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Common blow molding problems and countermeasures
1
Blow Molding Overview

Blow molding: It is a molding method for manufacturing hollow plastic products. It is a secondary molding technology that uses a gas pressure to inflate a parison that is still in a semi-molten state in a cavity into a hollow product.

Hollow blow molding is divided into injection blow molding, extrusion blow molding and stretch blow molding.

Injection blow molding uses injection molding to first make a plastic with a bottomed parison, and then moves the parison into a blow mold for blow molding. During the extrusion blow molding process, the tube blank is directly extruded by the extruder and hangs in a pre-separated cavity installed directly below the head; when the sagging parison reaches the specified length, the mold is closed immediately, and The incision of the mold cuts the tube blank; the compressed air is passed through the small holes on the parting surface of the mold, so that the parison is inflated against the mold wall and formed; maintaining the pressure, after the product is cooled and shaped in the cavity, the mold is opened to remove the product

A
Different classification according to tube forming method

(1) Extrusion-blow molding: simple production method, high output, low accuracy, and more applications.

(2) Injection-blow molding: high precision, good quality, high price, suitable for large-scale products.

B
Classified according to different molding processes

(1) ordinary blow molding

(2) Stretch blow molding: The product is stretched, with high strength and good air tightness.

a. Extrusion-stretching-blow molding (referred to as extrusion-stretching-blowing)

b. Injection-stretching-blow molding (referred to as note-pull-blow)

C
Different classification according to the number of tube blank layers

(1) Single-layer blow molding

(2) Multi-layer blow molding: good comprehensive performance, complex production, suitable for product packaging with high packaging requirements

Blow molding schematic

Extrusion blow molding

Injection blow molding

Stretch blow molding

2
Analysis of common problems
unusual phenomenon cause Solution
bubble Wet pellets
2. Air is mixed in from the hopper
3. Resin is too hot or has a long residence time
Fully dried raw materials
2. Increase the barrel pressure
a. Increase screw speed
b. Add filter plate screen
3. Check barrel and head temperature
Charred Resin is too hot or has a long residence time Check barrel, head temperature and thermocouple
Severe sag Resin temperature is too high
2. The parison extrusion speed is too slow
3. The mold closing speed is too slow
4. Raw materials absorb moisture
Resin temperature is too high
2. The parison extrusion speed is too slow
3. The mold closing speed is too slow
4. Raw materials absorb moisture
Parison blur Resin temperature is too low
2. Extrusion speed is too fast
3. Two raw materials are mixed
Increase the head and die temperature
2. Reduce extrusion speed
3. Changing material, cleaning barrel and machine head
Uneven wall thickness of the finished product in the circumferential direction 1. The center of the machine head is wrong, and the resin pressure in the machine head is different.
2. Uneven heating of the machine head
3. The center of the machine head is not consistent with the center of the molding die
4. Machine head or mold shaft is not vertical
1. Adjust the head clearance based on the wall thickness distribution
2. Check heater position and function
3. Align the center of the machine head with the center of the mold
4. Calibrate the head or mold to vertical
The wall thickness of the finished product is too thin compared to the die gap Parison sag elongation
2. The resin temperature is too high
3. The inflation ratio is too large
Increase extrusion temperature
2. Reduce resin temperature
3. Reduce the inflation ratio appropriately
The finished product is deformed in the mold Blow molding time is too short
2. Intake time is too slow
Extended blow molding time
2. Blow up immediately after closing the mold
Finished product blasting during mold opening The air pressure in the finished product is not eliminated Open the mold after eliminating the air pressure in the finished product
The cut edge of the finished product has dents 1. The flash angle of the cutting edge is not suitable
2. The mold closing speed is too fast
1. Correct the flash angle of the cutting edge
2. Slow the closing speed appropriately
Parison curling
a. inward
b. outward
a. Die temperature is too high
b. mold core temperature is too high
a. Die temperature is too high
b. mold core temperature is too high
Parison bending 1. The runner in the head is not suitable
2. The center of the nose is wrong
3. Uneven heating of the machine head
4. Extrusion speed is too fast
Check the head runner
2. Adjust the center of the machine head
3. Adjust the head heating temperature distribution
4. Reduce extrusion speed
Fracture during parison inflation Inflation ratio is too large
Eccentricity of parison
3. The parison extrusion speed is too slow
4. Blow molding is too slow after clamping
5. There are scars on the parison
6. Mixed with other raw materials or impurities
Use smaller inflation ratio
2. Adjust the head clearance
3. Increase extrusion speed
4. Blow up immediately after clamping
5. Check the machine head and shunt shuttle
6. Changing materials and cleaning
The cut edges are not completely fused Parison temperature is too low
2. The cutting edge is too blunt
1. Speed up mold closing and increase resin temperature
2. Correction of cutting edge width
The trimmed part is difficult to remove from the finished product Cutting edge is too wide
2. The cutting edge is not flat
3. Insufficient clamping pressure
Correction of cutting edge width
2. Check and correct the cutting edge
3. Increase clamping pressure
The parting line on the finished product is obvious Insufficient clamping pressure
2. Mold plane is not flat
Increase clamping pressure
2. Check and correct
Unsmoothing of finished product Tendon without slope
2. The bottom groove is too deep
Add slope to the tendon
2. The groove is as small as possible
Certain vertical streaks occur outside the weld line There are scars or impurities in the die Check the die runner

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