In this deep-dive, we take a musician-first look at what’s really going on with PET (Mylar™) timpani heads: why new heads “settle,” why some heads won’t hold a stable pitch center even when the lugs seem to match, why the bearing-edge crease turns brittle on well-worn heads, and what dimples from hard mallets actually mean. We also tackle the heat-gun question, why it sometimes “works” cosmetically, and why it can backfire acoustically. The goal is practical: fewer mystery tuning problems, faster diagnosis, and better decisions about when to adjust, when to re-seat, and when it’s simply time to replace the head.
Tension History: The Hidden Story Behind Mylar™ Head Behavior
A synthetic timpani head does not behave like a fresh, neutral sheet of plastic every time it is tuned. From the moment it is mounted, the head begins developing a tension history: a record of how it has been stretched, where it has been clamped, how evenly it seated, how hard it has been played, how far it has been cycled through the pedal range, and how much heat or environmental stress it has experienced. That history matters because PET/Mylar™ film is not perfectly “resettable.” It can settle, relax, creep, crease, and accumulate damage in specific areas, especially at the bearing-edge crease and at impact dimples. Once that history is built into the head, it affects everything that follows: false clears (where the head seems to clear at one pitch but cannot hold a stable principal tone), pitch stability, re-mounting success, low-range principal tone, and the decision to adjust, re-seat, or replace the head.
What this article is (and isn’t)
This is not a chemistry lesson. It’s a musician’s guide to the stuff you’ve probably seen:
- a new Mylar head that “takes a few days” to behave
- a head that looks fine but won’t hold a stable pitch center
- the bearing-edge crease that feels brittle when you remove an old head
- a used Mylar head that won’t clear well after it had been re-mounted
- dimples that show up after hard mallets and loud playing
- and the temptation (or advice) to “fix it with heat”
The goal is to connect those experiences to how PET film behaves so your decisions are faster and more reliable: mounting, maintenance, diagnosis, and when to stop chasing and replace.
Navigation: Glossary | References
First: what “Mylar™” actually means
Most synthetic timpani heads are made from biaxially oriented PET film (BOPET). The “oriented” part matters. That orientation is why the film is strong and stable, but it also means the film carries a manufacturing history, and heat or uneven stress can change how it behaves. (1)
Two practical facts to keep in mind:
- PET film has a glass transition temperature (Tg) commonly around 70–80°C. Near this region, polymers become much more “mobile,” and dimensional stability can change quickly. (1)(2)
- Industrial PET films are often heat-set to improve dimensional stability, basically “locking in” their shape and orientation. (2)
This doesn’t mean your head is going to melt at rehearsal or during a performance. It means: heat is not neutral on oriented film, and uneven stress/heat can create uneven behavior.
Also: manufacturers often say PET film does not become brittle “under normal conditions.” (3) Timpanists are often dealing with non-normal conditions: sharp bearing-edge creases, clamping pressure, friction, repeated tuning cycles, and impact damage. That’s exactly where brittleness shows up.
Why new heads “settle” (and why that’s normal)
Players describe it perfectly: new heads settle.
In materials terms, PET is viscoelastic. It behaves partly like a spring and partly like a slow-moving solid. Under tension, it shows:
- creep (it slowly stretches more with time under load), and
- stress relaxation (the tension can reduce over time even if you don’t touch anything). (5)
This is why:
- a head can sound better on day 2 or day 3, and
- your lugs can “feel the same,” yet the drum behaves differently after sitting.
That is also why you cannot simply re-tune to the same lug settings the next day and expect identical results. That is not superstition. That is time-dependent polymer behavior. (5)
Uneven stretch doesn’t reliably “self-correct”
Here’s the hard truth: a head can settle without becoming truly uniform.
If one sector was pulled harder early (uneven lug tension, binding at the bearing edge, off-centering), that part experiences higher stress, and it tends to creep/relax more. Viscoelastic behavior is tension-history dependent: high-stress zones change faster than low-stress zones. (5)
PET film shrink/relaxation is also time and temperature dependent and reflects the relaxation of internal strains from manufacturing. Once a film has shrunk at a given temperature, it generally doesn’t keep shrinking unless that temperature is reached again. (1) There’s even evidence that oriented PET can show measurable shrink/relaxation below Tg over multi-day exposures at warm temperatures. (7)
Musician translation:
A head can become “more stable tomorrow,” but it does not reliably “heal” a mechanically uneven stretch. That’s why you can have a head that:
- seems matched at a few points,
- still shimmers or beats,
- or clears at one pitch and not another.
