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KEPAMID, MAXMID, KEPEX

Injection molding guide
Equipment

Injection molding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. It is important to understand the process of injection molding to obtain high quality products. One cycle of injection molding process is simple. Material is dried at recommended drying condition, first, and then it is fed into a heated barrel, mixed and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. More specific guidelines are :

  1. 1. Non-return valve or check ring should be regularly checked to achieve holding pressure and proper amount of cushion, otherwise some troubles such as sink mark, variation of weight and dimension of the molded parts can occur.
  2. 2. Open nozzle is recommended for individual band heater on the cylinder in order for separate heat control.It should be noted that it is very dangerous if the nozzle cools and the melted material hardens because the solid material blocks the nozzle, which causes pressure increase inside of the cylinder.
  3. 3. The compression zone in screw is recommended in the range of 25~30 %. If compression zone is too small, the melted material can be decomposed due to excessive shear heating.
Injection molding

In mold fabrication, it is essential to previously review the dimensional precision, flow characteristics of the raw material, consistency of product, cost-effectiveness, etc.

4-2-1. Pre-drying Pre-drying of PA6 and PA66 is very important because they are hydrophilic material, which can easily absorb water and make a surface problem on the molded article. Sometimes injection molding process for these materials is difficult due to excessive drooling problem. In addition, even though PBT and PET belonging to polyester group rarely absorb water, they can be degraded while processing due to hydrolysis reaction. When hydrolysis reaction occurs, it can cause deterioration of the material’s tensile and impact strength and surface problem such as flash or silver streak on the molded article.To minimize the problem caused by water absorption, water content of the material’s pellet should be controlled below 0.02 %. Especially, because PET can absorb water faster than PBT, it should be dried thoroughly for good quality of molded article.

4-2-2. Melt temperature The melt temperature is generally higher than the set cylinder temperature due to shear heating of the screw rotation. The recommended cylinder temperatures are as follows.

Melt temperature
Polymer GRADE H4 H3 H2 H1
PA6 Unfilled 240 240 240 230
Impact 250 250 250 240
Glass fiber 260 250 250 250
Mineral filler 250 240 245 230
PA66 Unfilled 275 275 275 270
Impact 285 280 280 270
Glass fiber 290 285 285 280
Mineral filler 290 285 285 280
Unfilled FR* 275 265 270 260
GF, FR* 300 290 285 280
PBT Unfilled 250 240 240 230
Unfilled FR* 245 235 245 230
Glass fiber 250 250 240 230
GF, FR* 250 240 245 230
PET Glass fiber 280 280 270 260
GF, FR* 280 275 270 260

4-2-3. Injection pressure Injection pressure should be set high enough to achieve a high injection speed that may not be lowered be a low injection pressure. Appropriate injection pressure range are 800 ~ 2000 bar.

4-2-4. Mold temperature The mold temperature is one of the most important parameters for injection molding of crystalline polymer in particular. Mold temperature may widely be set up at 60 ~ 120°C, and a general recommendation is 70 ~ 90°C for general purpose of KEPAMID® and KEPEX®. If surface finish is important or the service temperature of finished part is expected to be high, higher mold temperature is recommended. To obtain a good quality product, the mold temperature must be consistently maintained so that the temperature distribution in the mold may be achieved uniformly.

4-2-5. Injection speed The injection speed should be determined by part geometry, gate size and location, surface feature, flow characteristics and mold temperature etc. In general, injection speed is set at high where there are flow marks, record marks and sink marks, on the other hand low injection rate is good to prevent jetting, flush, burn marks or gate smear, generated by high shear force against cavity wall.

4-2-6. Hold pressure Hold pressure plays a key role to make parts optimized in not only dimension but also in mechanical & physical properties. Because in hold stage (hold/pack), remaining melted for about 1~5 % of a cavity is forced to fill into the cavity to compensate for the volume contraction during cooling. The hold (pack) time must be set to slightly exceed the gate seal time (normally ½ to 1 sec) at which a gate is completely solidified so that a constant product may be obtained. As shown in Figure 4, the weight of a molded part increases upon the hold pressure time and then stops at a certain point.
At this time the gate of the part is solidified entirely and no more material can be incorporated. Finally part weight shows constant after the gate seal time. It is recommended that the hold pressure time be maintained until the gate seal is completed. Because the gate sealing time changes mostly upon the shape of cross-section and mold temperature, a proper hold pressure time must be determined such that the weight and dimension of a molded product are within a certain range. By setting optimum hold pressure, molded parts product with consistent dimensions can be produced. As a rule of thumb, the hold time can be simply calculated wall thickness (mm) times 8.
The hold pressure must be set in consideration of dimensional requirements. As a rule, hold pressure amounts to between 60-90 % of the injection pressure.

4-2-7. Plasticizing Because fast screw speed can make material decompose by high shear force, the reciprocating speed is preferably set as low as possible unless it does not affect cycle time. Generally, 80 ~ 120 screw rpm is recommended. But screw speed is dependent on the screw diameters, screw speed should be decreased with larger screw diameter.
A back pressure of 50 ~ 100 bar (hydraulic pressure usually 5 ~10 bar) is generally appropriate. However, to increase the efficiency of the dispersion of a color master-batch (color concentrates) or pigment, higher mixing by increasing back pressure may be required.
In addition high back pressure may be used to eliminate un-melted particles. In the case of glass fiber reinforced grades, high back pressure, proportional to rotational speed leads to breakage of the glass fiber, resulting in deterioration of mechanical strength. More importantly excessive back pressure gives rise to lower output along with longer cycle time. Therefore it should be taken into consideration in optimizing the back pressure.

4-2-8. Cooling Total cooling time is determined as sum of “hold pressure time + screw retraction time + a shot safety margin”.
Once material is solidified entirely, no additional cooling time is needed. Most of the time affecting the cooling time is the hold time. Therefore, if supposedly a hold pressure time is set appropriately, only screw retraction time needs to be taken into account.

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