To all our experienced injection molding machine adjusters, you likely share a common frustration in your daily work. I'm certain that 90% of machine adjusters in industry are plagued by same production problem every day: frequent occurrence of shrinkage defects in thick-walled areas of plastic products.

 

Those in injection molding industry are all too familiar with this dilemma; almost every workshop is caught in this vicious cycle. After molding, noticeable shrinkage in thick-walled areas leads to strict customer quality control. If product fails inspection, it's immediately returned for rework. To rectify shrinkage problem, our first instinct is to increase holding pressure. However, as soon as pressure is increased, before shrinkage is completely eliminated, flash appears in large quantities at product edges, parting line becomes wide and rough, and mold frequently suffers from impact damage.
High production costs cause significant headaches for factory owners, who repeatedly urge rectification and issue harsh criticisms. In desperation, we had no choice but to adjust holding pressure back to original value. The moment pressure dropped, shrinkage marks in thick-walled sections deepened and worsened, causing product defect rate to skyrocket. Days were spent repeatedly adjusting machine, overtime rework became norm. We had to meet both customer quality requirements and factory performance targets, leaving us exhausted and frustrated.
Having worked in industry for many years, I've seen countless experienced machine operators with over a decade of experience, possessing rich knowledge and excellent skills, yet they were stuck on conflict between shrinkage marks and flash, unable to find a proper solution. To completely resolve issue, we tried replacing screw, completely changing production process, even proactively communicating with mold makers to frequently repair and modify mold, consuming a great deal of manpower, resources, and money. However, final results were unsatisfactory, and core problem continued to recur.
Despite investing significant effort and resources, desired results remain elusive. Frustration and anxiety after repeated adjustments only exacerbate problem. This feeling of despair is deeply relatable to every frontline machine operator.
Today, we'll delve into this common industry issue and share a practical, hands-on method. This method requires no changes to mold structure, no replacement of raw materials, and no additional costs. It simply involves changing outdated pressure-holding debugging mindset used for over a decade and adjusting underlying pressure-holding logic. This can easily reduce product shrinkage defect rate from 8% to 1.5%.
This debugging solution not only perfectly solves shrinkage problem but also effectively reduces daily mold wear, lowers internal residual stress in product, and comprehensively improves overall product yield. Content is simple and easy to understand; it can be mastered in just a few minutes. Apply it directly to your machine next day for immediate results and easily overcome debugging difficulties.
For a long time, vast majority of machine operators have been unable to optimize holding pressure parameters, frequently encountering dilemma of shrinkage marks and flash. Ultimately, there is only one core reason: from very beginning, we have fallen into a misconception regarding holding pressure adjustment, using wrong holding pressure operation logic.
I believe that 99% of machine operators, upon discovering shrinkage marks in product, instinctively assume that holding pressure is insufficient. Their solution is to simply increase holding pressure, adopting a constant high-pressure mode, continuously holding pressure until gate has completely cooled and solidified. Little do they know that this is the biggest misconception in injection molding holding pressure adjustment, and root cause of countless adjustment failures.
We all know that core function of holding pressure settings is to compensate for shrinkage gaps caused by cooling of plastic, thus compensating for product shrinkage. Crystalline plastic raw materials such as PP, PA, and POM, which are most frequently used in daily production, have very distinct material characteristics: rapid cooling and solidification, a relatively large overall shrinkage rate, and significant volume changes during molding.
However, constant high-pressure holding method completely violates physical laws of natural cooling and molding of crystalline plastics. In the early stages of production, high pressure can indeed temporarily replenish a small amount of molten material and alleviate minor shrinkage marks. However, in mid-to-late stages of molding, plastic raw material gradually begins to crystallize and shrink. Continuing to apply high pressure at this stage not only fails to replenish mold cavity but also forces excess molten material into mold's tiny gaps, directly causing large-area flash.
Prolonged application of constant high pressure will also accumulate a large amount of internal stress within product. This stress is not directly visible to naked eye but can lead to a series of after-sales problems such as distortion, cracking, and breakage during later storage and use.
To put it simply, our debugging failures are not due to insufficient holding pressure, but rather to improper pressure application location, inaccurate pressure timing, and incorrect debugging methods, resulting in an imbalance between shrinkage marks and flash.
To fundamentally solve problem, it is essential to master scientific and standardized holding pressure methods. Core idea of correct holding pressure is to use a three-pronged debugging approach: gradient pressure combined with segmented duration, and gate solidification calibration. These three elements work together to form a complete debugging system.
This debugging method has been verified in practice by several reputable factories. One factory, specializing in automotive interior parts, completely reversed its production difficulties using this solution. Previously, this factory used a constant 90 MPa high-pressure holding mode, resulting in severe shrinkage marks on thick-walled products. The overall defect rate was as high as 8%, monthly losses due to product returns and customer compensation amounted to hundreds of thousands of yuan, severely impacting factory's profits.
Since fully adopting gradient holding pressure debugging method, factory's product shrinkage mark rate has dropped to 1.5%, product quality has significantly improved, mold wear has slowed down, overall mold lifespan has increased by 30%, production costs have been significantly reduced, and factory's overall efficiency has steadily increased.

