Resin flow and flow mark
- SANKO GOSEI
- Apr 19, 2023
- 2 min read
The flow of resin through the runner and molded product in the injection mold is shown in Figure (1).
As soon as the resin comes in contact with the mold, it cools and forms a solidified layer. If only L1 flows at the tip, only L2 flows at the periphery, so the surface area of the runner is different at the center and periphery. The resin flow is not like pushing out a tocolotene, but rather like a rubber balloon that spreads out infinitely.
As shown in Figure 2), the resin at the tip of the flow, a, spreads to the periphery as a1 to a5 as it progresses.
The surface of the molded product is not oriented because the flow rubs against the wall and stretches in the direction of the flow, but because the volume of a1 and a2 are equal, but their surface areas differ greatly, causing the resin on the surface layer to stretch and become oriented.
The resin at the tip of the cylinder forms the outer wall near the sprue, and the resin at the back of the cylinder (near the tip of the screw) forms the end of the molded product (far from the gate).
Resin at the tip of the screw was mixed at the shortest distance from the hopper, so it has insufficient viscosity and temperature and is prone to flow marks.
When the end of the molded product is filled, the flow velocity and pressure are reduced. The surrounding resin solidifies and contracts, causing the flow to temporarily stall (hesitation), and when the pressure is replenished and the flow resumes, adhesion and slippage are repeated.
The marks are left at right angles to the flow.
This can also be called chattering flow. This type of flow is more likely to occur when the injection volume is excessive compared to the cylinder's capacity, when the molding product has a large spread from the gate, when the mold is made with a low-temperature mold and low-temperature resin, or when the injection speed is slow.
Materials with a large temperature dependence of viscosity are particularly susceptible to flow marks.
The following measures can be taken to avoid these flow marks.
(1) Inject at close to the maximum injection speed or use a larger cylinder. Or change the machine.
(2) Raise the mold temperature.
(3) Raise the temperature of the resin.
In other words, the ability to perform high-speed injection is the key to preventing flow marks.
Translated with www.DeepL.com/Translator (free version)
~Related Article
Comments