Notes on UHMW Plastic Injection Molding Processing

Ultra-high molecular weight polyethylene (UHMW-PE) usually has a molecular weight of more than 1.5 million, with excellent wear resistance, impact resistance, self-lubrication and corrosion resistance. It is widely used in mechanical wear-resistant sliders, conveying guide rails, mining lining plates, special-shaped wear-resistant brackets and other parts. However, it is completely different from ordinary PE, ABS and PP materials, featuring extremely poor melt fluidity, narrow melting range, large shrinkage rate and easy internal stress deformation after cooling. It cannot be injected according to conventional plastic processes. Special requirements are required from raw material drying, machine parameters, mold structure, molding process to post-treatment. Improper operation will easily cause insufficient filling, shrinkage warpage, delamination cracking and surface powdering defects.

1. Raw Material Pre-treatment Notes(1) Drying and Dehumidification Control

UHMW itself has low water absorption, but it is easy to absorb moisture in the air during storage. Injection molding with moisture will cause surface silver lines, bubbles and delamination. Constant temperature drying must be adopted before processing with low temperature and long-time drying mode to prevent premature softening and agglomeration of materials. High-temperature rapid drying is strictly prohibited to avoid local degradation of materials and performance attenuation. Keep the hopper closed during processing to reduce moisture absorption of exposed raw materials.

(2) Raw Material Purity and Recycled Material Ratio

UHMW is very sensitive to impurities. Mixed other plastics and dust will directly cause brittle cracking of products and decline in wear resistance. The barrel and hopper must be thoroughly cleaned to avoid material mixing. The proportion of recycled materials should not be too high. Multiple recycling will damage the molecular chain structure, greatly reducing impact strength and wear resistance. Precision wear-resistant parts should adopt all-new raw materials as much as possible.

2. Injection Molding Machine and Screw Adaptation Requirements(1) Special Screw SelectionOrdinary standard screws are not suitable for UHMW. Excessive compression ratio will easily cause shear overheating, material degradation and whitening and delamination of products. Special polyethylene screws with low compression ratio and moderate length-diameter ratio must be selected, with deep screw groove and gentle shearing to reduce shear heat generation of melt and ensure stable melting of materials without decomposition.

(2) Zonal Control of Barrel TemperatureThe melting temperature range of UHMW is narrow. Too low temperature leads to poor melt fluidity and difficult mold filling; too high temperature easily causes thermal degradation, volatile substances and surface powdering of products. The barrel adopts the zoning temperature control principle of slightly higher middle section and lower front and rear sections. The nozzle temperature should not be too high to prevent drooling and material leakage, and avoid carbon deposition and material blockage at the nozzle.

(3) Matching of Injection Pressure and SpeedPoor fluidity is the biggest feature of UHMW. High injection pressure and medium-low speed filling mode must be adopted. Excessively fast injection speed will generate strong shear heat, causing melt delamination, large internal stress and later cracking; too slow speed will lead to premature condensation, insufficient flow and material shortage shrinkage. Maintain stable and uniform filling throughout the process, avoiding sudden speed changes.

3. Key Points of Mold Structure Design(1) Runner and Gate DesignThe runner should be thickened with smooth transition to reduce corner resistance and avoid melt retention and cooling. Prefer side gate and fan gate for the gate, increase the cross-sectional size of the gate. Small point gate and submarine gate cannot be used, which are easy to freeze and block materials with insufficient filling. The feeding position is arranged at the thick wall center of the product as much as possible to allow the melt to flow naturally from thick to thin, reducing shrinkage and depression.

(2) Demolding Draft and Ejection StructureUHMW has large friction coefficient and high shrinkage rate with strong mold clamping force. The demolding draft must be larger than ordinary PE, and the draft of side walls and rib positions should be enlarged to prevent scuffing and mold sticking. Large-area ejection such as top plate and top block should be adopted, and single-point ejector pins should be minimized to avoid local whitening, penetration and product deformation.

(3) Cooling and Exhaust LayoutUHMW has large cooling shrinkage. The mold cooling water channel should be evenly arranged and close to the cavity to ensure synchronous overall cooling and reduce warpage and distortion. Sufficient exhaust grooves must be set at the end of melt filling, dead corners and deep rib positions, otherwise it will easily cause air trapping burning, obvious weld lines and local material shortage.

4. Injection Molding Process Control Points(1) Back Pressure and Material Storage ControlThe back pressure should not be too high during processing. Excessive strong shear will damage the molecular chain and make the material brittle; too low back pressure will cause uneven mixing and inconsistent product density. Adopt low back pressure and slow material storage to soften and plasticize materials gently and ensure uniform and stable melt.

(2) Holding Pressure and Cooling Time SettingThe molding shrinkage of UHMW is much larger than ordinary plastics. Sufficient holding pressure and holding time must be set to replenish shrinkage in time and prevent surface shrinkage and internal voids. The cooling time should be appropriately extended. Forced demolding is not allowed, otherwise the internal stress of the product is not released, and bending and twisting deformation will occur soon after demolding.

(3) Prevention of Internal Stress and Delamination

Follow the principles of low shear, stable flow rate and balanced cooling throughout the process to avoid excessive fluctuation of process parameters. Once surface peeling and delamination occur, most of them are caused by excessive shear heat or mismatched temperature. Immediately reduce injection speed, slightly adjust barrel temperature, and check whether the screw shear is too strong.

5. Post-molding Treatment and Storage Requirements

(1) Shaping to Prevent DeformationUHMW products have high temperature and soft texture just after demolding, which are easy to bend and twist automatically. They should be placed on shaping fixtures for natural cooling and shaping, and random stacking and extrusion are not allowed to avoid permanent deformation.

(2) Aging Stress ReliefFor parts with high precision requirements such as long wear-resistant guide rails, normal temperature aging treatment is required after molding to release internal molding stress and prevent deformation and cracking during later use.

(3) Subsequent Cutting Processing NotesWhen subsequent turning, milling and drilling of UHMW products are carried out, the cutter should be sharp with moderate cutting speed to avoid high-temperature friction melting and tool sticking, ensuring flat cutting surface without burrs.

ConclusionThe core difficulties of UHMW injection molding are poor fluidity, narrow melting range, large shrinkage, easy stress deformation and shear degradation. In actual production, it is necessary to control raw material drying and purity, match special low-shear screws, reasonably set barrel temperature and injection parameters. The mold should be designed with large runner and large gate, enlarged demolding draft, uniform cooling and sufficient exhaust. Combined with reasonable holding pressure cooling and demolding shaping, it can solve common defects such as insufficient mold filling, shrinkage warpage, delamination cracking and surface powdering, and stably produce UHMW injection molded products with high strength and wear resistance.