Common problem

  1. 你的位置
  2. HOME
  3. News
  4. Common problem

Precautions for LLDPE Plastic Injection Molding

2026-04-30 11:55:27 Plastic Injection Molding
Linear Low Density Polyethylene (LLDPE) has excellent toughness, low temperature resistance, tear resistance and moderate fluidity, which is widely used in thin-walled products, packaging accessories, containers, plastic shells and daily injection molded parts. LLDPE has linear molecular structure with few branched chains and special crystallization rate. It is prone to molding problems such as shrinkage, warping, mold sticking, flow marks, insufficient filling and flash during injection molding. This paper systematically sorts out the full set of precautions for injection molding processing from five aspects: raw material characteristics, molding temperature, injection process, mold requirements and post-processing.

1. Raw Material Drying and Preliminary Control

LLDPE has extremely low water absorption and does not need long-term drying under normal environment, and can be directly fed for conventional production. However, if the storage environment is humid, stacked and damp or mixed with recycled materials, low-temperature drying treatment is still required, with the temperature controlled at 60～70℃ and the time of 1～2 hours. High-temperature drying is strictly prohibited to prevent material softening, agglomeration and premature degradation.

The mixing proportion of recycled materials should be strictly controlled, recommended not to exceed 20%. Recycled materials need to be screened for impurities and crushed evenly to avoid mixing hard materials and metal scraps causing screw wear and black spots and impurities on products. Do not randomly mix LLDPE of different brands. Excessive difference in melt index will cause uneven fluidity, resulting in flow marks, color difference and decline of mechanical properties. Check the matching degree of raw material melt index and brand before production.

injection mould

2. Key Points of Barrel and Mold Temperature Control

LLDPE has low melting point and large crystallization shrinkage, and the temperature setting must be stable and gradient. The barrel temperature increases sequentially from the rear section to the front section: rear section 160～170℃, middle section 175～185℃, nozzle 180～190℃. Do not set excessively high temperature; exceeding 200℃ is easy to cause decomposition, yellowing and smoke generation; too low temperature will lead to poor plasticization, difficult filling and dull matte surface.

Mold temperature control is particularly critical, conventional mold temperature 30～50℃. Higher mold temperature will increase crystallinity, improve product hardness but increase shrinkage and aggravate warping; too low mold temperature leads to fast cooling, easy to produce internal stress, surface flow lines and obvious weld lines. Thin-walled small parts appropriately increase mold temperature to facilitate mold filling; thick-walled large parts adopt low mold temperature to slow down cooling shrinkage and reduce shrinkage depression.

3. Key Points of Injection Pressure, Speed and Holding Pressure Process

LLDPE melt viscosity is sensitive to shear rate. Excessively fast injection speed is easy to produce jet lines, salivation and surface wave lines; too slow speed leads to insufficient filling and lack of glue. Adopt medium and low speed segmented injection molding: slow speed in the front section to fill the runner, uniform speed in the middle section to fill the cavity, deceleration and glue sealing in the end section to avoid high-speed air entrainment and melt turbulence.

Injection pressure should be moderate not too large. Excessively high pressure is easy to cause overflow flash and increase product internal stress; too low pressure leads to lack of glue and serious shrinkage. Holding pressure follows the principle of low pressure and long holding time. LLDPE has large crystallization shrinkage rate, so the holding time must be sufficient to compensate the shrinkage of thick wall areas and prevent surface depression and shrinkage pits; excessive holding pressure will cause mold sticking, whitening and demolding strain.
Screw back pressure is adjusted to a low level of 0.3～0.5MPa. Excessively high back pressure is easy to cause excessive shear heating and melt decomposition; too low back pressure leads to uneven mixing and product pattern.

4. Mold Structure and Molding Design Precautions

LLDPE has large shrinkage rate and obvious anisotropic shrinkage difference. The mold design must reserve sufficient shrinkage compensation in advance, with the conventional value of 1.5%～2.5%, to avoid small size and loose assembly after molding. The product structure should try to achieve uniform wall thickness, avoid local thick glue accumulation, and reduce glue at rib and column positions to eliminate shrinkage and warping deformation from the source.

Side gate and submarine gate are preferred for the gate. Direct large gate feeding is not recommended, which is easy to cause gate residual marks and jet lines; the gate position avoids the appearance surface and is set at corners or concealed positions. Mold exhaust grooves must be fully opened. LLDPE is easy to entrain air during filling. Poor exhaust will cause scorching, bubbles and weld lines. Exhaust must be done well at parting surface, cavity end and welding position.

injection mould

Appropriately increase demolding draft angle. LLDPE has high toughness, easy mold sticking and strain. The inner and outer wall draft angle is enlarged by 1°～2° compared with ordinary plastics. The cavity and core are fully polished to reduce demolding resistance; the thimble layout is uniform to avoid whitening and deformation caused by single-point ejection. Cooling water channels are arranged as conformal and uniform as possible to balance cooling speed and inhibit warping and distortion.

5. Demolding, Molding Defects and Post-processing Control

LLDPE has strong toughness and easy mold sticking. Demolding agent should be used as little as possible. If necessary, select neutral thin demolding agent. Excessive spraying is prohibited to avoid surface viscosity and oil spray, affecting subsequent printing and bonding. The product will continue to shrink for a period of time after molding. The just demolded products should be placed flat for shaping, not stacked and extruded to prevent bending and deformation caused by natural shrinkage.

Quick treatment of common defects: increase holding pressure time and optimize wall thickness for shrinkage; adjust symmetrical mold temperature front and rear and extend cooling for warping; reduce injection speed and stabilize plasticization for flow marks; lower clamping pressure and polish parting surface for flash; appropriately increase material temperature and injection pressure for lack of glue. In addition, LLDPE products are difficult to print and bond subsequently. No demolding agent residue is allowed on the molding surface. Flame or corona treatment can be used to improve adhesion if necessary.

Summary

The core control of LLDPE injection molding lies in five key points: low-temperature plasticization, medium and low speed injection, low pressure and long holding pressure, reasonable mold temperature and shrinkage allowance. Use its low water absorption characteristics to simplify the drying process, strictly control the temperature range of barrel and mold, avoid high-temperature decomposition and low-temperature poor plasticization; adopt segmented stable filling process to reduce flow marks, air entrainment and internal stress; reserve shrinkage allowance in advance for mold, increase demolding draft angle and improve exhaust and cooling; cooperate with reasonable recycled material control and flat shaping after molding. It can effectively avoid common defects such as shrinkage, warping, mold sticking and flow marks, stabilize product size, appearance and batch yield.

Home
Product
News
Contact