 

Traditional debugging methods tend to maintain a uniform pressure throughout process, ignoring pressure loss differences in different parts of mold cavity. In reality, different areas inside mold have significantly different holding pressure requirements. Uniform pressure debugging inevitably leads to situations where some areas pass test while others fail.
Near gate, molten plastic flows smoothly over a short distance with minimal pressure loss. If holding pressure here is set too high, excess pressure can easily cause flash and burrs to frequently appear. Conversely, further away from gate, molten plastic travels a longer distance, resulting in greater pressure loss along way. To achieve adequate shrinkage compensation, pressure setting needs to be adjusted appropriately to effectively eliminate shrinkage marks.
Industry-standard parameters are simple and easy to remember, and can be directly applied to all crystalline plastics. Near gate, holding pressure should be set to 85% of the overall injection pressure; at further end of product, holding pressure should be set to 65% of injection pressure.
For example, if the overall injection pressure is set to 100 MPa, then holding pressure near gate should be adjusted to 85 MPa, and at further end of product, to 65 MPa. Adjusting pressure according to this gradient perfectly balances molding state at both ends, fundamentally solving industry-wide problems of burrs near gate and shrinkage marks at further end of product.

A reasonable pressure gradient is fundamental, and precisely dividing holding time into stages is equally crucial to quality of finished product. A single fixed duration cannot adapt to cooling rhythm of plastic; it must be controlled segmented according to molding sequence, following the overall principle of high pressure for shrinkage compensation, medium pressure for stabilization, and low pressure for release.
The first stage is high-pressure shrinkage compensation period. At this time, plastic raw material has not yet completely cooled and solidified, and its fluidity is relatively high. High pressure is used to quickly fill thick-walled shrinkage cavities, filling gaps immediately and completely preventing shrinkage marks. The second stage sets a medium pressure for stable holding, allowing plastic raw material to cool at a uniform rate and smoothly complete crystallization process, ensuring a neat appearance and compact structure of product. The third stage switches to low pressure mode to slowly release accumulated stress inside product, avoiding potential quality problems such as deformation and cracking later on.
Using a 20mm thick PP material product as a reference standard, the overall total pressure holding time is set at 10 seconds. This time is specifically allocated as follows: 3 seconds for high-pressure compensation, 6 seconds for medium-pressure setting, 1 second for low-pressure unloading. During debugging, this can be flexibly adjusted according to actual thickness of product. For thicker products, medium-pressure holding time should be appropriately extended to ensure sufficient cooling and setting; for thinner products, high-pressure and medium-pressure times should be shortened to avoid excessive pressure and defects.

This step is the most crucial and essential part of the entire pressure holding debugging method, and it is also part most easily overlooked by most machine operators. In daily work, over 90% of operators set pressure holding time based entirely on personal experience and subjective judgment, arbitrarily entering values without any scientific basis.
If holding pressure time is set too short, gate will solidify prematurely, making it impossible to complete subsequent shrinkage compensation process, and shrinkage mark problem will still exist. If holding pressure time is set too long, it will not only delay production cycle and reduce workshop production efficiency, but also continuously generate flash due to prolonged pressure, increasing mold wear.
Weighing measurement method is currently the most accurate calibration method. Operation process is simple and anyone can operate it. During debugging, proceed gradually, increasing holding pressure time by 1 second each time. After each adjustment, a sample of finished product is weighed and recorded. Repeat test and comparison until product weight no longer changes. Recorded time at this point is standard time for gate to completely cool and solidify, and it is also optimal total holding pressure time for this product.
Automotive interior manufacturing plant mentioned earlier relied on repeated testing using weighing method, ultimately determining that product gate completely solidifies in 8 seconds. They directly fixed the total holding pressure time at 8 seconds, which is just right length to complete full shrinkage compensation process without wasting production time—a win-win situation.
In conclusion, all colleagues must remember that key to successful pressure holding process debugging is not who sets the highest pressure value, but who possesses a scientifically sound debugging mindset. Maintaining constant high pressure throughout the entire process is an outdated and incompatible method, no longer meeting today's high-quality production requirements.
For widely used crystalline plastics like PP, PA, and POM, a gradient pressure holding method, from high to low, is ultimate solution to completely resolve conflict between shrinkage marks and flash, easily balancing product appearance quality and production costs.
Finally, here's a simple practical exercise for all industry colleagues: Before leaving get off work, take a product with shrinkage defects that you are currently debugging and use a weighing method to accurately measure standard solidification time of gate. Record this information carefully. The first thing to do next day is to switch from long-used constant pressure holding method to a gradient pressure debugging method, and fine-tune various parameters in stages based on calculated solidification time.