The big misunderstanding: “soft” vs “brittle”
When timpanists say a worn head is brittle, they mean it tears easily, cracks start easily, and the bearing-edge crease feels fragile when you remove it. That’s not the same as stiffness, it’s mainly a loss of toughness (fracture resistance).
A polymer can behave “looser” for vibration/tension behavior and be brittle when it fails. Those are different properties.
Why the bearing-edge crease becomes brittle (and why it’s usually the worst spot)
The bearing-edge crease is a permanent crease where the film wraps over the bearing edge and is clamped. That zone sees:
- a tight bend radius (curvature),
- constant clamp pressure,
- friction and micro-slip during tuning/seating,
- and repeated stress cycles over the life of the head.
Film/packaging engineering has a blunt term for what repeated bending does: flex cracking, cracks produced by repeated flexing. (4) It’s so common there are standard definitions for “flex crack,” “flex strength,” and “folding endurance.” (4)
So when a worn head is removed and the bearing-edge crease feels “crispy,” that is not imagined: the crease zone has likely accumulated microdamage, lost toughness, and taken on a permanent mechanical “memory” of how it was seated on the drum. The head may still tune, but the bearing-edge crease becomes a common failure line.
This also helps explain why used plastic heads usually do not re-mount successfully. Once a synthetic head has been played in, the bearing-edge crease has already been formed under a specific set of conditions: a particular bowl diameter, bearing-edge shape, collar position, tension history, and pattern of wear. When the head is removed, that crease does not return to a fresh, neutral state. It retains the old set. On re-mounting, the head rarely reseats with the same symmetry and contact pattern, and even small mismatches can produce uneven seating, false clears, unstable pitch, and premature failure at the crease.
In other words, a used plastic head may look reusable, but the most important structural zone has already been permanently shaped and fatigued. For that reason, re-mounting a worn synthetic head is usually unreliable, even if the film still appears visually intact.
Dimples from hard mallets and fortissimo hits: not just cosmetic
Deep dimples are usually not “a dent you can rub out.” They often indicate:
- permanent set (plastic deformation),
- slight thinning,
- microdamage (tiny cracks/crazing).
And that damaged zone can behave differently in vibration, creep faster over time, and become a failure initiation point later. Visually, a shallow dent that disappears when the head is relaxed may be purely elastic (recoverable). A deep dimple that remains visible under zero tension is structurally compromised and will not fully recover.
Heat guns: can they “fix” dimples?
Sometimes they can make dimples look better. But “looks better” is not the same as “behaves better.”
What heating is doing
Heating increases chain mobility. In oriented PET films, that can allow a local dent to relax, but it can also allow orientation to relax and shrinkage stress to develop. (6)
Dimensional stability tends to change sharply near Tg, which is why Tg is a big deal in PET film testing. (2)
Why heat guns are risky in practice
- Hot spots
Heat guns heat unevenly. A single hot spot can create a local region with a different tension history than the surrounding film. - Local shrink / stiffness mismatch
Even a small change in orientation or shrink history can create an uneven stiffness/tension response—which is exactly what pitch stability depends on. - Sweeping a larger area is not automatically safer
Sweeping can reduce a sharp boundary between treated and untreated zones, but it increases the odds you warp a bigger area or drift too close to the collar/bearing-edge crease. - Heating while mounted is the worst version
Mounted heads are constrained at the collar and bearing-edge crease (clamped, stressed). Heating under constraint is how you create unpredictable residual stress and can accelerate crease brittleness. - Insert-ring heads are less forgiving
Insert rings constrain geometry more rigidly. If you cause local shrink or stiffness change, you can end up with a head that never seats the same way again.
Bottom line recommendation: If the dimple is deep enough that you’re reaching for heat, the head is already mechanically compromised. Heat is a high-risk, inconsistent intervention. It may improve appearance while making the head behave worse.
What to do with this information (practical takeaways)
For players
- Treat a new head’s first days as seating + settling, not final tuning. (5)
- Avoid extreme hard-mallet fortissimo on a brand-new head until it has seated and stabilized.
- If dimples appear and pitch stability starts slipping, don’t assume it’s “just you”, it may be structural impact damage.
- Be cautious about re-mounting used plastic heads. A head may look playable, but the bearing-edge crease has already taken a permanent set from its previous mounting.
For teachers
- Teach students this hierarchy: (1) geometry and seating, (2) friction and equalization, (3) then fine tempering.
