A cold slug well is generally provided at the end of main runner. Cold slug well is often equipped with a material pulling mechanism to pull out main runner condensate when mold is opened. Drawing rod form of cold slug well is shown in Figure 1. Figure 1a is a common form, which is also suitable for thermosetting plastics; Figure 1b is used for soft plastics, and is also suitable for hard or thermosetting plastics, but taper is small; Figure 1c is a general application form; Figure 1d is used in stripping mold plate without cold slug well;; Figure 1e is used when stripping mold plate has a cold slug well; Figure 1f is used in stripping mold without cold slug well; Figure 1g is used in demould of thermosetting plastics; Figure 1h is applied to thermosetting plastics with vertical parting.

 

Figure 1 Pull rod form of cold slug well

Exhaust is a problem that cannot be ignored in injection molds, especially development of rapid injection technology has stricter requirements on exhaust of injection molds. Function of exhaust groove mainly has two points: one is to remove air in cavity when molten material is injected; the other is to remove various gases generated during heating process of material. The thinner product, the farther away from gate, the more important role of exhaust groove. In addition, attention should be paid to opening of exhaust groove for small parts or precision parts, because it can avoid surface burns and insufficient injection volume of product, can also eliminate various defects of product and reduce mold pollution.

01 Exhaust method

There are many ways to vent mold, as shown in Figure 2. Figure 2a shows exhaust using gap of parting surface, Figure 2b, c, d, and e show exhaust using gap between movable parts, and Figure 2f shows exhaust with an exhaust groove on parting surface.

 

Figure 2 Several common exhaust methods

Size of exhaust tank should be designed to prevent materials from overflowing into tank while exhausting, and secondly to prevent blockage. Therefore, measured from inner surface of mold cavity to outer edge of cavity, size of exhaust groove is generally 1.5-6 mm long, 0.02-0.05 mm deep, depth of exhaust groove above 6-12 mm should be about 0.25. ~0.4 mm. See Table 1 for depth of exhaust grooves of different plastics.

 

03 Exhaust groove design during thermosetting plastic molding

Exhaust during thermosetting plastic molding is more important than thermoplastics. In the runner in front of gate, width of exhaust groove should be equal to width of runner with a depth of 0.12mm. Around mold cavity, exhaust grooves should be 25mm apart, with a width of 6.5mm, and a depth of 0.075~0.16mm. It depends on fluidity of material. Softer material should take a lower value. Ejector rod should be enlarged as much as possible, and in most cases, 3 to 4 flat surfaces with a height of 0.05mm should be ground on cylindrical surface of ejector rod, and direction of wear marks should be along length of ejector rod. Grinding is carried out with a fine-grained grinding wheel. End face of ejector rod should be ground with a chamfer of 0.12mm, so that if there is a flash formed, it will adhere to product.

(1) Poor melt flow results in annual ring-shaped wave flow marks on the surface of plastic part centered on gate. When low-temperature and high-viscosity melt with poor flow properties is injected into cavity in a semi-solidified and fluctuating state in injection port and runner, melt flows along surface of mold cavity and is squeezed by continuously injected subsequent melt to form backflow and stagnation, thereby generating annual ring-shaped wave flow marks centered on gate on the surface of plastic part.
In view of cause of this failure, temperature of mold and nozzle can be increased, injection rate and filling speed, injection pressure, holding pressure and time can be increased. A heater can also be installed at the gate to increase local temperature of gate. Cross-sectional area of gate and runner can also be appropriately enlarged.
Cross-section of gate and runner is best to be round, which can get the best mold filling. However, if gate is set in weak area of plastic part, a square section should be used. In addition, larger cold slug holes should be set at the bottom of injection port and end of runners. Material temperature has a greater influence on flow performance of melt. The more attention should be paid to size of cold slug hole, position of cold slug hole must be set at the end of melt along flow direction of injection port.
If main cause of annual ring-shaped wave flow marks is poor resin performance, a low-viscosity resin can be used when conditions permit.
(2) Poor flow of melt in flow channel leads to spiral wave flow marks on the surface of plastic part. When melt flows from narrow section of flow channel into cavity of larger section or mold flow channel is narrow and finish is poor, flow material is easy to form turbulence, resulting in formation of spiral wave flow marks on the surface of plastic part.
In this regard, injection speed can be appropriately reduced or injection speed can be controlled in stages by slow, fast, and slow. Gate of mold should be set in thick wall part or directly on wall side. Gate type is best to adopt handle, fan or diaphragm type. It is also possible to appropriately expand runner and gate section to reduce flow resistance of flow material.
In addition, flow of cooling water in mold should be controlled to keep mold at a higher temperature. If temperature of barrel and nozzle is appropriately increased within operating temperature range of process, it is beneficial to improve flow properties of melt.
(3) Volatile gas causes cloud-like wave flow marks on the surface of plastic parts. When using ABS or other copolymer resin raw materials, if processing temperature is high, volatile gas generated by resin and lubricant will cause cloud-like wave flow marks on the surface of plastic part.
In this regard, temperature of mold and barrel should be appropriately reduced, exhaust conditions of mold should be improved, material temperature and filling rate should be reduced, gate section should be appropriately expanded, and lubricant should be replaced or reduced in quantity.