Design for Manufacturing: designing with the mold in mind

Design for Manufacturing (DfM) is designing a product with the production method as the starting point. With injection molding, this means wall thickness, draft angles, ribs, gating, and material choices are included from the very first sketch, so the product can be made in the mold without costly changes. The difference with ordinary design is the order: you don’t first draw a shape and then look for a mold, but the mold guides the design. At QDP, that is the approach in every project.

No. DfM is not a separate service but a method that runs through the entire process, from concept to serial production. A DfM check afterwards on a completed design often gives a list of modifications that require redraws. Including it from the very first sketch avoids that extra round. If you don’t have your own design capacity, QDP takes over the design via the design outsourcing service and applies DfM from day one. If you supply a design yourself, we check it before the mold is built.

As early as possible, preferably already in the concept phase. The greatest part of the cost price and manufacturability is set in the design phase, not on the production floor. A choice that costs nothing in the sketch phase, such as a slightly different wall thickness or a shifted parting line, costs thousands of euros in steelwork and weeks of delay after the mold is milled. The later DfM starts, the less can still be influenced. That’s why QDP looks in from the very first concept, even if there is no drawing yet.

A lower unit price, fewer design iterations, and more stable quality in the series. DfM iterations typically save 10 to 30 percent on unit price by specifying smarter wall thickness, fewer components, and easier assembly. In addition, the mold is often simpler, with fewer slides, reducing investment and lead time. And because manufacturability is calculated up front, there are fewer surprises with the first shots. At QDP, the guarantee applies: if a product is not manufactured as designed, you get your investment back.

Yes, that is possible. For an existing product, DfM starts with an analysis of what is already there: the current design, the mold, the rejects and the assembly. Then you redesign for function, ergonomics, and manufacturability, with the aim of the same or a better product that is cheaper to make. Think of a thinner wall, merged parts, or a different material. Sometimes an adjustment to the existing mold suffices, sometimes a new one is worth it. This is the basis of QDP’s further development and cost-down projects.

That depends on the size, number of cavities, and how many slides the shape needs. A single cavity mold for a simple part is cheapest, and every undercut requiring a slide makes the mold more expensive and complex. Good DfM keeps the mold as simple as possible and thus the investment lower. At QDP, a mold starts from €3,000, with a concentration between €3,000 and €5,000, and extremes up to €60,000 for large, complex jobs. You get an indication within two minutes with the calculator.

Almost all technical plastics: ABS, PP, PA, PC, POM and glass fiber reinforced variants, plus biobased grades like PHA and PLA and recycled plastics. The material is part of the DfM consideration, not an afterthought, as it determines shrink, achievable wall thickness, cycle time and thus the entire mold around it. A mold is built for the shrinkage of one material, so switching after building costs money. QDP selects the material together with the design, based on function, environment, and cost price.

You reduce that risk before any steel is milled. With mold-flow analysis, you simulate filling; with material validation you check if the plastic does what the product needs, and with DfM checks you cover wall thickness, draft, and undercuts. A model to check fit is also part of it. If a detail turns out not to be manufacturable, you adjust the design while it's still cheap. At QDP, it also applies: if a product is not manufactured as designed, you get your investment back.

Most injection molded products have a wall thickness between 1.5 and 3 mm, depending on material and flow path. More important than the number is uniformity: thick zones cool more slowly and cause sink marks, shrink holes, and warping, while zones that are too thin don’t fill completely. Stiffness comes from ribs and form, not from mass. Where a thickness difference is needed, make the transition gradual. QDP calculates wall thickness for every design based on material, flow path, and cycle time.

That depends on material, size, and whether a dimension is in one mold half. Plastic shrinks 0.4 to 2.5 percent in the mold and that shrinkage varies per cycle. Dimensions over the parting line get 0.1 to 0.2 mm extra tolerance. Amorphous plastics such as ABS and PC shrink less and more evenly than semi-crystalline plastics like PP, PA, and POM. The rule of thumb: only set functional dimensions tightly and keep the rest general. QDP agrees on tolerances per functional dimension before building the mold.

A weld line occurs where two flows of molten plastic meet again, for example behind a hole. At that line, the material is slightly weaker and sometimes visible. Usually this is not a problem, but on a stressed or visible spot it is. A higher mold temperature or extra venting makes the line stronger and less visible. By consciously choosing the gating location, you determine where the line will be. QDP tests this in advance with mold-flow analysis.

Check for each detail if it can be formed in the opening direction of the mold. Often a hook, hole, or edge can be modified: move the parting line, create an opening under a snap tab, or rotate the feature a quarter turn. If not, slides or core pullers are needed. Every slide makes the mold more expensive, requires more maintenance, and may leave a visible line on the product. Therefore, check for each undercut if the function is really needed. QDP examines these details one by one with you during the DfM review.