Process parameter setting of TPE injection molding

Time：2022-09-23 08:36:58 / Popularity： / Source：

Before molding, plastic must be fully dried. After material containing moisture enters mold cavity, it will cause silver tape-like flaws on the surface of part, and even hydrolysis at high temperatures, resulting in deterioration of material. Therefore, material must be pretreated before molding process, so that material can maintain proper moisture.

Mold temperature setting

(1) Mold temperature affects molding cycle and molding quality. In actual operation, it is set from the lowest appropriate mold temperature of material used, then adjusted appropriately according to quality status.
(2) Correctly speaking, mold temperature refers to temperature of cavity surface when molding is performed. In mold design and molding engineering condition setting, it is important not only to maintain a proper temperature, but also to make it uniform distribution.
(3) Uneven mold temperature distribution will lead to uneven shrinkage and internal stress, which makes molding mouth prone to deformation and warping.
(4) Increasing mold temperature can obtain following effects;
1. Add crystallinity of molded product and a more uniform structure.
2. Make molding shrinkage more fully and reduce post-shrinkage.
3. Improve strength and heat resistance of molded products.
4. Reduce internal stress residue, molecular alignment and deformation.
5. Reduce flow resistance during filling and reduce pressure loss.
6. Make appearance of molded product more shiny and good.
7. Increase chance of burrs on molded products.
8. Increase location near gate and reduce chance of recession at far gate.
9. Reduce obvious degree of joint line.
10. Increase cooling time.

Metering and plasticization

(1) In molding process, control (metering) of injection volume and uniform melting (plasticization) of plastic are performed by plasticizing unit of injection machine.

Barrel Temperature

Although about 60~85% of melting of plastic is due to heat generated by rotation of screw, melting state of plastic is still greatly affected by temperature of heating cylinder, especially temperature near front of nozzle-when temperature in the front area is too high, it is easy to cause dripping and wire drawing when taking out product.

Screw speed

(1) Melting of plastic is mainly caused by heat generated by rotation of screw. Therefore, if screw speed is too fast, following effects will be caused:
1) Thermal decomposition of plastics.
2) Glass fiber (fiber-added plastic) is shortened.
3) Screw or heating cylinder wears faster.
(2) Speed setting can be measured by size of its circumferen-tial screw speed:
Circumferential speed = n (rotation speed) * d (diameter) * π (circularity)
Generally, for low-viscosity plastics with good thermal stability, circumferential speed of screw rod rotation can be set to about 1m/s, but for plastics with poor thermal stability, it should be as low as about 0.1.
(3) In practical applications, we can reduce screw speed as much as possible so that rotating feed can be completed before mold is opened.

BACK PRESSURE

(1) When screw rotates and feeds, pressure accumulated by melt advancing to front end of screw is called back pressure. During injection molding, it can be adjusted by adjusting return pressure of injection hydraulic cylinder. Back pressure can be as follows Effect:
1) Glue melts more evenly.
2) Toner and filler are more evenly dispersed.
3) Make gas exit from blanking port.
4) Metering of feed is accurate.
(2) Level of back pressure is determined by viscosity of plastic and its thermal stability. Too high back pressure will delay feeding time, increase in rotational shear force will easily cause plastic to overheat. Generally 5~15kg/cm2 is appropriate.

Suck back (SUCK BACK, DECOMPRESSION)

(1) After rod rotates and feeds, screw is properly retracted, melt pressure at the front of screw can be reduced. This is called loosening, and its effect can prevent dripping of nozzle.
(2) Insufficient, it is easy to cause sprue (SPRUE) to stick to mold; too much loosening can suck in air and cause air marks in molded product.

Stable molding number setting

(1) Confirm beforehand and prepare settings
Confirm that material is dry, mold temperature and heating cylinder temperature are correctly set and reach a processable state.
Check action and distance setting of opening and closing mold and ejection.
Injection pressure (P1) is set at 60% of maximum value.
Holding pressure (PH) is set at 30% of maximum value.
Injection speed (V1) is set at 40% of maximum value.
Screw speed (VS) is set at about 60RPM.
Back pressure (PB) is set at about 10kg/cm2.
Loose retreat is approximately set at 3mm.
Position of holding pressure switch is set at 30% of screw diameter. For example, for a screw with φ100mm, set 30mm.
Metering stroke is set slightly shorter than calculated value.
Total injection time is slightly shorter, and cooling time is slightly longer.
(2) Manual operation parameter correction
Mold is locked (confirm the rise of high pressure), and injection seat moves forward.
Inject manually until the screw stops completely, and pay attention to stop position.
Screw revolves back to feed material.
After cooling, open mold and take out molded product.
Repeat steps ⑴~⑷, screw will finally stop at 10%~20% of screw diameter, and molded product has no short shots, burrs, whitening, or cracks.
(3) Modification of semi-automatic operating parameters
Correction of metering stroke [metering end point]. Increase injection pressure to 99%, temporarily adjust holding pressure to 0, adjust metering end point S0 forward to occurrence of short shots, and then back to occurrence of burrs, with middle point as the selected position.
Correction of output speed restores PH to original level, adjusts injection speed up and down, finds out individual speeds where short shots and burrs occur, and uses middle point as appropriate speed. [At this stage, you can also enter parameter setting that corresponds to appearance problems with multiple speeds. set].
Correction of holding pressure. Adjust holding pressure up and down to find out individual pressures that cause surface depression and burrs, choose middle point to hold pressure.
Correction of holding pressure time [or injection time] gradually extends holding time until weight of molded product is obviously stable, which is a clear choice.
Correction of cooling time gradually reduces cooling time and confirms that following conditions can be met:
(1) Nolded product is ejected, clipped, trimmed, and the packaging will not be whitened, cracked or deformed.
(2) Mold temperature can be balanced and stable. A simple algorithm for cooling time of products with a meat thickness of more than 4mm:
1) Theoretical cooling time=S(1+2S)...... Mold temperature is below 60 degrees.
2) Theoretical cooling time=1.3S(1+2S)……. Mold 60 degrees or more [S represents maximum thickness of molded product].

Correction of plasticization parameters

(1) Confirm whether back pressure needs to be adjusted;
(2) Adjust screw speed to make metering time slightly shorter than cooling time;
(3) To confirm whether measurement time is stable, try to adjust temperature gradient of heating ring.
(4) Confirm whether there is dripping in nozzle, whether pig tails or sticky molds occur in main runner, whether finished product has air marks, etc., and adjust nozzle temperature or loosening distance appropriately.

Gud Mould Industry Co., Ltd