Stack mold, also known as double‑layer or tandem mold, is an efficient production solution that enables simultaneous injection molding in two upper and lower layers within one mold. Its production capacity is nearly twice that of ordinary molds, and it is widely used in mass production of standardized plastic parts. The process parameters of stack molds are quite different from those of ordinary single‑parting surface molds, focusing on balancing filling, holding, cooling and venting of the two layers of cavities, while considering clamping force, shot volume and mold opening stroke. Reasonable parameter setting can not only improve production capacity, but also avoid product defects such as short shots, sink marks, deformation and flash.
Barrel and Nozzle TemperatureStack molds usually have long runners and double‑layer cavities, so the melt flow path is longer. The temperature setting must ensure sufficient fluidity while preventing material degradation. The barrel temperature is 5–10°C higher than that of ordinary molds, adjusted according to materials: 180–220°C for PP and PE, 200–240°C for ABS, 250–280°C for PA66, and 280–310°C for PC. The middle section temperature is slightly higher than the front and rear sections to ensure uniform plasticization, and the rear section temperature is lower to prevent blockage of the feed port. The nozzle temperature is close to the front section temperature of the barrel to avoid drooling and wire drawing. For transparent parts and high‑temperature materials, the upper temperature limit is controlled within the allowable range of the material to prevent delamination, gas lines and yellowing.
Injection ParametersInjection is the most critical parameter for stack molds, with the core goal of simultaneous filling and synchronization of the upper and lower layers. The shot volume is calculated according to the total weight of the two layers of products: total shot volume = single cavity weight × number of cavities × 2 + runner weight, usually 60%–80% of the maximum shot volume of the equipment to ensure stable feeding. The injection pressure is 10%–20% higher than that of ordinary molds to overcome the resistance of long runners and double‑layer cavities and avoid short shots at the end. Multi‑stage injection control is particularly important: low speed for gate filling to stabilize mold filling; medium‑high speed for rapid filling of the main cavity to ensure synchronization of upper and lower layers; low speed for full cavity filling to reduce burrs and gas trapping. The injection speed follows the principle of "fast in the middle and slow at both ends" to prevent jetting and gas trapping burns. If one layer is full and the other is short, prioritize adjusting runner balance and mold venting, then fine‑tune injection speed and pressure.
Holding ParametersThe focus of stack mold holding is to control sink marks, deformation and dimensional consistency, and the two layers of products must be stably held. The holding pressure is 50%–70% of the injection pressure, which should not be too high to avoid stress concentration and difficult mold opening. The holding time is determined according to the product wall thickness: 3–6 seconds for thin‑walled parts, 6–12 seconds for medium‑thick parts, and 12–20 seconds for thick‑walled parts, based on no sink marks and stable product weight. Multi‑stage holding is adopted: higher pressure for shrinkage compensation in the first stage, and gradually reduced pressure in the second stage to reduce internal stress. The holding switch position is 95%–98% of cavity filling to avoid flash caused by overfilling. If the shrinkage of the upper and lower layers is inconsistent, first check mold cooling and runner size, then fine‑tune holding pressure and time.
Cooling Time and TemperatureStack molds have double‑layer cavities and large heat capacity, so sufficient cooling time is required to prevent top whitening and deformation. The cooling time is extended by 10%–30% compared to ordinary molds, based on no deformation of the product when ejected. PP and PE have shorter cooling time, while PC and ABS have longer time. The mold cooling water needs two independent controls, with separate upper and lower layer water circuits to ensure uniform temperature. The inlet water temperature is controlled at 30–50°C, with a temperature difference not exceeding 5°C. Extending cooling time can reduce internal stress and improve dimensional stability, especially for flat parts, gears and housings. Insufficient cooling will lead to inconsistent deformation of upper and lower layers, directly affecting yield.
Clamping Force and Mold Opening ParametersThe projected area of stack molds is twice that of ordinary molds, so the clamping force must be sufficient. The clamping force calculation formula is: total projected area × cavity pressure, where the total projected area includes the upper and lower layers of products and runners. In actual setting, add a 10%–15% safety margin on the basis of the calculated value to avoid mold expansion and flash. The mold opening speed adopts the "slow‑fast‑slow" mode, with sequential opening of the first and second parting surfaces, and stable speed to prevent cracking and whitening. The ejection speed and pressure are moderate, and the stack mold ejection mechanism is long to avoid ejector pin deformation caused by impact. Close the mold at low speed and low pressure to protect the mold parting surface and slide structure.
Back Pressure, Screw Speed and VentingThe back pressure is 5–10 bar higher than that of ordinary molds to improve plasticization uniformity and avoid inconsistent density of double‑layer products. The screw speed is moderate to ensure sufficient plasticization without excessive shear heat. Stack molds have long runners and many cavities, so venting must be smooth. Venting grooves are opened at the parting surface, weld line position and cavity end. Poor venting will lead to burning, short shots and bubbles, which can be solved by cooperating with low‑speed injection at the end.
SummaryThe core of stack mold parameter setting is balance, synchronization and stability: temperature ensures fluidity, injection ensures synchronous filling, holding ensures uniform dimensions, cooling ensures no deformation, and clamping ensures no flash. During debugging, prioritize ensuring consistent filling of the upper and lower layers, then optimize appearance and dimensions, and finally stabilize the cycle. Proficiency in stack mold parameter setting can greatly improve production capacity without adding equipment, making it an efficient solution for mass injection production.
