Multi Cavity Prototype Mold 4+4: How a Family Tool Actually Works

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  What a multi cavity prototype mold 4+4 really is
• 2
  Runner balance decides whether 4+4 works at all
• 3
  Cooling and ejection uniformity across eight cavities
• 4
  Shrinkage compensation and tolerance strategy
• 5
  When 4+4 is the wrong choice
• 6
  4+4 family tool vs separate prototype tools

The name is narrower than it sounds. A multi cavity prototype mold 4+4 is one tool base with eight cavities split into two groups: four cut to Part A, four cut to Part B. Both parts fill in the same shot from one machine, one barrel and one cycle. It is a family mold, not two small molds bolted together.

That distinction drives everything downstream. Because the two parts share a sprue, a runner system and a cooling circuit, they also share each other's problems. If Part A has a 3 mm wall and Part B has a 1.2 mm wall, the thin part packs out long before the thick one is done. The press does not know the difference.

Prototype runs use this layout for one reason: assembly validation. You get a housing and its cover, or a left and right bracket, from a single tool investment. Fit, snap features and cosmetic seams can be checked on real molded parts instead of machined lookalikes.

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  Same shot, two partsOne injection cycle fills all eight cavities.
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  Matched setBest when A and B assemble together.
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  Shared runnerBalance is the main design risk.

In a balanced runner, every cavity sees the same melt pressure and the same fill time. In an unbalanced one, the closest cavities fill first and over-pack while the far ones starve. On a 4+4 tool this shows up as a visible weight spread between cavities. A 2 percent spread is workable. A 10 percent spread means the tool will never hold consistent dimensions.

The fix is geometric, not a process tweak. Design the runner with symmetric path lengths from the sprue to each gate, or use a naturally balanced layout where each branch splits into equal halves. Hot runner systems with individual tip temperature control cost more but let you trim each cavity group separately, which helps when Part A and Part B want different fill rates.

Gate size matters too. Start near 60 to 80 percent of the nominal wall thickness at the gate location for the thicker part, then check the thinner part for freeze-off. A gate that seals too early leaves a short shot in the cavity that needed more time.

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  Check weight spreadWeigh parts from each cavity group and compare.
• 2
  Symmetric pathsEqual runner length beats a bigger sprue.
• 3
  Gate freeze-offUndersized gates starve the slow cavities.

Cooling time is set by the thickest section in the tool, not the average. If Part B has a 4 mm boss and Part A is a uniform 2 mm shell, the whole cycle waits on that boss. Add a baffle or a bubbler at the boss so heat leaves locally instead of soaking the surrounding steel.

Ejection is the second failure point. Eight cavities mean eight sets of ejector pins, and they must travel together. A pin that sits 0.1 mm proud on one cavity leaves a stress mark; a pin that lags leaves the part stuck. Run the ejector plate by hand before the first shot and watch for any cavity that resists.

Mold temperature control units should be split if the two parts want different surface temperatures. One circuit for the A side and one for the B side is cheap insurance during prototyping, and it keeps you from chasing a cosmetic defect that is really a thermal one.

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  Cycle set by thickest wallCool the boss, not the average wall.
• 2
  Pin travelCheck all eight cavities by hand before molding.
• 3
  Split circuitsSeparate A and B temperature zones if needed.

Every cavity is cut oversize to compensate for shrinkage. The trap in a 4+4 tool is assuming both parts shrink the same. Unfilled ABS might run 0.4 to 0.7 percent, glass-filled PA66 can drop to 0.2 to 0.4 percent in flow direction and differ across flow. Part A and Part B need their own shrinkage factors, and often their own directional factors.

For prototype work, cut steel to nominal plus the material supplier's mid-range shrinkage, then adjust by sparking or welding if the first samples drift. Leave 0.05 to 0.10 mm of stock on critical features so you can dial them in. Chasing a ±0.05 mm fit on a first-shot tool is usually wasted effort unless the feature is a locating boss.

Write down which dimensions matter. A cover that must sit flush needs its parting-line and step dimensions held. A non-critical rib can run loose. Prioritizing five or six dimensions keeps the tool adjustment cycle short.

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  Per-part shrinkageDo not share one factor across A and B.
• 2
  Leave stock0.05 to 0.10 mm on critical features.
• 3
  Name the critical dimsFive or six, not fifty.

The configuration fails when the two parts have very different volumes. A 5 g clip and a 50 g housing in the same tool will never balance. The press holds a single shot size, so the small part gets packed to the limit or the large part comes up short. Volume ratio above roughly 3:1 is a warning sign.

It also fails when one part needs a much longer cycle. A thick optical lens and a thin bezel cannot share a sensible hold time. The thin part flashes or burns while you wait for the lens to freeze.

Finally, skip 4+4 if the two parts are not actually a matched set. If they never assemble, you are carrying the balancing risk of a family tool for no benefit. Two single-cavity prototype tools, or one tool plus CNC-machined samples, are often faster and cheaper to iterate.

• 1
  Volume ratioKeep A and B within about 3:1.
• 2
  Cycle mismatchThick and thin parts fight each other.
• 3
  No assembly linkThen a family tool is extra risk.