Warpage in an HDPE part is uneven shrinkage, so you reduce it by making the part shrink the same amount on both faces and along its whole length. Start with the three levers you can change at the press. Bring the core and cavity steel to the same temperature. Hold pressure until the gate freezes, and cool the part longer before ejection. Gate position and wall thickness come after those, because changing them means cutting steel.
Read Which Way the Part Warps
The direction and pattern of the warp tell you which lever to check first:
- Bowed toward one face: the two mold halves are running at different temperatures. A flat part bends toward the hotter half. That rule was measured on flat coupons of an amorphous resin, so on HDPE treat it as the first direction to check.
- Shrinking more far from the gate: the part is under-packed at the end of the flow. Look at hold time and gate size.
- The same warp in every cavity: the cause is common to all cavities, such as the steel temperatures of the two halves, the wall design, the hold profile, or the resin and color. Warp in one cavity only points to that cavity’s gate or cooling circuit.
- Warp that started after a change: a modified cooling circuit, a new resin grade or lot, or a new color batch. Check what changed before you touch the settings.
Judge the bow on parts that have cooled to room temperature.
Balance Core and Cavity Cooling
Balance the steel temperatures of the two halves. Measure the core and cavity surfaces once the tool is running at a steady cycle. Adjust the water until both halves read within 10 °C (20 °F) of each other. The water setpoints may differ to get there.
Check each circuit as well. The water should warm by less than 2 °C (4 °F) between inlet and outlet; a larger rise means that circuit cools one end of the part more than the other. If the warp began after a cooling circuit was modified, measure both halves again.
Moving the whole mold colder or warmer is the weaker lever. A colder mold makes an HDPE part larger out of the press because it limits crystallization. Polyethylene keeps crystallizing at room temperature, so part of that gain is lost after molding.
Set Pack and Hold to Gate Freeze
Keep hold pressure on until the gate has frozen, and set the hold time a step past the freeze time that a gate-seal study finds.
Run the gate-seal study like this:
- Fix the cycle: keep the total cycle time constant. Set hold pressure by starting low and raising it until the part looks acceptable.
- Step the hold time: increase it in small steps and weigh the parts at each step to four significant figures.
- Read the freeze time: the gate has frozen at the hold time where part weight stops rising on a cold runner, or rises only slightly on a hot runner. On valve gates the curve can take any shape.
- Weigh the runner too: runner weight should keep rising after part weight stops. If it stops first, the sprue or a runner branch is undersized and froze before the gate.
- Set the timer: put the hold time a step above the freeze time, for example 6.5 s or more for a gate that freezes at 5.5 s.
On a multi-cavity tool, weigh the cavities separately when you suspect different cavity temperatures or gate sizes. Do not set hold pressure as a fixed share of injection pressure; step 1 sets it from the part.
Overpacking warps HDPE as well. If the gate takes a long time to freeze, either the gate is too deep or the hold pressure is high enough to keep pushing half-frozen material into the cavity.
Place and Size the Gate
Put the gate where it protects the dimension that must stay true. Gate a part that must stay round at its center, and gate a long, narrow part that must stay straight at one end.
Where the part has thick and thin areas, gate into the thickest section. Keep the flow length as short as the part allows.
Gate depth, the thinnest dimension of the gate, sets when the gate freezes. For polyethylene, start at a gate depth of 40–50 % of the wall thickness at the gate, toward 40 % on thin walls and toward 50 % on thick ones.
Start on the shallow side. A gate can be opened up later by removing steel, while a gate cut too deep cannot be closed. Very wide gates on thin parts, such as fan, film, ring, disk and diaphragm gates, slow the melt enough that it can freeze before packing ends.
In a multi-cavity tool every gate needs the same depth, width and land length, or the cavities pack unevenly. When the gate-seal study shows the gate freezing before the part is packed, take the depth you need to the toolmaker.
Keep Wall Thickness Uniform
Keep the wall the same thickness throughout the part, because a thick section cools more slowly and shrinks more than the thinner wall beside it.
Ribs and bosses thicker than the wall they join shrink more than that wall, and a rib on one face only pulls that face. On a deep part such as a cap or container, a core sitting off-center makes the wall thicker on one side. Section a few parts and measure the wall around the circumference. A wall that varies around the part points to the core, and no press setting removes it.
Cool the Part Before Ejection
Cool the part in the mold until it is stiff enough to keep its shape when ejected. The time depends on wall thickness and mold temperature, so find it on your own part. Add cooling time in steps and measure the warp once the parts have reached room temperature. Ejecting too hot, or pushing the part out unevenly, both cause warp.
In-mold cooling time for a flat wall follows this relation:
t = s² / (π² · a) · ln[(4/π) · (T_melt − T_mold) / (T_eject − T_mold)]
Here t is the cooling time in seconds, s the wall thickness in cm, a the thermal diffusivity (about 7.7 × 10⁻⁴ cm²/s for HDPE), T_melt the melt temperature, T_mold the mold wall temperature, and T_eject the temperature the middle of the wall must reach before ejection. The thickest section of the part sets the time. Back-calculate T_eject from the cooling time the stepping trial settles on, then use the relation to rescale that time when the wall thickness or mold temperature changes.
HDPE Resin Properties That Drive Warpage
Of the resin-side causes, check the color and additive package first. Some organic pigments, phthalocyanine blue the best known, act as nucleating agents for HDPE and change how much it shrinks along the flow compared with across it.
The UK National Physical Laboratory molded HDPE plates from one resin under the same conditions with different pigments. The laboratory concluded that the only way to control warp from phthalocyanine blue is to change the pigment’s chemistry.
In one granted patent’s examples, adding 0.15 % exfoliated talc, itself a nucleating agent, to a phthalocyanine blue HDPE raised its cross-flow shrinkage and cut the patent’s warpage index from 18 % to 8 %. Nucleation as such does not decide warp: in the same resin, one nucleating pigment raised it and a nucleating talc lowered it.
The test for any additive package is whether it keeps shrinkage along and across the flow equal. The carrier and wetting agent in a color masterbatch can change the pigment’s effect as well, so two masterbatches of the same shade may not mold the same.
When warp starts with a color change on an unchanged resin, tool and process, mold the uncolored HDPE and the colored batch back to back and compare them. The resin certificate will not show this cause, because it reports melt flow rate and density, which say nothing about how the resin crystallizes with your color.
Warpage in HDPE Bottle Caps
An HDPE cap warps in two separate ways: the top loses flatness and the skirt loses roundness.
Hold time improved both shape errors in a published trial on HDPE caps. A longer injection time made both worse.
Caps molded in different colors from the same resin and tool come out at different sizes and shapes, and in that trial one nucleating pigment gave the flattest tops while making the skirts less round. A change that fixes one shape error can worsen the other, so measure both whenever you change a color or a setting.
When a cap-grade HDPE runs in several colors through one cavity, tune the process for each color. When one color runs alone, the cavity can be cut oversize or unbalanced to absorb that color’s shrinkage.
Gate the cap at the center of the top, where the gate keeps the skirt round. If every cavity of a center-gated tool dishes the same way, the cause is common to all of them: check the steel temperatures under the top on core and cavity first, then the hold time.