- Teach them to recognize damage symptoms: persistent shimmer; stable at one pitch but unstable at another; weak principal tone (especially low); bearing-edge crease brittleness at removal.
- Make clear that “the head still fits” does not mean “the head will seat symmetrically.”
For techs and institutional care
- Heads don’t only fail by “age.” They fail by tension history.
- Replacement decisions should be based on pitch stability, seating behavior, and damage symptoms—not only “it still fits the drum.”
- Avoid treating used synthetic heads as interchangeable between drums. Once the bearing-edge crease has formed, it usually reflects one specific bowl, bearing edge, hoop, and tension history. Label heads when moving them between drums.
Conclusion
PET/Mylar heads are reliable when mounted symmetrically and allowed to settle, but they keep a memory of how they were stressed. Uneven early stretching, bearing-edge crease damage, and impact dimples can all create a head that still “tunes,” yet won’t support a stable pitch center. That same tension history also explains why used plastic heads usually do not re-mount successfully: the bearing-edge crease has already been formed, fatigued, and shaped by one specific drum.
Heat-gun dimple removal can sometimes improve appearance, but it often introduces new instability by creating localized shrink and tension histories, especially if done mounted or near the collar or bearing-edge crease. The practical musician’s approach is simple: mount for symmetry, let the head settle, avoid avoidable damage, be skeptical of re-mounting worn synthetic heads, and replace the head when the problem is structural rather than musical.
Glossary of Technical Terms
These brief definitions are tuned for musicians: enough to make the argument clear without turning the page into a materials-science course.
- Mylar™
- Common percussion shorthand for oriented PET film (often BOPET). “Mylar” is also a trademark name used for PET film products.
- PET
- Polyethylene terephthalate, the polymer used in many synthetic drumhead films.
- BOPET
- Biaxially oriented PET film. Orientation improves strength/stability but makes thermal/mechanical behavior history-dependent.
- Glass transition temperature (Tg)
- A temperature range where polymer chain mobility increases markedly; dimensional stability and damping behavior can change quickly near Tg.
- Heat-setting
- A manufacturing step used to stabilize oriented films by “locking in” dimensions/strain history; relates to shrink tendency when reheated.
- Viscoelastic
- Material behavior that is partly elastic (spring-like) and partly time-dependent (slow deformation/relaxation). Explains “settling.”
- Creep
- Gradual increase in deformation under constant load/stress over time (e.g., a head slowly stretching under tension).
- Stress relaxation
- Gradual decrease in stress/tension under constant deformation (e.g., tension dropping slightly over time even if lugs are untouched).
- Dimensional stability
- How well a film maintains its size/shape with time, stress, and temperature changes.
- Tension history
- The timpanist’s term for what polymer scientists call “stress history.” The accumulated record of how a head has been stretched, seated, clamped, tuned through its range, played, heated, and stressed over its lifetime. In everyday terms: every tuning session, every hard hit, every temperature shift leaves a permanent mark on the film. Tension history explains why PET/Mylar™ heads settle, why bearing-edge creases become brittle, why dimples do not fully “heal,” and why used synthetic heads usually do not re-mount reliably.
- Shrinkage stress
- Retractive forces that develop when oriented film is heated and tries to shrink as orientation relaxes; can create uneven tension histories.
- Toughness
- Resistance to crack initiation and crack growth. A head can “tune” yet lose toughness in the bearing-edge crease zone and become brittle at removal.
- Brittleness
- Low toughness; cracks/tears start and propagate easily, often observed at the bearing-edge crease and damaged impact zones of worn heads.
- Flex cracking
- Cracking in a film caused by repeated bending/flexing; common language in film engineering and relevant to the bearing-edge crease region.
- Bearing-edge crease / collar region
- The high-curvature, clamped zone where the head wraps over the bearing edge and is held by the hoop/ring; commonly becomes brittle with wear.
- Insert-ring head
- A synthetic head system with an internal ring structure that constrains seating geometry more rigidly; often less forgiving of local shrink or stiffness change.
- Impact damage
- Localized damage from hard mallets and high dynamics (e.g., dimples, microdamage) that can compromise stability and increase failure risk.
- Dimples
- Permanent set/indentation from strong impacts; often indicates local plastic deformation and/or microdamage that can affect stability and failure risk.
- False clear
- A term from the Duff Clearing Process describing a head that appears to clear at a given pitch but cannot hold a stable principal tone there; a sign of underlying mechanical unevenness or tension history in the head.
