Cold Slug Prevention Structure of Sprue for PLA Biodegradable Plastic Molds
PLA biodegradable plastics feature narrow melting temperature ranges from 155℃ to 175℃ and poor thermal stability. At the nozzle and main sprue sections of molds, melt PLA tends to cool down rapidly and solidify into hard cold slugs. Once cold slugs flow into mold cavities along with molten plastic, they will trigger multiple defects including reduced welding strength, air bubbles, silver streaks and brittle fracture on finished products. Blocked gates caused by solid slugs further lead to material shortage and abnormal injection pressure during molding. Unlike general-purpose plastics such as ABS and PP, PLA solidifies rapidly under low temperatures and forms rigid, fragile solids after cooling. Conventional cold slug wells cannot fully trap cold slugs, which easily break through retention structures and enter runner systems. Therefore, targeted sprue structures integrating heat preservation, heat storage, cold slug interception and flow diversion must be designed to stop cold slugs from entering product cavities and accommodate the mass injection molding of PLA biodegradable materials.
1. Extended Heat-storing Cold Slug Well for Main Sprue
Traditional spherical cold slug wells have limited depth and fail to contain PLA cold slugs effectively. The extended heat-storing cold slug well extends the depth of the cold trap at the bottom of the main sprue to 8mm–12mm, with a diameter 1.5 times that of the large end of the main sprue, constructed as an inverted cylinder paired with a spherical groove of 2mm radius at the bottom. During injection, low-temperature cold material ejected at the initial stage sinks into the extended cold slug well, and subsequently injected high-temperature molten PLA wraps the cold slug with its own heat to slow down solidification and prevent cold slugs from being carried forward by melt flow. A draft angle of 0.5° is machined on the side wall of the cold slug well, so cold slugs can be pulled out together with sprue condensate smoothly during mold opening without fragment residues blocking runners. For thin-walled PLA disposable tableware and degradable packaging film molds, a 2mm miniature ejector pin is installed at the bottom of the cold slug well to eject solid cold agglomerates instantly upon mold opening and avoid residual broken cold slugs causing continuous molding defects. This structure is mounted on the sprue bushing of the fixed mold without modifying cavity structures, featuring low reconstruction cost and compatibility with most small and medium-sized PLA injection molds.
2. Heated Sprue Bushing Insulation Structure to Eliminate Cold Slugs at Source
Rapid heat loss at the contact surface between the injection machine nozzle and mold sprue bushing is the primary cause of cold slug formation. A surrounding electric heating coil and thermocouple are installed on the outer wall of the sprue bushing to build a constant-temperature thermal insulation bushing, stabilizing the bushing temperature between 160℃ and 165℃ consistent with PLA melting temperature. Temperature differences at the joint are eliminated to prevent molten resin from cooling and solidifying locally. The heating coil is fully wrapped around the bushing outer wall, covered with aluminum silicate thermal insulation cotton externally to avoid heat transfer to the fixed mold plate and subsequent thermal deformation of the plate. The inner wall of the bushing is polished to Ra0.8 to reduce adhesion of PLA melt and prevent retained resin from degrading and carbonizing after long-term heating. The nozzle and bushing adopt spherical fitting surfaces treated with nitriding wear-resistant layers to avoid air intrusion through gaps and local cold zones. The combination of heated bushings and cold slug wells prevents cold slug generation in the main sprue and captures occasional cold slugs with cold traps simultaneously, balancing cold slug prevention and PLA thermal degradation inhibition, which is widely applied in molds for PLA tableware and medical degradable consumables requiring high appearance and mechanical strength.
3. Branch Cold Slug Retention Grooves at Terminals of Sub-runners
For multi-cavity PLA molds, independent miniature cold retention grooves are arranged at the terminal of each sub-runner, with a length of 5mm, width equal to the sub-runner width and depth of 3mm. Low-temperature cold material flowing along sub-runners accumulates in retention grooves first at the initial injection stage, while high-temperature melt flows into cavities to form qualified products afterwards, realizing separation of cold and hot melt. A thin partition rib of 0.1mm thickness is set between the retention groove and cavity side; high-temperature melt can break through the thin rib smoothly to enter cavities, while solid cold slugs are blocked inside retention grooves permanently. All retention grooves are arranged in waste runner areas and removed together with runner condensate after molding without leaving marks on finished products. During layout of multi-cavity molds, retention grooves face the mold center uniformly to guarantee identical feeding temperature of each cavity and avoid inconsistent product quality caused by cold slugs entering partial cavities. This structure is commonly adopted in multi-cavity molds for PLA disposable cutlery and degradable plastic housings.
4. Flow Limiting Buffering and Auxiliary Thermal Insulation Structures for Sprues
A tapered flow limiting buffer section is arranged at the connection between the main sprue and sub-runners, featuring an enlarged inlet and narrowed outlet to decelerate melt flow, so initial low-temperature cold slugs settle in the buffer zone temporarily while high-temperature melt flows steadily into each gate. A 2mm mica thermal insulation gasket is installed between the fixed mold plate and sprue bushing to block heat absorption by normal-temperature mold plates and temperature drop around the bushing. For point-gate PLA molds where extended cold slug wells cannot be deployed, concave cold storage pits are fabricated on moving mold inserts corresponding to gates to accommodate initial cold slugs, and cold slugs are ejected with runner condensate during mold opening. For high-speed automatic mass-production molds, miniature hot air circulation channels are added near the main sprue to feed 160℃ hot air during shutdown intervals and maintain sprue temperature, preventing PLA solidification inside sprue during frequent production start-stop cycles.
5. Supporting Process and Mold Maintenance Specifications
To sustain stable performance of cold slug prevention structures, matched molding processes and daily maintenance standards must be implemented. The temperature of heated sprue bushings shall not exceed 170℃ to avoid yellowing and embrittlement of PLA induced by long-time high-temperature degradation. When shutdown lasts over 15 minutes, temperatures of the material barrel and heated sprue bushing shall be lowered to 140℃ for heat preservation to prevent material degradation. The morphology of main sprue condensate is inspected every two hours during production; white hard blocks on the fracture surface of condensate indicate decreased interception efficiency of cold slug wells, requiring timely disassembly and cleaning of solid residues in retention grooves. Copper scrapers are used to remove carbonized adhesive layers on inner walls of cold slug wells and retention grooves instead of hard cutting tools, preventing scratches that trap PLA material and form black cold slugs after repeated thermal cycles. All cold slug prevention structures are designed as independent inserts for individual replacement upon severe wear and material accumulation, avoiding full mold disassembly and shortening maintenance downtime.
Conclusion
The cold slug prevention system for PLA mold sprues follows a three-layer protection logic: heat preservation at the source, interception in the middle section and collection at terminals. Heated insulated sprue bushings reduce heat loss at sprue positions to avoid cold slug formation of molten PLA, while extended heat-storing cold slug wells and terminal retention grooves capture generated cold slugs, cooperating with flow limiting buffer structures to block cold slugs flowing into cavities. Different from cold slug structures for ordinary plastics, PLA dedicated structures emphasize thermal insulation design and control temperature ranges strictly to avoid thermal degradation of materials. This integrated system eliminates defects including fracture and silver streaks caused by cold slugs mixed into products and retains intrinsic properties of degradable PLA materials. Proper application of this sprue cold slug prevention structure effectively improves the yield of PLA injection-molded products, supporting continuous mass production of degradable lunch boxes, disposable packaging and medical degradable supplies and reducing waste loss from cold slug-related defects.
