1. Plastic refining
Definition of plasticization: The phenomenon in which rubber changes from an elastic substance to a plastic substance under the influence of external factors is called plasticization
(1) Purpose of Refining
a. Enable the raw rubber to achieve a certain degree of plasticity, suitable for later stages of mixing and other processes
b. Unify the plasticity of raw rubber and ensure even quality of the rubber material
(2) Determination of plastic compound required: Mooney above 60 (theoretical) Mooney above 90 (actual)
(3) Plastic refining machine:
a. Open mill
Features: High labor intensity, low production efficiency, poor operating conditions, but it is relatively flexible, with low investment, and suitable for situations with many changes The speed ratio of the two drums of the open mill: front to back (1:1.15-1.27)
Operation methods: Thin pass plastic refining method, roll wrapping plastic refining method, climbing frame method, chemical plasticizer method
Operation time: The molding time should not exceed 20 minutes, and the parking time should be 4-8 hours
b. Internal mixer
Features: High production efficiency, easy operation, low labor intensity, and relatively uniform plasticity. However, high temperatures can cause a decrease in the physical and mechanical properties of the rubber material
Operation method: Weighing → Feeding → Plasticizing → Discharging → Reversing → Pressing → Cooling and unloading → Storage
Operation time: 10-15 minutes Parking time: 4-6 hours
(4) Regularly plasticized rubber
Rubber materials that often need to be molded include NR, hard NBR, hard rubber, and those with a Mooney rating of 90 or above
2. Mixing
The definition of mixing is to add various additives to rubber to make a mixed rubber
(1) Open the mixer for mixing
a. Wrapping roller: Wrap the raw rubber on the front roller and have a short preheating process of 3-5 minutes
b. Eating process: Add the additives that need to be added in a certain order. When adding, pay attention to the volume of accumulated glue. Less is difficult to mix, while more will roll and not be easy to mix
Feeding sequence: raw rubber → active agent, processing aid → sulfur → filling, softening agent, dispersant → processing aid → accelerator
c. Refining process: can mix better, faster, and more evenly
Knife method: a. Slant knife method (eight knife method) b. Triangle wrapping method c. Twisting operation method d. Gluing method (walking knife method)
d. The formula for calculating the loading capacity of the open mill is V=0.0065 * D * L, where V – volume D is the diameter of the roller (cm) and L is the length of the roller (cm)
e. Temperature of the roller: 50-60 degrees
f. Mixing time: There is no specific regulation, it depends on the operator’s proficiency
(2) Internal mixer mixing:
a. One stage mixing: After one stage of mixing, the mixing process is as follows: raw rubber → small material → reinforcing agent → softener → rubber discharge → addition of sulfur and accelerator to the tablet press → unloading → cooling and parking
b. Second stage mixing: Mixing in two stages. The first stage is raw rubber → small material → reinforcing agent → softener → rubber discharge → tablet pressing → cooling. The second stage is mother rubber → sulfur and accelerator → tablet pressing → cooling
(3) Common quality issues with mixed rubber
a. Compound agglomeration
The main reasons are: insufficient refining of raw rubber; Excessive roller pitch; Excessive adhesive capacity; Excessive roller temperature; The powdered compound contains coarse particles or clusters;
b. Excessive or insufficient specific gravity or uneven distribution
Reason: Inaccurate weighing of the compounding agent, incorrect mixing, omission, incorrect addition or omission during mixing
c. Spray frost
Mainly due to the excessive use of certain additives, which exceed their solubility in rubber at room temperature. When there is too much white filling, white substances will also be sprayed out, which is called powder spraying
d. Hardness too high, too low, uneven
The reason is that the weighing of vulcanizing agents, accelerators, softeners, reinforcing agents, and raw rubber is not accurate, and it is caused by incorrect or missed addition, resulting in uneven mixing and uneven hardness
e. Burn: The early vulcanization phenomenon of rubber materials
Reason: Improper combination of additives; Improper rubber mixing operation; Improper cooling and parking; Climate impacts, etc
3. Sulfurization
(1) Shortage of materials
a. The air between the mold and rubber cannot be discharged
b. Insufficient weighing
c. Insufficient pressure
d. Poor fluidity of rubber material
e. Excessive mold temperature and burnt rubber material
f. Early scorching of rubber material (dead material)
g. Insufficient material thickness and insufficient flow
(2) Bubbles and pores
a. Insufficient vulcanization
b. Insufficient pressure
c. Impurities or oil stains in the mold or rubber material
d. The vulcanization mold temperature is too high
e. Too little vulcanizing agent added, the vulcanization speed is too slow
(3) Heavy skin and cracking
a. The vulcanization speed is too fast, and the rubber flow is not sufficient
b. Dirty molds or adhesive stains
c. Too much isolation or release agent
d. Insufficient thickness of adhesive material
(4) Product demolding rupture
a. Excessive mold temperature or prolonged sulfur exposure
b. Excessive dosage of vulcanizing agent
c. The demolding method is incorrect
(5) Difficult to process
a. The tear strength of the product is too good (such as high tensile adhesive). This difficult processing is manifested by the inability to tear off the burrs
b. The strength of the product is too poor, manifested as brittle edges, which may tear the product together
Post time: Apr-16-2024
