IGES — that a mold maker can take directly into tooling design without needing to resolve geometric issues on their end. For a broader look at designing parts for the manufacturing process they’ll actually use, see our guide to design for manufacturing.
If you’re developing a part that will be injection molded and want to make sure the design is right before tooling costs are committed, get in touch. Early design review is the highest-leverage point in the process — and the cheapest place to fix problems. Call us at 718-557-9578.
