Plastic mold building tech index

Every plastic part shrinks as it cools. The only questions are how much and whether it shrinks evenly. Material suppliers publish rough shrinkage ranges, often 0.5 to 1.5%. The real number for your part depends on packing pressure, mold temperature, wall thickness, and gate seal time. Two identical parts can shrink differently if any of these change.

Two temperatures control shrinkage most. Barrel temperature sets how the melt behaves. Mold temperature sets how it freezes. Get both right and dimensions land where the drawing says they should.

What Actually Controls Shrinkage

Packing is the biggest lever. Holding pressure feeds extra melt into the cavity while the gate stays open. It compensates for volumetric contraction as the plastic cools. Packing time should run slightly past the gate seal point. Shorter than that, the part is starved. Longer, and you are just burning cycle time.

Mold temperature comes next. A warmer mold slows cooling and lets the part shrink more freely before ejection. A cold mold freezes the skin fast and locks in less shrinkage, but it also locks in more stress. Thermoplastic molds typically run 10 to 120°C. Where you sit in that range moves both dimensions and warpage.

Crystallinity sets the baseline. Semi-crystalline resins like PP and PA shrink more than amorphous ones like ABS and PC. Their molecules pack into ordered regions as they cool, which pulls the part in further. Fiber fillers restrain shrinkage along the flow but barely across it. That directional gap is a warpage source on its own.

Barrel Temperature by Material

Barrel temperature sets melt viscosity. Too cold and the melt will not fill or pack. Too hot and the resin degrades, which shows up as discoloration, odor, or brittle parts. The table below gives standard processing ranges. Stay inside them unless your material supplier says otherwise.

PC runs hottest at 280 to 310°C. That heat is needed to keep its high-viscosity melt flowing. PVC runs coolest at 160 to 180°C, and there is a reason. Overheated PVC releases corrosive gas that attacks the part and the mold steel. Watch residence time as closely as temperature. Melt sitting too long in a hot barrel degrades even inside the recommended range.

Mold Temperature by Material

Mold temperature decides surface quality, crystallinity, and cycle time in one stroke. Warmer molds give better surfaces and more complete crystallization in semi-crystalline resins. They also stretch cooling time, which usually sets the pace of the whole cycle.

The mold temperature column in the table above is the starting point, not the final answer. Thin-walled parts often need the warm end to fill completely. Thick parts may need the cool end to eject without distortion. Measure actual steel temperature with a contact probe during trials. Controller displays lie when water flow is restricted or circuits are imbalanced.

Keep both mold halves close in temperature. A 15°C split between cavity and core bows flat parts toward the hot side on every shot. Cooling water should enter near the gate, where the melt arrives hottest, and flow in the same direction as the melt path.

Drying Temperature and Time

Wet resin ruins dimensions before shrinkage even enters the picture. Moisture turns to steam in the barrel, leaving splay marks and internal bubbles. Hygroscopic resins must be dried to the supplier’s spec, every time.

PA and PC punish skipped drying the hardest. PA absorbs moisture fast and needs 4 to 6 hours at 80°C or more. PC wants 120°C for 2 to 4 hours. If splay appears mid-run on a previously good job, check the dryer first. A failed heater or a short residence time in the hopper is the usual culprit.

Cooling Channel Sizing

Channel diameter should grow with mold size and part thickness. Undersized channels restrict flow, and restricted flow means hot spots. Hot spots mean uneven shrinkage.

Bigger is not always better. Oversized channels near the cavity create their own temperature variance. Keep channels close enough to pull heat evenly, and keep flow turbulent. Water removes heat 8 to 10 times faster than oil, which is why water circuits dominate thin-wall and high-cavitation molds.

Flow Length, Wall Thickness, and Cavity Pressure

Thin walls need more pressure to fill. The ratio of flow length to wall thickness tells you how hard the job is. Higher ratios need higher cavity pressure, and pressure affects how much the melt packs, which feeds back into shrinkage.

PC is the stiffest case here: a 90:1 ratio already needs 50 MPa. LDPE flows far at low pressure. When a part runs at the edge of its ratio, gate and wall changes beat process tweaks. No amount of barrel heat fixes a flow length the resin cannot reach.

Practical Shrinkage Checklist

• Barrel temperature inside the supplier range; residence time watched.
• Mold temperature set per resin, verified on the steel, not just the controller.
• Both mold halves within a few degrees of each other.
• Hygroscopic resins dried to spec; dryer function checked when splay appears.
• Holding time past gate seal, confirmed by part weight.
• Cooling channels sized to the mold and kept turbulent.
• First-article dimensions measured after 24 hours, not straight off the press. Shrinkage continues as the part cools to room temperature.
• Steel cut with shrinkage allowance on the safe side; it is cheaper to polish off steel than to weld it back. Warpage from uneven shrinkage is covered in our internal stress guide, and surface defects from trapped air in our flow marks guide.