References
- Society of Vacuum Coaters. Polyester (PET) Film as a Substrate: a Tutorial. PDF. https://www.svc.org/clientuploads/directory/resource_library/07_692.pdf
- TA Instruments. TA388: Determination of the Dimensional Stability of Oriented PET Thin Films. PDF. https://www.tainstruments.com/pdf/literature/TA388.pdf
- DuPont Teijin Films. MYLAR® A Product Description (General Product Info). Datasheet PDF. https://marianinc.com/wp-content/uploads/2019/08/2019-08-16-DS-S-MylarA.pdf
- Filmquest Group. PET Film Glossary (flex crack, flex strength, folding endurance). https://www.petfilm.com/technical/glossary/
- Penn State MATSE 202. “Polymers as Viscoelastic Materials” (creep and stress relaxation overview). https://courses.ems.psu.edu/matse202/node/717
- Gupta, V. B., et al. “Shrinkage stress in oriented PET.” Polymer 35(12), 1994. PDF. https://cpsm.kpi.ua/polymer/1994/12/2560-2567.pdf
- Johnson, B. M. Relaxation of PET Orientation at Temperatures Below the Glass Transition. OhioLINK ETD, 2013. https://rave.ohiolink.edu/etdc/view?acc_num=toledo1384519910
Mind Map
Summary of the key concepts in this article. 
Click image to view full size.
Test Your Knowledge
Select a question to reveal the answer. Questions invite recall, analysis, application, or evaluation suitable for students and professionals.
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Q1: What specific type of plastic film are most synthetic timpani heads made from?
Answer: Biaxially oriented PET film (BOPET).
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Q2: Term — Glass transition temperature (Tg)
Answer: The temperature range where polymer chain mobility increases and dimensional stability can change rapidly.
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Q3: What is the typical range for the glass transition temperature (Tg) of PET film?
Answer: 70–80°C.
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Q4: What manufacturing process is used to “lock in” the shape and orientation of industrial PET films?
Answer: Heat-setting.
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Q5: In polymer science, the term _____ describes a material that behaves partly like a spring and partly like a slow-moving solid.
Answer: Viscoelastic.
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Q6: Term — Creep
Answer: The process where a material slowly stretches further over time while under a constant load.
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Q7: Term — Stress relaxation
Answer: The gradual reduction of tension over time while a material is held at a constant deformation.
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Q8: Why might a new timpani head require several days to “settle” and stabilize its pitch?
Answer: Because PET is viscoelastic, experiencing time-dependent creep and stress relaxation.
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Q9: How do high-stress zones in a PET head compare to low-stress zones in terms of relaxation speed?
Answer: High-stress zones change and relax faster than low-stress zones.
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Q10: Under what condition will a PET film generally continue to shrink after its initial manufacturing shrinkage?
Answer: Only if the specific temperature that caused the initial shrinkage is reached again.
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Q11: Why is a head that matches at several lug points still capable of “shimmering” or “beating”?
Answer: It may have a mechanically uneven stretch that does not reliably self-correct.
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Q12: In the context of drumhead wear, what does a loss of “toughness” signify?
Answer: A decrease in fracture resistance, making the material more prone to tearing or cracking.
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Q13: What causes the brittle, “crispy” feeling often found at the bearing-edge crease of an old head?
Answer: Accumulated microdamage and loss of toughness from tension history, not necessarily biological age.
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Q14: What is the technical term for cracks produced by the repeated bending of a film?
Answer: Flex cracking.
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Q15: Why is the bearing-edge crease considered a high-risk zone for head failure?
Answer: It experiences tight bend radii, constant clamping pressure, friction, and repeated stress cycles.
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Q16: Besides surface indentation, what do deep dimples from hard mallets typically indicate about the film’s structure?
Answer: Permanent plastic deformation, thinning, and microdamage such as tiny cracks.
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Q17: How does using a heat gun on a dimple affect the polymer chains of the PET film?
Answer: It increases chain mobility, allowing the orientation to relax.
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Q18: What is a primary risk of creating “hot spots” with a heat gun on a timpani head?
Answer: It creates a local region with a different tension history than the rest of the film.
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Q19: Why is it particularly risky to use a heat gun on a head while it is still mounted on the drum?
Answer: Heating under constraint creates unpredictable residual stress and accelerates brittleness at the crease.
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Q20: Why are insert-ring heads less forgiving of local shrinkage or stiffness changes than other head types?
Answer: The internal ring structure constrains the seating geometry more rigidly.
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Q21: What is the recommended approach for a player regarding a brand-new head’s first few days?
