Information about Mold Design Related to Test Molding

Molds can be classified in different ways, and each type has its own uniqueness. But what affects test molding the most is the mold structure—it impacts things like pressure loss during processing, temperature distribution, and injection speed. When the runner is long and the mold is large, you usually need to heat and cool the mold. The goal is to balance out pressure loss, get a reasonable temperature distribution, and if you pair that with the right gates and control the injection speed properly, you can get good-quality parts. Let’s go over a few mold types that affect test molding.

Hot runner molds

Hot runner molds can be split into insulated runner molds and heated runner molds.

1.Insulated runner molds: During test molding, you often have to run over a hundred parts before the runner inside the mold reaches a "non-solidifying" state. So, first, you need patience. Second, you have to watch for when that "non-solidifying" state kicks in—usually, you need to tweak the process parameters just right for it to happen. It has to meet a few conditions: ① The plastic isn’t heat-sensitive; ② Keep the molding cycle as short as possible; ③ The part should solidify and set quickly after injection, so thin-walled parts are best. If you’re using cooling water, don’t turn it on until the "non-solidifying" state is reached. Once it is, gradually increase the cooling water flow.

2. Heated runner molds: They have their own heating elements and might even have a cooling system, so the key adjustment is balancing heating and cooling. Usually, start by turning off the cooling water and cranking the heating temperature up to max. Once the runner hits that "non-solidifying" temperature, slowly increase the cooling water flow. Then, adjust the temperature controller step by step to a suitable level—you want the shortest possible cycle time, a runner that stays liquid, and parts that are within the acceptable deformation range.

Whether it’s an insulated or heated runner mold, they both need stable production. You can’t linger on one process step for too long.

Two-color molds

Right now, only countries like Germany can make four-color molds, but China can already make two-color molds. Two-color injection machines have two barrels, each injecting plastic of a different color. Usually, each barrel needs its own set of process parameters. If one barrel is perpendicular to the other, you have to make sure the mold doesn’t shift sideways when installing it. And during operation, the two sets of parameters need to work together, so you’ll have to do more coordination tests. But each barrel’s parameters can be adjusted independently.

Stack molds

Stack molds—like the ones for CD cases—need careful handling when installing. You have to adjust the mold opening/closing speed and distance just right. Thanks to better manufacturing standards, mold precision is much better now. Problems that used to happen a lot, like flash or warping, are rare these days. The main issues left are consistency and automation, but those are more about design than test molding, so they don’t matter as much here.

Specialized molds

Injection molds often have specialized designs for specific uses—like foam molds, micro-foam injection molds, rubber injection molds, and reaction injection molds. These specialized molds each have their own quirks during test molding, but I won’t go into each one here.

Gate types

There are lots of gate types, and they usually have a big impact on injection molding during testing. If the gate is too small, it’s usually hard to get the settings right. But you can’t make it too big either—big gates are a hassle to finish later and might even leave noticeable marks. Pin gates are easy to remove and leave small marks. Submarine gates hide the gate marks better, but they make the mold structure more complicated. For example, in molds for car bumpers, there might be multiple gates. During test molding, you might change some gates’ positions or numbers, and you should give designers feedback on that.

Runner conditions

Besides hot runners, the biggest issue with runners during test molding is making sure the sprue doesn’t get stuck in the fixed mold. You can fix this by adding a sprue puller. And during testing, never let the runner material break—if it does, it’ll cause trouble when closing the mold.

Plastic shrinkage

Plastic shrinkage is a really tricky problem. The shrinkage rate isn’t just about the plastic itself—it also depends on the part structure, processing methods, and more. To reduce shrinkage changes caused by the plastic, always use the same plastic for test molding that you’ll use in production.

In parts with uneven thickness, the thick sections show obvious sink marks. For parts with big ribs, the thick parts of those ribs also get noticeable sink marks on the surface—even corner areas with right angles can have sink marks or voids.

During test molding, just cranking up the injection pressure isn’t a solution. Usually, increasing the material temperature makes sink marks and voids worse, but heating the mold evenly can help reduce them. For heat-sensitive plastics, a small temperature increase can really boost melt flow, so a slight bump in material temperature might be worth trying.

For crystalline plastics like PP, sink marks and voids are worse under crystallization conditions. You can adjust the process parameters to reduce or prevent crystallization—like cooling quickly at the plastic’s crystallization temperature to stop it from crystallizing fully. If tweaking parameters doesn’t work, suggest changing the gate position or adding more gates to alter the mold shape—that can help reduce sink marks and voids.