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The difference between cold runner and hot runner

The cold runner is the part between the mold entrance and the product gate. The plastic is kept flowing in the runner by the injection pressure and its own heat, and the runner is part of the molded material, but not the product. Therefore, when we design the mold, we have to consider both the filling effect and how to save material by shortening and narrowing the runner, which is the ideal situation, but it is difficult to achieve the best of both worlds in practice.
 
The hot runner is a set of elements heated by resistance wire, which heats the molten plastic in the whole or part of the path (runner) between the gate sleeve of the injection mold and the inlet point of the product cavity, maintaining the heat balance and keeping the plastic in a constant molten state during the injection production process. Hot runner systems generally consist of hot nozzles, manifolds, temperature control boxes and accessories. Hot nozzles generally include two types: open nozzles and needle valve nozzles. Because the hot nozzle form directly determines the hot runner system selection and mold manufacturing, and therefore often correspondingly divided into open hot runner system and needle valve hot runner system.

Hot runner is an economical choice for mass-produced plastic parts (e.g., daily necessities, plastic cups) because there is no additional waste plastic generated, so it greatly reduces the material cost of the product, and for products where plastic materials are expensive (e.g., connector inserts used in automobiles). Compared to injection molds that do not use hot runners, it eliminates (or reduces) the waste generated by cold runners during the injection molding process and the extra work involved in recycling.

Advantages of hot runner system.
1. lower material cost, no scrap recycling problem, can save sprue scrap and grinding recycling cost.
2. Shorten filling time, cooling time, mold opening and closing stroke and molding cycle, especially for large molded parts with thin flesh and thick sprue.
3. No need to take out the sprue, it can automatically separate the sprue part (pin point sprue) and save the sprue taking out time.
4. Lower molding temperature and pressure, less stress on the finished product. 5.
5. Fully automatic molding is possible. 6.
6. Less pressure and temperature changes in the sprue, better quality of the finished product, less dents and flow marks.
7. No uneven mixing of raw materials or mixing of foreign materials.
8. Precise control of gate temperature and balanced filling of multiple mold cavities.
 
Disadvantages of hot runner system: 1.
1. Not suitable for plastic materials with fiber.
2. When changing color or plastic, it is inconvenient to change color molding. 3.
3. the cost is more expensive than the cold runner mold. 4.
4. less freedom of gate design than cold runner molds. 5.
5. relatively difficult to maintain and trouble to clean the blockage. 6.
6. difficult to make and process the mold. 7.
7. the mold structure is more complicated. 8.
8. not suitable for molding plastics with small temperature range. 9.
9. The molten resin will be retained in the tube, and there will be decomposition, deterioration and deterioration.
 
With decreasing resources, competition in the market, and significant increases in raw material prices, today's market is becoming more and more inclined to use hot runner systems. Although hot runner systems have many advantages, in order to ensure the right and most efficient injection molding process, it is necessary to consider the size of the product, aesthetic needs, production volume, raw materials and additives for the specific application in order to better select the right runner for the injection mold.