Introduction: A common problem – weld lines – can hide hundreds of thousands of dollars in hidden costs, and solution might be as simple as a minor product design change.
Those who have worked in injection molding industry for a long time are familiar with headache of "weld lines," especially in products with high aesthetic requirements such as home appliances, sanitary ware, and automotive exterior parts. Customers inspect products and say, "Weld lines are too obvious; we can't accept them."
In one sentence, months of effort on the entire mold are wasted.

Today, let's talk about real costs and where money is being wasted.
Material costs – good ABS costs tens of dollars per kilogram, and adjusting process requires at least three to five mold trials, sometimes many more;
Machine time and labor – once the machine is running, electricity meter spins rapidly, experienced technicians and their apprentices spend a whole day or even longer on process;
The most painful cost is mold repair – once it's determined that mold needs modification, welding, polishing, and structural changes can cost thousands to tens of thousands of dollars, so molds are generally not modified lightly.
This doesn't even include hidden losses from order delays. Customer's production line is waiting for parts, and this delay affects the entire supply chain, leading to penalties, reputational damage, and loss of future orders... These hidden costs can drive you crazy.
The story I'm sharing today is about Mr. Zhang, a mold factory owner, who suffered this loss before. A set of sanitary ware molds required four rounds of repairs due to weld lines, costing over 80,000 yuan in direct costs, and he also lost a long-term customer. He later said with a wry smile, "If I had known this, I should have paid closer attention during design phase."

How can you save money? Secret is: before designing mold, "pre-simulate" the entire injection molding process on computer.
This is power of mold flow analysis. It's like a "weather forecasting system" for injection molding industry – by inputting product's 3D model, plastic grade, gate location, and key injection molding parameters, it can predict how molten plastic will flow, where it will converge, where flow is fast or slow, and where problems are likely to occur.
Previously, we relied entirely on experience of senior technicians to solve problems, but when encountering new materials and new products, old methods were no longer effective. Now, with mold flow analysis, we can turn "unexpected" problems during mold trials into "inevitable" issues that can be foreseen during design phase. What used to require repeated adjustments during mold trials can now be verified with just a few clicks on computer.
Li, an injection molding technician with twenty years of experience, initially scoffed at mold flow analysis. Until one time, he spent three days trying to adjust a complex part without success. A technician, using a mold flow report, pointed out problem with gate location. After adjustment, mold trial was successful on the first try. Old Li's opinion changed completely: "This thing can really save us a lot of wasted effort."

That's principle, but how is it done? Let's take this bathroom casing case as an example.

 

A comparison using mold flow analysis makes problem clear: in original design, two streams of molten plastic converged like a "head-on collision," with a small angle and weak fusion. Resulting "seam" was long and obvious, and no matter how process was adjusted, weld line remained.
Solution was simple–just thicken middle area of product by 0.15mm. (For flow guidance)

 

This small change is like building a "buffer zone" before two highways intersect. Two streams of material no longer collide head-on, but instead fuse beforehand and then fill mold cavity. This almost completely eliminates weld line.

 

Sometimes, the most effective solution is precisely the simplest action. This is a true example of using minimal effort to achieve maximum results.

Some might ask, is this trick only useful for this specific case?
Let's broaden our thinking. This idea of "guiding flow and optimizing injection molding path," when used well, is a universal key. Similarly, we can design direction and thickness of reinforcing ribs to avoid surface sink marks; optimize product wall thickness distribution for more uniform cooling and reduced warping; pre-plan location of vents to address air entrapment and burning.
A true expert doesn't "fix" problems after they occur, but rather anticipates and prevents them beforehand. This is precisely competitive advantage in modern mold manufacturing.
A mold flow analysis investment of less than 10,000 yuan can reduce trial and mold repair costs by hundreds of thousands of yuan, as well as saving time and building customer trust throughout the entire project. This is a worthwhile investment by any measure.

Looking further ahead, we find that this transformation goes far beyond simply saving money; it's redefining value chain of mold manufacturing.
In traditional mold production, trial and error and mold repair are unavoidable "cost inputs," the most troublesome and unpredictable part of project budget. Through early mold flow analysis and design optimization, we can minimize this uncertainty, transforming mold manufacturing from a passive cost center into a predictable and controllable value-creating process.
This has a profound impact on company. Project managers can plan time and budgets more easily; sales personnel can provide customers with more reliable delivery dates; managers can allocate resources more rationally and improve overall efficiency; engineers can acquire a revenue-generating skill.
Next time during a review, consider asking: "Is resin flow smooth in this mold design?"
This valuable lesson teaches us that true cost control begins with the first design stroke, not the first mold trial.