Answer: Treat the period as seating and settling time rather than attempting final fine tuning.
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Q22: According to the hierarchy for teachers, what is the first step in head maintenance before addressing friction or tempering?
Answer: Geometry and seating.
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Q23: What pitch-related symptom suggests a timpani head has structural damage rather than a simple tuning issue?
Answer: The head is stable at one pitch but unstable at another.
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Q24: Term — Dimensional stability
Answer: The ability of a film to maintain its original size and shape under varying stress, time, and temperature.
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Q25: What forces are generated when an oriented film is heated and attempts to return to a less-ordered state?
Answer: Shrinkage stress.
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Q26: Why should extreme fortissimo playing with hard mallets be avoided on a brand-new timpani head?
Answer: The head has not yet seated and stabilized, making it more vulnerable to permanent impact damage.
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Q27: PET is the acronym for the polymer _____.
Answer: Polyethylene terephthalate.
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Q28: How does the “oriented” part of BOPET affect how the film responds to heat?
Answer: It ensures the film carries a memory of its manufacturing history, making thermal response non-neutral.
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Q29: In material science, low toughness that allows cracks to propagate easily is defined as _____.
Answer: Brittleness.
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Q30: What does a “weak principal tone,” especially in the low register, typically indicate about a timpani head?
Answer: The head is worn out or structurally compromised.
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Q31: How does “sweeping” a heat gun over a large area still pose a risk to the drumhead?
Answer: It increases the likelihood of warping a large section or damaging the critical collar and bearing-edge crease.
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Q32: True or False: According to the source, a PET head that looks fine visually will always hold a stable pitch center.
Answer: False.
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Q33: What technical term describes the high-curvature zone where the head wraps over the drum’s edge?
Answer: The bearing-edge crease (or collar region).
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Q34: Term — Impact damage
Answer: Localized structural compromise, such as dimples or micro-cracks, resulting from high-velocity hits.
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Q35: What is the primary factor that causes a head to behave differently after sitting, even if the lugs have not been moved?
Answer: Stress relaxation (and creep, both being time-dependent viscoelastic behaviors).
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Q36: Why is “looks better” an unreliable metric when using a heat gun to fix dimples?
Answer: The visual improvement may mask new localized stiffness mismatches and tension instabilities.
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Q37: What is the consequence of the “time-dependent” nature of polymer behavior in drumheads?
Answer: The drum’s behavior and tension can change even without user intervention.
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Q38: In the context of the source, what does the term “Mylar” typically represent to musicians?
Answer: Shorthand for oriented PET film.
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Q39: What is the “bottom line recommendation” when a head has a dimple deep enough to warrant heat treatment?
Answer: Replace the head, as it is already mechanically compromised.
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Q40: What should institutional replacement decisions be based on, rather than just the age of the head?
Answer: Pitch stability, seating behavior, and visible damage symptoms.
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Q41: How does PET film orientation relaxation occur at temperatures below the glass transition temperature (Tg)?
Answer: It can occur over multi-day exposures at warm temperatures.
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Q42: Why does a head “settle” instead of staying at its initial mounting tension?
Answer: The film undergoes creep (slow stretching) and stress relaxation under the tension of the lugs.
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Q43: In the hierarchy of drumhead setup, which step follows “friction and equalization”?
Answer: Fine tempering.
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Q44: What is the relationship between stiffness and vibration in a head treated with a heat gun?
Answer: The heat can cause local stiffness mismatches, leading to an uneven vibration and tension response.
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Q45: Term — Folding endurance
Answer: A measure of a material’s ability to withstand repeated bending before failing.
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Q46: Under what condition does an oriented PET film stop shrinking at a given temperature?
Answer: Once the film has already reached its maximum shrinkage for that specific temperature.
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Q47: Why is it difficult for a head to “heal” from an uneven pull at one sector during mounting?
Answer: Viscoelastic behavior is tension-history dependent, and high-stress zones deform differently than low-stress zones.
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Q48: What does a “crispy” texture at the bearing-edge crease imply about the safety of the head?
Answer: The crease has become a failure initiation point and may tear easily during handling.
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Q49: What is the goal of the “musician’s guide” provided in the source material?
Answer: To connect the physical behavior of PET film to practical decisions in mounting, maintenance, and replacement.
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Q50: How does the “biaxially oriented” manufacturing step affect the strength of the PET film?
Answer: It involves stretching the film in two directions at once, aligning polymer chains and locking in improved strength, dimensional stability, and resistance to deformation along both axes.